Identification and characterization of a new set of nucleolar ribonucleoproteins which line the chromosomes during mitosis

U3 snoRNA inter-regulates with DDX21 in the perichromosomal region to control mitosis

U3 snoRNA is essential for ribosome biogenesis during interphase. Upon mitotic onset, the nucleolus disassembles and U3 snoRNA relocates to the perichromosomal region (PR) to be considered as a chromosome passenger. Whether U3 controls mitosis remains unknown. Here, we demonstrate that U3 snoRNA is required for mitotic progression. We identified DDX21 as the predominant U3-binding protein during mitosis and confirmed that U3 snoRNA colocalizes with DDX21 in the PR. DDX21 knockdown induces mitotic catastrophe and similar mitotic defects caused by U3 snoRNA depletion. Interestingly, the uniform PR distribution of U3 snoRNA and DDX21 is interdependent. DDX21 functions in mitosis depending on its PR localization. Mechanistically, U3 snoRNA regulates DDX21 PR localization through maintaining its mobility. Moreover, Cy5-U3 snoRNA downsizes the fibrous condensates of His-DDX21 at proper molecular ratios in vitro. This work highlights the importance of the equilibrium between U3 snoRNA and DDX21 in PR formation and reveals the potential relationship between the PR assembly and mitotic regulation.

The intrinsically disorderly story of Ki-67

Ki-67 is one of the most famous marker proteins used by histologists to identify proliferating cells. Indeed, over 30 000 articles referring to Ki-67 are listed on PubMed. Here, we review some of the current literature regarding the protein. Despite its clinical importance, our knowledge of the molecular biology and biochemistry of Ki-67 is far from complete, and its exact molecular function(s) remain enigmatic. Furthermore, reports describing Ki-67 function are often contradictory, and it has only recently become clear that this proliferation marker is itself dispensable for cell proliferation. We discuss the unusual organization of the protein and its mRNA and how they relate to various models for its function. In particular, we focus on ways in which the intrinsically disordered structure of Ki-67 might aid in the assembly of the still-mysterious mitotic chromosome periphery compartment by controlling liquid-liquid phase separation of nucleolar proteins and RNAs.

Mapping the nucleolar proteome reveals a spatiotemporal organization related to intrinsic protein disorder

The nucleolus is essential for ribosome biogenesis and is involved in many other cellular functions. We performed a systematic spatiotemporal dissection of the human nucleolar proteome using confocal microscopy. In total, 1,318 nucleolar proteins were identified; 287 were localized to fibrillar components, and 157 were enriched along the nucleoplasmic border, indicating a potential fourth nucleolar subcompartment: the nucleoli rim. We found 65 nucleolar proteins (36 uncharacterized) to relocate to the chromosomal periphery during mitosis. Interestingly, we observed temporal partitioning into two recruitment phenotypes: early (prometaphase) and late (after metaphase), suggesting phase-specific functions. We further show that the expression of MKI67 is critical for this temporal partitioning. We provide the first proteome-wide analysis of intrinsic protein disorder for the human nucleolus and show that nucleolar proteins in general, and mitotic chromosome proteins in particular, have significantly higher intrinsic disorder level compared to cytosolic proteins. In summary, this study provides a comprehensive and essential resource of spatiotemporal expression data for the nucleolar proteome as part of the Human Protein Atlas.

Ki-67 and the Chromosome Periphery Compartment in Mitosis

The chromosome periphery is a complex network of proteins and RNA molecules (many derived from nucleoli) that covers the outer surface of chromosomes and whose function remains mysterious. Although it was first described over 130 years ago, technological advances and the recent discovery that Ki-67 acts as an organiser of this region have allowed the chromosome periphery to be dissected in previously unattainable detail, leading to a revival of interest in this obscure chromosomal compartment. Here, we review the most recent advances into the composition, structure and function of the chromosome periphery, discuss possible roles of Ki-67 during mitosis and consider why this structure is likely to remain the focus of ongoing attention in the future.

Ki67, PCNA, and MCM proteins: Markers of proliferation in the diagnosis of breast cancer

The proliferative activity of tumour cells represents an important prognostic marker in the diagnosis of cancer. One of the methods for assessing the proliferative activity of cells is the immunohistochemical detection of cell cycle-specific antigens. For example, Ki67, proliferating cell nuclear antigen (PCNA), and minichromosome maintenance (MCM) proteins are standard markers of proliferation that are commonly used to assess the growth fraction of a cell population. The function of Ki67, the widely used marker of proliferation, still remains unclear. In contrast, PCNA and MCM proteins have been identified as important participants of DNA replication. All three proteins only manifest their expression during the cell division of normal and neoplastic cells. Since the expression of these proliferative markers was confirmed in several malignant tumours, their prognostic and predictive values have been evaluated to determine their significance in the diagnosis of cancer. This review offers insight into the discovery of the abovementioned proteins, as well as their current molecular and biological importance. In addition, the functions and properties of all three proteins and their use as markers of proliferation in the diagnosis of breast cancer are described. This work also reveals new findings about the role of Ki67 during the mitotic phase of the cell cycle. Finally, information is provided about the advantages and disadvantages of using all three antigens in the diagnosis of cancer.

The non-coding RNA composition of the mitotic chromosome by 5'-tag sequencing

Mitotic chromosomes are one of the most commonly recognized sub-cellular structures in eukaryotic cells. Yet basic information necessary to understand their structure and assembly, such as their composition, is still lacking. Recent proteomic studies have begun to fill this void, identifying hundreds of RNA-binding proteins bound to mitotic chromosomes. However, by contrast, there are only two RNA species (U3 snRNA and rRNA) that are known to be associated with the mitotic chromosome, suggesting that there are many mitotic chromosome-associated RNAs (mCARs) not yet identified. Here, using a targeted protocol based on 5'-tag sequencing to profile the mammalian mCAR population, we report the identification of 1279 mCARs, the majority of which are ncRNAs, including lncRNAs that exhibit greater conservation across 60 vertebrate species than the entire population of lncRNAs. There is also a significant enrichment of snoRNAs and specific SINE RNAs. Finally, ∼40% of the mCARs are presently unannotated, many of which are as abundant as the annotated mCARs, suggesting that there are also many novel ncRNAs in the mCARs. Overall, the mCARs identified here, together with the previous proteomic and genomic data, constitute the first comprehensive catalogue of the molecular composition of the eukaryotic mitotic chromosomes.

Ki-67 is a PP1-interacting protein that organises the mitotic chromosome periphery

When the nucleolus disassembles during open mitosis, many nucleolar proteins and RNAs associate with chromosomes, establishing a perichromosomal compartment coating the chromosome periphery. At present nothing is known about the function of this poorly characterised compartment. In this study, we report that the nucleolar protein Ki-67 is required for the assembly of the perichromosomal compartment in human cells. Ki-67 is a cell-cycle regulated protein phosphatase 1-binding protein that is involved in phospho-regulation of the nucleolar protein B23/nucleophosmin. Following siRNA depletion of Ki-67, NIFK, B23, nucleolin, and four novel chromosome periphery proteins all fail to associate with the periphery of human chromosomes. Correlative light and electron microscopy (CLEM) images suggest a near-complete loss of the entire perichromosomal compartment. Mitotic chromosome condensation and intrinsic structure appear normal in the absence of the perichromosomal compartment but significant differences in nucleolar reassembly and nuclear organisation are observed in post-mitotic cells.DOI: http://dx.doi.org/10.7554/eLife.01641.001.

Assembly and disassembly of the nucleolus during the cell cycle

The nucleolus is a large nuclear domain in which transcription, maturation and assembly of ribosomes take place. In higher eukaryotes, nucleolar organization in three sub-domains reflects the compartmentation of the machineries related to active or inactive transcription of the ribosomal DNA, ribosomal RNA processing and assembly with ribosomal proteins of the two (40S and 60S) ribosomal subunits. The assembly of the nucleoli during telophase/early G(1) depends on pre-existing machineries inactivated during prophase (the transcription machinery and RNP processing complexes) and on partially processed 45S rRNAs inherited throughout mitosis. In telophase, the 45S rRNAs nucleate the prenucleolar bodies and order the dynamics of nucleolar assembly. The assembly/disassembly processes of the nucleolus depend on the equilibrium between phosphorylation/dephosphorylation of the transcription machinery and on the RNP processing complexes under the control of the CDK1-cyclin B kinase and PP1 phosphatases. The dynamics of assembly/disassembly of the nucleolus is time and space regulated.

Nucleolin functions in nucleolus formation and chromosome congression

A complex structure, designated the chromosome periphery, surrounds each chromosome during mitosis. Although several proteins have been shown to localize to the chromosome periphery, their functions during mitosis remain unclear. Here, we used a combination of high-resolution microscopy and RNA-interference-mediated depletion to study the functions of nucleolin, a nucleolar protein localized at the chromosome periphery, in interphase and mitosis. During mitosis, nucleolin was localized in the peripheral region including the vicinity of the outer kinetochore of chromosomes. Staining with an antibody specific for nucleolin phosphorylated by CDC2 revealed that nucleolin was also associated with the spindle poles from prometaphase to anaphase. Nucleolin depletion resulted in disorganization of the nucleoli at interphase. Furthermore, nucleolin-depleted cells showed a prolonged cell cycle with misaligned chromosomes and defects in spindle organization. The misaligned chromosomes showed syntelic kinetochore-microtubule attachments with reduced centromere stretching. Taken together, our results indicate that nucleolin is required for nucleolus formation, and is also involved in chromosome congression and spindle formation.

The nucleolus: a model for the organization of nuclear functions

Nucleoli are the prominent contrasted structures of the cell nucleus. In the nucleolus, ribosomal RNAs (rRNAs) are synthesized, processed and assembled with ribosomal proteins. The size and organization of the nucleolus are directly related to ribosome production. The organization of the nucleolus reveals the functional compartmentation of the nucleolar machineries that depends on nucleolar activity. When this activity is blocked, disrupted or impossible, the nucleolar proteins have the capacity to interact independently of the processing activity. In addition, nucleoli are dynamic structures in which nucleolar proteins rapidly associate and dissociate with nucleolar components in continuous exchanges with the nucleoplasm. At the time of nucleolar assembly, the processing machineries are recruited in a regulated manner in time and space, controlled by different kinases and form intermediate structures, the prenucleolar bodies. The participation of stable pre-rRNAs in nucleolar assembly was demonstrated after mitosis and during development but this is an intriguing observation since the role of these pre-rRNAs is presently unknown. A brief report on the nucleolus and diseases is proposed as well as of nucleolar functions different from ribosome biogenesis.

New Insights into Nucleolar Architecture and Activity

The nucleolus is the most obvious and clearly differentiated nuclear subcompartment. It is where ribosome biogenesis takes place and has been the subject of research over many decades. In recent years progress in our understanding of ribosome biogenesis has been rapid and is accelerating. This review discusses current understanding of how the biochemical processes of ribosome biosynthesis relate to an observable nucleolar structure. Emerging evidence is also described that points to other, unconventional roles for the nucleolus, particularly in the biogenesis of other RNA-containing cellular machinery, and in stress sensing and the control of cellular activity. Striking recent observations show that the nucleolus and its components are highly dynamic, and that the steady state structure observed by microscopical methods must be interpreted as the product of these dynamic processes. We still do not have detailed enough information to understand fully the organization and regulation of the various processes taking place in the nucleolus. However, the present power of light and electron microscopy (EM) techniques means that a description of nucleolar processes at the molecular level is now achievable, and the time is ripe for such an effort.

Nucleolus: from structure to dynamics

The nucleolus, a large nuclear domain, is the ribosome factory of the cells. Ribosomal RNAs are synthesized, processed and assembled with ribosomal proteins in the nucleolus, and the ribosome subunits are then transported to the cytoplasm. In this review, the structural organization of the nucleolus and the dynamics of the nucleolar proteins are discussed in an attempt to link both information. By electron microscopy, three main nucleolar components corresponding to different steps of ribosome biogenesis are identified and the nucleolar organization reflects its activity. Time-lapse videomicroscopy and fluorescent recovery after photobleaching (FRAP) demonstrate that mobility of GFP-tagged nucleolar proteins is slower in the nucleolus than in the nucleoplasm. Fluorescent recovery rates change with inhibition of transcription, decreased temperature and depletion of ATP, indicating that recovery is correlated with cell activity. At the exit of mitosis, the nucleolar processing machinery is first concentrated in prenucleolar bodies (PNBs). The dynamics of the PNBs suggests a steady state favoring residence of processing factors that are then released in a control- and time-dependent manner. Time-lapse analysis of fluorescence resonance energy transfer demonstrates that processing complexes are formed in PNBs. Finally, the nucleolus appears at the center of several trafficking pathways in the nucleus.

The perichromosomal layer

In addition to genetic information, mitotic chromosomes transmit essential components for nuclear assembly and function in a new cell cycle. A specialized chromosome domain, called the perichromosomal layer, perichromosomal sheath, chromosomal coat, or chromosome surface domain, contains proteins required for a variety of cellular processes, including the synthesis of messenger RNA, assembly of ribosomes, repair of DNA double-strand breaks, telomere maintenance, and apoptosis regulation. The layer also contains many proteins of unknown function and is a major target in autoimmune disease. Perichromosomal proteins are found along the entire length of chromosomes, excluding centromeres, where sister chromatids are paired and spindle microtubules attach. Targeting of proteins to the perichromosomal layer occurs primarily during prophase, and they generally remain associated until telophase. During interphase, perichromosomal proteins localize to nucleoli, the nuclear envelope, nucleoplasm, heterochromatin, centromeres, telomeres, and/or the cytoplasm. It has been suggested that the perichromosomal layer may contribute to chromosome structure, as several of the associated proteins have functions in chromatin remodeling during interphase. We review the identified proteins associated with this chromosome domain and briefly discuss their known functions during interphase and mitosis.

Tracking the interactions of rRNA processing proteins during nucleolar assembly in living cells

Reorganization of the nuclear machinery after mitosis is a fundamental but poorly understood process. Here, we investigate the recruitment of the nucleolar processing proteins in the nucleolus of living cells at the time of nucleus formation. We question the role of the prenucleolar bodies (PNBs), during migration of the processing proteins from the chromosome periphery to sites of rDNA transcription. Surprisingly, early and late processing proteins pass through the same PNBs as demonstrated by rapid two-color four-dimensional imaging and quantification, whereas a different order of processing protein recruitment into nucleoli is supported by differential sorting. Protein interactions along the recruitment pathway were investigated using a promising time-lapse analysis of fluorescence resonance energy transfer. For the first time, it was possible to detect in living cells the interactions between proteins of the same rRNA processing machinery in nucleoli. Interestingly interactions between such proteins also occur in PNBs but not at the chromosome periphery. The dynamics of these interactions suggests that PNBs are preassembly platforms for rRNA processing complexes.

Proliferation marker pKi-67 occurs in different isoforms with various cellular effects

The Ki-67 antigen, pKi-67, is a commonly used proliferation marker in research and pathology. It has been recognized that the protein exists in two different splice variants that differ in one exon. In the current work, we present three new splice variants of human pKi-67 consisting of two naturally occurring isoforms and one atypical version. Additionally, data is presented indicating that alternative splicing of the pKi-67 N-terminus is common in tumor cell lines. Analyzing 93 tissues mainly consisting of brain tumor specimens, we found evidence that long and short isoform can be expressed independently of each other. Induction of mitosis in human peripheral blood mononuclear cells revealed that short pKi-67 appears earlier in the cell cycle than the long isoform and reaches its expression maximum when transcription of the latter sets in. Finally, transfection of mammalian culture cells with exon 7 (specific for the long pKi-67 isoform and not present in the short isoform) in a tetracycline regulated expression system decreased the rate of cell proliferation without affecting the cell cycle. In summary, we present evidence that the pKi-67 N-terminus is differentially spliced resulting in at least five different isoforms with different functions.

Proliferation marker pKi-67 affects the cell cycle in a self-regulated manner

The proliferation marker pKi-67 is commonly used in research and pathology to detect proliferating cells. In a previous work, we found the protein to be associated with regulators of the cell cycle, controlling S-phase progression, as well as entry into and exit from mitosis. Here we investigate whether pKi-67 has a regulative effect on the cell cycle itself. For that purpose we cloned four fragments of pKi-67, together representing nearly the whole protein, and an N-terminal pKi-67 antisense oligonucleotide into a tetracycline inducible gene expression system. The sense fragments were C-terminally modified by addition of either a nuclear localization sequence (NLS) or a STOP codon to address the impact of their intracellular distribution. FACS based cell cycle analysis revealed that expression of nearly all pKi-67 domains and the antisense oligonucleotide led to a decreased amount of cells in S-phase and an increased number of cells in G(2)/M- and G(1)-phase. Subsequent analysis of the endogenous pKi-67 mRNA and protein levels revealed that the constructs with the most significant impact on the cell cycle were able to silence pKi-67 transcription as well. We conclude from the data that pKi-67 influences progression of S-phase and mitosis in a self-regulated manner and, therefore, effects the cell cycle checkpoints within both phases. Furthermore, we found pKi-67 mediates an anti-apoptotic effect on the cell and we verified that this marker, although it is a potential ribosomal catalyst, is not expressed in differentiated tissues with a high transcriptional activity.

Cell and Molecular Biology of Nucleolar Assembly and Disassembly

Nucleoli disassemble in prophase of the metazoan mitotic cycle, and they begin their reassembly (nucleologenesis) in late anaphase?early telophase. Nucleolar disassembly and reassembly were obvious to the early cytologists of the eighteenth and nineteenth centuries, and although this has lead to a plethora of literature describing these events, our understanding of the molecular mechanisms regulating nucleolar assembly and disassembly has expanded immensely just within the last 10-15 years. We briefly survey the findings of nineteenth-century cytologists on nucleolar assembly and disassembly, followed by the work of Heitz and McClintock on nucleolar organizers. A primer review of nucleolar structure and functions precedes detailed descriptions of modern molecular and microscopic studies of nucleolar assembly and disassembly. Nucleologenesis is concurrent with the reinitiation of rDNA transcription in telophase. The perichromosomal sheath, prenucleolar bodies, and nucleolar-derived foci serve as repositories for nucleolar processing components used in the previous interphase. Disassembly of the perichromosomal sheath along with the dynamic movements and compositional changes of the prenucleolar bodies and nucleolus-derived foci coincide with reactivation of rDNA synthesis within the chromosomal nucleolar organizers during telophase. Nucleologenesis is considered in various model organisms to provide breadth to our understanding. Nucleolar disassembly occurs at the onset of mitosis primarily as a result of the mitosis-specific phosphorylation of Pol I transcription factors and processing components. Although we have learned much regarding nucleolar assembly and disassembly, many questions still remain, and these questions are as vibrant for us today as early questions were for nineteenth- and early twentieth-century cytologists.

The proliferation marker pKi-67 organizes the nucleolus during the cell cycle depending on Ran and cyclin B

The proliferation marker pKi-67 ('Ki-67 antigen') is commonly used in clinical and research pathology to detect proliferating cells, as it is only expressed during cell-cycle progression. Despite the fact that this antigen has been known for nearly two decades, there is still no adequate understanding of its function. This study has therefore identified proteins that interact with pKi-67, using a yeast two-hybrid system. A mammalian two-hybrid system and immunoprecipitation studies were used to verify these interactions. Among other cell-cycle regulatory proteins, two binding partners associated with the small GTPase Ran were identified. In addition, DNA-structural and nucleolus-associated proteins binding to pKi-67 were found. Moreover, it was demonstrated that the N-terminal domain of pKi-67 is capable of self-binding to its own repeat region encoded by exon 13. Since RanBP, a protein involved in the transport of macromolecules over the nuclear lamina, was found to be a binding partner, a possible effect of pKi-67 on the localization of cell-cycle regulatory proteins was proposed. To test this hypothesis, a tetracycline-responsive gene expression system was used to induce the pKi-67 fragments previously used for the two-hybrid screens in HeLa cells. Subsequent immunostaining revealed the translocation of cyclin B1 from cytoplasm to nucleoli in response to this expression. It is suggested that pKi-67 is a Ran-associated protein with a role in the disintegration and reformation of the nucleolus and thereby in entry into and exit from the M-phase.

Pescadillo is essential for nucleolar assembly, ribosome biogenesis, and mammalian cell proliferation

Mutation of the zebrafish pescadillo gene blocks expansion of a number of tissues in the developing embryo, suggesting roles for its gene product in controlling cell proliferation. We report that levels of the pescadillo protein increase in rodent hepatocytes as they enter the cell cycle. Pescadillo protein localizes to distinct substructures of the interphase nucleus including nucleoli, the site of ribosome biogenesis. During mitosis pescadillo closely associates with the periphery of metaphase chromosomes and by late anaphase is associated with nucleolus-derived foci and prenucleolar bodies. Blastomeres in mouse embryos lacking pescadillo arrest at morula stages of development, the nucleoli fail to differentiate and accumulation of ribosomes is inhibited. We propose that in mammalian cells pescadillo is essential for ribosome biogenesis and nucleologenesis and that disruption to its function results in cell cycle arrest.

Isolation and characterization of a new nucleolar protein, Nrap, that is conserved from yeast to humans

BACKGROUND

The nucleolus is the site of rRNA synthesis and processing in eukaryotic cells, but its composition remains poorly understood.

RESULTS

We have identified a novel nucleolar RNA-associated protein (Nrap) which is highly conserved from yeast (Saccharomyces cerevisiae) to human, with homologues in mouse, Drosophila melanogaster, Caenorhabditis elegans, Arabidopsis thaliana, Schizosaccharomyces pombe, and other species. In the mouse, we have found that Nrap is ubiquitously expressed and is specifically localized in the nucleolus. We have also identified splice variants in human and mouse, and defined the intron-exon structure of the human Nrap gene. Nrap is inherited into daughter nuclei by associating with the condensed chromosomes during mitosis. RNase treatment of permeabilized cells indicated that the nucleolar localization of Nrap is RNA dependent. The effects of actinomycin D, cycloheximide and 5,6-dichloro-beta-d-ribofuranosyl-benzimidazole on Nrap expression and distribution in cultured cells suggest that Nrap is associated with the pre-rRNA transcript.

CONCLUSIONS

Nrap is a large nucleolar protein (of more than 1000 amino acids), and is a new class of protein with new structural and functional motifs. Nrap appears to be associated with ribosome biogenesis by interacting with pre-rRNA primary transcript.

Immunolocalization of nucleolar proteins during bovine oocyte growth, meiotic maturation, and fertilization

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.

Effects of anti-PM-Scl 100 (Rrp6p exonuclease) antibodies on prenucleolar body dynamics at the end of mitosis

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.

The nucleolar antigen Nop52, the human homologue of the yeast ribosomal RNA processing RRP1, is recruited at late stages of nucleologenesis

We report the molecular characterization of a novel nucleolar protein, Nop52, and its subcellular distribution during the cell cycle and nucleologenesis. This protein was originally identified with human autoantibodies which were subsequently used to clone its corresponding cDNA. Transfection experiments in mammalian cells have confirmed that this cDNA encodes a nucleolar protein that accumulates in the nucleoli and at the periphery of the chromosomes. Nop52 is the putative human homologue of the yeast ribosomal RNA processing protein RRP1 which is involved in pre-rRNA processing from 27S to 25S and 5.8S. In nucleoli, Nop52 is excluded from the ribosomal RNA transcription sites, accumulates in the granular external domain and mainly colocalizes with nucleolar proteins involved in the late processing step such as hPop1 and protein B23. During the building process of the nucleolus at the end of mitosis, a sequential order was observed in the assembly of nucleolar proteins of early and late processing mainly via the prenucleolar body pathway. The order is the following: fibrillarin, nucleolin, Nop52 together with protein B23 in the prenucleolar bodies, and simultaneously with hPop1, and finally Ki-67. The evolutionary conservation of Nop52 and the lethal effects observed in gene disruption experiments, predict a critical role for Nop52 in the generation of 28S rRNA.

Characterization of a novel nucleolar protein that transiently associates with the condensed chromosomes in mitotic cells

We report the isolation and characterization of a murine gene encoding a conserved mammalian nucleolar protein. The protein, called Tsg118, has a predicted molecular mass of 59.4 kDa and a high content of basic amino acids. A homologous human gene was localized to chromosome 16p12.3. The Tsg118 protein is predominantly expressed in proliferating somatic cells and in male germ cells. Indirect immunofluorescence microscopy analysis using an affinity-purified anti-Tsg118 serum shows colocalization of Tsg118 and a known nucleolar protein, fibrillarin, to the dense fibrillar component of the nucleolus. The nucleolar localization of the Tsg118 protein appears to be temporally restricted to the interphase stages of the somatic cell cycle and to the meiotic phase of spermatogenesis. We find that the Tsg118 protein localizes to the nucleolus in both proliferating and serum-starved cells. Interestingly, as the nucleolar signal disappears in mitotic cells, the Tsg118 protein instead becomes associated with the surface of the condensed chromosomes.

The eleven stages of the cell cycle, with emphasis on the changes in chromosomes and nucleoli during interphase and mitosis

Since we had subdivided the cell cycle into 11 stages--four for mitosis and seven for the interphase--and since we had experience in detecting DNA in the electron microscope (EN) by the osmium-amine procedure of Cogliati and Gauthier (Compt. Rend. Acad. Sci., 1973;276:3041-3044), we combined the two approaches for the analysis of DNA-containing structures at all stages of the cell cycle. Thin Epon sections of formaldehyde-fixed mouse duodenum were stained by osmium-amine for electron microscopic examination of the stages in the 12.3-hr long cell cycle of mouse duodenal crypt columnar cells. In addition, semi-thin Lowicryl sections of mouse duodenal crypts and cultured rat kidney cells were stained with the DNA-specific Hoechst 33258 dye and examined in the fluorescence microscope. The DNA detected by osmium-amine is in the form of nucleofilaments, seen at high magnification as long rows of 11 nm-wide rings (consisting of stained DNA encircling unstained histones). At all stages of the cycle as well as in nondividing cells, nucleofilaments are of three types: 'free,' 'attached' to chromatin accumulations, and 'compacted' in all chromatin accumulations, the form of dense spirals within. At stage I of the cycle, besides free and attached nucleofilaments, compacted ones are observed in the three heterochromatin forms (peripheral, nucleolus-associated, clumped). Soon after the S phase begins, chromatin 'aggregates' appear, which are small at stage II, mid-sized at stage III, and large at stage IV. Chromatin 'bulges' also appear at stage III and enlarge at stage IV, while heterochromatins disappear. At stage V, aggregates and bulges accrete into 'chromomeres,' a process responsible for the apparent chromosome condensation observed at prophase. The chromomeres gradually line up in rows and, at stage VIa (prometaphase), approach one another within each row and coalesce to build up the metaphase chromosomes which are fully formed at stage VIb (metaphase). Daughter chromosomes arising at stage VII (anaphase) are eventually packed into a chromosomal mass at each pole of the cell. During stage VIII (telophase), the chromosomal mass is split into large chunks. In the course of the G1 phase, the chunks thin out to give rise to irregular 'bands' at stage IX, the bands are then cleaved into central and peripheral fragments at stage X, and finally the central fragments are replaced by free nucleofilaments and clumps at stage XI, while the peripheral fragments are replaced by peripheral heterochromatin. The "nucleoli" at stages I-III are associated with stained heterochromatin but otherwise appear as unstained lucent areas, except for weakly stained patches composed of histone-free DNA filaments. During stage IV, nucleoli lose patches and associated heterochromatin, while weakly lucent, pale vesicles appear within nucleoli and in the nucleoplasm. By the end of substage VIa, nucleoli generally disappear, while pale vesicles persist around the chromosomes appearing at substage VIb. At stages VIII and IX, the vesicles seem to become strongly lucent and, at stages IX and X, they associate and fuse to yield homogeneous lucent areas, the 'prenucleolar bodies,' which include histone-free DNA patches. During stage XI, groups of these bodies associate to give rise to nucleoli. In conclusion, the cell cycle DNA changes can be classified into 4 broad periods (Fig. 6): 1) Stage I is a 2-hr long interphase "pause," during which the stained DNA shows no signs of either chromosome condensation or decondensation, while the overall nuclear pattern is similar to that in nondividing cell nuclei. Nucleoli are fully developed. 2) From stage II to VIa, the "chromosome condensation" period extends over about 7 hr, during which the events are interpreted as follows. Throughout the S phase (stages II-IV), newly-synthesized segments of nucleofilaments approach one another, adhere and thus build aggregates and later bulges on nuclear matrix sites. (ABSTRACT TRUNCATED)

Partially processed pre-rRNA is preserved in association with processing components in nucleolus-derived foci during mitosis

Previous studies showed that components implicated in pre-rRNA processing, including U3 small nucleolar (sno)RNA, fibrillarin, nucleolin, and proteins B23 and p52, accumulate in perichromosomal regions and in numerous mitotic cytoplasmic particles, termed nucleolus-derived foci (NDF) between early anaphase and late telophase. The latter structures were analyzed for the presence of pre-rRNA by fluorescence in situ hybridization using probes for segments of pre-rRNA with known half-lives. The NDF did not contain the short-lived 5'-external transcribed spacer (ETS) leader segment upstream from the primary processing site in 47S pre-rRNA. However, the NDF contained sequences from the 5'-ETS core, 18S, internal transcribed spacer 1 (ITS1), and 28S segments and also had detectable, but significantly reduced, levels of the 3'-ETS sequence. Northern analyses showed that in mitotic cells, the latter sequences were present predominantly in 45S-46S pre-rRNAs, indicating that high-molecular weight processing intermediates are preserved during mitosis. Two additional essential processing components were also found in the NDF: U8 snoRNA and hPop1 (a protein component of RNase MRP and RNase P). Thus, the NDF appear to be large complexes containing partially processed pre-rRNA associated with processing components in which processing has been significantly suppressed. The NDF may facilitate coordinated assembly of postmitotic nucleoli.

Effects of anti-fibrillarin antibodies on building of functional nucleoli at the end of mitosis

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.

The yeast nucleolar protein Cbf5p is involved in rRNA biosynthesis and interacts genetically with the RNA polymerase I transcription factor RRN3

Yeast Cbf5p was originally isolated as a low-affinity centromeric DNA binding protein (W. Jiang, K. Middleton, H.-J. Yoon, C. Fouquet, and J. Carbon, Mol. Cell. Biol. 13:4884-4893, 1993). Cbf5p also binds microtubules in vitro and interacts genetically with two known centromere-related protein genes (NDC10/CBF2 and MCK1). However, Cbf5p was found to be nucleolar and is highly homologous to the rat nucleolar protein NAP57, which coimmunoprecipitates with Nopp140 and which is postulated to be involved in nucleolar-cytoplasmic shuttling (U. T. Meier, and G. Blobel, J. Cell Biol. 127:1505-1514, 1994). The temperature-sensitive cbf5-1 mutant demonstrates a pronounced defect in rRNA biosynthesis at restrictive temperatures, while tRNA transcription and pre-rRNA and pre-tRNA cleavage processing appear normal. The cbf5-1 mutant cells are deficient in cytoplasmic ribosomal subunits at both permissive and restrictive temperatures. A high-copy-number yeast genomic library was screened for genes that suppress the cbf5-1 temperature-sensitive growth phenotype. SYC1 (suppressor of yeast cbf5-1) was identified as a multicopy suppressor of cbf5-1 and subsequently was found to be identical to RRN3, an RNA polymerase I transcription factor. A cbf5delta null mutant is not rescued by plasmid pNOY103 containing a yeast 35S rRNA gene under the control of a Pol II promoter, indicating that Cbf5p has one or more essential functions in addition to its role in rRNA transcription.

A class of nonribosomal nucleolar components is located in chromosome periphery and in nucleolus-derived foci during anaphase and telophase

. The subcellular location of several nonribosomal nucleolar proteins was examined at various stages of mitosis in synchronized mammalian cell lines including HeLa, 3T3, COS-7 and HIV-1 Rev-expressing CMT3 cells. Nucleolar proteins B23, fibrillarin, nucleolin and p52 as well as U3 snoRNA were located partially in the peripheral regions of chromosomes from prometaphase to early telophase. However, these proteins were also found in large cytoplasmic particles, 1-2 microm in diameter, termed nucleolus-derived foci (NDF). The NDF reached maximum numbers (as many as 100 per cell) during mid- to late anaphase, after which their number declined to a few or none during late telophase. The decline in the number of NDF approximately coincided with the appearance of prenucleolar bodies and reforming nucleoli. The HIV-1 Rev protein and a mutant Rev protein defective in its nuclear export signal were also found in the NDF. The mutant Rev protein precisely followed the pattern of localization of the above nucleolar proteins, whereas the wild-type Rev did not enter nuclei until G1 phase. The nucleolar shuttling phosphoprotein Nopp140 did not follow the above pattern of localization during mitosis: it dispersed in the cytoplasm from prometaphase through early telophase and was not found in the NDF. Although the NDF and mitotic coiled bodies disappeared from the cytoplasm at approximately the same time during mitosis, protein B23 was not found in mitotic coiled bodies, nor was p80 coilin present in the NDF. These results suggest that a class of proteins involved in preribosomal RNA processing associate with chromosome periphery and with NDF as part of a system to conserve and deliver preexisting components to reforming nucleoli during mitosis.

The nucleolar phosphoprotein P130 is a GTPase/ATPase with intrinsic property to form large complexes triggered by F- and Mg2+

We have previously identified a human nucleolar phosphoprotein p130 whose alterations during mitosis are correlated well with the nucleolar disassembly and reassembly. Further studies found that p130 in the cell lysates or after being purified by immunoprecipitation was able to form large complexes triggered by F- and Mg2+. These sodium dodecyl sulfate-insoluble p130 molecules were readily dissociated by adding EDTA to the complexes. It is known that F- and Mg2+ act on many GTPases and ATPases through the induction of a conformational transition mimicking the nucleoside triphosphate-bound state. These initial observations led us to discover that p130 functions as a GTP/ATP binding protein with intrinsic GTPase/ATPase activities. The rate of GTP hydrolysis by purified p130 under our experimental conditions was 0.8 mol/min/mol of p130. These results imply that p130, a novel nucleolar GTPase/ATPase, may switch its conformation in a nucleotide-dependent manner.

Nuclear components with microtubule-organizing properties in multicellular eukaryotes: functional and evolutionary considerations

The nucleus and the microtubular cytoskeleton of eukaryotic cells appear to be structurally and functionally interrelated. Together they constitute a "cell body". One of the most important components of this body is a primary microtubule-organizing center (MTOC-I) located on or near the nuclear surface and composed of material that, in addition to constitutive centrosomal material, also comprises some nuclear matrix components. The MTOC-I shares a continuity with the mitotic spindle and, in animal cells, with the centrosome also. Secondary microtubule-organizing centers (MTOC-IIs) are a special feature of walled plant cells and are found at the plasma membrane where they organize arrays of cortical MTs that are essential for ordered cell wall synthesis and hence for cellular morphogenesis. MTOC-IIs are held to be similar in origin to the MTOC-I, but their material has been translocated to the cell periphery, perhaps by MTs organized and radiating from the MTOC-I. Many intranuclear, matrix-related components have been identified to participate in MT organization during mitosis and cytokinesis; some of them also seem to be related to the condensation and decondensation of chromatin during the mitotic chromosome cycle.

Location of the HIV-1 Rev protein during mitosis: inactivation of the nuclear export signal alters the pathway for postmitotic reentry into nucleoli

The HIV-1 Rev protein localizes predominantly to the nucleolus of HIV-1-infected or Rev-expressing cells. The subcellular location of Rev during mitotic nucleolar disintegration was examined at various stages of mitosis in synchronized Rev-expressing CMT3 cells. During early prophase Rev was predominantly located in disintegrating nucleoli and began to accumulate at the peripheral regions of chromosomes in late prophase, eventually distributing uniformly on all chromosomes in prometaphase. In anaphase Rev remained associated with the perichromosomal regions, but significant amounts of Rev were also seen in numerous nucleolus-derived foci. The movement of Rev from disintegrating nucleoli to perichromosomal regions and foci was similar to that of nonribosomal nucleolar proteins, including fibrillarin, nucleolin, protein B23 and p52 of the granular component. During telophase Rev remained associated with perichromosomal regions and mitotic foci until the nuclear envelope started to reform. When nuclear envelope formation was complete in late telophase, nonribosomal nucleolar proteins were present in prenucleolar bodies (PNBs) which were eventually incorporated into nucleoli; at the same time, Rev was excluded from nuclei. In contrast, a trans-dominant negative Rev protein containing an inactive nuclear export signal reentered nuclei by the nonribosomal nucleolar protein pathway in late telophase, associating with PNBs and reformed nucleoli. Rev protein reentry into postmitotic nuclei was delayed until early G1 phase, but before the arrival of ribosomal protein S6. Thus, Rev behaves like a nonribosomal nucleolar protein through mitosis until early telophase; however, its nuclear reentry seems to require reestablishment of both a nuclear import system and active nucleoli.

cDNA cloning and characterization of a novel nucleolar protein

In an initial study of anti-nuclear antibodies in the chronic inflammatory bladder disease interstitial cystitis, we reported that 7% of interstitial cystitis patients studied had autoantibodies to the nucleolus. We now report that, using an autoimmune serum from a patient with interstitial cystitis, we have identified and partially characterized a novel protein with an M(r) of approximately 55 kDa (hereafter referred to as No55) localized to the granular component of the nucleolus. No55 was initially characterized by diffuse nucleolar immunofluorescence staining in interphase cells and by Western blotting as a 55-kDa doublet on whole-cell extracts. During mitosis, No55 was associated with chromosomes and appeared in prenucleolar bodies during telophase, but it did not colocalize with p80-coilin in coiled bodies. Immunoelectron microscopy revealed that No55 was localized uniformly throughout the granular component of the nucleolus compared with a more peripheral localization of nucleolar granular component protein B23. On segregation of the nucleolus with actinomycin D, No55 remained with the granular component of the segregated nucleolus, whereas protein B23 was found predominantly in the nucleoplasm. Finally, a cDNA expression library was screened with the human autoantibody against No55, and a 2.4-kb insert was isolated, subcloned to homogeneity, and then sequenced. Analysis of this sequence showed an open reading frame of approximately 1.3 kb coding for 437 amino acids with a predicted molecular weight of 50 kDa. A search of the gene sequence database indicated homology with SC65, a rat synaptonemal complex protein. Therefore, on the basis of molecular weight, nucleolar sublocalization, response to actinomycin D, and cDNA sequence determination, No55 is a novel protein of the interphase nucleolus.

Localisation of the Ki-67 antigen within the nucleolus. Evidence for a fibrillarin-deficient region of the dense fibrillar component

The Ki-67 antigen is detected in proliferating cells in all phases of the cell division cycle. Throughout most of interphase, the Ki-67 antigen is localised within the nucleous. To learn more about the relationship between the Ki-67 antigen and the nucleolus, we have compared the distribution of Ki-67 antibodies with that of a panel of antibodies reacting with nucleolar components by confocal laser scanning microscopy of normal human dermal fibroblasts in interphase stained in a double indirect immunofluorescence assay. During early G1, the Ki-67 antigen is detected at a large number of discrete foci throughout the nucleoplasm, extending to the nuclear envelope. During S-phase and G2, the antigen is located in the nucleolus. Double indirect immunofluorescence studies have revealed that during early to mid G1 the Ki-67 antigen is associated with reforming nucleoli within discrete domains which are distinct from domains containing two of the major nucleolar antigens fibrillarin and RNA polymerase I. Within mature nucleoli the Ki-67 antigen is absent from regions containing RNA polymerase I and displays only partial co-localisation within domains containing either fibrillarin or B23/nucleophosmin. Following disruption of nucleolar structure, induced by treatment of cells with the drug 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole or with actinomycin D, the Ki-67 antigen translocates to nucleoplasmic foci which are associated with neither fibrillarin nor RNA polymerase I. However, in treated cells the Ki-67 Ag remains associated with, but not co-localised to, regions containing B23/nucleophosmin. Our observations suggest that the Ki-67 antigen associates with a fibrillarin-deficient region of the dense fibrillar component of the nucleolus. Integrity of this region is lost following either nucleolar dispersal or nucleolar segregation.

Development-dependent localization of nuclear antigens in growing mouse oocytes

We have analyzed the distribution of nuclear and nucleolar proteins during the period of oocyte's growth. Oocytes were isolated mechanically or enzymatically from ovaries of juvenile mice of various ages (from 1 to 28 days after birth). Small nuclear ribonucleoproteins (snRNPs), the splicing factor SC-35, and a protein linked to cell proliferation (p-120) were detected by indirect immunofluorescence. snRNP distribution is consistent with the prophase state of oocyte's nuclei, while SC-35 (and p-120) exhibit a "speckled" distribution throughout the entire period of growth. The number of speckles (or foci) appears maximal around 10 days after birth, i.e., in the period of maximal transcriptional activity, and is sensitive to alpha-amanitin treatment. On the other hand, the immunofluorescent distribution of of nucleolin and p-103 (a nucleolar marker of the granular component) is compared to the ultrastructural distribution of the granular component analyzed by electron microscopy on oocytes of the same age.

Cell cycle related behavior of a chromosomal scaffold protein in MDCK epithelial cells

Because the mechanisms that govern mitosis are a key to the understanding of cell growth, the proteins associated with chromosomes specifically during this phase have received thorough attention. In the present work we report an Mr58000 protein in MDCK epithelial cells, recognized by a monoclonal antibody (LFM-1) that decorates chromosomes during M-phase. Cell fractionation methods followed by immunoblotting and immunofluorescence showed that this protein is associated with the nuclear fraction. Biochemical extraction procedures on isolated metaphase chromosomes from nocodazole-synchronized cells indicated that the Mr58000 protein behaves as a chromosomal scaffold protein, that is, it remains in the pellets after high salt (2M NaCl) or 3'-5' diiodosalicylic acid treatments, even in DNAse pre-digested samples. In addition, confocal microscopy of those chromosomes revealed the LFM-1 epitopes distributed on the external surface and the axis of chromatids. Parallel analysis of interphase nuclei revealed LFM-1 epitopes inside G1-, but excluded from G2-phase nuclei. These results were independently confirmed on nuclei sorted by flow cytometry and in cell populations synchronized by release of G1-/S-phase hydroxyurea arrest. The Mr58000 and a minor Mr38000 protein (which was enriched only in mitotic chromosomes of synchronized cells) were analyzed by Edman degradation. They shared the sequence at the amino-terminal end but failed to show total homology with known proteins. These results suggest that LFM-1 antigens fit some of the predictions of the licensing factor model, and may have a role in cell cycle dependent events.

Nuclear matrix proteins as structural and functional components of the mitotic apparatus

The eukaryotic nucleus is a membrane-enclosed compartment containing the genome and associated organelles supported by a complex matrix of nonhistone proteins. Identified as the nuclear matrix, this component maintains spatial order and provides the structural framework needed for DNA replication, RNA synthesis and processing, nuclear transport, and steroid hormone action. During mitosis, the nucleoskeleton and associated chromatin is efficiently dismantled, packaged, partitioned, and subsequently reassembled into daughter nuclei. The dramatic dissolution of the nucleus is accompanied by the assembly of a mitotic apparatus required to facilitate the complex events associated with nuclear division. Until recently, little was known about the fate or disposition of nuclear matrix proteins during mitosis. The availability of specific molecular probes and imaging techniques, including confocal microscopy and improved immunoelectron microscopy using resinless sections and related procedures, has enabled investigators to identify and map the distribution of nuclear matrix proteins throughout the cell cycle. This chapter will review the structure, function, and distribution of the protein NuMA (nuclear matrix mitotic apparatus) and other nuclear matrix proteins that depart the nucleus during the interphase/mitosis transition to become structural and functional components within specific domains of the mitotic apparatus.

The murine Ki-67 cell proliferation antigen accumulates in the nucleolar and heterochromatic regions of interphase cells and at the periphery of the mitotic chromosomes in a process essential for cell cycle progression

We have isolated the murine homologue of the human Ki-67 antigen. The Ki-67 antigen is used as a marker to assess the proliferative capacity of tumour cells; however, its cellular function is not known. The murine Ki-67 cDNA sequence (TSG126) was found to contain 13 tandem repeats, making up more than half of the total protein size. A comparison of this repetitive sequence block to its human counterpart, which contains 16 consecutive repeat units, revealed several conserved sequence motifs, including one motif frequently observed in proteins interacting with DNA. An antiserum developed against the product of the TSG126 cDNA clone identified a protein with an apparent molecular mass of 360 kDa, mainly expressed in proliferating cells. The TSG126 protein begins to accumulate during the late G1 stage of the cell cycle and is first seen as numerous small granules evenly distributed throughout the nucleus. During the S and the G2 phases, larger foci that overlap with the nucleoli and the heterochromatic regions are formed. At the onset of mitosis the TSG126 protein undergoes a dramatic redistribution process and becomes associated with the surface of the condensed chromosomes. The relative absence of the TSG126 protein from G1 interphase cells strongly argues against a model where the association of the TSG126 protein with mitotic chromosomes merely reflects a mechanism for the symmetrical distribution of nucleolar proteins between daughter cells. Instead, the intracellular distribution of the TSG126 protein during the cell cycle suggests that it could have a chromatin-associated function in both interphase and mitotic cells. Microinjection of anti-TSG126 antibodies into proliferating Swiss-3T3 fibroblasts was found to delay cell cycle progression, indicating that the TSG126 protein has an essential nuclear function.

Characterization of a cell cycle-dependent nuclear autoantigen

Analysis of reactivity to nuclear antigens in autoimmune sera revealed a serum that produced a previously undescribed cell cycle-dependent immunofluorescence staining pattern. By indirect immunofluorescence using HEp-2 cells as substrate, the serum generated a speckled and nucleolar pleomorphic staining pattern. This characteristic immunofluorescence pattern was detected in different cell lines from various species indicating that the antigen was highly conserved. This serum immunoprecipitated a 85 kDa protein using an extract from [35S]-labeled HeLa cells. Indirect immunofluorescence of proliferating mouse 3T3 cells displayed the characteristic pleomorphic staining which was not observed in serum-starved cells. Resting human and mouse peripheral blood lymphocytes were negative in immunofluorescence while mitogen-stimulated lymphocytes were positive. Germinal centers of mice two weeks after immunization with 2-phenyl-oxazolone showed speckled immunofluorescence staining in the dark zones whereas unimmunized mice were completely negative. Cell synchronization experiments showed a characteristic sequence of locations of the antigen during the cell cycle. In G1, cells were completely negative. In late G1, G1/S and S phase, speckles were visible. In early G2, speckles were visible, and later in G2, the nucleoli were positive. During mitosis chromosomes were stained. Further characterization of this antibody specificity and cloning of cDNA are in progress.

Cell-cycle-dependent alterations of a highly phosphorylated nucleolar protein p130 are associated with nucleologenesis

We identified a novel human nucleolar phosphoprotein p130 (130 kDa) using a strategy for selecting monoclonal antibodies against nuclear proteins which oscillate in the cell cycle. p130 is localized in interphase nucleoli in a dotted manner. Complete extraction of p130 required a high concentration of salt (0.5 M NaCl) indicating that it binds firmly to the nucleolar components via ionic interaction. p130 is heavily phosphorylated, since alkaline phosphatase treatment converted the purified p130 into a 95 kDa product; this was further supported by the in vitro demonstration that cellular phosphatase and casein kinase II activities were responsible for the interchange of these two forms. Extracts of mitotic cells had lower concentrations of p130 compared to those of interphase cells suggesting that a proportion of p130 might be degraded during mitosis. Moreover, all the remaining p130 in mitotic cells was further phosphorylated, likely by a cdc2 kinase, resulting in increase in its solubility, and its dispersion throughout the entire cytoplasm. Thus, p130 in metaphase and anaphase cells was unable to be detected by immunofluorescence microscopy. At telophase, p130 reappeared and aggregated into a granular structure, resembling the prenucleolar bodies. These granules migrated from the nucleoplasm to the nucleoli in early G1-phase. Actinomycin D was able to induce segregation of p130-containing granules into the nucleoplasm, similar to the well-known behavior of the fibrillarin-containing granules, indicating that p130 is localized in the dense fibrillar component, a subnucleolar region for pre-rRNA synthesis and processing. The cDNA sequence of p130 revealed a remarkable feature, that a serine-rich stretch interspersed with acidic residues is repeated ten times. Such a characteristic is shared with a rat nucleolar phosphoprotein Nopp140, which is thought to shuttle between the nucleolus and the cytoplasm. Although p130 shows 74% identity to Nopp140, our observations suggest that during mitosis the functions of p130 are related to nucleologenesis.

The chromosome periphery during mitosis

A complex structure, visible by electron microscopy, surrounds each chromosome during mitosis. The organization of this structure is distinct from that of the chromosomes and the cytoplasm. It forms a perichromosomal layer that can be isolated together with the chromosomes. This layer covers the chromosomes except in centromeric regions. The perichromosomal layer includes nuclear and nucleolar proteins as well as ribonucleoproteins (RNPs). The list of proteins and RNAs identified includes nuclear matrix proteins (perichromin, peripherin), nucleolar proteins (perichro-monucleolin, Ki-67 antigen, B23 protein, fibrillarin, p103, p52), ribosomal proteins (S1) and snRNAs (U3 RNAs). Only limited information is available about how and when the perichromosomal layer is formed. During early prophase, the proteins extend from the nucleoli towards the periphery of the nucleus. Thin cordon-like structures reach the nuclear envelope delimiting areas in which chromosomes condense. At telophase, the proteins are associated with the part of the chromosomes remaining condensed and accumulate in newly formed nucleoli in regions where chromatin is already decondensed. The perichromosomal layer contains several different classes of proteins and RNPs and it has been attributed various roles: (1) in chromosome organization, (2) as a barrier around the chromosomes, (3) involvement in compartmentation of the cells in prophase and telophase and (4) a binding site for chromosomal passenger proteins necessary to the early process of nuclear assembly.

Cell cycle redistribution of U3 snRNA and fibrillarin. Presence in the cytoplasmic nucleolus remnant and in the prenucleolar bodies at telophase

The distribution of the U3 small nuclear RNA during the cell cycle of the CHO cell line was studied by in situ hybridization using digoxigenin-labelled oligonucleotide probes. The location of the hybrids by immunofluorescence microscopy and at the ultrastructural level was correlated with the distribution of two nucleolar proteins, nucleolin and fibrillarin. The U3 snRNA molecules persist throughout mitosis in close association with the nucleolar remnant. U3 snRNA is present in the prenucleolar bodies (PNBs) and could participate in nucleologenesis in association with several nucleolar proteins such as nucleolin and fibrillarin. The interaction of U3 snRNP with the 5' external spacer of pre-RNA newly synthesized by active NORs is proposed to be the promoting event of nucleologenesis.

The nucleolus

Nucleoli are the sites of biosynthesis of the ribosomal precursors. They contain may copies of the genes for the main rRNAs (18S- and 28 S-rRNA) in the form of tandemly arranged repeats at the chromosomal nucleolar organizer regions (NORs). They also contain the small rRNA (5S-rRNA) that is synthesized outside the nucleolus, specific nucleolar proteins, among them the factors and enzymes necessary for transcription and transcript processing, and the precursor units of the ribosomes. In man as in may vertebrate species, three main components of nucleoli, besides chromatin, can be detected: fibrillar centres (FC), dense fibrillar component (DCF), and granular component (GC). Within a nucleolus the FCs are in many cases situated in its central region. The DFc forms a network of strands surrounding the FCs, but may sometimes reach for out towards the periphery of the nucleolus. The GC is usually situated in the peripheral regions of the nucleolus. In cells with a low level of ribosomal biosynthesis the nucleoli are small, usually with a single FC and little surrounding DFC and GC ("ring-shaped nucleolus"). In active cells the DFC forms a large network enclosing several, sometimes up to hundreds of FCs, and the GC covers a large area in the periphery ("compact nucleoli"). In cells at the onset of a new stimulation, the DFC is very prominent whereas the FCs are few and small, and the GC is also not very extensive ("reticulate nucleoli"). In some special cell types that are very active other arrangements of the structural components are found. In Sertoli cells, for instance, only one nucleolus is found, or occasionally two, each with a single large FC and a distinct area of GC, both areas being engulfed by DFC intermingled with some peripheral GC. Immunocytological and in situ hybridization studies to localize the rRNA genes within the nucleolus have so far led to divergent results. Both fibrillar components, the FCs and the DFC, have been claimed as the most probable candidates. Transcription of rDNA and the subsequent early steps of ribosome biosynthesis are localized in the DFC, whereas later steps (mature rRNA, preribosomes) are localized in the GC. The FCs may also serve as sites for the preparation of the rDNA for transcription, and as a store for certain nucleolar proteins. During mitosis, parts of the nucleolar proteins remain at the NORs. A direct contact between the nucleolus and the nuclear envelope is frequently observed but is not dependent on nucleolar activity.(ABSTRACT TRUNCATED AT 400 WORDS)