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Tani I, Oikawa Y, Doi S, Janairo JIB, Kamada R, Sakaguchi K. Ribosomal protein uL30 undergoes phase separation with nucleophosmin and regulates nucleolar formation in the absence of RNA. Biochem Biophys Res Commun 2023; 642:35-40. [PMID: 36543022 DOI: 10.1016/j.bbrc.2022.12.041] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/02/2022] [Accepted: 12/14/2022] [Indexed: 12/23/2022]
Abstract
The nucleolus is a membrane-less structure that exists in the nucleus of cells and plays a crucial role in ribosome biogenesis. It is known to be formed through liquid-liquid phase separation (LLPS) caused by the interaction of various nucleolar proteins and nucleic acids. Recently, many studies on LLPS with nucleolar proteins in the presence of RNA showed the importance of electrostatic interactions and cation-pi interactions among RNA and intrinsically disordered regions of proteins. However, it is reported that the initiation of nucleolar formation is RNA polymerase I-independent. The mechanism of nucleolar formation in the early stage remains obscure. In this study, we showed for the first time that the ribosomal protein uL30 and a major nucleolar protein, nucleophosmin (NPM) formed liquid droplets in vitro in the absence of RNA. The liquid droplet formation with uL30 and NPM may be derived from the interaction between the basic regions of uL30 and acidic regions of the oligomeric NPM. The knockdown of uL30 in cells significantly reduced the number of nucleoli, while it did not alter the protein level of NPM. The results showed that LLPS and nucleolar formation were affected by changes in uL30 levels. Our results suggest that the protein-protein interaction between nucleolar proteins may play an important role in nucleolar formation in the early stages when the rRNA content is very low.
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Affiliation(s)
- Itsumi Tani
- Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo, 060-0810, Japan
| | - Yui Oikawa
- Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo, 060-0810, Japan
| | - Seiyo Doi
- Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo, 060-0810, Japan
| | - Jose Isagani B Janairo
- Department of Biology, College of Science, De La Salle University, Manila, 0922, Philippines
| | - Rui Kamada
- Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo, 060-0810, Japan
| | - Kazuyasu Sakaguchi
- Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo, 060-0810, Japan.
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Kitaoka M, Smith OK, Straight AF, Heald R. Molecular conflicts disrupting centromere maintenance contribute to Xenopus hybrid inviability. Curr Biol 2022; 32:3939-3951.e6. [PMID: 35973429 PMCID: PMC9529917 DOI: 10.1016/j.cub.2022.07.037] [Citation(s) in RCA: 2] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/07/2022] [Revised: 06/15/2022] [Accepted: 07/15/2022] [Indexed: 11/22/2022]
Abstract
Although central to evolution, the causes of hybrid inviability that drive reproductive isolation are poorly understood. Embryonic lethality occurs when the eggs of the frog X. tropicalis are fertilized with either X. laevis or X. borealis sperm. We observed that distinct subsets of paternal chromosomes failed to assemble functional centromeres, causing their mis-segregation during embryonic cell divisions. Core centromere DNA sequence analysis revealed little conservation among the three species, indicating that epigenetic mechanisms that normally operate to maintain centromere integrity are disrupted on specific paternal chromosomes in hybrids. In vitro reactions combining X. tropicalis egg extract with either X. laevis or X. borealis sperm chromosomes revealed that paternally matched or overexpressed centromeric histone CENP-A and its chaperone HJURP could rescue centromere assembly on affected chromosomes in interphase nuclei. However, although the X. laevis chromosomes maintained centromeric CENP-A in metaphase, X. borealis chromosomes did not and also displayed ultra-thin regions containing ribosomal DNA. Both centromere assembly and morphology of X. borealis mitotic chromosomes could be rescued by inhibiting RNA polymerase I or preventing the collapse of stalled DNA replication forks. These results indicate that specific paternal centromeres are inactivated in hybrids due to the disruption of associated chromatin regions that interfere with CENP-A incorporation, at least in some cases due to conflicts between replication and transcription machineries. Thus, our findings highlight the dynamic nature of centromere maintenance and its susceptibility to disruption in vertebrate interspecies hybrids.
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Affiliation(s)
- Maiko Kitaoka
- Department of Molecular and Cell Biology, University of California, Berkeley, Berkeley, CA 94720-3200, USA
| | - Owen K Smith
- Department of Biochemistry, Stanford University School of Medicine, Stanford, CA 94305-5307, USA
| | - Aaron F Straight
- Department of Biochemistry, Stanford University School of Medicine, Stanford, CA 94305-5307, USA
| | - Rebecca Heald
- Department of Molecular and Cell Biology, University of California, Berkeley, Berkeley, CA 94720-3200, USA.
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Wang H, Luo X, You W, Dong Y, Ke C. Cytogenetic analysis and chromosomal characteristics of the polymorphic 18S rDNA of Haliotis discus hannai from Fujian, China. PLoS One 2015; 10:e0113816. [PMID: 25699679 PMCID: PMC4336138 DOI: 10.1371/journal.pone.0113816] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/26/2013] [Accepted: 10/31/2014] [Indexed: 11/27/2022] Open
Abstract
We report on novel chromosomal characteristics of Haliotis discus hannai from a breeding population at Fujian, China. The karyotypes of H. discus hannai we obtained from an abalone farm include a common type 2n = 36 = 10M + 8SM (82%) and two rare types 2n = 36 = 11M + 7SM (14%) and 2n = 36 = 10M + 7SM + 1ST (4%). The results of silver staining showed that the NORs of H. discus hannai were usually located terminally on the long arms of chromosome pairs 14 and 17, NORs were also sometimes located terminally on the short arms of other chromosomes, either metacentric or submetacentric pairs. The number of Ag-nucleoli ranged from 2 to 8, and the mean number was 3.61 ± 0.93. Among the scored interphase cells, 41% had 3 detectable nucleoli and 37% had 4 nucleoli. The 18S rDNA FISH result is the first report of the location of 18S rDNA genes in H. discus hannai. The 18S rDNA locations were highly polymorphic in this species. Copies of the gene were observed in the terminal of long or/and short arms of submetacentric or/and metacentric chromosomes. Using FISH with probe for vertebrate-like telomeric sequences (CCCTAA)3 displayed positive green FITC signals at telomere regions of all analyzed chromosome types. We found about 7% of chromosomes had breaks in prophase. A special form of nucleolus not previously described from H. discus hannai was observed in some interphase cells. It consists of many small silver-stained nucleoli gathered together to form a larger nucleolus and may correspond to prenucleolar bodies.
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Affiliation(s)
- Haishan Wang
- State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen, China
- Colleges of Ocean and Earth Sciences, Xiamen University, Xiamen, China
| | - Xuan Luo
- State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen, China
- Colleges of Ocean and Earth Sciences, Xiamen University, Xiamen, China
| | - Weiwei You
- State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen, China
- Colleges of Ocean and Earth Sciences, Xiamen University, Xiamen, China
| | - Yunwei Dong
- State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen, China
- Colleges of Ocean and Earth Sciences, Xiamen University, Xiamen, China
| | - Caihuan Ke
- State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen, China
- Colleges of Ocean and Earth Sciences, Xiamen University, Xiamen, China
- * E-mail:
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Cisterna B, Biggiogera M. Ribosome biogenesis: from structure to dynamics. INTERNATIONAL REVIEW OF CELL AND MOLECULAR BIOLOGY 2010; 284:67-111. [PMID: 20875629 DOI: 10.1016/s1937-6448(10)84002-x] [Citation(s) in RCA: 41] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/13/2022]
Abstract
In this chapter we describe the status of the research concerning the nucleolus, the major nuclear body. The nucleolus has been recognized as a dynamic organelle with many more functions than one could imagine. In fact, in addition to its fundamental role in the biogenesis of preribosomes, the nucleolus takes part in many other cellular processes and functions, such as the cell-cycle control and the p53 pathway: the direct or indirect involvement of the nucleolus in these various processes makes it sensitive to their alteration. Moreover, it is worth noting that the different nucleolar factors participating to independent mechanisms show different dynamics of association/disassociation with the nucleolar body.
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Affiliation(s)
- Barbara Cisterna
- Laboratory of Cell Biology and Neurobiology, Department of Animal Biology, University of Pavia, Pavia, Italy
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Handwerger KE, Wu Z, Murphy C, Gall JG. Heat shock induces mini-Cajal bodies in the Xenopus germinal vesicle. J Cell Sci 2002; 115:2011-20. [PMID: 11973343 DOI: 10.1242/jcs.115.10.2011] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
Cajal bodies are evolutionarily conserved nuclear organelles that are believed to play a central role in assembly of RNA transcription and processing complexes. Although knowledge of Cajal body composition and behavior has greatly expanded in recent years, little is known about the molecules and mechanisms that lead to the formation of these organelles in the nucleus. The Xenopus oocyte nucleus or germinal vesicle is an excellent model system for the study of Cajal bodies, because it is easy to manipulate and it contains 50-100 Cajal bodies with diameters up to 10 μm. In this study we show that numerous mini-Cajal bodies (less than 2 μm in diameter) form in the germinal vesicle after oocytes recover from heat shock. The mechanism for heat shock induction of mini-Cajal bodies is independent of U7 snRNA and does not require transcription or import of newly translated proteins from the cytoplasm. We suggest that Cajal bodies originate by self-organization of preformed components, preferentially on the surface of B-snurposomes.
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Affiliation(s)
- Korie E Handwerger
- Department of Embryology, Carnegie Institution of Washington, 115 West University Parkway, Baltimore, MD 21210, USA
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Verheggen C, Le Panse S, Almouzni G, Hernandez-Verdun D. Maintenance of nucleolar machineries and pre-rRNAs in remnant nucleolus of erythrocyte nuclei and remodeling in Xenopus egg extracts. Exp Cell Res 2001; 269:23-34. [PMID: 11525636 DOI: 10.1006/excr.2001.5304] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
The nuclear functions in erythrocytes are almost completely extinct. There is no RNA polymerase I transcription, although a remnant nucleolar structure is still present. The remnant nucleolus of Xenopus laevis erythrocytes maintains a morphologically organized structure, nearly exclusively fibrillar. In this inactive nucleolar remnant, we revealed the presence of a modified form of transcription factor UBF. Several proteins of the processing machinery such as fibrillarin, nucleolin and B23/NO38, snoRNAs U3 and U8, and partially processed preribosomal RNAs colocalized in these remnant structures. Attempts to reprogram these erythrocyte nuclei in Xenopus egg extract showed that import of several nucleolar proteins was induced while the nucleolar remnant was disorganized. UBF became abundant and showed a necklace-like distribution on the decondensed ribosomal genes. Fibrillarin, nucleolin, and snoRNAs U3 and U8, also largely imported from the extract, were associated in large prenuclear bodies scattered in the nucleoplasm. B23/NO38 was present in different small bodies formed only in the most decondensed nuclei. In these remodeled erythrocyte nuclei, there was no imported preribosomal RNA and the initial presence of a residual nucleolar structure containing several partners of ribosome biogenesis was not sufficient to promote reassembly of newly imported nucleolar machineries. These nuclei, which reproduce the early events of nucleogenesis are also transcriptionally silent and thus compare to the early embryonic nuclei of Xenopus laevis.
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Affiliation(s)
- C Verheggen
- Institut Jacques Monod, UMR 7592, Paris, France
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Fair T, Hyttel P, Lonergan P, Boland MP. Immunolocalization of nucleolar proteins during bovine oocyte growth, meiotic maturation, and fertilization. Biol Reprod 2001; 64:1516-25. [PMID: 11319160 DOI: 10.1095/biolreprod64.5.1516] [Citation(s) in RCA: 33] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/01/2022] Open
Abstract
During the growth phase of the bovine oocyte transcripts, polypeptides and ribosomes are accumulated in the oocyte to drive and sustain future meiotic maturation, fertilization, and early embryonic development. The oocyte also furnishes the early embryo with the components required to establish a functional transcriptionally active nucleolus at the time of maternal embryonic transition. The aim of the present study was to describe the behavior of key components of the nucleolus. The temporal localization of nucleolar proteins fibrillarin, nucleophosmin, nucleolin, RNA polymerase I (RNA pol I), topoisomerase I, upstream binding factor (UBF), and coilin 5P10 was investigated in growing and fully grown immature bovine oocytes during in vitro maturation and during the first postfertilization cell cycle using whole-mount immunocytochemistry and confocal microscopy. During the oocyte growth phase, fibrillarin, nucleophosmin, nucleolin, RNA pol I, and UBF were localized to the oocyte nucleolus. On completion of the growth phase, nucleolin and nucleophosmin appeared to migrate to the periphery of the nucleolus and into the nucleoplasm, and the proportion of oocytes displaying RNA pol I localization had decreased. Topoisomerase I was not detected at any stage. Fibrillarin appeared to be localized to large foci within the nucleolus and/or nucleoplasm. Nucleophosmin and nucleolin labeling was characterized by a homogeneous signal over the nucleolus. RNA pol I and UBF were characterized by the localization of the antibodies to individual or clustered foci in the nucleolus and/or nucleoplasm. Following oocyte nucleus breakdown (ONBD), the proteins appeared to disperse into the cytoplasm. All proteins were undetectable during meiotic maturation and were not relocalized until 5-10 h postinsemination (hpi). UBF was localized to the fertilizing sperm head of most zygotes at 5 hpi. By 10 hpi, all proteins were detected in most oocytes displaying two pronuclei. Nucleolar protein localization was exclusive to or more abundant in one pronucleus up to 20 hpi; thereafter, the pattern was more evenly distributed. Fibrillarin, nucleophosmin, nucleolin, UBF, and Pol I are present in the nuclei of growing and fully grown bovine oocytes until ONBD. They reappear at the late telophase stage of meiosis II and continue to be present up to the first mitotic division of embryo development.
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Affiliation(s)
- T Fair
- Department of Animal Science & Production and The Conway Institute of Biomedical and Biomolecular Research, University College Dublin, Lyons Research Farm, Newcastle, County Dublin, Ireland.
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Abstract
Cajal bodies are small nuclear organelles first described nearly 100 years ago by Ramón y Cajal in vertebrate neural tissues. They have since been found in a variety of animal and plant nuclei, suggesting that they are involved in basic cellular processes. Cajal bodies contain a marker protein of unknown function, p80-coilin, and many components involved in transcription and processing of nuclear RNAs. Among these are the three eukaryotic RNA polymerases and factors required for transcribing and processing their respective nuclear transcripts: mRNA, rRNA, and pol III transcripts. A model is discussed in which Cajal bodies are the sites for preassembly of transcriptosomes, unitary particles involved in transcription and processing of RNA. A parallel is drawn to the nucleolus and the preassembly of ribosomes, which are unitary particles involved in translation of proteins.
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Affiliation(s)
- J G Gall
- Department of Embryology, Carnegie Institution, Baltimore, Maryland 21210, USA.
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9
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Abstract
An understanding of the functional organization of nucleoli, the sites of ribosome biosynthesis, is limited by the present uncertainty about the topological arrangement of the transcribing rRNA genes. Since studies with “standard” nucleoli from somatic cells produced conflicting results, we have examined the amplified nucleoli of Xenopus oocytes. These nucleoli are unique in that they contain high copy numbers of rRNA genes, are not attached to chromosomes, lack non-ribosomal DNA and can be examined in light microscopic spread preparations of nuclear contents. By immunostaining and confocal microscopy we show that in growing stage IV oocytes the sites of rDNA are surrounded by the dense fibrillar component. The rDNA is actively transcribed as revealed by BrUTP injection into oocytes and localization of components of the nucleolar transcription machinery (RNA polymerase I and the transcription factor UBF). At the ultrastructural level, the rDNA sites correlate with the fibrillar centers of amplified nucleoli fixed in situ. The results provide clear evidence that the transcriptionally active rRNA genes are confined to the fibrillar centers of the oocyte nucleoli and open the possibility to analyze the protein composition of almost native, transcriptionally highly active nucleolar chromatin by immunofluorescence microscopy.
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Affiliation(s)
- C Mais
- Department of Cell and Developmental Biology, Biocenter of the University of Würzburg, Am Hubland, 97074 Würzburg, Germany
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10
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Verheggen C, Almouzni G, Hernandez-Verdun D. The ribosomal RNA processing machinery is recruited to the nucleolar domain before RNA polymerase I during Xenopus laevis development. J Cell Biol 2000; 149:293-306. [PMID: 10769023 PMCID: PMC2175160 DOI: 10.1083/jcb.149.2.293] [Citation(s) in RCA: 48] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/20/1999] [Accepted: 03/07/2000] [Indexed: 11/22/2022] Open
Abstract
Transcription and splicing of messenger RNAs are temporally and spatially coordinated through the recruitment by RNA polymerase II of processing factors. We questioned whether RNA polymerase I plays a role in the recruitment of the ribosomal RNA (rRNA) processing machinery. During Xenopus laevis embryogenesis, recruitment of the rRNA processing machinery to the nucleolar domain occurs in two steps: two types of precursor structures called prenucleolar bodies (PNBs) form independently throughout the nucleoplasm; and components of PNBs I (fibrillarin, nucleolin, and the U3 and U8 small nucleolar RNAs) fuse to the nucleolar domain before components of PNBs II (B23/NO38). This fusion process is independent of RNA polymerase I activity, as shown by actinomycin D treatment of embryos and by the lack of detectable RNA polymerase I at ribosomal gene loci during fusion. Instead, this process is concomitant with the targeting of maternally derived pre-rRNAs to the nucleolar domain. Absence of fusion was correlated with absence of these pre-rRNAs in nuclei where RNA polymerase II and III are inhibited. Therefore, during X. laevis embryogenesis, the recruitment of the rRNA processing machinery to the nucleolar domain could be dependent on the presence of pre-rRNAs, but is independent of either zygotic RNA polymerase I transcription or the presence of RNA polymerase I itself.
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Affiliation(s)
- Céline Verheggen
- Institut Jacques Monod, UMR 7592, 75251 Paris, France
- Institut Curie, Section de Recherche, UMR 144, 75248 Paris, France
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Fomproix N, Hernandez-Verdun D. Effects of anti-PM-Scl 100 (Rrp6p exonuclease) antibodies on prenucleolar body dynamics at the end of mitosis. Exp Cell Res 1999; 251:452-64. [PMID: 10471330 DOI: 10.1006/excr.1999.4578] [Citation(s) in RCA: 20] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Prenucleolar bodies (PNBs) are transitory structures which serve as building blocks for nucleoli at the transition mitosis/interphase. The assembly of PNBs and their pathway are not clearly understood. To better understand these events, the behavior of the PNB-containing PM-Scl 100 protein was compared with that of other PNB proteins. This nucleolar protein was chosen because its yeast homologue, Rrp6p exonuclease [1], is known to participate in late events in 5.8 S rRNA (ribosomal RNA) processing. There was a heterogeneous distribution of nucleolar proteins in different classes of PNBs. The PM-Scl 100 colocalized predominantly with protein B23. The PM-Scl-100-containing PNBs were translocated at later times to nucleoli as opposed to the fibrillarin-containing PNBs. Microinjections of antibodies directed against PM-Scl 100 during mitosis inhibited targeting of PM-Scl 100 to the nucleolus. However fibrillarin and protein B23 still participated in nucleolar assembly in early G1. We conclude that there are different kinds of PNBs whose translocation to the nucleoli follow ordered kinetics. Interestingly, proteins involved in late steps of processing as PM-Scl 100 are translocated late, suggesting that they are not cotranscriptionally associated with the rRNA precursors.
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Affiliation(s)
- N Fomproix
- Institut Jacques Monod, Cedex 05, Paris, 75251, France
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12
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Bell P, Scheer U. Developmental changes in RNA polymerase I and TATA box-binding protein during early Xenopus embryogenesis. Exp Cell Res 1999; 248:122-35. [PMID: 10094820 DOI: 10.1006/excr.1999.4411] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Xenopus early embryos are transcriptionally quiescent until the midblastula transition (MBT). We have examined the question of whether the absence of rRNA synthesis is related to a deficiency in the RNA polymerase I (pol I) transcription machinery. Previously we have demonstrated that the maternally provided pol I transcription factor UBF already binds to the inactive rRNA genes of pre-MBT embryos (P. Bell et al., 1997, J. Cell Sci. 110, 2053-2063). Here we have analyzed the fate of pol I and the TATA box-binding protein (TBP) through immunofluorescence and immunoblotting experiments. Pol I stockpiled in the egg is taken up by in vitro assembled pronuclei and concentrated into numerous distinct nuclear domains. Comparable storage sites of template-free pol I are also seen in nuclei of blastula to neurula stage embryos. In contrast, the amount of TBP is relatively low in oocytes and eggs but increases dramatically during the cleavage stages. Most of the newly synthesized TBP colocalizes with the stored form of pol I in the extranucleolar domains of blastula/gastrula embryos. The amount of TBP per embryo reaches peak values at the blastula/gastrula stage and then rapidly declines to normal somatic levels. The positive correlation of maximal TBP levels with the timing of the MBT suggests that overproduction of TBP is required for the formation of productive transcription complexes.
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Affiliation(s)
- P Bell
- Department of Cell and Developmental Biology, Biocenter, University of Würzburg, Am Hubland, Würzburg, D-97074, Germany
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Zatsepina OV, Rousselet A, Chan PK, Olson MO, Jordan EG, Bornens M. The nucleolar phosphoprotein B23 redistributes in part to the spindle poles during mitosis. J Cell Sci 1999; 112 ( Pt 4):455-66. [PMID: 9914158 DOI: 10.1242/jcs.112.4.455] [Citation(s) in RCA: 52] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
B23 is a major phosphoprotein in the interphasic nucleolus where it is involved in the assembly of pre-ribosomes. Using several cultured animal cells, we report that, in addition to the known redistribution of the protein during mitosis, B23 also becomes associated with mitotic spindle poles starting from early prometaphase onwards. Colocalization of B23 with the protein NuMA (Nuclear Mitotic Apparatus protein) was studied in mitotic cells and taxol-arrested cells. During the onset of mitosis, we observed that a fraction of B23 associates with, and dissociates from, the poles later than NuMA. At metaphase, both proteins are colocalized at the poles. The polar redistribution of both B23 and NuMA is mediated by microtubules. In taxol-treated cells, B23 is associated with the microtubule minus ends in the center of mitotic asters together with NuMA. Association of B23 with microtubule minus ends of mitotic asters was further confirmed with an in vitro assay, where B23 was found by western blotting to co-sediment with taxol-induced microtubule asters formed in a mitotic cell extract. Immunolabeling demonstrated that B23 and NuMA were both present at the center of the asters. Furthermore, an additional hyperphosphorylated form of B23 appeared when microtubule asters formed and associated with the asters. Immunodepletion of B23 from the mitotic extract revealed that taxol-induced microtubule asters were still observed in B23-immunodepleted mitotic extract, indicating that the presence of B23 at the poles is unlikely to be essential for spindle formation or stabilisation.
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Affiliation(s)
- O V Zatsepina
- Randall Institute, King's College London, London WC2B 5RL, UK.
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14
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Verheggen C, Le Panse S, Almouzni G, Hernandez-Verdun D. Presence of pre-rRNAs before activation of polymerase I transcription in the building process of nucleoli during early development of Xenopus laevis. J Cell Biol 1998; 142:1167-80. [PMID: 9732279 PMCID: PMC2149348 DOI: 10.1083/jcb.142.5.1167] [Citation(s) in RCA: 62] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/03/1998] [Revised: 06/16/1998] [Indexed: 11/22/2022] Open
Abstract
During the early development of Xenopus laevis, we followed in individual nuclei the formation of a nucleolus by examining simultaneously its structural organization and its transcriptional competence. Three distinct situations were encountered with different frequencies during development. During the first period of general transcriptional quiescence, the transcription factor UBF of maternal origin, was present in most nuclei at the ribosomal gene loci. In contrast, fibrillarin, a major protein of the processing machinery, was found in multiple prenucleolar bodies (PNBs) whereas nucleolin was dispersed largely in the nucleoplasm. During the second period, for most nuclei these PNBs had fused into two domains where nucleolin concentrated, generating a structure with most features expected from a transcriptionally competent nucleolus. However, RNA polymerase I-dependent transcription was not detected using run-on in situ assays whereas unprocessed ribosomal RNAs were observed. These RNAs were found to derive from a maternal pool. Later, during a third period, an increasing fraction of the nuclei presented RNA polymerase I-dependent transcription. Thus, the structural organization of the nucleolus preceded its transcriptional competence. We conclude that during the early development of X. laevis, the organization of a defined nucleolar structure, is not associated with the transcription process per se but rather with the presence of unprocessed ribosomal RNAs.
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Affiliation(s)
- C Verheggen
- Institut Jacques Monod, UMR 7592, Paris, France
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15
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Fomproix N, Gébrane-Younès J, Hernandez-Verdun D. Effects of anti-fibrillarin antibodies on building of functional nucleoli at the end of mitosis. J Cell Sci 1998; 111 ( Pt 3):359-72. [PMID: 9427684 DOI: 10.1242/jcs.111.3.359] [Citation(s) in RCA: 55] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
During mitosis some nuclear complexes are relocalized at the chromosome periphery and are then reintegrated into the re-forming nuclei in late telophase. To address questions concerning translocation from the chromosome periphery to nuclei, the dynamics of one nucleolar perichromosomal protein which is involved in the ribosomal RNA processing machinery, fibrillarin, was followed. In the same cells, the onset of the RNA polymerase I (RNA pol I) activity and translocation of fibrillarin were simultaneously investigated. In PtK1 cells, RNA pol I transcription was first detected at anaphase B. At the same mitotic stage, fibrillarin formed foci of increasing size around the chromosomes, these foci then gathered into prenucleolar bodies (PNBs) and later PNBs were targeted into the newly formed nucleoli. Electron microscopy studies enabled the visualization of the PNBs forming the dense fibrillar component (DFC) of new nucleoli. Anti-fibrillarin antibodies microinjected at different periods of mitosis blocked fibrillarin translocation at different steps, i.e. the formation of large foci, foci gathering in PNBs or PNB targeting into nucleoli, and thereby modified the ultrastructural organization of the nucleoli as well as of the PNBs. In addition, antibody-bound fibrillarin seemed localized with blocks of condensed chromatin in early G1 nuclei. It has been found that blocking fibrillarin translocation reduced or inhibited RNA pol I transcription. It is postulated that when translocation of proteins belonging to the processing machinery is inhibited or diminished, a negative feed-back effect is induced on nucleolar reassembly and transcriptional activity.
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