The beta-adrenergic receptor kinase (beta-ARK), which specifically phosphorylates only the agonist-occupied form of the beta-adrenergic and closely related receptors, appears to be important in mediating rapid agonist-specific (homologous) desensitization. The structure of this enzyme was elucidated by isolating clones from a bovine brain complementary DNA library through the use of oligonucleotide probes derived from partial amino acid sequence. The beta-ARK cDNA codes for a protein of 689 amino acids (79.7 kilodaltons) with a protein kinase catalytic domain that bears greatest sequence similarity to protein kinase C and the cyclic adenosine monophosphate (cyclic AMP)--dependent protein kinase. When this clone was inserted into a mammalian expression vector and transfected into COS-7 cells, a protein that specifically phosphorylated the agonist-occupied form of the beta 2-adrenergic receptor and phosphorylated, much more weakly, the light-bleached form of rhodopsin was expressed. RNA blot analysis revealed a messenger RNA of four kilobases with highest amounts in brain and spleen. Genomic DNA blot analysis also suggests that beta-ARK may be the first sequenced member of a multigene family of receptor kinases.

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G protein-coupled receptor kinase (GRK)-mediated receptor phosphorylation and ␤-arrestin binding uncouple G protein-coupled receptors (GPCRs) from their respective G proteins and initiates the process of receptor internalization. In the case of the ␤ 2 -adrenergic receptor and lysophosphatidic acid receptor, these processes can lead to ERK activation. Here we identify a novel mechanism whereby the activity of GRK2 is regulated by feedback inhibition. GRK2 is demonstrated to be a phosphoprotein in cells. Mass spectrometry and mutational analysis localize the site of phosphorylation on GRK2 to a carboxyl-terminal serine residue (Ser 670 ). Phosphorylation at Ser 670 impairs the ability of GRK2 to phosphorylate both soluble and membrane-incorporated receptor substrates and dramatically attenuates G␤␥-mediated activation of this enzyme. Ser 670 is located in a peptide sequence that conforms to an ERK consensus phosphorylation sequence, and in vitro, in the presence of heparin, ERK1 phosphorylates GRK2. Inhibition of ERK activity in HEK293 cells potentiates GRK2 activity, whereas, conversely, ERK activation inhibits GRK2 activity. The discovery that ERK phosphorylates and inactivates GRK2 suggests that ERK participates in a feedback regulatory loop. By negatively regulating GRK-mediated receptor phosphorylation, ␤-arrestin-mediated processes such as Src recruitment and clathrin-mediated internalization, which are required for GPCR-mediated ERK activation, are inhibited, thus dampening further ERK activation.