Identification of a single-stranded DNA binding protein from rat liver with high mobility group protein 1

HMGB1 in health and disease

Complex genetic and physiological variations as well as environmental factors that drive emergence of chromosomal instability, development of unscheduled cell death, skewed differentiation, and altered metabolism are central to the pathogenesis of human diseases and disorders. Understanding the molecular bases for these processes is important for the development of new diagnostic biomarkers, and for identifying new therapeutic targets. In 1973, a group of non-histone nuclear proteins with high electrophoretic mobility was discovered and termed high-mobility group (HMG) proteins. The HMG proteins include three superfamilies termed HMGB, HMGN, and HMGA. High-mobility group box 1 (HMGB1), the most abundant and well-studied HMG protein, senses and coordinates the cellular stress response and plays a critical role not only inside of the cell as a DNA chaperone, chromosome guardian, autophagy sustainer, and protector from apoptotic cell death, but also outside the cell as the prototypic damage associated molecular pattern molecule (DAMP). This DAMP, in conjunction with other factors, thus has cytokine, chemokine, and growth factor activity, orchestrating the inflammatory and immune response. All of these characteristics make HMGB1 a critical molecular target in multiple human diseases including infectious diseases, ischemia, immune disorders, neurodegenerative diseases, metabolic disorders, and cancer. Indeed, a number of emergent strategies have been used to inhibit HMGB1 expression, release, and activity in vitro and in vivo. These include antibodies, peptide inhibitors, RNAi, anti-coagulants, endogenous hormones, various chemical compounds, HMGB1-receptor and signaling pathway inhibition, artificial DNAs, physical strategies including vagus nerve stimulation and other surgical approaches. Future work further investigating the details of HMGB1 localization, structure, post-translational modification, and identification of additional partners will undoubtedly uncover additional secrets regarding HMGB1's multiple functions.

Functional dissection of an IFN-alpha/beta receptor 1 promoter variant that confers higher risk to chronic hepatitis B virus infection

BACKGROUND/AIMS

We previously demonstrated that two linked single nucleotide polymorphisms (SNPs) at -408 and -3 of type I interferon receptor 1 (IFNAR1) promoter are associated with susceptibility to chronic HBV infection. We aimed to elucidate the mechanism by which -3 and/or -408 C/T SNPs had such profound effects.

METHODS

A functional SNP in IFNAR1 promoter was defined by reporter gene assay, mutational analysis, flow cytometry analysis and gel shift assay. The nuclear protein binding to the essential polymorphic site was identified and its effect on transcriptional regulation of IFNAR1 was further demonstrated in a series of ex vivo and in vivo experiments.

RESULTS

We found C>T change at the -3 locus reduced the transcriptional activity of IFNAR1 promoter. High mobility group B protein 1 (HMGB1) and PARP-1 were co-recruited to the IFNAR1 promoter to regulate its transcription. We demonstrated HMGB1-binding affinity to IFNAR1 promoter was reduced in the -3T variant. Additionally, PARP-1, a cofactor for IFNAR1 transcription activation, was significantly suppressed by HBV.

CONCLUSION

Upon HBV infection, decreased binding affinity of HMGB1 to the IFNAR1 promoter -3T variant is aggravated by the suppressed PARP-1 expression caused by HBV, resulting in a further attenuated IFNAR1 expression. This compromises the antiviral and immuno-regulatory effects of IFN-alpha/beta, which may in turn affect the clinical outcome of HBV infection.

HMGB1 protein inhibits DNA replication in vitro: a role of the acetylation and the acidic tail

The high mobility group box (HMGB) 1 protein is a very abundant and conserved protein that is implicated in many key cellular events but its functions within the nucleus remain elusive. The role of this protein in replication of closed circular DNA containing a eukaryotic origin of replication has been studied in vitro by using native and recombinant HMGB1 as well as various modified HMGB1 preparations such as truncated protein, lacking its C-terminal tail, in vivo acetylated protein, and recombinant HMGB1 phosphorylated in vitro by protein kinase C (PKC). Native HMGB1 extracted from tumour cells inhibits replication and this effect is reduced upon acetylation and completely abolished upon removal of the acidic C-terminal tail. Recombinant HMGB1, however, fails to inhibit replication but it acquires such a property following in vitro phosphorylation by PKC.

The human HMGB1 promoter is modulated by a silencer and an enhancer-containing intron

The highly conserved, ubiquitous high mobility group protein HMGB1 (formerly named as HMG1) is an architectural transcription factor encoded by a single functional gene in human. HMGB1 is expressed in almost all cell or tissue types studied. In general, it is expressed at a basal level in most cells but at a slightly elevated level of 2--3-fold in actively proliferating tissues or estrogen stimulated breast cancer cells. To understand the regulatory mechanism controlling expression of the human HMGB1 gene, we cloned and analyzed the upstream region as well as the first intron of this gene. We found that transcription of the human HMGB1 gene in the breast cancer MCF-7 cells starts at one major site 57 nucleotides upstream from the first exon-intron boundary. Expression of the human HMGB1 gene is under the control of a very strong TATA-less promoter, which has an activity more than 18-fold that of the SV40 promoter. Immediately upstream, a silencer element is present. This silencer can repress the activity of the HMGB1 promoter down to just one-sixth. The first intron of the human HMGB1 gene contains enhancer elements, which can increase the human HMGB1 promoter activity by 2--3-fold. We postulate that the human HMGB1 gene is capable of being expressed at a very high level. The basal level of expression observed in most cells is probably a result of the strong promoter being held in check by the silencer. The 2--3-fold increase in HMGB1 expression observed in proliferating cells or breast cancer cells stimulated by estrogen may probably result from the action of the enhancer elements in intron 1.

DSP1, an HMG-like protein, is involved in the regulation of homeotic genes

The Drosophila dsp1 gene, which encodes an HMG-like protein, was originally identified in a screen for corepressors of Dorsal. Here we report that loss of dsp1 function causes homeotic transformations resembling those associated with loss of function in the homeotic genes Sex combs reduced (Scr), Ultrabithorax (Ubx), and Abdominal-B. The expression pattern of Scr is altered in dsp1 mutant imaginal discs, indicating that dsp1 is required for normal expression of this gene. Genetic interaction studies reveal that a null allele of dsp1 enhances trithorax-group gene (trx-G) mutations and partially suppresses Polycomb-group gene (Pc-G) mutations. On the contrary, overexpression of dsp1 induces an enhancement of the transformation of wings into halteres and of the extra sex comb phenotype of Pc. In addition, dsp1 male mutants exhibit a mild transformation of A4 into A5. Comparison of the chromatin structure at the Mcp locus in wild-type and dsp1 mutant embryos reveals that the 300-bp DNase I hypersensitive region is absent in a dsp1 mutant context. We propose that DSP1 protein is a chromatin remodeling factor, acting as a trx-G or a Pc-G protein depending on the considered function.

The chicken genome contains no HMG1 retropseudogenes but a functional HMG1 gene with long introns

We have cloned the genomic sequence coding for the high mobility group 1 (HMG1) protein in chickens. Multiple sequence alignment shows that the chicken HMG1 gene is highly homologous to the human and the mouse HMG1 genes. The gene structure of chicken HMG1 is similar to that of the mouse and the human HMG1 genes, with the same exon-intron boundaries. However, in contrast to other avian genes that have shorter introns, the chicken HMG1 gene has introns that are twice as long as their mammalian homologues. In addition to the functional, intron-containing HMG1 gene, all mammalian genomes contain more than 50 copies of HMG1 retropseudogenes each, while in the chicken genome there are no HMG1 retropseudogenes. This finding suggests that the HMG1 retropseudogenes arose in mammals after their divergence away from the birds.

DSP1 gene of Drosophila melanogaster encodes an HMG-domain protein that plays multiple roles in development

DSP1 is an HMG-box containing protein of Drosophila melanogaster which was first identified as a co-repressor of the Dorsal protein. Recently, the analysis of the structure of the gene has led us to propose that DSP1 is the Drosophila equivalent of the ubiquitous vertebrate HMG 1/2 proteins. In the present paper, the patterns of expression of DSP1 protein and RNA in adult flies and during development are reported. In the adults DSP1 protein is located in nurse cells of ovaries and in brain. During eggs development uniform expression of DSP1 protein persists until the end of germband retraction. At later stages, expression is restricted to the ventral nerve chord and brain. Using P-element mutagenesis, we have isolated a mutant deficient in DSP1 functions. Genetic studies of this mutant show that DSP1 protein is essential for the growth and the development of Drosophila. In addition to be a co-repressor of the transcriptional activator Dorsal our results provide compelling evidence that DSP1 is a regulator involved in several pathways necessary for the development of the fly.

Reexamination of the high mobility group-1 protein for self-association and characterization of hydrodynamic properties

Previous studies of the 25 kDa high mobility group-1 (HMG-1) protein have generated conflicting results regarding whether HMG-1 exists as a monomer or is capable of oligomerizing to (functional) tetramers. To resolve this question, sedimentation velocity analysis yielded a s20,w value of 2.59S, which is consistent with a monomeric protein. Equilibrium sedimentation data were obtained for three HMG-1 concentrations at two rotor speeds. The six sets of data were fit to both an ideal single component and monomer-dimer equilibrium model, with essentially identical fits produced for both models, with the latter indicating a low extent (7%) of dimerization. Reaction of HMG-1 with glutaraldehyde produced a small population of oligomers consistent with a low level of dimers. This supported the monomer-dimer equilibrium model. Surprisingly, gel permeation chromatography yielded an apparent molecular mass of approx. 55 kDa for both HMG-1 and HMG-2. This finding is considered anomalous and presumably due to the high negative charge density in the C terminus of HMG-1. The sedimentation data also permit one to model HMG-1 as a hydrated prolate ellipsoid with a major axis/minor axis ratio of 2. 79. The collective evidence from the sedimentation and chemical cross-linking studies strongly supports a moderately asymmetric monomer in solution and unequivocally eliminates the possibility of a highly extended shape for HMG-1 or the existence of any extensive oligomerization.

The acidic tail of the high mobility group protein HMG-D modulates the structural selectivity of DNA binding

HMG-D is one of the Drosophila counterparts of the vertebrate HMG1/2 class of abundant chromosomal proteins and contains three domains: an HMG domain followed by a basic region and a short acidic carboxyterminal tail. We show that the HMG domain of HMG-D does not bind to deformed DNA structures such as DNA bulges, cis-platinated DNA or four-way junctions but does bind tightly to DNA microcircles, suggesting that in vivo the natural ligands of this domain are tightly bent DNA loops. The flanking basic region substantially increases the DNA-binding activity of the HMG domain to DNA ligands other than microcircles. We demonstrate that the acidic tail alters the structural selectivity of DNA binding by increasing the affinity for deformed DNA and decreasing the affinity for linear B-DNA. Finally, we show that the acidic tail increases the efficiency of constraining preformed negative supercoils but conversely decreases the efficiency of supercoiling relaxed DNA in the presence of topoisomerase I.

Purification of a high-mobility-group 1 sea-urchin protein and cloning of cDNAs

The isolation of the sea urchin high-mobility-group 1 (HMG1) protein, the cloning of corresponding cDNA clones and the similarity to the human homologue are described. Sea urchin HMG1 was purified as one of the nuclear embryonic proteins which associate with an upstream regulatory element (E1) of the Strongylocentrotus purpuratus (Sp) CyIIIb actin-encoding gene. Using a synthetic oligodeoxyribonucleotide (oligo) which includes the E1 cis-acting element in a DNA affinity chromatography purification, the most prominent of the binding proteins was isolated and the N terminus sequenced. cDNA clones were isolated by screening an embryonic cDNA library with a synthetic oligo derived from the amino acid (aa) sequence. Comparison of the cDNAs ORF to known proteins revealed a 50% aa identity to the mammalian HMG1 and all the structural characteristics of this group of proteins. The sea urchin protein, SpHMG1, was synthesized in bacteria, as well as translated in vitro. Binding assays carried out with the recombinant SpHMG1 protein did not produce specific in vitro complexes with E1.

Differential expression of nuclear HMG1, HMG2 proteins and H1(zero) histone in various blood cells

Changes in the levels of chromosomal high-mobility group proteins HMG1, HMG2 and histone H1 zero were investigated in blood cells of various types, proliferation activity and stage of differentiation. The relative amounts of proteins HMG1, HMG2 and histone H1 zero were evaluated densitometrically by SDS-PAGE of 5 per cent w/v perchloric acid extracts of blood cells. Concerning the HMG1 and HMG2, the main conclusions were: the expression of these HMG proteins was higher in malignant cells, namely leukemia cell lines, then in lymphocytes or granulocytes and the distribution of HMG1 and HMG2 was highly cell-specific. In comparison with lymphoid cells, the levels of HMG1/2 were higher in myeloid cells. The results revealed that in myeloid cells HMG2 prevails over HMG1. There was no direct correlation between HMG1/2 expression and proliferation activity. The levels of HMG1/2 did not depend on the transcription of chromatin either. However, there was some connection between irreversibly differentiated nonproliferating cells and a loss of HMG1/2 proteins. Reversibly differentiated leukemic cells retain their HMG1/2 levels. Similarly to HMG1/2,H1 zero showed a strong cell specificity. The level of H1 zero was different in the various blood cell types. As compared with lymphoid cells, the level of H1 zero was several-fold higher in myeloid cells, regardless of whether they were normal or malignant. Moreover, there was an accumulation of H1 zero in differentiating HL-60 cells accompanied by only a slight decline in cell proliferation; this agrees with the idea that H1 zero expression is not directly associated with the inhibition of cell growth. Rather higher expression of H1 zero is related to changes during cell differentiation.

Specific interaction between H1 histone and high mobility protein HMG1

High mobility group proteins HMG1 and -2 and histone H1 are structural components of chromatin. Previously, we reported that HMG1 interacts with H1 histone in a way that modulates the ability of H1 to condense DNA in vitro, suggesting that these proteins may act together in vivo to regulate locally the condensation state of chromatin, possibly affecting replication and/or transcription. Here we show that reduced (native) HMG1 binds to H1 cooperatively at pH 6.0 as a tetramer with a dissociation constant of 3.4 x 10(-8) M, and at pH 7.5 as a monomer with a dissociation constant less than 10(-9) M. Denaturation through oxidation of sulfhydryl groups has a strong effect on the interaction of HMG1 with H1 histone, suggesting that the reduced state of HMG1 is critical to its function. Oxidized HMG1 failed to bind H1 at pH 7.5, and its binding at pH 6 was biphasic; the first three (or two) molecules of H1 were bound with a dissociation constant of 2 x 10(-8) M with negative cooperativity, and the last one (or two) H1's were bound cooperatively with KD = 1.8 x 10(-7) M. Regulation of the pH or the concentration of some other ion may be used in vivo to alter the interactions between HMG1 and -2, H1 histone, and DNA.

The human testis determining factor SRY: a new member of the HMG box protein family

The product of the sex-determining gene SRY is a member of the HMG box containing protein superfamily. The HMG box is a DNA-binding domain of about 80 amino acids shared by many proteins with diverse functions. It seems that the functions of the full length protein are restricted to the HMG box but their molecular basis remains to be determined. We have summarized here the properties of this binding domain described so far in the literature and, using a synthetic peptide mimicking the DNA binding domain (SRY80), we have confirmed the existence of DNA minor groove contacts with this domain. Using intrinsic fluorescence of the tryptophane, the interaction between SRY80 and the putative target sequence AACAAAT was also quantified. In conclusion, we also consider the possible putative action of SRY to fulfill its role in sex determination.

Purification and properties of a cruciform DNA binding protein from Ustilago maydis

A DNA binding protein with an M(r) of 11,000 was purified from Ustilago maydis. Its solubility in acid, amino acid composition, and mobility during gel electrophoresis are reminiscent of properties observed for the high mobility group nonhistone chromosomal proteins. The protein recognizes cruciform DNA made from oligonucleotides and also binds preferentially to a plasmid containing an extruded cruciform.

HMG-2 protein in developing rat brain cells

1. The distribution of HMG-2 protein was followed in unfractionated rat brain cells at different stages of development. Its amount gradually decreased and reached the lowest level in the terminally differentiated and non-proliferating cells. 2. In isolated oligodendrocyte nuclei the changes in the content of HMG-2 followed the same pattern of distribution which corresponded to their stage of development and proliferative activity, while in the terminally differentiated and non-proliferating cortical neurons a substantial amount of HMG-2 protein was present up to the twenty-eighth postnatal day. 3. In the presence of anti-HMG-2 antibodies the DNA synthetic activity of oligodendrocyte nuclei in vitro was significantly decreased. The treatment with antibodies affected mainly the DNA replicative activity of the nuclei, while their DNA repair activity remained unchanged.

A single-column chromatographic system for the analysis and preparation of high mobility group proteins 1 and 2 and other chromosomal proteins using nondenaturing solvents

One-step chromatography on a Mono S column allows the purification of high mobility group (HMG) proteins 1 and 2 under nondenaturing conditions. Chromatography of HMG1 and -2 on Mono S can be achieved with three of the most widely employed extraction techniques for chromosomal proteins, 0.35 M sodium chloride, 0.74 M perchloric acid, and 0.4 N sulfuric acid. In each case HMG1 and -2 are purified away from the other chromosomal proteins, histone H1, and core histones, and are resolved into both their reduced and oxidized forms. Additionally histone H1 and the core histones are fractionated on Mono S, thus the entire complement of chromosomal proteins can be analyzed in a single rapid chromatographic step.

Efficient large-scale purification of non-histone chromosomal proteins HMG1 and HMG2 by using Polybuffer-exchanger PBE94

A method for the efficient and practical large-scale purification of high-mobility group (HMG) non-histone chromosomal proteins, HMG1 and HMG2, from porcine thymus applying Polybuffer-exchanger PBE94 gel as anion-exchanger has been developed. This method affords higher resolution, purity and yield, than the conventional procedure of CM-Sephadex C-25 ion-exchange column chromatography. Furthermore, use of Polybuffer-exchanger PBE94 column chromatography led to direct preparation of HMG1 and HMG2 from loosely bound non-histone chromosomal protein fraction of chromatin without prefractional precipitation with trichloroacetic acid or prior extraction with perchloric acid. Thus, the application of PBE94 gel as an anion-exchanger to the subfractionation of other kinds of homologous protein is possible.

Tissue specificity of nucleo-cytoplasmic distribution of HMG1 and HMG2 proteins and their probable functions

The levels and distribution between nucleus and cytoplasm of HMG1 and HMG2 proteins have been investigated in different tissues of mammals. In lymphoid tissues and testis high amounts of these proteins are present in both nuclei and cytoplasm, while in the hepatic tissues and brain they accumulate in cytoplasm, mainly in the cytosol. In particular, very low amounts, if any, of HMG1 and 2 are present in the nuclei active for DNA replication (rat regenerating liver and primary hepatoma) or transcription (adult liver and brain). Therefore, it appears that HMG1 and 2 are not necessary for these processes. On the other hand, nuclear (chromosomal) HMG1 and 2 are characteristic for the tissues containing undifferentiated cells: lymphoid tissues, testis, neonatal liver. These proteins are bound to the chromatin regions solubilized early by sonication or DNase action. Comparison of the data obtained for different tissues shows an inverse correlation between the amounts of chromosomal HMG1 and 2, on the one hand, and of histone H1(0), on the other hand. These results suggest that chromosomal HMG1 and 2 take part in the processes that occur during cell differentiation, while histone H1(0) is induced to preserve differentiated cells from dedifferentiation.

Non-random reconstitution of HMG1 and HMG2 in chromatin. Determination of the histone contacts

We have studied how non-histone proteins HMG1 and HMG2 interact with rat liver chromatin using reconstitution and chemical cross-linking procedures. Both proteins were found to associate to chromatin only to some extent and always with a marked preference for short oligonucleosomes, mainly mono- and dinucleosomes. However, a slight reconstitution with the long polynucleosomal fraction can be observed in H1-depleted chromatin. Reconstitution is non-random and a clear preference for regions highly sensitive to staphylococcal nuclease (EC 3.1.31.1) is observed. Chemical cross-linking has allowed us to identify H1, H2A and H2B as the histones contacted by HMG1 and HMG2 upon reconstitution. Also, we present evidence that HMG1 and HMG2 interact with the nucleosomal particle without replacing H1 or any other histone.

A double-loop model for the replication of eukaryotic DNA

Coordinated DNA synthesis of both strands at the replication fork by a fixed 'replisome' may cause dynamic and topological problems. Based upon known properties of DNA helicase, DNA primase and DNA topoisomerases, and on novel properties of DNA polymerases and DNA ligase, we propose a 'double-loop' model for the replication of eukaryotic DNA that could minimize such problems.

Specific recognition of cruciform DNA by nuclear protein HMG1

Cruciform DNA, a non-double helix form of DNA, can be generated as an intermediate in genetic recombination as well as from palindromic sequences under the effect of supercoiling. Eukaryotic cells are equipped with a DNA-binding protein that selectively recognizes cruciform DNA. Biochemical and immunological data showed that this protein is HMG1, an evolutionarily conserved, essential, and abundant component of the nucleus. The interaction with a ubiquitous protein points to a critical role for cruciform DNA conformations.

Acetylated HMG1 protein interacts specifically with homologous DNA polymerase alpha in vitro

The acetylated, deacetylated and nonacetylated forms of HMG1 proteins from Guerin ascites tumour cells and calf thymus were separated and their in vitro interactions with homologous and heterologous DNA polymerases were studied. It has been found that only the acetylated form of HMG1 proteins forms a specific complex with homologous DNA polymerase alpha and stimulates its activity in vitro. The acetylation therefore is necessary for their possible function in DNA replication. This finding represents an evidence for a relationship between the acetylation of HMG1 proteins and their biological role.

Differences between HMG1 proteins isolated from normal and tumour cells

The properties of the non-histone chromosomal high-mobility-group 1 (HMG1) proteins from rat liver and Guerin ascites tumour cells (GAT cells) were compared and showed the following differences: (1) five spots were missing in the peptide map of HMG1 from GAT cells in comparison with that of HMG1 from rat liver; (2) HMG1 from GAT cells was about 5-times more poly(ADP)-ribosylated; (3) HMG1 from GAT cells which was found acetylated in vivo and incorporated [14C]acetate in vitro, whereas no incorporation of the label was detected in HMG1 from rat liver; (4) HMG1 from GAT cells exhibited pronounced ability to form oligomers at physiological ionic strength, while HMG1 from rat liver was predominantly in monomeric form. This property of HMG1 from GAT cells was lost upon deacetylation.

cis-Diamminedichloroplatinum(II) selectively cross-links high mobility group proteins 1 and 2 to DNA in micrococcal nuclease accessible regions of chromatin

The reaction of cis-diamminedichloroplatinum(II) with chicken erythrocyte nuclei produces covalent cross-linking of HMG proteins 1, 2 and E to DNA, in addition to cross-links amongst LMG proteins. This is supported by and consistent with the observations that all cross-links are chemically reversed by NaCN treatment, while only the cross-links involving the HMG proteins 1,2 and E are eliminated after a limited digestion with micrococcal nuclease. Having identified the subset of proteins selectively cross-linked to DNA by the bifunctional cis-(NH3)2PtCl2, a tentative model is proposed for the interactions between DNA and HMG proteins 1 and 2 in bulk chromatin. In addition, possible modes of action for this anti-neoplastic drug are suggested in light of these findings.

Lactate dehydrogenase and glyceraldehyde-phosphate dehydrogenase are single-stranded DNA-binding proteins that affect the DNA-polymerase-alpha-primase complex

Affinity chromatography on double-stranded (ds) and single-stranded (ss) DNA-cellulose columns was employed to find analogs of the Escherichia coli and T4 single-stranded DNA binding proteins (SSB proteins) in calf thymus. The interaction of several purified SSB proteins with the pure DNA-polymerase-alpha--primase complex on DNA synthesis on activated DNA and on primase-initiated M13 DNA served as a criterion for a possible involvement of one of these proteins in the process of DNA replication. Two SSB proteins were purified to essential homogeneity. These most abundant proteins exhibited apparent relative molecular masses of 35,000 (SSB-35) and 37,000 (SSB-37) for the protomers and 140,000 and 80,000 for the native enzymes. Both proteins resisted elution with 0.5 mg/ml dextran sulfate and were eluted from the ssDNA-cellulose with 0.2 M and 1 M NaCl, respectively. SSB-35 stimulated the DNA-polymerase-alpha--primase complex from the same organism up to fivefold over a broad range of DNA covering. By contrast, SSB-37 inhibited the primase-initiated replication of M13 DNA. Like most eukaryotic SSB proteins, these proteins showed a 300-fold preference for binding to ssDNA over dsDNA in a nitrocellulose filter binding assay, as well as strong binding to several DNA and RNA homopolymers. Furthermore, we provide evidence for a cooperative mode of binding for SSB-37. Although SSB-35 and SSB-37 behave as typically eukaryotic SSB proteins in all assays employed, we tested these SSB proteins for dehydrogenase activities as well. SSB-35 was found to be identical with lactate dehydrogenase and SSB-37 was identical with a dimeric form of glyceraldehyde-3-phosphate dehydrogenase. These results imply that further studies are mandatory in order to prove the authenticity of eukaryotic SSB proteins.

Native state of high mobility group chromosomal proteins 1 and 2 is rapidly lost by oxidation of sulfhydryl groups during storage

Oxidized forms of non-histone chromosomal proteins high mobility group 1 (HMG1) and HMG2 were detected by high-pressure liquid chromatography of preparations stored at 4 degrees C for 1 day. The oxidized form of each was found to have two free sulfhydryl groups, while the freshly prepared native form of each contained four. The native, reduced state could be maintained during storage by the addition of ethylenediaminetetraacetic acid or reducing agents.

Glyceraldehyde-3-phosphate dehydrogenase is a nonhistone protein and a possible activator of transcription in neurons

A single-stranded DNA-binding protein of Mr 35,000 (35K protein) was isolated from calf cerebral cortex by affinity chromatography on immobilized double-stranded and single-stranded DNA. Its localization in the nuclear compartment was demonstrated by immunohistochemistry. Previous studies had uncovered a homologous nonhistone chromosomal protein in the nuclei of rat cerebral cortex neurons, cerebellar neurons, oligodendrocytes, and liver cells. The rat protein accumulated in the nuclear compartment of neurons in exact temporal coincidence with the arrest of cell division and the initiation of terminal differentiation. Therefore, in the present work, the 35K protein was tested for an activating role in RNA transcription. During the course of this study we became aware that the 35K protein was identical to a glycolytic enzyme, glyceraldehyde-3-phosphate dehydrogenase (GAPDH, EC 1.2.1.12). When authentic GAPDH from rabbit skeletal muscle was injected into Xenopus laevis oocytes, it greatly stimulated RNA polymerase II transcription, whereas the 35K protein from calf brain did not. This apparent discrepancy was partially resolved by the finding that rabbit muscle GAPDH could be fractionated into two components by affinity chromatography on single-stranded DNA cellulose. Only 5% of the applied protein was retained on the column and could be eluted with a shallow salt gradient identical to the one used for the isolation of the 35K protein. This single-stranded DNA-binding component of rabbit muscle GAPDH did not stimulate transcription. Apparently, the 35K protein from calf brain corresponded to this single-stranded DNA-binding subfraction, which explained its failure to activate transcription.(ABSTRACT TRUNCATED AT 250 WORDS)

Detection and characterization of a novel factor that stimulates DNA polymerase alpha

A novel factor that stimulates DNA polymerase alpha activity on poly(dA) X oligo(dT) has been identified and partially purified from mouse FM3A cells. The assay system for the factor contained poly(ethylene glycol) 6000. The activities of DNA polymerase alpha on poly(dA) X oligo(dT) in the presence and absence of the stimulating factor were increased greatly by the addition of poly(ethylene glycol). Stimulation by the factor was observed at all the primer to template ratios tested from 0.01 to 0.3. The highest activity was observed at the ratio of 0.05, corresponding to about 3.3 primers on one template in the presence of the factor. The concentration of DNA polymerase alpha used in the assay affected the stimulation by the factor, and the stimulation became more prominent at concentrations of the enzyme lower than 0.04 unit per assay. The stimulating factor lowered the Km value of DNA polymerase alpha for the template-primer, though they had no effect on the Km value for dTTP substrate. The results of product analysis suggested that the stimulation by the factor is mainly due to the increase in the initiation frequency of DNA synthesis from the primers. The stimulating factor specifically stimulated DNA polymerase alpha but not DNA polymerases beta and gamma. Furthermore, the factor formed a complex with DNA polymerase alpha under a certain condition.

Identification of the core-histone-binding domains of HMG1 and HMG2

High mobility group (HMG) nonhistone chromosomal proteins are a group of abundant, conservative and highly charged nuclear proteins whose physiological role in chromatin is still unknown. To gain insight into the interactions of HMG1 and HMG2 with the fundamental components of chromatin we have introduced the methodology of photochemical crosslinking. This technique has allowed us to study the interaction of HMG1 and HMG2 with the core histones, in the form of an H2A X H2B dimer and an (H3 X H4)2 tetramer, for an effective time of crosslinking of less than 1 ms and under very mild conditions. This is achieved by using flash photolysis. With this procedure we found that both HMG1 and HMG2 interact with H2A X H2B and also with (H3 X H4)2. In the second case, they seem to do this through histone H3. To obtain more information about the interactions, we split HMG1 and HMG2 into their peptides using staphylococcal proteinase. The peptides obtained, which reflect the domain distribution of these proteins, were then used along with the histone oligomers to elucidate their interactions by means of photochemical crosslinking. Results obtained indicate that the domain of HMG1 and HMG2 involved in the interaction with H2A X H2B histones is the highly acidic C-terminal, whereas the N-terminal is involved in the interactions with (H3 X H4)2 histones. In all cases, the interactions found appear appreciably strong. Along with other data published in the literature, these proteins appear to have at least one binding site per domain for the chromatin components.

Protein HMG1 is different from a DNA helix unwinding protein in calf thymus

A number of criteria were used--chromatography on columns with single-stranded and double-stranded DNA, electrophoresis, peptide analysis, immunological tests and thermal denaturation of DNA--to show that protein (high mobility group) HMG1 and an unwinding protein from calf thymus are two distinct, unrelated proteins. While both proteins are thought to be related to DNA replication this might involve different mechanisms of action.

Age-dependent changes in the level of a 34 kDa DNA-binding protein in developing chick embryo liver

The relative amounts of a DNA-binding protein of 34 kDa increased during the early stages of development of chick embryo liver. The content of this protein reached a maximum in 18-19-day-old embryonic livers and decreased afterwards in older embryonic and post-natal chick livers. The 34 kDa polypeptide is the major DNA-binding protein (DBP) of embryonic liver and it preferentially binds to single-stranded DNA. The quantity of the 34 kDa DBP was relatively very low in embryonic muscle, heart and brain.

Isolation of high-mobility-group proteins HMG1 and HMG2 in non denaturing conditions and comparison of their properties with those of acid-extracted proteins

We describe a method for isolation and purification of the chromosomal proteins HMG1 and HMG2 in non-denaturing conditions which overcomes the difficulties of the published methods concerning yield and purity. The method is based on salt extraction, selective precipitation with ammonium sulfate and DEAE-cellulose chromatography. All studied properties of these proteins (formation of protein tetramers, enhancement of micrococcal nuclease digestion of DNA and chromatin, and protection of 165-basepair DNA in chromatosome) differ significantly from the properties of HMG1 and 2 isolated under denaturing conditions.

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.