Allosteric regulation of GAP activity by phospholipids in regulators of G-protein signaling

Regulators of G-protein signaling (RGS) proteins are GTPase-activating proteins (GAPs) for α subunits of the G _i andor G _q class of heterotrimeric G proteins. RGS GAP activity is inhibited by phosphatidic acid (PA), lysophosphatidic acid (LPA), and phosphatidylinositol 3,4,5-trisphosphate (PIP ₃) but not by other phospholipids, phosphoinositides, or diacylglycerol. Both PA and PIP ₃ can inhibit RGS4 GAP activity and their inhibition is additive, suggesting that PA and PIP ₃ interact with different domains of RGS4. The N terminus of RGS4 (1-57 amino acids) is required for PA binding and inhibition. Mutation at Lys20, far from the RGS domain of RGS4, decreases PA-mediated inhibition of RGS4 by more than 85%. Amino acid substitutions in helix 5 within the RGS domain of RGS4, opposite to the RGSGα protein contact face, reduce binding affinity and inhibition by PIP ₃. Calmodulin binds all RGS proteins tested in a Ca ²⁺-dependent manner at two sites, one in the N-terminal 33 amino acids and another in the RGS domain. Ca ²⁺calmodulin does not directly affect GAP activity of RGS4 but reverses PA and PIP ₃-mediated inhibition. In summary, these results demonstrate that phospholipids such as PA and PIP ₃ act as allosteric inhibitors of RGS proteins, and Ca ²⁺calmodulin competition with PA and PIP ₃ may provide an intracellular mechanism for feedback regulation of Ca ²⁺ signaling evoked by G-protein-coupled agonists.