Reciprocal Regulation of the Platelet-Derived Growth Factor Receptor-β and G Protein-Coupled Receptor Kinase 5 by Cross-Phosphorylation: Effects on Catalysis

Signaling by the platelet-derived growth factor receptor-beta (PDGFR beta) is diminished when the PDGFR beta is phosphorylated on seryl residues by G protein-coupled receptor kinase-5 (GRK5), but mechanisms for GRK5 activation by the PDGFR beta remain obscure. We therefore tested whether the PDGFR beta is able to tyrosine-phosphorylate and thereby activate GRK5. Purified GRK5 was tyrosine-phosphorylated by the wild-type PDGFR beta to a stoichiometry of 0.8 mol phosphate/mol GRK5, an extent similar to 5 times greater than observed with a Y857F PDGFR beta mutant that fails to phosphorylate exogenous substrates but autophosphorylates and activates Src normally. The degree of PDGFR beta-mediated phosphorylation of GRK5 correlated with GRK5 activity, as assessed by seryl phosphorylation of the PDGFR beta in purified protein preparations, in intact cells expressing a tyrosine-to-phenylalanine GRK5 mutant, and in GRK5 peptide phosphorylation assays. However, tyrosyl phosphorylation of GRK5 was not necessary for GRK5-mediated phosphorylation of the beta(2)-adrenergic receptor, even though beta(2)-adrenergic receptor activation promoted tyrosyl phosphorylation of GRK5 in smooth muscle cells. Phosphorylation of the PDGFR beta by GRK5 in smooth muscle cells or in purified protein preparations reduced PDGFR beta-mediated peptide phosphorylation. In contrast, phosphorylation of GRK5 by the PDGFR beta enhanced the V-max of GRK5-mediated peptide phosphorylation, by 3.4-fold, without altering the GRK5 K-M for peptide. We conclude that GRK5 tyrosyl phosphorylation is required for the activation of GRK5 by the PDGFR beta, but not by the beta(2)-adrenergic receptor, and that by activating GRK5, the PDGFR beta triggers its own desensitization.