G-PROTEIN DEACTIVATION IS RATE-LIMITING FOR SHUTOFF OF THE PHOTOTRANSDUCTION CASCADE

Photoreceptors detect light through a seven-helix receptor (rhodopsin) and heterotrimeric G protein (transducin) coupled to a cyclic GMP pho sphodiesterase(1,2). Similar pathways are used to amplify responses to hormones, taste and smell(3-5). The amplification of phototransductio n is reduced by a fall in cytoplasmic Ca2+ (refs 6-10), but it is not known how the deactivation of rhodopsin and transducin influence this response and hence the extent and duration of phosphodiesterase activi ty(11-14). Here we investigate this by recording the electrical respon se to flashes of light in truncated rod photoreceptors(10). By removin g ATP to block the deactivation of rhodopsin by phosphorylation(15), w e show that this reaction limits the amplitude of the response and beg ins within 3.2 s of a flash in a solution containing 1 mu M Ca2+, fall ing to 0.9 s in a zero-Ca2+ solution. In contrast, the activation and amplitude of the response were unaffected when transducin deactivation by GTP hydrolysis was blocked by replacing GTP with its nonhydrolysab le analogue GTP-gamma S-11, demonstrating that there is little GTP hyd rolysis occurring over the period in which photoexcited rhodopsin is q uenched. The rapid deactivation of rhodopsin is therefore a Ca2+-sensi tive step controlling the amplitude of the light response, whereas tra nsducin deactivation is slower and controls recovery.