SrSeptin6 is one of three septin GTPases of the colonial choanoflagellate Salpingoeca rosetta. Phylogenetic analysis places it in septin Group 1B, with the mammalian SEPT6 family. The 420-residue protein has a septin-type G domain with the G1, G3 and G4 GTP-binding motifs, a C-terminal coiled coil and a basic disordered tail. When expressed as a fluorescent fusion it is enriched at the basal pole of the cell, matching the paralogue SrSeptin2 (a Group 4, Cdc12-like septin), and the two proteins colocalise. Together with their sequence homology to septins known to form heteromeric filaments, this suggests that SrSeptin6 and SrSeptin2 assemble into a common septin structure at the basal pole. No loss-of-function or biochemical data exist, so whether it hydrolyses GTP and whether it contributes to rosette colony development are untested.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005525 GTP binding | IEA GO_REF:0000120 | ACCEPT | Summary: Combined InterPro/UniRule inference from the septin family (IPR016491) and septin-type G domain (IPR030379) signatures. SrSeptin6 has a complete septin-type G domain with G1, G3 and G4 motifs and PIRSR-predicted GTP contact residues. Nucleotide binding has not been measured. Reason: Guanine-nucleotide binding is a family-wide property of septin G domains, including those septins thought to have lost GTPase activity, so GTP binding is the appropriate molecular function to transfer. No GTPase activity is asserted. Supporting Evidence: file:SALRS/SrSeptin6/SrSeptin6-uniprot.txt PROSITE; PS51719; G_SEPTIN; 1. |
| GO:0031105 septin complex | IEA GO_REF:0000104 | ACCEPT | Summary: UniRule transfer from the septin family. SrSeptin6 (Group 1B) colocalises with SrSeptin2 (Group 4) at the basal pole, which the authors interpret as co-assembly into heteromeric filaments. No S. rosetta septin complex has been isolated. Reason: Heteromeric complex formation is a defining property of septins, and S. rosetta has septins from more than one group. The colocalisation data are consistent with this. The annotation is appropriate as electronic evidence. Supporting Evidence: PMID:30281390 Septin paralogues interact directly through their G-domains to form heteromeric filaments PMID:30281390 strongly suggest that SrSeptin2 and SrSeptin6 assemble together at the basal pole. |
| GO:0045178 basal part of cell | IDA PMID:30281390 Transfection of choanoflagellates illuminates their cell bio... | NEW | Summary: Live imaging of a transfected mTFP1-SrSeptin6 fusion shows enrichment at the basal pole of single cells, matching SrSeptin2. Reason: GOA has no experimental annotation for this protein. Basal enrichment is shown directly for SrSeptin6 (Fig. 4C). The rosette and microtubule panels of the same figure use SrSeptin2, so septin cytoskeleton and rosette-specific locations are not proposed for SrSeptin6. Supporting Evidence: PMID:30281390 mTFP1-SrSeptin6 mirrored the enrichment of mTFP1-SrSeptin2 at the basal pole PMID:30281390 found that SrSeptin6 displays the same basal localization as SrSeptin2 (Figure 4C). |
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Download this section (compressed HTML)Q: Does SrSeptin6 retain GTPase activity? Mammalian SEPT6-group septins are commonly described as GTPase-deficient; whether the choanoflagellate Group 1B septin shares this has not been examined.
Q: Is SrSeptin6 needed for the basal localisation of SrSeptin2, and for rosette development? No septin knockout has been reported.
Experiment: Inactivate PTSG_06009 and test rosette development and the localisation of tagged SrSeptin2.
Type: Gene knockout by genome editing
Experiment: Purify SrSeptin6 alone and with SrSeptin2 and measure nucleotide binding and GTP hydrolysis.
Type: In vitro GTPase assay
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