The G protein cascade of visual transduction: kinetics and regulation

In retinal rods photoexcited rhodopsin (R*) catalyses the activation of transducin (T) by GTP, which in turn activates the cGMP phosphodiesterase (PDE). The ensuing decrease in cGMP concentration reduces the cell membrane's channel conductance. To account for the kinetics of the response to light, all underlying biochemical reactions must reach maximum speed and be turned off within a second. Kinetic analysis of transducin activation suggests that because of the fast lateral diffusion of T, the rate-limiting step is not the collision between R* and T but the entry of GTP after the release fo GDP from the R*-bound T alpha. T alpha-GTP dissociates from both R* and T beta gamma and diffuses through the cytoplasm to activate PDE. In suspensions of bovine rod outer segments, time-resolved microcalorimetry yields rates of approximately 1-2 s-1 for the GTPase of T alpha and the correlated deactivation of PDE. But for isolated T alpha-GTP the single turnover GTPase rate measured by a stopped-flow technique is only 0.05 s-1. To activate PDE, T alpha-GTP binds tightly to the PDE gamma subunit. In vitro the soluble T alpha-GTP.PDE gamma complex dissociates from activated PDE alpha beta. Thus PDE gamma might be the GTPase activator of T alpha, but no GTPase acceleration was observed in isolated T alpha-GTP.PDE gamma. The GTPase activation must depend on the interaction of T alpha-GTP.PDE gamma with membrane-bound PDE alpha beta.