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

Abstract

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^1–3. 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^4,5. This ‘GTP-charged’ protein (G^GTP) is thought to activate PDE through an interaction in which an inhibitory protein is released from PDE^6–8. Assays of PDE activity in vitro have demonstrated that the hydrolysis of cyclic GMP occurs within milliseconds of light onset⁹, 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 G^p(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.