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

Regulators of G protein signaling (RGS) proteins accelerate GTP hydrolysis by G alpha subunits, thereby attenuating signaling. RGS4 is a GTPase-activa ting protein for G(i) and G(q) class alpha subunits. In the present study, we used knockouts of G(q) class genes in mice to evaluate the potency and s electivity of RGS4 in modulating Ca2+ signaling transduced by different G(q )-coupled receptors. RGS4 inhibited phospholipase C activity and Ca2+ signa ling in a receptor-selective manner in both permeabilized cells and cells d ialyzed with RGS4 through a patch pipette. Receptor-dependent inhibition of Ca2+ signaling by RGS4 was observed in acini prepared from the rat and mou se pancreas. The response of mouse pancreatic acini to carbachol was about 4- and 33-fold more sensitive to RGS4 than that of bombesin and cholecystok inin (CCK), respectively. RGS1 and RGS16 were also potent inhibitors of G(q )-dependent Ca2+ signaling and acted in a receptor-selective manner. RGS1 s howed approximately 1000-fold higher potency in inhibiting carbachol than C CK-dependent signaling. RGS16 was as effective as RGS1 in inhibiting carbac hol-dependent signaling but only partially inhibited the response to CCK. B y contrast, RGS2 inhibited the response to carbachol and CCK with equal pot ency. The same pattern of receptor-selective inhibition by RGS4 was observe d in acinar cells from wild type and several single and double G(q) class k nockout mice. Thus, these receptors appear to couple G(q) class alpha subun it isotypes equally. Difference in receptor selectivity of RGS proteins act ion indicates that regulatory specificity is conferred by interaction of RG S proteins with receptor complexes.