G protein control of Drosophila photoreceptor phospholipase C.

Light stimulates phosphatidylinositol bisphosphate phospholipase C (PLC) activity in Drosophila photoreceptors. We have investigated the mechanism of this reaction by assaying PLC activity in Drosophila head membranes using exogenous phospholipid substrates. PLC activation depends on the photoconversion of rhodopsin to metarhodopsin and is reduced in norpAEE5 PLC and ninaEP332 rhodopsin mutants. NorpA PLC is stimulated by light at free Ca2+ concentrations between 10 nM and 1 microM. This finding is consistent with a Ca(2+)-mediated positive feedback mechanism that contributes to the rapid temporal response of invertebrate photoreceptor cells. The guanyl nucleotide dependence of light-stimulated 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 subunit G beta e. These results indicate that G beta e functions as the beta subunit of the G protein coupling rhodopsin to NorpA PLC.