Spc29 is a component of the Spc-110 subcomplex and is required for the SPB (Spindle pole body) duplication. Spc29 acts as a linker between the central plaque component Spc42 to the inner plaque component Spc110 [].
Spc42 is a central plaque component of spindle pole body (SPB), which is the microtubule organising centre (MTOC) in the yeast Saccharomyces. Spc42 is involved in SPB duplication and facilitates the attachment of the SPB to the nuclear membrane [].
This N-terminal domain of Bbp1, a spindle pole body component, interacts directly, though transiently, with the polo-box domain of Cdc5. Full length Bbp1 localises at the cytoplasmic side of the central plaque periphery of the spindle pole body (SPB) and plays an important role in inserting a duplication plaque into the nuclear envelope and assembling a functional inner plaque []. Although not a membrane protein itself, Bbp1 binds to Mps2 as well as to Spc29 and the half-bridge protein Kar1, thus providing a model for how the SPB core is tethered within the nuclear envelope and to the half-bridge [].
This is the central coiled-coil region of cut12 also found in other fungi, barring S. cerevisiae. The full protein has two predicted coiled-coil regions, and one consensus phosphorylation site for p34cdc2 and two for MAP kinase. During fission yeast mitosis, the duplicated spindle pole bodies (SPBs) nucleate microtubule arrays that interdigitate to form the mitotic spindle. Cut12 is localised to the SPB throughout the cell cycle, predominantly around the inner face of the interphase SPB, adjacent to the nucleus []. Cut12 associates with Fin1 and is important in this context for the activity of Plo1 [].
Centrosome and spindle pole associated protein 1 (CSPP) interacts with MyoGEF (a guanine nucleotide exchange factor that localises to the central spindle and cleavage furrow) and may play a role in cell-cycle-dependent microtubule organisation [, ].
This C-terminal domain of Bbp1, a spindle pole body component, carries coiled-coils that are necessary for the localisation of Bbp1 to the spindle pole body (SPB) []. Although not a membrane protein itself, Bbp1 binds to Mps2 as well as to Spc29 and the half-bridge protein Kar1, thus providing a model for how the SPB core is tethered within the nuclear envelope and to the half-bridge [].
This entry represents the C-terminal domain of Spc110 (spindle pole body component 110). Its N terminus is shown to bind to gamma-tubulin small complex (g-TuSC), while its C-terminal domain is essential for calmodulin-binding. The C terminus of Spc110 is anchored to the SPB via a conserved PACT domain [].
S.cerevisiae Kre28 is asubunit of a kinetochore-microtubule binding complex that bridges centromeric heterochromatin and kinetochore MAPs and motors []. It may be regulated by sumoylation [].
This domain can be found in the C-terminal of the yeast spindle pole body-associated protein Vik1 and its paralogue, Cik1. This domain is a motor homology domain that retains microtubule binding properties but lacks a nucleotide binding site [, ].
This family includes nuclear envelope protein YPR174C and NAP1-binding protein (NBP1). Both proteins bind to the nuclear membrane. NBP1 has been shown in Saccharomyces cerevisiae to function in spindle pole body duplication []and YPR174C may be involved in the connection of the spindle pole body to the nuclear envelope [].
Centrosomal protein of 95kDa is localised to the centrosome and the spindle pole []. Its function is not clear. It is phosphorylated upon DNA damage, probably by ATM or ATR [, ].
Vertnin is reported to be a maternal factor involved in dorsoventral patterning in zebrafish. It is rapidly transported to the animal pole region after fertilisation and binds to a bmp2b regulatory region to prevent its transcription [].
This is a domain of fungal spindle pole body proteins that play a role in spindle body duplication. They contain binding sites for calmodulin-like proteins called centrins []which are present in microtubule-organising centres.
Mo25-like proteins are involved in both polarised growth and cytokinesis. In fission yeast Mo25 is localised alternately to the spindle pole body and to the site of cell division in a cell cycle dependent manner [, ].
Monopolar spindle protein 2 (Mps2) is a fungal transmembrane protein localised to the spindle pole body (SPB) []. It is required for the insertion of the nascent SPB into the nuclear envelope and for the proper execution of spindle pole body (SPB) duplication [, ]. The interaction between Mps2 and Spc24 may contribute to the localisation of Spc24 and other kinetochore components to the inner plaque of the SPB [].
The plasmid conjugative coupling protein TraD (also known as TrwB) is a basic integral inner-membrane nucleoside-triphosphate-binding protein. It is the structural prototype for the type IV secretion system coupling proteins, a family of proteins essential for macromolecular transport between cells []. This protein forms hexamers from six structurally very similar protomers []. This hexamer contains a central channel running from the cytosolic pole (formed by the all-α domains) to the membrane pole ending at the transmembrane pore shaped by 12 transmembrane helices, rendering an overall mushroom-like structure. The TrwB all-α domain appears to be the DNA-binding domain of the structure.
This entry contains GpsB (also known as YpsB), which is a cell cycle protein and a component of the divisome. It associates with the complex late in its assembly, after the Z-ring is formed, and is dependent on DivIC and PBP2B for its recruitment to the divisome. Together with EzrA, it is a key component of the system that regulates PBP1 localization during cell cycle progression. Its main role could be the removal of PBP1 from the cell pole after pole maturation is completed. GpsB also contributes to the recruitment of PBP1 to the division complex and is not essential for septum formation [, ].
Spore formation in Bacillus subtilis involves a highly asymmetric cell division where the chromosome destined to enter the spore needs to move to the extreme pole of the cell in order to be captured in the prespore cell. This is accomplished by a specialised that moves the oriC region of the chromosome close to the cell pole before septation. RacA, a DNA-binding protein, is part of this system and is required for proper chromosome separation [, ]. It binds in a dispersed manner throughout the chromosome but preferentially to sites clustered in the origin portion of the chromosome, causing condensation of the chromosome and its remodelling into an elongated, anchored structure.
This entry includes fission yeast Sid4 and its homologue, Ady3 and Cnm67, from budding yeasts. Side4 is required for activation of the spg1 GTPase signalling cascade which leads to the initiation of septation and the subsequent termination of mitosis. It may act as a scaffold at the spindle pole body to which other components of the spg1 signalling cascade attach in pombe [, , ].Ady3 and Cnm67 are paraglogues. Ady3 is required for spore wall formation []. Cnm67 serves as a spacer protein of the spindle pole body outer plaque [].
SPICE1 localises to spindle microtubules in mitosis and to centrioles throughout the cell cycle. Deletion of SPICE1 compromises the architecture of spindles, the integrity of the spindle pole and the process of aligning chromosomes on the spindle (chromosome congression) []. SPICE1 interacts with CEP120 in centriole duplication and elongation [].
Bud site selection protein 13, also known as pre-mRNA-splicing factor CWC26, belongs to the pre-mRNA retention and splicing (RES) complex. May also be involved in positioning the proximal bud pole signal [, , ]. The presence of RES subunit homologues in numerous eukaryotes suggests that its function is evolutionarily conserved [].
Mps1 is a serine/threonine-protein kinase involved in the phosphorylation of many mitotic regulators. It is required for spindle pole body (SPB) duplication and spindle checkpoint function [, ]. It is involved in sister chromatid biorientation in mitosis and meiosis []. The expression of Mps1 is regulated in a cell cycle-dependent manner, and the activity of Mps1 can be regulated through autophosphorylation [].
This domain is found in the N-terminal region of CLIP-associated proteins (CLASPs), which are widely conserved microtubule plus-end-tracking proteins that regulate the stability of dynamic microtubules [, ]. The domain is also found in other proteins involved in microtubule binding, including STU1, MOR1 and spindle pole body component Alp14.
Cdc14 is a component of the septation initiation network (SIN) and is required for the localisation and activity of Sid1. Sid1 is a protein kinase that localises asymmetrically to one spindle pole body (SPB) in anaphase disappears prior to cell separation [], [].
Ndc1 is a nucleoporin protein that is a component of the Nuclear Pore Complex, and, in fungi, also of the Spindle Pole Body. It consists of six transmembrane segments, three luminal loops, both concentrated at the N terminus and cytoplasmic domains largely at the C terminus, all of which are well conserved.
This is a domain of unknown function with homology to the CH domain. This domain can be found in CEP95CEP95 is localised to the centrosome and the spindle pole []. Its function is not clear. It is phosphorylated upon DNA damage, probably by ATM or ATR [, ].
Protein Topless is a plant transcriptional co-repressor. It may repress the expression of root-promoting genes in the top half of the embryo to allow proper differentiation of the shoot pole during the transition stage of embryogenesis [, , , ]. This entry represents Topless and related proteins belonging to the same family.
Bqt4 is a Schizosaccharomyces pombe protein that anchors telomeres to the inner nuclear membrane during both vegetative growth and meiosis. This is required for telomere clustering to the spindle pole body to form the bouquet arrangement of chromosomes during the meiotic prophase [].
This entry represents the OSK domain defined by Jeske and colleagues []. The domain is related to SGNH hydrolases but lacks the active site residues. The domain binds to RNA []. Proteins containing this domain include maternal effect protein oskar, which is required to keep nos RNA and staufen protein at the posterior pole in Drosophila melanogaster [].
This family consists of kinesin-associated protein 3 (KAP3, also known as SMAP). In human and mouse, KAP3 is involved in tethering the chromosomes to the spindle pole and in chromosome movement. It binds to the tail domain of the KIF3A/KIF3B heterodimer to form a heterotrimeric KIF3 complex and may regulate the membrane binding of this complex [, ].
Saccharomyces cerevisiae Kin4 is a central component of the spindle position checkpoint (SPOC), which monitors spindle position and regulates the mitotic exit network (MEN) [, ]. Kin4 associates with spindle pole bodies in mother cells to inhibit MEN signaling and delay mitosis until the anaphase nucleus is properly positioned along the mother-bud axis. Kin4 activity is regulated by both the bud neck-associated kinase Elm1 and protein phosphatase 2A [, ].The fission yeast homologue is known as ppk1 [].
This pleckstrin homology domain is found in eukaryotic proteins, including Mcp5, a fungal protein that anchors dynein at the cell cortex during the horsetail phase (prophase I) of meiosis. During prophase I of fission yeast all the telomeres become bundled at the spindle pole body and subsequently the nucleus undergoes a dynamic oscillation, resulting in elongated nuclear morphology known as "horsetail"nucleus. The pleckstrin homology domain is necessary for the cortical localisation of the Mcp5 protein during meiosis [].
PDCD2 is localized predominantly in the cytosol of cells situated at the opposite pole of the germinal centre from the centroblasts as well as in cells in the mantle zone. It has been shown to interact with BCL6, an evolutionarily conserved Kruppel-type zinc finger protein that functions as a strong transcriptional repressor and is required for germinal centre development. The rat homologue, Rp8, is associated with programmed cell death in thymocytes.
The MOB kinase activator family includes MOB1, an essential Saccharomyces cerevisiae protein, identified from a two-hybrid screen, that binds Mps1p, a protein kinase essential for spindle pole body duplication and mitotic checkpoint regulation. Conditional alleles of MOB1 cause a late nuclear division arrest at restrictive temperature []. This family also includes the MOB-like protein phocein, an intracellular protein that interacts with striatin and may play a role in membrane trafficking [].
UbcD4, also called ubiquitin carrier protein or ubiquitin-protein ligase, is a class II E2 ubiquitin-conjugating enzyme encoded by Drosophila E2 gene which is only expressed in pole cells in embryos. It is a putative E2 enzyme homologous to the Huntingtin interacting protein-2 (HIP2) of human. UbcD4 specifically interacts with the polyubiquitin-binding subunit of the proteasome. It contains a C-terminal ubiquitin-associated (UBA) domain in addition to an N-terminal catalytic ubiquitin-conjugating enzyme E2 (UBCc) domain [].
Exuperantia (Exu) is associated with localization of bicoid (bcd) mRNA and required for its localization at the anterior pole of the oocyte. Crystal structure of Exu reveals a dimeric assembly with each monomer consisting of a 3'-5' EXO-like domain and a sterile alpha motif (SAM)-like domain. The SAM-like domain interacts with its target RNA as a homodimer and is required for RNA binding activity [].
CfaE, the tip adhesin of enterotoxigenic Escherichia coli colonisation factor antigen I fimbriae (CFA/I), initiates binding of this enteropathogen to the small intestine. It comprises a stacked N-terminal β-sandwich adhesin domain (AD) and a C-terminal pilin domain (PD), with the putative receptor-binding pocket at one pole and an equatorial interdomain interface [, ].This superfamily represents the adhesin domain of CfaE, homologous to the pilus biogenesis initiator protein CbID and the major pilin subunits of CS pili.
This entry represents the GET complex subunit Get2 and its homologue, sif1 from Schizosaccharomyces pombe. The Golgi to ER traffic (GET) complex is composed of Get1, Get2 and Get3. The complex is involved in the post-translational delivery of tail-anchored (TA) proteins to the endoplasmic reticulum []. Get1 and Get2 form a transmembrane complex and interact with Get3, an ATPase which recognises and selectively binds the transmembrane domain of TA proteins in the cytosol [, ]. In Schizosaccharomyces pombe, sif1 interacts with Kms1 and Sad1, which are constitutive membrane-bound components of the SPB (spindle pole body) that interact with each other [].
The TACC protein family localises to the centrosome (the spindle pole body in fungi) and mitotic spindle, and plays a crucial role in bipolar spindle assembly [, ]. Furthermore, Alp7/TACC recruits kinesin-8-PP1 to the kinetochore to regulate timely mitotic progression and chromosome movement during anaphase A [].There is only one TACC protein in the nematode Caenorhabditis elegans (TAC-1), in Drosophila melanogaster (D-TACC), in Xenopus laevis (Maskin), and in fission yeast (Alp7 also known as Mia1p); by contrast, mammals have three TACC proteins (TACC1, TACC2 and TACC3) [].
The MOB kinase activator superfamily includes MOB1, an essential Saccharomyces cerevisiae protein, identified from a two-hybrid screen, that binds Mps1p, a protein kinase essential for spindle pole body duplication and mitotic checkpoint regulation. Conditional alleles of MOB1 cause a late nuclear division arrest at restrictive temperature []. This superfamily also includes the MOB-like protein phocein, an intracellular protein that interacts with striatin and may play a role in membrane trafficking []. Structurally, domains in this superfamily have a bromodomain-like fold, which consists of four helices arranged as a bundle with a minor mirror variant of up-and-down topology; they also contain a zinc-binding site.
This is the C-terminal domain found in components of the gamma-tubulin complex proteins (GCPs). Family members include spindle pole body (SBP) components such as Spc97 and Spc98 which function as the microtubule-organizing centre in yeast []. Furthermore, family members such as human GCP4 (gamma-tubulin complex component 4) have been structurally elucidated (). Structure-based sequence analysis revealed the existence of an exposed surfacearea conserved in all human GCPs and in GCP4 orthologues. This area is located in the C-terminal domain of GCP4, which was confirmed in vitro to bind directly to gamma-tubulin. Sequence alignment of human GCPs based on the GCP4 structure helped delineate conserved regions in the N- and C-terminal domains [].
This entry represents a conserved region approximately 60 residues long within the eukaryotic targeting protein for Xklp2 (TPX2). Xklp2 is a kinesin-like protein localised on centrosomes throughout the cell cycle and on spindle pole microtubules during metaphase. In Xenopus, it has been shown that Xklp2 protein is required for centrosome separation and maintenance of spindle bi-polarity [, ]. TPX2 is a microtubule-associated protein that mediates the binding of the C-terminal domain of Xklp2 to microtubules. It is phosphorylated during mitosis in a microtubule-dependent way [].
The N-terminal region of the fission yeast spindle pole body protein PPC89 has low similarity to the human Cep57 protein. The CLD or centrosome localisation domain of Cep57 and PPC89 is found towards the N terminus of the proteins. This region localises to the centrosome internally to gamma-tubulin, suggesting that it is either on both centrioles or on a centromatrix component. This N-terminal region can also multimerise with the N terminus of other Cep57 molecules. A microtubule-binding domain is found at the C terminus.
Casparian strip membrane proteins (CASPs) are four-membrane-span proteins that mediate the deposition of Casparian strips in the endodermis by recruiting the lignin polymerization machinery. CASP-like proteins (CASPLs) contain a CASP domain and are also able to form transmembrane scaffolds. The transmembrane domains, particularly the first (TM1) and the third (TM3) are highly conserved among these proteins. CASPLs can be classified in five groups (1-5); this family represents group 5 (CASPL5). In Arabidopsis, CASPLs showed specific expression in a variety of cell types, such as trichomes, abscission zone cells, peripheral root cap cells, and xylem pole pericycle cells [].
Sad1/UNC-84 (SUN)-domain proteins are inner nuclear membrane (INM) proteins that are part of bridging complexes linking cytoskeletal elements with the nucleoskeleton. Originally identified based on an ~150-amino acid region of homology between the C terminus of the Schizosaccharomyces pombe Sad1 protein and the Caenorhabditis elegans UNC-84 protein, SUN proteins are present in the proteomes of most eucaryotes. In addition to the SUN domain, these proteins contain a transmembrane sequence and at least one coiled-coil domain and localise to the inner nuclear envelope. SUN proteins are anchored in the inner nuclear envelope by their transmembrane segment and oriented in the membrane such that the C-terminal SUN domain is located in the space between the inner and outer nuclear membrane. Here, the SUN domain can interact with the C- terminal tail of an outer nuclear envelope protein that binds to the cytoskeleton, including the centrosome [, , ].Some proteins known to contain a SUN domain are listed below:Fission yeast spindle pole body-associated protein Sad1.Yeast spindle pole body assembly component MPS3, essential for nuclear division and fusion.Yeast uncharacterised protein SLP1.Caenorhabditis nuclear migration and anchoring protein UNC-84.Caenorhabditis SUN domain-containing protein 1 (sun-1), involved in centrosome attachment to the nucleus.Mammalian sperm-associated antigen 4 protein (SPAG4), may assist the organisation and assembly of outer dense fibres (ODFs), a specific structure of the sperm tail.Mammalian sperm-associated antigen 4-like protein (SPAG4L).Mammalian SUN1.Mammalian SUN2.Mammalian SUN3.Klaroid protein from Drosophila melanogaster [].
This domain (Cnn_1N) is a short conserved motif found in the N-terminal of a group of centrosome or spindle pole body (SPB) associated proteins, including Mto1 and Pcp1 from S. pombe [, ], centrosomin from flies [, ]and myomegalin/CDK5RAP2 from mammals [, ]. In Drosophila, centrosomin (Cnn) is an essential mitotic centrosome component required for assembly of all other known pericentriolar matrix (PCM) proteins to achieve microtubule-organising activity at mitosis. The Cnn_1N domain (motif 1 of Cnn) is necessary for proper recruitment of gamma-tubulin, D-TACC (the homologue of vertebrate transforming acidic coiled-coil proteins [TACC]), and Minispindles (Msps) to embryonic centrosomes but is not required for assembly of other centrosome components including Aurora A kinase and CP60 in Drosophila [].
This domain contains sequences that are similar to the N-terminal region of Red protein (). This and related proteins contain a RED repeat which consists of a number of RE and RD sequence elements []. The region in question has several conserved NLS sequences and a putative trimeric coiled-coil region [], suggesting that these proteins are expressed in the nucleus []. Protein RED (also known as IK) is found in the nucleus and is a component of the spliceosome []. It is also associated with the spindle pole where it co-localizes with and interacts with the spindle assembly checkpoint protein MAD1 during metaphase and anaphase. Depletion of RED shortens the mitotic cycle and MAD1 is incorrectly localized [].
This entry represents a coiled-coil region close to the C terminus of centrosomal proteins that is directly responsible for recruiting AKAP-450 and pericentrin to the centrosome. Hence the suggested name for this region is a PACT domain (pericentrin-AKAP-450 centrosomal targeting). This domain is also present at the C terminus of coiled-coil proteins from Drosophila and Schizosaccharomyces pombe (Fission yeast), and in the Drosophila protein it is sufficient for targeting to the centrosome in mammalian cells. The function of these proteins is unknown but they seem good candidates for having a centrosomal or spindle pole body location. The final 22 residues of this domain in AKAP-450 appear specifically to be a calmodulin-binding domain, indicating that this protein at least is likely to contribute to centrosome assembly [].
The BcsQ (also known as YhjQ) protein is encoded immediately upstream of bacterial cellulose synthase (bcs) genes in a broad range of bacteria, including both copies of the bcs locus in Klebsiella pneumoniae, and in several species is clearly part of the bcs operon. It is identified as a probable component of the bacterial cellulose metabolic process not only by gene location, but also by partial phylogenetic profiling, or Haft-Selengut algorithm [], based on a bacterial cellulose biosynthesis genome property profile.Cellulose plays an important role in biofilm formation and structural integrity in some bacteria. E. coli csQ may play a role in subcellular localization of an active cellulose biosynthesis apparatus at the bacterial cell pole [].
Proteins in this entry are guanine nucleotide exchange factors (GEF), including RAB3IL1/RAB3IP from animals, Sec2 from budding yeasts and Spo13 from fission yeasts. The mammalian guanine nucleotide exchange factor for Rab-3A (RAB3IL1) may activate RAB3A, a GTPase that regulates synaptic vesicle exocytosis []. The Rab-3A-interacting protein (RAB3IP) may activate RAB8A and RAB8B, which reorganizes actin and microtubules to form new cell surface domains [].In the yeast Saccharomyces cerevisiae, Rab guanine nucleotide exchange factor Sec2 activates Sec4 by catalyzing the dissociation of GDP from Sec4. Activation of Sec4 by Sec2 is needed for the directed transport of vesicles to sites of exocytosis []. In the yeast Schizosaccharomyces pombe, sporulation-specific protein 13 (Spo13) is required for sporulation. During sporulation the spindle pole body (SPB) forms outer plaques, an essential precursor to the assembly of the forespore membrane (FSM). SPB associates with Spo13 and Spo2, and without these genes the SPB is not modified and the FSM does not form [].
Protein RED (also known as IK) is found in the nucleus and is a component of the spliceosome []. It is also associated with the spindle pole where it co-localizes with and interacts with the spindle assembly checkpoint protein MAD1 during metaphase and anaphase. Depletion of RED shortens the mitotic cycle and MAD1 is incorrectly localized [].Proteins in this entry also include SPBC1539.02 from the fission yeast and smu-2 from C. elegans. SPBC1539.02 is required for chromosome segregation during meiosis []. SMU-2 and SMU-1 bind to each other as components of the spliceosome and modulate splice site selection of many pre-mRNAs [].Protein RED and related proteins contain an arginine/glutamic acid/aspartic acid repetitive sequence. The region in question has several conserved NLS sequences and a putative trimeric coiled-coil region, indicative that these proteins are expressed in the nucleus [].
Fin1 forms filaments between the spindle pole bodies of dividing cells []and localizes to spindle microtubules during anaphase. It contributes to spindle stability. The C-terminal half of Fin1 contains coiled-coil motifs that are required for its self-association. The N-terminal half of Fin1 can be phosphorylated at multiple sites by the cyclin-dependent kinase Clb5-Cdk1, and dephosphorylation in anaphase triggers its localization to the spindle []. Fin1 form a complex with phosphatase PP1, this complex is then recruited onto kinetochores promoting spindle assembly checkpoint (SAC) dis-assembly during anaphase [, ]. Fin1 has two RXXS/T sequences: S377 (RVTS), S526 (RKVS) that can be phosphorylated. Upon phosphorylation, interactions with other proteins such as Bmh1 and Bmh2 is promoted. However, de-phosphorylation during anaphase promotes the kinetochore recruitment of Fin1-PP1 [].
In Drosophila melanogaster, Nanos functions as a localised determinant of posterior pattern. Nanos RNA is localised to the posterior pole of the maturing egg cell and encodes a protein that emanates from this localised source. Nanos acts as a translational repressor and thereby establishes a gradient of the morphogen Hunchback []. Nanos comprises a non-conserved amino-terminus and highly conserved carboxy-terminal regions. The C-terminal region has two conserved Cys-Cys-His-Cys(CCHC)-type zinc-finger motifs that are indispensable for nanos function[]. Xcat-2 from Xenopus encodes a protein with a nanos-like zinc finger domain. It is found in the vegetal cortical region and is inherited by the vegetal blasomeres during development, and is degraded very early in development. The localised and maternally restricted expression of Xcat-2 RNA suggests a role for its protein in setting up regional differences in gene expression that occur early in development [].This entry consists of the Nanos protein and homologues, including Xcat-2.
This entry represents a domain found in actinobacterial proteins, including EspA and EspE from Mycobacteria.The ESX-1 operon is present in a number of Mycobacteria strains []. In Mycobacterium tuberculosis (Mtb) the type VII ESX-1 secretion apparatus is used to translocate the key virulence factors EsxA (ESAT-6) and EsxB (CFP-10). In Mtb, the EspA protein is encoded by the unlinked espACD operon []. The EspA protein of Mycobacterium tuberculosis is essential for the type VII ESX-1 protein secretion apparatus []. EspA, ESAT-6, and CFP-10 are each critical for virulence of pathogenic mycobacteria and secretion of these three proteins, is mutually dependent [, ]. EspA undergoes Cys138-mediated homodimerization, although the process is not required for EspA or ESAT-6 secretion []. EspE is encoded by the ESX-1 operon []. In M. smegmatis it has been shown to colocalise with SaeC at, or near, the cell pole []. In M. smegmatis secretion of the heterodimer of EsxAB (ESAT-6/CFP-10) is dependent on the co-secretion of proteins encoded from both esx1 (eg., EspE) and non-esx1 genes (eg., EspA) [].
This is the N-terminal domain found in components of the gamma-tubulin complex proteins (GCPs). Proteins containing this domain include spindle pole body (SBP) components such as Spc97 and Spc98 which function as the microtubule-organizing centre in yeast []. Proteins containing this domain also include human GCP4 (Gamma-tubulin complex component 4), which has been structurally elucidated []. Functional studies have shown that the N-terminal domain defines the functional identity of GCPs, suggesting that all GCPs are incorporated into the helix of gamma-tubulin small complexes (gTURCs) via lateral interactions between their N-terminal domains. Thereby, they define the direct neighbours and position the GCPs within the helical wall of gTuRC []. Sequence alignment of human GCPs based on the GCP4 structure helped delineate conserved regions in the N- and C-terminal domains []. In addition to the conserved sequences, the N-terminal domains carry specific insertions of various sizes depending on the GCP, i.e. internal insertions or N-terminal extensions. These insertions may equally contribute to the function of individual GCPs as they have been implied in specific interactions with regulatory or structural proteins. For instance, GCP6 carries a large internal insertion phosphorylated by Plk4 and containing a domain of interaction with keratins, whereas the N-terminal extension of GCP3 interacts with the recruitment protein MOZART1 [].
MreB proteins are essential for cell-shape maintenance and cell morphogenesis in most non-spherical bacteria [, ]. Most rod-shaped or non-spherical bacteria possess at least one mreB homologue. In Bacillus subtilis, sidewall elongation during vegetative growth is controlled by three MreB isoforms: MreB, Mbl and MreBH []. MreB proteins are found in rod-shaped bacteria, such as E. coli and B. subtilis, that grow by dispersed intercalation of new wall material along the long axis of the cell, as opposed to those that grow from the cell pole [].The crystal structure of MreB from Thermotoga maritima was resolved using X-ray crystallography, and the results suggested that MreB proteins form long filaments that wrap around the long axis of the cell close to the cell membrane, forming helix-like structures. These observations led to the idea that MreB proteins might have an actin-like cytoskeletal role in bacteria [, ]. However, this remains controversial [, ]. MreB and MreB-like proteins are thought to act as scaffolds, guiding the localization and activity of key peptidoglycan synthesizing proteins during cell elongation [, ]. MreB has also been implicated in chromosome segregation [].
The microtubule organizing centres (MTOCs) of eukaryotic cells are the sites of nucleation of microtubules, and are known as the centrosome in animal cells and the spindle pole body in yeast. Gamma-tubulin, which is 30% identical to alpha and beta tubulins that form microtubules, appears to be a key protein involved in nucleation of microtubules.Gamma tubulin can assemble into complexes of various sizes with members of the GCP family. In budding yeast, the gamma tubulin-containing small complex (gammaTuSC) contains gamma tubulin, GCP2 and GCP3 (also known as Spc97 and Spc98). In Drosophila and vertebrates, gamma tubulin forms much larger assemblies, termed gamma-tubulin ring complexes (gammaTuRCs), with gamma tubulin, GCP2, GCP3, GCP4, GCP5 and GCP6. The purified gammaTuSC and gammaTuRC complexes exhibit a 'lock washer' shape []. However, the purified gammaTuSC has been shown to have a much lower microtubule-nucleating activity than intact gammaTuRC []. Several models have been proposed to explain their assembly and nucleation mechanism []. This entry represents the GCP family, whose members include GCP2/3/4/5/6 and Spc97/98 []. They contain the GRIP1 and GRIP2 motifs, which are predicted to participate in protein-protein interactions []. They are gamma tubulin binding proteins that have similar protein structures [].
Nesprins (nuclear envelope spectrin-repeat proteins) are a family of giant spectrin-repeat containing proteins that act as versatile intracellular protein scaffolds []. They are characterised by a central extended spectrin-repeat (SR) and a C-terminal Klarsicht/ANC-1/Syne homology (KASH) domain that can associate with Sad1p/UNC-84 (SUN)-domain proteins of the inner nuclear membrane within the periplasmic space of the nuclear envelope (NE) [].This entry represents Nesprin-4 predominantly from mammals. Nesprin-4 links the nucleus to microtubules through its binding to kinesin-1 []. It is a component of the linker of the nucleoskeleton and cytoskeleton (LINC) complex, which plays critical roles in nuclear positioning, cell polarisation and cellular stiffness []. This entry also conbtains the karyogamy meiotic segregation protein 2 (kms2) from the fission yeast Schizosaccharomyces pombe. Kms2 contains a KASH domain and during interphase colocalizes within the nuclear envelope with the SUN domain-containing protein Sad1 at the site of attachment of the spindle pole body (SPB, the yeast version of the centrosome). Kms2 interacts with the SPB components Cut12 and Pcp1 and the Polo kinase Plo1 and is important for remodelling of the SPB and entry of the cell into mitosis [].
Toxoplasma gondii is an obligate intracellular apicomplexan protozoan parasite, with a complex lifestyle involving varied hosts []. It has two phases of growth: an intestinal phase in feline hosts, and an extra-intestinal phase in other mammals. Oocysts from infected cats develop into tachyzoites, and eventually, bradyzoites and zoitocysts in the extraintestinal host []. Transmission of the parasite occurs through contact with infected cats or raw/undercooked meat; in immunocompromised individuals, it can cause severe and often lethal toxoplasmosis. Acute infection in healthy humans can sometimes also cause tissue damage [].The protozoan utilises a variety of secretory and antigenic proteins to invade a host and gain access to the intracellular environment []. These originate from distinct organelles in the T. gondii cell, termed micronemes, rhoptries, and dense granules. They are released at specific times during invasion to ensure the proteins are allocated to their correct target destinations []. MIC1, a protein secreted from the microneme, is a 456-residue moiety involved in host cell recognition by the parasite []. The protein is released from the apical pole of T. gondii during infection, and attaches tohost-specific receptors []. Recent studies have demonstrated that Mic1 is a lactose-binding lectin, and utilises this to enhance its binding to host endothelial cells []. A homologue of Mic1 found in Neospora caninum interacts with sulphated host cell-surface glycosaminoglycans.
The drosophila Tudor protein, the founder of the Tudor domain family, is encoded by a 'posterior group' gene, which when mutated disrupt normal abdominal segmentation and pole cell formation. Another drosophila gene, homeless, is required for RNA localization during oogenesis. The tudor protein contains multiple repeats of a domain which is also found in homeless [, ].The tudor domain is found in many proteins that colocalise with ribonucleoprotein or single-strand DNA-associated complexes in the nucleus, in the mitochondrial membrane, or at kinetochores. At first it was not clear if the domain binds directly to RNA and ssDNA, or controls interactions with the nucleoprotein complexes but it is now known that this domain recognises and binds to methyl-arginine-lysine residues, playing important roles in diverse epigenetics, gene expression and the regulation of various small RNAs [, , ]. The tudor-containing protein homeless, also contains a zinc finger typical of RNA-binding proteins [].This domain has been implicated in protein-protein interactions in which methylated protein substrates bind to these domains. One example is the Tudor domain of Survival of Motor Neuron (SMN), linked to spinal muscular atrophy, which binds to symmetrically dimethylated arginines of arginine-glycine (RG) rich sequences found in the C-terminal tails of Sm proteins. The resolution of the solution structure of the Tudor domain of human SMN revealed that the Tudor domain forms a strongly bent antiparallel β-sheet with five strands forming a barrel-like fold. The structure exhibits a conserved negatively charged surface that interacts with the C-terminal Arg and Gly-rich tails of the spliceosomal Sm D1 and D3 proteins [, ].
Toxoplasma gondii is an obligate intracellular apicomplexan protozoan parasite, with a complex lifestyle involving varied hosts []. It has two phases of growth: an intestinal phase in feline hosts, and an extra-intestinal phase in other mammals. Oocysts from infected cats develop into tachyzoites, and eventually, bradyzoites and zoitocysts in the extraintestinal host []. Transmission of the parasite occurs through contact with infected cats or raw/undercooked meat; in immunocompromised individuals, it can cause severe and often lethal toxoplasmosis. Acute infection in healthy humans can sometimes also cause tissue damage [].The protozoan utilises a variety of secretory and antigenic proteins to invade a host and gain access to the intracellular environment []. These originate from distinct organelles in the T. gondii cell termed micronemes, rhoptries, and dense granules. They are released at specific times during invasion to ensure the proteins are allocated to their correct target destinations []. Dense granule antigens (GRAs) are released from the T. gondii tachyzoitewhile still encapsulated in a host vacuole.Gra2, one of these moieties, israpidly targeted to a network of membranous tubules that connect with thevacuolar membrane []. A recent study into the exact mechanism of Gra2secretion has revealed that the secondary structure of the protein plays amajor role in its targeting and release from the apical pole of the parasitic cell []. Two amphipathic α-helical regions ensure not onlythat association of Gra2 with the host vacuole membrane takes place, but also that it is correctly targeted to the cell posterior, where stabilisingnetworks of tubules form and hold the cell steady in the vacuole.
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 [].
