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.

p53 -Dependent and -Independent Nucleolar Stress Responses.

Abstract: The nucleolus has emerged as a cellular stress sensor and key regulator of p53-dependent and -independent stress responses. A variety of abnormal metabolic conditions, cytotoxic compounds, and physical insults induce alterations in nucleolar structure and function, a situation known as nucleolar or ribosomal stress. Ribosomal proteins, including RPL11 and RPL5, become increasingly bound to the p53 regulatory protein MDM2 following nucleolar stress. Ribosomal protein binding to MDM2 blocks its E3 ligase function leading to stabilization and activation of p53. In this review we focus on a number of novel regulators of the RPL5/RPL11-MDM2-p53 complex including PICT1 (GLTSCR2), MYBBP1A, PML and NEDD8. p53-independent pathways mediating the nucleolar stress response are also emerging and in particular the negative control that RPL11 exerts on Myc oncoprotein is of importance, given the role of Myc as a master regulator of ribosome biogenesis. We also briefly discuss the potential of chemotherapeutic drugs that specifically target RNA polymerase I to induce nucleolar stress.

Nucleolar targeting signal of human T-cell leukemia virus type I rex-encoded protein is essential for cytoplasmic accumulation of unspliced viral mRNA.

Abstract: The posttranscriptional regulator (rex) of human T-cell leukemia virus type I is known to be located predominantly in the cell nucleolus and to induce the accumulation of gag and env viral mRNAs. The N-terminal 19 amino acids of rex-encoded protein (Rex) has been shown to be sufficient to direct hybrid proteins to the cell nucleolus. We have studied the function of the nucleolar targeting signal (NOS) of rex by using full-length proviral DNA and mutant rex expression plasmids. Partial deletions of the NOS sequence abolished the accumulation of unspliced cytoplasmic mRNA, although the gene products of rex mutants were found in the nucleoplasm. These results indicate that NOS sequence, or nucleolar localization of Rex, is essential for Rex function.

Heterochromatin and the organisation of nucleoli in plants

Abstract: Low temperature treatment, with certain plant and animal species, allows a linear differentiation of somatic metaphase and anaphase chromosomes other than that which is characteristic at normal temperatures. Under the conditions of cold, heterochromatic parts become differentiated as segments of reduced diameter and, as shown with Feulgen, having a reduced nucleic acid charge (Darlington and La Cour, 1940, 1941 ; Geitler, io ; Callan, 1942 ; Haga, i; Haga and Kurabayashi, 1948). The chromomere-like differentiation obtained with special fixation methods by Shmargon (1938) in Secale and by Kakhidze (i) in Crepis capillaris, has been compared with the differentiation produced by this nucleic acid starvation (Wilson and Boothroyd, 1.4). There is, however, no resemblance either in the cause or in the effect. The segments starved of nucleic acid at metaphase agree in number and position with the segments overcharged with nucleic acid in the resting stage. Both were therefore defined as heterochromatin in a strict sense by Darlington and La Cour (ix). It should be noted that Heitz (1928, 1932) originally described heterochromatin as any part of the chromosome which was overstained during the resting stage. Since Heitz and others did not always use Feulgen staining some bodies may have been described by them as heterochromatin which were not overcharged with nucleic acid and therefore not heterochromatin in our stricter sense. Contrary to our findings, Wilson and Boothroyd (i) believe that revelation of the heterochromatic segments is dependent on a differential contraction of heterochromatin and euchromatin. Our microphotographs as well as camera lucida drawings, show clearly that this is not so. Such a view is also in conflict with that of Resende et al. ('iA) who have shown in Trillium sessile that the segments begin to differentiate at 9° C. And moreover, that with supercontraction of the chromosomes which happens below —6° C., the differentiation of the segments is in no way modified. However, these workers assume the segments to be either of nucleolar origin (which they clearly are not), or of an organisation intermediate between heterochromatin and nucleolar organiser (which

14 The Diverse World of Small Ribonucleoproteins

Abstract: Among the major players in the functioning of present-day eukaryotic cells are small complexes consisting of RNA and protein. Small ribonucleoprotein (RNP) particles, defined as tight complexes between one or more proteins and a small RNA molecule (chain length ≤ ~300 nucleotides), come in a variety of shapes and sizes. Since they can be very abundant (up to 10 7 copies per mammalian cell, as many as ribosomes) and are often highly conserved from yeast to man, deciphering their cellular roles has been an important challenge for molecular biologists. Small RNPs inhabit all cellular compartments that have been examined: the nucleoplasm, the nucleoli, the cytoplasm, and the mitochondria. Further classification of small RNPs (Table 1) has been made possible by the discovery that they are often targeted by autoantibodies found in the sera of patients with rheumatic disease, such as systemic lupus erythematosus. Usually the autoepitopes reside on the protein rather than the RNA moieties of small RNPs, meaning that particles of the same class possess common polypeptide constituents. The autoantibodies also provide potent tools for probing the structures and functions of small RNPs. The vast majority of small RNPs whose functions have been deciphered play roles somewhere along the pathway of gene expression (see Table 1). For example, small n uclear RNPs (snRNPs) of the nucleoplasm are involved in pre-mRNA processing or tRNA biogenesis. Small nucleolar RNPs contribute to the maturation of rRNA. S mall RNPs of the c ytoplasm (scRNPs) function in the control of translation or disposition of newly synthesized proteins.