Kinetics of synaptotagmin responses to Ca2+ and assembly with the core SNARE complex onto membranes

The synaptic vesicle protein synaptotagmin I binds Ca2+ and is required for efficient neurotransmitter release. Here, we measure the response time of the C2 domains of synaptotagmin to determine whether synaptotagmin is fast enough to function as a Ca2+ sensor for rapid exocytosis. We report that sy naptotagmin is "tuned" to sense Ca2+ concentrations that trigger neuronal e xocytosis. The speed of response is unique to synaptotagmin I and readily s atisfies the kinetic constraints of synaptic vesicle membrane fusion. We fu rther demonstrate that Ca2+ triggers penetration of synaptotagmin into memb ranes and simultaneously drives assembly of synaptotagmin onto the base of the ternary SNARE (soluble N-ethylmaleimide-sensitive fusion protein [NSF] attachment receptor) complex, near the transmembrane anchor of syntaxin. Th ese data support a molecular model in which synaptotagmin triggers exocytos is through its interactions with membranes and the SNARE complex.