Mutational analysis of Drosophila synaptotagmin demonstrates its essential role in Ca(2+)-activated neurotransmitter release.

Synaptotagmin (syt), a synaptic vesicle-specific protein known to bind Ca2+ in the presence of phospholipids, has been proposed to mediate Ca(2+)-dependent neurotransmitter release. We have addressed the role of syt in neurotransmitter release in vivo by generating mutations in synaptotagmin (syt) in the fruitfly and assaying the subsequent effects on neurotransmission. Most embryos that lack syt fail to hatch and exhibit very reduced, uncoordinated muscle contractions. Larvae with partial lack-of-function mutations show almost no evoked excitatory junctional potentials (EJPs) in 0.4 mM Ca2+ and a 15-fold reduction in EJP amplitude in 1.0 mM Ca2+ when compared with heterozygous controls. In contrast, we observe an increase in the frequency of spontaneous miniature EJPs in the mutants. These results provide in vivo evidence that syt plays a key role in Ca2+ activation of neurotransmitter release and indicate the existence of separate pathways for evoked and spontaneous neurotransmitter release.