Agonist-dependent phosphorylation of the G protein-coupled receptor kinase2 (GRK2) by Src tyrosine kinase

GRK2 is a member of the G protein-coupled receptor kinase (GRK) family, whi ch phosphorylates the activated form of a variety of G protein-coupled rece ptors (G;PCR) and plays an important role in GPCR modulation. It has been r ecently reported that stimulation of the mitogen-activated protein kinase c ascade by GPCRs involves tyrosine phosphorylation of docking proteins media ted by members of the Src tyrosine kinase family. In this report, we have i nvestigated the possible role of c-Src in modulating GRK2 function. We demo nstrate that c-Src can directly phosphorylate GRK2 on tyrosine residues, as shown by in vitro experiments with purified proteins. The phosphorylation reaction exhibits an apparent K-m for GRK2 of 12 nM, thus suggesting a phys iological relevance in living cells. Consistently, overexpression of the co nstitutively active c-Src Y527F mutant in COS-7 cells leads to tyrosine pho sphorylation of coexpressed GRK2, In addition, GRK2 can be detected in phos photyrosine immunoprecipitates from HEK-293 cells transiently transfected w ith this Src mutant. Interestingly, phosphotyrosine immunoblots reveal a ra pid and transient increase in GRK2 phosphorylation upon agonist stimulation of beta(2)-adrenergic receptors co-transfected with GRK2 and wild type c-S rc in COS-7 cells. This tyrosine phosphorylation is maximal within 5 min of isoproterenol stimulation and reaches values of similar to 5-fold over bas al conditions. Furthermore, GRK2 phosphorylation on tyrosine residues promo tes an increased kinase activity toward its substrates. Our results suggest that GRK2 phosphorylation by c-Src is inherent to GPCR activation and put forward a new mechanism for the regulation of GPCR signaling.