Synaptotagmin family proteins consist of a single N-terminal transmembrane domain and C-terminal tandem C2 domains (C2A and C2B), which have the ability to bind Ca2+ and phospholipid, and 15 synaptotagmin isoforms have been reported in the mouse and humans to date. The best-characterized isoform, synaptotagmin I, is essential for synaptic vesicle exocytosis and endocytosis.