G protein control of Drosophila photoreceptor phospholipase C

Abstract

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 norpA(EE5) PLC and ninaE(P332) rhodopsin mutants. NorpA PLC is stimulated by light at free Ca2+ concentrations between 10 nM and 1 μM. This finding is consistent with a Ca2+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 β subunit Gβe. These results indicate that Gβe functions as the β subunit of the G protein coupling rhodopsin to NorpA PLC.