Levels of chromosomal protein high mobility group 2 parallel the proliferative activity of testis, skeletal muscle, and other organs

Localization of HMG chromosomal proteins in the nucleus and cytoplasm by microinjection of functional antibody fragments into living fibroblasts

We have used microinjection and cell fractionation to localize the chromosomal high mobility group proteins (HMG) in human fibroblasts. Electrophoretic analysis of nuclear and cytoplasmic fractions from the fibroblasts indicates that the concentration of HMG-1,2 in the cytoplasm is 2.9 times larger than in the nucleus indicating that the majority of the cellular HMG-1,2 is present in the cytoplasm. In contrast, HMG-17 remains predominant in the nuclear fraction. We conclude that the cellular distribution of HMG-1,2 is significantly different from that of HMG-17. To avoid possible artifacts due to cell fractionation, fluoresceinated HMG-1 and HMG antibodies were microinjected into living fibroblasts. The cellular distribution of the injected proteins was monitored using fluorescent microscopy. Fluoresceinated HMG-1 microinjected into the cytoplasm moves very rapidly into the nucleus and concentrates in the nucleolus of living human fibroblasts. However, some control non-nuclear proteins also migrated into the nucleus raising the possibility that exogenous injected proteins do not always distribute in the same pattern as the endogenous proteins. The localization of microinjected F(ab)2 fragments derived from anti-HMG-1 was compared to that of microinjected F(ab)2 derived from anti-histones. Whereas the anti-histone F(ab)2 when injected into the cytoplasm migrated into the nucleus, the anti-HMG-1 F(ab)2 remained in the cytoplasm. Microinjection of anti-HMG-17 and anti-histone inhibited transcription in living cells, anti-HMG-1,2 did not. We conclude that HMG-1,2 proteins are present in both the nucleus and cytoplasm of living fibroblasts.

High mobility group proteins HMG1 and HMG2 do not decrease the melting temperature of DNA

High mobility group proteins 1 and 2 isolated in non-denaturing conditions cannot decrease the temperature of denaturation of DNA. When they are isolated or treated with tricloroacetic acid a hyperchromic peak below the melting temperature of free DNA appears in agreement with previous data ( Javaherian et al. (1979) Nucl . Acids Res. 6, 3569-3580). We show that this is due to light scattering of aggregated protein at submelting temperatures and not to melting of DNA.

Gene control by phosphoproteins: a theoretical model for eukarotic DNA regulation

Although gene regulatory mechanisms in eukaryotic cells are complex, some progress is being made in understanding them. Chromosomal proteins may play a significant role in genome function and gene control. More specifically, a central role may be played by the nuclear nonhistone proteins. It appears that both viral transformation and steroid hormone action may be associated with the phosphorylation of these proteins. Recent studies have revealed that some oncogenic viruses are capable of producing viral transforming proteins with protein kinase activity. This suggests how they may subvert normal gene regulatory mechanisms. Furthermore, the effects of the steroid-receptor complex on nuclear nonhistone proteins may be similar to the effects of these viral transforming proteins, but in a controlled sense. A model of gene-regulatory, nuclear phosphorylation reactions is formulated which suggests how some oncogenic viruses may control normal gene regulatory mechanisms and how steroid hormones may interact with these same mechanisms. Such a model may reveal how disruption of these same mechanisms leads to carcinogenesis.

A survey of H1o-and H5-like protein structure and distribution in higher and lower eukaryotes

A survey of H1o and H5-like proteins has been conducted through a range of higher and lower eukaryotic species. All mammals examined possessed H1o proteins, although in variable amounts, and the protein's structure was well conserved, though not invariant. The testis-specific histone Hlt (from rat) did not have an H1o-like structure and it appears that H1o does not occur in spermatocytes in any form. The results also show that Xenopus laevis contains H1o-like proteins, but lower, non-vertebrate eukaryotes (a crustacean, two fungi and a plant tissue) do not possess H1o or H5 proteins. The evidence suggests that H1o and H5 proteins may be considered as belonging to one family, distinct from H1 types. This H1o/H5 family may well be 'replacement histone' variants of H1. The results do not support suggestions of roles such as repression of DNA synthesis or of transcription for H1o/H5 proteins.

Release of ribonucleoprotein during digestion of rat testis chromatin with deoxyribonuclease II (3.1.4.6)

The composition of rat testis chromatin proteins in fractions produced by limited DNase II digestion followed by differential precipitation with MgCl2 has been studied. Over 50% of the acid-soluble proteins in the soluble chromatin fraction appeared to be quite similar to proteins which are associated with ribonucleoprotein (RNP) particles in HeLa cells. Although the ratios of the testis RNP protein components differed from those of HeLa RNP particles, the three major polypeptides were most similar to the HeLa components designated A2, B2, and C1. The soluble chromatin fraction was also enriched in the high mobility group proteins HMG1 and HMG2.

Isolation and partial sequence of bovine cDNA clones for the high-mobility-group protein (HMG-1)

Several cloned ds cDNAs containing bovine HMG-1 sequences have been isolated from a ds cDNA library prepared from the poly(A)+ mRNA fraction of bovine testis using a pool of synthetic 17-meric oligo-deoxyribonucleotides with the sequence 5'-GTAGTTAGTTCCACATTCTC-3' selected to be complementary to a region of the coding sequence corresponding to the relatively unambiguous amino acid sequence, Glu-Met-Trp-Asn-Asn-Thr. Determination of the DNA sequences in these clones indicates that they represent the 3' half of the HMG-1 message and contain an unusually long putative 3' untranslated region of 480 nucleotides. The sequence of the coding region corresponding to the 99 amino acids at the C-terminus of HMG-1 has been determined and largely confirms the published primary sequence in this region (Walker JM, (1982) in: The HMG Chromosomal Proteins, Academic Press, London & New York, pp. 69-88). In addition the cDNA sequence provides a complete sequence of the 30 residue polyacidic region and shows that the nucleotide sequence in this region is a repeating one and that the polyacidic domain comprises the C-terminus of the protein.

Gene control by phosphoproteins: a theoretical model for eukaryotic DNA regulation

Although gene regulatory mechanisms in eukaryotic cells are complex, some progress is being made in understanding them. Chromosomal proteins may play a significant role in genome function and gene control. More specifically, a central role may be played by the nuclear nonhistone proteins. It appears that both viral transformation and steroid hormone action may be associated with the phosphorylation of these proteins. Recent studies have revealed that some oncogenic viruses are capable of producing viral transforming proteins with protein kinase activity. This suggests how they may subvert normal gene regulatory mechanisms. Furthermore, the effects of the steroid-receptor complex on nuclear nonhistone proteins may be similar to the effects of these viral transforming proteins, but in a controlled sense. A model of gene-regulatory, nuclear phosphorylation reactions is formulated which suggests how some oncogenic viruses may control normal gene regulatory mechanisms and how steroid hormones may interact with these same mechanisms. Such a model may reveal how disruption of these same mechanisms leads to carcinogenesis.

Effect of X-radiation on DNA and histone synthesis in ataxia telangiectasia and normal lymphoblastoid cells

The possibility that the radiosensitivity of lymphoblastoid cell lines from patients with ataxia telangiectasia (A-T) is due to an aberrant content of histones has been examined. The histone pattern of lymphoblastoid cell lines derived from A-T patients was found to be indistinguishable from that obtained from normal individuals. X-ray irradiation led to a greater decrease in cell growth rate in the A-T cells than in the normal cells but was accompanied by a greater decrease of DNA synthesis rate in the normal cells. This difference in radiosensitivity was not reflected in differences in the content or rates of synthesis of histones or of major non-histone proteins in these cells. Reduction in the rate of DNA synthesis was not associated with the appearance of the lysine-rich histone variant H1. We conclude that the hypersensitivity to ionizing radiation in A-T cells is not due to fundamental differences in the composition or synthesis of the major chromosomal proteins.

Comparisons of the structures of the chromosomal high mobility group proteins HMG1 and HMG2 prepared under conditions of neutral and acidic pH

The chromosomal proteins HMG1 and 2 have been prepared by salt extraction and phosphocellulose chromatography at neutral pH (Isackson, P.J., Debold, W.A. and Reeck, G.R. (1980) FEBS Lett. 119, 337-342) to minimize protein denaturation. The structures of these phosphocellulose-prepared high mobility group proteins have been compared with those of high mobility group proteins using the previously described acid-extraction conditions which fully denature the proteins. When compared in the same solvent conditions the acid-extracted proteins did not refold to give the same level of alpha-helical and tertiary folded structures as the phosphocellulose-prepared proteins, suggesting that acid treatment can cause some irreversible damage to the proteins. This finding was supported by changes in the structure observed when phosphocellulose-prepared HMG1 was neutralized after exposure to acid. Gel filtration studies reveal no differences in the size of the high mobility group proteins, phosphocellulose-prepared and acid-extracted proteins both being largely monomeric in solution. Little difference was detected in the DNA-binding properties of the two types of protein, nor was there any difference in the oxidation state of the cysteines. However, isoelectric focusing analysis revealed differences in the subfractions of HMG2 prepared by the two methods.

Hl histones from mammalian testes. Hlt is associated with spermatogenesis in humans

Hlt is a testis-specific Hl histone variant associated with meiosis and post-meiotic stages of male germ cell development. We have now made tentative identification of Hlt in humans by a variety of criteria including electrophoretic and extractive properties. While Hlt was readily identified in extracts from normal testes, it was not detectable in extracts from aspermatogenic testes or from placenta. Identification of Hlt in humans confirms the widespread association of this unusual Hl variant with spermatogenesis among mammals ranging from rodents to primates.

Increased fraction of acid-soluble proteins in 0.35 M NaCl extracts of nuclei from rat liver tumors

1. The fraction of proteins extracted from nuclei with 0.35 M NaCl and soluble in 2% trichloroacetic acid was examined in five Morris hepatomas and rat liver. 2. This fraction was a much greater percentage of the total 0.35 M NaCl soluble proteins in the tumors than in normal or regenerating liver. 3. In part, this difference was due to proteins with molecular weights greater than high mobility group proteins. 4. The conditions for precipitation of high mobility group proteins 1 and 2 with trichloroacetic acid were found to differ in hepatoma and liver fractions.

The effect of isoproterenol and hydroxyurea on the presence of ubiquitin and protein A24 in the rat salivary gland

The in vivo administration of hydroxyurea for 12 h counteracts DNA synthesis and cell cycling stimulated by 72 h of isoproterenol treatment in rat salivary gland, as determined by fluorescence-activated flow cytometry. Hydroxyurea has little effect on [3H]leucine incorporation (protein synthesis) of the nuclear proteins soluble in 0.35 M NaCl, when examined by polyacrylamide gel chromatography and autoradiography from electrostatically sorted nuclei of (G0 + G1) and (G2 + M) phases of the in vivo cell cycle. Differential incorporation of [3H]leucine into nuclear proteins was observed during various phases of the cell cycle. Proteins 'X' and 'Z', observed in stained gel chromatographs of the 0.35 M NaCl-soluble nuclear proteins, were identified by biochemical analyses as ubiquitin and protein A24, respectively. Ubiquitin appeared transiently while A24 increased in gel chromatograms concomitant with progressive quiescence of the salivary gland induced by hydroxyurea.

The binding sites for large and small high-mobility-group (HMG) proteins. Studies on HMG-nucleosome interactions in vitro

Studies in vitro of binding high-mobility-group (HMG) proteins to nucleosomal particles that differ in their DNA contents reflect several aspects pertinent to their function in vivo. Two molecules of HMG 14 or 17 are accommodated by particles with 140 or 180 base pairs of DNA whereas HMG 1 or 2 are only bound by the larger specimens irrespective of the presence of HMG 14/17. It is concluded that one molecule of HMG 1 or 2 binds to the 40 base pairs of linker DNA whereas the HMG 14 or 17 molecules associate with the nucleosomal core. At physiological ionic strength, HMG 14 binding is cooperative, probably by triggering a conformational change in the nucleosomal particle. The phenomenon has been studied by two independent techniques. Besides the common gel-electrophoretic system, a centrifugation assay is described, which permits the derivation of a Hill coefficient nH = 1.3 and dissociation constants in the range of 30-90 nM at 0.15 M NaCl, pH 6.8.

The characterization of non-histone proteins whose amounts increase in chromatin from mouse hepatocarcinomas

A fraction containing liver- and hepatoma-specific non-histone proteins has been isolated from the chromatin of mice. Amino acid analysis of this fraction shows that it contains 16 mol of glutamic acid, 10 mol aspartic acid, 7 mol of both arginine and lysine per 100 mol and contains no cysteine or tyrosine. The proteins in this fraction are strongly associated with DNA and are co-extracted with histones from chromatin with 0.25 M HCl. In chromatin from age-related hepatomas, the amount of this fraction increased six-fold. This increase in concentrations of these chromatin proteins may be associated with changes of chromatin structure necessary to initiate malignant growth in liver cells.

Enhanced myocardial RNA synthesis in spontaneously hypertensive rats possible role of high-mobility group non-histone proteins

Cardiac hypertrophy in spontaneously hypertensive rats is associated with increased nuclear RNA polymerase activity. In order to explore mechanisms facilitating the interaction of the enzyme with its endogenous template, we compared the structure of nuclear chromatin from myocytes of 20-week-old spontaneously hypertensive rats and normotensive Wistar-Kyoto controls. Enhanced RNA synthesis in hypertensive rats was accompanied by increased susceptibility to digestion by deoxyribonuclease I. Nick translation of nuclei also resulted in higher nucleotide incorporation in hypertensive rats. Salt-extraction abolished the differences in deoxyribonuclease I sensitivity between the two animal groups. Reconstitution with either 0.35 M NaCl-extract or high mobility group (HMG) non-histone proteins restored digestion susceptibility but did not equalize SHR and WKY cells. SDS-polyacrylamide gel electrophoresis of 0.35 M NaCl-extracts and supernatants from deoxyribonuclease I digestion revealed the presence of HMG proteins which were preferentially released in hypertensive rats. There was a small but statistically significant increase in nuclear HMG protein content in hypertensive rats (0.12 +/- 0.02 mg/mg DNA vs. 0.09 +/- 0.02 mg/mg DNA in Wistar-Kyotos, P less than 0.05) but no difference in their electrophoretic appearance. These results indicate that chromatin structure is altered in the hypertrophied myocardium with resultant increase in deoxyribonuclease I susceptibility. This increase appears to be partly dependent on the high-mobility group non-histone proteins.

Chromosomal proteins of mouse teratocarcinoma cells

We have analyzed chromosomal proteins extracted from murine teratocarcinoma-derived stem cell lines (F9 and 12-1) and from their differentiated derivatives (12-1a) because of the differential sensitivity to DNase I digestion of these two cell types. The chromosomal DNA of stem cells is more sensitive to DNase I digestion than that of differentiated cells. Stem cell core histones are more highly acetylated than their differentiated counterparts, and certain high-mobility group (HMG) proteins from stem cells (HMG 1 and HMG 2) are found in greater amounts than in the differentiated cells though others (HMG 14 and HMG 17) occur in similar amounts. We have also identified a new HMG protein (HMG 9) that is present in stem cells and is lost following differentiation.

The effect of isoproterenol on nuclear protein synthesis in electrostatically sorted rat hepatocytes

Rat hepatocyte nuclei were electrostatically sorted and collected from specific regions of the cell cycle. Proteins were extracted from homogeneous populations of nuclei with cetyltrimethylammonium bromide and chromatographed by one- and two-dimensional acid-urea polyacrylamide gel electrophoresis. Small quantitative differences were observed in amido black stained two-dimensional patterns of the nuclear proteins of the G0 + G1 and G2 + M populations. However, comparison of the two-dimensional autoradiographs of 3H-leucine labeled nuclear proteins in these populations revealed dramatic differences. After isoproterenol administration, animals showed enhanced labeling of certain nuclear proteins in G2 + M populations, but no noticeable effect in protein labeling was observed in the G0 + G1 populations. Four days after the administration of isoproterenol, fluorescent histograms exhibited increased amounts of DNA in the G2 + M populations. Comparison of the total amino acid composition of two proteins from control animals revealed the distinct chemical contrast of these proteins. Both proteins possessed higher ratios of acidic to basic residues than anticipated for basic proteins. These procedures and findings should prove useful for further understanding of mechanisms of toxicant effects upon nuclear protein metabolism.