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. (Microbial infection) Receptor for C.botulinum neurotoxin type A (BoNT/A, botA); the toxin probably binds via extracellular loop 4 (PubMed:16543415). (Microbial infection) Possible receptor for C.botulinum neurotoxin type D (BoNT/D, botD); BoNT/D does not bind to extracellular loop 4 as do BoNT/A and BoNT/E (PubMed:21483489). (Microbial infection) Receptor for C.botulinum neurotoxin type E (BoNT/E); the toxin probably binds via extracellular loop 4 (PubMed:18815274). It probably requires glycosylation of Asn-573 (PubMed:18815274). 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 A (BoNT/A, botA). 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. Expressed in conventional synapses and cone ribbon synapses in the retina (at protein level) (PubMed:12687700). Expressed in diaphragm motor nerve terminals (at protein level) (PubMed:16543415). Expressed in hippocampus neurons (at protein level) (PubMed:18815274). Expressed during synaptogenesis in the retina (at protein level). Phosphorylation by CK1 of the N-terminal cytoplasmic domain regulates interaction with SYT1. N-glycosylated. Mice fail to grow, experience severe epileptic seizures and die immediately or shortly after birth probably due to multiple neural and endocrine deficits (PubMed:10624962). Mice lacking both Sv2a and Sv2b display a similar phenotype (PubMed:10624962). Diaphragm motor nerve terminals from mice with only a single copy of this gene and no Sv2b have decreased binding of C.botulinum neurotoxin type A (BoNT/A, botA) and are less sensitive to the toxin (PubMed:16543415). Hippocampal neurons from young mice lacking both Sv2a and Sv2b do not bind BoNT/A, nor do they take it up (PubMed:16543415, PubMed:18815274). Hippocampal neurons from young mice lacking both Sv2a and Sv2b do not bind C.botulinum neurotoxin type E (BoNT/E), nor do they take it up (PubMed:18815274). Hippocampal neurons from young mice lacking both Sv2a and Sv2b do not bind C.botulinum neurotoxin type BoNT/D (BoNT/D, botD), nor do they take it up (PubMed:21483489). Hippocampal neurons from young mice lacking both Sv2a and Sv2b take up C.botulinum neurotoxin type C (BoNT/C) and C.botulinum neurotoxin type F (BonT/F, botF) normally (PubMed:21483489). Belongs to the major facilitator superfamily.