Tyrosine phosphorylation of G-protein-coupled-receptor kinase 2 (GRK2) by c-Src modulates its interaction with Galphaq.

G-protein-coupled-receptor kinase 2 (GRK2) plays a key role in the modulation of G-protein-coupled-receptor (GPCR) signaling by both phosphorylating agonist-occupied GPCRs and by directly binding to activated Galphaq subunits, inhibiting downstream effectors activation. The GRK2/Galphaq interaction involves the N-terminal region of the kinase that displays homology to regulators of G-protein signaling (RGS) proteins. We have previously reported that upon GPCR stimulation, GRK2 can be phosphorylated by c-Src on tyrosine residues that are present in the RGS-homology (RH) region of this kinase. Here, we demonstrate that c-Src kinase activity increases the interaction between GRK2 and Galphaq. Tyrosine phosphorylation of GRK2 appears to be critically involved in the modulation of this interaction since the stimulatory effect of c-Src is not observed with a GRK2 mutant with impaired tyrosine phosphorylation (GRK2 Y13,86,92F), whereas a mutant that mimics GRK2 tyrosine phosphorylation in these residues displays an increased interaction with Galphaq. As evidence for a physiological role of this modulatory mechanism, activation of the muscarinic receptor M1, a Galphaq-coupled receptor, promotes an increase in GRK2/Galphaq co-immunoprecipitation that parallels the enhanced GRK2 phosphorylation on tyrosine residues. Moreover, c-Src activation enhances inhibition of the Galphaq/phospholipase Cbeta signaling pathway in intact cells, in a GRK2-tyrosine-phosphorylation-dependent manner. Our results suggest a feedback mechanism by which phosphorylation of GRK2 by c-Src increases both GRK2 kinase activity towards GPCRs and its specific interaction with Galphaq subunits, leading to a more rapid switch off of Galphaq-mediated signaling.