DNA binding and nuclear translocation of insect high-mobility-group- protein-1 (HMG1) proteins are inhibited by phosphorylation

Phosphorylation by Protein Kinase C Weakens DNA-Binding Affinity and Folding Stability of the HMGB1 Protein

The HMGB1 protein typically serves as a DNA chaperone that assists DNA-repair enzymes and transcription factors but can translocate from the nucleus to the cytoplasm or even to extracellular space upon some cellular stimuli. One of the factors that triggers the translocation of HMGB1 is its phosphorylation near a nuclear localization sequence by protein kinase C (PKC), although the exact modification sites on HMGB1 remain ambiguous. In this study, using spectroscopic methods, we investigated the HMGB1 phosphorylation and its impact on the molecular properties of the HMGB1 protein. Our nuclear magnetic resonance (NMR) data on the full-length HMGB1 protein showed that PKC specifically phosphorylates the A-box domain, one of the DNA binding domains of HMGB1. Phosphorylation of S46 and S53 was particularly efficient. Over a longer reaction time, PKC phosphorylated some additional residues within the HMGB1 A-box domain. Our fluorescence-based binding assays showed that the phosphorylation significantly reduces the binding affinity of HMGB1 for DNA. Based on the crystal structures of HMGB1-DNA complexes, this effect can be ascribed to electrostatic repulsion between the negatively charged phosphate groups at the S46 side chain and DNA backbone. Our data also showed that the phosphorylation destabilizes the folding of the A-box domain. Thus, phosphorylation by PKC weakens the DNA-binding affinity and folding stability of HMGB1.

Naturally Occurring HMGB1 Inhibitor, Glycyrrhizin, Modulates Chronic Seizures-Induced Memory Dysfunction in Zebrafish Model

Glycyrrhizin (GL) is a well-known pharmacological inhibitor of high mobility group box 1 (HMGB1) and is abundantly present in the licorice root (Glycyrrhiza radix). HMGB1 protein, a key mediator of neuroinflammation, has been implicated in several neurological disorders, including epilepsy. Epilepsy is a devastating neurological disorder with no effective disease-modifying treatment strategies yet, suggesting a pressing need for exploring novel therapeutic options. In the current investigation, using a second hit pentylenetetrazol (PTZ) induced chronic seizure model in adult zebrafish, regulated mRNA expression of HMGB1 was inhibited by pretreatment with GL (25, 50, and 100 mg/kg, ip). A molecular docking study suggests that GL establishes different binding interactions with the various amino acid chains of HMGB1 and Toll-like receptor-4 (TLR4). Our finding suggests that GL pretreatment reduces/suppresses second hit PTZ induced seizure, as shown by the reduction in the seizure score. GL also regulates the second hit PTZ induced behavioral impairment and rescued second hit PTZ related memory impairment as demonstrated by an increase in the inflection ratio (IR) at the 3 h and 24 h T-maze trial. GL inhibited seizure-induced neuronal activity as demonstrated by reduced C-fos mRNA expression. GL also modulated mRNA expression of BDNF, CREB-1, and NPY. The possible mechanism underlying the anticonvulsive effect of GL could be attributed to its anti-inflammatory activity, as demonstrated by the downregulated mRNA expression level of HMGB1, TLR4, NF-kB, and TNF-α. Overall, our finding suggests that GL exerts an anticonvulsive effect and ameliorates seizure-related memory disruption plausibly through regulating of the HMGB1-TLR4-NF-kB axis.

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.

Molecular and functional characterization of single-box high-mobility group B (HMGB) chromosomal protein from Aedes aegypti

High-mobility group B (HMGB) proteins have highly conserved, unique DNA-binding domains, HMG boxes, that can bind non-B-type DNA structures, such as bent, kinked and unwound structures, with high affinity. HMGB proteins also promote DNA bending, looping and unwinding. In this study, we determined the role of the Aedes aegypti single HMG-box domain protein AaHMGB; characterized its structure, spatiotemporal expression levels, subcellular localization, and nucleic acid binding activities; and compared these properties with those of its double-HMG-box counterpart protein, AaHMGB1. Via qRT-PCR, we showed that AaHMGB is expressed at much higher levels than AaHMGB1 throughout mosquito development. In situ hybridization results suggested a role for AaHMGB and AaHMGB1 during embryogenesis. Immunolocalization in the midgut revealed that AaHMGB is exclusively nuclear. Circular dichroism and fluorescence spectroscopy analyses showed that AaHMGB exhibits common features of α-helical structures and is more stably folded than AaHMGB1, likely due to the presence of one or two HMG boxes. Using several DNA substrates or single-stranded RNAs as probes, we observed significant differences between AaHMGB and AaHMGB1 in terms of their binding patterns, activity and/or specificity. Importantly, we showed that the phosphorylation of AaHMGB plays a critical role in its DNA-binding activity. Our study provides additional insight into the roles of single- versus double-HMG-box-containing proteins in nucleic acid interactions for better understanding of mosquito development, physiology and homeostasis.

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.

Effect of ulinastatin on the expression and distribution of high mobility group box 1 in human colon carcinoma cells in vitro

The aim of the present study was to investigate the in vitro effects of ulinastatin (UTI) on the proliferation, invasion, apoptosis, expression and distribution of high mobility group box 1 (HMGB1) and the expression of nuclear factor κB (NF‑κB) in human colon carcinoma LoVo cells. The cells were divided into control (untreated), UTI1 (400 U/ml UTI), UTI2 (800 U/ml UTI) and UTI3 (1,600 U/ml UTI) groups. The cell proliferation, invasion, apoptosis and the gene and protein expression of HMGB1 and NF‑κB were detected using a tetrazolium assay, Transwell cell invasion assays, a caspase‑3 activity assay, western blot analysis and reverse transcription quantitative polymerase chain reaction, respectively. The distribution of HMGB1 was detected using immunofluorescence. LoVo cell proilferation decreased the most in the UTI3 group followed, in order, by the UTI2, UTI1 and control groups. UTI inhibited invasion in LoVo cells and the inhibitory effect was enhanced as the UTI concentration increased. The activity of caspase‑3 increased the least in the control group followed, in order, by the UTI1, UTI2 and UTI3 groups. UTI inhibited the expression of HMGB1 and NF‑κB, and decreased the cytoplasmic distribution of HMGB1. Thus, UTI inhibited LoVo cell proliferation and induced LoVo cell apoptosis, the mechanism of which may be associated with a decreased in the expression of HMGB1 and NF‑κB, and the cytoplasmic distribution of HMGB1.

Structural insights into the mechanism of negative regulation of single-box high mobility group proteins by the acidic tail domain

The Drosophila and plant (maize) functional counterparts of the abundant vertebrate chromosomal protein HMGB1 (HMG-D and ZmHMGB1, respectively) differ from HMGB1 in having a single HMG box, as well as basic and acidic flanking regions that vary greatly in length and charge. We show that despite these variations, HMG-D and ZmHMGB1 exist in dynamic assemblies in which the basic HMG boxes and linkers associate with their intrinsically disordered, predominantly acidic, tails in a manner analogous to that observed previously for HMGB1. The DNA-binding surfaces of the boxes and linkers are occluded in “auto-inhibited” forms of the protein, which are in equilibrium with transient, more open structures that are “binding-competent.” This strongly suggests that the mechanism of auto-inhibition may be a general one. HMG-D and ZmHMGB1 differ from HMGB1 in having phosphorylation sites in their tail and linker regions. In both cases, in vitro phosphorylation of serine residues within the acidic tail stabilizes the assembled form, suggesting another level of regulation for interaction with DNA, chromatin, and other proteins that is not possible for the uniformly acidic (hence unphosphorylatable) tail of HMGB1.

The dengue vector Aedes aegypti contains a functional high mobility group box 1 (HMGB1) protein with a unique regulatory C-terminus

The mosquito Aedes aegypti can spread the dengue, chikungunya and yellow fever viruses. Thus, the search for key molecules involved in the mosquito survival represents today a promising vector control strategy. High Mobility Group Box (HMGB) proteins are essential nuclear factors that maintain the high-order structure of chromatin, keeping eukaryotic cells viable. Outside the nucleus, secreted HMGB proteins could alert the innate immune system to foreign antigens and trigger the initiation of host defenses. In this work, we cloned and functionally characterized the HMGB1 protein from Aedes aegypti (AaHMGB1). The AaHMGB1 protein typically consists of two HMG-box DNA binding domains and an acidic C-terminus. Interestingly, AaHMGB1 contains a unique alanine/glutamine-rich (AQ-rich) C-terminal region that seems to be exclusive of dipteran HMGB proteins. AaHMGB1 is localized to the cell nucleus, mainly associated with heterochromatin. Circular dichroism analyses of AaHMGB1 or the C-terminal truncated proteins revealed α-helical structures. We showed that AaHMGB1 can effectively bind and change the topology of DNA, and that the AQ-rich and the C-terminal acidic regions can modulate its ability to promote DNA supercoiling, as well as its preference to bind supercoiled DNA. AaHMGB1 is phosphorylated by PKA and PKC, but not by CK2. Importantly, phosphorylation of AaHMGB1 by PKA or PKC completely abolishes its DNA bending activity. Thus, our study shows that a functional HMGB1 protein occurs in Aedes aegypt and we provide the first description of a HMGB1 protein containing an AQ-rich regulatory C-terminus.

High mobility group box-1 is phosphorylated by protein kinase C zeta and secreted in colon cancer cells

High mobility group box-1 (HMGB1), a nuclear protein, is overexpressed and secreted in cancer cells. Phosphorylation on two different nuclear localization signal regions are known to be important for the nuclear-to-cytoplasmic transport and secretion of HMGB1. However, little is known about the biochemical mechanism of HMGB1 modifications and its subsequent secretion from cancer cells. To identify the specific enzyme and important sites for HMGB1 phosphorylation, we screened the protein kinase C (PKC) family in a colon cancer cell line (HCT116) for HMGB1 binding by pull-down experiments using a 3XFLAG-HMGB1 construct. Strong interactions between atypical PKCs (PKC-ζ, λ, and ι) and cytoplasmic HMGB1 were observed in HCT116 cells. We further identified the most critical PKC isotype that regulates HMGB1 secretion is PKC-ζ by using PKC inhibitors and siRNA experiments. The serine residues at S39, S53 and S181 of HMGB1 were related to enhancing HMGB1 secretion. We also demonstrated overexpression and activation of PKC-ζ in colon cancer tissues. Our findings suggest that PKC-ζ is involved in the phosphorylation of HMGB1, and the phosphorylation of specific serine residues in the nuclear localization signal regions is related to enhanced HMGB1 secretion in colon cancer cells.

The effect of PKC phosphorylation on the "architectural" properties of HMGB1 protein

High mobility group box (HMGB)1 protein acts as an architectural element, promoting the assembly of active nucleoprotein complexes due to its ability to bend DNA and to bind preferentially to distorted DNA structures. The behavior of HMGB1 as an "architect" of chromatin defines it as an important factor in many cellular processes such as repair, replication and remodeling. It was shown that the post-synthetic acetylation of HMGB1 at Lys2 modulated its essential properties as a structure-specific nuclear protein. We studied the role of PKC phosphorylation on the "architectural" properties of HMGB1, (i) the effect for the formation of a stable complex with DNA damaged by the anti-tumour drug cis-platinum and (ii) the influence on the ability of HMGB1 protein to bend short DNA fragments. PKC-phosphorylated recombinant HMGB1 increased about an order of magnitude its affinity to cis-platinated DNA, a finding that has already been reported for in vivo acetylated protein. Regarding the effect on the protein's DNA bending ability, it was enhanced upon phosphorylation as demonstrated by the stimulation of DNA circularization. We showed also that PKC phosphorylated the recombinant protein in vitro simultaneously at two target sites. Our results demonstrate that the PKC phosphorylation of HMGB1 has a considerable effect on the fundamental properties of the protein; therefore this post-synthetic modification may serve as a modulator of the HMGB1 participation in different nuclear processes.

CK2 phosphorylation of Schistosoma mansoni HMGB1 protein regulates its cellular traffic and secretion but not its DNA transactions

Background

The helminth Schistosoma mansoni parasite resides in mesenteric veins where fecundated female worms lay hundred of eggs daily. Some of the egg antigens are trapped in the liver and induce a vigorous granulomatous response. High Mobility Group Box 1 (HMGB1), a nuclear factor, can also be secreted and act as a cytokine. Schistosome HMGB1 (SmHMGB1) is secreted by the eggs and stimulate the production of key cytokines involved in the pathology of schistosomiasis. Thus, understanding the mechanism of SmHMGB1 release becomes mandatory. Here, we addressed the question of how the nuclear SmHMGB1 can reach the extracellular space.

Principal Findings

We showed in vitro and in vivo that CK2 phosphorylation was involved in the nucleocytoplasmic shuttling of SmHMGB1. By site-directed mutagenesis we mapped the two serine residues of SmHMGB1 that were phosphorylated by CK2. By DNA bending and supercoiling assays we showed that CK2 phosphorylation of SmHMGB1 had no effect in the DNA binding activities of the protein. We showed by electron microscopy, as well as by cell transfection and fluorescence microscopy that SmHMGB1 was present in the nucleus and cytoplasm of adult schistosomes and mammalian cells. In addition, we showed that treatments of the cells with either a phosphatase or a CK2 inhibitor were able to enhance or block, respectively, the cellular traffic of SmHMGB1. Importantly, we showed by confocal microscopy and biochemically that SmHMGB1 is significantly secreted by S. mansoni eggs of infected animals and that SmHMGB1 that were localized in the periovular schistosomotic granuloma were phosphorylated.

Conclusions

We showed that secretion of SmHMGB1 is regulated by phosphorylation. Moreover, our results suggest that egg-secreted SmHMGB1 may represent a new egg antigen. Therefore, the identification of drugs that specifically target phosphorylation of SmHMGB1 might block its secretion and interfere with the pathogenesis of schistosomiasis.

Nucleocytoplasmic distribution of the Arabidopsis chromatin-associated HMGB2/3 and HMGB4 proteins

High mobility group (HMG) proteins of the HMGB family are chromatin-associated proteins that as architectural factors are involved in the regulation of transcription and other DNA-dependent processes. HMGB proteins are generally considered nuclear proteins, although mammalian HMGB1 can also be detected in the cytoplasm and outside of cells. Plant HMGB proteins studied so far were found exclusively in the cell nucleus. Using immunofluorescence and fluorescence microscopy of HMGB proteins fused to the green fluorescent protein, we have examined the subcellular localization of the Arabidopsis (Arabidopsis thaliana) HMGB2/3 and HMGB4 proteins, revealing that, in addition to a prominent nuclear localization, they can be detected also in the cytoplasm. The nucleocytoplasmic distribution appears to depend on the cell type. By time-lapse fluorescence microscopy, it was observed that the HMGB2 and HMGB4 proteins tagged with photoactivatable green fluorescent protein can shuttle between the nucleus and the cytoplasm, while HMGB1 remains nuclear. The balance between the basic amino-terminal and the acidic carboxyl-terminal domains flanking the central HMG box DNA-binding domain critically influences the nucleocytoplasmic distribution of the HMGB proteins. Moreover, protein kinase CK2-mediated phosphorylation of the acidic tail modulates the intranuclear distribution of HMGB2. Collectively, our results show that, in contrast to other Arabidopsis HMGB proteins such as HMGB1 and HMGB5, the HMGB2/3 and HMGB4 proteins occur preferentially in the cell nucleus, but to various extents also in the cytoplasm.

Post-synthetic acetylation of HMGB1 protein modulates its interactions with supercoiled DNA

High mobility group box (HMGB) proteins 1 and 2 are abundant non-histone nuclear proteins that regulate chromatin structure because of their structure-specific binding to DNA. Here, we have investigated how the post-synthetic acetylation of HMGB1 affects its interaction with negatively supercoiled DNA by employing monoacetylated at Lys2 protein, isolated from butyrate-treated cells. Our data reveal that this modification enhances three reaction parameters: binding affinity, supercoiling activity and capacity to protect the supercoiled DNA from relaxation by topoisomerase I. We show that monoacetylation at Lys2 mimics the effect of acidic tail removal but to a lesser extent thus demonstrating that in vivo acetylated HMGB1 is capable of modulating its interaction with negatively supercoiled DNA.

Post-translational methylation of high mobility group box 1 (HMGB1) causes its cytoplasmic localization in neutrophils

High mobility group box 1 (HMGB1) protein plays multiple roles in transcription, replication, and cellular differentiation. HMGB1 is also secreted by activated monocytes and macrophages and passively released by necrotic or damaged cells, stimulating inflammation. HMGB1 is a novel antigen of anti-neutrophil cytoplasmic antibodies (ANCA) observed in the sera of patients with ulcerative colitis and autoimmune hepatitis, suggesting that HMGB1 is secreted from neutrophils to the extracellular milieu. However, the actual distribution of HMGB1 in the cytoplasm of neutrophils and the mechanisms responsible for it are obscure. Here we show that HMGB1 in neutrophils is post-translationally mono-methylated at Lys42. The methylation alters the conformation of HMGB1 and weakens its DNA binding activity, causing it to become largely distributed in the cytoplasm by passive diffusion out of the nucleus. Thus, post-translational methylation of HMGB1 causes its cytoplasmic localization in neutrophils. This novel pathway explains the distribution of nuclear HMGB1 to the cytoplasm and is important for understanding how neutrophils release HMGB1 to the extracellular milieu.

Nucleocytoplasmic shuttling of HMGB1 is regulated by phosphorylation that redirects it toward secretion

The high mobility group box 1 (HMGB1) protein can be secreted by activated monocytes and macrophages and functions as a late mediator of sepsis. HMGB1 contains two nuclear localization signals (NLSs) for controlled nuclear transport, and acetylation of both NLSs of HMGB1 is involved in nuclear transport toward secretion. However, phosphorylation of HMGB1 and its relation to nuclear transport have not been shown. We show here that HMGB1 is phosphorylated and dynamically shuttled between cytoplasmic and nuclear compartments according to its phosphorylation state. Phosphorylation of HMGB1 was detected by metabolic labeling and Western blot analysis after treatments with TNF-alpha and okadaic acid, a phosphatase inhibitor. Hyperphosphorylated HMGB1 in RAW 264.7 and human monocytes was relocated to the cytoplasm. In a nuclear import assay, phosphorylated HMGB1 in the cytoplasm did not enter the nucleus. We mutated serine residues of either or both NLSs of HMGB1 to glutamic acid to simulate a phosphorylated state and examined the binding of HMGB1 to karyopherin-alpha1, which was identified as the nuclear import protein for HMGB1 in this study. Substitution to glutamic acid in either NLSs decreased the binding with karyopherin-alpha1 by approximately 50%; however, substitution of both NLSs showed no binding, and HMGB1 was relocated to the cytoplasm and subsequently secreted. These data support the hypothesis that HMGB1 could be phosphorylated and that the direction of transport is regulated by phosphorylation of both NLS regions.

High mobility group proteins of the plant HMGB family: dynamic chromatin modulators

In plants, the chromosomal high mobility group (HMG) proteins of the HMGB family typically contain a central HMG-box DNA-binding domain that is flanked by a basic N-terminal and an acidic C-terminal domain. The HMGB proteins are abundant and highly mobile proteins in the cell nucleus that influence chromatin structure and enhance the accessibility of binding sites to regulatory factors. Due to their remarkable DNA bending activity, HMGB proteins can increase the structural flexibility of DNA, promoting the assembly of nucleoprotein complexes that control DNA-dependent processes including transcription. Therefore, members of the HMGB family act as versatile modulators of chromatin function.

Beyond Linker Histones and High Mobility Group Proteins: Global Profiling of Perchloric Acid Soluble Proteins

Extraction with HClO(4) provides an easy method for efficient enrichment of both histone H1 and HMG proteins from a variety of tissues. Usually, the histone and the HMG proteins are the most abundant components of the extracts, however, other proteins have frequently been observed but only seldom studied in more detail. Here we describe a study aimed at global characterization of HClO(4) extractable proteins from breast cancer cell lines. We report identification of 150 unique proteins by liquid chromatography tandem mass spectrometry including almost all major histone H1 variants and canonical members of the HMG protein families. In the extracts, diverse proteins with HMG-like amino acid composition were identified and their post-translational modifications were mapped. Importantly, those include multiple proteins known or supposed to be related to cell proliferation and cancer. Since purification of these proteins as well as low abundant variants of histone and HMG proteins is difficult due to their metabolic instability, characterization of these proteins from crude extracts can facilitate studies aimed at better understanding of their function.

Upregulation of HMG1 leads to melanoma inhibitory activity expression in malignant melanoma cells and contributes to their malignancy phenotype

Malignant transformation of melanocytes to melanoma cells closely parallels activation of melanoma inhibitory activity (MIA) expression. We have previously shown that upregulation of MIA occurs on a transcriptional level and involves the highly conserved region (HCR) promoter element. We further observed that the HCR element interacts with the melanoma-associated transcription factor (MATF) and thereby confers strong promoter activation. In this study we identify the peptide sequence of MATF and show that it is identical with the transcription factor HMG1. HMG1 was upregulated in malignant melanoma cells and further activated by hypophosphorylation. Stable antisense-HMG1 expression in melanoma cells led to the reduction of MIA promoter activity and protein expression, indicating that HMG1 is a potent regulator of MIA expression. Interestingly, chromatin immunoprecipitation and electrophoretic mobility shift experiments indicated that HMG1 and the NF-kappa B family member p65 both interact and bind to the HCR promoter element. In summary, our study proves HMG1 and p65 to be important factors in MIA regulation and melanoma progression.

Phosphorylation of maize and Arabidopsis HMGB proteins by protein kinase CK2alpha

In plants, a variety of chromatin-associated high mobility group (HMG) proteins belonging to the HMGB family have been identified. We have examined the phosphorylation of the HMGB proteins from the monocotyledonous plant maize and the dicotyledonous plant Arabidopsis by protein kinase CK2alpha. Maize CK2alpha phosphorylates the maize HMGB1 and HMGB2/3 proteins and the Arabidopsis HMGB1, HMGB2/3, and HMGB4 proteins. Maize HMGB4 and HMGB5 and Arabidopsis HMGB5 are not phosphorylated by CK2alpha. Depending on the HMGB protein up to five amino acid residues are phosphorylated in the course of the phosphorylation reaction. The HMGB1 proteins from both plants are markedly more slowly phosphorylated by CK2alpha than the other HMGB substrate proteins, indicating that certain HMGB proteins are clearly preferred substrates for CK2alpha. The rate of the phosphorylation reaction appears to be related to the ease of interaction between CK2alpha and the HMGB proteins, as indicated by chemical cross-linking experiments. MALDI/TOF mass spectrometry analyses demonstrate that the HMGB1 and HMGB2/3 proteins occur in various phosphorylation states in immature maize kernels. Thus, HMGB1 exists as monophosphorylated, double-phosphorylated, triple-phosphorylated, and tetraphosphorylated protein in kernel tissue, and the tetraphosphorylated form is the most abundant version. The observed in vivo phosphorylation states indicate that protein kinase(s) other than CK2alpha contribute(s) to the modification of the plant HMGB proteins. The fact that the HMGB proteins are phosphorylated to various extents reveals that the existence of differentially modified forms increases the number of distinct HMGB protein variants in plant chromatin that may be adapted to certain functions.

Mitotic phosphorylation of chromosomal protein HMGN1 inhibits nuclear import and promotes interaction with 14.3.3 proteins

Progression through mitosis is associated with reversible phosphorylation of many nuclear proteins including that of the high-mobility group N (HMGN) nucleosomal binding protein family. Here we use immunofluorescence and in vitro nuclear import studies to demonstrate that mitotic phosphorylation of the nucleosomal binding domain (NBD) of the HMGN1 protein prevents its reentry into the newly formed nucleus in late telophase. By microinjecting wild-type and mutant proteins into the cytoplasm of HeLa cells and expressing these proteins in HmgN1(-/-) cells, we demonstrate that the inability to enter the nucleus is a consequence of phosphorylation and is not due to the presence of negative charges. Using affinity chromatography with recombinant proteins and nuclear extracts prepared from logarithmically growing or mitotically arrested cells, we demonstrate that phosphorylation of the NBD of HMGN1 promotes interaction with specific 14.3.3 isotypes. We conclude that mitotic phosphorylation of HMGN1 protein promotes interaction with 14.3.3 proteins and suggest that this interaction impedes the reentry of the proteins into the nucleus during telophase. Taken together with the results of previous studies, our results suggest a dual role for mitotic phosphorylation of HMGN1: abolishment of chromatin binding and inhibition of nuclear import.

Protein kinase CK2 differentially phosphorylates maize chromosomal high mobility group B (HMGB) proteins modulating their stability and DNA interactions

The high mobility group (HMG) proteins of the HMGB family are architectural factors in eukaryotic chromatin, which are involved in the regulation of various DNA-dependent processes. We have examined the post-translational modifications of five HMGB proteins from maize suspension cultured cells, revealing that HMGB1 and HMGB2/3, but not HMGB4 and HMGB5, are phosphorylated by protein kinase CK2. The phosphorylation sites have been mapped to the acidic C-terminal domains by analysis of tryptic peptides derived from HMGB1 and HMGB2/3 using nanospray ion trap mass spectrometry. In native HMGB1, Ser(149) is constitutively phosphorylated, whereas Ser(133) and Ser(136) are differentially phosphorylated. The functional significance of the CK2-mediated phosphorylation of HMGB proteins was analyzed by circular dichroism measurements showing that the phosphorylation increases the thermal stability of the HMGB proteins. Electrophoretic mobility shift assays demonstrate that the phosphorylation reduces the affinity of the HMGB proteins for linear DNA. The specific recognition of DNA minicircles is not affected by the phosphorylation, but a different pattern of protein-DNA complexes is formed. Collectively, these findings show that phosphorylation of residues within the acidic C-terminal domain of the HMGB proteins can modulate protein stability and the DNA binding properties of the HMGB proteins.

The modulation of the biological activities of mitochondrial histone Abf2p by yeast PKA and its possible role in the regulation of mitochondrial DNA content during glucose repression

The mitochondrial histone, Abf2p, of Saccharomyces cerevisiae is essential for the maintenance of mitochondrial DNA (mtDNA) and appears to play an important role in the recombination and copy number determination of mtDNA. Abf2p, encoded by a nuclear gene, is a member of HMG1 DNA-binding protein family and has two HMG1-Box domains, HMG1-Box A and B. To investigate the role of Abf2p in the control of mtDNA copy number, we asked if the in vivo functions of Abf2p are regulated by the possible modification such as phosphorylation. We found that the N-terminal extended segment (KRPT(21)S(22)) of HMG1-Box A is rapidly and specifically phosphorylated by cAMP-dependent protein kinase (PKA) in vitro. The phosphorylation in this region inhibits the binding of Abf2p to all kinds of DNA including four-way junction DNA and the supercoiling activity of Abf2p itself. The abf2 mutant cells with an abf2(T21A/S22A) allele defective in the phosphorylation site have a severe defect in the regulation of mtDNA content during glucose repression in vivo. These observations suggest that the phosphorylation via PKA, that is activated during glucose repression, may regulate the in vivo functions of Abf2p for the control of mtDNA content during shift from gluconeogenic to fermentative growth.

Consecutive steps of phosphorylation affect conformation and DNA binding of the chironomus high mobility group A protein

The high mobility group (HMG) proteins of the AT-hook family (HMGA) lie downstream in regulatory networks with protein kinase C, Cdc2 kinase, MAP kinase, and casein kinase 2 (CK2) as final effectors. In the cells of the midge Chironomus, almost all of the HMGA protein (cHMGA) is phosphorylated by CK2 at two adjacent sites. 40% of the protein population is additionally modified by MAP kinase. Using spectroscopic and protein footprinting techniques, we analyzed how individual and consecutive steps of phosphorylation change the conformation of an HMGA protein and affect its contacts with poly(dA-dT).poly(dA-dT) and a fragment of the interferon-beta promoter. We demonstrate that phosphorylation of cHMGA by CK2 alters its conformation and modulates its DNA binding properties such that a subsequent phosphorylation by Cdc2 kinase changes the organization of the protein-DNA complex. In contrast, consecutive phosphorylation by MAP kinase, which results in a dramatic change in cHMGA conformation, has no direct effect on the complex. Because the phosphorylation of the HMGA proteins attenuates binding affinity and reduces the extent of contacts between the DNA and protein, it is likely that this process mirrors the dynamics and diversity of regulatory processes in chromatin.

Characterization of bovine angiogenin-1 and lactogenin-like protein as glycyrrhizin-binding proteins and their in vitro phosphorylation by C-kinase

Angiogenin-1 (p15, an angiogenesis inducer with RNase activity) and lactogenin-like protein (p17) isolated from partially purified bovine lactoferrin (bLF) preparations were characterized as glycyrrhizin (GL)-binding proteins (gbPs). As expected, bLF-affinity column chromatography confirmed these two gbPs to be bLF-binding proteins. These two purified gbPs exhibited RNase activities when incubated with poly(C) as a substrate. Both GL and glycyrrhetinic acid (GA) at 100 microM significantly inhibited RNase activities of these two gbPs, both of which functioned as phosphate acceptors of C-kinase in vitro. Phosphorylation of p15 and p17 by C-kinase was inhibited by GA in a dose-dependent manner with the 50% inhibition dose (ID50) of approx. 10 microM, whereas GL required a relatively high dose (300 microM) to inhibit significantly it. A GA derivative (oGA, ID50=approx. 0.3 microM) was found to be a potent inhibitor of the C-kinase-mediated phosphorylation of these two gbPs in vitro. In addition, a possible physiological significance of C-kinase on the physiological interaction between bLF and two bLF-binding proteins (p15 and p17) is noted.

Biochemical characterization of cholesterol-3-sulfate as the sole effector for the phosphorylation of HMG1 by casein kinase I in vitro

Phosphorylation of high mobility group protein 1 (HMG1) by casein kinase I (CK-I) and potent effectors (inhibitors and activators) of this phosphorylation were investigated in vitro. We found that (i) CK-I phosphorylates specifically threonine residues on HMG1 when incubated with cholesterol-3-sulfate (CH-3S), but no phosphorylation of HMG1 is detected in the presence of other cholesterol related compounds or their sulfated derivatives; (ii) this phosphorylation is selectively inhibited by heparin, but stimulated significantly by 3',4',7-trihydroxy-isofavone at low doses (0.1-3 microM); and (iii) CH-3S directly induces a drastic conformational change in HMG1. The latter finding provides a mechanism to explain how CH-3S alone can induce the phosphorylation of HMG1 by CK-I in vitro.

Point mutations within AT-hook domains of the HMGI homologue HMGIYL1 affect binding to gene promoter but not to four-way junction DNA

High-mobility group I/Y (HMGI/Y) proteins are chromosomal proteins involved in gene and chromatin regulation. Elevated levels of HMGI/Y proteins were reported in diverse malignant tumors, and rearrangements of their genes are casually involved in the development of benign tumors. In humans, the chromosomal locus Xp22 has been often found to be affected in diverse benign mesenchymal tumors. Recent studies revealed that this region contains a retropseudogene HMGIYL1 which potentially can be activated in a way of "exonization" upon aberrations involving this region. The coding sequence of the HMGIY-L1 is highly homologous to the HMGI(Y) gene. On the protein level, both HMGIYL1 and HMGI differ at few amino acid residues, including their putative DNA-binding domains (DBDs). Here we have approached the question of whether the HMGIYL1 product would be able to adopt a role of HMGI in the context of binding to gene promoters and chromatin. Comparative binding studies, employing protein footprinting technique, revealed that HMGIYL1 has lost the ability to bind to the promoter of the interferon beta gene, but retained its high affinity for the four-way junction DNA. Our results stress the importance of particular residues within the DBDs for DNA binding and demonstrate that tight binding of HMGI/Y proteins to the four-way junction DNA can be achieved in alternative ways. The binding of HMGIYL1 to four-way junction DNA suggests that activation of the HMGIYL1 gene would yield a protein sharing some binding properties with HMG1-box proteins and histone H1. Thus, the HMGIYL1 could interplay together with these components in chromatin regulation.

Cdc2 and mitogen-activated protein kinases modulate DNA binding properties of the putative transcriptional regulator Chironomus high mobility group protein I

Cells of the dipteran insect Chironomus contain a high mobility group protein that is homologous to the mammalian high mobility group proteins I/Y (HMGI/Y). These proteins facilitate the assembly of higher order nucleoprotein complexes. In proliferating cells, >30% of Chironomus HMGI was found to be phosphorylated. The phosphorylation sites were mapped to Ser3, Ser22, and Ser72 and were found to be substrates for the kinases Cdc2 (and mitogen-activated protein (MAP)), MAP, and Ca2+/phospholipid-dependent protein kinase, respectively. In mitotically arrested cells, the extent of phosphorylation at Ser3 increased, whereas phosphorylation at Ser22 remained unchanged. In nondividing cells, phosphorylation at Ser3 and Ser22 was strongly reduced. The DNA binding affinity of Chironomus HMGI was not influenced by single phosphorylation at Ser3 or Ser22. In contrast, phosphorylation at both of these sites resulted in a 10-fold weakening of the binding activity and altered the mode of protein-DNA interaction. Since both human and murine HMGI/Y proteins, similarly to the insect HMGI protein, possess phosphorylation sites for Cdc2 and MAP kinases that intersperse the AT-hook DNA-binding motifs, our results may reflect a general mechanism that regulates the properties and function of this class of putative transcriptional regulators.

Conformational changes of DNA induced by binding of Chironomus high mobility group protein 1a (cHMG1a). Regions flanking an HMG1 box domain do not influence the bend angle of the DNA

High mobility group (HMG) proteins are thought to facilitate assembly of higher order chromatin structure through modulation of DNA conformation. In this work we investigate the bending of a 30-base pair DNA fragment induced by Chironomus HMG1 (cHMG1a), and HMGI (cHMGI) proteins. The DNA bending was measured in solution by monitoring the end-to-end distance between fluorescence probes attached to opposite ends of the DNA fragment. The distance was measured by fluorescence energy transfer using a novel europium chelate as a fluorescence donor. These measurements revealed that the end-to-end distance in the 30-base pair DNA was decreased from approximately 100 A in free DNA to approximately 50.5 A in cHMG1a. DNA complex. The most probable DNA bending angle consistent with these distance measurements is about 150 degrees. The deletion of the charged regulatory domains located close to the C terminus of the HMG1 box domain of cHMG1a protein had no effect on the induced bend angle. The ability to induce a large DNA bend distinguishes the cHMG1 from the cHMGI protein. Only small perturbation of the DNA conformation was observed upon binding of the cHMGI protein. A strong DNA bending activity of cHMG1a and its relative abundance in the cell suggests that this protein plays a very important role in modulation of chromatin structure.

Nuclear accumulation of HMG2 protein is mediated by basic regions interspaced with a long DNA-binding sequence, and retention within the nucleus requires the acidic carboxyl terminus

High mobility group 2 (HMG2) protein is ubiquitously distributed in the nucleus of higher eukaryotic cells. Accumulation of an HMG2-beta-galactosidase fusion protein expressed in COS-7 cells suggested active transport of HMG2 protein into the nucleus after translation in the cytoplasm. Deletion analysis of the HMG2 sequence in the HMG2-beta-galactosidase fusion protein indicated that basic regions interspaced with the long DNA-binding sequence in HMG2, called the HMG1/2 box, are necessary for the nuclear accumulation of HMG2. The close configuration of basic regions at both ends of the DNA-binding sequence in the tertiary structure may function as the nuclear localization signal. This novel nuclear localization signal structure is different from typical ones such as the single or bipartite basic cluster in many nuclear proteins. A portion of the HMG2 molecule remained in the cytoplasm after translation. Interspecies heterokaryon assay demonstrated that the acidic carboxyl terminus of HMG2 was necessary for retention of the protein in the nucleus.

High mobility group proteins cHMG1a, cHMG1b, and cHMGI are distinctly distributed in chromosomes and differentially expressed during ecdysone dependent cell differentiation

The mammalian high mobility group proteins HMGI/Y and HMG1/2 are thought to play an architectural role in assembly of nucleoprotein structures. Counterparts to these proteins have recently been found in the cells of the Dipteran insect Chironomus. In this report we investigate the distribution of three abundant HMG proteins in interphase giant chromosomes of the midge, Chironomus. By means of the indirect immunofluorescence technique the cHMG1b and cHMGI proteins were localized in chromosomal puffs, suggesting their involvement in the organization of transcriptionally active chromatin. In contrast, the highly abundant protein cHMG1a was rather uniformly distributed in the chromosomes. The cHMGI protein, but not cHMG1a or cHMG1b, was detected in nucleoli, which may indicate a role in the transcription of ribosomal genes. The regions of the interphase chromosomes containing AT-rich DNA did not contain higher levels of the cHMGI and cHMG1b proteins. A correlation between the specific location of these proteins in chromatin and their synthesis and turnover rates was observed.

The cardiac-muscle specific enhancer-promoter of slow/cardiac troponin C binds HMG-2

The cardiac muscle-specific enhancer-promoter of the slow/cardiac troponin C (cTnC) gene contains five protein binding regions, four of which bind cardiac-myocyte specific proteins. We screened a stage 11 chick embryo expression library with a double-stranded oligonucleotide probe consisting of one of these regions, CEF-1. One of the clones obtained was the chicken high mobility group protein, HMG-2. An electrophoretic gel mobility shift assay (EMSA) showed a specific binding interaction between the HMG-2 protein and the dsDNA CEF-1 probe. The cardiac-specific enhancer region of cTnC contains at least one possible HMG binding region and it is in the CEF-1 sequence overlapping a known GATA-4 binding site. Mutation of the nucleotide sequence of this HMG binding region diminishes its protein binding ability and markedly decreases its cardiac specific transcriptional activity. HMG-2 is a DNA bending protein that is predominantly found in the nucleus in proliferating cells and in the cytoplasm of terminally differentiated cells. It is an integral and stabilizing factor in the transcription activation nucleoprotein complex and is often described as an 'architectural transcription factor'. It markedly stimulates the transcription of many genes, often in association with tissue-specific transcription factors. We believe that the presence of HMG-2 in the enhancer-promoter binding protein complex of cTnC augments DNA bending and facilitates the DNA binding and interaction of other tissue-specific factors (e.g. GATA-4, which also binds to this region). This would result in increased transcription of the cTnC gene during the proliferation phase of embryonic cardiac myocyte development.

Structural and functional consequences of mutations within the hydrophobic cores of the HMG1-box domain of the Chironomus high-mobility-group protein 1a

The high-mobility-group protein 1 box domain (HMG1-BD) is a structural element found in several DNA-binding proteins in eukaryotic cells. Its structure is dominated by three alpha-helices. The spatial arrangement of these helices into an L-shaped molecule is maintained by a number of apolar residues organized into a main and a secondary hydrophobic core. To analyze the significance of these residues for proper folding, conformational stability, and ability to bind and bend DNA, we have mutated the highly conserved Trp14 of the Chironomus HMG1a protein and have synthesized a series of N-terminally truncated forms. The observed alterations in DNA-binding and DNA-bending characteristics were correlated with structural consequences, as revealed by CD spectroscopy, limited trypsin digestion, and transverse urea gradient gel electrophoresis. Mutation of the Trp14 residue (Chironomus [W14A]HMG1a) and deletion of the seven N-terminal residues, respectively, which are members of the main and the secondary core of Chironomus HMG1a, both resulted in a substantial unfolding of the protein. Unexpectedly, these mutants still retained their ability to bind and bend DNA. Conformational analysis of wild-type cHMG1a and [W14A]cHMG1a showed that the proteins unfold at 2-4 M urea. In contrast, their DNA complexes persisted even at 6-8 M of the denaturant. Multiple contacts between the HMG1-BD and the DNA are probably responsible for the unusual stability of the complexes.

Molecular mimicry in development: identification of ste11+ as a substrate and mei3+ as a pseudosubstrate inhibitor of ran1+ kinase

ran1+ (pat1+) kinase inhibits exit from the mitotic cell cycle and entry into meiosis. Inactivation of ran1+ by mei3+ is sufficient to precipitate the entire meiotic developmental program. Here, we show that the ste11+ transcription factor is a substrate for ran1+ in vitro and that this reaction is directly inhibited by mei3+. Sequence comparison reveals that ste11+ contains two domains homologous to each other and to a domain of mei3+. Mutagenesis studies reveal that the regions of homology contain substrate specificity determinants. These results identify sequences critical for phosphorylation of ste11+ by ran1+ and suggest that mei3+ employs a pseudosubstrate mechanism for its inhibitory function.