Plays a role in the control of regulated secretion in neural and endocrine cells, enhancing selectively low-frequency neurotransmission. Positively regulates vesicle fusion by maintaining the readily releasable pool of secretory vesicles (By similarity). (Microbial infection) Receptor for the C.botulinum neurotoxin type A2 (BoNT/A, botA); glycosylation is not essential but enhances the interaction (PubMed:29649119). Probably also serves as a receptor for the closely related C.botulinum neurotoxin type A1. Interacts with SYT1/synaptotagmin-1 in a calcium-dependent manner. Binds the adapter protein complex AP-2 (By similarity). (Microbial infection) Interacts with C.botulinum neurotoxin type A2 (BoNT/A, botA) (PubMed:29649119). Interaction is improved by glycosylation of SV2 (PubMed:29649119). Enriched in chromaffin granules, not present in adrenal microsomes. Associated with both insulin granules and synaptic-like microvesicles in insulin-secreting cells of the pancreas (By similarity). Colocalizes with ATP2B1 at photoreceptor synaptic terminals. Phosphorylation by CK1 of the N-terminal cytoplasmic domain regulates interaction with SYT1. N-glycosylated. Identified as the brain binding-site for the antiepileptic drug levetiracetam/lev. Belongs to the major facilitator superfamily. Extended N-terminus.