Immunofluorescent localization of the proteins of nuclear ribonucleoprotein complexes

Antibodies were raised in chickens against heterogeneous nuclear RNA (hnRNA)-binding proteins from 30S ribonucleoprotein (RNP) complexes of mouse Taper hepatoma ascites cell nuclei. The antibody preparations were characterized for immunological specificity and purity by double- diffusion gels, binding to specific bands in SDS polyacrylamide gels, and crossed immunoelectrophoresis. Antibodies raised against either whole 30S RNP complexes or purified RNP core proteins had a strong selective affinity for the four 34,000- to 40,000-dalton polypeptides which comprise the major structural proteins of hnRNP. The intracellular distribution of 30S RNP antigens in mouse ascites cells was determined by indirect immunofluorescence microsacopy. In interphase cells immunofluorescent sites were restricted to the nucleus, and nucleoli were free of fluorescence. The chicken anti-mouse- RNP antibodies were also able to react with cells from many different vertebrate species, showing a similar nucleus-restricted localization of the reacting sites. The antibodies also bound chick 30S RNP-proteins and reacted with the nuclei of chick cells. An exception to this was the failure of the antibody to bind to adult chick erythrocytes, suggesting that these major hnRNA binding proteins may be found only in nuclei capable of RNA synthesis.     

Acceptor proteins for (ADP-ribose)n in the HeLa S3 cell cycle

The acceptor proteins for (ADP-ribose)n were investigated by using nuclei or chromosomes isolated from specific phases of the cell cycle of HeLa S3 cells. Analysis of HMG proteins and histone H1 by acetic acid/urea polyacrylamide gel electrophoresis demonstrated that the (ADP-ribosyl)n-ation of HMG 14 and 17 and histone H1 increased by 12- and 5-fold, respectively, in the metaphase chromosomes as compared with that in the G1 phase cell nuclei. The degree of (ADP-ribosyl)n-ation of these proteins in the S phase cell nuclei was as low as that in G1 phase cell nuclei. In the G2 phase cell nuclei, the degrees of (ADP-ribosyl)n-ation of HMG 14 and 17 and histone H1 were about 5- and 2-fold greater, respectively, as compared with that in the G1 phase cell nuclei. The (ADP-ribosyl)n-ation of HMG 1 and 2 was constant through the cell cycle except for a slight decrease in the S phase. The data may imply that the (ADP-ribosyl)n-ation of HMG 14 and 17 and histone H1 is linked to chromatin structural changes in mitosis.     

Induction of permeability changes and death of vertebrate cells is modulated by the virulence of Entamoeba spp. isolates

Although Entamoeba histolytica is capable of inducing an apoptotic response in vertebrate cells in vitro (Cell. Microbiol. 2 (2000) 617), it is not known whether vertebrate cell death requires direct amoeba-vertebrate cell contact or simply the presence of amoebae in the area of the vertebrate cells. In addition, Entamoeba spp. vary in their virulence and pathogenicity. The potential effects of these critical parameters also have not been elucidated. We tested the virulent HM-1:IMSS isolate and the non-virulent Rahman isolate of E. histolytica, and the non-virulent E. dispar CYNO16:TPC isolate against two vertebrate cell lines, HeLa and Chinese hamster ovary cells in vitro using ethidium homodimer as a fluorescent indicator of changes in vertebrate cell permeability. Fluorescence appeared in vertebrate cell nuclei within approximately 2-3 min of contact between HM-1 amoebae and vertebrate cells independent of vertebrate cell type. However, vertebrate cells in the immediate vicinity of but not contacted by HM-1 amoebae were not affected. In contrast, although both E. histolytica Rahman and E. dispar CYNO16 amoebae moved freely among and contacted vertebrate cells, the nuclei of the vertebrate cells never fluoresced implying that the cells remained alive and impermeant to the ethidium homodimer. This is the first demonstration that direct contact between virulent amoebae and vertebrate cells is required to kill vertebrate cells and that the process is restricted to virulent Entamoeba isolates. An understanding at the molecular level of the processes involved could help to reduce the pathology associated with this parasite.     

Nuclear antigens in the HeLa cell cycle

Summary Antisera to 0.35 M NaCl extracts and residues of S phase HeLa nuclei were reacted with electrophoretically separated proteins from the nuclei or nuclear material of HeLa cells synchronized in G₁, S, G₂ or M phases of the cell cycle. Quantitative evaluation of the peroxidase-antiperoxidase stained nitrocellulose transfers (Western blots) revealed significant changes in the quantities of nuclear non-histone proteins during the cell cycle. Immunochemical staining of electrophoretically separated nuclear antigens permits their selective detection in minute quantities and in the presence of many additional proteins.     

Regulation of interkinetic nuclear migration by cell cycle-coupled active and passive mechanisms in the developing brain

A hallmark of neurogenesis in the vertebrate brain is the apical-basal nuclear oscillation in polarized neural progenitor cells. Known as interkinetic nuclear migration (INM), these movements are synchronized with the cell cycle such that nuclei move basally during G1-phase and apically during G2-phase. However, it is unknown how the direction of movement and the cell cycle are tightly coupled. Here, we show that INM proceeds through the cell cycle-dependent linkage of cell-autonomous and non-autonomous mechanisms. During S to G2 progression, the microtubule-associated protein Tpx2 redistributes from the nucleus to the apical process, and promotes nuclear migration during G2-phase by altering microtubule organization. Thus, Tpx2 links cell-cycle progression and autonomous apical nuclear migration. In contrast, in vivo observations of implanted microbeads, acute S-phase arrest of surrounding cells and computational modelling suggest that the basal migration of G1-phase nuclei depends on a displacement effect by G2-phase nuclei migrating apically. Our model for INM explains how the dynamics of neural progenitors harmonize their extensive proliferation with the epithelial architecture in the developing brain.     

Mutation of the hamster cell cycle gene RCC1 is complemented by the homologous genes of Drosophila and S.cerevisiae.

The RCC1 gene has been isolated from several vertebrates, including human, hamster and Xenopus. Genes similar to RCC1, namely BJ1 and SRM1/PRP20, have been isolated from the insect Drosophila and from the budding yeast Saccharomyces cerevisiae. A mutation of the RCC1 gene in the hamster BHK21 cell line, tsBN2, confers pleiotropic phenotypes, including G1 arrest and premature induction of mitosis in cells synchronized at the G1/S boundary. Similarly, mutations of the SRM1/PRP20 gene are pleiotropic; the srm1 mutant shows G1 arrest and suppression of the mating defect of mutants lacking pheromone receptors, and the prp20 mutant shows an alteration in mRNA metabolism. Here we show that both BJ1 and SRM1/PRP20 complement the temperature sensitive phenotype of the tsBN2 cells. Like RCC1 proteins of vertebrates, the protein products of the Drosophila and yeast RCC1 homologues were located in the nuclei of the mammalian cells. These results suggest that the BJ1 and SRM1/PRP20 genes are functionally equivalent to the vertebrate RCC1 genes, and that the RCC1 gene plays an important role in the regulation of gene expression in the eukaryotic cell cycle.     

MPF Induction of Microtubule Assembly at Interphase Kinetochore of CHO Cells

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Proteins associated with heterogeneous nuclear RNA in eukaryotic cells

When HeLa cell nuclei axe mechanically disrupted in either hypotonic or isotonic buffers, heterogeneous nuclear RNA is recovered from the post-nucleolar fraction in the form of EDTA-resistant ribonucleoprotein particles, which sediment between 40 S and 250 S in sucrose gradients containing 0.01 m or 0.15 m-NaCl. That the RNA in these particles is HnRNA² is indicated by its heterodisperse sedimentation (20 to 80 S) and its continued synthesis in concentrations of actinomycin D that selectively inhibit the synthesis of ribosomal RNA. The specificity of the HnRNA-protein complexes is evidenced by the failure of deliberate attempts to generate artificial RNP by the addition of deproteinized HnRNA to intact or disrupted nuclei at low ionic strength.The proteins bound to HnRNA are complex. In HeLa cells, HnRNP particles contain proteins with molecular weights from 39,000 to approximately 180,000 (sodium dodecyl sulfate-polyacrylamide gel electrophoresis) and isoelectric points between 4.9 and 8.3 (analytical isoelectric focusing). They are readily distinguishable from proteins in other cell fractions, including those in chromatin.Exposure of HeLa HnRNP particles to 0.5 m-NaCl reduces their average sedimentation velocity by approximately 30%. CsCl density-gradient analysis reveals that this is accompanied by the loss of a major portion of the proteins. However, a significant fraction of the HnRNP (25 to 30%) is resistant to high salt concentrations and continues to band at the same density as native HnRNP (1.43 g/cm³). This is true even after prolonged exposure (24 h) to high salt. The salt-resistant HnRNP is enriched for proteins above 60,000 molecular weight. In at least these two respects, this sub-class of HnRNP resembles “messenger RNP” prepared from cytoplasmic polyribosomes, which is also salt-stable and contains relatively high molecular weight proteins.HnRNP particles can also be recovered from HeLa cell nuclei lysed in high salt but these contain many extra proteins, notably histones, and sediment much faster in sucrose gradients than particles prepared as above. HnRNP is not liberated by extracting HeLa nuclei in 0.14 m-NaCl, pH 8.0 (Samarina et al., 1967) unless the temperature is 20 °C or higher. In this case the particles are converted to 45 S structures, which contain partially degraded HnRNA. 45 S particles can also be produced by subjecting 40 to 250 S HnRNP to a very limited digestion with pancreatic ribonuclease (1 to 2 hits/molecule).HnRNP particles have similar sedimentation velocities (40 to 300 S) when isolated under physiological ionic conditions from a variety of mammalian cells, including WI38 human diploid fibroblasts, mouse L-cells, monkey kidney cells and rat liver. However, electrophoresis reveals a distinct pattern of HnRNP proteins for each cell type. It is proposed that this cell-specificity reflects a situation in which HnRNA molecules that differ in nucleotide sequence are complexed with different sets of proteins, so that the resulting HnRNP particles are biochemically distinct at each genetic locus. This hypothesis is discussed in relation to the cytology of lampbrush and polytene chromosomes.     

Co-expressed Cyclin D variants cooperate to regulate proliferation of germline nuclei in a syncytium

The role of the G1-phase Cyclin D-CDK 4/6 regulatory module in linking germline stem cell (GSC) proliferation to nutrition is evolutionarily variable. In invertebrate Drosophila and C. elegans GSC models, G1 is nearly absent and Cyclin E is expressed throughout the cell cycle, whereas vertebrate spermatogonial stem cells have a distinct G1 and Cyclin D1 plays an important role in GSC renewal. In the invertebrate, chordate, Oikopleura, where germline nuclei proliferate asynchronously in a syncytium, we show a distinct G1-phase in which 2 Cyclin D variants are co-expressed. Cyclin Dd, present in both somatic endocycling cells and the germline, localized to germline nuclei during G1 before declining at G1/S. Cyclin Db, restricted to the germline, remained cytoplasmic, co-localizing in foci with the Cyclin-dependent Kinase Inhibitor, CKIa. These foci showed a preferential spatial distribution adjacent to syncytial germline nuclei at G1/S. During nutrient-restricted growth arrest, upregulated CKIa accumulated in arrested somatic endoreduplicative nuclei but did not do so in germline nuclei. In the latter context, Cyclin Dd levels gradually decreased. In contrast, the Cyclin Dbβ splice variant, lacking the Rb-interaction domain and phosphodegron, was specifically upregulated and the number of cytoplasmic foci containing this variant increased. This upregulation was dependent on stress response MAPK p38 signaling. We conclude that under favorable conditions, Cyclin Dbβ-CDK6 sequesters CKIa in the cytoplasm to cooperate with Cyclin Dd-CDK6 in promoting germline nuclear proliferation. Under nutrient-restriction, this sequestration function is enhanced to permit continued, though reduced, cycling of the germline during somatic growth arrest.     

Ionophore A23187 raises cyclic AMP levels in macrophages by stimulating prostaglandin E formation.

A class of non-histone chromatin proteins that were bound tightly to DNA and could not be dissociated from the chromatin by high salt and urea was isolated from HeLa cell nuclei and separated from DNA by DNase digestion. These ‘tight’ proteins retained their ability to bind to single- and double-stranded DNA as assayed by nitrocellulose filter binding. Polyacrylamide gel electrophoresis showed that the most prominent proteins possessed molecular weights of about 60 000 D. In asynchronously growing HeLa cell cultures about $\text{1}\text{3}$ of the cell nuclei were immunoreactive to fluorescein-labeled antinucleoside antibodies. The immunoreactive cells were the fraction in S phase. Cycloheximide treatment of the cultures raised the fraction of immunoreactive nuclei to over $\text{sol2}\text{3}$. Exposure of the fixed cycloheximide-treated cell to tight proteins prior to staining with the antibody reduced the fraction of immunoreactive cells to the normal S phase level. Immuno-reactivity induced by X-irradiation or by the intercalating mutagen hycanthone was also suppressed by tight proteins. Cycloheximide treatment preferentially reduced the cellular content of tight proteins, suggesting that these proteins undergo a metabolic turnover with a half-life of about 5 h.     

A nuclear protein that binds preferentially to methylated DNA in vitro may play a role in the inaccessibility of methylated CpGs in mammalian nuclei

The effects of DNA methylation on gene expression and chromatin structure suggest the existence of a mechanism in the nucleus capable of distinguishing methylated and non-methylated sequences. We report the finding of a nuclear protein in several vertebrate tissues and cell lines that binds preferentially to methylated DNA in vitro. Its lack of sequence-specific requirements makes it potentially capable of binding to any methylated sequence in mammalian nuclei. An in vivo counterpart of these results is that methylated CpGs are inaccessible to nucleases within nuclei. In contrast, non-methylated CpG sites, located mainly at CpG islands, and restriction sites not containing this dinucleotide, are relatively accessible. The possibility that DNA methylation acts through binding to specific proteins that could alter chromatin structure is discussed.     

Removal of a single pore subcomplex results in vertebrate nuclei devoid of nuclear pores

The vertebrate nuclear pore complex, 30 times the size of a ribosome, assembles from a library of soluble subunits and two membrane proteins. Using immunodepletion of Xenopus nuclear reconstitution extracts, it has previously been possible to assemble nuclei lacking pore subunits tied to protein import, export, or mRNA export. However, these altered pores all still possessed the bulk of pore structure. Here, we immunodeplete a single subunit, the Nup107-160 complex, using antibodies to Nup85 and Nup133, two of its components. The resulting reconstituted nuclei are severely defective for NLS import and DNA replication. Strikingly, they show a profound defect for every tested nucleoporin. Even the integral membrane proteins POM121 and gp210 are absent or unorganized. Scanning electron microscopy reveals pore-free nuclei, while addback of the Nup107-160 complex restores functional pores. We conclude that the Nup107-160 complex is a pivotal determinant for vertebrate nuclear pore complex assembly.     

Localisation of 26S proteasomes with different subunit composition in insect muscles undergoing programmed cell death

The 26S proteasome is a large multisubunit complex involved in degrading both cytoplasmic and nuclear proteins. We have investigated the subcellular distribution of four regulatory ATPase subunits (S6 (TBP7/MS73), S6' (TBP1), S7 (MSS1), and S10b (SUG2)) together with components of 20S proteasomes in the intersegmental muscles (ISM) of Manduca sexta during developmentally programmed cell death (PCD). Immunogold electron microscopy shows that S6 is located in the heterochromatic part of nuclei of ISM fibres. S6' is present in degraded material only outside intact fibres. S7 can be detected in nuclei, cytoplasm and also in degraded material. S10b, on the other hand, is initially found in nuclei and subsequently in degraded cytoplasmic locations during PCD. 20S proteasomes are present in all areas where ATPase subunits are detected, consistent with the presence of intact 26S proteasomes. These results are discussed in terms of heterogeneity of 26S proteasomes, 26S proteasome disassembly and the possible role of ATPases in non-proteasome complexes in the process of PCD. Cell Death and Differentiation (2000) 7, 1210 - 1217.     

Coordinated nuclear export of 60S ribosomal subunits and NMD3 in vertebrates

60S and 40S ribosomal subunits are assembled in the nucleolus and exported from the nucleus to the cytoplasm independently of each other. We show that in vertebrate cells, transport of both subunits requires the export receptor CRM1 and Ran.GTP. Export of 60S subunits is coupled with that of the nucleo- cytoplasmic shuttling protein NMD3. Human NMD3 (hNMD3) contains a CRM-1-dependent leucine-rich nuclear export signal (NES) and a complex, dispersed nuclear localization signal (NLS), the basic region of which is also required for nucleolar accumulation. When present in Xenopus oocytes, both wild-type and export-defective mutant hNMD3 proteins bind to newly made nuclear 60S pre-export particles at a late step of subunit maturation. The export-defective hNMD3, but not the wild-type protein, inhibits export of 60S subunits from oocyte nuclei. These results indicate that the NES mutant protein competes with endogenous wild-type frog NMD3 for binding to nascent 60S subunits, thereby preventing their export. We propose that NMD3 acts as an adaptor for CRM1-Ran.GTP-mediated 60S subunit export, by a mechanism that is conserved from vertebrates to yeast.