Nucleolus

About: Nucleolus is a research topic. Over the lifetime, 5873 publications have been published within this topic receiving 232435 citations. The topic is also known as: GO:0005730 & cell nucleolus.

Silver staining, immunofluorescence, and immunoelectron microscopic localization of nucleolar phosphoproteins B23 and C23

Abstract: Nucleolar organizer region (NOR)-specific silver staining and immunolocalization of nucleolar phosphoproteins B23 and C23 were compared in Novikoff hepatoma ascites cells. Silver staining and protein C23 immunostaining were both localized in the fibrillar shell surrounding the fibrillar center and in the fibrillar center. During mitosis, silver staining and protein C23 were localized at the NORs. Therefore, protein C23 and the silver-staining protein both seem to be associated with rDNA-containing structures (Mirre and Stahl 1981). A comparison of toluidine blue staining specific for RNA and B23 immunostaining demonstrated that protein B23 was associated with RNA-containing regions of the nucleolus and was absent from the fibrillar centers. Localization of these proteins and their functions are discussed in relation to the organization of the nucleolus.

Nucleolar KKE/D repeat proteins Nop56p and Nop58p interact with Nop1p and are required for ribosome biogenesis.

Abstract: Different point mutations in the nucleolar protein fibrillarin (Nop1p in Saccharomyces cerevisiae) can inhibit different steps in ribosome synthesis. A screen for mutations that are synthetically lethal (sl) with the nop1-5 allele, which inhibits pre-rRNA processing, identified NOP56. An independent sl mutation screen with nop1-3, which inhibits pre-rRNA methylation, identified a mutation in NOP58. Strikingly, Nop56p and Nop58p are highly homologous (45% identity). Both proteins were found to be essential and localized to the nucleolus. A temperature-sensitive lethal mutant allele, nop56-2, inhibited many steps in pre-rRNA processing, particularly on the pathway of 25S/5.8S rRNA synthesis, and led to defects in 60S subunit assembly. Epitope-tagged constructs show that both Nop56p and Nop58p are associated with Nop1p in complexes, Nop56p and Nop1p exhibiting a stoichiometric association. These physical interactions presumably underlie the observed sl phenotypes. Well-conserved homologs are present in a range of organisms, including humans (52% identity between human hNop56p and yeast Nop56p), suggesting that these complexes have been conserved in evolution.

Accurate Processing of a Eukaryotic Precursor Ribosomal RNA by Ribonuclease MRP in Vitro

Abstract: Very few of the enzymes required for eukaryotic precursor ribosomal RNA (pre-rRNA) processing have been identified. Ribonuclease (RNase) MRP was characterized as a nuclease that cleaves mitochondrial replication primers, but it is predominantly nucleolar. Previous genetic evidence revealed that this ribonucleoprotein is required, directly or indirectly, for cleavage of the yeast pre-rRNA in vivo at site A3. Here, an in vitro processing system that accurately reproduces this cleavage is described. Biochemical purification and the use of extracts depleted of the MRP RNA demonstrate that endonucleolytic cleavage of the pre-rRNA is directly mediated by RNase MRP. This establishes a role for RNase MRP in the nucleolus.

The many facets of H/ACA ribonucleoproteins.

Abstract: The H/ACA ribonucleoproteins (RNPs) are known as one of the two major classes of small nucleolar RNPs They predominantly guide the site-directed pseudouridylation of target RNAs, such as ribosomal and spliceosomal small nuclear RNAs In addition, they process ribosomal RNA and stabilize vertebrate telomerase RNA Taken together, the function of H/ACA RNPs is essential for ribosome biogenesis, pre-mRNA splicing, and telomere maintenance Every cell contains 100-200 different species of H/ACA RNPs, each consisting of the same four core proteins and one function-specifying H/ACA RNA Most of these RNPs reside in nucleoli and Cajal bodies and mediate the isomerization of specific uridines to pseudouridines Catalysis of the reaction is mediated by the putative pseudouridylase NAP57 (dyskerin, Cbf5p) Unexpectedly, mutations in this housekeeping enzyme are the major determinants of the inherited bone marrow failure syndrome dyskeratosis congenita This review details the many diverse functions of H/ACA RNPs, some yet to be uncovered, with an emphasis on the role of the RNP proteins The multiple functions of H/ACA RNPs appear to be reflected in the complex phenotype of dyskeratosis congenita

Subnuclear shuttling of human telomerase induced by transformation and DNA damage.

Abstract: The telomerase ribonucleoprotein complex caps chromosome ends by adding telomeric repeats. Here we show that catalytically active human telomerase has a regulated intranuclear localization that is dependent on the cell-cycle stage, transformation and DNA damage. In primary cell lines, low expression of a fusion protein of green fluorescent protein and telomerase reverse transcriptase (GFP-hTERT) increases telomerase activity and stabilizes the maintenance of telomere length. Confocal microscopy shows that the release of telomerase to the nucleoplasm from sequestration at nucleolar sites is enhanced at the expected time of telomere replication. By contrast, in tumour and transformed cells, there is an almost complete dissociation of telomerase from nucleoli at all stages of the cell cycle. Transfection of the simian virus 40 genome into a primary cell line is sufficient to mobilize telomerase from nucleoli to the nucleoplasm. Conversely, ionizing radiation induces the reassociation of telomerase with nucleoli in both primary and transformed cells. These findings show that transformation and DNA damage have opposite effects on the cellular regulation of active telomerase, affecting the enzyme's access to both telomeric and nontelomeric substrates.