Ca2+/Calmodulin Reverses Phosphatidylinositol 3,4,5-Trisphosphate-dependent Inhibition of Regulators of G Protein-signaling GTPase-activating Protein Activity*

Regulators of G protein signaling (RGS proteins) are GTPase-activating proteins (GAPs) for Gi and/or Gq class G protein α subunits. RGS GAP activity is inhibited by phosphatidylinositol 3,4,5-trisphosphate (PIP3) but not by other lipid phosphoinositides or diacylglycerol. Both the negatively charged head group and long chain fatty acids (C16) are required for binding and inhibition of GAP activity. Amino acid substitutions in helix 5 within the RGS domain of RGS4 reduce binding affinity and inhibition by PIP3 but do not affect inhibition of GAP activity by palmitoylation. Conversely, the GAP activity of a palmitoylation-resistant mutant RGS4 is inhibited by PIP3. Calmodulin binds all RGS proteins we tested in a Ca2+-dependent manner but does not directly affect GAP activity. Indeed, Ca2+/calmodulin binds a complex of RGS4 and a transition state analog of Gαi1-GDP-AlF4 −. Ca2+/calmodulin reverses PIP3-mediated but not palmitoylation-mediated inhibition of GAP activity. Ca2+/calmodulin competition with PIP3 may provide an intracellular mechanism for feedback regulation of Ca2+ signaling evoked by G protein-coupled agonists.

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