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.

Subcellular compartmentalization of adeno-associated virus type 2 assembly.

Abstract: Using immunofluorescence and in situ hybridization techniques, we studied the intracellular localization of adeno-associated virus type 2 (AAV-2) Rep proteins, VP proteins, and DNA during the course of an AAV-2/ adenovirus type 2 coinfection. In an early stage, the Rep proteins showed a punctate distribution pattern over the nuclei of infected cells, reminiscent of replication foci. At this stage, no capsid proteins were detectable. At later stages, the Rep proteins were distributed more homogeneously over the nuclear interior and finally became redistributed into clusters slightly enriched at the nuclear periphery. During an intermediate stage, they also appeared at an interior part of the nucleolus for a short period, whereas most of the time the nucleoli were Rep negative. AAV-2 DNA colocalized with the Rep proteins. All three capsid proteins were strongly enriched in the nucleolus in a transient stage of infection, when the Rep proteins homogeneously filled the nucleoplasm. Thereafter, they became distributed over the whole nucleus and colocalized in nucleoplasmic clusters with the Rep proteins and AAV-2 DNA. While VP1 and VP2 strongly accumulated in the nucleus, VP3 was almost equally distributed between the nucleus and cytoplasm. Capsids, visualized by a conformationspecific antibody, were first detectable in the nucleoli and then spread over the whole nucleoplasm. This suggests that nucleolar components are involved in initiation of capsid assembly whereas DNA packaging occurs in the nucleoplasm. Expression of a transfected full-length AAV-2 genome followed by adenovirus infection showed all stages of an AAV-2/adenovirus coinfection, whereas after expression of the cap gene alone, capsids were restricted to the nucleoli and did not follow the nuclear redistribution observed in the presence of the whole AAV-2 genome. Coexpression of Rep proteins released the restriction of capsids to the nucleolus, suggesting that the Rep proteins are involved in nuclear redistribution of AAV capsids during viral infection. Capsid formation was dependent on the concentration of expressed capsid protein.

Identification and characterization of a yeast nucleolar protein that is similar to a rat liver nucleolar protein.

Abstract: We have produced monoclonal antibodies against purified nuclei from the yeast Saccharomyces cerevisiae and have characterized three different antibodies that recognize a protein with an apparent molecular weight of 38,000, termed p38. Subcellular fractionation shows that virtually all of p38 occurs in the nuclear fraction. High concentrations of salt (1 M) or urea (6 M) effectively solubilize p38 from a nuclear envelope fraction prepared by digestion of nuclei with DNase. Indirect immunofluorescence demonstrates a crescent shaped distribution of p38 at the inner periphery of the nucleus, with p38 extending between dividing pairs of cells during (closed) mitosis. Postembedding immunogold electron microscopy shows decoration of the densely stained "crescent" region of the yeast nucleus, confirming the localization of p38 to the nucleolus. One of the monoclonals, D77, cross reacts on immunoblots with a single protein of molecular weight 37,000 from purified rat liver nuclei. Indirect immunofluorescence localizes this protein to the nucleolus, and shows that it is dispersed throughout the cell during mitosis. The yeast and rat liver nucleolar proteins behave similarly when electrophoresed in two dimensions, and appear to have basic pI values. Analysis of immunological cross-reactivity using D77, and antibodies specific for nucleolar proteins from other sources, suggests that the rat liver protein is fibrillarin, and demonstrates that p38 shares epitopes with fibrillarin, as well as with other vertebrate nucleolar proteins.

Clustering of yeast tRNA genes is mediated by specific association of condensin with tRNA gene transcription complexes

Abstract: The 274 tRNA genes in Saccharomyces cerevisiae are scattered throughout the linear maps of the 16 chromosomes, but the genes are clustered at the nucleolus when compacted in the nucleus. This clustering is dependent on intact nucleolar organization and contributes to tRNA gene-mediated (tgm) silencing of RNA polymerase II transcription near tRNA genes. After examination of the localization mechanism, we find that the chromosome-condensing complex, condensin, is involved in the clustering of tRNA genes. Conditionally defective mutations in all five subunits of condensin, which we confirm is bound to active tRNA genes in the yeast genome, lead to loss of both pol II transcriptional silencing near tRNA genes and nucleolar clustering of the genes. Furthermore, we show that condensin physically associates with a subcomplex of RNA polymerase III transcription factors on the tRNA genes. Clustering of tRNA genes by condensin appears to be a separate mechanism from their nucleolar localization, as microtubule disruption releases tRNA gene clusters from the nucleolus, but does not disperse the clusters. These observations suggest a widespread role for condensin in gene organization and packaging of the interphase yeast nucleus.

Autoantibody to RNA polymerase I in scleroderma sera.

Abstract: Autoantibodies to components of the nucleolus are a unique serological feature of patients with scleroderma. There are autoantibodies of several specificities; one type produces a speckled pattern of nucleolar staining in immunofluorescence. In actinomycin D and 5,6-dichloro-beta-D-ribofuranosylbenzimidazole-treated Vero cells, staining was restricted to the fibrillar and not the granular regions. By double immunofluorescence, specific rabbit anti-RNA polymerase I antibodies stained the same fibrillar structures in drug-segregated nucleoli as scleroderma sera. Scleroderma sera immunoprecipitated 13 polypeptides from [35S]methionine-labeled HeLa cell extract with molecular weights ranging from 210,000 to 14,000. Similar polypeptides were precipitated by rabbit anti-RNA polymerase I antibodies, and their common identities were confirmed in immunoabsorption experiments. Microinjection of purified IgG from a patient with speckled nucleolar staining effectively inhibited ribosomal RNA transcription. Autoantibodies to RNA polymerase I were restricted to certain patients with scleroderma and were not found in other autoimmune diseases.

What the nucleolus says to a tumour pathologist

Abstract: The importance of nucleolar changes in cancer cells is underestimated in tumour pathology. There is evidence that the nucleolus is the mirror of a series of metabolic changes that characterize cancer cells. Cell entry into the cell cycle is always associated with up-regulation of the nucleolar function and increased nucleolar size, which are also directly dependent on the rapidity of cell cycle progression. Furthermore, alterations of the major tumour suppressor retinoblastoma (Rb) and p53 pathways also contribute to the stimulation of nucleolar function and to nucleolar enlargement. High cell growth fraction, high cell growth rate and disruption of the Rb and p53 pathways are responsible for greater aggressiveness of cancer tissues. Therefore, the evaluation of nucleolar size allows one to obtain reliable information on the clinical outcome of the cancer: the greater the nucleolar size, the worse the tumour prognosis. Indeed, a series of studies carried out on numerous human tumours has shown that nucleolar hypertrophy (prominent nucleolus) was an independent predictive and prognostic parameter of a fatal clinical outcome.