The core domain of a new retina specific RGS protein stimulates the GTPase activity of transducin in vitro

GTP hydrolysis by the transducin a subunit is stimulated by a membrane-bound protein. The identity of this GTPase-activating protein (GAP) is not yet known, but the recent identification of a new gene family encoding regulator of G protein signaling (RGS) proteins raises the possibility that the transducin GAP is an RGS protein. Biochemical evidence shows that RGS proteins act as GAPs for alpha subunits of the Gi subfamily of G proteins. To identify an RGS protein that could be a GAP for the transducin alpha subunit, we investigated the expression of RGS proteins in the retina and identified a new RGS domain, RET-RGS-d, which is specifically expressed in the retina. In situ RNA hybridization analyses revealed that RET-RGS-d is expressed in photoreceptor cells as well as in other cells of the retina. Recombinant RET-RGS-d accelerates single turnover hydrolysis of GTP by transducin. We used RET-RGS-d to isolate a full-length cDNA, RET-RGS1, encoding a new RGS protein with a C terminus that corresponds to RET-RGS-d. The N-terminal half of RET-RGS1 contains a putative transmembrane domain and a string of nine cysteines that are potential substrates for multiple palmitoylation. These findings suggest that RET-RGS1 is an integral membrane protein and that it is a candidate for the membrane-associated protein responsible for the GAP activity detected in photoreceptor membranes.