Phosphorylation-independent inhibition of parathyroid hormone receptor signaling by G protein-coupled receptor kinases

Homologous desensitization of G protein-coupled receptors is thought to occur in several steps: binding of G protein-coupled receptor kinases (GRKs) to receptors, receptor phosphorylation, kinase dissociation, and finally binding of beta-arrestins to phosphorylated receptors. It generally is assumed that only the last step inhibits receptor signaling. Investigating the parathyroid hormone (PTH) receptor --> inositol phosphate pathway, we report here that GRKs can inhibit receptor signaling already under nonphosphorylating conditions. GRKs phosphorylated the PTH receptor in membranes and in intact cells; the order of efficacy was GRK2>GRK3>GRK5. Transient transfection of GRKs with the PTH receptor into COS-1 cells inhibited PTH-stimulated inositol phosphate generation. Such an inhibition also was seen with the kinase-negative mutant GRK2-K220R and also for a C-terminal truncation mutant of the PTH receptor that could not be phosphorylated. Several lines of evidence indicated that this phosphorylation-independent inhibition was exerted by an interaction between GRKs and receptors: (a) this inhibition was not mimicked by proteins binding to G proteins, phosducin, and GRK2 C terminus, (b) GRKs caused an agonist-dependent inhibition (= desensitization) of receptor-stimulated G protein GTPase-activity (this effect also was seen with the kinase-inactive GRK2-mutant and the phosphorylation-deficient receptor mutant), and (c) GRKs bound directly to the PTH receptor. These data suggest that signaling by the PTH receptor already is inhibited by the first step of homologous desensitization, the binding of GRKs to the receptors.