A PRODUCT OF THE DROSOPHILA STONED LOCUS REGULATES NEUROTRANSMITTER RELEASE

The Drosophila stoned locus encodes two novel gene products termed sto nedA and stonedB, which possess sequence motifs shared by proteins inv olved in intracellular vesicle traffic. A specific requirement for sto ned in the synaptic vesicle cycle has been suggested by synthetic gene tic interactions between stoned and shibire, a gene essential for syna ptic vesicle recycling (Petrovich et al., 1993). A synaptic role of st oned gene products also is suggested by altered synaptic transients in electroretinograms recorded from stoned mutant eyes (Petrovich et at. , 1993). We show here that the stonedA protein is highly enriched at D rosophila nerve terminals. Mutant alleles that affect stonedA disrupt the normal regulation of synaptic vesicle exocytosis at neuromuscular synapses of Drosophila. Spontaneous neurotransmitter release is enhanc ed dramatically, and evoked release is reduced substantially in such s toned mutants. Ultrastructural studies reveal no evidence of major dis organization at stoned mutant nerve terminals. Thus, our data indicate a direct role for stonedA in regulating synaptic vesicle exocytosis. However, genetic and morphological observations suggest additional, su btle effects of stoned mutations on synaptic vesicle recycling. Remark ably, almost all phenotypes of stoned mutants are similar to those pre viously described for mutants of synaptotagmin, a protein postulated t o regulate both exocytosis and the recycling of synaptic vesicles. We propose a model in which stonedA functions together with synaptotagmin to regulate synaptic vesicle cycling.