Abstract

Many Regulators of G proteinSignaling (RGS) proteins accelerate the intrinsic GTPase activity of G_iα and G_qα-subunits [i.e., behave as GTPase-activating proteins (GAPs)] and several act as G_qα-effector antagonists. RGS3, a structurally distinct RGS member with a unique N-terminal domain and a C-terminal RGS domain, and an N-terminally truncated version of RGS3 (RGS3CT) both stimulated the GTPase activity of G_iα (except G_zα) and G_qα but not that of G_sα or G_12α. RGS3 and RGS3CT had G_qα GAP activity similar to that of RGS4. RGS3 impaired signaling through G_q-linked receptors, although RGS3CT invariably inhibited better than did full-length RGS3. RGS3 potently inhibited G_qαQ209L- and G_11αQ209l-mediated activation of a cAMP-response element-binding protein reporter gene and G_qαQ209L induced inositol phosphate production, suggesting that RGS3 efficiently blocks G_qα from activating its downstream effector phospholipase C-β. Whereas RGS2 and to a lesser extent RGS10 also inhibited signaling by these GTPase-deficient G proteins, other RGS proteins including RGS4 did not. Mutation of residues in RGS3 similar to those required for RGS4 G_iα GAP activity, as well as several residues N terminal to its RGS domain impaired RGS3 function. A greater percentage of RGS3CT localized at the cell membrane than the full-length version, potentially explaining why RGS3CT blocked signaling better than did full-length RGS3. Thus, RGS3 can impair Gi- (but not Gz-) and Gq-mediated signaling in hematopoietic and other cell types by acting as a GAP for G_iα and G_qα subfamily members and as a potent G_qα subfamily effector antagonist.