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 (PubMed:30389919). May also play a role in the regulation of cell fate in the intestine (PubMed:30389919). Positive regulator of apoptosis involved in hematopoietic stem cell homeostasis; via its interaction with XIAP (PubMed:20952537). 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 (PubMed:23788729). Plays an important role in male fertility and sperm motility (PubMed:15737930, PubMed:15737931). 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 (PubMed:15737930, PubMed:15737931). Involved in the migration of cortical neurons and the formation of neuron leading processes during embryonic development (PubMed:20181826). Required for dopaminergic metabolism in presynaptic autoreceptors; potentially via activity as a presynaptic scaffold protein (PubMed:17296554). 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 (By similarity). 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 (By similarity). Interacts with SEPTIN14 (via C-terminus) (PubMed:20181826). Interacts with DYRK1A (PubMed:18938227). Interacts with SLC6A3/DAT and SNCA/alpha-synuclein (PubMed:17296554). Interacts with STX1A; in the striatum (PubMed:17296554). Interacts with XIAP (via BIR3 domain) following the induction of apoptosis (PubMed:30389919). Interacts with AREL1 (via HECT domain); in the cytoplasm following induction of apoptosis (PubMed:23479728). Interacts with DPYSL5. Found in the sperm annulus, a fibrous ring structure connecting the midpiece and the principal piece of the sperm flagellum (PubMed:15737930, PubMed:15737931, PubMed:27811987). In platelets, found in areas surrounding alpha-granules (By similarity). Expressed and colocalized with SLC6A3 and SNCA in axon terminals, especially at the varicosities (PubMed:17296554). In retinoic acid-treated P19 cells, found first in the mitochondria and at later times, as neuronal differentiation proceeds, in the cytosol. Expressed in the cerebral cortex, striatum, midbrain, cerebellum and spinal cord (at protein level). Expressed in the substantia nigra pars compacta, ventral tegmental area, projection fiber bundles and in axon terminals surrounding striatal neurons (at protein level) (PubMed:17296554). Expressed in hair follicle stem cells (at protein level) (PubMed:23788729). Expressed in small intestinal crypts; abundantly expressed at the crypt base (at protein level) (PubMed:30389919). Widely expressed in the brain and to a lesser extent in the testis, lung and liver (PubMed:21679490, PubMed:17296554). Highly expressed in the brain and testis and, to a lesser extent in the heart, lung and kidney (PubMed:12106288, PubMed:12581152, PubMed:15737931). In the brain, abundant in areas of high cell density, particularly in the stria terminalis (PubMed:12106288). Expressed in the entorhinal, temporal and visual cortices and the hippocampus of the brain where is colocalizes with DYRK1A in postnatal day 1 and adult mice. Expressed and extensively colocalizes with DYRK1A in apical dendrites of pyramidal cells (PubMed:18938227). Predominantly expressed in embryonic brain and dorsal root ganglion neurons. Expressed in LGR5-positive intestinal stem cells and lysozyme-positive Paneth cells (at protein level) (PubMed:30389919). Expressed in the brain and testis (PubMed:15737931). Expressed in the intermediate zone and cortical plate of the cortex at 15.5 dpc. Widely expressed in embryos at 7 dpc. Predominantly expressed in the brain at 14 dpc and on postnatal day 1. Induced by S.japonicum egg-mediated liver fibrosis at the site of egg granulomas; expression peaks at 12 weeks post infection with expression decreasing thereafter. Phosphorylated by DYRK1A. Ubiquitinated by AREL1. May be phosphorylated. Knockout mice have a decreased basal prepulse startle response and an enhanced acoustic startle response (PubMed:17296554). Decreased concentration of TH and SLC6A3/DAT in dopaminergic axons and axon terminals, does not affect axon morphology (PubMed:17296554). Male mice are sterile due to immotile and structurally defective sperm (PubMed:15737930, PubMed:15737931). Sperms exhibit defective mitochondrial architecture, bent and nonmotile tails, absence of the annulus (a fibrous ring structure connecting the midpiece and the principal piece of the sperm flagellum), lower ATP consumption, impaired capacitation and defects in the removal of residual cytoplasm (PubMed:15737930, PubMed:15737931). Increased in intestinal crypt diameter, length and cell number (PubMed:30389919). Decreased apoptosis and increased maintenance of colon crypt architecture in response to intestinal barrier damage, as a result of improved Lgr5-positive intestinal stem cell-mediated regeneration (PubMed:30389919). Decrease in differentiated intestinal goblet cells (PubMed:30389919). Increase proliferation of Lgr5-positive intestinal stem cells and lysozyme-positive granular Paneth cells in the crypt base of both the small and large intestines (PubMed:30389919). Increase in Reg4-positive cells in colon epithelium and nuclear localization of CTNNB1 in cells of the intestinal crypt base (PubMed:30389919). Increased incidence of spontaneous hematopoietic malignancies, splenomegaly and a third of mice developed spontaneous neoplasia at 11 to 15 months old (PubMed:20952537). Increased number of B-linage progenitors, immature B cells and increased functional hematopoietic progenitor cells in the bone marrow of 6 to 13 week old mice, increased hematopoietic progenitor cell levels persisted in 11 to 15 month old mice (PubMed:20952537). Normal hair follicle bulge morphology but twice as many epithelial progenitor cells and an elongated tail epithelial strand (PubMed:23788729). Significantly improved dorsal skin wound repair which included an increased amount of proliferating hair follicle stem cells in the wound bed (PubMed:23788729). Decrease in apoptosis of hematopoietic progenitors in response to irradiation (PubMed:20952537). SEPTIN4 and XIAP double knockout mice show no differences in apoptosis or lymphoproliferation in hematopoietic stem and progenitor cells (PubMed:20952537). Delayed dermal wound repair and hair follicle regeneration, via increased apoptosis of hair follicle stem cells (PubMed:23788729). May act as a tumor suppressor in both hematopoietic and neoplastic malignancies. Belongs to the TRAFAC class TrmE-Era-EngA-EngB-Septin-like GTPase superfamily. Septin GTPase family.