G Protein-Coupled Receptors

The M3-Muscarinic Receptor Is Phosphorylated and Regulated by Casein Kinase 2

Science's STKE  17 Apr 2007:
Vol. 2007, Issue 382, pp. tw134
DOI: 10.1126/stke.3822007tw134

After binding to ligands, almost all G protein-coupled receptors (GPCRs) are rapidly phosphorylated--a modification with important functional consequences, such as desensitization and internalization. Many of the kinases involved in the phosphorylation of GPCRs are members of the family of GPCR kinases (GRKs). GPCRs are often phosphorylated at multiple sites and by more than one kinase, so the regulation of GPCRs by phosphorylation is quite complex. Torrecilla et al. have extended their previous studies by examining the role of casein kinase 2 (CK2) in the phosphorylation and regulation of the M3-muscarinic receptor. Both knockdown of CK2 with siRNA and blocking of its activity with specific inhibitors reduced the amount of phosphorylation of the M3-muscarinic receptor in CHO cells, as measured by immunoprecipitation assays. The use of mutant M3-muscarinic receptors showed that the sites phosphorylated by CK2 were in the third intracellular loop. This region is also targeted by GRK2 and GRK6, although further analysis showed that the specific sites phosphorylated by CK2 and GRK2 or -6 did not significantly overlap. Inhibitors of CK2 reduced the phosphorylation of the M3-muscarinic receptor in mouse cerebellar granule (CG) neurons. GRK-mediated phosphorylation causes internalization of the M3-muscarinic receptor, but CK2-mediated phosphorylation had no such effect. Whereas knockdown of CK2 did not change the ability of the M3-muscarinic receptor to activate extracellular signal-regulated kinase 1 and 2 (ERK1/2), activation of the c-Jun pathway was increased. Analysis of phosphopeptides from digested M3-muscarinic receptors from either CHO cells or CG neurons revealed different patterns of phosphorylation, implying that the phosphorylation of a given receptor is cell type-specific, potentially leading to different functional outcomes.

I. Torrecilla, E. J. Spragg, B. Poulin, P. J. McWilliams, S. C. Mistry, A. Blaukat, A. B. Tobin, Phosphorylation and regulation of a G protein-coupled receptor by protein kinase CK2. J. Cell Biol. 177, 127-137 (2007). [Abstract] [Full Text]