Metabotropic glutamate receptors (mGluRs) are G protein-coupled receptors (GPCRs) that can be desensitized following prolonged exposure to agonist. Dhami et al. investigated the mechanism by which GPCR kinases (GRKs) contribute to desensitization of mGluR1a and mGluR1b (a splice variant with a shorter COOH-terminal cytoplasmic tail). When expressed in human embryonic kidney cells (HEK 293), wild-type mGluR1a is robustly phosphorylated in the presence and absence of heterologously expressed GRK2. Expression of a deletion mutant of mGluR1a lacking the last 333 amino acids (mGluR1Δ866) or expression of mGluR1b with GRK2 demonstrated that these two proteins were not phosphorylated in the presence or absence of agonist despite the overexpression of the kinase. However, the ability of agonist to stimulate inositol phosphate production was attenuated by overexpression of GRK2. Also, GRK2 coprecipitated with both the wild-type receptor, mGluR1Δ866, and mGluR1b. A kinase-inactive mutant of GRK2 also promoted desensitization of the mGluR proteins, supporting the concept that phosphorylation is not essential for GRK2-mediated desensitization of these receptors. An NH2-terminal truncation mutant of GRK2 containing only the regulator of G protein signaling (RGS) domain was effective at inhibiting the receptors and could be coprecipitated with the receptors, indicating that this region is responsible for mediating the phosphorylation-independent desensitization.
G. K. Dhami, P. H. Amborgh, L. B. Dale, R. Sterne-Marr, S. S. G. Ferguson, Phosphorylation-independent regulation of metabotropic glutamate receptor signaling by G protein-coupled receptor kinase 2. J. Biol. Chem. 277, 25266-25272 (2002). [Abstract] [Full Text]