Regulation of G protein-mediated signal transduction by RGS proteins

Abstract

RGS proteins form a new family of regulatory proteins of G protein signaling. They contain homologous core domains (RGS domains) of about 120 amino acids. RGS domains interact with activated Gα subunits. Several RGS proteins have been shown biochemically to act as GTPase activating proteins (GAPs) for their interacting Gα subunits. Other than RGS domains, RGS proteins differ significantly in size, amino acid sequences, and tissue distribution. In addition, many RGS proteins have other protein-protein interaction motifs involved in cell signaling. We have shown that p115RhoGEF, a newly identified GEF(guanine nucleotide exchange factor) for RhoGTPase, has a RGS domain at its N-terminal region and this domain acts as a specific GAP for Gα12 and Gα13. Furthermore, binding of activated Gα13 to this RGS domain stimulated GEF activity of p115RhoGEF. Activated Gα12 inhibited Gα13-stimulated GEF activity. Thus p115RhoGEF is a direct link between heterotrimeric G protein and RhoGTPase and it functions as an effector for Gα12 and Gα13 in addition to acting as their GAP. We also found that RGS domain at N-terminal regions of G protein receptor kinase 2 (GRK2) specifically interacts with Gαq/11 and inhibits Gαq-mediated activation of PLC-β, apparently through sequestration of activated Gαq. However, unlike other RGS proteins, this RGS domain did not show significant GAP activity to Gαq. These results indicate that RGS proteins have far more diverse functions than acting simply as GAPs and the characterization of function of each RGS protein is crucial to understand the G protein signaling network in cells.