REGULATION OF G-PROTEIN-COUPLED RECEPTOR KINASE-5 (GRK5) BY ACTIN

Gr protein-coupled receptor kinases (GRKs) initiate pathways leading t o the desensitization of agonist-occupied G-protein-coupled receptors (GPCRs), Here we report that the cytoskeletal protein actin binds and inhibits GRK5, Actin inhibits the kinase activity directly, reducing G RK5-mediated phosphorylation of both membrane-bound GPCRs and soluble substrates, GRK5 binds actin monomers with a K-d of 0.6 mu M and actin filaments with a K-d of 0.2 mu M. Mutation of 6 amino acids near the amino terminus of GRK5 eliminates actin-mediated inhibition of GRK5. C almodulin has previously been shown to bind to the amino terminus of G RK5 (Pronin, A. N., and Benovic, J. L. (1997) J. Biol, Chem. 272, 3806 -3812) and here we show calmodulin displaces GRK5 from actin. Calmodul in inhibits GRK5-mediated phosphorylation of GPCRs, but not soluble su bstrates such as casein. Thus in the presence of actin, calmodulin det ermines the substrate specificity of GRK5 by preferentially allowing p hosphorylation of soluble substrates over membrane-bound substrates.