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. May mediate Ca(2+)-regulation of exocytosis in acrosomal reaction in sperm (By similarity). Binds 3 Ca(2+) ions per subunit. The ions are bound to the C2 domains. Isoform 1: Homodimer; disulfide-linked via the cysteine motif. Isoform 1: Can also form heterodimers with SYT3, SYT7, SYT9 and SYT10. Isoform 1: Interacts with STX1A, STX1B and STX2; the interaction is Ca(2+)-dependent. Isoform 2: Is not able to form homodimer and heterodimers. Localized predominantly to endoplasmic reticulum (ER) and/or Golgi-like perinuclear compartment (By similarity). The cysteine motif mediates homo- or heterodimer formation via formation of disulfide bonds. Belongs to the synaptotagmin family.