The drosophila dgq gene encodes a G α protein that mediates phototransduction

We examined the roles of the Drosophila G qα 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 qα immunoglobulin blocked the light-dependent GTP hydrolysis activity associated with Drosophila head membranes in vitro, suggesting that rhodopsin stimulated DG q Dominantly active DG q, 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 q, 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.