Synaptotagmin-like protein 2 (SYTL2) belongs to the synaptotagmin-like protein family, which contains a N-terminal RabBD (Rab-binding) domain and two C-terminal C2 domains. RabBD domain mediates interaction with RAB27A and recruitment on to vesicular structures in cytotoxic T-lymphocytes (CTL) [, ]. The C2 domain mediates binding to phosphatidylserine (PS) and phosphatidylinositol 4,5-bisphosphate (PIP2) and localisation to the cell membrane [, ]. SYTL2 has several isoforms produced by alternative splicing, and different isoforms may have different functions.In humans, SYTL2 isoform 1 acts as a RAB27A effector protein and plays a role in cytotoxic granule exocytosis in lymphocytes. It is required for cytotoxic granule docking at the immunologic synapse. Isoform 4 binds phosphatidylserine (PS) and phosphatidylinositol-4,5-bisphosphate (PIP2) and promotes the recruitment of glucagon-containing granules to the cell membrane in pancreatic alpha cells. Its binding to PS is inhibited by Ca2+ while binding to PIP2 is Ca2+ insensitive [, , ]. In mice, SYTL2 isoform 1 may play a role in melanosome transport and vesicle trafficking. It controls melanosome distribution in the cell periphery and regulates melanocyte morphology [, ]. Isoform 1 also acts as a positive mediator of mucus secretion by the surface mucus cells of the stomach. It mediates basal mucus secretion by gastric surface cells by promoting the proper granule biognesis and docking of mucus granules with the apical plasma membrane []. Isoform 11 acts as a RAB27A effector protein and plays a role in cytotoxic granule exocytosis in lymphocytes. It is required for cytotoxic granule docking at the immunologic synapse. Isoform 1 may play a role in melanosome transport and vesicle trafficking. It controls melanosome distribution in the cell periphery and regulates melanocyte morphology.
Members of this family are Class III phosphatidylinositol 3-kinases (PI3Ks) (catalytic subunits). PI3K is a lipid kinase and a key signalling enzyme involving in cell survival and proliferation, cell motility and adhesion, cytoskeletal rearrangement and vesicle trafficking []. The different PI3K isoforms have cell-specific functions. In yeast, VPS34 is a key enzyme required for cell division, vacuolar protein sorting, and vacuole segregation []. The major components of the yeast VPS intracellular trafficking complex are conserved in humans [].There are three major classes of PI3Ks, I and III (Class I is also subdivided into Ia and Ib), and a more distantly relatedClass IV which contains Ser/Thr kinases. The different classes of PI3K catalyse phosphorylation of the 3'-OH position of phosphatidyl myo-inositol (PtdIns) lipids, generating different 3'-phosphorylated lipid products that act as secondary messengers. The classification of PI3Ks is based upon sequence analysis and domain architecture of the catalytic subunits, but the divisions also reflect the biochemical properties and the differential association with a variety of regulatory adaptor subunits. This division is mirrored not only in their specialised functions but also in the different modes of regulation of the different enzymes in the family []. Furthermore, each of the PI3K classes differ in their preferred lipid substrate. Class III PI3Ks use only phosphatidylinositol as substrate, whereas Class Ia and Ib PI3K activity is focused upon phosphatidylinositol (4,5)-bisphosphate as substrate in vivo []. These substrate-related differences are presumed to result from subtle variations in the structures of the active sites of the different PI3Ks.Class III PI3Ks contain only the core catalytic subunit, which consists of the N-terminal C2 domain, the helical domain and the double-lobed catalytic domain. Class I and II contain an additional Ras-binding domain. The N-terminal C2 domain interacts mainly with the scaffolding helical domain of the enzyme []. The function of the central helical domain is not clear, it has been suggested to be involved in substrate presentation []. The C-terminal catalytic domain is shared by the PI3- and PI4-kinases. This domain is distantly related to the catalytic domain of protein kinases, with a global similarity of the core topological features but significantdifferences in the substrate-binding sites [].For additional information please see [].
Membrane contacts sites (MCSs), regions where two organelles come in closeproximity to one another, act as molecular hubs for the exchange of smallmolecules (e.g. lipids) and signals (e.g. calcium ions). Synaptotagmin-likeMitochondrial lipid-binding Proteins (SMP) domains are exclusively found atMCSs between different organelles such as endoplasmic reticulum (ER)-Mitochondrion, ER-Plasma membrane (PM) and Nucleus-Vacuole junctions. The SMPdomain is able to homo- or heterodimerize, harbors lipids in a hydrophobiccavity and mediates lipid transfer between the two adjacent bilayersindependently of membrane fusion and fission reactions. SMP proteins arewidespread amongst eukaryotic species with a particular enrichment in plantsand features suggestive of species-specific functional variations. SMP domain-containing proteins have been classified into four broad groups: C2 domainsynaptotagmin-like, PH domain-containing HT-008, PDZK8 and mitochondrialprotein families [, , , , , ].The SMP domain consists of 6 β-strands and 3 helices arranged to form abarrel whose interior is lined almost exclusively by hydrophobic residues. The resulting elongated barrel-shaped cylindrical structureharbors a lateral opening and a central hydrophobic cavity where phospholipidscan bind. It dimerizes in an anti-parallel fashion to form a cylindertraversed by a deep hydrophobic groove [, , ]. The SMP domain belongs to theTULIP (for TUbular LIPid-binding) protein superfamily of lipid transferproteins [].
RGS3 is a member of R4 subfamily of RGS family, a diverse group of multifunctional proteins that regulate cellular signalling events downstream of G-protein coupled receptors (GPCRs) []. Signalling is initiated when GPCRs bind to their ligands, triggering the replacement of GDP bound to the G-alpha subunits of heterotrimeric G proteins with GTP. RGSs inhibit signal transduction by increasing the GTPase activity of G protein alpha subunits, thereby driving them into their inactive GDP-bound form. This activity defines them as GTPase activating proteins (GAPs).Regulator of G protein signaling 3 (RGS3) contains a membrane-targeting C2 domain, one PDZ domain, and an RGS (Regulator of G-protein Signalling) domain. RGS3 has been identified to inhibit Galpha-q and Galpha-i-mediated signaling by acting as a GTPase-activating protein []. RGS3 exits as several splice isoforms []. A short form, RGS3S, induced apoptosis when overexpressed [], whereas PDZ-RGS3 has been linked to cell migration through interaction with Ephrin receptors []. RGS3 interacts with neuroligin, estrogen receptor-alpha, and 14-3-3 outside of the GPCR pathways [, , ].This entry represents the RGS domain of RGS3.
A large group of bacterial exotoxins are referred to as "A/B toxins", essentially because they are formed from two subunits. The "A"subunitpossesses enzyme activity, and is transferred to the host cell followinga conformational change in the membrane-bound transport "B"subunit [].Clostridial species are one of the major causes of food poisoning/gastro-intestinal illnesses. They are Gram-positive, spore-forming rods that occurnaturally in the soil []. Included in the family are: Clostridium botulinum, which produces one of the most potent toxins in existence; Clostridium tetani, causative agent of tetanus; and Clostridium perfringens, commonly found in wound infections and diarrhoea cases. Among the toxins produced by certain Clostridium spp. are the binary exotoxins. These proteins consist of two independent polypeptides, whichcorrespond to the A/B subunit moieties. The enzyme component (A) enters the cell through endosomes produced by the oligomeric binding/translocationprotein (B), and prevents actin polymerisation through ADP-ribosylation of monomeric G-actin [, , ].Members of the "A"binary toxin family include C. perfringens iota toxin Ia[], C. botulinum C2 toxin CI [], and Clostridium difficile ADP-ribosyltransferase []. Other homologous proteins have been found in Clostridium spiroforme [, ], and related bacteria such as Bacillus species.
The VASt (VAD1 Analog of StAR-related lipid transfer) domain is conserved across eukaryotes and is structurally related to Bet v1-like domains, including START lipid-binding domains. The ~190-amino acid VASt domain is predominantly associated with lipid bindingdomains such as GRAM, C2 and PH domains. The VASt domain is likely to have a function in binding large hydrophobic ligands and may be specific for sterol [, ].The predicted structure of the VASt domain is a two-layer sandwich α/β fold, also called "helix grip fold", containing three α-helices (α1 to3), six β-sheets (β1 to 6) and two loops (ω1 and 2) numbered from N to C terminus [].Some proteins known to contain a VASt domain are listed below:Plant vascular associated death1 (VAD1), a regulator of programmed cell death (PCD) harboring a GRAM putative lipid-binding domain.Yeast SNF1 Interacting Protein 3 (SIP3), may be involved in sterol transfer between intracellular membranes.Yeast Suicide Protein 1 (YSP1), a mitochondrial protein specifically required for the mitochondrial thread-grain transition, de-energization, and the cell death. May be involved in sterol transfer between intracellular membranes.Yeast Suicide Protein 2 (YSP2), a mitochondrial membrane protein involved in mitochondrial fragmentation. May be involved in sterol transfer between intracellular membranes.Human GramD1a-c.
Phosphoinositide-specific phospholipase C (PI-PLC), also known as 1-phosphatidylinositol 4,5-bisphosphate phosphodiesterase, plays a role in the inositol phospholipid signaling by hydrolysing phosphatidylinositol-4,5-bisphosphate to produce the second messengers inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG). These cause the increase of intracellular calcium concentration and the activation of protein kinase C (PKC), respectively.The PLC family in murine or human species is comprised of multiple subtypes. On the basis of their structure, they have been divided into five classes, beta (beta-1, 2, 3 and 4), gamma (gamma-1 and 2), delta (delta-1, 3 and 4), epsilon, zeta, and eta types [, ].PLC-delta-3 is essential for trophoblast and placental development []. It locates at the cleavage furrow where it may participate in cytokinesis []. PI-PLC-delta3 contains a core set of domains, including an N-terminal pleckstrin homology (PH) domain, four atypical EF-hand motifs, a PLC catalytic core, and a single C-terminal C2 domain. The PLC catalytic core domain is a TIM barrel with two highly conserved regions (X and Y) split by a highly degenerate linker sequence. In addition, PI-PLC-delta3 possesses a classical leucine-rich nuclear export sequence (NES) located in the EF hand motifs, which may be responsible transporting PI-PLC-delta3 from the cell nucleus [].
Doc2a (double C2-like domain-containing protein alpha) and Doc2b (double C2-like domain-containing protein beta) are members of the double C2 domain protein family. Doc2a is expressed in neuronal cells and has been implicated in Ca2+-dependent neurotransmitter release [, ]. Doc2a exhibits Ca2+-dependent phospholipid-binding activity through its C2A domain, which is thought to be important for the regulation of Ca2+-dependent exocytosis. The C2B domain is required for binding of syntaxin-1a/synaptosome-associated protein of 25kDa (SNAP-25) heterodimer []. Doc2a is also expressed in pancreatic islets, and has been implicated together with Doc2b in the synergistic regulation of glucose-stimulated insulin secretion [].Doc2b contains two calcium and phospholipid binding domains in its C terminus []. It interacts with the SNARE (soluble N-ethylmaleimide-sensitive factor attached protein receptor) complex composed of SNAP25, STX1A and VAMP2. Doc2b regulates SNARE-dependent fusion of vesicles with membranes in a calcium-dependent manner. It is involved in calcium-dependent spontaneous release of neurotransmitter, with a function analogous to synaptotagmin-1, but with a higher Ca2+ sensitivity []. Doc2b is a positive SNARE regulator for glucose transporter GLUT4 vesicle fusion and mediates insulin-stimulated glucose transport in adipocytes []. It is involved in both insulin-stimulated glucose uptake in adipocytes and glucose-stimulated insulin secretion in pancreatic cells, as well as insulin responsiveness in skeletal muscle [, ].
In Saccharomyces cerevisiae, Mid1 is a yeast plasma membrane protein required for Ca2+ influx induced by the mating pheromone alpha-factor during the mating process [, , ]. The protein is composed of 548-amino-acid residues, contains four hydrophobic regions (H1, H2, H3 and H4) and two cysteine-rich regions (C1 and C2) at the C terminus. H1 appears to be a signal sequence necessary for the alpha-factor-induced delivery to the plasma membrane. The region from H1 to H3 is required for the localisation of Mid1 in the plasma and ER membranes. C1 and C2 are thought to be involved in oligomerisation via the formation of disulphide bonds. Trafficking of Mid1-GFP to the plasma membrane is dependent on the N-glycosylation of Mid1 and the transporter protein Sec12. This suggests that the trafficking of Mid1-GFP to the plasma membrane requires a Sec12-dependent pathway from the ER to the Golgi, and that Mid1 is recruited via a Sec6- and Sec7-independent pathway from the Golgi to the plasma membrane.This entry also includes Ehs1 from Schizosaccharomyces pombe. Ehs1 is required for Ca2+ influx and for vitality of cells in a late, pheromone-induced event of the mating process requiring calcium-induced signaling [].
This entry represents the catalytic domain of PI3Kbeta (also known as p110beta), which is a Class IA phosphoinositide-3-kinase (PI3K) that phosphorylates PtdIns (Phosphatidylinositol), PtdIns4P (Phosphatidylinositol 4-phosphate) and PtdIns(4,5)P2 (Phosphatidylinositol 4,5-bisphosphate) to generate phosphatidylinositol 3,4,5-trisphosphate (PIP3). It has been shown to regulate chromosome segregation in mitosis [].PI3Ks can be divided into three main classes (I, II, and III), defined by their substrate specificity, regulation, and domain structure. Class I PI3Ks are the only enzymes capable of converting PtdIns(4,5)P2 to the critical second messenger PtdIns(3,4,5)P3. Class I enzymes are heterodimers and exist in multiple isoforms consisting of one catalytic subunit (p110alpha, beta, gamma or delta) and one of several regulatory subunits (p85alpha, beta or gamma). They are further classified into class IA (alpha, beta and delta) and IB (gamma). Class IA enzymes contain an N-terminal p85 binding domain, a Ras binding domain, a lipid binding C2 domain, a PI3K homology domain of unknown function, and a C-terminal ATP-binding cataytic domain. They associate with a regulatory subunit of the p85 family and are activated by tyrosine kinase receptors [].
This entry represents the catalytic domain of PI3Kdelta (also known as p110delta), which is a Class IA phosphoinositide-3-kinase (PI3K) that that phosphorylates PftdIns(4,5)P2 (Phosphatidylinositol 4,5-bisphosphate) to generate phosphatidylinositol 3,4,5-trisphosphate (PIP3). PI3Kdelta is mainly expressed in immune cells and plays an important role in cellular and humoral immunity. It plays a major role in antigen receptor signaling in B-cells, T-cells, and mast cells. It regulates the differentiation of peripheral helper T-cells and controls the development and function of regulatory T-cells [].PI3Ks can be divided into three main classes (I, II, and III), defined by their substrate specificity, regulation, and domain structure. Class I PI3Ks are the only enzymes capable of converting PtdIns(4,5)P2 to the critical second messenger PtdIns(3,4,5)P3. Class I enzymes are heterodimers and exist in multiple isoforms consisting of one catalytic subunit (p110alpha, beta, gamma or delta) and one of several regulatory subunits (p85alpha, beta or gamma). They are further classified into class IA (alpha, beta and delta) and IB (gamma). Class IA enzymes contain an N-terminal p85 binding domain, a Ras binding domain, a lipid binding C2 domain, a PI3K homology domain of unknown function, and a C-terminal ATP-binding cataytic domain. They associate with a regulatory subunit of the p85 family and are activated by tyrosine kinase receptors [].
This entry represents the catalytic domain of PI3Kalpha (also known as p110alpha), which is a Class IA phosphoinositide-3-kinase (PI3K) that that phosphorylates PtdIns (Phosphatidylinositol), PtdIns4P (Phosphatidylinositol 4-phosphate) and PtdIns(4,5)P2 (Phosphatidylinositol 4,5-bisphosphate) to generate phosphatidylinositol 3,4,5-trisphosphate (PIP3). PI3Kalpha plays an important role in insulin signaling []. It also mediates cardiac growth induced by exercise training []. Mutations in the PI3Kalpha gene have been linked to various cancers []. PI3Ks can be divided into three main classes (I, II, and III), defined by their substrate specificity, regulation, and domain structure. Class I PI3Ks are the only enzymes capable of converting PtdIns(4,5)P2 to the critical second messenger PtdIns(3,4,5)P3. Class I enzymes are heterodimers and exist in multiple isoforms consisting of one catalytic subunit (p110alpha, beta, gamma or delta) and one of several regulatory subunits (p85alpha, beta or gamma). They are further classified into class IA (alpha, beta and delta) and IB (gamma). Class IA enzymes contain an N-terminal p85 binding domain, a Ras binding domain, a lipid binding C2 domain, a PI3K homology domain of unknown function, and a C-terminal ATP-binding cataytic domain. They associate with a regulatory subunit of the p85 family and are activated by tyrosine kinase receptors [].
Protein kinases C (PKCs) constitute a family of Ser/Thr kinases. PKCs are classified into three groups (classical, atypical, and novel) depending on their mode of activation and the structural characteristics of their regulatory domain [, ]. Novel PKCs (nPKCs) comprise delta, epsilon, eta, and theta isoforms, which have tandem C1 domains and a C2 domain that does not bind calcium []. nPKCs are calcium-independent, but require DAG (1,2-diacylglycerol) and phosphatidylserine (PS) for activity. PKC-theta is selectively expressed in T-cells and plays an important and non-redundant role in several aspects of T-cell biology []. Although T-cells also express other PKC isoforms, PKC-theta is unique in that upon antigen stimulation, it is translocated to the plasma membrane at the immunological synapse, where it mediates signals essential for T-cell activation []. It is essential for TCR-induced proliferation, cytokine production, T-cell survival, and the differentiation and effector function of T-helper (Th) cells, particularly Th2 and Th17. PKC-theta is being developed as a therapeutic target for Th2-mediated allergic inflammation and Th17-mediated autoimmune diseases [].
In Gram-negative bacteria, growth on methanol is dependent on the soluble, periplasmic quinoprotein methanol dehydrogenase, which oxidises methanol to formaldehyde. The electrons generated by this reaction are transferred from the reduced enzyme to the unusual cytochrome cL, which is subsequently oxidised itself by cytochrome c2 (also known as cytochrome cH), which then transfers the electrons to a membrane-bound cytochrome oxidase [].This entry represents cytochrome cL (also known as cytochrome C551i in some species), whose amino acid sequence is distinct from that of other c-type cytochromes and does not fit into any established amino acid sequence class. Despite its lack of homology to other proteins, many of its properties, eg the low-spin haem prosthetic group, are similar to those of class 1 cytochrome c proteins. Other properties, such as its large size and acidic nature are distinct to cytochrome cL. The core of this protein has a structure typical of class I cytochrome c proteins, consisting of compact alpha helices enclosing the haem c prosthetic group with one edge of the haem exposed []. Unusually, there is a tightly bound calcium close to the haem group which is thought to help stabilise redox potential and may be involved in the transfer of electrons from methanol dehydrogenase to the haem group.
This entry represents the SH2 domain found in tensin-like proteins. The tensins are a family of intracellular proteins that interact with receptor tyrosine kinases (RTKs), integrins, and actin. They are thought act as signaling bridges between the extracellular space and the cytoskeleton. There are four homologues: tensin1, tensin2 (TENC1, C1-TEN), tensin3 and tensin4 (cten), all of which contain a C-terminal tandem SH2-PTB domain pairing, as well as actin-binding regions that may localize them to focal adhesions. The isoforms of tensin2 and tensin3 contain N-terminal C1 domains, which are atypical and not expected to bind to phorbol esters. Tensins 1-3 contain a phosphatase (PTPase) and C2 domain pairing which resembles PTEN (phosphatase and tensin homologue deleted on chromosome 10) protein [].PTEN is a lipid phosphatase that dephosphorylates phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P3) to yield phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2). As PtdIns(3,4,5)P3 is the product of phosphatidylinositol 3-kinase (PI3K) activity, PTEN is therefore a key negative regulator of the PI3K pathway []. Because of their PTEN-like domains, the tensins may also possess phosphoinositide-binding or phosphatase capabilities. However, only tensin2 and tensin3 have the potential to be phosphatases since only their PTPase domains contain a cysteine residue that is essential for catalytic activity. In general SH2 domains are involved in signal transduction. They typically bind pTyr-containing ligands via two surface pockets, a pTyr and hydrophobic binding pocket, allowing proteins with SH2 domains to localize to tyrosine phosphorylated sites [, , ].
The D-galactoside binding lectin purified from sea urchin (Anthocidaris crassispina) eggs exists as a disulphide-linked homodimer of two subunits; the dimeric form is essential for hemagglutination activity []. The sea urchin egg lectin (SUEL) forms a new class of lectins. Although SUEL was first isolated as a D-galactoside binding lectin, it was latter shown that it bind to L-rhamnose preferentially [, ]. L-rhamnose and D-galactose share the same hydroxyl group orientation at C2 and C4 of the pyranose ring structure.A cysteine-rich domain homologous to the SUEL protein has been identified in the following proteins [, , ]:Plant beta-galactosidases () (lactases).Mammalian latrophilin, the calcium independent receptor of alpha-latrotoxin (CIRL). The galactose-binding lectin domain is not required for alpha-latratoxin binding [].Human lectomedin-1.Rhamnose-binding lectin (SAL) from catfish (Silurus asotus, Namazu) eggs. This protein is composed of three tandem repeat domains homologous to the SUEL lectin domain. All cysteine positions of each domain are completely conserved [].The hypothetical B0457.1, F32A7.3A and F32A7.3B proteins from Caenorhabditis elegans.The human KIAA0821 protein.
This entry represents a conserved region within a number of eukaryotic dedicator of cytokinesis proteins (DOCK), which are guanine nucleotide exchange factors (GEFs) [, , ], that activate some small GTPases by exchanging bound GDP for free GTP such as Rac. DOCK proteins are required during several cellular processes, such as cell motility and phagocytosis []. These proteins have a DOCK-homology region 1 (DHR-1, also known as DOCK-type C2 domain) at the N-terminal and a DHR-2 (also known as DOCKER domain) at the C-terminal. The DOCKER domain () is a GEF catalytic domain organised into three lobes, A, B and C, with the Rho-family binding site and catalytic centre generated entirely from lobes B and C. This entry represents Lobe B, which adopts an unusual architecture of two antiparallel beta sheets disposed in a loosely packed orthogonal arrangement. This lobe changes its position relative to lobe C and the bound GTPase, which suggests that lobe B distinguishes between the switch 1 conformations of the small GTPases Rac1 and Cdc42 [, ].
Protein kinases C (PKCs) constitute a family of Ser/Thr kinases. PKCs are classified into three groups (classical, atypical, and novel) depending on their mode of activation and the structural characteristics of their regulatory domain [, ]. Novel PKCs (nPKCs) comprise delta, epsilon, eta, and theta isoforms, which have tandem C1 domains and a C2 domain that does not bind calcium []. nPKCs are calcium-independent, but require DAG (1,2-diacylglycerol) and phosphatidylserine (PS) for activity. PKC-epsilon has been shown to behave as an oncoprotein [, ]. Its overexpression contributes to neoplastic transformation depending on the cell type. It contributes to oncogenesis by inducing disordered cell growth and inhibiting cell death. It also plays a role in tumour invasion and metastasis[, ]. PKC-epsilon has also been found to confer cardioprotection against ischemia and reperfusion-mediated damage [, ]. Other cellular functions include the regulation of gene expression, cell adhesion, and cell motility [].This entry also includes PKCs from invertebrates, such as Pkc98E from Drosophila, which exhibits sequence identity to PKC-epsilon [].
Proteins containing this domain are a family of phosphoinositide phosphatases with substrates that include phosphatidylinositol-4,5-diphosphate and phosphatidylinositol-3,4,5-trisphosphate. This family is conserved in deuterostomes; VSP was first identified as a sperm flagellar plasma membrane protein in Ciona intestinalis []. Gene duplication events in primates resulted in the presence of paralogs, transmembrane phosphatase with tensin homology (TPTE) and TPTE2, that retain protein domain architecture but, in the case of TPTE, have lost catalytic activity. TPTE, also called cancer/testis antigen 44 (CT44), may play a role in the signal transduction pathways of the endocrine or spermatogenic function of the testis. TPTE2, also called TPTE and PTEN homologous inositol lipid phosphatase (TPIP), occurs in several differentially spliced forms; TPIP alpha displays phosphoinositide 3-phosphatase activity and is localized on the endoplasmic reticulum, while TPIP beta is cytosolic and lacks detectable phosphatase activity [, ]. VSP/TPTE proteins contain an N-terminal voltage sensor consisting of four transmembrane segments, a protein tyrosine phosphatase (PTP)-like phosphoinositide phosphatase catalytic domain, followed by a regulatory C2 domain [].
Semaphorins were first cloned as recognised mediators of cellular guidance, and consist of a large family of phylogenetically conserved secreted and transmembrane signalling proteins. Among the best-characterised vertebrate Semaphorins are the five secreted Class 3 members that contain an approximately 500 amino acid N-terminal Semaphorin domain, a C2 type immunoglobulin domain, and a highly basic C-terminal tail []. Two receptor families have been implicated in mediating the actions of class 3 semaphorins: the Neuropilins and Plexins. The nine known vertebrate Plexins are divided into four subfamilies (A through D) based on structure []. Several Plexins have been shown to interact directly with some class 4, 7 and V Semaphorins, but class 3 Semaphorins, however, do not appear to bind Plexins directly. Rather, the functional receptors for these Semaphorins are complexes of Neuropilins and A-type Plexins, with the former serving as the ligand-binding moiety and the latter the signal-transducing component [, ]. There are two Neuropilins (NP-1 and NP-2) that bind the five class 3 Semaphorins preferentially. In particular, Sema3A binds NP-1, whereas Sema3F utilises NP-2, while NP-1 and NP-2 heterodimers are thought to serve as functional receptors for Sema3C [].Semaphorin 4F may be involved in the injury response of intramedullary axotomized motoneurons [].
Semaphorins were first cloned as recognised mediators of cellular guidance, and consist of a large family of phylogenetically conserved secreted and transmembrane signalling proteins. Among the best-characterised vertebrate Semaphorins are the five secreted Class 3 members that contain an approximately 500 amino acid N-terminal Semaphorin domain, a C2 type immunoglobulin domain, and a highly basic C-terminal tail []. Two receptor families have been implicated in mediating the actions of class 3 semaphorins: the Neuropilins and Plexins. The nine known vertebrate Plexins are divided into four subfamilies (A through D) based on structure []. Several Plexins have been shown to interact directly with some class 4, 7 and V Semaphorins, but class 3 Semaphorins, however, do not appear to bind Plexins directly. Rather, the functional receptors for these Semaphorins are complexes of Neuropilins and A-type Plexins, with the former serving as the ligand-binding moiety and the latter the signal-transducing component [, ]. There are two Neuropilins (NP-1 and NP-2), which bind the five class 3 Semaphorins preferentially. In particular, Sema3A binds NP-1, whereas Sema3F utilises NP-2, while NP-1 and NP-2 heterodimers are thought to serve as functional receptors for Sema3C [].Recent work suggests a possible role of Gallus gallus (Chicken) Sema3E/collapsin-5 in restricting growth of retinal ganglion cell axons to the optic fibre layer [].
Semaphorins were first cloned as recognised mediators of cellular guidance, and consist of a large family of phylogenetically conserved secreted and transmembrane signalling proteins. Among the best-characterised vertebrate Semaphorins are the five secreted Class 3 members that contain an approximately 500 amino acid N-terminal Semaphorin domain, a C2 type immunoglobulin domain, and a highly basic C-terminal tail []. Two receptor families have been implicated in mediating the actions of class 3 semaphorins: the Neuropilins and Plexins. The nine known vertebrate Plexins are divided into four subfamilies (A through D) based on structure []. Several Plexins have been shown to interact directly with some class 4, 7 and V Semaphorins, but class 3 Semaphorins, however, do not appear to bind Plexins directly. Rather, the functional receptors for these Semaphorins are complexes of Neuropilins and A-type Plexins, with the former serving as the ligand-binding moiety and the latter the signal-transducing component [, ]. There are two Neuropilins (NP-1 and NP-2) that bind the five class 3 Semaphorins preferentially. In particular, Sema3A binds NP-1, whereas Sema3F utilises NP-2, while NP-1 and NP-2 heterodimers are thought to serve as functional receptors for Sema3C [].Recent microarray studies have suggested a role for Sema 6C in dental mesenchyme-induced neurite repulsion [].
Synaptotagmins are synaptic vesicle membrane proteins found in abundance in nerve cells and some endocrine cells [, ]. The amino acid sequence of synaptotagmin comprises a single transmembrane region with a short vesicular N-terminal region, and a cytoplasmic C-terminal region containing 2 internal repeats similar to the C2 regulatory domain of protein kinase C. The protein is believed to be important in the docking and fusion of synaptic vesicles with the plasma membrane, i.e. with neurotransmitter release [, ].Synaptotagmin 1 (originally called p65) and synaptotagmin 2 were the first to members identified in the synaptotagmin family [, ]. Synaptotagmin 1 may have a regulatory role in the membrane interactions during trafficking of synaptic vesicles at the active zone of the synapse []. It binds acidic phospholipids with a specificity that requires the presence of both an acidic head group and a diacyl backbone [, ]. It has been shown to function as a Ca2+ sensor on the synaptic vesicle surface, therefore to regulate Ca2+ dependent neurotransmitter release [, ].
Ras GTPase-activating protein 1 (also known as p120-RasGAP) is an inhibitory regulator of the Ras-cyclic AMP pathway [, ]. Its C-terminal catalytic domain promotes GTP hydrolysis and plays a key role in the regulation of Ras-GTP bound []. Its N-terminal part contains two SH2, SH3, PH (pleckstrin homology) and CaLB/C2 (calcium-dependent phospholipid-binding domain) domains, which allow various functions such as anti-/pro-apoptosis, proliferation and cell migration [].Alternative splicing results in two isoforms. The shorter isoform which lacks the N-terminal hydrophobic region, has the same activity, and is expressed in placental tissues. In general the longer isoform contains two SH2 domains, an SH3 domain, a pleckstrin homology (PH) domain, and a calcium-dependent phospholipid-binding C2 domain. The C terminus contains the catalytic domain of RasGap which catalyzes the activation of Ras by hydrolyzing GTP-bound active Ras into an inactive GDP-bound form of Ras [].This entry represents the SH3 domain of RasGAP []. The SH3 domain of RasGAP is unable to bind proline-rich sequences but have been shown to interact with protein partners such as the G3BP protein, Aurora kinases, and the Calpain small subunit 1. The RasGAP SH3 domain is necessary for the downstream signaling of Ras and it also influences Rho-mediated cytoskeletal reorganization [].
Phosphatidylinositol-specific phospholipase C (), an eukaryotic intracellular enzyme, plays an important role in signal transduction processes [](see ). It catalyzes the hydrolysis of 1-phosphatidyl-D-myo-inositol-3,4,5-triphosphate into the second messenger molecules diacylglycerol and inositol-1,4,5-triphosphate. This catalytic process is tightly regulated by reversible phosphorylation and binding of regulatory proteins [, , ].In mammals, there are at least 6 different isoforms of PI-PLC, they differ in their domain structure, their regulation, and their tissue distribution. Lower eukaryotes also possess multiple isoforms of PI-PLC.All eukaryotic PI-PLCs contain two regions of homology, sometimes referred to as 'X-box' (see ) and 'Y-box'. The order of these two regions is always the same (NH2-X-Y-COOH), but the spacing is variable. In most isoforms, the distance between these two regions is only 50-100 residues but in the gamma isoforms one PH domain, two SH2 domains, and one SH3 domain are inserted between the two PLC-specific domains. The two conserved regions have been shown to be important for the catalytic activity. At the C-terminal of the Y-box, there is a C2 domain (see ) possibly involved in Ca-dependent membrane attachment.
The D-galactoside binding lectin purified from sea urchin (Anthocidaris crassispina) eggs exists as a disulphide-linked homodimer of two subunits; the dimeric form is essential for hemagglutination activity []. The sea urchin egg lectin (SUEL) forms a new class of lectins. Although SUEL was first isolated as a D-galactoside binding lectin, it was latter shown that it bind to L-rhamnose preferentially [, ]. L-rhamnose and D-galactose share the same hydroxyl group orientation at C2 and C4 of the pyranose ring structure.A cysteine-rich domain homologous to the SUEL protein has been identified in the following proteins [, , ]:Plant beta-galactosidases () (lactases).Mammalian latrophilin, the calcium independent receptor of alpha-latrotoxin (CIRL). The galactose-binding lectin domain is not required for alpha-latratoxin binding [].Human lectomedin-1.Rhamnose-binding lectin (SAL) from catfish (Silurus asotus, Namazu) eggs. This protein is composed of three tandem repeat domains homologous to the SUEL lectin domain. All cysteine positions of each domain are completely conserved [].The hypothetical B0457.1, F32A7.3A and F32A7.3B proteins from Caenorhabditis elegans.The human KIAA0821 protein.Structurally, the rhamnose-binding lectin domain (also known as the N-terminal lectin domain, Lec) is composed of five β-strands , a single, long α-helix, and two small helical elements. The overall fold is that of a β-sandwich with two antiparallel sheets [].
Cysteine protease activity is dependent on an active dyad of cysteine andhistidine, the order and spacing of these residues varying in the 20 or soknown families. Cysteine proteases have been grouped into two clans (CA andCB). Families C1, C2 and C10 are loosely termed papain-like and belong to clan CA; five cysteine proteases belong to clan CB; other families havenot been assigned to clans. Nearly half of all cysteine proteases are found exclusively in viruses. The order of catalytic cysteine and histidine residues within the primary structure differs between the families and is an indication of convergent evolution [, ].Bacteria produce a number of protein precursors that undergo post-translational methylation and proteolysis prior to secretion as activeproteins. Type IV prepilin leader peptidases, which belong to the C20 familyof cysteine proteases, are enzymes that mediate this type of post-translational modification. Type IV pilin is a protein found on the surfaceof Pseudomonas aeruginosa, Neisseria gonorrhoeae and other Gram-negativepathogens. Pilin subunits attach the infecting organism to the surface of host epithelial cells. They are synthesised as prepilin subunits, which differ from mature pilin by virtue of containing a 6-8 residue leaderpeptide consisting of charged amino acids. Mature type IV pilins alsocontain a methylated N-terminal phenylalanine residue. Prepilin leader peptidases are found on the cytosolic membrane surface,where they have dual activity, involving cleavage of glycine-phenylalaninebonds and methylation of the newly-revealed N-terminal phenylalanine. Theconsensus sequence for the site of proteolytic cleavage is -G+F-T-L/I-, inwhich the Gly P1 residue is essential []. The peptidases are suseptible to thiol blocking reagents. Site directed mutagenesis has indicated four highlyconserved cysteine residues that affect both the protease and methylase activity.
This group of cysteine peptidases belong to the MEROPS peptidase family C2 (calpain family, clan CA). A type example is calpain, which is an intracellular protease involved in many important cellular functions that are regulated by calcium [, ]. The protein is a complex of 2 polypeptide chains (light and heavy), with eleven known active peptidases in humans and two non-peptidase homologues known as calpamodulin and androglobin []. These include a highly calcium-sensitive (i.e., micro-molar range) form known as mu-calpain, mu-CANP or calpain I; a form sensitive to calcium in the milli-molar range, known as m-calpain, m-CANP or calpain II; and a third form, known as p94, which is found in skeletal muscle only [].All forms have identical light but different heavy chains. Both mu- and m-calpain are heterodimers containing an identical 28kDa subunit and an 80kDa subunit that shares 55-65% sequence homology between the two proteases [, ]. The crystallographic structure of m-calpain reveals six "domains"in the 80kDa subunit [, ]: A 19-amino acid NH2-terminal sequence;Active site domain IIa;Active site domain IIb. Domain 2 showslow levels of sequence similarity to papain; although the catalytic His hasnot been located by biochemical means, it is likely that calpain and papainare related [].Domain III;An 18-amino acid extended sequence linking domain III to domain IV;Domain IV, which resembles the penta EF-hand family of polypeptides, binds calcium and regulates activity []. Ca2+-binding causes a rearrangement of the protein backbone, the net effect of which is that a Trp side chain, which acts as a wedge between catalytic domains IIa and IIb in the apo state, moves away from the active sitecleft allowing for the proper formation of the catalytic triad []. This superfamily describes domain III. Calpains are activated via rearrangement of the catalytic domain II induced by cooperative binding of Ca2+ to several sites of the molecule. A cluster of acidic residues in domain III, the acidic loop, has been proposed to function as part of an electrostatic switch in the activation process [].
This entry represents the C-terminal SH2 domain of phosphatidylinositol-4, 5-bisphosphate phosphodiesterase gamma (PLC-gamma).PLC-gamma is a signaling molecule that is recruited to the C-terminal tail of the receptor upon autophosphorylation of a highly conserved tyrosine. PLC-gamma is composed of a pleckstrin homology (PH) domain followed by an elongation factor (EF) domain, two catalytic regions of PLC domains that flank two tandem SH2 domains (N-SH2, C-SH2), and ending with a SH3 domain and C2 domain. N-SH2 domain-mediated interactions represent a crucial step in transmembrane signaling by receptor tyrosine kinases []. SH2 domains recognize phosphotyrosine (pY) in the context of particular sequence motifs in receptor phosphorylation sites. Both N-SH2 and C-SH2 have a very similar binding affinity to pY. But in growth factor stimulated cells these domains bind to different target proteins. N-SH2 binds to pY containing sites in the C-terminal tails of tyrosine kinases and other receptors. Recently it has been shown that this interaction is mediated by phosphorylation-independent interactions between a secondary binding site found exclusively on the N-SH2 domain and a region of the FGFR1 tyrosine kinase domain. This secondary site on the SH2 cooperates with the canonical pY site to regulate selectivity in mediating a specific cellular process. C-SH2 binds to an intramolecular site on PLC-gamma itself which allows it to hydrolyze phosphatidylinositol-4,5-bisphosphate into diacylglycerol and inositol triphosphate. These then activate protein kinase C and release calcium []. In general SH2 domains are involved in signal transduction. They typically bind pTyr-containing ligands via two surface pockets, a pTyr and hydrophobic binding pocket, allowing proteins with SH2 domains to localize to tyrosine phosphorylated site [].
This entry represents the N-terminal SH2 domain of phosphatidylinositol-4, 5-bisphosphate phosphodiesterase gamma (PLC-gamma).PLC-gamma is a signaling molecule that is recruited to the C-terminal tail of the receptor upon autophosphorylation of a highly conserved tyrosine. PLC-gamma is composed of a pleckstrin homology (PH) domain followed by an elongation factor (EF) domain, two catalytic regions of PLC domains that flank two tandem SH2 domains (N-SH2, C-SH2), and ending with a SH3 domain and C2 domain. N-SH2 domain-mediated interactions represent a crucial step in transmembrane signaling by receptor tyrosine kinases []. SH2 domains recognize phosphotyrosine (pY) in the context of particular sequence motifs in receptor phosphorylation sites. Both N-SH2 and C-SH2 have a very similar binding affinity to pY. But in growth factor stimulated cells these domains bind to different target proteins. N-SH2 binds to pY containing sites in the C-terminal tails of tyrosine kinases and other receptors. Recently it has been shown that this interaction is mediated by phosphorylation-independent interactions between a secondary binding site found exclusively on the N-SH2 domain and a region of the FGFR1 tyrosine kinase domain. This secondary site on the SH2 cooperates with the canonical pY site to regulate selectivity in mediating a specific cellular process. C-SH2 binds to an intramolecular site on PLC-gamma itself which allows it to hydrolyze phosphatidylinositol-4,5-bisphosphate into diacylglycerol and inositol triphosphate. These then activate protein kinase C and release calcium []. In general SH2 domains are involved in signal transduction. They typically bind pTyr-containing ligands via two surface pockets, a pTyr and hydrophobic binding pocket, allowing proteins with SH2 domains to localize to tyrosine phosphorylated site [].
Bacteriophage lambda C1 repressor controls the expression of viral genes as part of the lysogeny/lytic growth switch. C1 is essential for maintaining lysogeny, where the phage replicates non-disruptively along with the host. If the host cell is threatened, then lytic growth is induced. The Lambda C1 repressor consists of two domains connected by a linker: an N-terminal DNA-binding domain that also mediates interactions with RNA polymerase, and a C-terminal dimerisation domain []. The DNA-binding domain consists of four helices in a closed folded leaf motif. Several different phage repressors from different helix-turn-helix families contain DNA-binding domains that adopt a similar topology. These include the Lambda Cro repressor, Bacteriophage 434 C1 and Cro repressors, P22 C2 repressor, and Bacteriophage Mu Ner protein.The DNA-binding domain of Bacillus subtilis spore inhibition repressor SinR is identical to that of phage repressors []. SinR represses sporulation, which only occurs in response to adverse conditions. This provides a possible evolutionary link between the two adaptive responses of bacterial sporulation and prophage induction.Other DNA-binding domains also display similar structural folds to that of Lambda C1. These include bacterial regulators such as the purine repressor (PurR), the lactose repressor (Lacr) and the fructose repressor (FruR), each of which has an N-terminal DNA-binding domain that exhibits a fold similar to that of lambda C1, except that they lack the first helix [, , ]. POU-specific domains found in transcription factors such as in Oct-1, Pit-1 and Hepatocyte nuclear factor 1a (LFB1/HNF1) display four-helical fold DNA-binding domains similar to that of Lambda C1 [, , ]. The N-terminal domain of cyanase has an α-helix bundle motif similar to Lambda C1, but it probably does not bind DNA. Cyanase is an enzyme found in bacteria and plants that catalyses the reaction of cyanate with bicarbonate to produce ammonia and carbon dioxide in response to extracellular cyanate [].
This group of cysteine peptidases belong to the MEROPS peptidase family C2 (calpain family, clan CA). A type example is calpain, which is an intracellular protease involved in many important cellular functions that are regulated by calcium [, ]. The protein is a complex of 2 polypeptide chains (light and heavy), with eleven known active peptidases in humans and two non-peptidase homologues known as calpamodulin and androglobin []. These include a highly calcium-sensitive (i.e., micro-molar range) form known as mu-calpain, mu-CANP or calpain I; a form sensitive to calcium in the milli-molar range, known as m-calpain, m-CANP or calpainII; and a third form, known as p94, which is found in skeletal muscle only [].All forms have identical light but different heavy chains. Both mu- and m-calpain are heterodimers containing an identical 28kDa subunit and an 80kDa subunit that shares 55-65% sequence homology between the two proteases [, ]. The crystallographic structure of m-calpain reveals six "domains"in the 80kDa subunit [, ]: A 19-amino acid NH2-terminal sequence;Active site domain IIa;Active site domain IIb. Domain 2 showslow levels of sequence similarity to papain; although the catalytic His hasnot been located by biochemical means, it is likely that calpain and papainare related [].Domain III;An 18-amino acid extended sequence linking domain III to domain IV;Domain IV, which resembles the penta EF-hand family of polypeptides, binds calcium and regulates activity []. Ca2+-binding causes a rearrangement of the protein backbone, the net effect of which is that a Trp side chain, which acts as a wedge between catalytic domains IIa and IIb in the apo state, moves away from the active site cleft allowing for the proper formation of the catalytic triad []. Calpain-like mRNAs have been identified in other organisms including bacteria, but the molecules encoded by these mRNAs have not been isolated, so little is known about their properties. How calpain activity is regulated in these organisms cells is still unclear In metazoans, the activity of calpain is controlled by a single proteinase inhibitor, calpastatin (). The calpastatin gene can produce eight or more calpastatin polypeptides ranging from 17 to 85kDa by use of different promoters and alternative splicing events. The physiological significance of these different calpastatins is unclear, although all bind to three different places on the calpain molecule; binding to at least two of the sites is Ca2+ dependent. The calpains ostensibly participate in a variety of cellular processes including remodelling of cytoskeletal/membrane attachments, different signal transduction pathways, and apoptosis. Deregulated calpain activity following loss of Ca2+ homeostasis results in tissue damage in response to events such as myocardial infarcts, stroke, and brain trauma [].
Cyclotides (cyclo peptides) are plant peptides of ~30 amino acids with a head to-tail cyclic backbone and six cysteine residues involved in three disulphide bonds. The cyclotides are extremely resistant to proteolysis and are remarkably stable. Cyclotides display a diverse range of biological activities, including uterotonic activity, inhibition of neurotensin binding, hemolytic, anti-HIV and anti-microbial activity. This range of biological activities makes cyclotides amenable to potential pharmaceutical and agricultural applications. Although their precise role in plants has not yet been reported, it appears that they are most likely present as defence molecules [, , , ].The three-dimensional structure of cyclotides is compact and contains a number of β-turns, three β-strands arranged in a distorted triple-stranded β-sheet, a short helical segment, and a network of disulphide bonds which form a cystine knot. The cystine knot consists of an embedded ring in the structure, formed by two disulphide bonds and their connecting backbone segments is threaded by a third disulphide bond. Although the cystine knot motif is now well known in a wide variety of proteins, the cyclotides remain as the only example in which a cystine knot is embedded within a circular protein backbone, a motif that is referred to as the cyclic cystine knot (CCK) [, , , ].Cyclotides can be separated into two sub-families, one of which tends to contain a larger number of positively charged residues and has a bracelet-like circularisation of the backbone. The second subfamily contains a backbone twist due to a cis-Pro peptide bond and may conceptually be regarded as a molecular Moebius strip [, ]. Bracelet and Moebius families of cyclotides possess a Knottin scaffold. The cyclotide family of proteins is abundant in plants from the Rubiaceae and Violaceae families and includes:Kalata B1.Circulins.Cyclopsychotride A.Cycloviolacin O1.Also included in this entry are cliotides from the leguminous plant Clitoria ternatea. These are cyclotides with known medicinal properties that have antimicrobial activities against Escherichia coli and are cytotoxic to HeLa cells [].This entry also includes chassatides. There are 18 chassatides peptides: 14 new cyclotides and 4 uncyclotides from the Rubiaceae family. Uncyclotides are the most potent chassatides for antimicrobial, cytotoxic, and hemolytic activities. All uncyclotides belong to the bracelet subfamily, all lacking the Asn/Asp residue at their C termini, which is crucial for backbone cyclization. Genetic characterization of novel cyclotides revealed that their precursors are highly shortened. They consist of five bracelet, two Möbius, and two hybrid cyclotides. Two Met-oxidized derivatives of chassatide C2 and C11 have been isolated, while we know that oxydation of methionine to methionine sulfoxide (MetO) causes a complete loss of biological activities [].
The Rab11 GTPase regulates recycling of internalized plasma membrane receptors and is essential for completion of cytokinesis. A family of Rab11 interacting proteins (FIPs) that conserve a C-terminal Rab-binding domain (RBD) selectively recognise the active form of Rab11. FIPs are diverse in sequence length and composition toward their N-termini, presumably a feature that underpins their specific roles in Rab11-mediated vesicle trafficking. They have been divided into three subfamilies (classe I, II, and III)on the basis of domain architecture. Class I FIPs comprises a subfamily of three proteins (Rip11/pp75/FIP5, Rab-coupling protein (RCP), and FIP2) that possess an N-terminal C2 domain, localize to recycling endosomes, and regulate plasma membrane recycling. The class II subfamily consists of two proteins (FIP3/eferin/arfophilin and FIP4) with tandem EF hands and a proline-rich region. Class II FIPs localize to recycling endosomes, the trans-Golgi network, and have been implicated in the regulation of membrane trafficking during cytokinesis. The class III subfamily consists of a single protein, FIP1, which does not contain obvious homology domains or motifs other than the FIP-RBD [, , , ].The FIP-RBD domain is also found in Rab6-interacting protein Erc1/Elks. Erc1 is the regulatory subunit of the IKK complex and probably recruits IkappaBalpha/NFKBIA to the complex []. It may be involved in the organisation of the cytomatrix at the nerve terminals active zone (CAZ) which regulates neurotransmitter release. It may also be involved in vesicle trafficking at the CAZ, as well as in Rab-6 regulated endosomes to Golgi transport [].The FIB-RBD domain consists of an N-terminal long α-helix, followed by a 90 degrees bend at a conserved proline residue, a 3(10) helix and a C-terminal short β-strand, adopting an "L"shape. The long α-helix forms a parallel coiled-coil homodimer that symmetrically interacts with two Rab11 molecules on both sides, forming a quaternary Rab11-(FIP)2-Rab11 complex. The Rab11-interacting region of FIP-RBD is confined to the C-terminal 24 amino acids, which cover the C-terminal half of the long α-helix and the short β-strand [, , , ]. This entry represents the FIP-RBD C-terminal domain.
This group of cysteine peptidases belong to the MEROPS peptidase family C2 (calpain family, clan CA). A type example is calpain, which is an intracellular protease involved in many important cellular functions that are regulated by calcium [, ]. The protein is a complex of 2 polypeptide chains (light and heavy), with eleven known active peptidases in humans and two non-peptidase homologues known as calpamodulin and androglobin []. These include a highly calcium-sensitive (i.e., micro-molar range) form known as mu-calpain, mu-CANP or calpain I; a form sensitive to calcium in the milli-molar range, known as m-calpain, m-CANP or calpain II; and a third form, known as p94, which is found in skeletal muscle only [].All forms have identical light but different heavy chains. Both mu- and m-calpain are heterodimers containing an identical 28kDa subunit and an 80kDa subunit that shares 55-65% sequence homology between the two proteases [, ]. The crystallographic structure of m-calpain reveals six "domains"in the 80kDa subunit [, ]: A 19-amino acid NH2-terminal sequence;Active site domain IIa;Active site domain IIb. Domain 2 showslow levels of sequence similarity to papain; although the catalytic His hasnot been located by biochemical means, it is likely that calpain and papainare related [].Domain III;An 18-amino acid extended sequence linking domain III to domain IV;Domain IV, which resembles the penta EF-hand family of polypeptides, binds calcium and regulates activity []. Ca2+-binding causes a rearrangement of the protein backbone, the net effect of which is that a Trp side chain, which acts as a wedge between catalytic domains IIa and IIb in the apo state, moves away from the active site cleft allowing for the proper formation of the catalytic triad []. Calpain-like mRNAs have been identified in other organisms including bacteria, but the molecules encoded by these mRNAs have not been isolated, so little is known about their properties. How calpain activity is regulated in these organisms cells is still unclear In metazoans, the activity of calpain is controlled by a single proteinase inhibitor, calpastatin (). The calpastatin gene can produce eight or more calpastatin polypeptides ranging from 17 to 85kDa by use of different promoters and alternative splicing events. The physiological significance of these different calpastatins is unclear, although all bind to three different places on the calpain molecule; binding to at least two of the sites is Ca2+ dependent. The calpains ostensibly participate in a variety of cellular processes including remodelling of cytoskeletal/membrane attachments, different signal transduction pathways, and apoptosis. Deregulated calpain activity following loss of Ca2+ homeostasis results in tissue damage in response to events such as myocardial infarcts, stroke, and brain trauma []. This entry includes subdomain III of typical and atypical calpains.
The membrane attack complex/perforin (MACPF) domain is conserved in bacteria, fungi, mammals and plants. It was originally identified and named as being common to five complement components (C6, C7, C8-alpha, C8-beta, and C9) and perforin. These molecules perform critical functions in innate and adaptive immunity. The MAC family proteins and perforin are known to participate in lytic pore formation. In response to pathogen infection, a sequential and highly specific interaction between the constituent elements occurs to form transmembrane channels which are known as the membrane-attack complex (MAC).Only a few other MACPF proteins have been characterised and several are thought to form pores for invasion or protection [, , ]. Examples are proteins from malarial parasites [], the cytolytic toxins from sea anemones [], and proteins that provide plant immunity [, ]. Functionally uncharacterised MACPF proteins are also evident in pathogenic bacteria such as Chlamydia spp []and Photorhabdus luminescens (Xenorhabdus luminescens) [].The MACPF domain is commonly found to be associated with other N- and C-terminal domains, such as TSP1 (see ), LDLRA (see ), EGF-like (see ),Sushi/CCP/SCR (see ), FIMAC or C2 (see ). They probably control or target MACPF function [, ]. The MACPF domain oligomerizes, undergoes conformational change, and is required for lytic activity.The MACPF domain consists of a central kinked four-stranded antiparallel beta sheet surrounded by alpha helices and beta strands, forming two structural segments. Overall, the MACPF domain hasa thin L-shaped appearance. MACPF domains exhibit limited sequence similarity but contain a signature [YW]-G-[TS]-H-[FY]-x(6)-G-G motif [, , ].Some proteins known to contain a MACPF domain are listed below:Vertebrate complement proteins C6 to C9. Complement factors C6 to C9 assemble to form a scaffold, the membrane attack complex (MAC), that permits C9 polymerisation into pores that lyse Gram-negative pathogens [, ].Vertebrate perforin. It is delivered by natural killer cells and cytotoxic T lymphocytes and forms oligomeric pores (12 to 18 monomers) in the plasma membrane of either virus-infected or transformed cells.Arabidopsis thaliana (Mouse-ear cress) constitutively activated cell death 1 (CAD1) protein. It is likely to act as a mediator that recognises plant signals for pathogen infection [].Arabidopsis thaliana (Mouse-ear cress) necrotic spotted lesions 1 (NSL1) protein [].Venomous sea anemone Phyllodiscus semoni (Night anemone) toxins PsTX-60A and PsTX-60B [].Venomous sea anemone Actineria villosa (Okinawan sea anemone) toxin AvTX-60A [].Plasmodium sporozoite microneme protein essential for cell traversal 2 (SPECT2). It is essential for the membrane-wounding activity of the sporozoite and is involved in its traversal of the sinusoidal cell layer prior to hepatocyte-infection [].P. luminescens Plu-MACPF. Although nonlytic, it was shown to bind to cell membranes [].Chlamydial putative uncharacterised protein CT153 [].
The membrane attack complex/perforin (MACPF) domain is conserved in bacteria, fungi, mammals and plants. It was originally identified and named as being common to five complement components (C6, C7, C8-alpha, C8-beta, and C9) and perforin. These molecules perform critical functions in innate and adaptive immunity. The MAC family proteins and perforin are known to participate in lytic pore formation. In response to pathogen infection, a sequential and highly specific interaction between the constituent elements occurs to form transmembrane channels which are known as the membrane-attack complex (MAC).Only a few other MACPF proteins have been characterised and several are thought to form pores for invasion or protection [, , ]. Examples are proteins from malarial parasites [], the cytolytic toxins from sea anemones [], and proteins that provide plant immunity [, ]. Functionally uncharacterised MACPF proteins are also evident in pathogenic bacteria such as Chlamydia spp []and Photorhabdus luminescens (Xenorhabdus luminescens) [].The MACPF domain is commonly found to be associated with other N- and C-terminal domains, such as TSP1 (see ), LDLRA (see ), EGF-like (see ),Sushi/CCP/SCR (see ), FIMAC or C2 (see ). They probably control or target MACPF function [, ]. The MACPF domain oligomerizes, undergoes conformational change, and is required for lytic activity.The MACPF domain consists of a central kinked four-stranded antiparallel beta sheet surrounded by alpha helices and beta strands, forming two structural segments. Overall, the MACPF domain has a thin L-shaped appearance. MACPF domainsexhibit limited sequence similarity but contain a signature [YW]-G-[TS]-H-[FY]-x(6)-G-G motif [, , ].Some proteins known to contain a MACPF domain are listed below:Vertebrate complement proteins C6 to C9. Complement factors C6 to C9 assemble to form a scaffold, the membrane attack complex (MAC), that permits C9 polymerisation into pores that lyse Gram-negative pathogens [, ].Vertebrate perforin. It is delivered by natural killer cells and cytotoxic T lymphocytes and forms oligomeric pores (12 to 18 monomers) in the plasma membrane of either virus-infected or transformed cells.Arabidopsis thaliana (Mouse-ear cress) constitutively activated cell death 1 (CAD1) protein. It is likely to act as a mediator that recognises plant signals for pathogen infection [].Arabidopsis thaliana (Mouse-ear cress) necrotic spotted lesions 1 (NSL1) protein [].Venomous sea anemone Phyllodiscus semoni (Night anemone) toxins PsTX-60A and PsTX-60B [].Venomous sea anemone Actineria villosa (Okinawan sea anemone) toxin AvTX-60A [].Plasmodium sporozoite microneme protein essential for cell traversal 2 (SPECT2). It is essential for the membrane-wounding activity of the sporozoite and is involved in its traversal of the sinusoidal cell layer prior to hepatocyte-infection [].P. luminescens Plu-MACPF. Although nonlytic, it was shown to bind to cell membranes [].Chlamydial putative uncharacterised protein CT153 [].
Cyclotides (cyclo peptides) are plant peptides of ~30 amino acids with a head to-tail cyclic backbone and six cysteine residues involved in three disulphide bonds. The cyclotides are extremely resistant to proteolysis and are remarkably stable. Cyclotides display a diverse range of biological activities, including uterotonic activity, inhibition of neurotensin binding, hemolytic, anti-HIV and anti-microbial activity. This range of biological activities makes cyclotides amenable to potential pharmaceutical and agricultural applications. Although their precise role in plants has not yet been reported, it appears that they are most likely present as defence molecules [, , , ].The three-dimensional structure of cyclotides is compact and contains a number of β-turns, three β-strands arranged in a distorted triple-stranded β-sheet, a short helical segment, and a network of disulphide bonds which form a cystine knot. The cystine knot consists of an embedded ring in the structure, formed by two disulphide bonds and their connecting backbone segments is threaded by a third disulphide bond. Although the cystine knot motif is now well known in a wide variety of proteins, the cyclotides remain as the only example in which a cystine knot is embedded within a circular protein backbone, a motif that is referred to as the cyclic cystine knot (CCK) [, , , ].Cyclotides can be separated into two sub-families, one of which tends to contain a larger number of positively charged residues and has a bracelet-like circularisation of the backbone. The second subfamily contains a backbone twist due to a cis-Pro peptide bond and may conceptually be regarded as a molecular Moebius strip [, ]. Bracelet and Moebius families of cyclotides possess a Knottin scaffold. The cyclotide family of proteins is abundant in plants from the Rubiaceae and Violaceae families and includes:Kalata B1.Circulins.Cyclopsychotride A.Cycloviolacin O1.Also included in this entry are cliotides from the leguminous plant Clitoria ternatea. These are cyclotides with known medicinal properties that have antimicrobial activities against Escherichia coli and are cytotoxic to HeLa cells [].This entry also includes chassatides. There are 18 chassatides peptides: 14 new cyclotides and 4 uncyclotides from the Rubiaceae family. Uncyclotides are the most potent chassatides for antimicrobial, cytotoxic, and hemolytic activities. All uncyclotides belong to the bracelet subfamily, all lacking the Asn/Asp residue at their C termini, which is crucial for backbone cyclization. Genetic characterization of novel cyclotides revealed that their precursors are highly shortened. They consist of five bracelet, two Möbius, and two hybrid cyclotides. Two Met-oxidized derivatives of chassatide C2 and C11 have been isolated, while we know that oxydation of methionine to methionine sulfoxide (MetO) causes a complete loss of biological activities [].
The Rab11 GTPase regulates recycling of internalized plasma membrane receptors and is essential for completion of cytokinesis. A family of Rab11 interacting proteins (FIPs) that conserve a C-terminal Rab-binding domain (RBD) selectively recognise the active form of Rab11. FIPs are diverse in sequence length and composition toward their N-termini, presumably a feature that underpins their specific roles in Rab11-mediated vesicle trafficking. They have been divided into three subfamilies (classe I, II, and III)on the basis of domain architecture. Class I FIPs comprises a subfamily of three proteins (Rip11/pp75/FIP5, Rab-coupling protein (RCP), and FIP2) that possess an N-terminal C2 domain, localize to recycling endosomes, and regulate plasma membrane recycling. The class II subfamily consists of two proteins (FIP3/eferin/arfophilin and FIP4) with tandem EF hands and a proline-rich region. Class II FIPs localize to recycling endosomes, the trans-Golgi network, and have been implicated in the regulation of membrane trafficking during cytokinesis. The class III subfamily consists of a single protein, FIP1, which does not contain obvious homology domains or motifs other than the FIP-RBD [, , , ].The FIP-RBD domain is also found in Rab6-interacting protein Erc1/Elks. Erc1 is the regulatory subunit of the IKK complex and probably recruits IkappaBalpha/NFKBIA to the complex []. It may be involved in the organisation of the cytomatrix at the nerve terminals active zone (CAZ) which regulates neurotransmitter release. It may also be involved in vesicle trafficking at the CAZ, as well as in Rab-6 regulated endosomes to Golgi transport [].The FIB-RBD domain consists of an N-terminal long α-helix, followed by a 90 degrees bend at a conserved proline residue, a 3(10) helix and a C-terminal short β-strand, adopting an "L"shape. The long α-helix forms a parallel coiled-coil homodimer that symmetrically interacts with two Rab11 molecules on both sides, forming a quaternary Rab11-(FIP)2-Rab11 complex. The Rab11-interacting region of FIP-RBD is confined to the C-terminal 24 amino acids, which cover the C-terminal half of the long α-helix and the short β-strand [, , , ]. This entry represents the FIP-RBD C-terminal domain.
The Ubiquitin Interacting Motif (UIM), or 'LALAL-motif', is a stretch of about 20 amino acid residues, which was first described in the 26S proteasome subunit PSD4/RPN-10 that is known to recognise ubiquitin [, ]. In addition, the UIM is found, often in tandem or triplet arrays, in a variety of proteins either involved in ubiquitination and ubiquitin metabolism, or known to interact with ubiquitin-like modifiers. Among the UIM proteins are two different subgroups of the UBP (ubiquitin carboxy-terminal hydrolase) family of deubiquitinating enzymes, one F-box protein, one family of HECT-containing ubiquitin-ligases (E3s) from plants, and several proteins containing ubiquitin-associated UBA and/or UBX domains []. In most of these proteins, the UIM occurs in multiple copies and in association with other domains such as UBA (), UBX (), ENTH, EH (), VHS (), SH3 (), HECT (), VWFA (), EF-hand calcium-binding, WD-40 (), F-box (), LIM (), protein kinase (), ankyrin (), PX (), phosphatidylinositol 3- and 4-kinase (), C2 (), OTU (), dnaJ (), RING-finger () or FYVE-finger (). UIMs have been shown to bind ubiquitin and to serve as a specific targeting signal important for monoubiquitination. Thus, UIMs may have several functions in ubiquitin metabolism each of which may require different numbers of UIMs [, , ]. The UIM is unlikely to form an independent folding domain. Instead, based on the spacing of the conserved residues, the motif probably forms a short α-helix that can be embedded into different protein folds []. Some proteins known to contain an UIM are listed below: Eukaryotic PSD4/RPN-10/S5, a multi-ubiquitin binding subunit of the 26S proteasome. Vertebrate Machado-Joseph disease protein 1 (Ataxin-3), which acts as a histone-binding protein that regulates transcription; defects in Ataxin-3 cause the neurodegenerative disorder Machado-Joseph disease (MJD).Vertebrate epsin and epsin2. Vertebrate hepatocyte growth factor-regulated tyrosine kinase substrate (HRS). Mammalian epidermal growth factor receptor substrate 15 (EPS15), which is involved in cell growth regulation. Mammalian epidermal growth factor receptor substrate EPS15R. Drosophila melanogaster (Fruit fly) liquid facets (lqf), an epsin. Yeast VPS27 vacuolar sorting protein, which is required for membrane traffic to the vacuole.
This group of cysteine peptidases belong to the MEROPS peptidase family C2 (calpain family, clan CA). A type example is calpain, which is an intracellular protease involved in many important cellular functions that are regulated by calcium [, ]. The protein is a complex of 2 polypeptide chains (light and heavy), with eleven known active peptidases in humans and two non-peptidase homologues known as calpamodulin and androglobin []. These include a highly calcium-sensitive (i.e., micro-molar range) form known as mu-calpain, mu-CANP or calpain I; a form sensitive to calcium in the milli-molar range, known as m-calpain, m-CANP or calpain II; and a third form, known as p94, which is found in skeletal muscle only [].All forms have identical light but different heavy chains. Both mu- and m-calpain are heterodimers containing an identical 28kDa subunit and an 80kDa subunit that shares 55-65% sequence homology between the two proteases [, ]. The crystallographic structure of m-calpain reveals six "domains"in the 80kDa subunit [, ]: A 19-amino acid NH2-terminal sequence;Active site domain IIa;Active site domain IIb. Domain 2 showslow levels of sequence similarity to papain; although the catalytic His hasnot been located by biochemical means, it is likely that calpain and papainare related [].Domain III;An 18-amino acid extended sequence linking domain III to domain IV;Domain IV, which resembles the penta EF-hand family of polypeptides, binds calcium and regulates activity []. Ca2+-binding causes a rearrangement of the protein backbone, the net effect of which is that a Trp side chain, which acts as a wedge between catalytic domains IIa and IIb in the apo state, moves away from the active site cleft allowing for the proper formation of the catalytic triad []. Calpain-like mRNAs have been identified in other organisms including bacteria, but the molecules encoded by these mRNAs have not been isolated, so little is known about their properties. How calpain activity is regulated in these organisms cells is still unclear In metazoans, the activity of calpain is controlled by a single proteinase inhibitor, calpastatin (). The calpastatin gene can produce eight or more calpastatin polypeptides ranging from 17 to 85kDa by use of different promoters and alternative splicing events. The physiological significance of these different calpastatins is unclear, although all bind to three different places on the calpain molecule; binding to at least two of the sites is Ca2+ dependent. The calpains ostensibly participate in a variety of cellular processes including remodelling of cytoskeletal/membrane attachments, different signal transduction pathways, and apoptosis. Deregulated calpain activity following loss of Ca2+ homeostasis results in tissue damage in response to events such as myocardial infarcts, stroke, and brain trauma []. This entry represents domain III. The function of the domain III and I are currently unknown. Domain II is a cysteine protease and domain IV is a calcium binding domain. Calpains are believed to participate in intracellular signaling pathways mediated by calcium ions.
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 residues that 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.This group of cysteine peptidases belong to the MEROPS peptidase family C2 (calpain family, clan CA). A type example is calpain, which is an intracellular protease involved in many important cellular functions that are regulated by calcium [, ]. The protein is a complex of 2 polypeptide chains (light and heavy), with eleven known active peptidases in humans and two non-peptidase homologues known as calpamodulin and androglobin []. These include a highly calcium-sensitive (i.e., micro-molar range) form known as mu-calpain, mu-CANP or calpain I; a form sensitive to calcium in the milli-molar range, known as m-calpain, m-CANP or calpain II; and a third form, known as p94, which is found in skeletal muscle only [].All forms have identical light but different heavy chains. Both mu- and m-calpain are heterodimers containing an identical 28kDa subunit and an 80kDa subunit that shares 55-65% sequence homology between the two proteases [, ]. The crystallographic structure of m-calpain reveals six "domains"in the 80kDa subunit [, ]: A 19-amino acid NH2-terminal sequence;Active site domain IIa;Active site domain IIb. Domain 2 showslow levels of sequence similarity to papain; although the catalytic His hasnot been located by biochemical means, it is likely that calpain and papainare related [].Domain III;An 18-amino acid extended sequence linking domain III to domain IV;Domain IV, which resembles the penta EF-hand family of polypeptides, binds calcium and regulates activity []. Ca2+-binding causes a rearrangement of the protein backbone, the net effect of which is that a Trp side chain, which acts as a wedge between catalytic domains IIa and IIb in the apo state, moves away from the active site cleft allowing for the proper formation of the catalytic triad []. Calpain-like mRNAs have been identified in other organisms including bacteria, but the molecules encoded by these mRNAs have not been isolated, so little is known about their properties. How calpain activity is regulated in these organisms cells is still unclear In metazoans, the activity of calpain is controlled by a single proteinase inhibitor, calpastatin (). The calpastatin gene can produce eight or more calpastatin polypeptides ranging from 17 to 85kDa by use of different promoters and alternative splicing events. The physiological significance of these different calpastatins is unclear, although all bind to three different places on the calpain molecule; binding to at least two of the sites is Ca2+ dependent. The calpains ostensibly participate in a variety of cellular processes including remodelling of cytoskeletal/membrane attachments, different signal transduction pathways, and apoptosis. Deregulated calpain activity following loss of Ca2+ homeostasis results in tissue damage in response to events such as myocardial infarcts, stroke, and brain trauma [].
This group of cysteine peptidases belong to the MEROPS peptidase family C2 (calpain family, clan CA). A type example is calpain, which is an intracellular protease involved in many important cellular functions that are regulated by calcium [, ]. The protein is a complex of 2 polypeptide chains (light and heavy), with eleven known active peptidases in humans and two non-peptidase homologues known as calpamodulin and androglobin []. These include a highly calcium-sensitive (i.e., micro-molar range) form known as mu-calpain, mu-CANP or calpain I; a form sensitive to calcium in the milli-molar range, known as m-calpain, m-CANP or calpain II; and a third form, known as p94, which is found in skeletal muscle only [].All forms have identical light but different heavy chains. Both mu- and m-calpain are heterodimers containing an identical 28kDa subunit and an 80kDa subunit that shares 55-65% sequence homology between the two proteases [, ]. The crystallographic structure of m-calpain reveals six "domains"in the 80kDa subunit [, ]: A 19-amino acid NH2-terminal sequence;Active site domain IIa;Active site domain IIb. Domain 2 showslow levels of sequence similarity to papain; although the catalytic His hasnot been located by biochemical means, it is likely that calpain and papainare related [].Domain III;An 18-amino acid extended sequence linking domain III to domain IV;Domain IV, which resembles the penta EF-hand family of polypeptides, binds calcium and regulates activity []. Ca2+-binding causes a rearrangement of the protein backbone, the net effect of which is that a Trp side chain, which acts as a wedge between catalytic domains IIa and IIb in the apo state, moves away from the active site cleft allowing for the proper formation of the catalytic triad []. Calpain-like mRNAs have been identified in other organisms including bacteria, but the molecules encoded by these mRNAs have not been isolated, so little is known about their properties. How calpain activity is regulated in these organisms cells is still unclear In metazoans, the activity of calpain is controlled by a single proteinase inhibitor, calpastatin (). The calpastatin gene can produce eight or more calpastatin polypeptides ranging from 17 to 85kDa by use of different promoters and alternative splicing events. The physiological significance of these different calpastatins is unclear, although all bind to three different places on the calpain molecule; binding to at least two of the sites is Ca2+ dependent. The calpains ostensibly participate in a variety of cellular processes including remodelling of cytoskeletal/membrane attachments, different signal transduction pathways, and apoptosis. Deregulated calpain activity following loss of Ca2+ homeostasis results in tissue damage in response to events such as myocardial infarcts, stroke, and brain trauma []. Calpains are a family of cytosolic cysteine proteinases (see ). Members of the calpain family are believed to function in various biological processes, including integrin-mediated cell migration, cytoskeletal remodeling, cell differentiation and apoptosis [, ].The calpain family includes numerous members from C. elegans to mammals and with homologues in yeast and bacteria. The best characterised members are the m- and mu-calpains, both proteins are heterodimer composed of a large catalytic subunit and a small regulatory subunit. The large subunit comprises four domains (dI-dIV) while the small subunit has two domains (dV-dVI). Domain dI is a short region cleaved by autolysis, dII is the catalytic core, dIII is a C2-like domain, dIV consists of five calcium binding EF-hand motifs [].The crystal structure of calpain has been solved [, ]. The catalytic region consists of two distinct structural domains (dIIa and dIIb). dIIa contains a central helix flanked on three faces by a cluster of α-helices and is entirely unrelated to the corresponding domain in the typical thiol proteinases. The fold of dIIb is similar to the corresponding domain in other cysteine proteinases and contains two three-stranded anti-parallel β-sheets. The catalytic triad residues (C,H,N) are located in dIIa and dIIb. The activation of the domain is dependent on the binding of two calcium atoms in two non EF-hand calcium binding sites located in the catalytic core, one close to the Cys active site in dIIa and one at the end of dIIb. Calcium-binding induced conformational changes in the catalytic domain which align the active site [][].The profile covers the whole catalytic domain.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 residues that 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.
