Turnover of ribosomal proteins in regenerating rat liver after partial hepatectomy

The rates of synthesis and degradation of ribosomal proteins, prelabelled with [14C]bicarbonate, were determined as an index of the rate of ribosome turnover in regenerating rat liver. The half-life of ribosomes is about 178 and 75 hr in regenerating and normal liver, respectively. The comparison of turnover rates of ribosomal proteins with the corrected values of rRNA, based on re-utilization of nucleotides, suggests that ribosomes are metabolized as a unit in vivo. There is at least 70% overestimation for ribosome half-life when orotate-labelled RNA is used for turnover determinations. The absolute rate of synthesis is estimated as 3925 and 1081 ribosomes/min per cell in 24 hr regenerating and normal rat liver, respectively.     

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

Electrophoresis of ribosomal proteins from 1-day regenerating and sham-operated rat livers, doubly-labeled with two iodine isotopes, revealed that the relative stoichiometric ratios of proteins S6 from the 40S subunit and L5, L18 and L27 from the 60S subunit, vary significantly from those obtained with the other proteins. The ratios of the altered proteins return to normal within three days after partial hepatectomy. Polypeptide chain elongation and binding of aminoacyl-tRNA were slightly greater with regenerating preparations than with control preparation, when 80S ribosomes were used, but differences were not observed when the experiments were carried out with purified subunits and exogenous template.     

Phosphorylation of nuclear proteins in rat regenerating liver.

In studies of the phosphorylated proteins in rat liver and Walker-256, it was established that the ratio of various fractions of P-N linkages to P-O linkages varies from 0.6 to 3.1. In rat regenerating liver nuclei, the ratio of P-N and P-O varies with time after partial hepatectomy. Using [3H]-lysine and 32Pi, it is shown that phosphoryllysine forms in some new and, presumably, some preexisting H1 molecules. Using [3H]histidine and 32Pi, it is shown that phosphohistidine forms exclusively in preexisting H4. The half-life of H4 phosphohistidine appears to be about 2 h.     

Yeast ribosomal proteins. VIII. Isolation of two proteins and sequence characterization of twenty-four proteins from cytoplasmic ribosomes

Summary Two proteins, YL41 and YL43, were isolated from 80S ribosomes of Saccharomyces cerevisiae by filtration through a Sephacryl S-200 column and by chromatography on a column of carboxymethylcellulose. Their amino acid compositions are presented. Twenty-four proteins including these two proteins were subjected to sequence analyses by automated Edman degradation. Amino-terminal amino acid sequences were determined for 17 proteins, YS3, YS9, YS23, YS24, YS29, YL6, YL8, YL11, YL15, YL17, YL23, YL28, YL33, YL37, YL39, YL41, and YL43. YL41, which has a 72.7% lysine and arginine content, was found to be particular to eukaryotic ribosomes. The aminotermini of another seven proteins, YS2, YS5, YS8, YS12, YS13, YS20, and YS27, were suggested to be blocked.Comparison of the amino-terminal sequences with all other ribosomal protein sequences so far available indicates that YS9 shows sequence homology to rat liver ribosomal protein S8 (Wittmann-Liebold et al. 1979).     

Preferential biosynthesis of ribosomal structural proteins by free and loosely bound polysomes from regenerating rat liver.

Homologous cell-free systems were prepared using free, total bound, tightly bound or KCl-sensitive loosely bound (KCl-sensitive) polysomes from regenerating rat liver. [14C]Leucine was incubated with one kind of polysomes and [3H]leucine with another kind. The reaction mixtures were then combined, and ribosomal structural proteins were purified as described previously [4], using two-dimensional polyacrylamide gel electrophoresis as the final step [5]. The 3H to 14C ratios of the purified fractions were estimated to compare the activities of the two kinds of polysomes for biosynthesis of ribosomal structural proteins. The following results were obtained: (1) The activity of free polysomes for biosynthesis of ribosomal structural proteins was about 3.6 or 2.4 times higher than that of total bound polysomes in two experiments in which 14C and 3H labeling was reversed. The radioactivities incorporated by free polysomes into most of the proteins separated on two-dimensional gel were found to be definitely higher than those in the surrounding areas, suggesting that most of the ribosomal structural proteins were synthesized by free polysomes. The activity of free polysomes for biosynthesis of ribosomal structural proteins was about 7 times higher than that of tightly bound polysomes, which were prepared by washing the microsomal membrane fraction with 0.5 M KCl. The radioactivities incorporated by tightly bound polysomes into the proteins separated on two-dimensional gel were only slightly higher than those in the surrounding areas, indicating that these polysomes had very low synthetic activity. (2) Preferential synthesis of histones by free polysomes was also shown using the same procedures. (3) KCl-sensitive polysomes which were released by washing the microsomal membrane fraction with 0.5 M KCl, were shown to have definitely higher activity than tightly bound polysomes for biosynthesis of ribosomal structural proteins. (4) From these results, it is concluded that most of the ribosomal structural proteins are preferentially synthesized by free and KCl-sensitive polysomes in regenerating rat liver.