Synaptotagmin I functions as a calcium sensor to synchronize neurotransmitter release

To characterize Ca2+-mediated synaptic vesicle fusion, we analyzed Drosophila synaptotagmin I mutants deficient in specific interactions mediated by its two Ca2+ binding C2 domains. In the absence of synaptotagmin 1, synchronous release is abolished and a kinetically distinct delayed asynchronous release pathway is uncovered. Synapses containing only the C2A domain of synaptotagmin partially recover synchronous fusion, but have an abolished Ca2+ cooperativity. Mutants that disrupt Ca2+ sensing by the C2B domain have synchronous release with normal Ca2+ cooperativity, but with reduced release probability. Our data suggest the Ca2+ cooperativity of neurotransmitter release is likely mediated through synaptotagmin-SNARE interactions, while phospholipid binding and oligomerization trigger rapid fusion with increased release probability. These results indicate that synaptotagmin is the major Ca2+ sensor for evoked release and functions to trigger synchronous fusion in response to Ca2+, while suppressing asynchronous release.