β2-Adrenergic Receptor Stimulated, G Protein-Coupled Receptor Kinase 2 Mediated, Phosphorylation of Ribosomal Protein P2

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Abstract
G protein-coupled receptor kinases are well characterized for their ability to phosphorylate and desensitize G protein-coupled receptors (GPCRs). In addition to phosphorylating the β2-adrenergic receptor (β2AR) and other receptors, G protein-coupled receptor kinase 2 (GRK2) can also phosphorylate tubulin, a nonreceptor substrate. To identify novel nonreceptor substrates of GRK2, we used two-dimensional gel electrophoresis to find cellular proteins that were phosphorylated upon agonist-stimulation of the β2AR in a GRK2-dependent manner. The ribosomal protein P2 was identified as an endogenous HEK-293 cell protein whose phosphorylation was increased following agonist stimulation of the β2AR under conditions where tyrosine kinases, PKC and PKA, were inhibited. P2 along with its other family members, P0 and P1, constitutes a part of the elongation factor-binding site connected to the GTPase center in the 60S ribosomal subunit. Phosphorylation of P2 is known to regulate protein synthesis in vitro. Further, P2 and P1 are shown to be good in vitro substrates for GRK2 with KM values approximating 1 μM. The phosphorylation sites in GRK2-phosphorylated P2 are identified (S102 and S105) and are identical to the sites known to regulate P2 activity. When the 60S subunit deprived of endogenous P1 and P2 is reconstituted with GRK2-phosphorylated P2 and unphosphorylated P1, translational activity is greatly enhanced. These findings suggest a previously unrecognized relationship between GPCR activation and the translational control of gene expression mediated by GRK2 activation and P2 phosphorylation and represent a potential novel signaling pathway responsible for P2 phosphorylation in mammals.