Phosphorylation and desensitization of G protein-coupled receptors : Regulation by G protein betagamma subunits.

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Muscarinic acetylcholine receptor m2 subtypes (m2 receptors) are phosphorylated by G protein-coupled receptor kinase (GRK) in an agonist-dependent manner. We have located the phosphorylation sites in the central part of intracellular loop (I3) of m2 receptors. Seven to eight serine and three to four threonine residues in this region were phosphorylated. The phosphorylation of I3-GST (glutathione S-transferase) fusion protein by GRK was found to be synergistically stimulated by G protein betagamma subunits and mastoparan. In addition, the phosphorylation of I3-GST was found to be stimulated by carbamylcholine in the presence of m2 or I3-deleted m2 receptors. These results indicate that the agonist-bound m2 receptors serve both as substrates and activators of GRK.The interaction of m2 receptors with G proteins, Gi or Go, in in vitro reconstitution system of purified proteins was not affected by phosphorylation of m2 receptors by GRK or by deletion of I3 from m2 receptors. Agonist-dependent phosphorylation in vivo of m2 receptors expressed in COS7 cells was stimulated by coexpressed GRK2 and was inhibited by coexpressed dominant negative form of GRK2. In addition, the sequestration of m2 receptors was found to be facilitated by GRK2 and attenuated by dominant negative form of GRK2.