PIP(3) inhibition of RGS protein and its reversal by Ca(2+)/calmodulin mediate voltage-dependent control of the G protein cycle in a cardiac K(+) channel

Regulators of G protein signaling (RGS) accelerate intrinsic GTP hydrolysis on α subunits of trimeric G proteins and play crucial roles in the physiological regulation of G protein-mediated cell signaling. The control mechanisms of the action of RGS proteins per se are poorly clarified, however. We recently showed a physiological mode of action of a RGS protein in cardiac myocytes. The voltage-dependent formation of Ca(2+)/calmodulin facilitated the GTPase activity of RGS by an unidentified mechanism, which underlay the “relaxation” behavior of G protein-gated K(+) (K(G)) channels. Here we report the mechanism which is the reversal by Ca(2+)/calmodulin of phosphatidylinositol-3,4,5,-trisphosphate (PIP(3))-mediated inhibition of RGS. Purified RGS4 protein alone inhibited GTP-induced K(G) channel activity in inside-out patches from atrial myocytes. The inhibitory effect of RGS4 was reduced by PIP(3) and restored by addition of Ca(2+)/calmodulin. The intracellular application of anti-PIP(3) antibody abolished the RGS-dependent relaxation behavior of K(G) current in atrial myocytes. This study, therefore, reveals a general physiological control mechanism of RGS proteins by lipid–protein interaction.