Abstract
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.
Users
Please
log in to take part in the discussion (add own reviews or comments).