Injection of GTP-binding protein or cyclic GMP phosphodiesterase hyperpolarizes retinal rods

Light-induced activation of cyclic GMP phosphodiesterase (PDE) in the outer segments of vertebrate rod photoreceptors has been suggested as a step in the transduction process linking the absorption of light by rhodopsin with a hyperpolarization of the plasma membrane. Activation of PDE is mediated by a 'GTP-binding protein' (G). As a result of interaction with a rhodopsin photoproduct (possibly metarhodopsin II380), this GTP-binding protein exchanges a previously bound GDP for a GTP. This 'GTP-charged' protein (GGTP) is thought to activate PDE through an interaction in which an inhibitory protein is released from PDE. Assays of PDE activity in vitro have demonstrated that the hydrolysis of cyclic GMP occurs within milliseconds of light onset, which suggests that this process is rapid enough to be an intermediate step in transduction. To test this idea electrophysiologically, we have injected purified GTP-binding protein that was binding a hydrolysis-resistant analogue of GTP, guanylyl imidodiphosphate (p(NH)ppG), termed Gp(NH)ppG, as well as partially purified PDE, into toad rod outer segments while recording membrane voltage. We report here that injection of either of these biochemically active proteins induces a reversible hyperpolarization of the rod membrane, while injection of inactive proteins seems to have no effect on membrane voltage.