Filament-forming cytoskeletal GTPase (By similarity). May play a role in cytokinesis (Potential). May play a role in platelet secretion (By similarity). 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. In platelets, associated with a complex containing STX4. Interacts with PRKN; this interaction leads to SEPTIN5 ubiquitination and degradation (By similarity). Interacts with DYRK1A (By similarity). Interacts with STX1A; in the cerebellar cortex (By similarity). In platelets, found in areas surrounding alpha-granules. Expressed at high levels in the CNS, as well as in heart and platelets (at protein level). Phosphorylated by DYRK1A (By similarity). In platelets, phosphorylated in response to thrombin, phorbol-12-myristate-13-acetate and collagen. In a heterologous system, SEPTIN5 overexpression has been shown to exert dopamine-dependent neurotoxicity. As wild-type PRKN, but not familial-linked PRKN mutants, ubiquitinates mouse SEPTIN5 and promotes its degradation, it has been suggested that a deficiency in SEPTIN5 degradation may contribute to the development of early onset Parkinson disease 2 (PARK2). Belongs to the TRAFAC class TrmE-Era-EngA-EngB-Septin-like GTPase superfamily. Septin GTPase family.