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Septin Edit Wikipedia article
|Cell division/GTP binding protein|
Septins are a group of highly-conserved GTP binding proteins found in eukaryotes. In yeast cells, they build scaffolding to provide structural support during cell division and compartmentalize parts of the cell. Recent research in human cells suggests that septins build cages around bacterial pathogens, immobilizing the harmful microbes and preventing them from invading other cells.
 Septins in Saccharomyces cerevisiae
The septins were discovered in 1970 by Leland H. Hartwell and colleagues in a screen for temperature-sensitive mutants affecting cell division (cdc mutants). The screen revealed four mutants which prevented cytokinesis at restrictive temperature. The corresponding genes represent the four original septins, ScCDC3, ScCDC10, ScCDC11, and ScCDC12. Despite disrupted cytokinesis, the cells continued budding, DNA synthesis, and nuclear division, which resulted in large multinucleate cells with multiple, elongated buds. In 1976, analysis of electron micrographs revealed ~20 evenly spaced striations of 10-nm filaments around the mother-bud neck in wild-type but not in septin-mutant cells. Immunofluorescence studies revealed that the septin proteins colocalize into a septin ring at the neck. The localization of all four septins is disrupted in conditional Sccdc3 and Sccdc12 mutants, indicating interdependence of the septin proteins. Strong support for this finding was provided by biochemical studies: The four original septins co-purified on affinity columns, together with a fifth septin protein, encoded by ScSEP7 or ScSHS1. Purified septins from budding yeast, Drosophila, Xenopus, and mammalian cells are able to self associate in vitro to form highly ordered, filamentous structures. How the septins interact in vitro to form heteropentamers that assemble into filaments was studied in detail in S. cerevisiae. Based on these and former studies, the septins are composed of a variable N-terminus with a basic phosphoinositide binding motif, a conserved core comprising a GTP-binding domain, a septin-unique element and a C-terminal extension including a predicted coiled coil.
Micrographs of purified filaments raised the possibility that the septins are organized in parallel to the mother-bud axis. The 10-nm striations seen on electron micrographs may be the result of lateral interaction between the filaments. Mutant strains lacking factors important for septin organization support this view. Instead of continuous rings, the septins form bars oriented along the mother-bud axis in deletion mutants of ScGIN4, ScNAP1 and ScCLA4.
The septins act as a scaffold, recruiting many proteins. These protein complexes are involved in cytokinesis, chitin deposition, cell polarity, spore formation, in the morphogenesis checkpoint, spindle alignment checkpoint and bud site selection.
Budding yeast cytokinesis is driven through two septin dependent, redundant processes: recruitment and contraction of the actomyosin ring and formation of the septum by vesicle fusion with the plasma membrane. In contrast to septin mutants, disruption of one single pathway only leads to a delay in cytokinesis, not complete failure of cell division. Hence, the septins are predicted to act at the most upstream level of cytokinesis.
 Cell polarity
After the apical-isotropic switch in budding yeast, cortical components, supposedly of the exocyst and polarisome, are delocalized from the apical pole to the entire plasma membrane of the bud, but not the mother cell. The septin ring at the neck serves as a cortical barrier that prevents membrane diffusion of these factors between the two compartments. This asymmetric distribution is abolished in septin mutants.
Some conditional septin mutants do not form buds at their normal axial location. Moreover, the typical localization of some bud-site-selection factors in a double ring at the neck is lost or disturbed in these mutants. This indicates that the septins may serve as anchoring site for such factors in axially budding cells.
It seems that one single septin organization should not be sufficient to fulfill such a variety of tasks. Accordingly, the septin cortex undergoes several changes throughout the cell cycle: The first visible septin structure is a distinct ring which appears ~15 min before bud emergence. After bud emergence, the ring broadens to assume the shape of an hourglass around the mother-bud neck. During cytokinesis, the septin cortex splits into a double ring which eventually disappears. How can the septin cortex undergo such dramatic changes, although some of its functions may require it to be a stable structure? FRAP analysis has revealed that the turnover of septins at the neck undergoes multiple changes during the cell cycle. The predominant, functional conformation is characterized by a low turnover rate (frozen state), during which the septins are phosphorylated. Structural changes require a destabilization of the septin cortex (fluid state) induced by dephosphorylation prior to bud emergence, ring splitting and cell separation.
The composition of the septin cortex does not only vary throughout the cell cycle but also along the mother-bud axis. This inherent polarity of septin filaments allows concentration of some proteins primarily to the mother side of the neck, some to the center and others to the bud site.
 Septins in filamentous fungi
Since their discovery in S. cerevisiae, septin homologues have been found throughout the eukaryotic kingdom, with the exception of plants. The variety of different shapes that septins can assume within a single cell is especially apparent in filamentous fungi, where they control aspects of filamentous morphology.
 Candida albicans
The genome of C. albicans encodes homologues to all S. cerevisiae septins (CaCDC3, CaCDC10, CaCDC11, CaCDC12, CaSEP7). They form a diffuse band at the base of emerging hyphae, a bright double ring at septation sites, an extended diffuse cap at hyphal tips and elongated filaments stretching around the spherical chlamydospores. As an effect of maturation, double rings reflect hyphal polarity by disassembling the tip proximal ring. CaCdc3p and CaCdc12p are essential for proliferation in yeast or hyphal forms. Cacdc10Δ and Cacdc11Δ deletion mutants are viable but show aberrant chitin localization and cannot properly maintain hyphal growth direction.
 Aspergillus nidulans
Five septins are found in A. nidulans (AnAspAp, AnAspBp, AnAspCp, AnAspDp, AnAspEp). AnAspBp forms single rings at septation sites that eventually split into double rings. Additionally, AnAspBp forms a ring at sites of branch emergence which broadens into a band as the branch grows. Like in C. albicans, double rings reflect polarity of the hypha, but by disassembling the more basal ring. Bases for the various patterns of septin organization could be different modifications and/or localization of different septin interaction partners. Conditional mutants of the essential AnAspBp display diffuse chitin deposition and a hyper-branching phenotype.
 Ashbya gossypii
The ascomycete A. gossypii possesses homologues to all S. cerevisiae septins, with one being duplicated (AgCDC3, AgCDC10, AgCDC11A, AgCDC11B, AgCDC12, AgSEP7). In vivo studies of AgSep7p-GFP have revealed that septins assemble into discontinuous hyphal rings close to growing tips and sites of branch formation and into asymmetric structures at the base of branching points. Rings are made of filaments which are long and diffuse close to growing tips and short and compact further away from the tip. During septum formation, the septin ring splits into two to form a double ring. Agcdc3Δ, Agcdc10Δ and Agcdc12Δ deletion mutants display aberrant morphology and are defective for actin-ring formation, chitin-ring formation, and sporulation. Due to the lack of septa, septin deletion mutants are highly sensitive, and damage of a single hypha can result into complete lysis of a young mycelium.
 Human septins
|This section does not cite any references or sources. (December 2011)|
However, most studies of septins, or guanosine-5′-triphosphate (GTP) binding proteins, have been confined to yeast cells. The latest research in human cells suggests that septins build 'cages' around bacterial pathogens, immobilizing the harmful microbes and preventing them from invading other healthy cells. This cellular defence system could be explored to create therapies for dysentery and other illnesses. “This is a new way for cells to control an infection,” Shigella, a bacterium that causes sometimes lethal diarrhoea in humans and other primates. To propagate from cell to cell, Shigella bacteria develop actin-polymer 'tails', which propel the microbes around and allow them to force their way into neighbouring host cells. To counterattack, human cells produce a cell-signalling protein called TNF-α. The researchers found that when TNF-α is present, thick bundles of septin filaments encircle the microbes. This, in turn, interferes with tail formation and stops Shigella in its tracks. Microbes that become trapped in septin cages are broken down in a stage of the cell's life cycle called autophagy. “Autophagy is more efficient because of the septin cage, and the septin cage does not occur if you do not have the autophagy. Many research groups are working on understand the link between septins and autophagy, and to determine how important septins are in humans in vivo. Disruptions in septins and mutations in the genes that code for them could be involved in causing leukaemia, colon cancer and neurodegenerative conditions such as Parkinson's disease and Alzheimer's disease. Potential therapies for these, as well as for bacterial conditions such as dysentery caused by Shigella, might bolster the body’s immune system with drugs that mimic the behaviour of TNF-α and allow the septin cages to proliferate.
 In mitochondria
- Mascarelli A (December 2011). "Septin proteins take bacterial prisoners: A cellular defence against microbial pathogens holds therapeutic potential". Nature. doi:10.1038/nature.2011.9540.
- Takahashi S, Inatome R, Yamamura H, Yanagi S (February 2003). "Isolation and expression of a novel mitochondrial septin that interacts with CRMP/CRAM in the developing neurones". Genes Cells 8 (2): 81–93. doi:10.1046/j.1365-2443.2003.00617.x. PMID 12581152.
 Further reading
- Longtine MS, DeMarini DJ, Valencik ML, Al-Awar OS, Fares H, De Virgilio C, Pringle JR (February 1996). "The septins: roles in cytokinesis and other processes". Curr. Opin. Cell Biol. 8 (1): 106–19. doi:10.1016/S0955-0674(96)80054-8. PMID 8791410.
- Gladfelter AS, Pringle JR, Lew DJ (December 2001). "The septin cortex at the yeast mother-bud neck". Curr. Opin. Microbiol. 4 (6): 681–9. doi:10.1016/S1369-5274(01)00269-7. PMID 11731320.
- Faty M, Fink M, Barral Y (June 2002). "Septins: a ring to part mother and daughter". Curr. Genet. 41 (3): 123–31. doi:10.1007/s00294-002-0304-0. PMID 12111093.
- Versele M, Gullbrand B, Shulewitz MJ, Cid VJ, Bahmanyar S, Chen RE, Barth P, Alber T, Thorner J (October 2004). "Protein-protein interactions governing septin heteropentamer assembly and septin filament organization in Saccharomyces cerevisiae". Mol. Biol. Cell 15 (10): 4568–83. doi:10.1091/mbc.E04-04-0330. PMC 519150. PMID 15282341.
- Douglas LM, Alvarez FJ, McCreary C, Konopka JB (September 2005). "Septin function in yeast model systems and pathogenic fungi". Eukaryotic Cell 4 (9): 1503–12. doi:10.1128/EC.4.9.1503-1512.2005. PMC 1214204. PMID 16151244.
- Gladfelter AS (March 2006). "Control of filamentous fungal cell shape by septins and formins". Nat. Rev. Microbiol. 4 (3): 223–9. doi:10.1038/nrmicro1345. PMID 16429163.
- "Hall PA, Russell SEH, Pringle JR, (2008). The septins. Oxford: John Wiley-Blackwell. p. 370. ISBN 0-470-51969-X.
- Gonzalez-Novo A, Vázquez de Aldana CR, Jimenez J (2009). "Fungal septins: one ring to rule it all?". Cent. Eur. J. Biol. 4 (3): 274–289. doi:10.2478/s11535-009-0032-2.
This tab holds the annotation information that is stored in the Pfam database. As we move to using Wikipedia as our main source of annotation, the contents of this tab will be gradually replaced by the Wikipedia tab.
Septin Provide feedback
Members of this family include CDC3, CDC10, CDC11 and CDC12/Septin. Members of this family bind GTP. As regards the septins, these are polypeptides of 30-65kDa with three characteristic GTPase motifs (G-1, G-3 and G-4) that are similar to those of the Ras family. The G-4 motif is strictly conserved with a unique septin consensus of AKAD. Most septins are thought to have at least one coiled-coil region, which in some cases is necessary for intermolecular interactions that allow septins to polymerise to form rod-shaped complexes. In turn, these are arranged into tandem arrays to form filaments. They are multifunctional proteins, with roles in cytokinesis, sporulation, germ cell development, exocytosis and apoptosis .
Casamayor A, Snyder M; , Mol Cell Biol 2003;23:2762-2777.: Molecular dissection of a yeast septin: distinct domains are required for septin interaction, localization, and function. PUBMED:12665577 EPMC:12665577
External database links
This tab holds annotation information from the InterPro database.
InterPro entry IPR000038
Septins constitute a eukaryotic family of guanine nucleotide-binding proteins, most of which polymerise to form filaments [PUBMED:14611653]. Members of the family were first identified by genetic screening for Saccharomyces cerevisiae (Baker's yeast) mutants defective in cytokinesis [PUBMED:4950437]. Temperature-sensitive mutations in four genes, CDC3, CDC10, CDC11 and CDC12, were found to cause cell-cycle arrest and defects in bud growth and cytokinesis. The protein products of these genes localise at the division plane between mother and daughter cells, indicating a role in mother-daughter separation during cytokinesis [PUBMED:3316985]. Members of the family were therefore termed septins to reflect their role in septation and cell division. The identification of septin homologues in higher eukaryotes, which localise to the cleavage furrow in dividing cells, supports an orthologous function in cytokinesis. Septins have since been identified in most eukaryotes, except plants [PUBMED:10805747].
Septins are approximately 40-50 kDa in molecular mass, and typically comprise a conserved central core domain (more than 35% sequence identity between mammalian and yeast homologues) flanked by more divergent N- and C-termini. Most septins possess a P-loop motif in their N-terminal domain (which is characteristic of GTP-binding proteins), and a predicted C-terminal coiled-coil domain [PUBMED:10481176].
A number of septin interaction partners have been identified in yeast, many of which are components of the budding site selection machinery, kinase cascades or of the ubiquitination pathway. It has been proposed that septins may act as a scaffold that provides an interaction matrix for other proteins [PUBMED:10805747, PUBMED:10481176]. In mammals, septins have been shown to regulate vesicle dynamics [PUBMED:11942624]. Mammalian septins have also been implicated in a variety of other cellular processes, including apoptosis, carcinogenesis and neurodegeneration [PUBMED:9203580].
This entry represents a variety of septins and homologous sequences involved in the cell division process.
The mapping between Pfam and Gene Ontology is provided by InterPro. If you use this data please cite InterPro.
|Molecular function||GTP binding (GO:0005525)|
|Biological process||cell cycle (GO:0007049)|
Below is a listing of the unique domain organisations or architectures in which this domain is found. More...
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1Cannot generate PP/Heatmap alignments for seeds; no PP data available
Key: available, not generated, — not available.
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This page displays the phylogenetic tree for this family's seed alignment. We use FastTree to calculate neighbour join trees with a local bootstrap based on 100 resamples (shown next to the tree nodes). FastTree calculates approximately-maximum-likelihood phylogenetic trees from our seed alignment.
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|Seed source:||Pfam-B_440 (release 2.1)|
|Number in seed:||14|
|Number in full:||2609|
|Average length of the domain:||234.50 aa|
|Average identity of full alignment:||37 %|
|Average coverage of the sequence by the domain:||62.73 %|
|HMM build commands:||
build method: hmmbuild -o /dev/null HMM SEED
search method: hmmsearch -Z 23193494 -E 1000 --cpu 4 HMM pfamseq
|Family (HMM) version:||13|
|Download:||download the raw HMM for this family|
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The tree is built by considering the taxonomic lineage of each sequence that has a match to this family. For each node in the resulting tree, we draw an arc in the sunburst. The radius of the arc, its distance from the root node at the centre of the sunburst, shows the taxonomic level ("superkingdom", "kingdom", etc). The length of the arc represents either the number of sequences represented at a given level, or the number of species that are found beneath the node in the tree. The weighting scheme can be changed using the sunburst controls.
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Unmapped species names
The tree is built by looking at each sequence in the full alignment for the family. We take the name of the species given by UniProt and try to map that to the full taxonomic tree from NCBI. In some cases, the name chosen by UniProt does not map to any node in the NCBI tree, perhaps because the chosen name is listed as a synonym or a misspelling in the NCBI taxonomy.
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Since we reduce the species tree to only the eight main taxonomic levels, sequences that are mapped to the sub-species level in the tree would not normally be shown. Rather than leave out these species, we map them instead to their parent species. So, for example, for sequences belonging to one of the Vibrio cholerae sub-species in the NCBI taxonomy, we show them instead as belonging to the species Vibrio cholerae.
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The tree shows the occurrence of this domain across different species. More...
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For all of the domain matches in a full alignment, we count the number that are found on all sequences in the alignment. This total is shown in the purple box.
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We use the NCBI species tree to group organisms according to their taxonomy and this forms the structure of the displayed tree. Note that in some cases the trees are too large (have too many nodes) to allow us to build an interactive tree, but in most cases you can still view the tree in a plain text, non-interactive representation. Those species which are represented in the seed alignment for this domain are highlighted.
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There is 1 interaction for this family. More...
We determine these interactions using iPfam, which considers the interactions between residues in three-dimensional protein structures and maps those interactions back to Pfam families. You can find more information about the iPfam algorithm in the journal article that accompanies the website.
For those sequences which have a structure in the Protein DataBank, we use the mapping between UniProt, PDB and Pfam coordinate systems from the PDBe group, to allow us to map Pfam domains onto UniProt sequences and three-dimensional protein structures. The table below shows the structures on which the Septin domain has been found. There are 15 instances of this domain found in the PDB. Note that there may be multiple copies of the domain in a single PDB structure, since many structures contain multiple copies of the same protein seqence.
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