Small GTPases form an independent superfamily within the larger class of regulatory GTP hydrolases. This superfamily contains proteins that control a vast number of important processes and possess a common, structurally preserved GTP-binding domain [, ]. Sequence comparisons of small G proteins from various species have revealed that they are conserved in primary structures at the level of 30-55% similarity [].Crystallographic analysis of various small G proteins revealed the presence of a 20kDa catalytic domain that is unique for the whole superfamily [, ]. The domain is built of five alpha helices (A1-A5), six β-strands (B1-B6) and five polypeptide loops (G1-G5). A structural comparison of the GTP- and GDP-bound form, allows one to distinguish two functional loop regions: switch I and switch II that surround the gamma-phosphate group of the nucleotide. The G1 loop (also called the P-loop) that connects the B1 strand and the A1 helix is responsible for the binding of the phosphate groups. The G3 loop provides residues for Mg2 and phosphate binding and is located at the N terminus of the A2 helix. The G1 and G3 loops are sequentially similar to Walker A and Walker B boxes that are found in other nucleotide binding motifs. The G2 loop connects the A1 helix and the B2 strand and contains a conserved Thr residue responsible for Mg2 binding. The guanine base is recognised by the G4 and G5 loops. The consensus sequence NKXD of the G4 loop contains Lys and Asp residues directly interacting with the nucleotide. Part of the G5 loop located between B6 and A5 acts as a recognition site for the guanine base [].The small GTPase superfamily can be divided into at least 8 different families, including:Arf small GTPases. GTP-binding proteins involved in protein trafficking by modulating vesicle budding and uncoating within the Golgi apparatus.Ran small GTPases. GTP-binding proteins involved in nucleocytoplasmic transport. Required for the import of proteins into the nucleus and also for RNA export.Rab small GTPases. GTP-binding proteins involved in vesicular traffic.Rho small GTPases. GTP-binding proteins that control cytoskeleton reorganisation.Ras small GTPases. GTP-binding proteins involved in signalling pathways.Sar1 small GTPases. Small GTPase component of the coat protein complex II (COPII) which promotes the formation of transport vesicles from the endoplasmic reticulum (ER).Mitochondrial Rho (Miro). Small GTPase domain found in mitochondrial proteins involved in mitochondrial trafficking.Roc small GTPases domain. Small GTPase domain always found associated with the COR domain.This entry represents the Uncharacterized protein MJ1339 from Methanocaldococcus jannaschii, thought to be part of the wider small GTPase superfamily.
Small GTPases form an independent superfamily within the larger class of regulatory GTP hydrolases. This superfamily contains proteins that control a vast number of important processes and possess a common, structurally preserved GTP-binding domain [, ]. Sequence comparisons of small G proteins from various species have revealed that they are conserved in primary structures at the level of 30-55% similarity [].Crystallographic analysis of various small G proteins revealed the presence of a 20kDa catalytic domain that is unique for the whole superfamily [, ]. The domain is built of five alpha helices (A1-A5), six β-strands (B1-B6) and five polypeptide loops (G1-G5). A structural comparison of the GTP- and GDP-bound form, allows one to distinguish two functional loop regions: switch I and switch II that surround the gamma-phosphate group of the nucleotide. The G1 loop (also called the P-loop) that connects the B1 strand and the A1 helix is responsible for the binding of the phosphate groups. The G3 loop provides residues for Mg2 and phosphate binding and is located at the N terminus of the A2 helix. The G1 and G3 loops are sequentially similar to Walker A and Walker B boxes that are found in other nucleotide binding motifs. The G2 loop connects the A1 helix and the B2 strand and contains a conserved Thr residue responsible for Mg2 binding. The guanine base is recognised by the G4 and G5 loops. The consensus sequence NKXD of the G4 loop contains Lys and Asp residues directly interacting with the nucleotide. Part of the G5 loop located between B6 and A5 acts as a recognition site for the guanine base [].The small GTPase superfamily can be divided into at least 8 different families, including:Arf small GTPases. GTP-binding proteins involved in protein trafficking by modulating vesicle budding and uncoating within the Golgi apparatus.Ran small GTPases. GTP-binding proteins involved in nucleocytoplasmic transport. Required for the import of proteins into the nucleus and also for RNA export.Rab small GTPases. GTP-binding proteins involved in vesicular traffic.Rho small GTPases. GTP-binding proteins that control cytoskeleton reorganisation.Ras small GTPases. GTP-binding proteins involved in signalling pathways.Sar1 small GTPases. Small GTPase component of the coat protein complex II (COPII) which promotes the formation of transport vesicles from the endoplasmic reticulum (ER).Mitochondrial Rho (Miro). Small GTPase domain found in mitochondrial proteins involved in mitochondrial trafficking.Roc small GTPases domain. Small GTPase domain always found associated with the COR domain.
Gamma-aminobutyric acid type A (GABAA) receptors are members of the neurotransmitter ligand-gated ion channels: they mediate neuronal inhibition on binding GABA. The effects of GABA on GABAA receptors are modulated by a range of therapeutically important drugs, including barbiturates, anaesthetics and benzodiazepines (BZs) []. The BZs are a diverse range of compounds, including widely prescribed drugs, such as librium and valium, and their interaction with GABAA receptors provides the most potent pharmacological means of distinguishing different GABAA receptor subtypes.GABAA receptors are pentameric membrane proteins that operate GABA-gated chloride channels []. Eight types of receptor subunit have been cloned, with multiple subtypes within some classes: alpha 1-6, beta 1-4, gamma 1-4, delta, epsilon, pi, rho 1-3 and theta [, ]. Subunits are typically 50-60kDa in size and comprise a long N-terminal extracellular domain, containing a putative signal peptide and a disulphide-bonded beta structural loop; 4 putative transmembrane (TM) domains; and a large cytoplasmic loop connecting the third and fourth TM domains. Amongst family members, the large cytoplasmic loop displays the most divergence in terms of primary structure, the TM domains showing the highest level of sequence conservation [].Most GABAA receptors contain one type of alpha and beta subunit, and a single gamma polypeptide in a ratio of 2:2:1 [], though in some cases other subunits such as epsilon or delta may replace gamma. The BZ binding site is located at the interface of adjacent alpha and gamma subunits; therefore, the type of alpha and gamma subunits present is instrumental in determining BZ selectivity and sensitivity. Receptors can be categorised into 3 groups based on their alpha subunit content and, hence, sensitivity to BZs: alpha 1-containing receptors have greatest sensitivity towards BZs (type I); alpha 2, 3 and 5-containing receptors have similar but distinguishable properties (type II); and alpha 4- and 6-containing assemblies have very low BZ affinity []. A conserved histidine residue in the alpha subunit of type I and II receptors is believed to be responsible for BZ affinity []. This entry also includes glycine receptor subunit alpha []; subunits of the acetylcholine-gated chloride channel (ACC), Acetylcholine-gated ion channel acc-4 and Serotonin-gated chloride channel mod-1 in the nematode Caenorhabditis elegans; and Glutamate-gated chloride channel from Drosophila melanogaster (DrosGluCl). ACC is a heteropentamer consisting of ACC-1 and ACC-3, ACC-1 and ACC-4, ACC-2 and ACC-3 or ACC-2 and ACC-4. It is triggered in response to acetylcholine, but not GABA, glutamate, glycine, histamine or dopamine []. DrosGluCl plays an important role in the visual response by regulating the activity of ON/OFF-selective neurons [].
Guanine nucleotide binding proteins (G proteins) are membrane-associated, heterotrimeric proteins composed of three subunits: alpha (), beta () and gamma () []. G proteins and their receptors (GPCRs) form one of the most prevalent signalling systems in mammalian cells, regulating systems as diverse as sensory perception, cell growth and hormonal regulation []. At the cell surface, the binding of ligands such as hormones and neurotransmitters to a GPCR activates the receptor by causing a conformational change, which in turn activates the bound G protein on the intracellular-side of the membrane. The activated receptor promotes the exchange of bound GDP for GTP on the G protein alpha subunit. GTP binding changes the conformation of switch regions within the alpha subunit, which allows the bound trimeric G protein (inactive) to be released from the receptor, and to dissociate into active alpha subunit (GTP-bound) and beta/gamma dimer. The alpha subunit and the beta/gamma dimer go on to activate distinct downstream effectors, such as adenylyl cyclase, phosphodiesterases, phospholipase C, and ion channels. These effectors in turn regulate the intracellular concentrations of secondary messengers, such as cAMP, diacylglycerol, sodium or calcium cations, which ultimately lead to a physiological response, usually via the downstream regulation of gene transcription. The cycle is completed by the hydrolysis of alpha subunit-bound GTP to GDP, resulting in the re-association of the alpha and beta/gamma subunits and their binding to the receptor, which terminates the signal []. The length of the G protein signal is controlled by the duration of the GTP-bound alpha subunit, which can be regulated by RGS (regulator of G protein signalling) proteins or by covalent modifications [].G protein alpha subunits are 350-400 amino acids in length and havemolecular weights in the range 40-45kDa. Seventeen distinct types ofalpha subunit have been identified in mammals. These fall into 4 main groups on the basis of both sequence similarity and function: alpha-S (),alpha-Q (), alpha-I ()and alpha-12() [].The specific combination of subunits in heterotrimeric G proteins affects not only which receptor it can bind to, but also which downstream target is affected, providing the means to target specific physiological processes in response to specific external stimuli [, ]. G proteins carry lipid modifications on one or more of their subunits to target them to the plasma membrane and to contribute to protein interactions.This family consists of the alpha-12 group of G proteins, which includes both alpha-12 group of G proteins and alpha-13 group of G proteins. G alpha-12 and G alpha-13 are ubiquitously expressed and can induce many cellular responses, including phospholipase C-epsilon activation, phospholipase D activation, cytoskeletal change, oncogenic response, apoptosis, MAP kinase activation and Na/H-exchange activation. G alpha-12 and G alpha-13 can activate several effectors, including small GTPases such as Rho [].
Small GTPases form an independent superfamily within the larger class ofregulatory GTP hydrolases. This superfamily contains proteins that control avast number of important processes and possess a common, structurallypreserved GTP-binding domain [, ]. Sequence comparisons of small G proteinsfrom various species have revealed that they are conserved in primarystructures at the level of 30-55% similarity [].Crystallographic analysis of various small G proteins revealed the presence of a 20kDa catalytic domain that is unique for the whole superfamily [, ]. The domain is built of five alpha helices (A1-A5), sixβ-strands (B1-B6) and five polypeptide loops (G1-G5). A structuralcomparison of the GTP- and GDP-bound form, allows one to distinguish twofunctional loop regions: switch I and switch II that surround thegamma-phosphate group of the nucleotide. The G1 loop (also called the P-loop)that connects the B1 strand and the A1 helix is responsible for the binding ofthe phosphate groups. The G3 loop provides residues for Mg(2+) and phosphatebinding and is located at the N terminus of the A2 helix. The G1 and G3 loopsare sequentially similar to Walker A and Walker B boxes that are found inother nucleotide binding motifs. The G2 loop connects the A1 helix and the B2strand and contains a conserved Thr residue responsible for Mg(2+) binding.The guanine base is recognised by the G4 and G5 loops. The consensus sequenceNKXD of the G4 loop contains Lys and Asp residues directly interacting withthe nucleotide. Part of the G5 loop located between B6 and A5 acts as arecognition site for the guanine base [].The small GTPase superfamily can be divided in 8 different families:Arf small GTPases. GTP-binding proteins involved in protein trafficking bymodulating vesicle budding and un-coating within the Golgi apparatusRan small GTPases. GTP-binding proteins involved in nucleocytoplasmictransport. Required for the import of proteins into the nucleus and alsofor RNA exportRab small GTPases. GTP-binding proteins involved in vesicular traffic.Rho small GTPases. GTP-binding proteins that control cytoskeletonreorganisationRas small GTPases. GTP-binding proteins involved in signaling pathwaysSar1 small GTPases. Small GTPase component of the coat protein complex II(COPII) which promotes the formation of transport vesicles from theendoplasmic reticulum (ER)Mitochondrial Rho (Miro). Small GTPase domain found in mitochondrialproteins involved in mitochondrial traffickingRoc small GTPases domain. Small GTPase domain always found associated withthe COR domain.This entry represents the Roc small GTPase domain.
This entry represents the N-terminal PH domain of FGD1.In general, FGDs (including FGD1, FGD2, FGD3 and FGD4/Frabin) have a RhoGEF (DH) domain, followed by an N-terminal PH domain, a FYVE domain and a C-terminal PH domain. All FGDs are guanine nucleotide exchange factors that activates the Rho GTPase Cdc42, an important regulator of membrane trafficking. The RhoGEF domain is responsible for GEF catalytic activity, while the N-terminal PH domain is involved in intracellular targeting of the DH domain []. Mutations in the FGD1 gene are responsible for the X-linked disorder known as faciogenital dysplasia (FGDY) []. Both FGD1 and FGD3 are targeted by the ubiquitin ligase SCF(FWD1/beta-TrCP) upon phosphorylation of two serine residues in its DSGIDS motif and subsequently degraded by the proteasome. However, FGD1 and FGD3 induced significantly different morphological changes in HeLa Tet-Off cells and while FGD1 induced long finger-like protrusions, FGD3 induced broad sheet-like protrusions when the level of GTP-bound Cdc42 was significantly increased by the inducible expression of FGD3. They also reciprocally regulated cell motility in inducibly expressed in HeLa Tet-Off cells, FGD1 stimulated cell migration while FGD3 inhibited it. FGD1 and FGD3 therefore play different roles to regulate cellular functions, even though their intracellular levels are tightly controlled by the same destruction pathway through SCF(FWD1/beta-TrCP) [, ].PH domains have diverse functions, but in general are involved in targeting proteins to the appropriate cellular location or in the interaction with a binding partner []. They share little sequence conservation, but all have a common fold, which is electrostatically polarized. Less than 10% of PH domains bind phosphoinositide phosphates (PIPs) with high affinity and specificity []. PH domains are distinguished from other PIP-binding domains by their specific high-affinity binding to PIPs with two vicinal phosphate groups: PtdIns(3,4)P2, PtdIns(4,5)P2 or PtdIns(3,4,5)P3 which results in targeting some PH domain proteins to the plasma membrane []. A few display strong specificity in lipid binding. Any specificity is usually determined by loop regions or insertions in the N terminus of the domain, which are not conserved across all PH domains. PH domains are found in cellular signaling proteins such as serine/threonine kinase, tyrosine kinases, regulators of G-proteins, endocytotic GTPases, adaptors, as well as cytoskeletal associated molecules and in lipid associated enzymes [].
This entry represents the C-terminal PH domain of FGD1-4.In general, FGDs (including FGD1, FGD2, FGD3 and FGD4/Frabin) have a RhoGEF (DH) domain, followed by an N-terminal PH domain, a FYVE domain and a C-terminal PH domain. All FGDs are guanine nucleotide exchange factors that activates the Rho GTPase Cdc42, an important regulatorof membrane trafficking. The RhoGEF domain is responsible for GEF catalytic activity, while the N-terminal PH domain is involved in intracellular targeting of the DH domain []. Mutations in the FGD1 gene are responsible for the X-linked disorder known as faciogenital dysplasia (FGDY) []. Both FGD1 and FGD3 are targeted by the ubiquitin ligase SCF(FWD1/beta-TrCP) upon phosphorylation of two serine residues in its DSGIDS motif and subsequently degraded by the proteasome. However, FGD1 and FGD3 induced significantly different morphological changes in HeLa Tet-Off cells and while FGD1 induced long finger-like protrusions, FGD3 induced broad sheet-like protrusions when the level of GTP-bound Cdc42 was significantly increased by the inducible expression of FGD3. They also reciprocally regulated cell motility in inducibly expressed in HeLa Tet-Off cells, FGD1 stimulated cell migration while FGD3 inhibited it. FGD1 and FGD3 therefore play different roles to regulate cellular functions, even though their intracellular levels are tightly controlled by the same destruction pathway through SCF(FWD1/beta-TrCP) [, ].PH domains have diverse functions, but in general are involved in targeting proteins to the appropriate cellular location or in the interaction with a binding partner []. They share little sequence conservation, but all have a common fold, which is electrostatically polarized. Less than 10% of PH domains bind phosphoinositide phosphates (PIPs) with high affinity and specificity []. PH domains are distinguished from other PIP-binding domains by their specific high-affinity binding to PIPs with two vicinal phosphate groups: PtdIns(3,4)P2, PtdIns(4,5)P2 or PtdIns(3,4,5)P3 which results in targeting some PH domain proteins to the plasma membrane []. A few display strong specificity in lipid binding. Any specificity is usually determined by loop regions or insertions in the N terminus of the domain, which are not conserved across all PH domains. PH domains are found in cellular signaling proteins such as serine/threonine kinase, tyrosine kinases, regulators of G-proteins, endocytotic GTPases, adaptors, as well as cytoskeletal associated molecules and in lipid associated enzymes [].
Small GTPases form an independent superfamily within the larger class of regulatory GTP hydrolases. This superfamily contains proteins that control a vast number of important processes and possess a common, structurally preserved GTP-binding domain [, ]. Sequence comparisons of small G proteins from various species have revealed that they are conserved in primary structures at the level of 30-55% similarity [].Crystallographic analysis of various small G proteins revealed the presence of a 20kDa catalytic domain that is unique for the whole superfamily [, ]. The domain is built of five alpha helices (A1-A5), six β-strands (B1-B6) and five polypeptide loops (G1-G5). A structural comparison of the GTP- and GDP-bound form, allows one to distinguish two functional loop regions: switch I and switch II that surround the gamma-phosphate group of the nucleotide. The G1 loop (also called the P-loop) that connects the B1 strand and the A1 helix is responsible for the binding of the phosphate groups. The G3 loop provides residues for Mg2 and phosphate binding and is located at the N terminus of the A2 helix. The G1 and G3 loops are sequentially similar to Walker A and Walker B boxes that are found in other nucleotide binding motifs. The G2 loop connects the A1 helix and the B2 strand and contains a conserved Thr residue responsible for Mg2 binding. The guanine base is recognised by the G4 and G5 loops. The consensus sequence NKXD of the G4 loop contains Lys and Asp residues directly interacting with the nucleotide. Part of the G5 loop located between B6 and A5 acts as a recognition site for the guanine base [].The small GTPase superfamily can be divided into at least 8 different families, including:Arf small GTPases. GTP-binding proteins involved in protein trafficking by modulating vesicle budding and uncoating within the Golgi apparatus.Ran small GTPases. GTP-binding proteins involved in nucleocytoplasmic transport. Required for the import of proteins into the nucleus and also for RNA export.Rab small GTPases. GTP-binding proteins involved in vesicular traffic.Rho small GTPases. GTP-binding proteins that control cytoskeleton reorganisation.Ras small GTPases. GTP-binding proteins involved in signalling pathways.Sar1 small GTPases. Small GTPase component of the coat protein complex II (COPII) which promotes the formation of transport vesicles from the endoplasmic reticulum (ER).Mitochondrial Rho (Miro). Small GTPase domain found in mitochondrial proteins involved in mitochondrial trafficking.Roc small GTPases domain. Small GTPase domain always found associated with the COR domain.This entry represents a branch of the small GTPase superfamily that includes the ADP ribosylation factor Arf, Arl (Arf-like), Arp(Arf-related proteins) and the remotely related Sar (Secretion-associated and Ras-related) proteins. Arf proteins are major regulators of vesicle biogenesis in intracellular traffic []. They cycle between inactive GDP-bound and active GTP-bound forms that bind selectively to effectors. The classical structural GDP/GTP switch is characterised by conformational changes at the so-called switch 1 and switch 2 regions, which bind tightly to the gamma-phosphate of GTP but poorly or not at all to the GDP nucleotide. Structural studies of Arf1 and Arf6 have revealed that although these proteins feature the switch 1 and 2 conformational changes, they depart from other small GTP-binding proteins in that they use an additional, unique switch to propagate structural information from one side of the protein to the other. The GDP/GTP structural cycles of human Arf1 and Arf6 feature a unique conformational change that affects the beta2-beta3 strands connecting switch 1 and switch 2 (interswitch) and also the amphipathic helical N terminus. In GDP-boundArf1 and Arf6, the interswitch is retracted and forms a pocket to which the N-terminal helix binds, the latter serving as a molecular hasp to maintain the inactive conformation. In the GTP-bound form of these proteins, the interswitch undergoes a two-residue register shift that pulls switch 1 and switch 2 up, restoring an active conformation that can bind GTP. In this conformation, the interswitch projects out of the protein and extrudes the N-terminal hasp by occluding its binding pocket.
Small GTPases form an independent superfamily within the larger class of regulatory GTP hydrolases. This superfamily contains proteins that control a vast number of important processes and possess a common, structurally preserved GTP-binding domain [, ]. Sequence comparisons of small G proteins from various species have revealed that they are conserved in primary structures at the level of 30-55% similarity [].Crystallographic analysis of various small G proteins revealed the presence of a 20kDa catalytic domain that is unique for the whole superfamily [, ]. The domain is built of five alpha helices (A1-A5), six β-strands (B1-B6) and five polypeptide loops (G1-G5). A structural comparison of the GTP- and GDP-bound form, allows one to distinguish two functional loop regions: switch I and switch II that surround the gamma-phosphate group of the nucleotide. The G1 loop (also called the P-loop) that connects the B1 strand and the A1 helix is responsible for the binding of the phosphate groups. The G3 loop provides residues for Mg2 and phosphate binding and is located at the N terminus of the A2 helix. The G1 and G3 loops are sequentially similar to Walker A and Walker B boxes that are found in other nucleotide binding motifs. The G2 loop connects the A1 helix and the B2 strand and contains a conserved Thr residue responsible for Mg2 binding. The guanine base is recognised by the G4 and G5 loops. The consensus sequence NKXD of the G4 loop contains Lys and Asp residues directly interacting with the nucleotide. Part of the G5 loop located between B6 and A5 acts as a recognition site for the guanine base [].The small GTPase superfamily can be divided into at least 8 different families, including:Arf small GTPases. GTP-binding proteins involved in protein trafficking by modulating vesicle budding and uncoating within the Golgi apparatus.Ran small GTPases. GTP-binding proteins involved in nucleocytoplasmic transport. Required for the import of proteins into the nucleus and also for RNA export.Rab small GTPases. GTP-binding proteins involved in vesicular traffic.Rho small GTPases. GTP-binding proteins that control cytoskeleton reorganisation.Ras small GTPases. GTP-binding proteins involved in signalling pathways.Sar1 small GTPases. Small GTPase component of the coat protein complex II (COPII) which promotes the formation of transport vesicles from the endoplasmic reticulum (ER).Mitochondrial Rho (Miro). Small GTPase domain found in mitochondrial proteins involved in mitochondrial trafficking.Roc small GTPases domain. Small GTPase domain always found associated with the COR domain.The SAR1 [, ]protein, first identified in budding yeast, is a 21kDa GTP-binding protein involved in vesicular transport between the endoplasmicreticulum and the Golgi []. It is a GTP-binding protein that takes part in theformation of secretory vesicles by binding to an ER type II membraneprotein, Sec12p []. It is evolutionary conserved and seems to be presentin all eukaryotes.SAR1 is generally included in the RAS 'superfamily' of small GTP-bindingproteins, but it is only slightly related to other RAS proteins. It alsodiffers from RAS proteins in that it lacks cysteine residues at the C terminusand is therefore not subject to prenylation. SAR1 is slightly related to ARFs.
Small GTPases form an independent superfamily within the larger class of regulatory GTP hydrolases. This superfamily contains proteins that control a vast number of important processes and possess a common, structurally preserved GTP-binding domain [, ]. Sequence comparisons of small G proteins from various species have revealed that they are conserved in primary structures at the level of 30-55% similarity [].Crystallographic analysis of various small G proteins revealed the presence of a 20kDa catalytic domain that is unique for the whole superfamily [, ]. The domain is built of five alpha helices (A1-A5), six β-strands (B1-B6) and five polypeptide loops (G1-G5). A structural comparison of the GTP- and GDP-bound form, allows one to distinguish two functional loop regions: switch I and switch II that surround the gamma-phosphate group of the nucleotide. The G1 loop (also called the P-loop) that connects the B1 strand and the A1 helix is responsible for the binding of the phosphate groups. The G3 loop provides residues for Mg2 and phosphate binding and is located at the N terminus of the A2 helix. The G1 and G3 loops are sequentially similar to Walker A and Walker B boxes that are found in other nucleotide binding motifs. The G2 loop connects the A1 helix and the B2 strand and contains a conserved Thr residue responsible for Mg2 binding. The guanine base is recognised by the G4 and G5 loops. The consensus sequence NKXD of the G4 loop contains Lys and Asp residues directly interacting with the nucleotide. Part of the G5 loop located between B6 and A5 acts as a recognition site for the guanine base [].The small GTPase superfamily can be divided into at least 8 different families, including:Arf small GTPases. GTP-binding proteins involved in protein trafficking by modulating vesicle budding and uncoating within the Golgi apparatus.Ran small GTPases. GTP-binding proteins involved in nucleocytoplasmic transport. Required for the import of proteins into the nucleus and also for RNA export.Rab small GTPases. GTP-binding proteins involved in vesicular traffic.Rho small GTPases. GTP-binding proteins that control cytoskeleton reorganisation.Ras small GTPases. GTP-binding proteins involved in signalling pathways.Sar1 small GTPases. Small GTPase component of the coat protein complex II (COPII) which promotes the formation of transport vesicles from the endoplasmic reticulum (ER).Mitochondrial Rho (Miro). Small GTPase domain found in mitochondrial proteins involved in mitochondrial trafficking.Roc small GTPases domain. Small GTPase domain always found associated with the COR domain.This entry represents a branch of the small GTPase superfamily that includes the ADP ribosylation factor Arf, Arl (Arf-like), and Arp(Arf-related proteins). Arf proteins are major regulators of vesicle biogenesis in intracellular traffic []. They cycle between inactive GDP-bound and active GTP-bound forms that bind selectively to effectors. The classical structural GDP/GTP switch is characterised by conformational changes at the so-called switch 1 and switch 2 regions, which bind tightly to the gamma-phosphate of GTP but poorly or not at all to the GDP nucleotide. Structural studies of Arf1 and Arf6 have revealed that although these proteins feature the switch 1 and 2 conformational changes, they depart from other small GTP-binding proteins in that they use an additional, unique switch to propagate structural information from one side of the protein to the other. The GDP/GTP structural cycles of human Arf1 and Arf6 feature a unique conformational change that affects the beta2-beta3 strands connecting switch 1 and switch 2 (interswitch) and also the amphipathic helical N terminus. In GDP-boundArf1 and Arf6, the interswitch is retracted and forms a pocket to which the N-terminal helix binds, the latter serving as a molecular hasp to maintain the inactive conformation. In the GTP-bound form of these proteins, the interswitch undergoes a two-residue register shift that pulls switch 1 and switch 2 up, restoring an active conformation that can bind GTP. In this conformation, the interswitch projects out of the protein and extrudes the N-terminal hasp by occluding its binding pocket.
Small GTPases form an independent superfamily within the larger class of regulatory GTP hydrolases. This superfamily contains proteins that control a vast number of important processes and possess a common, structurally preserved GTP-binding domain [, ]. Sequence comparisons of small G proteins from various species have revealed that they are conserved in primary structures at the level of 30-55% similarity [].Crystallographic analysis of various small G proteins revealed the presence of a 20kDa catalytic domain that is unique for the whole superfamily [, ]. The domain is built of five alpha helices (A1-A5), six β-strands (B1-B6) and five polypeptide loops (G1-G5). A structural comparison of the GTP- and GDP-bound form, allows one to distinguish two functional loop regions: switch I and switch II that surround the gamma-phosphate group of the nucleotide. The G1 loop (also called the P-loop) that connects the B1 strand and the A1 helix is responsible for the binding of the phosphate groups. The G3 loop provides residues for Mg2 and phosphate binding and is located at the N terminus of the A2 helix. The G1 and G3 loops are sequentially similar to Walker A and Walker B boxes that are found in other nucleotide binding motifs. The G2 loop connects the A1 helix and the B2 strand and contains a conserved Thr residue responsible for Mg2 binding. The guanine base is recognised by the G4 and G5 loops. The consensus sequence NKXD of the G4 loop contains Lys and Asp residues directly interacting with the nucleotide. Part of the G5 loop located between B6 and A5 acts as a recognition site for the guanine base [].The small GTPase superfamily can be divided into at least 8 different families, including:Arf small GTPases. GTP-binding proteins involved in protein trafficking by modulating vesicle budding and uncoating within the Golgi apparatus.Ran small GTPases. GTP-binding proteins involved in nucleocytoplasmic transport. Required for the import of proteins into the nucleus and also for RNA export.Rab small GTPases. GTP-binding proteins involved in vesicular traffic.Rho small GTPases. GTP-binding proteins that control cytoskeleton reorganisation.Ras small GTPases. GTP-binding proteins involved in signalling pathways.Sar1 small GTPases. Small GTPase component of the coat protein complex II (COPII) which promotes the formation of transport vesicles from the endoplasmic reticulum (ER).Mitochondrial Rho (Miro). Small GTPase domain found in mitochondrial proteins involved in mitochondrial trafficking.Roc small GTPases domain. Small GTPase domain always found associated with the COR domain.Ras proteins are small GTPases that regulate cell growth, proliferation anddifferentiation. The different Ras isoforms: H-ras, N-ras and K-ras, generate distinct signaloutputs, despite interactingwith a common set of activators and effectors. Ras is activated by guanine nucleotide exchange factors (GEFs) thatrelease GDP and allow GTP binding. Many RasGEFs have been identified.These are sequestered in the cytosol until activation by growth factorstriggers recruitment to the plasma membrane or Golgi, where the GEFcolocalizes with Ras. Active GTP-bound Ras interacts with severaleffector proteins: among the best characterised are the Raf kinases,phosphatidylinositol 3-kinase (PI3K), RalGEFs and NORE/MST1. Ras proteins are synthesized ascytosolic precursors that undergo post-translational processing to be ableto associate with cell membranes []. First, protein farnesyl transferase, a cytosolicenzyme, attaches a farnesyl group to the cysteine residue of the CAAXmotif. Second, the farnesylated CAAX sequence targets Ras to thecytosolic surface of the ER where an endopeptidase removes the AAX tripeptide. Third, the α-carboxyl group on the now carboxy-terminal farnesylcysteine ismethylated by isoprenylcysteine carboxyl methyltransferase. Finally, after methylation, Ras proteins take one oftwo routes to the cell surface, which is dictated by a second targetingsignal that is located immediately amino-terminal to the farnesylatedcysteine. N-ras and H-ras are expressedstably on the plasma membrane, on Golgi in transfected cells, and at least transiently on the ER. Ras has also been visualized on endosomes.
Neurotransmitter ligand-gated ion channels are transmembrane receptor-ion channel complexes that open transiently upon binding of specific ligands, allowing rapid transmission of signals at chemical synapses [, ]. Five of these ion channel receptor families have been shown to form a sequence-related superfamily:Nicotinic acetylcholine receptor (AchR), an excitatory cation channel in vertebrates and invertebrates; in vertebrate motor endplates it is composed of alpha, beta, gamma and delta/epsilon subunits; in neurons it is composed of alpha and non-alpha (or beta) subunits [].Glycine receptor, an inhibitory chloride ion channel composed of alpha and beta subunits [].Gamma-aminobutyric acid (GABA) receptor, an inhibitory chloride ion channel; at least four types of subunits (alpha, beta, gamma and delta) are known [].Serotonin 5HT3 receptor, of which there are seven major types (5HT3-5HT7) [].Glutamate receptor, an excitatory cation channel of which at least three types have been described (kainate, N-methyl-D-aspartate (NMDA) and quisqualate) [].These receptors possess a pentameric structure (made up of varying subunits), surrounding a central pore. All known sequences of subunits from neurotransmitter-gated ion-channels are structurally related. They are composed of a large extracellular glycosylated N-terminal ligand-binding domain, followed by three hydrophobic transmembrane regions which form the ionic channel, followed by an intracellular region of variable length. A fourth hydrophobic region is found at the C-terminal of the sequence [, ].Gamma-aminobutyric acid type A (GABAA) receptors are members of the neurotransmitter ligand-gated ion channels: they mediate neuronal inhibition on binding GABA. The effects of GABA on GABAA receptors are modulated by a range of therapeutically important drugs, including barbiturates, anaesthetics and benzodiazepines (BZs) []. The BZs are a diverse range of compounds, including widely prescribed drugs, such as librium and valium, and their interaction with GABAA receptors provides the most potent pharmacological means of distinguishing different GABAA receptor subtypes.GABAA receptors are pentameric membrane proteins that operate GABA-gated chloride channels []. Eight types of receptor subunit have been cloned, with multiple subtypes within some classes: alpha 1-6, beta 1-4, gamma 1-4, delta, epsilon, pi, rho 1-3 and theta [, ]. Subunits are typically 50-60kDa in size and comprise a long N-terminal extracellular domain, containing a putative signal peptide and a disulphide-bonded beta structural loop; 4 putative transmembrane (TM) domains; and a large cytoplasmic loop connecting the third and fourth TM domains. Amongst family members, the large cytoplasmic loop displays the most divergence in terms of primary structure, the TM domains showing the highest level of sequence conservation [].Most GABAA receptors contain one type of alpha and beta subunit, and a single gamma polypeptide in a ratio of 2:2:1 [], though in some cases other subunits such as epsilon or delta may replace gamma. The BZ binding site is located at the interface of adjacent alpha and gamma subunits; therefore, the type of alpha and gamma subunits present is instrumental in determining BZ selectivity and sensitivity. Receptors can be categorised into 3 groups based on their alpha subunit content and, hence, sensitivity to BZs: alpha 1-containing receptors have greatest sensitivity towards BZs (type I); alpha 2, 3 and 5-containing receptors have similar but distinguishable properties (type II); and alpha 4- and 6-containing assemblies have very low BZ affinity []. A conserved histidine residue in the alpha subunit of type I and II receptors is believed to be responsible for BZ affinity []. A cDNA encoding the human GABAA receptor beta 2 subunit has been cloned andsequenced []. Expression of recombinant human GABAA receptors containingdifferent beta subunits (beta 1, beta 2 or beta 3) in both transfectedcells and Xenopus laevis oocytes, has revealed the influence of the beta subunit onthe pharmacology of the receptor. For a number of benzodiazepine bindingsite compounds, a barbiturate, and several neurosteroids, neither theaffinity nor the efficacy of the compounds is influenced by the type ofbeta subunit present in the receptor molecule []. These observationssuggest that the beta subunit does not significantly influence thebenzodiazepine, barbiturate, or steroid site pharmacologies of human GABAAreceptor subtypes [].
Small GTPases form an independent superfamily within the larger class of regulatory GTP hydrolases. This superfamily contains proteins that control a vast number of important processes and possess a common, structurally preserved GTP-binding domain [, ]. Sequence comparisons of small G proteins from various species have revealed that they are conserved in primary structures at the level of 30-55% similarity [].Crystallographic analysis of various small G proteins revealed the presence of a 20kDa catalytic domain that is unique for the whole superfamily [, ]. The domain is built of five alpha helices (A1-A5), six β-strands (B1-B6) and five polypeptide loops (G1-G5). A structural comparison of the GTP- and GDP-bound form, allows one to distinguish two functional loop regions: switch I and switch II that surround the gamma-phosphate group of the nucleotide. The G1 loop (also called the P-loop) that connects the B1 strand and the A1 helix is responsible for the binding of the phosphate groups. The G3 loop provides residues for Mg2 and phosphate binding and is located at the N terminus of the A2 helix. The G1 and G3 loops are sequentially similar to Walker A and Walker B boxes that are found in other nucleotide binding motifs. The G2 loop connects the A1 helix and the B2 strand and contains a conserved Thr residue responsible for Mg2 binding. The guanine base is recognised by the G4 and G5 loops. The consensus sequence NKXD of the G4 loop contains Lys and Asp residues directly interacting with the nucleotide. Part of the G5 loop located between B6 and A5 acts as a recognition site for the guanine base [].The small GTPase superfamily can be divided into at least 8 different families, including:Arf small GTPases. GTP-binding proteins involved in protein trafficking by modulating vesicle budding and uncoating within the Golgi apparatus.Ran small GTPases. GTP-binding proteins involved in nucleocytoplasmic transport. Required for the import of proteins into the nucleus and also for RNA export.Rab small GTPases. GTP-binding proteins involved in vesicular traffic.Rho small GTPases. GTP-binding proteins that control cytoskeleton reorganisation.Ras small GTPases. GTP-binding proteins involved in signalling pathways.Sar1 small GTPases. Small GTPase component of the coat protein complex II (COPII) which promotes the formation of transport vesicles from the endoplasmic reticulum (ER).Mitochondrial Rho (Miro). Small GTPase domain found in mitochondrial proteins involved in mitochondrial trafficking.Roc small GTPases domain. Small GTPase domain always found associated with the COR domain.This entry includes Tem1 from budding yeasts and Spg1 from fission yeasts. They are GTPases involved in the regulation of the cell cycle. In Schizosaccharomyces pombe, Spg1 is required for the localisation of Cdc7 (part of the septation initiation network) to the spindle pole body (SPB) []. It is regulated negatively by a GTPase-activating protein (GAP) comprising two subunits - Byr4 and Cdc16. In anaphase B, Spg1 is localised on the new SPB []. In Saccharomyces cerevisiae, Tem1 is associated with the mitotic exit network (MEN). It is involved in termination of M phase of the cell cycle [].
Neurotransmitter ligand-gated ion channels are transmembrane receptor-ion channel complexes that open transiently upon binding of specific ligands, allowing rapid transmission of signals at chemical synapses [, ]. Five of these ion channel receptor families have been shown to form a sequence-related superfamily:Nicotinic acetylcholine receptor (AchR), an excitatory cation channel in vertebrates and invertebrates; in vertebrate motor endplates it is composed of alpha, beta, gamma and delta/epsilon subunits; in neurons it is composed of alpha and non-alpha (or beta) subunits [].Glycine receptor, an inhibitory chloride ion channel composed of alpha and beta subunits [].Gamma-aminobutyric acid (GABA) receptor, an inhibitory chloride ion channel; at least four types of subunits (alpha, beta, gamma and delta) are known [].Serotonin 5HT3 receptor, of which there are seven major types (5HT3-5HT7) [].Glutamate receptor, an excitatory cation channel of which at least three types have been described (kainate, N-methyl-D-aspartate (NMDA) and quisqualate) [].These receptors possess a pentameric structure (made up of varying subunits), surrounding a central pore. All known sequences of subunits from neurotransmitter-gated ion-channels are structurally related. They are composed of a large extracellular glycosylated N-terminal ligand-binding domain, followed by three hydrophobic transmembrane regions which form the ionic channel, followed by an intracellular region of variable length. A fourth hydrophobic region is found at the C-terminal of the sequence [, ].Gamma-aminobutyric acid type A (GABAA) receptors are members of the neurotransmitter ligand-gated ion channels: they mediate neuronal inhibition on binding GABA. The effects of GABA on GABAA receptors are modulated by a range of therapeutically important drugs, including barbiturates, anaesthetics and benzodiazepines (BZs) []. The BZs are a diverse range of compounds, including widely prescribed drugs, such as librium and valium, and their interaction with GABAA receptors provides the most potent pharmacological means of distinguishing different GABAA receptor subtypes.GABAA receptors are pentameric membrane proteins that operate GABA-gated chloride channels []. Eight types of receptor subunit have been cloned, with multiple subtypes within some classes: alpha 1-6, beta 1-4, gamma 1-4, delta, epsilon, pi, rho 1-3 and theta [, ]. Subunits are typically 50-60kDa in size and comprise a long N-terminal extracellular domain, containing a putative signal peptide and a disulphide-bonded beta structural loop; 4 putative transmembrane (TM) domains; and a large cytoplasmic loop connecting the third and fourth TM domains. Amongst family members, the large cytoplasmic loop displays the most divergence in terms of primary structure, the TM domains showing the highest level of sequence conservation [].Most GABAA receptors contain one type of alpha and beta subunit, and a single gamma polypeptide in a ratio of 2:2:1 [], though in some cases other subunits such as epsilon or delta may replace gamma. The BZ binding site is located at the interface of adjacent alpha and gamma subunits; therefore, the type of alpha and gamma subunits present is instrumental in determining BZ selectivity and sensitivity. Receptors can be categorised into 3 groups based on their alpha subunit content and, hence, sensitivity to BZs: alpha 1-containing receptors have greatest sensitivity towards BZs (type I); alpha 2, 3 and 5-containing receptors have similar but distinguishable properties (type II); and alpha 4- and 6-containing assemblies have very low BZ affinity []. A conserved histidine residue in the alpha subunit of type I and II receptors is believed to be responsible for BZ affinity []. Amongst type II members, alpha 5-containing receptors exhibit an altered BZselectivity. Affinity towards the compound zolpidem distinguishes alpha 5from alpha 2- and alpha 3-containing receptor subtypes []. The alpha 5subunit is expressed primarily in the hippocampus.
Neurotransmitter ligand-gated ion channels are transmembrane receptor-ion channel complexes that open transiently upon binding of specific ligands, allowing rapid transmission of signals at chemical synapses [, ]. Five of these ion channel receptor families have been shown to form a sequence-related superfamily:Nicotinic acetylcholine receptor (AchR), an excitatory cation channel in vertebrates and invertebrates; in vertebrate motor endplates it is composed of alpha, beta, gamma and delta/epsilon subunits; in neurons it is composed of alpha and non-alpha (or beta) subunits [].Glycine receptor, an inhibitory chloride ion channel composed of alpha and beta subunits [].Gamma-aminobutyric acid (GABA) receptor, an inhibitory chloride ion channel; at least four types of subunits (alpha, beta, gamma and delta) are known [].Serotonin 5HT3 receptor, of which there are seven major types (5HT3-5HT7) [].Glutamate receptor, an excitatory cation channel of which at least three types have been described (kainate, N-methyl-D-aspartate (NMDA) and quisqualate) [].These receptors possess a pentameric structure (made up of varying subunits), surrounding a central pore. All known sequences of subunits from neurotransmitter-gated ion-channels are structurally related. They are composed of a large extracellular glycosylated N-terminal ligand-binding domain, followed by three hydrophobic transmembrane regions which form the ionic channel, followed by an intracellular region of variable length. A fourth hydrophobic region is found at the C-terminal of the sequence [, ].Gamma-aminobutyric acid type A (GABAA) receptors are members of the neurotransmitter ligand-gated ion channels: they mediate neuronal inhibition on binding GABA. The effects of GABA on GABAA receptors are modulated by a range of therapeutically important drugs, including barbiturates, anaesthetics and benzodiazepines (BZs) []. The BZs are a diverse range of compounds, including widely prescribed drugs, such as librium and valium, and their interaction with GABAA receptors provides the most potent pharmacological means of distinguishing different GABAA receptor subtypes.GABAA receptors are pentameric membrane proteins that operate GABA-gated chloride channels []. Eight types of receptor subunit have been cloned, with multiple subtypes within some classes: alpha 1-6, beta 1-4, gamma 1-4, delta, epsilon, pi, rho 1-3 and theta [, ]. Subunits are typically 50-60kDa in size and comprise a long N-terminal extracellular domain, containing a putative signal peptide and a disulphide-bonded beta structural loop; 4 putative transmembrane (TM) domains; and a large cytoplasmic loop connecting the third and fourth TM domains. Amongst family members, the large cytoplasmic loop displays the most divergence in terms of primary structure, the TM domains showing the highest level of sequence conservation [].Most GABAA receptors contain one type of alpha and beta subunit, and a single gamma polypeptide in a ratio of 2:2:1 [], though in some cases other subunits such as epsilon or delta may replace gamma. The BZ binding site is located at the interface of adjacent alpha and gamma subunits; therefore, the type of alpha and gamma subunits present is instrumental in determining BZ selectivity and sensitivity. Receptors can be categorised into 3 groups based on their alpha subunit content and, hence, sensitivity to BZs: alpha 1-containing receptors have greatest sensitivity towards BZs (type I); alpha 2, 3 and 5-containing receptors have similar but distinguishable properties (type II); and alpha 4- and 6-containing assemblies have very low BZ affinity []. A conserved histidine residue in the alpha subunit of type I and II receptors is believed to be responsible for BZ affinity []. Alpha subunitslargely determine benzodiazepine binding properties []. Mutagenesis and agonist/antagonist binding studies have suggested a close functional and structural association of alpha-subunits with the agonist/antagonist binding site, and involvement of N-terminal portions of the extracellular domains of all subunits in the gating of the channel [].
Neurotransmitter ligand-gated ion channels are transmembrane receptor-ion channel complexes that open transiently upon binding of specific ligands, allowing rapid transmission of signals at chemical synapses [, ]. Five of these ion channel receptor families have been shown to form a sequence-related superfamily:Nicotinic acetylcholine receptor (AchR), an excitatory cation channel in vertebrates and invertebrates; in vertebrate motor endplates it is composed of alpha, beta, gamma and delta/epsilon subunits; in neurons it is composed of alpha and non-alpha (or beta) subunits [].Glycine receptor, an inhibitory chloride ion channel composed of alpha and beta subunits [].Gamma-aminobutyric acid (GABA) receptor, an inhibitory chloride ion channel; at least four types of subunits (alpha, beta, gamma and delta) are known [].Serotonin 5HT3 receptor, of which there are seven major types (5HT3-5HT7) [].Glutamate receptor, an excitatory cation channel of which at least three types have been described (kainate, N-methyl-D-aspartate (NMDA) and quisqualate) [].These receptors possess a pentameric structure (made up of varying subunits), surrounding a central pore. All known sequences of subunits from neurotransmitter-gated ion-channels are structurally related. They are composed of a large extracellular glycosylated N-terminal ligand-binding domain, followed by three hydrophobic transmembrane regions which form the ionic channel, followed by an intracellular region of variable length. A fourth hydrophobic region is found at the C-terminal of the sequence [, ].Gamma-aminobutyric acid type A (GABAA) receptors are members of the neurotransmitter ligand-gated ion channels: they mediate neuronal inhibition on binding GABA. The effects of GABA on GABAA receptors are modulated by a range of therapeutically important drugs, including barbiturates, anaesthetics and benzodiazepines (BZs) []. The BZs are a diverse range of compounds, including widely prescribed drugs, such as librium and valium, and their interaction with GABAA receptors provides the most potent pharmacological means of distinguishing different GABAA receptor subtypes.GABAA receptors are pentameric membrane proteins that operate GABA-gated chloride channels []. Eight types of receptor subunit have been cloned, with multiple subtypes within some classes: alpha 1-6, beta 1-4, gamma 1-4, delta, epsilon, pi, rho 1-3 and theta [, ]. Subunits are typically 50-60kDa in size and comprise a long N-terminal extracellular domain, containing a putative signal peptide and a disulphide-bonded beta structural loop; 4 putative transmembrane (TM) domains; and a large cytoplasmic loop connecting the third and fourth TM domains. Amongst family members, the large cytoplasmic loop displays the most divergence in terms of primary structure, the TM domains showing the highest level of sequence conservation [].Most GABAA receptors contain one type of alpha and beta subunit, and a single gamma polypeptide in a ratio of 2:2:1 [], though in some cases other subunits such as epsilon or delta may replace gamma. The BZ binding site is located at the interface of adjacent alpha and gamma subunits; therefore, the type of alpha and gamma subunits present is instrumental in determining BZ selectivity and sensitivity. Receptors can be categorised into 3 groups based on their alpha subunit content and, hence, sensitivity to BZs: alpha 1-containing receptors have greatest sensitivity towards BZs (type I); alpha 2, 3 and 5-containing receptors have similar but distinguishable properties (type II); and alpha 4- and 6-containing assemblies have very low BZ affinity []. A conserved histidine residue in the alpha subunit of type I and II receptors is believed to be responsible for BZ affinity []. Alpha 4- and 6-containing receptors are often referred to as `diazepam-insensitive' receptors, since inclusion of such subunits in place of type Iand II polypeptides virtually eliminates sensitivity to BZs. The alpha 6subunit is found exclusively in the cerebellar granule cells. It alsoundergoes alternative splicing, generating a non-functional isoform thatlacks 10 residues from the N-terminal domain [Ashcroft, F.M.GABA(A) Receptors. In Ion Channels and Disease. Academic Press 2000, PP.325-336.].
Neurotransmitter ligand-gated ion channels are transmembrane receptor-ion channel complexes that open transiently upon binding of specific ligands, allowing rapid transmission of signals at chemical synapses [, ]. Five of these ion channel receptor families have been shown to form a sequence-related superfamily:Nicotinic acetylcholine receptor (AchR), an excitatory cation channel in vertebrates and invertebrates; in vertebrate motor endplates it is composed of alpha, beta, gamma and delta/epsilon subunits; in neurons it is composed of alpha and non-alpha (or beta) subunits [].Glycine receptor, an inhibitory chloride ion channel composed of alpha and beta subunits [].Gamma-aminobutyric acid (GABA) receptor, an inhibitory chloride ion channel; at least four types of subunits (alpha, beta, gamma and delta) are known [].Serotonin 5HT3 receptor, of which there are seven major types (5HT3-5HT7) [].Glutamate receptor, an excitatory cation channel of which at least three types have been described (kainate, N-methyl-D-aspartate (NMDA) and quisqualate) [].These receptors possess a pentameric structure (made up of varying subunits), surrounding a central pore. All known sequences of subunits from neurotransmitter-gated ion-channels are structurally related. They are composed of a large extracellular glycosylated N-terminal ligand-binding domain, followed by three hydrophobic transmembrane regions which form the ionic channel, followed by an intracellular region of variable length. A fourth hydrophobic region is found at the C-terminal of the sequence [, ].Gamma-aminobutyric acid type A (GABAA) receptors are members of the neurotransmitter ligand-gated ion channels: they mediate neuronal inhibition on binding GABA. The effects of GABA on GABAA receptors are modulated by a range of therapeutically important drugs, including barbiturates, anaesthetics and benzodiazepines (BZs) []. The BZs are a diverse range ofcompounds, including widely prescribed drugs, such as librium and valium, and their interaction with GABAA receptors provides the most potent pharmacological means of distinguishing different GABAA receptor subtypes.GABAA receptors are pentameric membrane proteins that operate GABA-gated chloride channels []. Eight types of receptor subunit have been cloned, with multiple subtypes within some classes: alpha 1-6, beta 1-4, gamma 1-4, delta, epsilon, pi, rho 1-3 and theta [, ]. Subunits are typically 50-60kDa in size and comprise a long N-terminal extracellular domain, containing a putative signal peptide and a disulphide-bonded beta structural loop; 4 putative transmembrane (TM) domains; and a large cytoplasmic loop connecting the third and fourth TM domains. Amongst family members, the large cytoplasmic loop displays the most divergence in terms of primary structure, the TM domains showing the highest level of sequence conservation [].Most GABAA receptors contain one type of alpha and beta subunit, and a single gamma polypeptide in a ratio of 2:2:1 [], though in some cases other subunits such as epsilon or delta may replace gamma. The BZ binding site is located at the interface of adjacent alpha and gamma subunits; therefore, the type of alpha and gamma subunits present is instrumental in determining BZ selectivity and sensitivity. Receptors can be categorised into 3 groups based on their alpha subunit content and, hence, sensitivity to BZs: alpha 1-containing receptors have greatest sensitivity towards BZs (type I); alpha 2, 3 and 5-containing receptors have similar but distinguishable properties (type II); and alpha 4- and 6-containing assemblies have very low BZ affinity []. A conserved histidine residue in the alpha subunit of type I and II receptors is believed to be responsible for BZ affinity []. Amongst type II members, alpha 2-containing receptors have a very similar BZaffinity profile to those containing the alpha 3 subunit. Differences inefficacy can be exploited to differentiate such subtypes: for example, thecompound flunitrazepam has a greater efficacy on the alpha 2 subtype. Alpha3-containing receptors are less widely distributed and are found primarilyin the cerebellum granule layer [].
Neurotransmitter ligand-gated ion channels are transmembrane receptor-ion channel complexes that open transiently upon binding of specific ligands, allowing rapid transmission of signals at chemical synapses [, ]. Five of these ion channel receptor families have been shown to form a sequence-related superfamily:Nicotinic acetylcholine receptor (AchR), an excitatory cation channel in vertebrates and invertebrates; in vertebrate motor endplates it is composed of alpha, beta, gamma and delta/epsilon subunits; in neurons it is composed of alpha and non-alpha (or beta) subunits [].Glycine receptor, an inhibitory chloride ion channel composed of alpha and beta subunits [].Gamma-aminobutyric acid (GABA) receptor, an inhibitory chloride ion channel; at least four types of subunits (alpha, beta, gamma and delta) are known [].Serotonin 5HT3 receptor, of which there are seven major types (5HT3-5HT7) [].Glutamate receptor, an excitatory cation channel of which at least three types have been described (kainate, N-methyl-D-aspartate (NMDA) and quisqualate) [].These receptors possess a pentameric structure (made up of varying subunits), surrounding a central pore. All known sequences of subunits from neurotransmitter-gated ion-channels are structurally related. They are composed of a large extracellular glycosylated N-terminal ligand-binding domain, followed by three hydrophobic transmembrane regions which form the ionic channel, followed by an intracellular region of variable length. A fourth hydrophobic region is found at the C-terminal of the sequence [, ].Gamma-aminobutyric acid type A (GABAA) receptors are members of the neurotransmitter ligand-gated ion channels: they mediate neuronal inhibition on binding GABA. The effects of GABA on GABAA receptors are modulated by a range of therapeutically important drugs, including barbiturates, anaesthetics and benzodiazepines (BZs) []. The BZs are a diverse range of compounds, including widely prescribed drugs, such as librium and valium, and their interaction with GABAA receptors provides the most potent pharmacological means of distinguishing different GABAA receptor subtypes.GABAA receptors are pentameric membrane proteins that operate GABA-gated chloride channels []. Eight types of receptor subunit have been cloned, with multiple subtypes within some classes: alpha 1-6, beta 1-4, gamma 1-4, delta, epsilon, pi, rho 1-3 and theta [, ]. Subunits are typically 50-60kDa in size and comprise a long N-terminal extracellular domain, containing a putative signal peptide and a disulphide-bonded beta structural loop; 4 putative transmembrane (TM) domains; and a large cytoplasmic loop connecting the third and fourth TM domains. Amongst family members, the large cytoplasmic loop displays the most divergence in terms of primary structure, the TM domains showing the highest level of sequence conservation [].Most GABAA receptors contain one type of alpha and beta subunit, and a single gamma polypeptide in a ratio of 2:2:1 [], though in some cases other subunits such as epsilon or delta may replace gamma. The BZ binding site is located at the interface of adjacent alpha and gamma subunits; therefore, the type of alpha and gamma subunits present is instrumental in determining BZ selectivity and sensitivity. Receptors can be categorised into 3 groups based on their alpha subunit content and, hence, sensitivity to BZs: alpha 1-containing receptors have greatest sensitivity towards BZs (type I); alpha 2, 3 and 5-containing receptors have similar but distinguishable properties (type II); and alpha 4- and 6-containing assemblies have very low BZ affinity []. A conserved histidine residue in the alpha subunit of type I and II receptors is believed to be responsible for BZ affinity []. Alpha 4- and 6-containing receptors are often referred to as `diazepam- insensitive' receptors, since inclusion of such subunits in place of type I and II polypeptides virtually eliminates sensitivity to BZs. The alpha 4 receptor subtype is localised primarily in the thalamus [].
Neurotransmitter ligand-gated ion channels are transmembrane receptor-ion channel complexes that open transiently upon binding of specific ligands, allowing rapid transmission of signals at chemical synapses [, ]. Five of these ion channel receptor families have been shown to form a sequence-related superfamily:Nicotinic acetylcholine receptor (AchR), an excitatory cation channel in vertebrates and invertebrates; in vertebrate motor endplates it is composed of alpha, beta, gamma and delta/epsilon subunits; in neurons it is composed of alpha and non-alpha (or beta) subunits [].Glycine receptor, an inhibitory chloride ion channel composed of alpha and beta subunits [].Gamma-aminobutyric acid (GABA) receptor, an inhibitory chloride ion channel; at least four types of subunits (alpha, beta, gamma and delta) are known [].Serotonin 5HT3 receptor, of which there are seven major types (5HT3-5HT7) [].Glutamate receptor, an excitatory cation channel of which at least three types have been described (kainate, N-methyl-D-aspartate (NMDA) and quisqualate) [].These receptors possess a pentameric structure (made up of varying subunits), surrounding a central pore. All known sequences of subunits from neurotransmitter-gated ion-channels are structurally related. They are composed of a large extracellular glycosylated N-terminal ligand-binding domain, followed by three hydrophobic transmembrane regions which form the ionic channel, followed by an intracellular region of variable length. A fourth hydrophobic region is found at the C-terminal of the sequence [, ].Gamma-aminobutyric acid type A (GABAA) receptors are members of the neurotransmitter ligand-gated ion channels: they mediate neuronal inhibition on binding GABA. The effects of GABA on GABAA receptors are modulated by a range of therapeutically important drugs, including barbiturates, anaesthetics and benzodiazepines (BZs) []. The BZs are a diverse range of compounds, including widely prescribed drugs, such as librium and valium, and their interaction with GABAA receptors provides the most potent pharmacological means of distinguishing different GABAA receptor subtypes.GABAA receptors are pentameric membrane proteins that operate GABA-gated chloride channels []. Eight types of receptor subunit have been cloned, with multiple subtypes within some classes: alpha 1-6, beta 1-4, gamma 1-4, delta, epsilon, pi, rho 1-3 and theta [, ]. Subunits are typically 50-60kDa in size and comprise a long N-terminal extracellular domain, containing a putative signal peptide and a disulphide-bonded beta structural loop; 4 putative transmembrane (TM) domains; and a large cytoplasmic loop connecting the third and fourth TM domains. Amongst family members, the large cytoplasmic loop displays the most divergence in terms of primary structure, the TM domains showing the highest level of sequence conservation [].Most GABAA receptors contain one type of alpha and beta subunit, and a single gamma polypeptide in a ratio of 2:2:1 [], though in some cases other subunits such as epsilon or delta may replace gamma. The BZ binding site is located at the interface of adjacent alpha and gamma subunits; therefore, the type of alpha and gamma subunits present is instrumental in determining BZ selectivity and sensitivity. Receptors can be categorised into 3 groups based on their alpha subunit content and, hence, sensitivity to BZs: alpha 1-containing receptors have greatest sensitivity towards BZs (type I); alpha 2, 3 and 5-containing receptors have similar but distinguishable properties (type II); and alpha 4- and 6-containing assemblies have very low BZ affinity []. A conserved histidine residue in the alpha subunit of type I and II receptors is believed to be responsible for BZ affinity []. The alpha 1 subunit is the most widely expressed of the alpha family membersand most commonly found in receptors comprising a gamma 2 and beta subunits.Such subtypes display `classical' BZ affinity and efficacy.
Neurotransmitter ligand-gated ion channels are transmembrane receptor-ion channel complexes that open transiently upon binding of specific ligands, allowing rapid transmission of signals at chemical synapses [, ]. Five of these ion channel receptor families have been shown to form a sequence-related superfamily:Nicotinic acetylcholine receptor (AchR), an excitatory cation channel in vertebrates and invertebrates; in vertebrate motor endplates it is composed of alpha, beta, gamma and delta/epsilon subunits; in neurons it is composed of alpha and non-alpha (or beta) subunits [].Glycine receptor, an inhibitory chloride ion channel composed of alpha and beta subunits [].Gamma-aminobutyric acid (GABA) receptor, an inhibitory chloride ion channel; at least fourtypes of subunits (alpha, beta, gamma and delta) are known [].Serotonin 5HT3 receptor, of which there are seven major types (5HT3-5HT7) [].Glutamate receptor, an excitatory cation channel of which at least three types have been described (kainate, N-methyl-D-aspartate (NMDA) and quisqualate) [].These receptors possess a pentameric structure (made up of varying subunits), surrounding a central pore. All known sequences of subunits from neurotransmitter-gated ion-channels are structurally related. They are composed of a large extracellular glycosylated N-terminal ligand-binding domain, followed by three hydrophobic transmembrane regions which form the ionic channel, followed by an intracellular region of variable length. A fourth hydrophobic region is found at the C-terminal of the sequence [, ].Gamma-aminobutyric acid type A (GABAA) receptors are members of the neurotransmitter ligand-gated ion channels: they mediate neuronal inhibition on binding GABA. The effects of GABA on GABAA receptors are modulated by a range of therapeutically important drugs, including barbiturates, anaesthetics and benzodiazepines (BZs) []. The BZs are a diverse range of compounds, including widely prescribed drugs, such as librium and valium, and their interaction with GABAA receptors provides the most potent pharmacological means of distinguishing different GABAA receptor subtypes.GABAA receptors are pentameric membrane proteins that operate GABA-gated chloride channels []. Eight types of receptor subunit have been cloned, with multiple subtypes within some classes: alpha 1-6, beta 1-4, gamma 1-4, delta, epsilon, pi, rho 1-3 and theta [, ]. Subunits are typically 50-60kDa in size and comprise a long N-terminal extracellular domain, containing a putative signal peptide and a disulphide-bonded beta structural loop; 4 putative transmembrane (TM) domains; and a large cytoplasmic loop connecting the third and fourth TM domains. Amongst family members, the large cytoplasmic loop displays the most divergence in terms of primary structure, the TM domains showing the highest level of sequence conservation [].Most GABAA receptors contain one type of alpha and beta subunit, and a single gamma polypeptide in a ratio of 2:2:1 [], though in some cases other subunits such as epsilon or delta may replace gamma. The BZ binding site is located at the interface of adjacent alpha and gamma subunits; therefore, the type of alpha and gamma subunits present is instrumental in determining BZ selectivity and sensitivity. Receptors can be categorised into 3 groups based on their alpha subunit content and, hence, sensitivity to BZs: alpha 1-containing receptors have greatest sensitivity towards BZs (type I); alpha 2, 3 and 5-containing receptors have similar but distinguishable properties (type II); and alpha 4- and 6-containing assemblies have very low BZ affinity []. A conserved histidine residue in the alpha subunit of type I and II receptors is believed to be responsible for BZ affinity []. Amongst type II members, alpha 2-containing receptors have a very similar BZaffinity profile to those containing the alpha 3 subunit. Differences inefficacy can be exploited to differentiate such subtypes: for example, thecompound flunitrazepam has a greater efficacy on the alpha 2 subtype. Alpha2-containing receptors are widely distributed throughout the CNS and arealso found in some motor neurones and the pancreas [].
Neurotransmitter ligand-gated ion channels are transmembrane receptor-ion channel complexes that open transiently upon binding of specific ligands, allowing rapid transmission of signals at chemical synapses [, ]. Five of these ion channel receptor families have been shown to form a sequence-related superfamily:Nicotinic acetylcholine receptor (AchR), an excitatory cation channel in vertebrates and invertebrates; in vertebrate motor endplates it is composed of alpha, beta, gamma and delta/epsilon subunits; in neurons it is composed of alpha and non-alpha (or beta) subunits [].Glycine receptor, an inhibitory chloride ion channel composed of alpha and beta subunits [].Gamma-aminobutyric acid (GABA) receptor, an inhibitory chloride ion channel; at least four types of subunits (alpha, beta, gamma and delta) are known [].Serotonin 5HT3 receptor, of which there are seven major types (5HT3-5HT7) [].Glutamate receptor, an excitatory cation channel of which at least three types have been described (kainate, N-methyl-D-aspartate (NMDA) and quisqualate) [].These receptors possess a pentameric structure (made up of varying subunits), surrounding a central pore. All known sequences of subunits from neurotransmitter-gated ion-channels are structurally related. They are composed of a large extracellular glycosylated N-terminal ligand-binding domain, followed by three hydrophobic transmembrane regions which form the ionic channel, followed by an intracellular region of variable length. A fourth hydrophobic region is found at the C-terminal of the sequence [, ].Gamma-aminobutyric acid type A (GABAA) receptors are members of the neurotransmitter ligand-gated ion channels: they mediate neuronal inhibition on binding GABA. The effects of GABA on GABAA receptors are modulated by a range of therapeutically important drugs, including barbiturates, anaesthetics and benzodiazepines (BZs) []. The BZs are a diverse range of compounds, including widely prescribed drugs, such as librium and valium, and their interaction with GABAA receptors provides the most potent pharmacological means of distinguishing different GABAA receptor subtypes.GABAA receptors are pentameric membrane proteins that operate GABA-gated chloride channels []. Eight types of receptor subunit have been cloned, with multiple subtypes within some classes: alpha 1-6, beta 1-4, gamma 1-4, delta, epsilon, pi, rho 1-3 and theta [, ]. Subunits are typically 50-60kDa in size and comprise a long N-terminal extracellular domain, containing a putative signal peptide and a disulphide-bonded beta structural loop; 4 putative transmembrane (TM) domains; and a large cytoplasmic loop connecting the third and fourth TM domains. Amongst family members, the large cytoplasmic loop displays the most divergence in terms of primary structure, the TM domains showing the highest level of sequence conservation [].Most GABAA receptors contain one type of alpha and beta subunit, and a single gamma polypeptide in a ratio of 2:2:1 [], though in some cases other subunits such as epsilon or delta may replace gamma. The BZ binding site is located at the interface of adjacent alpha and gamma subunits; therefore, the type of alpha and gamma subunits present is instrumental in determining BZ selectivity and sensitivity. Receptors can be categorised into 3 groups based on their alpha subunit content and, hence, sensitivity to BZs: alpha 1-containing receptors have greatest sensitivity towards BZs (type I); alpha 2, 3 and 5-containing receptors have similar but distinguishable properties (type II); and alpha 4- and 6-containing assemblies have very low BZ affinity []. A conserved histidine residue in the alpha subunit of type I and II receptors is believed to be responsible for BZ affinity []. The epsilon subunit was first identified in 1997. Northern blot analysis of several humanbrain tissues showed that epsilon transcripts were relatively enriched inamygdala and thalamus, compared to whole brain, and particularly abundant inthe subthalmic nucleus. Heteromeric recombinant receptors containing theepsilon subunit in place of the more usual gamma subunit were found to be insenstitive to the potentiating effects ofanaesthetic agents [].
Neurotransmitter ligand-gated ion channels are transmembrane receptor-ion channel complexes that open transiently upon binding of specific ligands, allowing rapid transmission of signals at chemical synapses [, ]. Five of these ion channel receptor families have been shown to form a sequence-related superfamily:Nicotinic acetylcholine receptor (AchR), an excitatory cation channel in vertebrates and invertebrates; in vertebrate motor endplates it is composed of alpha, beta, gamma and delta/epsilon subunits; in neurons it is composed of alpha and non-alpha (or beta) subunits [].Glycine receptor, an inhibitory chloride ion channel composed of alpha and beta subunits [].Gamma-aminobutyric acid (GABA) receptor, an inhibitory chloride ion channel; at least four types of subunits (alpha, beta, gamma and delta) are known [].Serotonin 5HT3 receptor, of which there are seven major types (5HT3-5HT7) [].Glutamate receptor, an excitatory cation channel of which at least three types have been described (kainate, N-methyl-D-aspartate (NMDA) and quisqualate) [].These receptors possess a pentameric structure (made up of varying subunits), surrounding a central pore. All known sequences of subunits from neurotransmitter-gated ion-channels are structurally related. They are composed of a large extracellular glycosylated N-terminal ligand-binding domain, followed by three hydrophobic transmembrane regions which form the ionic channel, followed by an intracellular region of variable length. A fourth hydrophobic region is found at the C-terminal of the sequence [, ].Gamma-aminobutyric acid type A (GABAA) receptors are members of the neurotransmitter ligand-gated ion channels: they mediate neuronal inhibition on binding GABA. The effects of GABA on GABAA receptors are modulated by a range of therapeutically important drugs, including barbiturates, anaesthetics and benzodiazepines (BZs) []. The BZs are a diverse range of compounds, including widely prescribed drugs, such as librium and valium, and their interaction with GABAA receptors provides the most potent pharmacological means of distinguishing different GABAA receptor subtypes.GABAA receptors are pentameric membrane proteins that operate GABA-gated chloride channels []. Eight types of receptor subunit have been cloned, with multiple subtypes within some classes: alpha 1-6, beta 1-4, gamma 1-4, delta, epsilon, pi, rho 1-3 and theta [, ]. Subunits are typically 50-60kDa in size and comprise a long N-terminal extracellular domain, containing a putative signal peptide and a disulphide-bonded beta structural loop; 4 putative transmembrane (TM) domains; and a large cytoplasmic loop connecting the third and fourth TM domains. Amongst family members, the large cytoplasmic loop displays the most divergence in terms of primary structure, the TM domains showing the highest level of sequence conservation [].Most GABAA receptors contain one type of alpha and beta subunit, and a single gamma polypeptide in a ratio of 2:2:1 [], though in some cases other subunits such as epsilon or delta may replace gamma. The BZ binding site is located at the interface of adjacent alpha and gamma subunits; therefore, the type of alpha and gamma subunits present is instrumental in determining BZ selectivity and sensitivity. Receptors can be categorised into 3 groups based on their alpha subunit content and, hence, sensitivity to BZs: alpha 1-containing receptors have greatest sensitivity towards BZs (type I); alpha 2, 3 and 5-containing receptors have similar but distinguishable properties (type II); and alpha 4- and 6-containing assemblies have very low BZ affinity []. A conserved histidine residue in the alpha subunit of type I and II receptors is believed to be responsible for BZ affinity []. Delta cDNA was first reported in rat, mouse and humans []. Delta mRNAwas found to be present in regions of the brain that were low in gamma 2, and insensitivity towards "classical"BZs was observed in receptorscontaining the delta subunit. Furthermore, delta subunits are thought to preferentially pair with the alpha 6 polypeptides over other subtypes, and are often found in place of gamma subunits.
Neurotransmitter ligand-gated ion channels are transmembrane receptor-ion channel complexes that open transiently upon binding of specific ligands, allowing rapid transmission of signals at chemical synapses [, ]. Five of these ion channel receptor families have been shown to form a sequence-related superfamily:Nicotinic acetylcholine receptor (AchR), an excitatory cation channel in vertebrates and invertebrates; in vertebrate motor endplates it is composed of alpha, beta, gamma and delta/epsilon subunits; in neurons it is composed of alpha and non-alpha (or beta) subunits [].Glycine receptor, an inhibitory chloride ion channel composed of alpha and beta subunits [].Gamma-aminobutyric acid (GABA) receptor, an inhibitory chloride ion channel; at least four types of subunits (alpha, beta, gamma and delta) are known [].Serotonin 5HT3 receptor, of which there are seven major types (5HT3-5HT7) [].Glutamate receptor, an excitatory cation channel of which at least three types have been described (kainate, N-methyl-D-aspartate (NMDA) and quisqualate) [].These receptors possess a pentameric structure (made up of varying subunits), surrounding a central pore. All known sequences of subunits from neurotransmitter-gated ion-channels are structurally related. They are composed of a large extracellular glycosylated N-terminal ligand-binding domain, followed by three hydrophobic transmembrane regions which form the ionic channel, followed by an intracellular region of variable length. A fourth hydrophobic region is found at the C-terminal of the sequence [, ].Gamma-aminobutyric acid type A (GABAA) receptors are members of the neurotransmitter ligand-gated ion channels: they mediate neuronal inhibition on binding GABA. The effects of GABA on GABAA receptors are modulated by a range of therapeutically important drugs, including barbiturates, anaesthetics and benzodiazepines (BZs) []. The BZs are a diverse range of compounds, including widely prescribed drugs, such as librium and valium, and their interaction with GABAA receptors provides the most potent pharmacological means of distinguishing different GABAA receptor subtypes.GABAA receptors are pentameric membrane proteins that operate GABA-gated chloride channels []. Eight types of receptor subunit have been cloned, with multiple subtypes within some classes: alpha 1-6, beta 1-4, gamma 1-4, delta, epsilon, pi, rho 1-3 and theta [, ]. Subunits are typically 50-60kDa in size and comprise a long N-terminal extracellular domain, containing a putative signal peptide and a disulphide-bonded beta structural loop; 4 putative transmembrane (TM) domains; and a large cytoplasmic loop connecting the third and fourth TM domains. Amongst family members, the large cytoplasmic loop displays the most divergence in terms of primary structure, the TM domains showing the highest level of sequence conservation [].Most GABAA receptors contain one type of alpha and beta subunit, and a single gamma polypeptide in a ratio of 2:2:1 [], though in some cases other subunits such as epsilon or delta may replace gamma. The BZ binding site is located at the interface of adjacent alpha and gamma subunits; therefore, the type of alpha and gamma subunits present is instrumental in determining BZ selectivity and sensitivity. Receptors can be categorised into 3 groups based on their alpha subunit content and, hence, sensitivity to BZs: alpha 1-containing receptors have greatest sensitivity towards BZs (type I); alpha 2, 3 and 5-containing receptors have similar but distinguishable properties (type II); and alpha 4- and 6-containing assemblies have very low BZ affinity []. A conserved histidine residue in the alpha subunit of type I and II receptors is believed to be responsible for BZ affinity []. Identification and characterisation of the theta subunit was first reportedin 1999 []. Cloning of the full-length cDNA was performed using a humanwhole-brain library, yielding a deduced open reading frame of 627 aminoacids. This polypeptide was found to be most similar to the beta 1 subunitwith regard to sequence identity, and was able to co-assemble with alpha 2,beta 1 and gamma 1 subunits, yielding heteromeric assemblies with a 4-foldincrease in sensitivity towards GABA. Furthermore, theta mRNA was found to have a unique spatial distribution, with significant expression withinmonoaminergic neurons of both human and monkey brain.
Neurotransmitter ligand-gated ion channels are transmembrane receptor-ion channel complexes that open transiently upon binding of specific ligands, allowing rapid transmission of signals at chemical synapses [, ]. Five of these ion channel receptor families have been shown to form a sequence-related superfamily:Nicotinic acetylcholine receptor (AchR), an excitatory cation channel in vertebrates and invertebrates; in vertebrate motor endplates it is composed of alpha, beta, gamma and delta/epsilon subunits; in neurons it is composed of alpha and non-alpha (or beta) subunits [].Glycine receptor, an inhibitory chloride ion channel composed of alpha and beta subunits [].Gamma-aminobutyric acid (GABA) receptor, an inhibitory chloride ion channel; at least four types of subunits (alpha, beta, gamma and delta) are known [].Serotonin 5HT3 receptor, of which there are seven major types (5HT3-5HT7) [].Glutamate receptor, an excitatory cation channel of which at least three types have been described (kainate, N-methyl-D-aspartate (NMDA) and quisqualate) [].These receptors possess a pentameric structure (made up of varying subunits), surrounding a central pore. All known sequences of subunits from neurotransmitter-gated ion-channels are structurally related. They are composed of a large extracellular glycosylated N-terminal ligand-binding domain, followed by three hydrophobic transmembrane regions which form the ionic channel, followed by an intracellular region of variable length. A fourth hydrophobic region is found at the C-terminal of the sequence [, ].Gamma-aminobutyric acid type A (GABAA) receptors are members of the neurotransmitter ligand-gated ion channels: they mediate neuronal inhibition on binding GABA. The effects of GABA on GABAA receptors are modulated by a range of therapeutically important drugs, including barbiturates, anaesthetics and benzodiazepines (BZs) []. The BZs are a diverse range of compounds, including widely prescribed drugs, such as librium and valium, and their interaction with GABAA receptors provides the most potent pharmacological means of distinguishing different GABAA receptor subtypes.GABAA receptors are pentameric membrane proteins that operate GABA-gated chloride channels []. Eight types of receptor subunit have been cloned, with multiple subtypes withinsome classes: alpha 1-6, beta 1-4, gamma 1-4, delta, epsilon, pi, rho 1-3 and theta [, ]. Subunits are typically 50-60kDa in size and comprise a long N-terminal extracellular domain, containing a putative signal peptide and a disulphide-bonded beta structural loop; 4 putative transmembrane (TM) domains; and a large cytoplasmic loop connecting the third and fourth TM domains. Amongst family members, the large cytoplasmic loop displays the most divergence in terms of primary structure, the TM domains showing the highest level of sequence conservation [].Most GABAA receptors contain one type of alpha and beta subunit, and a single gamma polypeptide in a ratio of 2:2:1 [], though in some cases other subunits such as epsilon or delta may replace gamma. The BZ binding site is located at the interface of adjacent alpha and gamma subunits; therefore, the type of alpha and gamma subunits present is instrumental in determining BZ selectivity and sensitivity. Receptors can be categorised into 3 groups based on their alpha subunit content and, hence, sensitivity to BZs: alpha 1-containing receptors have greatest sensitivity towards BZs (type I); alpha 2, 3 and 5-containing receptors have similar but distinguishable properties (type II); and alpha 4- and 6-containing assemblies have very low BZ affinity []. A conserved histidine residue in the alpha subunit of type I and II receptors is believed to be responsible for BZ affinity []. The existence of a pi subunit was first reported in 1997, where it wasdetected in a number of human and rat tissues. The subunit shares 30-40%amino acid identity with other members of the GABAA receptor subunit family.The polypeptide is found in several peripheral tissues, including theuterus, where its function appears to be related to tissue contractility: pisubunits can co-assemble with other GABAA receptor subunits to formrecombinant receptors with altered sensitivity to pregnenalone [].
Small GTPases form an independent superfamily within the larger class ofregulatory GTP hydrolases. This superfamily contains proteins that control avast number of important processes and possess a common, structurallypreserved GTP-binding domain [, ]. Sequence comparisons of small G proteinsfrom various species have revealed that they are conserved in primarystructures at the level of 30-55% similarity [].Crystallographic analysis of various small G proteins revealed the presence of a 20kDa catalytic domain that is unique for the whole superfamily [, ]. The domain is built of five alpha helices (A1-A5), sixβ-strands (B1-B6) and five polypeptide loops (G1-G5). A structuralcomparison of the GTP- and GDP-bound form, allows one to distinguish twofunctional loop regions: switch I and switch II that surround thegamma-phosphate group of the nucleotide. The G1 loop (also called the P-loop)that connects the B1 strand and the A1 helix is responsible for the binding ofthe phosphate groups. The G3 loop provides residues for Mg(2+) and phosphatebinding and is located at the N terminus of the A2 helix. The G1 and G3 loopsare sequentially similar to Walker A and Walker B boxes that are found inother nucleotide binding motifs. The G2 loop connects the A1 helix and the B2strand and contains a conserved Thr residue responsible for Mg(2+) binding.The guanine base is recognised by the G4 and G5 loops. The consensus sequenceNKXD of the G4 loop contains Lys and Asp residues directly interacting withthe nucleotide. Part of the G5 loop located between B6 and A5 acts as arecognition site for the guanine base [].The small GTPase superfamily can be divided in 8 different families:Arf small GTPases. GTP-binding proteins involved in protein trafficking bymodulating vesicle budding and un-coating within the Golgi apparatusRan small GTPases. GTP-binding proteins involved in nucleocytoplasmictransport. Required for the import of proteins into the nucleus and alsofor RNA exportRab small GTPases. GTP-binding proteins involved in vesicular traffic.Rho small GTPases. GTP-binding proteins that control cytoskeletonreorganisationRas small GTPases. GTP-binding proteins involved in signaling pathwaysSar1 small GTPases. Small GTPase component of the coat protein complex II(COPII) which promotes the formation of transport vesicles from theendoplasmic reticulum (ER)Mitochondrial Rho (Miro). Small GTPase domain found in mitochondrialproteins involved in mitochondrial traffickingRoc small GTPases domain. Small GTPase domain always found associated withthe COR domain.Ran (or TC4), is an evolutionary conserved member of the Ras superfamily of small GTPases that regulates all receptor-mediated transport between the nucleus and the cytoplasm. Ran has been implicated in a large number of processes, including nucleocytoplasmic transport, RNA synthesis, processing and export and cell cycle checkpoint control [, ]. Ran plays a crucial role in both import/export pathways and determines the directionality of nuclear transport. Import receptors (importins) bind their cargos in the cytoplasm where the concentration of RanGTP is low (due to action of RanGAP), and release their cargos in the nucleus where the concentration of RanGTP is high (due to action of RanGEF) [, ]. Export receptors (exportins) respond to RanGTP in the opposite manner. Furthermore, it has been shown that nuclear transport factor 2 (NTF2, ) stimulates efficient nuclear import of a cargo protein. NTF2 binds specifically to RanGDP and to the FXFG repeat containing nucleoporins []. Ran is generally included in the RAS 'superfamily' of small GTP-binding proteins [], but it is only slightly related to the other RAS proteins. It also differs from RAS proteins in that it lacks cysteine residues at its C-terminal and is therefore not subject to prenylation. Instead, Ran has an acidic C terminus. It is, however, similar to RAS family members in requiring a specific guanine nucleotide exchange factor (GEF) and a specific GTPase activating protein (GAP) as stimulators of overall GTPase activity.Ran consists of a core domain that is structurally similar to the GTP-binding domains of other small GTPases but, in addition, Ran has a C-terminal extension consisting of an unstructured linker and a 16 residue α-helix that is located opposite the "Switch I"region in the RanGDP structure []. Three regions of Ran change conformation depending on the nucleotide bound, the Switch I and II regions, which interact with the bound nucleotide, as well as the C-terminal extension. In RanGDP, the C-terminal extension contacts the core of the protein, while in RanGTP, the extension is extending away from the core, most likely due to a steric clash between the switch I region and the linker part of the C-terminal extension. This suggests that the C-terminal extension in RanGDP is crucial for shielding residues in the core domain and preventing the switch regions from adopting a GTP-like form. This prevents binding of transport factors to RanGDP that would otherwise lead to uncoordinated interaction between importin beta-like proteins and cellular factors.
Neurotransmitter ligand-gated ion channels are transmembrane receptor-ion channel complexes that open transiently upon binding of specific ligands, allowing rapid transmission of signals at chemical synapses [, ]. Five of these ion channel receptor families have been shown to form a sequence-related superfamily:Nicotinic acetylcholine receptor (AchR), an excitatory cation channel in vertebrates and invertebrates; in vertebrate motor endplates it is composed of alpha, beta, gamma and delta/epsilon subunits; in neurons it is composed of alpha and non-alpha (or beta) subunits [].Glycine receptor, an inhibitory chloride ion channel composed of alpha and beta subunits [].Gamma-aminobutyric acid (GABA) receptor, an inhibitory chloride ion channel; at least four types of subunits (alpha, beta, gamma and delta) are known [].Serotonin 5HT3 receptor, of which there are seven major types (5HT3-5HT7) [].Glutamate receptor, an excitatory cation channel of which at least three types have been described (kainate, N-methyl-D-aspartate (NMDA) and quisqualate) [].These receptors possess a pentameric structure (made up of varying subunits), surrounding a central pore. All known sequences of subunits from neurotransmitter-gated ion-channels are structurally related. They are composed of a large extracellular glycosylated N-terminal ligand-binding domain, followed by three hydrophobic transmembrane regions which form the ionic channel, followed by an intracellular region of variable length. A fourth hydrophobic region is found at the C-terminal of the sequence [, ].Gamma-aminobutyric acid type A (GABAA) receptors are members of the neurotransmitter ligand-gated ion channels: they mediate neuronal inhibition on binding GABA. The effects of GABA on GABAA receptors are modulated by a range of therapeutically important drugs, including barbiturates, anaesthetics and benzodiazepines (BZs) []. The BZs are a diverse range of compounds, including widely prescribed drugs, such as librium and valium, and their interaction with GABAA receptors provides the most potent pharmacological means of distinguishing different GABAA receptor subtypes.GABAA receptors are pentameric membrane proteins that operate GABA-gated chloride channels []. Eight types of receptor subunit have been cloned, with multiple subtypes within some classes: alpha 1-6, beta 1-4, gamma 1-4, delta, epsilon, pi, rho 1-3 and theta [, ]. Subunits are typically 50-60kDa in size and comprise a long N-terminal extracellular domain, containing a putative signal peptide and a disulphide-bonded beta structural loop; 4 putative transmembrane (TM) domains; and a large cytoplasmic loop connecting the third and fourth TM domains. Amongst family members, the large cytoplasmic loop displays the most divergence in terms of primary structure, the TM domains showing the highest level of sequence conservation [].Most GABAA receptors contain one type of alpha and beta subunit, and a single gamma polypeptide in a ratio of 2:2:1 [], though in some cases other subunits such as epsilon or delta may replace gamma. The BZ binding site is located at the interface of adjacent alpha and gamma subunits; therefore, the type of alpha and gamma subunits present is instrumental in determining BZ selectivity and sensitivity. Receptors can be categorised into 3 groups based on their alpha subunit content and, hence, sensitivity to BZs: alpha 1-containing receptors have greatest sensitivity towards BZs (type I); alpha 2, 3 and 5-containing receptors have similar but distinguishable properties (type II); and alpha 4- and 6-containing assemblies have very low BZ affinity []. A conserved histidine residue in the alpha subunit of type I and II receptors is believed to be responsible for BZ affinity []. Three mammalian gamma subunits have been identified (gamma 1 to 3), each encoded by a separate gene, plus an avian gamma 4 subunit. The presence of a gamma 2 subunit, together with alpha 1, confers `classical' BZ-binding activity to GABAA receptors; substitution for gamma 1 or 3 leads to an altered binding profile for BZs []. The gamma 2 gene undergoes alternative exon splicing leading to the generation of two isoforms that differ by an additional 24-bp exon in the large putative cytoplasmic domain []. The isoforms are termed gamma 2L (long) and gamma 2S (short), and are ubiquitously expressed. The gamma 2L splice variant has been implicated in potentiation of GABAA receptors by ethanol.This entry represents Gamma-aminobutyric acid receptor subunit gamma-1/4 found in GABAA receptors, mainly found in Chordates.
Neurotransmitter ligand-gated ion channels are transmembrane receptor-ion channel complexes that open transiently upon binding of specific ligands, allowing rapid transmission of signals at chemical synapses [, ]. Five of these ion channel receptor families have been shown to form a sequence-related superfamily:Nicotinic acetylcholine receptor (AchR), an excitatory cation channel in vertebrates and invertebrates; in vertebrate motor endplates it is composed of alpha, beta, gamma and delta/epsilon subunits; in neurons it is composed of alpha and non-alpha (or beta) subunits [].Glycine receptor, an inhibitory chloride ion channel composed of alpha and beta subunits [].Gamma-aminobutyric acid (GABA) receptor, an inhibitory chloride ion channel; at least four types of subunits (alpha, beta, gamma and delta) are known [].Serotonin 5HT3 receptor, of which there are seven major types (5HT3-5HT7) [].Glutamate receptor, an excitatory cation channel of which at least three types have been described (kainate, N-methyl-D-aspartate (NMDA) and quisqualate) [].These receptors possess a pentameric structure (made up of varying subunits), surrounding a central pore. All known sequences of subunits from neurotransmitter-gated ion-channels are structurally related. They are composed of a large extracellular glycosylated N-terminal ligand-binding domain, followed by three hydrophobic transmembrane regions which form the ionicchannel, followed by an intracellular region of variable length. A fourth hydrophobic region is found at the C-terminal of the sequence [, ].Gamma-aminobutyric acid type A (GABAA) receptors are members of the neurotransmitter ligand-gated ion channels: they mediate neuronal inhibition on binding GABA. The effects of GABA on GABAA receptors are modulated by a range of therapeutically important drugs, including barbiturates, anaesthetics and benzodiazepines (BZs) []. The BZs are a diverse range of compounds, including widely prescribed drugs, such as librium and valium, and their interaction with GABAA receptors provides the most potent pharmacological means of distinguishing different GABAA receptor subtypes.GABAA receptors are pentameric membrane proteins that operate GABA-gated chloride channels []. Eight types of receptor subunit have been cloned, with multiple subtypes within some classes: alpha 1-6, beta 1-4, gamma 1-4, delta, epsilon, pi, rho 1-3 and theta [, ]. Subunits are typically 50-60kDa in size and comprise a long N-terminal extracellular domain, containing a putative signal peptide and a disulphide-bonded beta structural loop; 4 putative transmembrane (TM) domains; and a large cytoplasmic loop connecting the third and fourth TM domains. Amongst family members, the large cytoplasmic loop displays the most divergence in terms of primary structure, the TM domains showing the highest level of sequence conservation [].Most GABAA receptors contain one type of alpha and beta subunit, and a single gamma polypeptide in a ratio of 2:2:1 [], though in some cases othersubunits such as epsilon or delta may replace gamma. The BZ binding site is located at the interface of adjacent alpha and gamma subunits; therefore, the type of alpha and gamma subunits present is instrumental in determining BZ selectivity and sensitivity. Receptors can be categorised into 3 groups based on their alpha subunit content and, hence, sensitivity to BZs: alpha 1-containing receptors have greatest sensitivity towards BZs (type I); alpha 2, 3 and 5-containing receptors have similar but distinguishable properties (type II); and alpha 4- and 6-containing assemblies have very low BZ affinity []. A conserved histidine residue in the alpha subunit of type I and II receptors is believed to be responsible for BZ affinity []. Three mammalian gamma subunits have been identified (gamma 1 to 3), each encoded by a separate gene, plus an avian gamma 4 subunit. The presence of a gamma 2 subunit, together with alpha 1, confers `classical' BZ-binding activity to GABAA receptors; substitution for gamma 1 or 3 leads to an altered binding profile for BZs []. The gamma 2 gene undergoes alternative exon splicing leading to the generation of two isoforms that differ by an additional 24-bp exon in the large putative cytoplasmic domain []. The isoforms are termed gamma 2L (long) and gamma 2S (short), and are ubiquitously expressed. The gamma 2L splice variant has been implicated in potentiation of GABAA receptors by ethanol.This entry represents the gamma 1 subunit.
Neurotransmitter ligand-gated ion channels are transmembrane receptor-ion channel complexes that open transiently upon binding of specific ligands, allowing rapid transmission of signals at chemical synapses [, ]. Five of these ion channel receptor families have been shown to form a sequence-related superfamily:Nicotinic acetylcholine receptor (AchR), an excitatory cation channel in vertebrates and invertebrates; in vertebrate motor endplates it is composed of alpha, beta, gamma and delta/epsilon subunits; in neurons it is composed of alpha and non-alpha (or beta) subunits [].Glycine receptor, an inhibitory chloride ion channel composed of alpha and beta subunits [].Gamma-aminobutyric acid (GABA) receptor, an inhibitory chloride ion channel; at least four types of subunits (alpha, beta, gamma and delta) are known [].Serotonin 5HT3 receptor, of which there are seven major types (5HT3-5HT7) [].Glutamate receptor, an excitatory cation channel of which at least three types have been described (kainate, N-methyl-D-aspartate (NMDA) and quisqualate) [].These receptors possess a pentameric structure (made up of varying subunits), surrounding a central pore. All known sequences of subunits from neurotransmitter-gated ion-channels are structurally related. They are composed of a large extracellular glycosylated N-terminal ligand-binding domain, followed by three hydrophobic transmembrane regions which form the ionic channel, followed by an intracellular region of variable length. A fourth hydrophobic region is found at the C-terminal of the sequence [, ].Gamma-aminobutyric acid type A (GABAA) receptors are members of the neurotransmitter ligand-gated ion channels: they mediate neuronal inhibition on binding GABA. The effects of GABA on GABAA receptors are modulated by a range of therapeutically important drugs, including barbiturates, anaesthetics and benzodiazepines (BZs) []. The BZs are a diverse range of compounds, including widely prescribed drugs, such as librium and valium, and their interaction with GABAA receptors provides the most potent pharmacological means of distinguishing different GABAA receptor subtypes.GABAA receptors are pentameric membrane proteins that operate GABA-gated chloride channels []. Eight types of receptor subunit have been cloned, with multiple subtypes within some classes: alpha 1-6, beta 1-4, gamma 1-4, delta, epsilon, pi, rho 1-3 and theta [, ]. Subunits are typically 50-60kDa in size and comprise a long N-terminal extracellular domain, containing a putative signal peptide and a disulphide-bonded beta structural loop; 4 putative transmembrane (TM) domains; and a large cytoplasmic loop connecting the third and fourth TM domains. Amongst family members, the large cytoplasmic loop displays the most divergence in terms of primary structure, the TM domains showing the highest level of sequence conservation [].Most GABAA receptors contain one type of alpha and beta subunit, and a single gamma polypeptide in a ratio of 2:2:1 [], though in some cases other subunits such as epsilon or delta may replace gamma. The BZ binding site is located at the interface of adjacent alpha and gamma subunits; therefore, the type of alpha and gamma subunits present is instrumental in determining BZ selectivity and sensitivity. Receptors can be categorised into 3 groups based on their alpha subunit content and, hence, sensitivity to BZs: alpha 1-containing receptors have greatest sensitivity towards BZs (type I); alpha 2, 3 and 5-containing receptors have similar but distinguishable properties (type II); and alpha 4- and 6-containing assemblies have very low BZ affinity []. A conserved histidine residue in the alpha subunit of type I and II receptors is believed to be responsible for BZ affinity []. Three mammalian gamma subunits have been identified (gamma 1 to 3), each encoded by a separate gene, plus an avian gamma 4 subunit. The presence of a gamma 2 subunit, together with alpha 1, confers `classical' BZ-binding activity to GABAA receptors; substitution for gamma 1 or 3 leads to an altered binding profile for BZs []. The gamma 2 gene undergoes alternative exon splicing leading to the generation of two isoforms that differ by an additional 24-bp exon in the large putative cytoplasmic domain []. The isoforms are termed gamma 2L (long) and gamma 2S (short), and are ubiquitously expressed. The gamma 2L splice variant has been implicated in potentiation of GABAA receptors by ethanol.This entry represents the gamma 2 subunit.
Neurotransmitter ligand-gated ion channels are transmembrane receptor-ion channel complexes that open transiently upon binding of specific ligands, allowing rapid transmission of signals at chemical synapses [, ]. Five of these ion channel receptor families have been shown to form a sequence-related superfamily:Nicotinic acetylcholine receptor (AchR), an excitatory cation channel in vertebrates and invertebrates; in vertebrate motor endplates it is composedof alpha, beta, gamma and delta/epsilon subunits; in neurons it is composed of alpha and non-alpha (or beta) subunits [].Glycine receptor, an inhibitory chloride ion channel composed of alpha and beta subunits [].Gamma-aminobutyric acid (GABA) receptor, an inhibitory chloride ion channel; at least four types of subunits (alpha, beta, gamma and delta) are known [].Serotonin 5HT3 receptor, of which there are seven major types (5HT3-5HT7) [].Glutamate receptor, an excitatory cation channel of which at least three types have been described (kainate, N-methyl-D-aspartate (NMDA) and quisqualate) [].These receptors possess a pentameric structure (made up of varying subunits), surrounding a central pore. All known sequences of subunits from neurotransmitter-gated ion-channels are structurally related. They are composed of a large extracellular glycosylated N-terminal ligand-binding domain, followed by three hydrophobic transmembrane regions which form the ionic channel, followed by an intracellular region of variable length. A fourth hydrophobic region is found at the C-terminal of the sequence [, ].Gamma-aminobutyric acid type A (GABAA) receptors are members of the neurotransmitter ligand-gated ion channels: they mediate neuronal inhibition on binding GABA. The effects of GABA on GABAA receptors are modulated by a range of therapeutically important drugs, including barbiturates, anaesthetics and benzodiazepines (BZs) []. The BZs are a diverse range of compounds, including widely prescribed drugs, such as librium and valium, and their interaction with GABAA receptors provides the most potent pharmacological means of distinguishing different GABAA receptor subtypes.GABAA receptors are pentameric membrane proteins that operate GABA-gated chloride channels []. Eight types of receptor subunit have been cloned, with multiple subtypes within some classes: alpha 1-6, beta 1-4, gamma 1-4, delta, epsilon, pi, rho 1-3 and theta [, ]. Subunits are typically 50-60kDa in size and comprise a long N-terminal extracellular domain, containing a putative signal peptide and a disulphide-bonded beta structural loop; 4 putative transmembrane (TM) domains; and a large cytoplasmic loop connecting the third and fourth TM domains. Amongst family members, the large cytoplasmic loop displays the most divergence in terms of primary structure, the TM domains showing the highest level of sequence conservation [].Most GABAA receptors contain one type of alpha and beta subunit, and a single gamma polypeptide in a ratio of 2:2:1 [], though in some cases other subunits such as epsilon or delta may replace gamma. The BZ binding site is located at the interface of adjacent alpha and gamma subunits; therefore, the type of alpha and gamma subunits present is instrumental in determining BZ selectivity and sensitivity. Receptors can be categorised into 3 groups based on their alpha subunit content and, hence, sensitivity to BZs: alpha 1-containing receptors have greatest sensitivity towards BZs (type I); alpha 2, 3 and 5-containing receptors have similar but distinguishable properties (type II); and alpha 4- and 6-containing assemblies have very low BZ affinity []. A conserved histidine residue in the alpha subunit of type I and II receptors is believed to be responsible for BZ affinity []. Three mammalian gamma subunits have been identified (gamma 1 to 3), each encoded by a separate gene, plus an avian gamma 4 subunit. The presence of a gamma 2 subunit, together with alpha 1, confers `classical' BZ-binding activity to GABAA receptors; substitution for gamma 1 or 3 leads to an altered binding profile for BZs []. The gamma 2 gene undergoes alternative exon splicing leading to the generation of two isoforms that differ by an additional 24-bp exon in the large putative cytoplasmic domain []. The isoforms are termed gamma 2L (long) and gamma 2S (short), and are ubiquitously expressed. The gamma 2L splice variant has been implicated in potentiation of GABAA receptors by ethanol.This entry represents the gamma 3 subunit.
This group of cysteine peptidases correspond to MEROPS peptidase family C58 (clan CA). They are found in bacteria that include plant pathogens (Pseudomonas syringae), root nodule bacteria, and intracellular pathogens (e.g. Yersinia pestis, Haemophilus ducreyi, Pasteurella multocida, Chlamydia trachomatis) of animal hosts. The peptidase domain features a catalytic triad of Cys, His, and Asp. Sequences can be extremely divergent outside of a few well-conserved motifs. YopT, a virulence effector protein of Y. pestis, cleaves and releases host cell Rho GTPases from the membrane, thereby disrupting the actin cytoskeleton. Members of the family from pathogenic bacteria are likely to be pathogenesis factors [].Secretion of virulence factors in Gram-negative bacteria involves transportation of the protein across two membranes to reach the cell exterior. There have been four secretion systems described in animal enteropathogens such as Salmonella and Yersinia, with further sequence homologies in plant pathogens like Ralstonia and Erwinia []. The type III secretion system is of great interest, as it is used to transport virulence factors from the pathogen directly into the host cell and is only triggered when the bacterium comes into close contact with the host. The protein subunits of the system are very similar to those of bacterial flagellar biosynthesis. However, while the latter forms a ring structure to allow secretion of flagellin and is an integral part of the flagella itself, type III subunits in the outer membrane translocate secreted proteins through a channel-like structure [].Exotoxins secreted by the type III system do not possess a secretion signal, and are considered unique because of this []. Y. pestis secretes such a protein, YopT []. YopT is injected into the host cell upon contact, and is therefore considered to be a virulence factor. Haemophilus spp. express a similar toxin on their surface, a 76kDa antigen [].A cysteine peptidase is a proteolytic enzyme that hydrolyses a peptide bond using the thiol group of a cysteine residue as a nucleophile. Hydrolysis involves usually a catalytic triad consisting of the thiol group of the cysteine, the imidazolium ring of a histidine, and a third residue, usually asparagine or aspartic acid, to orientate and activate the imidazolium ring. In only one family of cysteine peptidases, is the role of the general base assigned to a residue other than a histidine: in peptidases from family C89 (acid ceramidase) an arginine is the general base. Cysteine peptidases can be grouped into fourteen different clans, with members of each clan possessing a tertiary fold unique to the clan. Four clans of cysteine peptidases share structural similarities with serine and threonine peptidases and asparagine lyases. From sequence similarities, cysteine peptidases can be clustered into over 80 different families []. Clans CF, CM, CN, CO, CP and PD contain only one family.Cysteine peptidases are often active at acidic pH and are therefore confined to acidic environments, such as the animal lysosome or plant vacuole. Cysteine peptidases can be endopeptidases, aminopeptidases, carboxypeptidases, dipeptidyl-peptidases or omega-peptidases. They are inhibited by thiol chelators such as iodoacetate, iodoacetic acid, N-ethylmaleimide or p-chloromercuribenzoate.Clan CA includes proteins with a papain-like fold. There is a catalytic triad which occurs in the order: Cys/His/Asn (or Asp). A fourth residue, usually Gln, is important for stabilising the acyl intermediate that forms during catalysis, and this precedes the active site Cys. The fold consists of two subdomains with the active site between them. One subdomain consists of a bundle of helices, with the catalytic Cys at the end of one of them, and the other subdomain is a β-barrel with the active site His and Asn (or Asp). There are over thirty families in the clan, and tertiary structures have been solved for members of most of these. Peptidases in clan CA are usually sensitive to the small molecule inhibitor E64, which is ineffective against peptidases from other clans of cysteine peptidases [].Clan CD includes proteins with a caspase-like fold. Proteins in the clan have an α/β/α sandwich structure. There is a catalytic dyad which occurs in the order His/Cys. The active site His occurs in a His-Gly motif and the active site Cys occurs in an Ala-Cys motif; both motifs are preceded by a block of hydrophobic residues []. Specificity is predominantly directed towards residuesthat occupy the S1 binding pocket, so that caspases cleave aspartyl bonds, legumains cleave asparaginyl bonds, and gingipains cleave lysyl or arginyl bonds.Clan CE includes proteins with an adenain-like fold. The fold consists of two subdomains with the active site between them. One domain is a bundle of helices, and the other a β-barrell. The subdomains are in the opposite order to those found in peptidases from clan CA, and this is reflected in the order of active site residues: His/Asn/Gln/Cys. This has prompted speculation that proteins in clans CA and CE are related, and that members of one clan are derived from a circular permutation of the structure of the other.Clan CL includes proteins with a sortase B-like fold. Peptidases in the clan hydrolyse and transfer bacterial cell wall peptides. The fold shows a closed β-barrel decorated with helices with the active site at one end of the barrel []. The active site consists of a His/Cys catalytic dyad.Cysteine peptidases with a chymotrypsin-like fold are included in clan PA, which also includes serine peptidases. Cysteine peptidases that are N-terminal nucleophile hydrolases are included in clan PB. Cysteine peptidases with a tertiary structure similar to that of the serine-type aspartyl dipeptidase are included in clan PC. Cysteine peptidases with an intein-like fold are included in clan PD, which also includes asparagine lyases.
