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35  structures 909  species 1  interaction 7962  sequences 85  architectures

Family: Septin (PF00735)

Summary: Septin

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Septin Edit Wikipedia article

Cell division/GTP binding protein
Symbol Cell_Div_GTP_bd
Pfam PF00735
Pfam clan CL0023
InterPro IPR000038

Septins are a group of GTP-binding proteins found primarily in eukaryotic cells of fungi and animals, but also in some green algae.[1][2] Different septins form protein complexes with each other. These complexes can further assemble into filaments, rings and gauzes. Assembled as such, septins function in cells by localizing other proteins, either by providing a scaffold to which proteins can attach, or by preventing diffusion of molecules from one compartment of the cell to another.[1][2][3][4]

Septins have been implicated in the localization of cellular processes at the site of cell division, at the plasma membrane, at sites where specialized structures like cilia or flagella are attached to the cell body.[3] In yeast cells, they compartmentalize parts of the cell and build scaffolding to provide structural support during cell division at the septum, from which they derive their name.[2] Recent research in human cells suggests that septins build cages around bacterial pathogens, immobilizing the harmful microbes and preventing them from invading other cells.[5]

As filament forming proteins, septins can be considered part of the cytoskeleton.[3] Apart from forming non-polar filaments, septins associate with cell membranes, actin filaments and microtubules.[3] Although present in most eukaryotes, septins have not been observed in green plants,[4] but they have been reported in red algae.[6]


schematic domain structure of septin polypeptide chain
a) schematic of septin molecule with GTP binding domain to one side and the N and C termini of the polypeptide chain to the other
b) schematic of septin heterohexameric complex (of human septins), where different septins bind to each other via their GTP binding domains or via the N and C termini. Note the symmetry of the complex
c) schematic how septin complexes could align to form septin filaments

Septins are P-Loop-NTPase proteins that range in weight from 30-65 kDa. Septins are highly conserved between different eukaryotic species. They are composed of a variable-length proline rich N-terminus with a basic phosphoinositide binding motif important for membrane association, a GTP-binding domain, a highly conserved Septin Unique Element domain, and a C-terminal extension including a coiled coil domain of varying length.[3]

Septins interact either via their respective GTP-binding domains, or via both their N- and C-termini. Different organisms express a different number of septins, and from those symmetric oligomeres are formed. For example, in humans Sept7-Sept6-Sept2-Sept2-Sept6-Sept7 form one complex, and in yeast Cdc11-Cdc12-Cdc3-Cdc10-Cdc10-Cdc3-Cdc12-Cdc11 form another one. These complexes then associate to form non-polar filaments, filament bundles, cages or ring structures in cells.[3]


Septins are found in fungi, animals, some green algae, and some red algae, but not in higher plants.

In Saccharomyces cerevisiae

Septins in Saccharomyces cerevisiae (fluorescent micrograph)
• Green: septins (AgSEP7-GFP)
• Red: cell outline (phase contrast)
• Scale bar: 10 μm

There are seven different septins in Saccharomyces cerevisiae. Five of those are involved in mitosis, while two (Spr3 and Spr28) are specific to sporulation.[1][2] Mitotic septins (Cdc3, Cdc10, Cdc11, Cdc12, Shs1) form a ring structure at the bud neck during cell division.[1][3] They are involved in the selection of the bud-site, the positioning of the mitotic spindle, polarized growth, and cytokinesis. The sporulating septins (Spr3, Spr28) localize together with Cdc3 and Cdc11 to the edges of prospore membranes.[1]


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.[2]

The composition of the septin cortex does not only vary throughout the cell cycle but also along the mother-bud axis. This polarity of the septin network allows concentration of some proteins primarily to the mother side of the neck, some to the center and others to the bud site.



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 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.

In Filamentous fungi

Since their discovery in S. cerevisiae, septin homologues have been found in other eukaryotic species, including filamentous fungi. Septins in filamentous fungi display a variety of different shapes within single cells, where they control aspects of filamentous morphology.[7]

Candida albicans

The genome of C. albicans encodes homologues to all S. cerevisiae septins. Without Cdc3 and Cdc12 genes Candida albicans cannot proliferate, other septins affect morphology and chitin deposition, but are not essential. Candida albicans can display different morphologies of vegetative growth, which determines the appearance of septin structures. Newly forming hyphae form a septin ring at the base, Double rings form at sites of hyphal septation, and a septin cap forms at hyphal tips. Elongated septin-filaments encircle the spherical chlamydospores. Double rings of septins at the septation site also bear growth polarity, with the growing tip ring disassembling, while the basal ring remaining intact.[7]

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. In the case of Aspergillus nidulans polarity is conveyed by disassembly of the more basal ring (the ring further away from the hyphal growth tip), leaving the apical ring intact, potentially as a growth guidance cue.[1][7]

Ashbya gossypii

Septins in Ashbya gossypii (fluorescent micrograph) • Green: septins (AgSEP7-GFP)
• Red: cell outline (phase contrast)
• Inlay: 3D reconstruction of a discontinuous septin ring
• Scale bars: 10 μm

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,[1] 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 in complete lysis of a young mycelium.

In metazoa

In contrast to septins in yeast, and in contrast to other cytoskeletal components of metazoa, septins do not form a continuous network in metazoan cells, but several dispersed ones in the cortical cytoplasm. These are integrated with actin bundles and microtubules. For example, the actin bundling protein anillin is required for correct spatial control of septin organization.[4] In the sperm cells of mammals, septins form a stable ring called annulus in the tail. In mice (and potentially in humans, too), defective annulus formation leads to male infertility.[3][4]


In humans, septins are involved in cytokinesis, cilium formation and neurogenesis through the capability to recruit other proteins or serve as a diffusion barrier. There are 13 different human genes coding for septins. The septin proteins produced by these genes are grouped into four subfamilies each named after its founding member: (i) SEPT2 (SEPT1, SEPT4, SEPT5), (ii) SEPT3 (SEPT9, SEPT12), (iii) SEPT6 (SEPT8, SEPT10, SEPT11, SEPT14), and (iv) SEPT7. Septin protein complexes are assembled to form either hetero-hexamers (incorporating monomers selected from three different groups and the monomer from each group is present in two copies; 3 x 2 = 6) or hetero-octamers (monomers from four different groups, each monomer present in two copies; 4 x 2 = 8). These hetero-oligomers in turn form higher-order structures such as filaments and rings.[3][4]

Septins form cage-like structures around bacterial pathogens, immobilizing harmful microbes and preventing them from invading healthy cells. This cellular defence system could potentially be exploited to create therapies for dysentery and other illnesses. For example, Shigella is a bacterium that causes lethal diarrhoea in humans. To propagate from cell to cell, Shigella bacteria develop actin-polymer 'tails', which propel the microbes and allow them to gain entry into neighbouring host cells. As part of the immune response, human cells produce a cell-signalling protein called TNF-α which trigger thick bundles of septin filaments to encircle the microbes within the infected host cell.[8] Microbes that become trapped in these septin cages are broken down by autophagy.[9] 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.[5]

Caenorhabditis elegans

In the nematode worm Caenorhabditis elegans there are two genes coding for septins, and septin complexes contain the two different septins in a tetrameric UNC59-UNC61-UNC61-UNC59 complex. Septins in C.elegans concentrate at the cleavage furrow and the spindle midbody during cell division. Septins are also involved in cell migration and axon guidance in C.elegans.[1]

In mitochondria

The septin localized in the mitochondria is called mitochondrial septin (M-septin). It was identified as a CRMP/CRAM-interacting protein in developing mouse brain.[10]


The septins were discovered in 1970 by Leland H. Hartwell and colleagues in a screen for temperature-sensitive mutants affecting cell division (cdc mutants) in yeast (Saccharomyces cerevisiae). The screen revealed four mutants which prevented cytokinesis at restrictive temperature. The corresponding genes represent the four original septins, ScCDC3, ScCDC10, ScCDC11, and ScCDC12.[2][3] 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.[2][3][7] Immunofluorescence studies revealed that the septin proteins colocalize into a septin ring at the neck.[3][7] 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 filaments.[7] How the septins interact in vitro to form heteropentamers that assemble into filaments was studied in detail in S. cerevisiae.

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.


  1. ^ a b c d e f g h i Weirich CS, Erzberger JP, Barral Y (2008). "The septin family of GTPases: architecture and dynamics". Nat. Rev. Mol. Cell Biol. 9 (6): 478–89. doi:10.1038/nrm2407. PMID 18478031. 
  2. ^ a b c d e f g Douglas LM, Alvarez FJ, McCreary C, Konopka JB (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 1214204Freely accessible. PMID 16151244. 
  3. ^ a b c d e f g h i j k l m Mostowy S, Cossart P (2012). "Septins: the fourth component of the cytoskeleton". Nat. Rev. Mol. Cell Biol. 13 (3): 183–94. doi:10.1038/nrm3284. PMID 22314400. 
  4. ^ a b c d e Kinoshita M (2006). "Diversity of septin scaffolds". Curr. Opin. Cell Biol. 18 (1): 54–60. doi:10.1016/ PMID 16356703. 
  5. ^ a b 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. 
  6. ^ Brawley, Blouin; et al. (2017). "Insights into the red algae and eukaryotic evolution from the genome of Porphyra umbilicalis (Bangiophyceae, Rhodophyta)". PNAS: E6361–E6370. PMID 28716924. 
  7. ^ a b c d e f Gladfelter AS (2006). "Control of filamentous fungal cell shape by septins and formins". Nat. Rev. Microbiol. 4 (3): 223–9. doi:10.1038/nrmicro1345. PMID 16429163. 
  8. ^ Mostowy S, Bonazzi M, Hamon MA, Tham TN, Mallet A, Lelek M, Gouin E, Demangel C, Brosch R, Zimmer C, Sartori A, Kinoshita M, Lecuit M, Cossart P (2010). "Entrapment of intracytosolic bacteria by septin cage-like structures". Cell Host Microbe. 8 (5): 433–44. doi:10.1016/j.chom.2010.10.009. PMID 21075354. 
  9. ^ Mostowy S, Sancho-Shimizu V, Hamon MA, Simeone R, Brosch R, Johansen T, Cossart P (2011). "p62 and NDP52 proteins target intracytosolic Shigella and Listeria to different autophagy pathways". J. Biol. Chem. 286 (30): 26987–95. doi:10.1074/jbc.M111.223610. PMC 3143657Freely accessible. PMID 21646350. 
  10. ^ 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

This page is based on a Wikipedia article. The text is available under the Creative Commons Attribution/Share-Alike License.

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 [2].

Literature references

  1. 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

  2. Kinoshita M; , Genome Biol 2003;4:236.: The septins. PUBMED:14611653 EPMC:14611653

Internal database links

This tab holds annotation information from the InterPro database.

InterPro entry IPR030379

The P-loop guanosine triphosphatases (GTPases) control a multitude of biological processes, ranging from cell division, cell cycling, and signal transduction, to ribosome assembly and protein synthesis. GTPases exert their control by interchanging between an inactive GDP-bound state and an active GTP-bound state, thereby acting as molecular switches. The common denominator of GTPases is the highly conserved guanine nucleotide-binding (G) domain that is responsible for binding and hydrolysis of guanine nucleotides.

Septins are a family of eukaryotic cytoskeletal proteins conserved from yeasts to humans. The septin family belongs to the guanosine-triphosphate (GTP)ase superclass of P-loop nucleoside triphosphate (NTP)ases. Septins participate in diverse cellular functions including cytokinesis, vesicle trafficking, vesicle fusion, axonal guidance and migration, diffusion barrier, scaffolds, pathogenesis and others. Septin monomers form homo- and hetero-oligomeric complexes that assemble into filaments. Structurally all septins have a GTP-binding domain flanked by N- and C-terminal regions of variable length. The GTP-binding domain is the most highly conserved and is characterised by the presence of three of the five classical GTP-binding motifs. The G1 motif (or Walker A box, GxxxxGKS/T) forms the P-loop, which interacts directly with the nucleotide, whereas the G3 (DxxG) and G4 (xKxD) motifs are respectively essential for Mg(2+) binding and for conferring GTP binding specificity over other nucleotides. The basic structure of the septin-type G domain closely resembles the canonical G domain exemplified by Ras, with six beta-strands and five alpha-helices. A unique feature of the septin-type G domain is the presence of four additional elements compared to Ras. These are the helix alpha5' between alpha4 and beta6, the two antiparallel strands beta7 and beta8, and the alpha6 C-terminal helix that points away from the G domain at a 90deg angle relative to the axis of interaction between subunits [PUBMED:11916378, PUBMED:16009555, PUBMED:17637674, PUBMED:23163726, PUBMED:24367716].

This entry represents the septin-type G domain.

Gene Ontology

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Domain organisation

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Pfam Clan

This family is a member of clan P-loop_NTPase (CL0023), which has the following description:

AAA family proteins often perform chaperone-like functions that assist in the assembly, operation, or disassembly of protein complexes [2].

The clan contains the following 229 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_5 AAA_6 AAA_7 AAA_8 AAA_9 AAA_PrkA ABC_ATPase ABC_tran ABC_tran_Xtn Adeno_IVa2 Adenylsucc_synt ADK AFG1_ATPase AIG1 APS_kinase Arf ArsA_ATPase ATP-synt_ab ATP_bind_1 ATP_bind_2 ATPase ATPase_2 Bac_DnaA BCA_ABC_TP_C Beta-Casp Cas_Csn2 Cas_St_Csn2 CbiA CBP_BcsQ CDC73_C CENP-M CFTR_R CLP1_P CMS1 CoaE CobA_CobO_BtuR CobU cobW CPT CSM2 CTP_synth_N Cytidylate_kin Cytidylate_kin2 DAP3 DBINO DEAD DEAD_2 DLIC DNA_pack_C DNA_pack_N DNA_pol3_delta DNA_pol3_delta2 DnaB_C dNK DUF1611 DUF1726 DUF2075 DUF2326 DUF2478 DUF257 DUF2791 DUF2813 DUF3584 DUF463 DUF815 DUF853 DUF87 DUF927 Dynamin_N Dynein_heavy Elong_Iki1 ELP6 ERCC3_RAD25_C Exonuc_V_gamma FeoB_N Fer4_NifH Flavi_DEAD FTHFS FtsK_SpoIIIE G-alpha Gal-3-0_sulfotr GBP GBP_C GTP_EFTU Gtr1_RagA Guanylate_kin GvpD HDA2-3 Helicase_C Helicase_C_2 Helicase_C_4 Helicase_RecD Herpes_Helicase Herpes_ori_bp Herpes_TK HSA HydF_dimer HydF_tetramer Hydin_ADK IIGP IPPT IPT IstB_IS21 KAP_NTPase KdpD Kinase-PPPase Kinesin KTI12 LAP1C Lon_2 LpxK MCM MeaB MEDS Mg_chelatase Microtub_bd MipZ MMR_HSR1 MMR_HSR1_C MobB MukB MutS_V Myosin_head NACHT NB-ARC NOG1 NTPase_1 NTPase_P4 ORC3_N ParA Parvo_NS1 PAXNEB PduV-EutP PhoH PIF1 Ploopntkinase1 Ploopntkinase2 Ploopntkinase3 Podovirus_Gp16 Polyoma_lg_T_C Pox_A32 PPK2 PPV_E1_C PRK PSY3 Rad17 Rad51 Ras RecA ResIII RHD3 RHSP RNA12 RNA_helicase Roc RsgA_GTPase RuvB_N SbcCD_C SecA_DEAD Septin Sigma54_activ_2 Sigma54_activat SKI SMC_N SNF2_N Spore_IV_A SRP54 SRPRB SulA Sulfotransfer_1 Sulfotransfer_2 Sulfotransfer_3 Sulfotransfer_4 Sulphotransf SWI2_SNF2 T2SSE 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 TsaE UvrB UvrD-helicase UvrD_C UvrD_C_2 Viral_helicase1 VirC1 VirE Zeta_toxin Zot


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Curation and family details

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Curation View help on the curation process

Seed source: Pfam-B_440 (release 2.1)
Previous IDs: GTP_CDC;
Type: Domain
Sequence Ontology: SO:0000417
Author: Bateman A
Number in seed: 13
Number in full: 7962
Average length of the domain: 230.40 aa
Average identity of full alignment: 35 %
Average coverage of the sequence by the domain: 60.83 %

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HMM build commands:
build method: hmmbuild -o /dev/null HMM SEED
search method: hmmsearch -Z 45638612 -E 1000 --cpu 4 HMM pfamseq
Model details:
Parameter Sequence Domain
Gathering cut-off 26.2 26.2
Trusted cut-off 26.2 26.2
Noise cut-off 26.1 26.1
Model length: 281
Family (HMM) version: 18
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Species distribution

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Archea Archea Eukaryota Eukaryota
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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 35 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 sequence.

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