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.

Association of the Herpes Simplex Virus Type 1 Us11 Gene Product with the Cellular Kinesin Light-Chain-Related Protein PAT1 Results in the Redistribution of Both Polypeptides

Abstract: The herpes simplex virus type 1 (HSV-1) Us11 gene encodes a multifunctional double-stranded RNA (dsRNA)-binding protein that is expressed late in infection and packaged into the tegument layer of the virus particle. As a tegument component, Us11 associates with nascent capsids after its synthesis late in the infectious cycle and is delivered into newly infected cells at times prior to the expression of viral genes. Us11 is also an abundant late protein that regulates translation through its association with host components and contains overlapping nucleolar retention and nuclear export signals, allowing its accumulation in both nucleoli and the cytosol. Thus, at various times during the viral life cycle and in different intracellular compartments, Us11 has the potential to execute discrete tasks. The analysis of these functions, however, is complicated by the fact that Us11 is not essential for viral replication in cultured cells. To discover new host targets for the Us11 protein, we searched for cellular proteins that interact with Us11 and have identified PAT1 as a Us11-binding protein according to multiple, independent experimental criteria. PAT1 binds microtubules, participates in amyloid precursor protein trafficking, and has homology to the kinesin light chain (KLC) in its carboxyl terminus. The carboxyl-terminal dsRNA-binding domain of Us11, which also contains the nucleolar retention and nuclear export signals, binds PAT1, whereas 149 residues derived from the KLC homology region of PAT1 are important for binding to Us11. Both PAT1 and Us11 colocalize within a perinuclear area in transiently transfected and HSV-1-infected cells. The 149 amino acids derived from the KLC homology region are required for colocalization of the two polypeptides. Furthermore, although PAT1 normally accumulates in the nuclear compartment, Us11 expression results in the exclusion of PAT1 from the nucleus and its accumulation in the perinuclear space. Similarly, Us11 does not accumulate in the nucleoli of infected cells that overexpress PAT1. These results establish that Us11 and PAT1 can associate, resulting in an altered subcellular distribution of both polypeptides. The association between PAT1, a cellular trafficking protein with homology to KLC, and Us11, along with a recent report demonstrating an interaction between Us11 and the ubiquitous kinesin heavy chain (R. J. Diefenbach et al., J. Virol. 76:3282-3291, 2002), suggests that these associations may be important for the intracellular movement of viral components.

Saccharomyces cerevisiae nucleolar protein Nop7p is necessary for biogenesis of 60S ribosomal subunits.

Abstract: To identify new gene products that participate in ribosome biogenesis, we carried out a screen for mutations that result in lethality in combination with mutations in DRS1, a Saccharomyces cerevisiae nucleolar DEAD-box protein required for synthesis of 60S ribosomal subunits. We identified the gene NOP7that encodes an essential protein. The temperature-sensitive nop7-1 mutation or metabolic depletion of Nop7p results in a deficiency of 60S ribosomal subunits and accumulation of halfmer polyribosomes. Analysis of pre-rRNA processing indicates that nop7 mutants exhibit a delay in processing of 27S pre-rRNA to mature 25S rRNA and decreased accumulation of 25S rRNA. Thus Nop7p, like Drs1p, is required for essential steps leading to synthesis of 60S ribosomal subunits. In addition, inactivation or depletion of Nop7p also affects processing at the A0, A1, and A2 sites, which may result from the association of Nop7p with 35S pre-rRNA in 90S pre-rRNPs. Nop7p is localized primarily in the nucleolus, where most steps in ribosome assembly occur. Nop7p is homologous to the zebrafish pescadillo protein necessary for embryonic development. The Nop7 protein contains the BRCT motif, a protein-protein interaction domain through which, for example, the human BRCA1 protein interacts with RNA helicase A.

Reversible disassembly of somatic nucleoli by the germ cell proteins FRGY2a and FRGY2b

Abstract: Egg cytoplasm has the capability to reprogramme differentiated somatic nuclei, as shown by nuclear transplantation in animal cloning 1 , 2 . The nucleoli of donor nuclei are rapidly disassembled on injection into interphase eggs and are correctly reassembled when donor transcription initiates in the early embryos of frogs and mammals, recapitulating the physiological nucleolar dynamics of early embryogenesis 3 - 6 . This is one of the most remarkable structural reorganizations of somatic nuclei in nuclear cloning. Despite the long history of nuclear cloning, almost nothing is known about the molecular mechanism of nucleolar disassembly in egg cytoplasm. Here we show that the Xenopus germ cell proteins FRGY2a and FRGY2b 7 - 9 reversibly disassemble somatic nucleoli in egg cytoplasm, independently of continuing ribosomal RNA transcription. The carboxy-terminal domain of FRGY2a, which localizes to the nucleoli, is sufficient for nucleolar disassembly in transfected cells. Our results show that a single protein fragment can trigger reversible disassembly of the complex nucleolar structure.

The nucleolar architecture of polymerase i transcription and processing

Abstract: The nucleolus, the site of transcription and processing of the major ribosomal genes, generally reveals three distinct ultrastructural components in conventional thin-section electron micrographs (fibrillar centres, dense fibrillar component and granular component) We show here that different parts of the transcription and transcript processing pathway can be mapped to the different nucleolar components in pea root cells This study shows the full three-dimensional arrangement of the different domains by in situ hybridization and confocal microscopy, and their correspondence with the major ultrastructural components of the nucleolus is revealed by parallel serial section electron microscopy The active rDNA is widely dispersed in discrete foci, the larger of which, at least, correspond to well-defined fibrillar centres A probe to the external transcribed spacer (ETS) sequence of the pre-rRNA transcripts labels clearly demarcated regions surrounding the foci of rDNA, and which we show correspond to the dense fibrillar component Finally, a probe to the entire 45S transcript shows a higher concentration in regions corresponding to the granular component, surrounding the dense fibrillar component labelled by the ETS probe The changes in structure that occur with heat shock show that nucleolar organization is dynamic and dependent upon transcriptional activity These results show that the various RNA processing events are spatially highly organized and suggest a vectorial or radial model of transcription and transcript processing, where nascent and newly completed transcripts occupy zones surrounding the genes, which are in turn surrounded by regions containing the older more mature transcripts