High mobility group proteins 1 and 2 function as general class II transcription factors

HMGB1-Mediated Activation of the Inflammatory-Reparative Response Following Myocardial Infarction

Different cell types belonging to the innate and adaptive immune system play mutually non-exclusive roles during the different phases of the inflammatory-reparative response that occurs following myocardial infarction. A timely and finely regulation of their action is fundamental for the process to properly proceed. The high-mobility group box 1 (HMGB1), a highly conserved nuclear protein that in the extracellular space can act as a damage-associated molecular pattern (DAMP) involved in a large variety of different processes, such as inflammation, migration, invasion, proliferation, differentiation, and tissue regeneration, has recently emerged as a possible regulator of the activity of different immune cell types in the distinct phases of the inflammatory reparative process. Moreover, by activating endogenous stem cells, inducing endothelial cells, and by modulating cardiac fibroblast activity, HMGB1 could represent a master regulator of the inflammatory and reparative responses following MI. In this review, we will provide an overview of cellular effectors involved in these processes and how HMGB1 intervenes in regulating each of them. Moreover, we will summarize HMGB1 roles in regulating other cell types that are involved in the different phases of the inflammatory-reparative response, discussing how its redox status could affect its activity.

Biological functions and theranostic potential of HMGB family members in human cancers

The high mobility group box (HMGB) protein family consists of four members: HMGB1, 2, 3, and 4. They share similar amino acid sequences and identical functional regions, especially HMGB1, 2, and 3. The homology in structure may lead to similarity in function. In fact, though their targets may be different, they all possess the fundamental function of binding and distorting target DNAs. However, further research confirmed they are distributed differently in tissues and involved in various distinct physiological and pathological cellular processes, including cell proliferation, division, migration, and differentiation. Recently, the roles of HMGB family members in carcinogenesis has been widely investigated; however, systematic discussion on their functions and clinical values in malignant tumors is limited. In this review, we mainly review and summarize recent advances in knowledge of HMGB family members in terms of structure, distribution, biochemical cascades, and specific mechanisms regarding tumor progression. Importantly, the diagnostic, prognostic, and therapeutic value of these proteins in cancers is discussed. Finally, we envisage the orientation and challenges of this field in further studies.

Nucleotide-Binding Oligomerization Domain-Like Receptor 3 Deficiency Attenuated Isoproterenol-Induced Cardiac Fibrosis via Reactive Oxygen Species/High Mobility Group Box 1 Protein Axis

Nucleotide-binding oligomerization domain-like receptor 3 (NLRP3) is involved in fibrosis of multiple organs, such as kidney, liver, lung, and the like. However, the role of NLRP3 in cardiac fibrosis is still controversial and remains unclear. The study aims to investigate the role of NLRP3 on cardiac fibrosis induced by isoproterenol (ISO). In vivo, NLRP3 knockout and wild-type mice were subcutaneously injected with ISO to induce the cardiac fibrosis model. The results showed that NLRP3 deficiency alleviated the cardiac fibrosis and inflammation induced by ISO. In vitro, neonatal rat ventricular myocytes (NRVMs) and primary adult mouse cardiac fibroblasts of NLRP3 knockout and wild-type mice were isolated and challenged with ISO. Adenovirus (Ad-) NLRP3 and small interfering RNAs targeting NLRP3 were used to transfect NRVMs to overexpress or knockdown NLRP3. We found that NLRP3 could regulate high-mobility group box 1 protein (HMGB1) secretion via reactive oxygen species production in NRVMs and the HMGB1 secreted by NRVMs promoted the activation and proliferation of cardiac fibroblasts. Thus, we concluded that the NLRP3/reactive oxygen species/HMGB1 pathway could be the underlying mechanism of ISO-induced cardiac fibrosis.

HMGB1 in kidney diseases

High mobility group box 1 (HMGB1) is a highly conserved nucleoprotein involving in numerous biological processes, and well known to trigger immune responses as the damage-associated molecular pattern (DAMP) in the extracellular environment. The role of HMGB1 is distinct due to its multiple functions in different subcellular location. In the nucleus, HMGB1 acts as a chaperone to regulate DNA events including DNA replication, repair and nucleosome stability. While in the cytoplasm, it is engaged in regulating autophagy and apoptosis. A great deal of research has explored its function in the pathogenesis of renal diseases. This review mainly focuses on the role of HMGB1 and summarizes the pathway and treatment targeting HMGB1 in the various renal diseases which may open the windows of opportunities for the development of desirable therapeutic ends in these pathological conditions.

Alarmins in Osteoporosis, RAGE, IL-1, and IL-33 Pathways: A Literature Review

Alarmins are endogenous mediators released by cells following insults or cell death to alert the host’s innate immune system of a situation of danger or harm. Many of these, such as high-mobility group box-1 and 2 (HMGB1, HMGB2) and S100 (calgranulin proteins), act through RAGE (receptor for advanced glycation end products), whereas the IL-1 and IL-33 cytokines bind the IL-1 receptors type I and II, and the cellular receptor ST2, respectively. The alarmin family and their signal pathways share many similarities of cellular and tissue localization, functions, and involvement in various physiological processes and inflammatory diseases including osteoporosis. The aim of the review was to evaluate the role of alarmins in osteoporosis. A bibliographic search of the published scientific literature regarding the role of alarmins in osteoporosis was organized independently by two researchers in the following scientific databases: Pubmed, Scopus, and Web of Science. The keywords used were combined as follows: “alarmins and osteoporosis”, “RAGE and osteoporosis”, “HMGB1 and osteoporosis”, “IL-1 and osteoporosis”, “IL 33 and osteopororsis”, “S100s protein and osteoporosis”. The information was summarized and organized in the present review. We highlight the emerging roles of alarmins in various bone remodeling processes involved in the onset and development of osteoporosis, as well as their potential role as biomarkers of osteoporosis severity and progression. Findings of the research suggest a potential use of alarmins as pharmacological targets in future therapeutic strategies aimed at preventing bone loss and fragility fractures induced by aging and inflammatory diseases.

The Role of HMGB1, a Nuclear Damage-Associated Molecular Pattern Molecule, in the Pathogenesis of Lung Diseases

Significance: High-mobility group protein box 1 (HMGB1), a ubiquitous nuclear protein, regulates chromatin structure and modulates the expression of many genes involved in the pathogenesis of lung cancer and many other lung diseases, including those that regulate cell cycle control, cell death, and DNA replication and repair. Extracellular HMGB1, whether passively released or actively secreted, is a danger signal that elicits proinflammatory responses, impairs macrophage phagocytosis and efferocytosis, and alters vascular remodeling. This can result in excessive pulmonary inflammation and compromised host defense against lung infections, causing a deleterious feedback cycle. Recent Advances: HMGB1 has been identified as a biomarker and mediator of the pathogenesis of numerous lung disorders. In addition, post-translational modifications of HMGB1, including acetylation, phosphorylation, and oxidation, have been postulated to affect its localization and physiological and pathophysiological effects, such as the initiation and progression of lung diseases. Critical Issues: The molecular mechanisms underlying how HMGB1 drives the pathogenesis of different lung diseases and novel therapeutic approaches targeting HMGB1 remain to be elucidated. Future Directions: Additional research is needed to identify the roles and functions of modified HMGB1 produced by different post-translational modifications and their significance in the pathogenesis of lung diseases. Such studies will provide information for novel approaches targeting HMGB1 as a treatment for lung diseases.

HMGB1 and repair: focus on the heart

High-mobility group box 1 (HMGB1) is one of the most abundant proteins in eukaryotes and the best characterized damage-associated molecular pattern (DAMP). The biological activities of HMGB1 depend on its subcellular location, context and post-translational modifications. Inside the nucleus, HMGB1 is engaged in many DNA events such as DNA repair, transcription regulation and genome stability; in the cytoplasm, its main function is to regulate the autophagic flux while in the extracellular environment, it possesses more complicated functions and it is involved in a large variety of different processes such as inflammation, migration, invasion, proliferation, differentiation and tissue regeneration. Due to this pleiotropy, the role of HMGB1 has been vastly investigated in various pathological diseases and a large number of studies have explored its function in cardiovascular pathologies. However, in this contest, the precise mechanism of action of HMGB1 and its therapeutic potential are still very controversial since is debated whether HMGB1 is involved in tissue damage or plays a role in tissue repair and regeneration. The main focus of this review is to provide an overview of the effects of HMGB1 in different ischemic heart diseases and to discuss its functions in these pathological conditions.

HMGB1 Protein: A Therapeutic Target Inside and Outside the Cell

High-mobility group box 1 protein (HMGB1) is a nonhistone chromosomal protein discovered more than 30 years ago. It is an abundant nuclear protein that has a dual function-in the nucleus, it binds DNA and participates in practically all DNA-dependent processes serving as an architectural factor. Outside the cell, HMGB1 plays a different role-it acts as an alarmine that activates a large number of HMGB1-"competent" cells and mediates a broad range of physiological and pathological responses. This universality makes it an attractive target for innovative therapeutic strategies in the treatment of various diseases. Here we present an overview of the major nuclear and extracellular properties of HMGB1 and describe its interaction with different molecular partners as specific receptors or inhibitors, which are important for its role as a target in multiple diseases. We highlight its pivotal role as a target for cancer treatment at two aspects: first in terms of its substantial impact on the repair capacity of cancer cells, thus affecting the effectiveness of chemotherapy with the antitumor drug cis-platinum and, second, the possibility to be targeted by microRNAs influencing different pathways of human diseases, thus making it a promising candidate for a new strategy for therapeutic interventions against various pathological conditions but mainly cancer.

Rituximab-induced HMGB1 release is associated with inhibition of STAT3 activity in human diffuse large B-cell lymphoma

Treatment with rituximab plus cyclophosphamide, doxorubicin, vincristine, and prednisone (R-CHOP) has greatly improved clinical outcomes in patients with diffuse large B-cell lymphoma (DLBCL) compared with CHOP. The mechanism of rituximab-induced cell death is poorly understood. We found that rituximab does not enhance the directly killing efficacy of CHOP, as tested on a panel of DLBCL cell lines. Rituximab induced a rapid release of HMGB1 (High mobility group protein B 1). This release is independent of cell death but significantly correlated with an inhibition on STAT3 activity. In the resting state, HMGB1 co-localizes and interacts with STAT3 in the nucleus of DLBCL cells. Treatment with rituximab breaks this binding and triggers HMGB1 release. Treatment with R-CHOP but not CHOP significantly increased plasma HMGB1 and decreased IL-10 concentrations in DLBCL patients compared with controls. The conditioned medium from rituximab-treated DLBCL cells is able to trigger dendritic cell maturation, phagocytosis, and IFN-g secretion by cytotoxic T cells. In conclusion, our results demonstrate that rituximab induces an inhibition on STAT3 activity, leading to increased HMGB1 release and decreased IL-10 secretion, which elicits immune responses, suggesting that indirect effects on the immune system rather than direct killing contribute to elimination of DLBCL.

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.

Human HMGB1 directly facilitates interactions between nucleotide excision repair proteins on triplex-directed psoralen interstrand crosslinks

Psoralen is a chemotherapeutic agent that acts by producing DNA interstrand crosslinks (ICLs), which are especially cytotoxic and mutagenic because their complex chemical nature makes them difficult to repair. Proteins from multiple repair pathways, including nucleotide excision repair (NER), are involved in their removal in mammalian cells, but the exact nature of their repair is poorly understood. We have shown previously that HMGB1, a protein involved in chromatin structure, transcriptional regulation, and inflammation, can bind cooperatively to triplex-directed psoralen ICLs with RPA, and that mammalian cells lacking HMGB1 are hypersensitive to psoralen ICLs. However, whether this effect is mediated by a role for HMGB1 in DNA damage recognition is still unknown. Given HMGB1's ability to bind to damaged DNA and its interaction with the RPA protein, we hypothesized that HMGB1 works together with the NER damage recognition proteins to aid in the removal of ICLs. We show here that HMGB1 is capable of binding to triplex-directed psoralen ICLs with the dedicated NER damage recognition complex XPC-RAD23B, as well as XPA-RPA, and that they form a higher-order complex on these lesions. In addition, we demonstrate that HMGB1 interacts with XPC-RAD23B and XPA in the absence of DNA. These findings directly demonstrate interactions between HMGB1 and the NER damage recognition proteins, and suggest that HMGB1 may affect ICL repair by enhancing the interactions between NER damage recognition factors.

Expression of a high mobility group protein isolated from Cucumis sativus affects the germination of Arabidopsis thaliana under abiotic stress conditions

Although high mobility group B (HMGB) proteins have been identified from a variety of plant species, their importance and functional roles in plant responses to changing environmental conditions are largely unknown. Here, we investigated the functional roles of a CsHMGB isolated from cucumber (Cucumis sativus L.) in plant responses to environmental stimuli. Under normal growth conditions or when subjected to cold stress, no differences in plant growth were found between the wild-type and transgenic Arabidopsis thaliana overexpressing CsHMGB. By contrast, the transgenic Arabidopsis plants displayed retarded germination compared with the wild-type plants when grown under high salt or dehydration stress conditions. Germination of the transgenic plants was delayed by the addition of abscisic acid (ABA), implying that CsHMGB affects germination through an ABA-dependent way. The expression of CsHMGB had affected only the germination stage, and CsHMGB did not affect the seedling growth of the transgenic plants under the stress conditions. The transcript levels of several germination-responsive genes were modulated by the expression of CsHMGB in Arabidopsis. Taken together, these results suggest that ectopic expression of a CsHMGB in Arabidopsis modulates the expression of several germination-responsive genes, and thereby affects the germination of Arabidopsis plants under different stress conditions.

High-mobility group box proteins modulate tumor necrosis factor-alpha expression in osteoclastogenesis via a novel deoxyribonucleic acid sequence

We have previously shown that mice lacking the TSH receptor (TSHR) exhibit osteoporosis due to enhanced osteoclast formation. The fact that this enhancement is not observed in double-null mice of TSHR and TNFalpha suggests that TNFalpha overexpression in osteoclast progenitors (macrophages) may be involved. It is unknown how TNFalpha expression is regulated in osteoclastogenesis. Here, we describe a receptor activator for nuclear factor-kappaB ligand (RANKL)-responsive sequence (CCG AGA CAG AGG TGT AGG GCC), spanning from -157 to -137 bp of the 5'-flanking region of the TNFalpha gene, which functions as a cis-acting regulatory element. We further show how RANKL treatment stimulates the high-mobility group box proteins (HMGB) HMGB1 and HMGB2 to bind the RANKL-responsive sequence and up-regulates TNFalpha transcription. Exogenous HMGB elicits the expression of cytokines, including TNFalpha, as well as osteoclast formation. Conversely, TSH inhibits the expression of HMGB and TNFalpha and the formation of osteoclasts. These results suggest that HMGB play a pivotal role in osteoclastogenesis. We also show a direct correlation between the expression of HMGB and TNFalpha and osteoclast formation in TSHR-null mice and TNFalpha-null mice. Taken together, we conclude that HMGB and TNFalpha play critical roles in the regulation of osteoclastogenesis and the remodeling of bone.

Chromosomal high mobility group (HMG) proteins of the HMGB-type occurring in the moss Physcomitrella patens

High mobility group (HMG) proteins of the HMGB family are chromatin-associated proteins that act as architectural factors in nucleoprotein structures, which regulate DNA-dependent processes including transcription. Members of the HMGB family have been characterised from various mono-and dicot plants, but not from lower plant species. Here, we have identified three candidate HMGB proteins encoded in the genome of the moss Physcomitrella patens. The structurally similar HMGB2 and HMGB3 proteins display the typical overall structure of higher plant HMGB proteins consisting of a central HMG-box DNA-binding domain that is flanked by a basic N-terminal and an acidic C-terminal domain. The HMGB1 protein differs from higher plant HMGB proteins by having a very extensive N-terminal domain and by lacking the acidic C-terminal domain. Like higher plant HMGB proteins, HMGB3 localises to the cell nucleus, but HMGB1 is targeted to plastids. Analysis of the HMG-box domains of HMGB1 and HMGB3 by CD revealed that HMGB1box and the HMGB3box have an alpha-helical structure. While the HMGB3box interacts with DNA comparable to typical higher plant counterparts, the HMGB1box has only a low affinity for DNA. Cotransformation assays in Physcomitrella protoplasts demonstrated that expression of HMGB3 resulted in repression of reporter gene expression. In summary, our data show that functional HMGB-type proteins occur in Physcomitrella and most likely in other lower plant species.

Characterization of Transgenic Arabidopsis Plants Overexpressing High Mobility Group B Proteins under High Salinity, Drought or Cold Stress

High mobility group B (HMGB) proteins found in the nuclei of higher eukaryotes play roles in various cellular processes such as replication, transcription and nucleosome assembly. The Arabidopsis thaliana genome contains eight genes encoding HMGB proteins, the functions of which remain largely unknown in the transcriptional regulation of plant stress responses. To understand better the functions of HMGB proteins in the responses of plants to environmental stimuli, we examined the effect of various abiotic stresses on germination and growth of transgenic Arabidopsis plants that overexpress a single isoform of HMGB. The expression of HMGB2, HMGB3 and HMGB4 was up-regulated by cold stress, whereas the expression of HMGB2 and HMGB3 was markedly down-regulated by drought or salt stress. Under salt or drought stress, the transgenic Arabidopsis plants that overexpress HMGB2 displayed retarded germination and subsequent growth compared with wild-type plants. Overexpression of HMGB4 had no impact on seed germination and seedling growth of the plants under the stress conditions tested. In contrast to no significant stress-related phenotypes of HMGB5-overexpressing plants, loss-of-function mutants of HMGB5 displayed retarded germination and subsequent growth compared with wild-type plants under stress conditions. Although transcript levels of various stress-responsive genes were not modulated by the expression of HMGB2, expression of several germination-responsive genes was modulated by HMGB2 under salt stress. Taken together, these results provide a novel basis for understanding the biological functions of HMGB protein family members that differently affect germination and seedling growth of Arabidopsis plants under various stress conditions.

High-mobility group protein 2 may be involved in the locus control region regulation of the beta-globin gene cluster

Expression regulation of the beta-globin gene cluster is a result of synergistic interactions between cis-elements and trans-acting factors. Previous studies usually concentrated on the core sequence of each hypersensitive site in the locus control region of the beta-globin gene cluster. But more and more evidence illustrates that the flanking regions are indispensable also. Using electrophoretic mobility shift assay and solid-phase DNase I footprinting methods, we identified a small nuclear protein from K562 cells that binds specifically to the first AT-rich region flanking the hypersensitive site 2 core sequence of the human beta-globin gene locus control region. N-terminal sequencing of the enriched protein proved that it is a member of the high-mobility group protein 2 family. This indicates that the AT-rich region in human hypersensitive site 2 may take part in the regulation of the beta-globin gene cluster by facilitating DNA bending, which is a prerequisite for the looping mechanism in this region.

TFIIA abrogates the effects of inhibition by HMGB1 but not E1A during the early stages of assembly of the transcriptional preinitiation complex

Successful assembly of the transcriptional preinitiation complex (PIC) is prerequisite to transcriptional initiation. At each stage of PIC assembly, regulation may occur as repressors and activators compete with and influence the incorporation of general transcription factors (GTFs). Both TFIIA and HMGB1 bind individually to the TATA-binding protein (TBP) to increase the rate of binding and to stabilize TBP binding to the TATA element. The competitive binding between these two cofactors for TBP/TATA was examined to show that TFIIA binds preferentially to TBP and inhibits HMGB1 binding. TFIIA can also readily dissociate HMGB1 from the preestablished HMGB1/TBP/TATA complex. This suggests that TFIIA and HMGB1 may bind to the same or overlapping sites on TBP and/or compete for similar DNA sites that are 5' to the TATA element. In addition, EMSA studies show that adenovirus E1A(13S) oncoprotein is unable to disrupt either the preestablished TFIIA/TBP/TATA or TFIIA/TFIIB/TBP/TATA complexes, but does inhibit complex formation when all transcription factors were simultaneously added. The inhibitory effect of E1A(13S) on the assembly of the PIC is overcome when excess TBP is added back in the reaction, while addition of either excess TFIIA or TFIIB were ineffective. This shows that the main target for E1A(13S) is free TBP and emphasizes the primary competition between E1A and the TATA-element for unbound TBP. This may be the principal point, if not the only point, at which E1A can target TBP to exert its inhibitory effect. This work, coupled with previous findings in our laboratory, indicates that TFIIA is much more effective than TFIIB in reversing the inhibitory effect of HMGB1 binding in the early stages of PIC assembly, which is consistent with the in vitro transcription results.

Physical interaction of tumour suppressor p53/p73 with CCAAT-binding transcription factor 2 (CTF2) and differential regulation of human high-mobility group 1 (HMG1) gene expression

The CCAAT-binding transcription factor (CTF)/nuclear factor I (NF-I) group of cellular DNA-binding proteins recognizes the sequence GCCAAT and is implicated in eukaryotic transcription, as well as DNA replication. Molecular analysis of human CTF/NF-I cDNA clones revealed multiple mRNA species that contain alternative coding regions, apparently as a result of differential splicing. Expression and functional analysis established that individual gene products can bind to GCCAAT recognition sites and serve as both promoter-selective transcriptional activators and initiation factors for DNA replication. The interaction between CTF2 and p53/p73 was shown to modulate their ability to regulate transcription of their respective target genes. In the present paper, we report that p53 down-regulates the activity of the high mobility group 1 (HMG1) gene promoter, whereas p73alpha up-regulates the activity of this promoter. Furthermore, CTF2 transactivates p53-induced p21 promoter activity, but inhibits p73alpha-induced p21 promoter activity. Using deletion mutants, we found that the DNA-binding domains of both p53 and p73alpha are required for physical interaction with CTF2 via the regions between amino acid residues 161 and 223, and 228 and 312 respectively. CTF2 enhances the DNA-binding activity of p53 and inhibits the DNA-binding activity of p73alpha. These results provide novel information on the functional interplay between CTF2 and p53/p73 as important determinants of their function in cell proliferation, apoptosis, DNA repair and cisplatin resistance.

Involvement of HMGB1 and HMGB2 proteins in exogenous DNA integration reaction into the genome of HeLa S3 cells

High mobility group 1 and 2 proteins (HMGB1 and HMGB2) are abundant chromosomal proteins in eukaryotic cells. We examined the involvement of HMGB1 and HMGB2 in nonhomologous illegitimate recombination. The HMGB1 or HMGB2 expression plasmid, carrying the neo(r) gene as a selection marker, was introduced into HeLa S3 cells to obtain stably-transfected cells. The number of G418-resistant colonies was about 10 times the number of colonies of control cells transfected with plasmids not carrying the HMGB genes. The copy number of the stably-integrated neo(r) gene was higher in the cells transfected with the HMGB expression plasmids than in control cells. The exogenous DNA integration was suggested to have occurred by nonhomologous illegitimate recombination. On the contrary, the introduction of the HMGB antisense RNA expression plasmid with a reporter plasmid carrying the neo(r) gene into HeLa S3 cells decreased the number of G418-resistant colonies. These results indicate that HMGB1 and HMGB2 each have a novel function as stimulators of stable integration of plasmid DNA into the host genome and that they may be important for the process of spontaneous DNA integration in living cells.

Specificity of the stimulatory interaction between chromosomal HMGB proteins and the transcription factor Dof2 and its negative regulation by protein kinase CK2-mediated phosphorylation

The high mobility group (HMG) proteins of the HMGB family are chromatin-associated proteins that can contribute to transcriptional control by interaction with certain transcription factors. Using the transcription factor Dof2 and five different maize HMGB proteins, we have examined the specificity of the HMGB-transcription factor interaction. The HMG-box DNA binding domain of HMGB1 is sufficient for the interaction with Dof2. Although all tested HMGB proteins can interact with Dof2, the various HMGB proteins stimulate the binding of Dof2 to its DNA target site with different efficiencies. The HMGB5 protein is clearly the most potent facilitator of Dof2 DNA binding. Maximal stimulation of the DNA binding by the HMGB proteins requires association of HMGB and Dof2 prior to DNA binding. HMGB5 and Dof2 form a ternary complex with the DNA, but within the protein-DNA complex the interaction of HMGB5 and Dof2 is different from that in solution, as in contrast to the proteins in solution, they cannot be cross-linked with glutaraldehyde when bound to DNA. Phosphorylation of HMGB1 by protein kinase CK2 abolishes the interaction with Dof2 and the stimulation of Dof2 DNA binding. These findings indicate that transcription factors may recruit certain members of the HMGB family as assistant factors.

Immunomodulation by parasites: high mobility group 2 (HMG-2) protein is a putative intracellular mediator for fucosylated sugars of Schistosoma mansoni

Fucosylated sugars in Schistosoma mansoni possess immunomodulatory properties. In order to gain insights to the mechanisms involved, attempts were made to identify host immune cell molecules that specifically recognize these sugars. On Western blots, specific binding of synthetic biotinylated fucose sugars to proteins of approximately 25-27kDa was observed. Three proteins were isolated by affinity chromatography and subjected to protein sequencing. The determined N-terminal sequences and that of tryptic peptides of two proteins did not show homology to known sequences in the NCBI database. The third was identified as a member of the high mobility group 2 (HMG-2) proteins. In vitro stimulation of mouse spleen cells with Lewis(x) sugars up-regulated the expression of HMG-2 mRNA. These data suggest that HMG-2 protein may function as a putative intracellular receptor/mediator for fucosylated sugars of parasites.

Dual roles for HMGB1: DNA binding and cytokine

Effective therapies against overwhelming Gram-negative bacteremia, or sepsis, have eluded successful development. The discovery that tumor necrosis factor (TNF), a host-derived inflammatory mediator, was both necessary and sufficient to recapitulate Gram-negative sepsis raised cautious optimism for developing a targeted therapeutic. However, the rapid kinetics of the TNF response to infection defined an extremely narrow window of opportunity during which anti-TNF therapeutics could be successfully administered. HMGB1 was previously studied as a DNA-binding protein involved in DNA replication, repair, and transcription; and as a membrane-associated protein that mediates neurite outgrowth. A decade-long search has culminated in our identification of HMGB1 as a late mediator of endotoxemia. HMGB1 is released by macrophages upon exposure to endotoxin, activates many other pro-inflammatory mediators, and is lethal to otherwise healthy animals. Elevated levels of HMGB1 are observed in the serum of patients with sepsis, and the highest levels were found in those patients that died. The delayed kinetics of HMGB1 release indicate that it may be useful to target this toxic cytokine in the development of future therapies.

HMG proteins and DNA flexibility in transcription activation

The relative stiffness of naked DNA is evident from measured values of longitudinal persistence length (approximately 150 bp) and torsional persistence length (approximately 180 bp). These parameters predict that certain arrangements of eukaryotic transcription activator proteins in gene promoters should be much more effective than others in fostering protein-protein interactions with the basal RNA polymerase II transcription apparatus. Thus, if such interactions require some kind of DNA looping, DNA loop energies should depend sensitively on helical phasing of protein binding sites, loop size, and intrinsic DNA curvature within the loop. Using families of artificial transcription templates where these parameters were varied, we were surprised to find that the degree of transcription activation by arrays of Gal4-VP1 transcription activators in HeLa cell nuclear extract was sensitive only to the linear distance separating a basal promoter from an array of bound activators on DNA templates. We now examine the hypothesis that this unexpected result is due to factors in the extract that act to enhance apparent DNA flexibility. We demonstrate that HeLa cell nuclear extract is rich in a heat-resistant activity that dramatically enhances apparent DNA longitudinal and torsional flexibility. Recombinant mammalian high-mobility group 2 (HMG-2) protein can substitute for this activity. We propose that the abundance of HMG proteins in eukaryotic nuclei provides an environment in which DNA is made sufficiently flexible to remove many constraints on protein binding site arrangements that would otherwise limit efficient transcription activation to certain promoter geometries.

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.

Chromatin-mediated transcriptional regulation by the yeast architectural factors NHP6A and NHP6B

The Saccharomyces cerevisiae NHP6A and NHP6B proteins are chromatin architectural factors, functionally and structurally related to the mammalian high mobility group (HMG)-1 and -2 proteins, a family of non-sequence-specific DNA binding proteins. nhp6a nhp6b mutants have various morphological defects and are defective in the induced expression of several RNA polymerase II-transcribed genes. We found that NHP6A/B proteins are also required for full induction of the yeast CHA1 gene. Importantly, CHA1 basal level expression is increased 10-fold in an nhp6a nhp6b double deletion mutant. Micrococcal nuclease and DNase I analysis of the CHA1 gene in this strain showed an open promoter structure, characteristic of the activated state of this promoter, even under non-inducing conditions. To address the possible function of the NHP6A/B proteins in chromatin-mediated gene regulation, we performed whole-genome transcriptional profiling of a Deltanhp6a Deltanhp6b yeast strain. Our results suggest that NHP6A/B proteins play an important regulatory role, repressing as well as potentiating expression of genes involved in several cellular processes, and that NHP6A/B control is exerted at the level of the individual gene.

The effect of the acidic tail on the DNA-binding properties of the HMG1,2 class of proteins: insights from tail switching and tail removal

The high-mobility group (HMG) proteins HMG1, HMG2 and HMG2a are relatively abundant vertebrate DNA-binding and bending proteins that bind with structure specificity, rather than sequence specificity, and appear to play an architectural role in the assembly of nucleoprotein complexes. They have two homologous "HMG-box" DNA-binding domains (which show about 80 % homology) connected by a short basic linker to an acidic carboxy-terminal tail that differs in length between HMG1 and 2. To gain insights into the role of the acidic tail, we examined the DNA-binding properties of HMG1, HMG2b and HMG2a from chicken erythrocytes (corresponding to HMG1, HMG2 and HMG2a in other vertebrates). HMG1, with the longest acidic tail, is less effective than HMG2a and 2b (at a given molar input ratio) in supercoiling relaxed, closed circular DNA, in inducing ligase-mediated circularisation of an 88 bp DNA fragment, and in binding to four-way DNA junctions in a gel-shift assay. Removal of the acidic tail increases the affinity of the HMG boxes for DNA and largely abolishes the differences between the three species. Switching the acidic tail of HMG1 for that of HMG2a or 2b gives hybrid proteins with essentially the same DNA-binding properties as HMG2a, 2b. The length (and possibly sequence) of the acidic tail thus appears to be the dominant factor in mediating the differences in properties between HMG1, 2a and 2b and finely tunes the rather similar DNA-binding properties of the tandem HMG boxes, presumably to fulfill different cellular roles. The tail is essential for structure-selective DNA-binding of the HMG boxes to DNA minicircles in the presence of equimolar linear DNA, and has little effect on the affinity for this already highly distorted DNA ligand, in contrast to binding to linear and four-way junction DNA.

Influence of HMG-1 and adenovirus oncoprotein E1A on early stages of transcriptional preinitiation complex assembly

The TATA-binding protein (TBP) in the TFIID complex binds specifically to the TATA-box to initiate the stepwise assembly of the preinitiation complex (PIC) for RNA polymerase II transcription. Transcriptional activators and repressors compete with general transcription factors at each step to influence the course of the assembly. To investigate this process, the TBP.TATA complex was titrated with HMG-1 and the interaction monitored by electrophoretic mobility shift assays. The titration produced a ternary HMG-1.TBP. TATA complex, which exhibits increased mobility relative to the TBP. TATA complex. The addition of increasing levels of TFIIB to this complex results in the formation of the TFIIB.TBP.TATA complex. However, in the reverse titration, with very high mole ratios of HMG-1 present, TFIIB is not dissociated off and a complex is formed that contains all factors. The simultaneous addition of E1A to a mixture of TBP and TATA; or HMG-1, TBP, and TATA; or TFIIB, TBP, and TATA inhibits complex formation. On the other hand, E1A added to the pre-established complexes shows a significantly reduced capability to disrupt the complex. In add-back experiments with all complexes, increased levels of TBP re-established the complexes, indicating that the primary target for E1A in all complexes is TBP.

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.

Poly(ADP-ribosyl)ation reactions in the regulation of nuclear functions

Poly(ADP-ribosyl)ation is a post-translational modification of proteins. During this process, molecules of ADP-ribose are added successively on to acceptor proteins to form branched polymers. This modification is transient but very extensive in vivo, as polymer chains can reach more than 200 units on protein acceptors. The existence of the poly(ADP-ribose) polymer was first reported nearly 40 years ago. Since then, the importance of poly(ADP-ribose) synthesis has been established in many cellular processes. However, a clear and unified picture of the physiological role of poly(ADP-ribosyl)ation still remains to be established. The total dependence of poly(ADP-ribose) synthesis on DNA strand breaks strongly suggests that this post-translational modification is involved in the metabolism of nucleic acids. This view is also supported by the identification of direct protein-protein interactions involving poly(ADP-ribose) polymerase (113 kDa PARP), an enzyme catalysing the formation of poly(ADP-ribose), and key effectors of DNA repair, replication and transcription reactions. The presence of PARP in these multiprotein complexes, in addition to the actual poly(ADP-ribosyl)ation of some components of these complexes, clearly supports an important role for poly(ADP-ribosyl)ation reactions in DNA transactions. Accordingly, inhibition of poly(ADP-ribose) synthesis by any of several approaches and the analysis of PARP-deficient cells has revealed that the absence of poly(ADP-ribosyl)ation strongly affects DNA metabolism, most notably DNA repair. The recent identification of new poly(ADP-ribosyl)ating enzymes with distinct (non-standard) structures in eukaryotes and archaea has revealed a novel level of complexity in the regulation of poly(ADP-ribose) metabolism.

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 MSN1 and NHP6A genes suppress SWI6 defects in Saccharomyces cerevisiae

Ankyrin (ANK) repeats were first found in the Swi6 transcription factor of Saccharomyces cerevisiae and since then were identified in many proteins of eukaryotes and prokaryotes. These repeats are thought to serve as protein association domains. In Swi6, ANK repeats affect DNA binding of both the Swi4/Swi6 and Mbp1/Swi6 complexes. We have previously described generation of random mutations within the ANK repeats of Swi6 that render the protein temperature sensitive in its ability to activate HO transcription. Two of these SWI6 mutants were used in a screen for high copy suppressors of this phenotype. We found that MSN1, which encodes a transcriptional activator, and NHP6A, which encodes an HMG-like protein, are able to suppress defective Swi6 function. Both of these gene products are involved in HO transcription, and Nhp6A may also be involved in CLN1 transcription. Moreover, because overexpression of NHP6A can suppress caffeine sensitivity of one of the SWI6 ANK mutants, swi6-405, other SWI6-dependent genes may also be affected by Nhp6A. We hypothesize that Nhp6A and Msn1 modulate Swi6-dependent gene transcription indirectly, through effects on chromatin structure or other transcription factors, because we have not been able to demonstrate that either Msn1 or Nhp6A interact with the Swi4/Swi6 complex.

The high mobility group protein 1 enhances binding of the estrogen receptor DNA binding domain to the estrogen response element

We have examined the ability of the high-mobility group protein 1 (HMG1) to alter binding of the estrogen receptor DNA-binding domain (DBD) to the estrogen response element (ERE). HMG1 dramatically enhanced binding of purified, bacterially expressed DBD to the consensus vitellogenin A2 ERE in a dose-dependent manner. The ability of HMG1 to stabilize the DBD-ERE complex resulted in part from a decrease in the dissociation rate of the DBD from the ERE. Antibody supershift experiments demonstrated that HMG1 was also capable of forming a ternary complex with the ERE-bound DBD in the presence of HMG1-specific antibody. HMG1 did not substantially affect DBD-ERE contacts as assessed by methylation interference assays, nor did it alter the ability of the DBD to induce distortion in ERE-containing DNA fragments. Because HMG1 dramatically enhanced estrogen receptor DBD binding to the ERE, and the DBD is the most highly conserved region among the nuclear receptor superfamily members, HMG1 may function to enhance binding of other nuclear receptors to their respective response elements and act in concert with coactivator proteins to regulate expression of hormone-responsive genes.

High mobility group protein-1 (HMG-1) is a unique activator of p53

The binding of p53 protein to DNA is stimulated by its interaction with covalent as well as noncovalent modifiers. We report the identification of a factor from HeLa nuclear extracts that activates p53 DNA binding. This factor was purified to homogeneity and identified as the high mobility group protein, HMG-1. HMG-1 belongs to a family of highly conserved chromatin-associated nucleoproteins that bend DNA and facilitate the binding of various transcription factors to their cognate DNA sequences. We demonstrate that recombinant His-tagged HMG-1 enhances p53 DNA binding in vitro and also that HMG-1 and p53 can interact directly in vitro. Unexpectedly, HMG-1 also stimulates DNA binding by p53Delta30, a carboxy-terminally deleted form of the protein that is considered to be constitutively active, suggesting that HMG-1 stimulates p53 by a mechanism that is distinct from other known activators of p53. Finally, using transient transfection assays we show that HMG-1 can increase p53 and p53Delta30-mediated transactivation in vivo. HMG-1 promotes the assembly of higher order p53 nucleoprotein structures, and these data, along with the fact that HMG-1 is capable of bending DNA, suggest that HMG-1 may activate p53 DNA binding by a novel mechanism involving a structural change in the target DNA.

Dof DNA-binding domains of plant transcription factors contribute to multiple protein-protein interactions

Dof proteins are a family of plant transcription factors that have a strongly conserved DNA-binding domain, designated the Dof domain. This domain has the potential to form a single zinc finger. This report describes the self-association of a maize Dof protein, Dof1 (previously designated MNB1a). Affinity chromatography revealed that Dof1 also interacted with another maize Dof protein, Dof2, as well as with high-mobility-group (HMG) protein 1. Results of mapping of the region required for the protein-protein interactions of Dof1 suggested that these interactions may be mediated by the Dof domain. When gel mobility shift assays were performed with purified recombinant Dof proteins, homomeric and heteromeric complexes of Dof proteins on DNA were detected. It seems possible that formation of complexes of different Dof proteins through direct protein-protein interactions might be involved in the regulation of transcription. Evidence is also presented that HMG1 has an effect on the binding of Dof1 to DNA. Therefore, it appears that the Dof domain is a multifunctional domain that is involved not merely in binding to DNA but also in multiple protein-protein interactions.

Stimulation of V(D)J cleavage by high mobility group proteins

V(D)J recombination requires a pair of signal sequences with spacer lengths of 12 and 23 bp between the conserved heptamer and nonamer elements. The RAG1 and RAG2 proteins initiate the reaction by making double-strand DNA breaks at both signals, and must thus be able to operate on these two different spatial arrangements. We show that the DNA-bending proteins HMG1 and HMG2 stimulate cleavage and RAG protein binding at the 23 bp spacer signal. These findings suggest that DNA bending is important for bridging the longer spacer, and explain how a similar array of RAG proteins could accommodate a signal with either a 12 or a 23 bp spacer. An additional effect of HMG proteins is to stimulate coupled cleavage greatly when both signal sequences are present, suggesting that these proteins also aid the formation of a synaptic complex.

Characterization of the cloned promoter of the human initiation factor 4AI gene

Protein synthesis initiation factor 4AI (eIF-4AI) is ubiquitously expressed in eucaryotic cells. We characterized the eIF-4AI gene promoter cloned from human fibroblasts. The minimal promoter, localized to a region in the 5'-noncoding region adjacent to the first exon, consisted of approximately 300 base pairs (bp), 80% of which were identical with those in the corresponding mouse promoter. The minimal promoter contained TATA and CAAT motifs and consensus sequences binding to SP1 (three sites) and AP2 (one site). Deletion analyses of the promoter revealed that a 24-bp region near 5'-end of the minimal promoter was essential for the efficient transcription, although the AP2 site in the region was dispensable. A fluorescence polarization assay suggested that the plus strand of the 24-bp region, despite the lack of known consensus sequences binding to transcription factors except for AP2, bound to unknown nuclear protein(s) in a sequence specific manner.

High mobility group 1 protein is not stably associated with the chromosomes of somatic cells

High mobility group 1 (HMG1) protein is an abundant and conserved component of vertebrate nuclei and has been proposed to play a structural role in chromatin organization, possibly similar to that of histone H1. However, a high abundance of HMG1 had also been reported in the cytoplasm and on the surface of mammalian cells. We conclusively show that HMG1 is a nuclear protein, since several different anti-HMG1 antibodies stain the nucleoplasm of cultured cells, and epitope-tagged HMG1 is localized in the nucleus only. The protein is excluded from nucleoli and is not associated to specific nuclear structures but rather appears to be uniformly distributed. HMG1 can bind in vitro to reconstituted core nucleosomes but is not stably associated to chromatin in live cells. At metaphase, HMG1 is detached from condensed chromosomes, contrary to histone H1. During interphase, HMG1 readily diffuses out of nuclei after permeabilization of the nuclear membranes with detergents, whereas histone H1 remains associated to chromatin. These properties exclude a shared function for HMG1 and H1 in differentiated cells, in spite of their similar biochemical properties. HMG1 may be stably associated only to a very minor population of nucleosomes or may interact transiently with nucleosomes during dynamic processes of chromatin remodeling.

Extracellular high-mobility group 1 protein is essential for murine erythroleukaemia cell differentiation

A high-mobility group 1 (HMG1) protein type isolated from murine erythroleukaemia (MEL) cells promotes acceleration of the differentiation process when added to a MEL cell culture together with the inducer hexamethylene bisacetamide. We now provide direct evidence that the presence of HMG1 protein in the extracellular medium is essential for terminal erythroid differentiation. An extracellular function for HMG1 protein in MEL cell is further supported by a demonstration that this protein is released from MEL cells exposed to the chemical inducer and that the addition of an anti-(HMG1 protein) monoclonal antibody to the cell culture inhibits the differentiation process almost completely. The release of HMG1 protein from MEL cells is modulated by compounds affecting cell calcium homoeostasis, such as a calcium ionophore or verapamil. In fact, in the presence of the ionophore an increased rate of differentiation is accompanied by an enhanced extracellular release of HMG1 protein, whereas in the presence of verapamil both phenomena are significantly decreased.

Yeast HMG proteins NHP6A/B potentiate promoter-specific transcriptional activation in vivo and assembly of preinitiation complexes in vitro

Nonhistone proteins 6A and 6B (NHP6A/B) are nonsequence-specific DNA-binding proteins from Saccharomyces cerevisiae that are related structurally and functionally to the mammalian high mobility group proteins 1 and 2. These DNA architectural proteins distort DNA structure severely and have been shown to promote assembly of specialized recombination complexes. Here we show that the yeast NHP6A/B proteins are required for the induction of a subset of genes transcribed by RNA polymerase II (pol II). Activation of the CUP1, CYC1, GAL1, and DDR2 genes was decreased or abolished completely in the delta nhp6A/B strain. No significant change in basal expression was observed for any of the 10 genes examined. Analysis of chimeric gene constructs localized the regions dependent on NHP6A/B to be primarily at the core promoters, although the GAL1 UAS also requires NHP6A/B for activity. In vitro, NHP6A stimulated transcription by pol II at the GAL1 promoter three- to fivefold above the level of activation by GAL4-VP16 alone. Gel mobility shift assays showed that NHP6A promotes the formation of a complex with TBP and TFIIA at the TATA box that has enhanced affinity for TFIIB.

The conserved lymphokine element-0 in the IL5 promoter binds to a high mobility group-1 protein

The conserved lymphokine elements-0 (CLE0) in the IL5 promoter is essential for the expression of IL-5. Here, we report the cloning and expression of a cDNA encoding a novel CLE0-binding protein, CLEBP-1 from a mouse Th2 clone, D10.G4.1. Interestingly, it was found that the CLEBP1 cDNA sequence was almost identical to the sequences of known high mobility group-1 (HMG1) cDNAs. When expressed as a recombinant fusion protein in Escherichia coli, CLEBP-1 was shown to bind to the IL5-CLE0 element in electrophoretic mobility-shift assays (EMSA) and southwestern blot analysis. The CLEBP-1 fusion protein cross-reacts with and-HMG-1/2 in Western blot analysis. It also binds to the CLE0 elements of IL4, GMCSF and GCSF genes. CLEBP-1 and closely related HMG-1 and HMG-2 proteins may play key roles in facilitating the expression of the lymphokine genes that contain CLE0 elements.

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.

Stimulation of transcription accompanying relaxation of chromatin structure in cells overexpressing high mobility group 1 protein

We developed murine C-127 cell lines that stationarily overexpress high mobility group (HMG) proteins 1 and 2 by transfecting them with the bovine papilloma virus plasmid carrying their respective cDNA sequences. Using these cell lines, we examined the effects of these HMG proteins on the modulation of chromatin structure that accompanied transcription. The levels of HMG1 mRNA and protein in cells overexpressing HMG1 protein were enhanced about 7- and 3-fold, respectively, in comparison with control cells, whereas those in cells overexpressing HMG2 protein were enhanced about 17- and 9-fold. The expression of reporter genes transfected into the cells was enhanced approximately 2-fold in cells overexpressing HMG1, but not HMG2, in comparison with those in control cells, irrespective of the sources of the genes and promoters. The minichromosome derived from the reporter plasmid in cells overexpressing HMG1 protein was more susceptible to micrococcal nuclease digestion than those in cells overexpressing HMG2 protein and control cells. The enhanced accessibility to micrococcal nuclease was not restricted to the expressing gene and promoter but involved the entire minichromosome, suggesting that the enhancement of gene expression resulted from changes in the condensation of the entire minichromosomal region by HMG1 protein. Minichromosomes in cells overexpressing HMG contained enhanced amounts of the respective HMG proteins and simultaneously reduced amounts of histone H1s. These results suggest that HMG1 and -2 proteins have different functions in the modulation of chromatin structure, and that HMG1 protein may sustain the structure of the respective gene to ensure that its activity as a template is expressed fully. These observations on the modulation of chromatin structure accompanying gene transcription in cells overexpressing HMG protein may provide important information on the function of these proteins.

DNA looping by Saccharomyces cerevisiae high mobility group proteins NHP6A/B. Consequences for nucleoprotein complex assembly and chromatin condensation

The formation of higher order protein.DNA structures often requires bending of DNA strands between specific sites, a process that can be facilitated by the action of nonspecific DNA-binding proteins which serve as assembly factors. A model for this activity is the formation of the invertasome, an intermediate structure created in the Hin-mediated site-specific DNA inversion reaction, which is stimulated by the prokaryotic nucleoid-associated protein HU. Previously, we have shown that the mammalian HMG1/2 proteins substitute for HU in this system and display efficient DNA wrapping activity in vitro. In the present work, we isolate the primary sources of assembly factor activity in Saccharomyces cerevisiae, as measured by the ability to stimulate invertasome formation, and show that these are the previously identified NHP6A/B proteins. NHP6A/B have comparable or greater activity in DNA binding, bending, and supercoiling with respect to HU and HMG1 and appear to form more stable protein.DNA complexes. In addition, expression of NHP6A in mutant Escherichia coli cells lacking HU and Fis restores normal morphological appearance to these cells, specifically in nucleoid condensation and segregation. From these data we predict diverse architectural roles for NHP6A/B in manipulating chromosome structure and promoting the assembly of multicomponent protein.DNA complexes.

The C-terminal domain of SIN1 in yeast interacts with a protein that binds the URS1 region of the yeast HO gene

A protein or protein complex has previously been identified in Saccharomyces cerevisiae which both binds a short DNA sequence in URS1 of HO and interacts with SIN1. SIN1, which has some sequence similarity to mammalian HMG1, is an abundant chromatin protein in yeast and is thought to participate in the transcriptional repression of a specific family of genes. SIN1 binds DNA weakly, though it has no DNA binding specificity. Here we address the nature of the interaction between SIN1 and the specific DNA binding protein(s) to HO DNA. We show that the isolated C-terminal region of SIN1 can interact in vitro with the DNA binding protein, causing a supershift in a gel mobility shift assay. Interestingly, inclusion of the region in SIN1 which contains two acidic sequences, precludes the binding of recombinant protein to the DNA/protein complex.