The tandem C2 domains of synaptotagmin contain redundant Ca2+ binding sites that cooperate to engage t-SNAREs and trigger exocytosis.

Real-time voltammetry measurements from cracked PC12 cells were used to analyze the role of synaptotagmin-SNARE interactions during Ca2+-triggered exocytosis. The isolated C2A domain of synaptotagmin I neither binds SNAREs nor inhibits norepinephrine secretion. In contrast, two C2 domains in tandem (either C2A-C2B or C2A-C2A) bind strongly to SNAREs, displace native synaptotagmin from SNARE complexes, and rapidly inhibit exocytosis. The tandem C2 domains of synaptotagmin cooperate via a novel mechanism in which the disruptive effects of Ca2+ ligand mutations in one C2 domain can be partially alleviated by the presence of an adjacent C2 domain. Complete disruption of Ca2+-triggered membrane and target membrane SNARE interactions required simultaneous neutralization of Ca2+ ligands in both C2 domains of the protein. We conclude that synaptotagmin-SNARE interactions regulate membrane fusion and that cooperation between synaptotagmin's C2 domains is crucial to its function.