Endogenous phosphorylation of ribosomal proteins from membrane-free rat liver polysomes

Beta 2-adrenergic receptor stimulated, G protein-coupled receptor kinase 2 mediated, phosphorylation of ribosomal protein P2

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 beta2-adrenergic receptor (beta2AR) 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 beta2AR 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 beta2AR 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 K(M) values approximating 1 microM. 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.

hCG-Increased phosphorylation of proteins in primary culture of Leydig cells: further characterization

Previous studies using monodimensional gel electrophoresis of total cell proteins have shown that hCG was able to increase the phosphorylation of at least 6 proteins in cultured porcine Leydig cells. By using subcellular fractionation and 2D gel electrophoresis, we now show that most of these proteins, whose phosphorylation was increased by hCG, are basic nuclear proteins. In addition, hCG also increases the phosphorylation of microsomal proteins in particular ribosomal protein S6. Moreover no phosphoproteins were detected in the cytosol.

Protein kinase activity tightly bound to liver polysomes

Rat liver polysomes washed with 0.5-1.5 M KCl at 37 degrees C keep a constant protein kinase activity revealed only by auto-phosphorylation of ribosomal proteins. The enzyme catalyzes the transfer of the gamma-phosphate group from ATP to serine (75%) but also to threonine residues (25%). It is released when polysomes are dissociated into subunits using centrifugation through a sucrose gradient containing a high K+/Mg++ ratio. Its properties have been compared with those of the two other enzymatic activities which are, in contrast, washed out during salt treatment of polysomes. After release upon polysome dissociation, this third activity is able to phosphorylate histone II A. Protection of the enzyme in the polysome structure against salt treatment, suggests that it is located at the junction of the two subunits.

Spot position of rat liver ribosomal proteins by four different two-dimensional electrophoreses in polyacrylamide gel

Separation of the proteins from rat liver 40S and 60S ribosomal subunits and polysomes was done in four different two-dimensional polyacrylamide gel electrophoresis systems. The first dimension was run at acidic or basic pH, the second dimension either with sodium dodecyl sulphate or at acidic pH in 18% acrylamide. The position of each individual protein of both subunits and polysomes was determined in each system. This identification resulted from a new method avoiding any pervious purification of individual proteins. The new "proposed uniform nomenclature for mammalian ribosomal proteins" (McConkey et al. in press) was used for numbering the proteins in the four systems.

Occurrence of phosphorylated forms of an acidic protein in the large ribosomal subunit of rat liver

An acidic protein from rat liver 60-S ribosomal subunits was selectively extracted with 50% ethanol. It was revealed as three different spots by two-dimensional gel electrophoresis, two of them being attributable to phosphorylated forms since they disappeared after alkaline phosphatase treatment. The relationship between this protein and similar acidic proteins found in eucaryotic cells is discussed.