Filament-forming cytoskeletal GTPase (By similarity). Involved in cytokinesis (Potential). May play a role in platelet secretion. Septins polymerize into heterooligomeric protein complexes that form filaments, and can associate with cellular membranes, actin filaments and microtubules. GTPase activity is required for filament formation (By similarity). Interacts with SEPTIN2 and SEPTIN5. Interaction with SEPTIN4 not detected. In platelets, associated with a complex containing STX4 (By similarity). Interacts with PRKN; this interaction leads to SEPTIN5 ubiquitination and degradation (PubMed:11078524). Interacts with DYRK1A (PubMed:18938227). Interacts with STX1A; in the cerebellar cortex (PubMed:17296554). Detected at 17 dpc in the brain. Expression increases during postnatal life and reaches a plateau at approximately P10 (at protein level). Phosphorylated by DYRK1A. Mice have a normal life-span and show no apparent abnormalities, including synaptic properties and hippocampal neuron growth. In platelets, hyperresponsive degranulation phenotype. Belongs to the TRAFAC class TrmE-Era-EngA-EngB-Septin-like GTPase superfamily. Septin GTPase family.