 In particular, GRKs phosphorylate intracellular domains of G protein-coupled receptors (GPCRs).GRKs function in tandem with arrestin proteins to regulate the sensitivity of GPCRs for stimulating downstream heterotrimeric G protein and G protein-independent signaling pathways.GRKs reside normally in an inactive state, but their kinase activity is stimulated by binding to a ligand-activated GPCR (rather than by regulatory phosphorylation as is common in other AGC kinases).