Synaptotagmin I Functions as a Calcium Sensor to Synchronize Neurotransmitter Release

Abstract To characterize Ca 2+ -mediated synaptic vesicle fusion, we analyzed Drosophila synaptotagmin I mutants deficient in specific interactions mediated by its two Ca 2+ binding C2 domains. In the absence of synaptotagmin I, 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 Ca 2+ cooperativity. Mutants that disrupt Ca 2+ sensing by the C2B domain have synchronous release with normal Ca 2+ cooperativity, but with reduced release probability. Our data suggest the Ca 2+ 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 Ca 2+ sensor for evoked release and functions to trigger synchronous fusion in response to Ca 2+ , while suppressing asynchronous release.