Modules in the photoreceptor RGS9-1 center dot G(beta 5L) GTPase-accelerating protein complex control effector coupling, GTPase acceleration, proteinfolding, and stability

RGS (regulators of G protein signaling proteins regulate G protein signalin g by accelerating GTP hydrolysis, but little is known about regulation of G TPase-accelerating protein (GAP) activities or roles of domains and subunit s outside the catalytic cores. RGS9-1 is the GAP required for rapid recover y of light responses in vertebrate photoreceptors and the only mammalian RG S protein with a defined physiological function. It belongs to an RGS subfa mily whose members have multiple domains, including G gamma -like domains t hat bind G(beta5) proteins. Members of this subfamily play important roles in neuronal signaling, Within the GAP complex organized around the RGS doma in of RGS9-1, we have identified a functional role for the G gamma -like-G( beta 5L) complex in regulation of GAP activity by an effector subunit, cGMP phosphodiesterase gamma and in protein folding and stability of RGS9-1, Th e C-terminal domain of RGS9-1 also plays a major role in conferring effecto r stimulation. The sequence of the RGS domain determines whether the sign o f the effector effect will be positive or negative. These roles were observ ed in, vitro using full-length proteins or fragments for RGS9-1, RGS7, G(be ta 5S), and G(beta 5s), The dependence of RGS9-1 on Gp, co-expression for f olding, stability, and function has been confirmed in vivo using transgenic Xenopus laevis, These results reveal how multiple domains and regulatory p olypeptides work together to fine tune G(t alpha) inactivation.