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

The rat liver single-stranded DNA binding protein, S25 and HD25, isolated by differential DNA cellulose affinity chromatography was compared to the high mobility group proteins, HMG1 and HMG2, isolated from rat liver chromatin by the technique of Goodwin et al. (Goodwin, G. H., Sanders, C., and Johns, E. W. (1973) Eur. J. Biochem. 38, 14-19). Analysis of their amino acid composition, electrophoretic mobility, and tryptic peptide map reveal the identity of the single-stranded DNA binding protein with HMG1 protein, implying that the rat liver HMG1 protein becomes able both to destabilize a double helix of DNA and to stimulate homologous DNA polymerases only when rat liver cells enter a phase of DNA synthesis, possibly after a specific modification.     

Monoclonal antibody against non-histone chromosomal protein high mobility group 1 Co-migrates with high mobility group 1 into the nucleus

Non-histone chromosomal protein high mobility group 1 (HMG-1) rapidly migrates into the nucleus when injected into the cytoplasm of bovine fibroblasts and HeLa cells by red cell-mediated microinjection (Rechsteiner, M., and Kuehl, L. (1979) Cell 16, 901-908). We isolated hybridomas secreting monoclonal antibodies against HMG-1. One of these monoclonal antibodies, FR-1, inhibited in vitro binding of 125I-HMG-1 to chromatin isolated from FL cells. When 125I-HMG-1 was co-introduced with antibody FR-1 by red cell-mediated microinjection, antibody FR-1 did not prevent the accumulation of 125I-HMG-1 in the nucleus. When 125I-antibody FR-1 or fluorescein isothiocyanate antibody FR-1 was introduced into the cytoplasm of FL cells, most of the antibody did not accumulate in the nucleus. But when 125I- or fluorescein isothiocyanate antibody FR-1 was co-introduced with HMG-1 into the cytoplasm of FL cells, it did migrate into the nucleus.     

Interplay between human high mobility group protein 1 and replication protein A on psoralen-cross-linked DNA

Human high mobility group box (HMGB) 1 and -2 proteins are highly conserved and abundant chromosomal proteins that regulate chromatin structure and DNA metabolism. HMGB proteins bind preferentially to DNA that is bent or underwound and to DNA damaged by agents such as cisplatin, UVC radiation, and benzo[a]pyrenediol epoxide (BPDE). Binding of HMGB1 to DNA adducts is thought to inhibit nucleotide excision repair (NER), leading to cell death, but the biological roles of these proteins remain obscure. We have used psoralen-modified triplex-forming oligonucleotides (TFOs) to direct a psoralen-DNA interstrand cross-link (ICL) to a specific site to determine the effect of HMGB proteins on recognition of these lesions. Our results reveal that human HMGB1 (but not HMGB2) binds with high affinity and specificity to psoralen ICLs, and interacts with the essential NER protein, replication protein A (RPA), at these lesions. RPA, shown previously to bind tightly to these lesions, also binds in the presence of HMGB1, without displacing HMGB1. A discrete ternary complex is formed, containing HMGB1, RPA, and psoralen-damaged DNA. Thus, HMGB1 has the ability to recognize ICLs, can cooperate with RPA in doing so, and likely modulates their repair by the NER machinery. The abundance of HMGB1 suggests that it may play an important role in determining the sensitivity of cells to DNA damage under physiological, experimental, and therapeutic conditions.     

Synthesis of high mobility group proteins in regenerating rat liver.

Incorporation of [3H]lysine into the non-histone chromosomal proteins HMG1, HMG2, and HMG17 and into each of the five major classes of histones was measured in rat liver at various times after partial hepatectomy. Histone synthesis was closely coupled temporally to that of DNA, although a small amount of histone was shown to be produced before DNA replication began. In contrast, the incorporation curves for the high mobility group (HMG) proteins showed little correlation with that for DNA. At 4 h after partial hepatectomy, protein synthesis had virtually ceased. Thereafter, the rates of synthesis of the HMG proteins rose steadily so that by 12 h, well before the onset of DNA replication they had reached about two-thirds of the maximum rates attained during the first cell division cycle. Histones had only reached about one-sixth of their maximum rates at this time. The lack of coupling betweeen the synthesis of the HMG proteins and DNA was confirmed by experiments with inhibitors of DNA replication. Reduction of DNA synthesis to less than 10% of the uninhibited rate had little or no effect on incorporation into the HMG proteins, whereas, under similar conditions, the rate of synthesis of histones was reduced by more than 50%.     

Characterization of high mobility group protein levels during spermatogenesis in the rat

The distribution, quantitation, and synthesis of high mobility group (HMG) proteins during spermatogenesis in the rat have been determined. HMG1, -2, -14, and -17 were isolated from rat testes by Bio-Rex 70 chromatography combined with preparative gel electrophoresis. Amino acid analysis revealed that each rat testis HMG protein was similar to its calf thymus analogue. Tryptic peptide maps of somatic and testis HMG2 showed no differences and, therefore, failed to detect an HMG2 variant. Testis levels of HMG proteins, relative to DNA content, were equivalent to other tissues for HMG1 (13 micrograms/mg of DNA), HMG14 (3 micrograms/mg of DNA), and HMG17 (5 micrograms/mg of DNA). The testis was distinguished in that it contained a substantially higher level of HMG2 than any other rat tissue (32 micrograms/mg of DNA). HMG protein levels were determined from purified or enriched populations of testis cells representing the major stages of spermatogenesis; spermatogonia and early primary spermatocytes, pachytene spermatocytes, early spermatids, and late spermatids; and testicular somatic cells. High levels of HMG2 in the testis were due to pachytene spermatocytes and early spermatids (56 +/- 4 and 47 +/- 6 micrograms/mg of DNA, respectively). Mixtures of spermatogonia and early primary spermatocytes showed lower levels of HMG2 (12 +/- 3 micrograms/mg of DNA) similar to proliferating somatic tissues, whereas late spermatids had no detectable HMG proteins. The somatic cells of the testis, including isolated populations of Sertoli and Leydig cells, showed very low levels of HMG2 (2 micrograms/mg of DNA), similar to those in nonproliferating somatic tissues. HMG proteins were synthesized in spermatogonia and primary spermatocytes, but not in spermatids. Rat testis HMG2 exhibited two bands on acid-urea gels. A "slow" form comigrated with somatic cell HMG2, while the other "fast" band migrated ahead of the somatic form and appeared to be testis-specific. The "fast" form of HMG2 accounted for the large increase of HMG2 levels in rat testes. These results show that the very high level of HMG2 in testis is not associated with proliferative activity as previously hypothesized.     

Structural homology between rbs repressor and ribose binding protein implies functional similarity.

The deduced amino acid sequence of the rbs repressor, RbsR, of Escherichia coli is homologous over its C-terminal 272 residues to the entire sequence of the periplasmic ribose binding protein. RbsR is also homologous to a family of bacterial repressor proteins including LacI. This implies that the structure of the repressor consists of a two-domain binding protein portion attached to a DNA-binding domain having the four-helix structure of the LacI headpiece. The implications of these relationships to the mechanism of this class of repressors are discussed.     

The Saccharomyces cerevisiae high mobility group box protein HMO1 contains two functional DNA binding domains

High mobility group box (HMGB) proteins are architectural proteins whose HMG DNA binding domains confer significant preference for distorted DNA, such as 4-way junctions. HMO1 is one of 10 Saccharomyces cerevisiae HMGB proteins, and it is required for normal growth and plasmid maintenance and for regulating the susceptibility of yeast chromatin to nuclease. Using electrophoretic mobility shift assays, we have shown here that HMO1 binds 26-bp duplex DNA with K(d) = 39.6 +/- 5.0 nm and that its divergent box A domain participates in DNA interactions, albeit with low affinity. HMO1 has only modest preference for DNA with altered conformations, including DNA with nicks, gaps, overhangs, or loops, as well as for 4-way junction structures and supercoiled DNA. HMO1 binds 4-way junctions with half-maximal saturation of 19.6 +/- 2.2 nm, with only a modest increase in affinity in the absence of magnesium ions (half-maximal saturation 6.1 +/- 1.1 nm). Whereas the box A domain contributes modest structure-specific binding, the box B domain is required for high affinity binding. HMO1 bends DNA, as measured by DNA cyclization assays, facilitating cyclization of 136-, 105-, and 87-bp DNA, but not 75-bp DNA, and it has a significantly longer residence time on DNA minicircles compared with linear duplex DNA. The unique DNA binding properties of HMO1 are consistent with global roles in the maintenance of chromatin structure.     

Structural basis for the proinflammatory cytokine activity of high mobility group box 1

High mobility group box 1 (HMGB), a ubiquitous DNA-binding protein, has been implicated as a proinflammatory cytokine and late mediator of lethal endotoxemia. HMGB1 is released by activated macrophages. It amplifies and extends the inflammatory response by inducing cytokine release and mediating acute lung injury, anorexia, and the inflammatory response to tissue necrosis. The kinetics of HMGB1 release provide a wide therapeutic window for endotoxemia because extracellular levels of HMGB1 begin to increase 12 to 24 h after exposure to inflammatory stimuli. Here, we demonstrate that a DNA-binding domain of HMGB1, the B box, recapitulates the cytokine activity of full length HMGB1 and efficiently activates macrophages to release tumor necrosis factor (TNF) and other proinflammatory cytokines. Truncation of the B box revealed that the TNF-stimulating activity localizes to 20 amino acids (HMGB1 amino acids 89 to 108). Passive immunization of mice with antibodies raised against B box conferred significant protection against lethal endotoxemia or sepsis, induced by cecal perforation. These results indicate that a proinflammatory domain of HMGB1 maps to the highly conserved DNA-binding B box, making this primary sequence a suitable target in the design of therapeutics.     

Structural/functional homology between the bacterial and eukaryotic cytoskeletons

Structural proteins are now known to be as necessary for controlling cell division and cell shape in prokaryotes as they are in eukaryotes. Bacterial ParM and MreB not only have atomic structures that resemble eukaryotic actin and form similar filaments, but they are also equivalent in function: the assembly of ParM drives intracellular motility and MreB maintains the shape of the cell. FtsZ resembles tubulin in structure and in its dynamic assembly, and is similarly controlled by accessory proteins. Bacterial MinD and eukaryotic dynamin appear to have similar functions in membrane control. In dividing eukaryotic organelles of bacterial origin, bacterial and eukaryotic proteins work together.     

Domain-domain interactions in high mobility group 1 protein (HMG1).

The high mobility group protein HMG1 is a conserved chromosomal protein with two homologous DNA-binding domains, A and B, and an acidic carboxy-terminal tail, C. The structure of isolated domains A and B has been previously determined by NMR, but the interactions of the different domains within the complete protein were unknown. By means of differential scanning calorimetry and circular dichroism we have investigated the thermal stability of HMG1, of the truncated protein A-B (HMG1 without the acidic tail C) and of the isolated domains A and B. In 3 mm sodium acetate buffer, pH 5, the thermal melting of domains A and B are identical (transition temperature tm = 43 degrees C and 41 degrees C, denaturation enthalpies DeltaH = 46 kcal.mol-1). The thermal melting of protein A-B presents two nearly identical transitions (tm = 40 degrees C and 41 degrees C, DeltaH = 44 kcal.mol-1 and 46 kcal.mol-1, respectively). We conclude that the two domains A and B within protein A-B behave as independent domains. The thermal melting of HMG1 is biphasic. The two transitions have a different value of tm (38 degrees C and 55 degrees C) and corresponding values of DeltaH around 40 kcal.mol-1. We conclude that within HMG1, the acidic tail C is interacting with one of the two domains A and B, however, the two domains A and B do not interact with each other. At 37 degrees C, one of the two domains A and B, within HMG1, is partly unfolded, whereas the other which interacts with the acidic tail C, is fully native. The interaction free energy of the acidic tail C is estimated to be in the range of 2.5 kcal.mol-1 based on simulations of the thermograms of HMG1 as a function of the interaction free energy.     

Sequence of a cDNA encoding turtle high mobility group 1 protein

In order to understand sequence information about turtle HMG1 gene, a cDNA encoding HMG1 protein of the Chinese soft-shell turtle (Pelodiscus sinensis) was amplified by RT-PCR from kidney total RNA, and was cloned, sequenced and analyzed. The results revealed that the open reading frame (ORF) of turtle HMG1 cDNA is 606 bp long. The ORF codifies 202 amino acid residues, from which two DNA-binding domains and one polyacidic region are derived. The DNA-binding domains share higher amino acid identity with homologues sequences of chicken (96.5%) and mammalian (74%) than homologues sequence of rainbow trout (67%). The polyacidic region shows 84.6% amino acid homology with the equivalent region of chicken HMG1 cDNA. Turtle HMG1 protein contains 3 Cys residues located at completely conserved positions. Conservation in sequence and structure suggests that the functions of turtle HMG1 cDNA may be highly conserved during evolution. To our knowledge, this is the first report of HMG1 cDNA sequence in any reptilian.     

Synthesis and phosphorylation of histone H1 and high mobility group proteins in the regenerating rat liver after X irradiation

A rat was irradiated to the upper abdomen including the liver and then partially hepatectomized. The subsequent synthesis and phosphorylation of histone H1 and nonhistone chromosomal high mobility group (HMG) proteins were investigated. Incorporation of [3H]lysine into histone H1 was increased and reached its peak at 27 h after hepatectomy, and 14 Gy of X rays inhibited the increase. Increase in the incorporation of [3H]lysine into HMG (1 + 2), 14, and 17 which occurred around 27 h after hepatectomy was not inhibited by 14 Gy irradiation. Phosphorylation of histone H1, measured with 32Pi incorporation in vivo, was maximal between 21 and 24 h, and it was inhibited by 4.8 Gy of X rays and delayed with 1.9 Gy. Phosphorylation of HMG 14, which was the only HMG protein phosphorylated under present conditions, was not affected by X irradiation. The [3H]thymidine incorporation into nuclear DNA started increasing at 21 h and reached its maximum at 27 h after hepatectomy. X irradiation with 4.8 Gy inhibited the incorporation, and 1.9 Gy lowered it.     

Structural homology between mouse liver and horse spleen ferritins

Mouse liver ferritin is composed almost exclusively of polypeptide chains similar in molecular mass (22 kDa) to that characteristic of the major chain (H) found in heart ferritin isolated from human, horse or rat. In these species the predominant polypeptide of liver (L) is smaller (about 20 kDa). Here we show that mouse liver and horse spleen ferritins and apoferritins exhibit extensive structural homology as judged by the similarity in the diffraction patterns of their crystals grown from cadmium sulphate solutions. Implications of this finding are discussed.     

Phosphorylation of human high mobility group N1 protein by protein kinase CK2

High mobility group (HMG) N1 protein, formerly known as HMG 14, is a member of the chromosomal HMG protein family. Protein kinase CK2 was previously reported to be able to phosphorylate bovine HMGN1 in vitro; Ser89 and Ser99, corresponding to Ser88 and Ser98 in human HMGN1, were shown to be major and minor recognition sites, respectively. In this report, we employed mass spectrometry and examined both the extent and the sites of phosphorylation in HMGN1 protein catalyzed by recombinant human protein kinase CK2. We found that five serine residues, i.e., Ser6, Ser7, Ser85, Ser88, and Ser98, in HMGN1 can be phosphorylated by the kinase in vitro. All five sites were previously shown to be phosphorylated in MCF-7 human breast cancer cells in vivo. Among these five sites, Ser6, Ser7, and Ser85 were new sites of phosphorylation induced by protein kinase CK2 in vitro.     

Inhibition of nucleotide excision repair by high mobility group protein HMGA1

The mammalian non-histone "high mobility group" A (HMGA) proteins are the primary nuclear proteins that bind to the minor groove of AT-rich DNA. They may, therefore, influence the formation and/or repair of DNA lesions that occur in AT-rich DNA, such as cyclobutane pyrimidine dimers (CPDs) induced by UV radiation. Employing both stably transfected lines of human MCF7 cells containing tetracycline-regulated HMGA1 transgenes and primary Hs578T tumor cells, which naturally overexpress HMGA1 proteins, we have shown that cells overexpressing HMGA1a protein exhibit increased UV sensitivity. Moreover, we demonstrated that knockdown of intracellular HMGA1 concentrations via two independent methods abrogated this sensitivity. Most significantly, we observed that HMGA1a overexpression inhibited global genomic nucleotide excision repair of UV-induced CPD lesions in MCF-7 cells. Consistent with these findings in intact cells, DNA repair experiments employing Xenopus oocyte nuclear extracts and lesion-containing DNA substrates demonstrated that binding of HMGA1a markedly inhibits removal of CPDs in vitro. Furthermore, UV "photo-foot-printing" demonstrated that CPD formation within a long run of Ts (T(18)-tract) in a DNA substrate changes significantly when HMGA1 is bound prior to UV irradiation. Together, these results suggest that HMGA1 directly influences both the formation and repair of UV-induced DNA lesions in intact cells. These findings have important implications for the role that HMGA protein overexpression might play in the accumulation of mutations and genomic instabilities associated with many types of human cancers.     

Hyperlipidemia stimulates the extracellular release of the nuclear high mobility group box 1 protein

Our aim was to evaluate the effect of hyperlipidemia on the activation of endogenous alarmin, the high mobility group box 1 (HMGB1) protein, related to systemic inflammation associated with the progression of experimental atherosclerosis and to establish whether statin treatment regulates the HMGB1 signaling pathway. Hyperlipidemia was induced in vivo in golden Syrian hamsters and in monocyte cell culture (U937) by feeding the animals with a high-fat Western diet and by exposing the cells to hyperlipidemic serum. Blood samples, heart, lung and cells were harvested for biochemical, morphological, Western blot, quantitative polymerase chain reaction and enzyme-linked immunosorbent assay analyses. The data revealed that, in the atherosclerotic animal model, the protein HMGB1 and its gene expression were increased and that fluvastatin treatment significantly reduced the release of HMGB1 into the extracellular space. The cell culture experiments demonstrated the relocation of HMGB1 protein from the nucleus to cytoplasm under hyperlipidemic stress. The high level of detected HMGB1 correlated positively with the up-regulation of the advanced glycation end product receptors (RAGE) in the lung tissue from hyperlipidemic animals. During hyperlipidemic stress, the AKT signaling pathway could be activated by HMGB1-RAGE interaction. These results support the existence of a direct correlation between experimentally induced hyperlipidemia and the extracellular release of HMGB1 protein; this might be controlled by statin treatment. Moreover, the data suggest new potentials for statin therapy, with improved effects on patients with systemic inflammation induced by hyperlipidemia.     

Detection by chemical cross-linking of interaction between high mobility group protein 1 and histone oligomers in free solution

Among the more abundant non-histone proteins is the high mobility group (HMG), with an unknown role in chromatin. We have investigated, by chemical cross-linking, the interaction of the protein HMG 1 with the histone dimer H2A X H2B and the histone tetramer (H3 X H4)2 in free solution. Cross-linking with dimethyl suberimidate, 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide, and the cleavable cross-linker dimethyl-3,3'-dithiobispropionimidate, by two-dimensional electrophoresis reveals the existence of an interaction between HMG 1 and the histone dimer, and also between HMG 1 and the histone tetramer. In the case of the H2A X H2B dimer, the analysis of the patterns of the cross-linking products shows the presence of a trimer, (H2A X H2B) X HMG 1, and of another oligomer of higher molecular weight which also contains H2A X H2B and HMG 1. Non-histone HMG 1 has been found to interact with (H3 X H4)2, both by cross-linking kinetics and also by gel permeation chromatography, displaying a stoichiometry of one HMG 1/histone tetramer. The results have been interpreted as indicating the existence of an interaction between HMG 1 and both oligomers through two different binding sites.     

Systemic release of high mobility group box 1 protein during severe murine influenza

Hypercytokinemia is gaining recognition as the mechanism of fatality from influenza. No work to date has addressed the role of high mobility group box 1 protein (HMGB1) in influenza, the parallel being that in other severe proinflammatory cytokine syndromes (e.g., sepsis and malaria) levels of circulating HMGB1 are elevated and may correlate with death. Using a commercially available ELISA for HMGB1, we found that HMGB1 was not increased in the plasma of influenza virus-infected mice (A/Japan/305/57) on day 7 post infection, about the time of peak mortality, and peak levels of HMGB1 in the plasma did not occur until relatively late in infection, on day 9 post infection. In keeping with the late peak of HMGB1 being unassociated with mortality, administration of ethyl pyruvate, which inhibits active secretion but not passive release of HMGB1, to influenza virus-infected mice, did not affect their survival. Further work is required to determine whether influenza virus infection induces passive release of HMGB1, and whether HMGB1 neutralization with a specific Ab would improve survival.