What role does calcium play in exocytosis?

Calcium plays a key role in triggering the exocytosis of neurotransmitter and hormone-containing vesicles. Spanning both the vesicle and cell membrane are v-SNARE and t-SNARE protein complexes, respectively, that drive the biological fusion of the two membranes. The v-SNARE complex, which is embedded in the membrane of vesicles, is comprised of synaptobrevin and the calcium-sensitive protein synaptotagmin. When calcium binds to synaptotagmin it enhances the interactions between proteins of the v-SNARE and t-SNARE complexes causing the two membranes to fuse together.