Analysis by two-dimensional polyacrylamide gel electrophoresis of the in vivo phosphorylation of ribosomal proteins derived from free and membrane-bound polysomes

Phosphorylation controls binding of acidic proteins to the ribosome

The replacement of each one of the eight serine residues present in the amino acid sequence of the Saccharomyces cerevisiae acidic ribosomal phosphoprotein YP2 beta (L45) by different amino acids has been performed by heteroduplex site-directed mutagenesis in the cloned gene. The mutated DNA was used to transform a yeast strain previously deprived of the original protein YP2 beta (L45) by gene disruption. The replacement of serine in position 19 by either alanine, aspartic acid, or threonine prevents in vivo phosphorylation of the protein and its interaction with the ribosome. The serine-19 mutated gene is unable to rescue the negative effect on the growth rate caused by elimination of the original protein in YP2 beta (L45) gene disrupted strains. The mutation of any one of the other seven serine residues has no effect on either the phosphorylation or the ribosome binding capacity of the protein, although replacement of serine-72 seems to increase the sensitivity of the polypeptide to degradation. These results provide strong evidence indicating that ribosomal protein phosphorylation plays an important part in the activity of the particle and that it supports the existence of a control mechanism of protein synthesis, which would regulate the level of phosphorylation of acidic proteins.

Starvation-induced alterations of ribosomal protein phosphorylation in Bombyx mori L. Evidence for different phosphorylation kinetics in free and membrane-bound ribosomes

In the posterior silk gland of Bombyx mori, ribosomal protein S1, homologous to S6 in mammals, is partially phosphorylated in a normally fed animal. Before the first meal of the fifth larval instar, S1 is completely dephosphorylated. Likewise, starvation induces rapid dephosphorylation of the protein in both free and membrane-bound ribosomes. Upon refeeding after 48 h of starvation, S1 becomes phosphorylated again, first on membrane-bound ribosomes, then on free ribosomes, with a lag time of about 3 h. Following 48 h of refeeding, the most highly phosphorylated form of S1 predominates in both populations of ribosomes. These variations in phosphorylation are correlated with the level of protein synthesis in the posterior silk gland, 70% of the ribosomes occurring in polysomes upon feeding and only 30% upon starvation [Prudhomme, J.-C. & Couble, P. (1979) Biochimie (Paris) 61, 215-227]. After in vivo 32P labelling, the phosphopeptides of S1 from free and membrane-bound ribosomes were found to be identical and phosphoserine (only) was found in each S1. These results suggest the involvement of S1 phosphorylation in the regulation of protein synthesis at the translational level and the existence of at least two different pathways controlling this phosphorylation: one for the free ribosomes, the other for the membrane-bound ribosomes.

Number and molecular weights of the basic proteins of rat liver ribosomes

1) Efficient separation of the proteins from rat liver ribosomes can by achieved by two-dimensional polyacrylamide gel electrophoresis. Complete separation of all components, however, is not possible with one system only. Comparison of the results obtained with different systems suggests further heterogeneity of S15, L22, L28, L33 and L35 and enables identification of S15a, S15b, L22a, L22b, L28a, L28b, L33, L33a, L35a and L36b. 2) Ribosomal proteins were substituted with iodoacetamide prior to electrophoresis or handled in all steps of the procedure in the presence of reducing agents. These procedures prevent the formation of oxidation products described erroneously as ribosomal proteins S5, S6, L15, L17 and L32 in earlier papers. 3) Estimation of the molecular weights was performed by two-dimensional separation of the small and large subunit proteins using sodium dodecyl sulphate in the second dimension. The positions of the 70 basic proteins in the 2-D patterns were identified. 4) The small and large subunit proteins have molecular weights in the range of 8000 to 35,000 and 11,000 to 55,500 Dalton, respectively. The number average molecular weights for the small and large subunit proteins are 22,500 and 26,500 Dalton, respectively. The sum of the molecular weights is 0.67 x 10(6) Dalton for the proteins of the small subunit and 1.05 x 10(6) Dalton for the proteins of the large subunit.

Influence of 9-beta-D-arabinofuranosyladenine on total protein synthesis and on differential gene expression of unique proteins in the rodent malarial parasite Plasmodium berghei

The antibiotic 9-beta-D-arabinofuranosyladenine is a drug with a broad spectrum of activity against animal viruses, with little or no effect on mammalian cells, when administered in vivo or in vitro. Here we report that the antibiotic markedly inhibited the incorporation of [35S]methionine into malarial protein. Inhibition was apparent when the parasites were either exposed to the drug in vivo during the course of infection or incubated with the drug in vitro. Moreover, the antibiotic induced pronounced changes in the spectrum of proteins synthesized. Some proteins that are prominently apparent in the control disappear from the drug-treated parasites; others specific for drug-treated parasites appear, indicating changes in the commitment for gene expression as manifested by the appearance of the final protein product. Proteins synthesized were analyzed by two-dimensional polyacrylamide gels; the first dimension used isoelectric focusing in cylinder gels and the second dimension used electrophoresis in a lithium dodecyl sulfate slab gel. Proteins were visualized by radioautography.

Comparison of the protein content of free and membrane-bound rat liver polysomes and of the derived subunits

Ribosomal proteins from pure free and membrane-bound rat liver polysomes were analyzed with a highly resolutive two-dimensional gel electrophoresis technique, using sodium dodecyl sulfate in the second dimension. Three acidic proteins found in free polysomes were always absent from the membrane-bound polysomes. Their molecular weights were estimated to be 20 000, 19 500 and 18 500. When free ribosomes were dissociated into subunits, the three protein spots were still found in the 60S subunit pattern, but they were weaker than in polysomes. A possible involvement of these three proteins in the attachment of ribosomal structures to the membranes is proposed.