Plays an essential role in hormone and neurotransmitter calcium-dependent exocytosis and endocytosis (PubMed:17502420, PubMed:28596237, PubMed:28031464). Part of the SNARE (Soluble NSF Attachment Receptor) complex composed of SNAP25, STX1A and VAMP2 which mediates the fusion of synaptic vesicles with the presynaptic plasma membrane. STX1A and SNAP25 are localized on the plasma membrane while VAMP2 resides in synaptic vesicles. The pairing of the three SNAREs from the N-terminal SNARE motifs to the C-terminal anchors leads to the formation of the SNARE complex, which brings membranes into close proximity and results in final fusion. Participates in the calcium-dependent regulation of acrosomal exocytosis in sperm (PubMed:12101244). Also plays an important role in the exocytosis of hormones such as insulin or glucagon-like peptide 1 (GLP-1) (PubMed:17502420, PubMed:28596237, PubMed:28031464). Part of the SNARE core complex containing SNAP25, VAMP2 and STX1A; this complex constitutes the basic catalytic machinery of the complex neurotransmitter release apparatus (PubMed:28031464, PubMed:19196426). The SNARE complex interacts with CPLX1 (By similarity). Interacts with STXBP1 (PubMed:28031464). Interacts (via C-terminus) with KCNB1 (via C-terminus); the interaction increases in a calcium-dependent manner and induces a pore-independent enhancement of exocytosis in neuroendocrine cells, chromaffin cells, pancreatic beta cells and from the soma of dorsal root ganglia (DRG) neurons (By similarity). Interacts with SYTL4 (PubMed:12101244). Interacts with STXBP6 (By similarity). Interacts with PLCL1 (via C2 domain) (PubMed:23341457). Interacts with OTOF (PubMed:17055430). Interacts with LGI3 (PubMed:18760330). Interacts with SLC6A4 (By similarity). Interacts with SYT6 and SYT8; the interaction is Ca(2+)-dependent (PubMed:15774481). Interacts with VAMP8 (PubMed:28031464). Interacts with SNAP23 (PubMed:28031464). Interacts with VAPA and SYBU (By similarity). Interacts with PRRT2 (PubMed:29056747). Interacts with SEPT8 (By similarity). Interacts with STXBP5L (PubMed:21998599). Interacts with synaptotagmin-1/SYT1 (By similarity).Interacts with SEPTIN5; in the cerebellar cortex (PubMed:17296554). Interacts with SEPTIN4; in the striatum (PubMed:17296554). Colocalizes with KCNB1 at the cell membrane. Expressed in the striatum (at protein level) (PubMed:17296554). Expressed in the ileum (PubMed:28596237). Phosphorylated by CK2. Phosphorylation at Ser-188 by DAPK1 significantly decreases its interaction with STXBP1 (By similarity). Phosphorylated by CK2 (By similarity). Phosphorylation at Ser-188 by DAPK1 significantly decreases its interaction with STXBP1 (By similarity). Sumoylated, sumoylation is required for regulation of synaptic vesicle endocytosis. Mutant mice show impaired glucose tolerance without any marked hyperglycemia. Belongs to the syntaxin family.