The phosphorylation of rat liver ribosomes following administration of dimethylnitrosamine

The protein synthetic activity in vitro of ribosomes differing in the extent of phosphorylation of their ribosomal proteins

We describe a re-examination of the cell-free protein synthetic activity of eukaryotic ribosomes having proteins phosphorylated to different extents. Ribosomal 40 S subunits were isolated both from a variety of cells in which there is relatively little phosphorylation of ribosomal protein S6, and from cells subjected in vivo to different stimuli that promote the extensive phosphorylation of protein S6. The ability of these subunits to bind Met-tRNA as well as the second amino acyl-tRNA (Val-tRNA) was compared in the presence of highly purified initiation factors, elongation factor EF-1 at various concentrations of 60S subunits, 9 S globin mRNA and potassium ions. The ability of the subunits to synthesize polyphenylalanine was also studied using highly purified elongation factors. In no case was any significant difference in activity observed between ribosomes with protein S6 phosphorylated to different extents. Similar, though less extensive, studies were preformed comparing 60 S ribosomal subunits differing in the extent of phosphorylation of the acidic phosphoprotein, L gamma , and of L14. No difference in activity was observed between these ribosomes.

Increased phosphorylation of a liver ribosomal protein in mice carrying ascites tumours

The extent of phosphorylation of ribosomal protein S6 in the livers of mice carrying the Krebs II ascites tumour has been examined by two-dimensional gel electrophoresis. Seven days after injection, an increase was observed in the amount of stained material migrating at the position of the phosphorylated derivatives of ribosomal protein S6, and there was also an increase in the proportion of ribosomes sedimenting as monosomes. The extent of phosphorylation of ribosomal protein S6 in the ascites cells themselves was low. The findings are discussed in relation to the phosphorylation of ribosomal protein S6 in transformed cells and in other pathological states of the liver.

Interaction of synthetic polynucleotides with small rat liver ribosomal subunits possessing low and highly phosphorylated protein S6

The interaction of some synthetic mRNAs (polyuridylate, polyadenylate, polycytidylate) with small rat liver ribosomal subunits which have protein S6 in different states of phosphorylation was studied by BioGel column chromatography, affinity chromatography on poly(U)-Sepharose 4B, and continuous diafiltration at 4 degrees C. 40-S subunits with low phosphorylated protein S6 (isolated from normal liver) and small subunits with highly phosphorylated protein S6 (from galactosamine-, thioacetamide-, dimethylnitrosamine-, puromycin-, and cycloheximide-treated livers) bind initially equal amounts of poly(U) but the dissociation of the radioactive polyuridylate occurs much more rapidly and to a greater extent from the low than from the highly phosphorylated type of subunits. From control- and galactosamine-4-S subunits 62% and 22%, respectively, of originally bound [3H]poly(U) was removed. The release of initially bound poly(A) from 40-S subunits of galactosamine-treated liver ws retarded but reached finally the same level as with control liver ribosomal subunits (removal of 40% of bound [3H]poly(A)). No differences between low and highly phosphorylated subunits were observed with poly(C). If the dissociation reaction was performed at 22 degrees C instead of 4 degrees C the differences in the release of poly(U) described above disappeared.

Two-dimensional electrophoretic analysis of ribosomal proteins from chronically injured liver

Proteins of the small and large ribosomal subunit, isolated at various times from long-term thioacetamide-damaged rat livers, were analysed by two-dimensional polyacrylamide gel electrophoresis and compared with those from normal liver. Chronic hepatic injury induced a number of time-dependent changes of the structural proteins of the small subunit, whereas the proteins of the large subparticle were essentially unaffected. The most significant alterations were an anodical dislocation of protein S6, a strong diminution in the amounts of proteins S9 and S10 and the occurrence of 3 to 4 additional small subunit proteins. By autoradiographic studies it was established that the modifications of S6 were brought about by an enhanced phosphorylation of this protein, which was the earliest sign of a ribosomal alteration in injured liver tissue.