Kinetic Mechanism of RGS9-1 Potentiation by R9AP

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Abstract
The duration of the photoreceptor's response to a light stimulus determines the speed at which an animal adjusts to ever-changing conditions of the visual environment. One critical component which regulates the photoresponse duration on the molecular level is the complex between the ninth member of the regulators of G protein signaling family (RGS9-1) and its partner, type 5 G protein β-subunit (Gβ5L). RGS9-1·Gβ5L is responsible for the activation of the GTPase activity of the photoreceptor-specific G protein, transducin. Importantly, this function of RGS9-1·Gβ5L is regulated by its membrane anchor, R9AP, which drastically potentiates the ability of RGS9-1·Gβ5L to activate transducin GTPase. In this study, we address the kinetic mechanism of R9AP action and find that it consists primarily of a direct increase in the RGS9-1·Gβ5L activity. We further showed that the binding site for RGS9-1·Gβ5L is located within the N-terminal putative trihelical domain of R9AP, and even though this domain is sufficient for binding, it takes the entire R9AP molecule to potentiate the activity of RGS9-1·Gβ5L. The mechanism revealed in this study is different from and complements another well-established mechanism of regulation of RGS9-1·Gβ5L by the effector enzyme, cGMP phosphodiesterase, which is based entirely on the enhancement in the affinity between RGS9-1·Gβ5L and transducin. Together, these mechanisms ensure timely transducin inactivation in the course of the photoresponse, a requisite for normal vision.