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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.
After the isotropic-apical 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.
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.
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.
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.
|This section does not cite any references or sources. (December 2011)|
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.
- 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.
- 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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AAA family proteins often perform chaperone-like functions that assist in the assembly, operation, or disassembly of protein complexes .
The clan contains the following 198 members:6PF2K AAA AAA-ATPase_like AAA_10 AAA_11 AAA_12 AAA_13 AAA_14 AAA_15 AAA_16 AAA_17 AAA_18 AAA_19 AAA_2 AAA_21 AAA_22 AAA_23 AAA_24 AAA_25 AAA_26 AAA_27 AAA_28 AAA_29 AAA_3 AAA_30 AAA_31 AAA_32 AAA_33 AAA_34 AAA_35 AAA_4 AAA_5 AAA_6 AAA_7 AAA_8 AAA_9 AAA_PrkA ABC_ATPase ABC_tran ABC_tran_2 Adeno_IVa2 Adenylsucc_synt ADK AFG1_ATPase AIG1 APS_kinase Arch_ATPase Arf ArgK ArsA_ATPase ATP-synt_ab ATP_bind_1 ATP_bind_2 Bac_DnaA CbiA CMS1 CoaE CobA_CobO_BtuR CobU cobW CPT CTP_synth_N Cytidylate_kin Cytidylate_kin2 DAP3 DEAD DEAD_2 DLIC DNA_pack_C DNA_pack_N DNA_pol3_delta DNA_pol3_delta2 DnaB_C dNK DUF1253 DUF1611 DUF2075 DUF2478 DUF258 DUF2791 DUF2813 DUF3584 DUF463 DUF815 DUF853 DUF87 DUF927 Dynamin_N Exonuc_V_gamma FeoB_N Fer4_NifH Flavi_DEAD FTHFS FtsK_SpoIIIE G-alpha Gal-3-0_sulfotr GBP GTP_EFTU GTP_EFTU_D2 GTP_EFTU_D4 Gtr1_RagA Guanylate_kin GvpD HDA2-3 Helicase_C Helicase_C_2 Helicase_C_4 Helicase_RecD Herpes_Helicase Herpes_ori_bp Herpes_TK IIGP IPPT IPT IstB_IS21 KaiC KAP_NTPase Kinesin Kinesin-relat_1 Kinesin-related KTI12 LpxK MCM MEDS Mg_chelatase Mg_chelatase_2 MipZ Miro MMR_HSR1 MobB MukB MutS_V Myosin_head NACHT NB-ARC NOG1 NTPase_1 ParA Parvo_NS1 PAXNEB PduV-EutP PhoH PIF1 Podovirus_Gp16 Polyoma_lg_T_C Pox_A32 PPK2 PPV_E1_C PRK Rad17 Rad51 Ras RecA ResIII RHD3 RHSP RNA12 RNA_helicase RuvB_N SbcCD_C SecA_DEAD Septin Sigma54_activ_2 Sigma54_activat SKI SMC_N SNF2_N Spore_IV_A SRP54 SRPRB Sulfotransfer_1 Sulfotransfer_2 Sulfotransfer_3 Sulphotransf T2SE T4SS-DNA_transf Terminase_1 Terminase_3 Terminase_6 Terminase_GpA Thymidylate_kin TIP49 TK TniB Torsin TraG-D_C tRNA_lig_kinase TrwB_AAD_bind UPF0079 UvrD-helicase UvrD_C UvrD_C_2 Viral_helicase1 VirC1 VirE YhjQ Zeta_toxin Zot
We store a range of different sequence alignments for families. As well as the seed alignment from which the family is built, we provide the full alignment, generated by searching the sequence database using the family HMM. We also generate alignments using four representative proteomes (RP) sets, the NCBI sequence database, and our metagenomics sequence database. More...
There are various ways to view or download the sequence alignments that we store. We provide several sequence viewers and a plain-text Stockholm-format file for download.
We make a range of alignments for each Pfam-A family:
- the curated alignment from which the HMM for the family is built
- the alignment generated by searching the sequence database using the HMM
- Representative Proteomes (RPs) at 15%, 35%, 55% and 75% co-membership thresholds
- alignment generated by searching the NCBI sequence database using the family HMM
- alignment generated by searching the metagenomics sequence database using the family HMM
You can see the alignments as HTML or in three different sequence viewers:
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You can download (or view in your browser) a text representation of a Pfam alignment in various formats:
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We make a range of alignments for each Pfam-A family. You can see a description of each above. You can view these alignments in various ways but please note that some types of alignment are never generated while others may not be available for all families, most commonly because the alignments are too large to handle.
1Cannot generate PP/Heatmap alignments for seeds; no PP data available
Key: available, not generated, — not available.
Format an alignment
We make all of our alignments available in Stockholm format. You can download them here as raw, plain text files or as gzip-compressed files.
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MyHits provides a collection of tools to handle multiple sequence alignments. For example, one can refine a seed alignment (sequence addition or removal, re-alignment or manual edition) and then search databases for remote homologs using HMMER3.
HMM logos is one way of visualising profile HMMs. Logos provide a quick overview of the properties of an HMM in a graphical form. You can see a more detailed description of HMM logos and find out how you can interpret them here. More...
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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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Curation and family details
This section shows the detailed information about the Pfam family. You can see the definitions of many of the terms in this section in the glossary and a fuller explanation of the scoring system that we use in the scores section of the help pages.
|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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This visualisation provides a simple graphical representation of the distribution of this family across species. You can find the original interactive tree in the More....
This chart is a modified "sunburst" visualisation of the species tree for this family. It shows each node in the tree as a separate arc, arranged radially with the superkingdoms at the centre and the species arrayed around the outermost ring.
How the sunburst is generated
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.
In order to reduce the complexity of the representation, we reduce the number of taxonomic levels that we show. We consider only the following eight major taxonomic levels:
Colouring and labels
Segments of the tree are coloured approximately according to their superkingdom. For example, archeal branches are coloured with shades of orange, eukaryotes in shades of purple, etc. The colour assignments are shown under the sunburst controls. Where space allows, the name of the taxonomic level will be written on the arc itself.
As you move your mouse across the sunburst, the current node will be highlighted. In the top section of the controls panel we show a summary of the lineage of the currently highlighed node. If you pause over an arc, a tooltip will be shown, giving the name of the taxonomic level in the title and a summary of the number of sequences and species below that node in the tree.
Anomalies in the taxonomy tree
There are some situations that the sunburst tree cannot easily handle and for which we have work-arounds in place.
Missing taxonomic levels
Some species in the taxonomic tree may not have one or more of the main eight levels that we display. For example, Bos taurus is not assigned an order in the NCBI taxonomic tree. In such cases we mark the omitted level with, for example, "No order", in both the tooltip and the lineage summary.
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.
So that these nodes are not simply omitted from the sunburst tree, we group them together in a separate branch (or segment of the sunburst tree). Since we cannot determine the lineage for these unmapped species, we show all levels between the superkingdom and the species as "uncategorised".
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.
Too many species/sequences
For large species trees, you may see blank regions in the outer layers of the sunburst. These occur when there are large numbers of arcs to be drawn in a small space. If an arc is less than approximately one pixel wide, it will not be drawn and the space will be left blank. You may still be able to get some information about the species in that region by moving your mouse across the area, but since each arc will be very small, it will be difficult to accurately locate a particular species.
The tree shows the occurrence of this domain across different species. More...
We show the species tree in one of two ways. For smaller trees we try to show an interactive representation, which allows you to select specific nodes in the tree and view them as an alignment or as a set of Pfam domain graphics.
Unfortunately we have found that there are problems viewing the interactive tree when the it becomes larger than a certain limit. Furthermore, we have found that Internet Explorer can become unresponsive when viewing some trees, regardless of their size. We therefore show a text representation of the species tree when the size is above a certain limit or if you are using Internet Explorer to view the site.
If you are using IE you can still load the interactive tree by clicking the "Generate interactive tree" button, but please be aware of the potential problems that the interactive species tree can cause.
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.
We also count the number of unique sequences on which each domain is found, which is shown in green. Note that a domain may appear multiple times on the same sequence, leading to the difference between these two numbers.
Finally, we group sequences from the same organism according to the NCBI code that is assigned by UniProt, allowing us to count the number of distinct sequences on which the domain is found. This value is shown in the pink boxes.
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.
You can use the tree controls to manipulate how the interactive tree is displayed:
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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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