G-PROTEIN CONTROL OF DROSOPHILA PHOTORECEPTOR PHOSPHOLIPASE-C

Light stimulates phosphatidylinositol bisphosphate phospholipase C (PL C) activity in Drosophila photoreceptors. We have investigated the mec hanism of this reaction by assaying PLC activity in Drosophila head me mbranes using exogenous phospholipid substrates. PLC activation depend s on the photoconversion of rhodopsin to metarhodopsin and is reduced in norpA(EE5) PLC and ninaE(P332) rhodopsin mutants. NorpA PLC is stim ulated by light at free Ca2+ concentrations between 10 nM and 1 mu M. This finding is consistent with a Ca2+ mediated positive feedback mech anism that contributes to the rapid temporal response of invertebrate photoreceptor cells. The guanyl nucleotide dependence of light-stimula ted PLC activity indicates that a G protein regulates NorpA. This was confirmed by the observation that light stimulation of PLC activity is deficient in mutants that lack the eye-specific G protein beta subuni t G beta e. These results indicate that G beta e functions as the beta subunit of the G protein coupling rhodopsin to NorpA PLC.