Filament-forming cytoskeletal GTPase (Probable). Pro-apoptotic protein involved in LGR5-positive intestinal stem cell and Paneth cell expansion in the intestines, via its interaction with XIAP (By similarity). May also play a role in the regulation of cell fate in the intestine (By similarity). Positive regulator of apoptosis involved in hematopoietic stem cell homeostasis; via its interaction with XIAP (By similarity). Negative regulator of repair and hair follicle regeneration in response to injury, due to inhibition of hair follicle stem cell proliferation, potentially via its interaction with XIAP (By similarity). Plays an important role in male fertility and sperm motility (By similarity). During spermiogenesis, essential for the establishment of the annulus (a fibrous ring structure connecting the midpiece and the principal piece of the sperm flagellum) which is a requisite for the structural and mechanical integrity of the sperm (By similarity). Involved in the migration of cortical neurons and the formation of neuron leading processes during embryonic development (By similarity). Required for dopaminergic metabolism in presynaptic autoreceptors; potentially via activity as a presynaptic scaffold protein (By similarity). Required for the induction of cell death mediated by TGF-beta and possibly by other apoptotic stimuli (PubMed:11146656, PubMed:15837787). Induces apoptosis through binding and inhibition of XIAP resulting in significant reduction in XIAP levels, leading to caspase activation and cell death (PubMed:15029247). Mediates the interaction between BCL2 and XIAP, thereby positively regulating the ubiquitination and degradation of BCL2 and promoting apoptosis (PubMed:29020630). 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. Interacts with SEPTIN8 (PubMed:15116257). In a mesenchymal cell line, interacts with SEPTIN9 isoform 2 variants HNA Trp-106 and Phe-111, but not the wild type SEPTIN9 (PubMed:17546647). Component of a septin core octomeric complex consisting of SEPTIN12, SEPTIN7, SEPTIN6 and SEPTIN2 or SEPTIN4 in the order 12-7-6-2-2-6-7-12 or 12-7-6-4-4-6-7-12 (PubMed:25588830). Interacts with SEPTIN14 (via C-terminus) (By similarity). Interacts with DYRK1A (By similarity). Interacts with SLC6A3/DAT and SNCA/alpha-synuclein (By similarity). Interacts with STX1A; in the striatum (By similarity). Interacts with XIAP (via BIR3 domain) following the induction of apoptosis (By similarity). Interacts with AREL1 (via HECT domain); in the cytoplasm following induction of apoptosis (PubMed:23479728). Part of a complex composed of SEPTIN4 isoform ARTS, XIAP and BCL2, within the complex interacts with both BCL2 (via BH3 domain) and XIAP, ARTS acts as a scaffold protein and stabilizes the complex (PubMed:29020630). Interacts with XIAP (via BIR3 domain) following the induction of apoptosis (PubMed:15029247, PubMed:21695558). In platelets, found in areas surrounding alpha-granules (PubMed:15116257). Found in the sperm annulus, a fibrous ring structure connecting the midpiece and the principal piece of the sperm flagellum (PubMed:25588830). Expressed and colocalized with SLC6A3 and SNCA in axon terminals, especially at the varicosities (By similarity). While predominantly localized in the mitochondria under resting conditions, translocates into the nucleus after TGF-beta treatment and apoptosis induction. Widely expressed in adult and fetal tissues with highest expression in adult brain (at protein level), heart, liver and adrenal gland and fetal heart, kidney, liver and lung. Expressed in presynaptic terminals of dopaminergic neurons projecting from the substantia nigra pars compacta to the striatum (at protein level) (PubMed:17296554). Expressed in axonal varicosities in dopaminergic nerve terminals (at protein level) (PubMed:17296554). Expressed in the putamen and in the adjacent cerebral cortex (at protein level) (PubMed:17296554). Expressed in colonic crypts (at protein level) (PubMed:30389919). Also expressed in colorectal cancers and malignant melanomas. Expressed in platelets. Highly expressed in the brain and heart. Phosphorylated by DYRK1A. Ubiquitinated by AREL1. Colocalizes with alpha-synuclein in Lewy bodies in the substantia nigra pars compacta of Parkinson disease patients (PubMed:17296554). Shows reduced expression in dopaminergic nerve terminals of the striatum in sporadic Parkinson disease (PubMed:17296554). May be defective in GTP-binding. Belongs to the TRAFAC class TrmE-Era-EngA-EngB-Septin-like GTPase superfamily. Septin GTPase family.