Localization of the 70 000 dalton heat-induced protein in the nuclear matrix of BHK cells

Heat shock enhances the susceptibility of BHK cells to rotavirus infection through the facilitation of entry and post-entry virus replication steps

Rotavirus infection is known to induce several cellular stress proteins, although their possible involvement in the replication cycle of the virus has not been studied. In addition, the heat shock cognate protein hsc70 has been shown to function as a post-attachment receptor during virus entry. In this work we have studied the effect of heat shock on the susceptibility of cells to rotavirus infection. BHK cells, which are largely refractory to the virus, became about 100-fold more susceptible when heat-treated, while the rotavirus highly susceptible MA104 cells did not significantly modified their susceptibility upon heat stress, suggesting that heat shock induces factors that are rate-limiting the replication of rotaviruses in BHK but not in MA104 cells. The heat treatment was shown to facilitate the rotavirus infection of BHK cells at the penetration and post-penetration levels, and each of these stages seems to contribute comparably to the overall observed 100-fold increase in infectivity. Since the binding of the virus to the cell surface was not affected, the caloric stress probably facilitates the penetration and/or uncoating of the virus. The pathway of virus entry into heat-shocked BHK cells seems to be similar to that used in MA104 cells, since treatments that affect MA104 cell infection also affected rotavirus infectivity in heat-treated BHK cells.

A proteomic study of the arabidopsis nuclear matrix

The eukaryotic nucleus has been proposed to be organized by two interdependent nucleoprotein structures, the DNA-based chromatin and the RNA-dependent nuclear matrix. The functional composition and molecular organization of the second component have not yet been resolved. Here, we describe the isolation of the nuclear matrix from the model plant Arabidopsis, its initial characterization by confocal and electron microscopy, and the identification of 36 proteins by mass spectrometry. Electron microscopy of resinless samples confirmed a structure very similar to that described for the animal nuclear matrix. Two-dimensional gel electrophoresis resolved approximately 300 protein spots. Proteins were identified in batches by ESI tandem mass spectrometry after resolution by 1D SDS-PAGE. Among the identified proteins were a number of demonstrated or predicted Arabidopsis homologs of nucleolar proteins such as IMP4, Nop56, Nop58, fibrillarins, nucleolin, as well as ribosomal components and a putative histone deacetylase. Others included homologs of eEF-1, HSP/HSC70, and DnaJ, which have also been identified in the nucleolus or nuclear matrix of human cells, as well as a number of novel proteins with unknown function. This study is the first proteomic approach towards the characterization of a higher plant nuclear matrix. It demonstrates the striking similarities both in structure and protein composition of the operationally defined nuclear matrix across kingdoms whose unicellular ancestors have separated more than one billion years ago.

The nuclear matrix is a thermolabile cellular structure

Heat shock sensitizes cells to ionizing radiation, cells heated in S phase have increased chromosomal aberrations, and both Hsp27 and Hsp70 translocate to the nucleus following heat shock, suggesting that the nucleus is a site of thermal damage. We show that the nuclear matrix is the most thermolabile nuclear component. The thermal denaturation profile of the nuclear matrix of Chinese hamster lung V79 cells, determined by differential scanning calorimetry (DSC), has at least 2 transitions at Tm = 48 degrees C and 55 degrees C with an onset temperature of approximately 40 degrees C. The heat absorbed during these transitions is 1.5 cal/g protein, which is in the range of enthalpies for protein denaturation. There is a sharp increase in 1-anilinonapthalene-8-sulfonic acid (ANS) fluorescence with Tm = 48 degrees C, indicating increased exposure of hydrophobic residues at this transition. The Tm = 48 degrees C transition has a similar Tm to those predicted for the critical targets for heat-induced clonogenic killing (Tm = 46 degrees C) and thermal radiosensitization (Tm = 47 degrees C), suggesting that denaturation of nuclear matrix proteins with Tm = 48 degrees C contribute to these forms of nuclear damage. Following heating at 43 degrees C for 2 hours, Hsc70 binds to isolated nuclear matrices and isolated nuclei, probably because of the increased exposure of hydrophobic domains. In addition, approximately 25% of exogenous citrate synthase also binds, indicating a general increase in aggregation of proteins onto the nuclear matrix. We propose that this is the mechanism for increased association of nuclear proteins with the nuclear matrix observed in nuclei Isolated from heat-shocked cells and is a form of indirect thermal damage.

Flow cytometric studies of the nuclear matrix

We have devised a method to measure the protein and nucleic acid content of the nuclear matrix using flow cytometry. Nuclear matrices were prepared from nuclei by DNase I digestion followed by 3 M NaCl extraction. The resulting particles were stained with fluorescein isothiocyanate (FITC) for protein and propidium iodide (PI) for double-stranded nucleic acids, and fluorescence as well as forward angle light scatter was detected. The matrices were also subjected to additional chemical or enzymatic perturbations, and changes in the above parameters were measured. Results showed that matrices from heat-shocked cells not only retained the majority of heat-induced excess nuclear protein, but also exhibited higher PI signals than controls after RNase A digestion. This observation did not hold if RNase A digestion preceded high-salt extraction, suggesting that a salt-extractable moiety had been replaced or altered by heat so that double-stranded RNA was protected from the nucleolytic attack. The residual PI fluorescence in matrices from heated cells bore a linear relationship to the increased protein content in those matrices, indicating that the excess protein sequesters matrix-associated RNA. Polyacrylamide gel electrophoresis of matrix polypeptides revealed increased amounts of many proteins as a result of heat as well as the appearance of several new proteins, one of which comigrates with the HSP72/73 heat-shock proteins. The results of these studies show that flow cytometry can be used to study the nuclear matrix and is capable of detecting changes that result from alterations in its protein composition.

Synthesis of shock proteins in cultured fetal mouse myocardial cells

We examined the synthesis of shock proteins in cultured fetal mouse myocytes. The preparation is free from fibroblasts, and the cells are vital and morphologically intact with respect to beat frequency and electron microscopy. Cultured myocytes from fetal mouse heart respond to heat shock and cadmium chloride, H2O2, allylamine, cyclosporine, and azathioprine exposure with the synthesis of shock proteins. Heat shock induces the de novo synthesis of two proteins of 71 and 68 kDa; cadmium chloride induces, in addition, a protein of 30 kDa. The other substances tested provoke the synthesis only of the 30-kDa polypeptide. The formation of heat shock proteins is concentration-dependent: Cyclosporine provokes the de novo synthesis of the 30-kDa polypeptide at concentrations above 10 ng/ml, whereas azathioprine causes the same effect at concentrations above 50 micrograms/ml. Hence cyclosporine might be cardiotoxic already at concentrations below the pharmacological dosages while azathioprine influences the myocytes only at concentrations much higher than the therapeutic level. Our results indicate that heat shock protein expression in cultured myocytes may be a useful tool to monitor cardiotoxicity.

Control of ribosome biosynthesis in plant cell cultures under heat-shock conditions. Ribosomal RNA

The immediate block of ribosome biosynthesis in heat-shocked tomato cell cultures is primarily caused by the complete inhibition of pre-rRNP processing. Depending on the heat-shock conditions synthesis of pre-rRNP goes on, though at a reduced level. Synthesis and/or preservation of pre-rRNP during heat shock as well as its efficient processing in the recovery period are thoroughly improved by preconditioning of cells to the hyperthermic treatment. Such preinduced cultures are characterized by their content of preformed heat-shock proteins, whose dominant representative (hsp 70) becomes highly enriched in the characteristic granular rRNP material observed in nucleoli of heat-shocked cells. This is shown by immune fluorescence staining and microautoradiography.

Heat shock (45 degrees C) results in an increase of nuclear matrix protein mass in HeLa cells

The nuclear matrix from HeLa cells heated at 45 degrees C was isolated to determine the effect of thermal shock on its composition and structure. The matrix from unheated cells contained about 10 per cent of total cell protein and was observed to be spherical particle with a diameter ranging from 3 to 5 microns with the major constituent polypeptides having molecular weights of 45, 47, 55, 57, 59 and 65 kilodaltons. The nuclear-matrix protein mass increased linearly with increasing exposure time at 45 degrees C with no observable change in its size or shape. The additional proteins were observed in general to have molecular weights greater than 45 kilodaltons, with marked increases in polypeptides of 28.5, 38.5, 60, 66, 75, 81, 88, 100 and 115 kilodaltons. An exponential relationship was observed between heat-induced cytotoxicity and the nuclear matrix protein mass increase. A 15 per cent increase in matrix protein mass was sustained prior to the onset of cytotoxicity, while a 35 per cent increase in matrix protein content was associated with a 63 per cent probability of cell killing. The results indicate that redistribution of cell protein or alterations in the mass or structure of the nuclear matrix may be involved in heat-induced cytotoxicity.

Intracellular distribution of 73,000 and 72,000 dalton heat shock proteins in HeLa cells

Intracellular localization of 73,000 and 72,000 dalton heat shock proteins (HSP73/72) in HeLa cells that were heat shocked or treated with chemical stressors was investigated using indirect immunofluorescent staining. The antiserum used specifically recognized the HSP73/72 in HeLa cells, and HSPs were increased by heating cells at 42 degrees C for 2 or 4 h and by prior treatment with chemical stressors (sodium arsenite, cadmium chloride, 8-hydroxyquinoline and ethanol). There was diffuse cytoplasmic staining at 37 degrees C, whereas nucleoli were stained brightly when cells were heated at 42 degrees C for 2 h. This rapid accumulation of HSP73/72 in the nucleoli was not inhibited by cycloheximide (50 micrograms/ml). Translocation of HSPs to the nucleoli was specific for heat because no translocation was induced by treatment with chemical stressors. When the cells were returned to 37 degrees C after heating, the HSPs in their nucleoli disappeared rapidly and diffuse cytoplasmic staining was present after 6-9 h. Our results suggest that the transient accumulation of HSP73/72 in HeLa cell nucleoli that is induced by heat shock is not correlated with the development of thermotolerance obtained in other cell systems.

Expression of a Drosophila heat shock protein in mammalian cells: transient association with nucleoli after heat shock

We transformed mouse L cells with a cloned Drosophila hsp70 gene and obtained cells with either heat-inducible or constitutively expressed copies of the gene. The distribution of hsp70 in these cells was examined by indirect immunofluorescence with monoclonal antibodies specific to the Drosophila protein. In constitutive cells, hsp70 was present in both cytoplasm and nucleus. After heat shock, the nuclear hsp70 was transiently concentrated in nucleoli, from which it had previously been excluded; the cytoplasmic hsp70 moved to a perinuclear location, a result consistent with it being associated with intermediate filaments of the cytoskeleton. The nucleolar migration took several hours and was partly inhibited by actinomycin D, but was independent of protein synthesis; it may reflect binding to newly-synthesized rRNA. Drosophila hsp70 was also expressed from replicating plasmids in monkey COS cells, and was found to be concentrated in nuclei even at low temperature. Migration to nucleoli occurred after heat shock. These results indicate that a single protein can have multiple interactions with cellular components, and form the basis for future studies of these interactions by in vitro mutagenesis and expression of the hsp70 gene.

Characterization of lamin proteins in BHK cells

Lamins are structural proteins found in rat liver nuclear envelope and are major constituents of the nuclear matrix. 2-D gel electrophoresis indicates that BHK cell nuclear matrix is composed of four major proteins (62 kD, 68 kD, 70 kD and 72 kD). Three of these proteins are very similar to lamins A, B and C of rat liver nuclear envelope according to their molecular mass and isoelectric points. An anti-serum specific to BHK matrix proteins has been raised. On 2-D immunoblot, this serum detects all the 62, 68 and 72 kD polypeptide isovariants but only one of the two isovariants of the 70 kD polypeptide. Rat lamins A, B and C react with the anti-BHK matrix serum. However, when a monoclonal antibody to rat liver lamins A, B and C is used (Burke, B, Tooze, J & Warren, G, EMBO j 2 (1983) 361 [23]), only the 72 kD (lamin A-like) and the 62 kD (lamin C-like) BHK polypeptides are detected. Our results suggest that although a strong similarity exists between BHK and rat lamins, there is no identical cross-reactivity between the two species.