THE DROSOPHILA DGQ GENE ENCODES A G(ALPHA)-PROTEIN THAT MEDIATES PHOTOTRANSDUCTION

We examined the roles of the Drosophila G(q alpha) proteins (DG(q)) in the phototransduction pathway. The DG(q) proteins immunolocalized to the ocelli and all eight retinular photoreceptor cell rhabdomeres. An affinity-purified anti-DG(q alpha) immunoglobulin blocked the light-dependent GTP hydrolysis activity associated with Drosophila head membranes in vitro, suggesting that rhodopsin stimulated DG(q). Dominantly active DG(q1) mutants exhibited a light-independent GTPase activity and abnormal electrophysiological light responses, such as reduced retinal sensitivity and slow response kinetics compared with wild-type flies. Dominant DG(q2) mutants exhibited a light-independent GTPase activity with normal electrophysiological light responses. Retinas of double mutants of DG(q1), but not DG(q2), with the light-dependent retinal degeneration mutant rdgB degenerated even in the dark. DG(q1) stimulation of rdgB retinal degeneration in the dark was norpA-dependent. These results indicate that DG(q1) mediates the stimulation by light-activated rhodopsin of the norpA-encoded phospholipase C in the visual transduction cascade.