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

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 o f photoresponse in vertebrate rods and cones. In photoreceptors, the slow i ntrinsic rate of transducin GTPase is accelerated by the complex of the nin th member of the regulators of G protein signaling family with the long spl ice variant of type 5 G protein beta subunit (RGS9-G beta 5L). However, phy siologically rapid GTPase is observed only when transducin forms a complex with its effector, the gamma subunit of cGMP phosphodiesterase (PDE gamma), In this study, we addressed the mechanism by which PDE gamma regulates the rate of transducin GTPase, We found that RGS9-G beta 5L alone has a signif icant ability to activate transducin GTPase, but its affinity for transduci n is low. PDEy acts by enhancing the affinity between activated transducin and RGS9-G beta 5L by more than 15-fold, which is evident both from kinetic measurements of transducin GTPase rate and from protein binding assays wit h immobilized transducin. Furthermore, our data indicate that a single RGS9 -G beta 5L molecule is capable of accelerating the GTPase activity of simil ar to 100 transducin molecules/s. This rate is faster than the rates report ed previously for any RGS protein and is sufficient for timely photorecepto r recovery in both rod and cone photoreceptors.