Immunochemical studies of high mobility group non-histone chromatin proteins HMG 1 and HMG 2

Transcriptional frequency and cell determination

The relative base composition of DNA regulatory sequences of certain genes of undetermined multipotent progenitor cells may account for the frequency of transcription of these genes in cell determination. The sequences of these regulatory regions of cell determination genes that are more AT-rich would create the potential for transcription at a higher frequency due to their lower melting temperature, as well as propensity to bend. An increase of one or more of the high mobility group (HMG) chromatin proteins would preferentially bind the more AT-rich regulatory sequences, thereby increasing the rate of transcription. The amount of unphosphorylated H1 histone reacting with these same regulatory sites may decrease transcription frequency. The level of cell growth, i.e. total protein synthesis of a cell, is correlated positively with the synthesis of HMG proteins. H1 histone synthesis is linked to DNA replication. Unbalanced growth would alter the amounts of HMG proteins and H1 histone, thus changing transcriptional frequency. The greater the enrichment of AT sequences in the regulatory regions of the cell determination genes, the greater may be the extent of evolutionary conservation. Higher frequency of transcription of the cell determination genes with the more AT-rich regulatory sequences could account for the earlier expression of the more conserved cell determination genes during embryonic development. Preferential binding of H1 histone to the more AT-rich regulatory sequences would subsequently restrict their transcription before that of less conserved cell determination genes.

Polyspecific reactivity of a murine monoclonal antibody that binds to nuclear matrix-associated, chromatin-bound autoantigens

To investigate polyspecific autoantibody interactions, we have characterized the binding of a cloned murine monoclonal IgM antibody termed (RTE-23) of strain BALB/c origin. By indirect immunofluorescence this antibody displayed a nuclear speckled and peripheral pattern in interphase cells from human and rodent cell lines. In contrast, in mitotic cells, antibody RTE-23 bound to the periphery of individual chromosomes. Immunoblot analysis of soluble and insoluble nuclear proteins from purified rat fibroblast nuclei showed that antibody RTE-23 bound to molecules of 28, 29, and 33 kDa. Furthermore, antibody RTE-23 demonstrated marked polyspecificity and reacted with cytoskeletal proteins (vimentin, keratin, actin), single-stranded DNA, specific synthetic polynucleotides, and cardiolipin. Antibody RTE-23 also showed a lupus anticoagulant-like activity. Screening of sera of autoimmune disease patients with antinuclear antibodies revealed two patients, both with SLE, whose sera blocked antibody RTE-23 binding to nuclei and recognized nuclear proteins identical to those recognized by antibody RTE-23. These results suggested that antibody RTE-23 displays a pattern of self-antigen binding that is represented as well in SLE patient sera.

Concentrations of high-mobility-group proteins in the nucleus and cytoplasm of several rat tissues

Nuclear and cytoplasmic fractions were isolated from various tissues of the rat by a nonaqueous technique. The high-mobility-group (HMG) proteins were extracted from these fractions with acid and separated by one- and two-dimensional PAGE. The concentrations of high-mobility-group proteins HMG1, HMG2, and HMG17 in the nucleus and cytoplasm were then estimated from the staining intensities of the electrophoretic bands. The cytoplasmic concentrations of these proteins were very low--usually less than 1/30 of those present in the corresponding nuclear fractions. For the tissues studied (liver, kidney, heart, and lung), the concentrations of HMG proteins in the nucleus did not differ significantly from one tissue to another. Averaged over the four tissues investigated, there were 0.28 molecule of HMG1, 0.18 molecule of HMG2, and 0.46 molecule of HMG17 per nucleosome. These values are considerably higher than those that have been reported previously.

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.

Comparison of the high-mobility-group chromosomal proteins in rainbow-trout (Salmo gairdnerii) liver and testis

Chromatography and characterization of the proteins extracted by 5% (w/v) HClO4 from rainbow-trout (Salmo gairdnerii) liver and testis show that the two tissues present a characteristically different spectrum of high-mobility-group (HMG) proteins. A variant subfraction of HMG C is found in liver, but is not detectable in testis, where even the main fraction of HMG C is present in only very low quantity. A protein, F, which appears to be related to protein H6 has similarly been isolated only from liver and not from testis. Quantification of the HMG proteins in total 5%-HClO4 extracts of trout liver and testis nuclei shows that, in relation to DNA, levels of HMG T1 and T2, and D are more than 2-fold, and C, 20-fold higher in liver than in testis. However, these differences do not result merely from the sequential withdrawal of HMG proteins at the same time that histones are replaced by protamines in the developing spermatid, since in testis, at some stages of maturation, levels of H6 are almost 2-fold higher than in liver. The implications of these findings for the function of HMG proteins are discussed.

High mobility group proteins of amphibian oocytes: a large storage pool of a soluble high mobility group-1-like protein and involvement in transcriptional events

Oocytes of several amphibian species (Xenopus laevis, Rana temporaria, and Pleurodeles waltlii) contained a relatively large pool of nonchromatin-bound, soluble high mobility group (HMG) protein with properties similar to those of calf thymus proteins HMG-1 and HMG-2 (protein HMG-A; A, amphibian). About half of this soluble HMG-A was located in the nuclear sap, the other half was recovered in enucleated ooplasms. This protein was identified by its mobility on one- and two-dimensional gel electrophoresis, by binding of antibodies to calf thymus HMG-1 to polypeptides electrophoretically separated and blotted on nitrocellulose paper, and by tryptic peptide mapping of radioiodinated polypeptides. Most, if not all, of the HMG-A in the soluble nuclear protein fraction, preparatively defined as supernatant obtained after centrifugation at 100,000 g for 1 h, was in free monomeric form, apparently not bound to other proteins. On gel filtration it eluted with a mean peak corresponding to an apparent molecular weight of approximately 25,000; on sucrose gradient centrifugation it appeared with a very low S value (2-3 S), and on isoelectric focusing it appeared in fractions ranging from pH approximately 7 to 9. This soluble HMG-A was retained on DEAE-Sephacel but could be eluted already at moderate salt concentrations (0.2 M KCl). In oocytes of various stages of oogenesis HMG-A was accumulated in the nucleus up to concentrations of approximately 14 ng per nucleus (in Xenopus), corresponding to approximately 0.2 mg/ml, similar to those of the nucleosomal core histones. This nuclear concentration is also demonstrated using immunofluorescence microscopy. When antibodies to bovine HMG-1 were microinjected into nuclei of living oocytes of Pleurodeles the lateral loops of the lampbrush chromosomes gradually retracted and the whole chromosomes condensed. As shown using electron microscopy of spread chromatin from such injected oocyte nuclei, this process of loop retraction was accompanied by the appearance of variously-sized and irregularly-spaced gaps within transcriptional units of chromosomal loops but not of nucleoli, indicating that the transcription of non-nucleolar genes was specifically inhibited by this treatment and hence involved an HMG-1-like protein. These data show that proteins of the HMG-1 and -2 category, which are usually chromatin-bound components, can exist, at least in amphibian oocytes, in a free soluble monomeric form, apparently not bound to other molecules. The possible role of this large oocyte pool of soluble HMG-A in early embryogenesis is discussed as well as the possible existence of soluble HMG proteins in other cells.

Immunological specificity of Novikoff hepatoma chromatin: isolation of three antigenic proteins

1. Immunization of rabbits with dehistonized Novikoff hepatoma or normal rat liver chromatin elicited specific antibodies. 2. Chromosomal nonhistone proteins from Novikoff hepatoma were fractionated and three proteins (approx. mol. wt 39,000, 49,0000 and 56,000) were identified with the hepatoma antigenic complexes. 3. All three proteins showed significant reactivity of PAS staining suggesting that they were glycosylated. 4. Proteins migrating similar to these three Novikoff hepatoma fractions were detected by polyacrylamide gel electrophoresis in the chromosomal nonhistone protein fraction obtained from normal rat liver. 5. However, the three proteins from normal liver were PAS negative, and did not react immunologically with antiserum to dehistonized Novikoff hepatoma chromatin.

Comparative studies on microinjected high-mobility-group chromosomal proteins, HMG1 and HMG2

The nonhistone chromosomal proteins, HMG1 and HMG2, were iodinated and introduced into HeLa cells, bovine fibroblasts, or mouse 3T3 cells by erythrocyte-mediated microinjection. Autoradiographic analysis of injected cells fixed with glutaraldehyde consistently showed both molecules concentrated within nuclei. Fixation with methanol, on the other hand, resulted in some leakage of the microinjected proteins from the nuclei so that more autoradiographic grains appeared over the cytoplasm or outside the cells. Both injected and endogenous HMG1 and HMG2 partitioned unexpectedly upon fractionation of bovine fibroblasts, HeLa, or 3T3 cells, appearing in the cytoplasmic fractions. However, in calf thymus, HMG1 and HMG2 molecules appeared in the 0.35 M NaCl extract of isolated nuclei, as expected. These observations show that the binding of HMG1 and HMG2 to chromatin differs among cell types or that other tissue-specific components can influence their binding. Coinjection of [125I]HMG1 and [131I]HMG2 into HeLa cells revealed that the two molecules display virtually equivalent distributions upon cell fractionation, identical stability, identical intracellular distributions, and equal rates of equilibration between nuclei. In addition, HMG1 and HMG2 did not differ in their partitioning upon fractionation nor in their stability in growing vs. nongrowing 3T3 cells. Thus, we have not detected any significant differences in the intracellular behavior of HMG1 and HMG2 after microinjection into human, bovine, or murine cells.

Heterogeneous binding of high mobility group chromosomal proteins to nuclei

A dramatic difference is observed in the intracellular distribution of the high mobility group (HMG) proteins when chicken embryo fibroblasts are fractionated into nucleus and cytoplasm by either mass enucleation of cytochalasin-B-treated cells or by differential centrifugation of mechanically disrupted cells. Nuclei (karyoplasts) obtained by cytochalasin B treatment of cells contain more than 90 percent of the HMG 1, while enucleated cytoplasts contain the remainder. A similar distribution between karyoplasts and cytoplasts is observed for the H1 histones and the nucleosomal core histones as anticipated. The presence of these proteins, in low amounts, in the cytoplast preparation can be accounted for by the small percentage of unenucleated cells present. In contrast, the nuclei isolated from mechanically disrupted cells contain only 30-40 percent of the total HMGs 1 and 2, the remainder being recovered in the cytosol fraction. No histone is observed in the cytosol fraction. Unike the higher molecular weight HMGs, most of the HMGs 14 and 17 sediment with the nuclei after cell lysis by mechanical disruption. The distribution of HMGs is unaffected by incubating cells with cytochalasin B and mechanically fractionating rather than enucleating them. Therefore, the dramatic difference in HMG 1 distribution observed using the two fractionation techniques cannot be explained by a cytochalasin-B-induced redistribution. On reextraction and sedimentation of isolated nuclei obtained by mechanical cell disruption, only 8 percent of the HMG 1 is released to the supernate. Thus, the majority of the HMG 1 originally isolated with these nuclei, representing 35 percent of the total HMG 1, is stably bound, as is all the HMGs 14 and 17. The remaining 65 percent of the HMGs 1 and 2 is unstably bound and leaks to the cytosol fraction under the conditions of mechanical disruption. It is suggested that the unstably bound HMGs form a protein pool capable of equilibrating between cytoplasm and stably bound HMGs.

Nerve growth factor mediates phosphorylation of specific proteins

Nerve growth factor (NGF), epidermal growth factor (EGF), insulin, cholera toxin (CT) and cAMP all stimulate the phosphorylation of proteins in the PC12 nerve-like cell line. NGF, CT and cAMP enhance phosphorylation of the same set of proteins including tyrosine hydroxylase, ribosomal protein S6, histones H1 and H3, and the nonhistone chromosomal and cytoplasmic high mobility group (HMG) 17 protein, and reduce phosphorylation of H2A. EGF but not insulin enhances the phosphorylation of tyrosine hydroxylase. Insulin but not EGF enhances the phosphorylation of histone H3 and decreases the phosphorylation of H2A. EGFD and insulin each enhance phosphorylations of both ribosomal protein S6 and histone H1, but neither hormone induces phosphorylation of HMG 17. The extent of these effects depends upon the ligand concentration and is half-maximal at physiological concentrations of the hormones (beta-NGF, 2 ng/ml; EGF, 1 ng/ml. insulin, 0.5 microunits/ml). Maximal effects of NGF are seen within 15 min and persist even after 3 days of culture in the presence of NGF. When phosphorylation of ribosomal protein S6 is maximally stimulated by NGF, no further stimulation can be achieved by adding saturating quantities of either cAMP or CT. However, simultaneous addition of saturating quantities of NGF and either EGF or insulin results in an enhancement of phosphorylation that is equal to the sum of that achieved when the two ligands are added separately. These results suggest that the enhanced phosphorylation of S6 achieved by NGF or cAMP occurs through a common mechanism which differs from those which mediate EGF or insulin-enhanced phosphorylation. The data also provide strong evidence that the action of NGF included protein phosphorylation mediated by cAMP-dependent protein kinase. The phosphorylation of each of these proteins in response to NGF may play an important role in NGF action.

Human-specific nuclear protein that associates with the polar region of the mitotic apparatus: distribution in a human/hamster hybrid cell

We describe the first example of a predominantly nuclear protein which during mitosis becomes part of the mitotic apparatus. This protein has been termed the nuclear-mitotic apparatus (NuMA) protein. In interphase cells NuMA protein is restricted to the nucleus and is a constituent of isolated nuclear matrices, but in mitotic cells it is observed by indirect immunofluorescence microscopy to be concentrated at the polar regions of the mitotic apparatus. This mitotic localization is dependent on the integrity of the spindle, since treatments which disrupt the spindle result in dispersion of NuMA protein throughout the cell. Comparison to the subcellar distribution of tubulin at different stages of the cell cycle indicates that NuMA protein is distinct from the previously identified components of the mitotic spindle. Its association with the nuclear matrix and its localization during mitosis to the site of nuclear reassembly suggest the interesting possibility that NuMA protein could be representative of a class of proteins involved in the early events of nuclear reassembly. NuMA is present in the nuclei and mitotic spindle of all types of human cells that have been examined, but proteins of similar molecular weight (300,000 daltons in dissociating solvents) or immunological specificity are not detected in cells of other species (including monkey). However, the NuMA protein is synthesized in a human/Chinese hamster hybrid cell containing a reduced number of human chromosomes. Immunofluorescence studies of this hybrid cell showed that the distribution of NuMA protein is equivalent to that in human cells. These results suggest that the human gene coding for NuMA protein, unlike other genes coding for human specific nuclear proteins, can be expressed in human/hamster hybrid cells and that the cell hybrids will be useful in further characterization of NuMA protein.