Regulators of G protein signaling (RGS) accelerate intrinsic GTP hydrolysis on ␣ 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 voltagedependent 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 ؉ (KG) channels. Here we report the mechanism which is the reversal by Ca 2؉ ͞calmodulin of phosphatidylinositol-3,4,5,-trisphosphate (PIP3)-mediated inhibition of RGS. Purified RGS4 protein alone inhibited GTP-induced KG channel activity in inside-out patches from atrial myocytes. The inhibitory effect of RGS4 was reduced by PIP3 and restored by addition of Ca 2؉ ͞calmodulin. The intracellular application of anti-PIP3 antibody abolished the RGS-dependent relaxation behavior of KG current in atrial myocytes. This study, therefore, reveals a general physiological control mechanism of RGS proteins by lipidprotein interaction. H eterotrimeric G proteins mediate different intracellular signaling cascades and regulate many cellular functions (1). Although numerous kinds of G protein-coupled receptors and effectors have been identified, the existence of regulators of the G protein cycle has been neglected for a long period. Recently, a family of cytosolic proteins named ''regulators of G protein signaling'' (RGS proteins) has been identified (2, 3). These proteins share the conserved ''RGS domain'' of Ϸ120 aa which is responsible for accelerating GTPase activity on the G protein ␣ subunit (4-6). RGS proteins are thought to play a central role in the physiological regulation of the G protein cycle, and their importance has been confirmed in the immune response (7) and sensory perception (8, 9). To date, more than 20 mammalian RGS proteins have been identified. These RGS proteins vary in their molecular structure, tissue distribution, and intracellular localization, and thus may play divergent functional roles in different tissues.We recently found that RGS proteins were responsible not only for the acceleration of the deactivation time course upon agonist washout (10), but also for the characteristic gating behavior of G protein-activated inward-rectifier K ϩ channels (K G ) in cardiac atrial myocytes (11-13). K G channels are directly activated by the ␥ subunits released from pertussis toxinsensitive G proteins upon receptor stimulation, and contribute to acetylcholine (ACh)-induced deceleration of heart beat and neurotransmitter-evoked slow inhibitory postsynaptic potentials in neurons (14). When membrane potential is suddenly hyperpolarized from positive potentials, the K G current in myocytes first instantaneously and then slowly increases. The latter timedependent current change is termed ''relaxation'' and is characteristic for native K G current (15). ...