The effector enzyme regulates the duration of G protein signaling in vertebrate photoreceptors by increasing the affinity between transducin and RGS protein

Abstract

The photoreceptor-specific G protein transducin acts as a molecular switch, stimulating the activity of its downstream effector in its GTP-bound form and inactivating the effector upon GTP hydrolysis. This activity makes the rate of transducin GTPase an essential factor in determining the duration of photoresponse in vertebrate rods and cones. In photoreceptors, the slow intrinsic rate of transducin GTPase is accelerated by the complex of the ninth member of the regulators of G protein signaling family with the long splice variant of type 5 G protein β subunit (RGS9·Gβ5L). However, physiologically rapid GTPase is observed only when transducin forms a complex with its effector, the γ subunit of cGMP phosphodiesterase (PDEγ). In this study, we addressed the mechanism by which PDEγ regulates the rate of transducin GTPase. We found that RGS9·Gβ5L alone has a significant ability to activate transducin GTPase, but its affinity for transducin is low. PDEγ acts by enhancing the affinity between activated transducin and RGS9·Gβ5L by more than 15-fold, which is evident both from kinetic measurements of transducin GTPase rate and from protein binding assays with immobilized transducin. Furthermore, our data indicate that a single RGS9·Gβ5L molecule is capable of accelerating the GTPase activity of ~100 transducin molecules/s. This rate is faster than the rates reported previously for any RGS protein and is sufficient for timely photoreceptor recovery in both rod and cone photoreceptors.