May be involved in Ca(2+)-dependent exocytosis of secretory vesicles through Ca(2+) and phospholipid binding to the C2 domain or may serve as Ca(2+) sensors in the process of vesicular trafficking and exocytosis. Binds 3 Ca(2+) ions per subunit. The ions are bound to the C2 domains. Homodimer; disulfide-linked via the cysteine motif (PubMed:10531343). Can also form heterodimers with SYT3, SYT6, SYT7 and SYT10 (PubMed:10531343, PubMed:10531344, PubMed:10871604). Interacts with DNAJC5 and SNAP25, but not with HSC70 (PubMed:20847230). The interaction with DNAJC5 is stimulated tenfold in presence of calcium while the interaction with SNAP25 is inhibited (PubMed:20847230). The cysteine motif mediates homo- or heterodimer formation via formation of disulfide bonds. Belongs to the synaptotagmin family.