The products of the Drosophila stoned locus interact with synaptic vesicles via synaptotagmin

The stoned locus of Drosophila melanogaster encodes two novel proteins, sto nedA (STNA) and stonedB (STNB), both of which are expressed in the nervous system. Flies with defects at the stoned locus have abnormal behavior and a ltered synaptic transmission. Genetic interactions, in particular with the shibire (dynamin) mutation, indicated a presynaptic function for stoned and suggested an involvement in vesicle cycling. Immunological studies reveale d colocalization of the stoned proteins at the neuromuscular junction with the integral synaptic vesicle protein synaptotagmin (SYT). We show here tha t stoned interacts genetically with synaptotagmin to produce a lethal pheno type. The STNB protein is found by co-immunoprecipitation to be associated with synaptic vesicles, and glutathione S-transferase pull-downs demonstrat e an in vitro interaction between the mu2-homology domain of STNB and the C 2B domain of the SYTI isoform. The STNA protein is also found in associatio n with vesicles, and it too exhibits an in vitro association with SYTI. How ever, we find that the bulk of STNA is in a nonmembranous fraction. By usin g the shibire mutant to block endocytosis, STNB is shown to be present on s ome synaptic vesicles before exocytosis. However, STNB is not associated wi th all synaptic vesicles. We hypothesize that STNB specifies a subset of sy naptic vesicles with a role in the synaptic vesicle cycle that is yet to be determined.