Figure 1.

Major players in calcium-triggered exocytosis. Formation of a complex between vesicular v- (red) and plasma membrane t-SNAREs (red) drives membrane fusion. Complex assembly is arrested at an intermediate stage by the synergistic action of Synaptotagmin (light orange) and complexin (black). Membrane depolarization opens voltage-gated calcium channels and allows rapid calcium influx. Tandem C2 domains of Synaptotagmin bind 2–3 calcium ions each and bury their hydrophobic residues at the tips of the calcium-binding loops into the bilayer. Other rearrangements likely allow further SNARE assembly and contribute to fusion pore opening.