Abstract

Signaling by the platelet-derived growth factor receptor-β (PDGFRβ) is diminished when the PDGFRβ is phosphorylated on seryl residues by G protein-coupled receptor kinase-5 (GRK5), but mechanisms for GRK5 activation by the PDGFRβ remain obscure. We therefore tested whether the PDGFRβ is able to tyrosine-phosphorylate and thereby activate GRK5. Purified GRK5 was tyrosine-phosphorylated by the wild-type PDGFRβ to a stoichiometry of 0.8 mol phosphate/mol GRK5, an extent ∼5 times greater than observed with a Y857F PDGFRβ mutant that fails to phosphorylate exogenous substrates but autophosphorylates and activates Src normally. The degree of PDGFRβ-mediated phosphorylation of GRK5 correlated with GRK5 activity, as assessed by seryl phosphorylation of the PDGFRβ 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 β₂-adrenergic receptor, even though β₂-adrenergic receptor activation promoted tyrosyl phosphorylation of GRK5 in smooth muscle cells. Phosphorylation of the PDGFRβ by GRK5 in smooth muscle cells or in purified protein preparations reduced PDGFRβ-mediated peptide phosphorylation. In contrast, phosphorylation of GRK5 by the PDGFRβ 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 PDGFRβ, but not by the β₂-adrenergic receptor, and that by activating GRK5, the PDGFRβ triggers its own desensitization.