Plays an essential role in hormone and neurotransmitter calcium-dependent exocytosis and endocytosis (PubMed:7901002, PubMed:17301173, PubMed:18167541, PubMed:20484665, PubMed:22411134, PubMed:28813412). 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 (PubMed:14665625, PubMed:16888141, PubMed:19571812). 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 (PubMed:14665625, PubMed:16888141, PubMed:19571812). Participates in the calcium-dependent regulation of acrosomal exocytosis in sperm (By similarity). Also plays an important role in the exocytosis of hormones such as insulin or glucagon-like peptide 1 (GLP-1) (By similarity). 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:14665625, PubMed:16888141, PubMed:19196426, PubMed:9759724, PubMed:11533035, PubMed:11832227, PubMed:12496247, PubMed:19571812, PubMed:28813412, PubMed:18337752, PubMed:21785414, PubMed:26280336). The SNARE complex interacts with CPLX1 (PubMed:21785414, PubMed:28813412). Interacts with STXBP1 (PubMed:10746715, PubMed:12730201, PubMed:18337752). 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 (PubMed:17301173, PubMed:18167541, PubMed:20484665, PubMed:22411134). Interacts with SYTL4 (By similarity). Interacts with STXBP6 (PubMed:12145319). Interacts with PLCL1 (via C2 domain) (PubMed:23341457). Interacts with OTOF (By similarity). Interacts with LGI3 (By similarity). Interacts with SLC6A4 (PubMed:11709063). Interacts with SYT6 and SYT8; the interaction is Ca(2+)-dependent (By similarity). Interacts with VAMP8 (PubMed:10336434). Interacts with SNAP23 (PubMed:9507000). Interacts with VAPA and SYBU (By similarity). Interacts with PRRT2 (By similarity). Interacts with SEPT8 (PubMed:19196426). Interacts with STXBP5L (By similarity). Interacts with synaptotagmin-1/SYT1 (PubMed:26280336, PubMed:28813412). Interacts with SEPTIN5; in the cerebellar cortex (By similarity). Interacts with SEPTIN4; in the striatum (By similarity). Colocalizes with KCNB1 at the cell membrane (PubMed:17301173). Colocalizes with PLCL1 at the cell membrane (PubMed:22673903). Expressed predominantly in cerebral cortex, hippocampus, cerebellum, adrenal medulla and retina with weak expression detected in non-neuronal tissues. Phosphorylated by CK2. Phosphorylation at Ser-188 by DAPK1 significantly decreases its interaction with STXBP1. (Microbial infection) Targeted and hydrolyzed by C.botulinum neurotoxin type C (BoNT/C), which hydrolyzes the 253-Lys-|-Ala-254 bond (PubMed:7737992). Cleavage inhibits neurotransmitter release (PubMed:7901002). 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. Belongs to the syntaxin family. Extended N-terminus.