RGS proteins determine signaling specificity of G(q)-coupled receptors

Abstract

Regulators of G protein signaling (RGS) proteins accelerate GTP hydrolysis by Gα subunits, thereby attenuating signaling. RGS4 is a GTPase- activating protein for G(i) and G(q) class α subunits. In the present study, we used knockouts of Gq class genes in mice to evaluate the potency and selectivity of RGS4 in modulating Ca2+ signaling transduced by different G(q)-coupled receptors. RGS4 inhibited phospholipase C activity and Ca2+ signaling in a receptor-selective manner in both permeabilized cells and cells dialyzed with RGS4 through a patch pipette. Receptor-dependent inhibition of Ca2+ signaling by RGS4 was observed in acini prepared from the rat and mouse pancreas. The response of mouse pancreatic acini to carbachol was about 4- and 33-fold more sensitive to RGS4 than that of bombesin and cholecystokinin (CCK), respectively. RGS1 and RGS16 were also potent inhibitors of G(q)-dependent Ca2+ signaling and acted in a receptor- selective manner. RGS1 showed approximately 1000-fold higher potency in inhibiting carbachol than CCK-dependent signaling. RGS16 was as effective as RGS1 in inhibiting carbachol-dependent signaling but only partially inhibited the response to CCK. By contrast, RGS2 inhibited the response to carbachol and CCK with equal potency. The same pattern of receptor-selective inhibition by RGS4 was observed in acinar cells from wild type and several single and double G(q) class knockout mice. Thus, these receptors appear to couple G(q) class α subunit isotypes equally. Difference in receptor selectivity of RGS proteins action indicates that regulatory specificity is conferred by interaction of RGS proteins with receptor complexes.