Studies on the structure and function of ribosomes from regenerating rat 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.

Differences between newly formed and recycled free small ribosome subunits in liver cytoplasm

Heterogeneity among the free small ribosomal subunits of rat liver was studied. Newly made free small subunits differ from the bulk of the free subunits in the following ways. (1) The newly made free small subunits sediment slightly faster than most free small subunits and form a band at slightly lower densities. (2) Mature free small subunits, prepared under conditions which minimize the amount of loosely bound protein, readily dimerize in solutions of low ionic strength. Little dimerization is found with newly formed subunits. This permits the separation of fractions enriched in newly formed subunits. (3) The fraction enriched in newly formed subunits contains a distinctive additional protein as compared the most of the free small subunits.