SEPTIN1

SEPTIN1 (SEPT1) is a GTPase-family septin that assembles into heteromeric septin complexes and acts as a scaffold supporting cell division, membrane trafficking, and cytoskeletal organization (PMID:23087102, PMID:30709970, PMID:32392324). During mitosis and meiosis it localizes to the spindle pole and midbody, where it is required for spindle assembly, chromosome congression, and cytokinesis progression (PMID:16179162, PMID:19799413, PMID:21932310); its mitotic activity is governed by Aurora-B kinase, which physically binds SEPT1 and phosphorylates it at Ser248, Ser307, and Ser315 (PMID:16179162). At the Golgi, SEPT1 forms a scaffold needed for Golgi ribbon integrity and anterograde membrane traffic, acting through the matrix protein GM130 and centrosomal CEP170/γ-TuRC components to promote microtubule nucleation and perinuclear Golgi positioning (PMID:30709970). SEPT1 also localizes actin-dependently to lamellipodia, where it is required for cell spreading and forms a complex with SEPT5 (PMID:23087102), and it participates in higher-order septin complexes with SEPT2, SEPT10, SEPT11, and SEPT12 at the sperm neck whose disruption produces acephalic, immotile spermatozoa (PMID:32392324). In neurons, SEPT1 is developmentally upregulated postnatally and localizes to synaptic fractions (PMID:32146512).

1. 2005 High

   Established that SEPT1 is an Aurora-B substrate and a mitotic component, linking a septin to the kinase machinery controlling cell division.

   Evidence Yeast two-hybrid, reciprocal Co-IP, GST pull-down, in vitro kinase assay with phosphosite mapping, and immunofluorescence in mammalian cells

   PMID:16179162

   Open questions at the time

   • Functional consequence of Ser248/307/315 phosphorylation on SEPT1 activity or assembly not tested
   • Whether phosphorylation regulates spindle-pole or midbody localization unknown
2. 2006 Low

   Provided the structural starting point for SEPT1 by crystallizing a low-aggregation serine-to-alanine mutant.

   Evidence Recombinant expression, site-directed mutagenesis, crystallization and X-ray diffraction to 2.5 Å

   PMID:16511282

   Open questions at the time

   • No refined structure or mechanistic conclusion drawn
   • Mutations may perturb native nucleotide or interface behavior
3. 2007 Low

   Identified the septin1 ortholog as a Parkin ubiquitination substrate, hinting at degradative regulation of septins.

   Evidence In vivo ubiquitination assay and Co-IP in Drosophila

   PMID:17456438

   Open questions at the time

   • Drosophila ortholog finding without site mapping or mutagenesis
   • Not confirmed for human SEPT1
   • Functional consequence of ubiquitination undefined
4. 2009 Medium

   Demonstrated a direct functional requirement for SEPT1 in completing cytokinesis, beyond mere midbody localization.

   Evidence Immunofluorescence midbody localization and siRNA knockdown quantifying cytokinesis defects

   PMID:19799413

   Open questions at the time

   • Molecular partners at the midbody not defined
   • Single lab; no rescue control reported
5. 2011 Medium

   Extended SEPT1's division role to meiosis, showing it is needed for spindle assembly and chromosome congression in oocytes.

   Evidence siRNA knockdown in mouse oocytes with polar body extrusion and spindle morphology analysis

   PMID:21932310

   Open questions at the time

   • Mechanism linking SEPT1 to spindle/chromosome behavior unresolved
   • Single lab
6. 2013 Medium

   Placed SEPT1 at lamellipodia in an actin-dependent manner and tied it to cell spreading, broadening its role to cell-substrate interaction.

   Evidence Immunofluorescence, Co-IP showing SEPT5 complex, and siRNA knockdown with cell spreading assay in squamous carcinoma cells

   PMID:23087102

   Open questions at the time

   • How SEPT1/SEPT5 complex couples to actin or adhesion machinery unknown
   • Single cell line
7. 2019 High

   Defined SEPT1 as a Golgi scaffold required for ribbon integrity and anterograde traffic, and placed it in a GM130–CEP170–γ-TuRC microtubule-nucleating pathway.

   Evidence siRNA depletion, immunofluorescence, membrane traffic assays, and co-localization with GM130/CEP170/γ-TuRC

   PMID:30709970

   Open questions at the time

   • Direct biochemical interactions among SEPT1, GM130, CEP170 not mapped
   • Whether the same SEPT1 pool serves Golgi and mitotic functions unknown
8. 2020 Medium

   Showed SEPT1 is a member of a sperm-neck septin complex whose disruption causes structural sperm defects, and separately documented postnatal synaptic localization in neurons.

   Evidence Co-IP and immunofluorescence defining a SEPT1/2/10/11/12 complex plus a SEPT12 D197N knock-in mouse with EM (sperm); developmental Western blotting, fractionation, and imaging (neurons)

   PMID:32146512 PMID:32392324

   Open questions at the time

   • Specific contribution of SEPT1 within the multi-septin complex not isolated
   • Synaptic function of neuronal SEPT1 not tested functionally

• How GTP binding/hydrolysis and Aurora-B phosphorylation control SEPT1 complex assembly and switching between its mitotic, Golgi, lamellipodial, and synaptic roles remains unresolved.
• No demonstrated GTPase activity or nucleotide-dependent regulation in the corpus
• No structure-function link between phosphosites and assembly
• Mechanism partitioning SEPT1 across compartments unknown