RGS-R, A RETINAL SPECIFIC RGS PROTEIN, BINDS AN INTERMEDIATE CONFORMATION OF TRANSDUCIN AND ENHANCES RECYCLING

G proteins regulate intracellular signaling by coupling a cycle of gua nine nucleotide binding and hydrolysis to transient changes of cellula r functions. The mechanisms that control the recycling of transducin, the ''pacesetting'' G protein that regulates mammalian phototransducti on, are unclear. We show that a novel retinal specific RGS-motif prote in specifically binds to an intermediate conformation involved in GTP hydrolysis by transducin and accelerates phosphate release and the rec ycling of transducin. This specific interaction further rationalizes t he kinetics of the phototransduction cascade and provides a general hy pothesis to explain the mechanism of interaction of RGS proteins with other G proteins.