Calcium sensor that participates in triggering neurotransmitter release at the synapse (PubMed:30107533). May have a regulatory role in the membrane interactions during trafficking of synaptic vesicles at the active zone of the synapse. It binds acidic phospholipids with a specificity that requires the presence of both an acidic head group and a diacyl backbone. A Ca(2+)-dependent interaction between synaptotagmin and putative receptors for activated protein kinase C has also been reported. It can bind to at least three additional proteins in a Ca(2+)-independent manner; these are neurexins, syntaxin and AP2. Plays a role in dendrite formation by melanocytes. (Microbial infection) Receptor for C.botulinum neurotoxin type B (BoNT/B, botB); interaction is improved in the presence of gangliosides (PubMed:8144634, PubMed:14504267, PubMed:17167418). BoNT/B toxin binds to the membrane proximal vesicular domain of Syt1 (residues 32-51) (PubMed:14504267, PubMed:17167421). (Microbial infection) Receptor for C.botulinum neurotoxin type G (BoNT/G, botG); unlike the case with BoNT/B, interaction is not improved in the presence of gangliosides (PubMed:15123599, PubMed:20219474). BoNT/G toxin binds to the vesicular domain of Syt1 (residues 32-53) (PubMed:15123599, PubMed:20219474). Binds 3 Ca(2+) ions per subunit. The ions are bound to the C2 domains. Homotetramer (Probable). Interacts with SCAMP5 (By similarity). Interacts with STON2 (By similarity). Forms a complex with SV2B, syntaxin 1 and SNAP25 (By similarity). Interacts with SV2A, SV2B and SV2C (PubMed:8910372, PubMed:15866046). Interacts with RIMS1 (PubMed:11438518). Interacts with PRRT2 (By similarity). Interacts with DNAJC5 in a phosphorylation-dependent manner (PubMed:11931641). Interacts (via N-terminus) with RAB3A (PubMed:28057568). Interacts with SYT12 (PubMed:17190793). Interacts with calmodulin (By similarity). (Microbial infection) Interacts with C.botulinum neurotoxin type B (BoNT/B, botB). Has lower affinity for BoNT/B than Syt2; mutating its residues to match those in Syt2 increases its affinity (PubMed:17167421, PubMed:17167418). (Microbial infection) Interacts with C.botulinum neurotoxin type G (BoNT/G, botG). Expressed in the brain (at protein level) (PubMed:17190793). Predominantly expressed in rostral, phylogenetically younger brain regions, and in some endocrine tissues. The first C2 domain mediates Ca(2+)-dependent phospholipid binding. The second C2 domain mediates interaction with SV2A and probably with STN2. Glycosylated. Belongs to the synaptotagmin family.