Assembly Of Ribosomal Proteins Research Articles

Rapamycin inhibits ribosomal protein synthesis and induces G1 prolongation in mitogen-activated T lymphocytes.

We investigated the effects of rapamycin (RAP) on cell cycle progression and protein synthesis in mitogen-activated primary T lymphocytes. Stimulation of resting human T lymphocytes with phorbol ester and calcium ionophore rendered cells capable of initiating DNA synthesis within 30 h; roughly 60% of the cells entered the first G2/M phase of the cell cycle within 96 h. Addition of RAP delayed the entry into S phase by 9 h, although a similar percentage (approximately 50%) of cells entered the first G2/M phase and proliferated. On this basis, we concluded that RAP primarily induced a G1 prolongation without blocking cell cycle progression. Addition of the co-mitogens to resting T lymphocytes up-regulated the translation of ribosomal protein mRNA concurrent with activation of p70s6k. RAP inhibited this translational up-regulation of ribosomal protein mRNA as well as the activation of p70s6k without affecting translation of nonribosomal protein mRNA. RAP also prevented the synthesis and accumulation of ribosomal proteins. Further, this failure to increase ribosomal proteins, which probably reflects the failure to increase numbers of ribosomes, resulted in suppression of the synthesis of total cellular protein and a delay in the escalation of cell size. RAP-treated cells eventually initiated DNA synthesis when cell size became equivalent to that of the control cells entering S phase of the cell cycle. Thus, inhibition of protein synthesis caused by the primary inhibition of ribosomal protein mRNA translation probably explains the effect of RAP on cell cycle progression of mitogen-activated resting T lymphocytes.

Nucleolar proteins that bind NLSs: a role in nuclear import or ribosome biogenesis?

In a search for proteins that bind nuclear localization sequences (NLSs), a number of nucleolar proteins with diverse functions were found. It is thought that the assay fortuitously uncovered a novel domain that mediates the interaction between these nucleolar proteins and ribosomal proteins containing NLS-like sequences. The domain is highly acidic and contains a number of serines forming putative casein kinase II sites. Here, we propose a model in which the nucleolar proteins catalyse the assembly of ribosomal proteins with pre-rRNA.

Depletion of yeast ribosomal proteins L16 or rp59 disrupts ribosome assembly.

Two strains of Saccharomyces cerevisiae were constructed that are conditional for synthesis of the 60S ribosomal subunit protein, L16, or the 40S ribosomal subunit protein, rp59. These strains were used to determine the effects of depriving cells of either of these ribosomal proteins on ribosome assembly and on the synthesis and stability of other ribosomal proteins and ribosomal RNAs. Termination of synthesis of either protein leads to diminished accumulation of the subunit into which it normally assembles. Depletion of L16 or rp59 has no effect on synthesis of most other ribosomal proteins or ribosomal RNAs. However, most ribosomal proteins and ribosomal RNAs that are components of the same subunit as L16 or rp59 are rapidly degraded upon depletion of L16 or rp59, presumably resulting from abortive assembly of the subunit. Depletion of L16 has no effect on the stability of most components of the 40S subunit. Conversely, termination of synthesis of rp59 has no effect on the stability of most 60S subunit components. The implications of these findings for control of ribosome assembly and the order of assembly of ribosomal proteins into the ribosome are discussed.

A constitutive nucleolar protein identified as a member of the nucleoplasmin family.

Using monoclonal antibodies we have localized a polypeptide, appearing on gel electrophoresis with a Mr of approximately 38,000 and a pI of approximately 5.6, to the granular component of the nucleoli of Xenopus laevis oocytes and a broad range of cells from various species. The protein (NO38) also occurs in certain distinct nucleoplasmic particles but is not detected in ribosomes and other cytoplasmic components. During mitosis NO38-containing material dissociates from the nucleolar organizer region and distributes over the chromosomal surfaces and the perichromosomal cytoplasm; in telophase it re-populates the forming nucleoli. With these antibodies we have isolated from a X. laevis ovary lambda gt11 expression library a cDNA clone encoding a polypeptide which, on one- and two-dimensional gel electrophoresis, co-migrates with authentic NO38. The amino acid sequence deduced from this clone defines a polypeptide of 299 amino acids of mol. wt 33,531 which is characterized by the presence of two domains exceptionally rich in aspartic and glutamic acid, one of them flanked by two putative karyophilic signal heptapeptides. Comparison with other protein sequences shows that NO38 is closely related to the histone-binding, karyophilic protein nucleoplasmin: the first 124 amino acids have 58 amino acid positions in common. Protein NO38 also shows striking homologies to the phosphopeptide region of rat nucleolar protein B23 and the carboxyterminal region of human B23. We propose that protein NO38, which forms distinct homo-oligomers of approximately 7S and Mr of approximately 230,000, is a member of a family of karyophilic proteins, the 'nucleoplasmin family'. It is characterized by its specific association with the nucleolus and might be involved in nuclear accumulation, nucleolar storage and pre-rRNA assembly of ribosomal proteins in a manner similar to that discussed for the role of nucleoplasmin in histone storage and chromatin assembly.

The Pools of Ribosomal Proteins and Ribosomal Ribonucleic Acids During Relaxed Control of Escherichia coli A19 (Hfr, rel met rns)

The soluble fraction extracted from Escherichia coli A19 (Hfr, rel met rns) during early and late times of phenotypic and genotypic induced relaxed control have been examined for the possible accumulation of ribosomal proteins (r-proteins) and rRNA species during this time of unbalanced macromolecular synthesis. Ribosomal proteins and rRNA species were not found to accumulate within the soluble fraction at any time during this period of relaxed control; even after the typical rRNA accumulation had ceased, r-proteins did not accumulate. It is concluded, from these and related observations, that the r-proteins and rRNA species known to be produced during relaxation must immediately associate to form the unusual ribonucleoprotein particles (e.g. 'relaxed particles' and 'chloramphenicol particles') characteristic of periods of relaxed control. Since r-proteins do not accumulate even when net RNA accumulation halts, it appears that some elements of the normal, basic co-ordination between rRNA and r-protein synthesis/stability persist even during relaxed control.

Synthesis and assembly of ribosomal proteins at the onset of pyrimidine starvation

Ribosomal proteins from a pyrimidine auxotroph of Escherichia coli were pulse labelled with 14C-leucine prior to and following depletion of uridine. The incorporation of label into different 70S ribosomal protein bands decreased differentially at the onset of starvation. Readdition of uridine following the 14C-leucine pulse did not alter the labelling pattern. A sequential turn-off in the synthesis of different ribosomal proteins and a tight coupling between synthesis and assembly were indicated.