RECIPROCAL REGULATION OF RGS PROTEINS BYPHOSPHOLIPID AND CA~(2+)/CALMODULIN INCARDIOMYOCYTES: IMPLICATION FOR CHOLINERGICREGULATION OF HEART RATES

摘要

G-protein-gated potassium (Kg) channels, which are directly activated by G-protein pv subunits released from pertussis toxin-sensitive G-proteins, exist on atrial myocytes, and are responsible for acetylcholine (ACh)-induced deceleration of heart beats. A characteristic gating property of native KG currents is time-dependent current increase during hyperpolarizing pulses, named "relaxation". We have previously shown that this relaxation gating is apparent voltage-dependent control of trimeric G-protein cycle by a. family of proteins, designated 'regulators of G-protein-signaling (RGS)' proteins. The voltage-dependent formation of Ca~(2+)/calmodulin (CaM) facilitated the GTPase-accelerating activity of the RGS protein by removing intrinsic inhibition mediated by a kind of phospholipid, phosphatidylmositol-3,4,5,-trisphosphate (PI(3,4,5)P3). Here we show the molecular mechanism of this reciprocal control of RGS-action. We detected the specific interaction between RGS4 and PI(3,4,5)P3, which was abolished by Ca~(2+)/CaM. This modulation exclusively occurred within RGS domain, which is responsible for GTPase-accelerating activity. We identified the cluster of positively charged residues in RGS domain, which lie just behind the residues crucial for RGS-Ga interaction, as a candidate of the molecular switch of PI(3,4,5)P_3/CaM-modulation. Because these residues are conserved in almost all RGS protein subtypes expressed in cardiac myocytes, this allosteric modulation of RGS proteins should be important in the physiological control of G-protein mediated cell signalings in the heart. Their possible roles in adrenergic- and cholinergic- regulation of cardiac functions are discussed.