Recombinant human chromosomal proteins HMG-14 and HMG-17

The spectrum of anti-chromatin/nucleosome autoantibodies: independent and interdependent biomarkers of disease

Autoantibodies directed to chromatin components date back to the discovery of the LE cell and the LE cell phenomenon circa 1950, and subsequent evidence that major components of that reaction were chromatin components and histones in particular. Over time, immunoassays ranging from ELISA and line immunoassays to more modern bead-based assays incorporated histone and DNA mixtures, purified histones, and purified nucleosomes leading to a more thorough understanding of the genesis and pathogenetic relationships of antibodies to chromatin components in systemic lupus erythematosus and other autoimmune conditions. More recently, interest has focussed on other components of chromatin such as high mobility group (HMG) proteins both as targets of B cell responses and pro-inflammatory mediators. This review will focus on immunoassays that utilize chromatin components, their clinical relationships, and newer evidence implicating HMG proteins and DNA neutrophil extracellular traps (NETs) as important players in systemic autoimmune rheumatic diseases.

Nucleosome structural changes induced by binding of non-histone chromosomal proteins HMGN1 and HMGN2

Interactions between the nucleosome and the non-histone chromosomal proteins (HMGN1 and HMGN2) were studied by circular dichroism (CD) spectroscopy to elucidate structural changes in the nucleosome induced by HMGN binding. Unlike previous studies that used a nucleosome extracted from living cells, in this study we utilized a nucleosome reconstituted from unmodified recombinant histones synthesized in Escherichia coli and a 189-bp synthetic DNA fragment harboring a nucleosome positioning sequence. This DNA fragment consists of 5′-TATAAACGCC-3′ repeats that has a high affinity to the histone octamer. A nucleosome containing a unique octamer-binding sequence at a specific location on the DNA was produced at sufficiently high yield for spectroscopic analysis. CD data have indicated that both HMGN1 and HMGN2 can increase the winding angle of the nucleosome DNA, but the extent of the structural changes induced by these proteins differs significantly. This suggests HMGN1 and HMGN2 would have different abilities to facilitate nucleosome remodeling.

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A nucleosome was reconstituted from recombinant histones and a synthetic DNA.

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Nucleosomes were produced at sufficiently high yield for spectroscopic analysis.

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A nucleosome with and without HMGN proteins was analyzed using CD spectroscopy.

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CD data indicate that HMGN proteins increase the winding angle of the nucleosome DNA.

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HMGN1 and HMGN2 may have different abilities to facilitate nucleosome remodeling.

Developmental role of HMGN proteins in Xenopus laevis

HMGN proteins are architectural chromatin proteins that reduce the compaction of the chromatin fiber, facilitate access to nucleosomes and modulate replication and transcription processes. Here we demonstrate that in Xenopus laevis, the expression and cellular location of the HMGN proteins are developmentally regulated and that their misexpression leads to gross developmental defects in post-blastula embryos. HMGN transcripts and proteins are present throughout oogenesis; however, the proteins stored in the cytoplasm are not associated with lampbrush chromosomes, and are rapidly degraded when oocytes mature into eggs. During embryogenesis, HMGN expression is first detected in blastula stages and progresses to a tissue-specific expression reaching relative high levels in the mesodermal and neuroectodermal regions of tadpoles. Only after midblastula transition (MBT), alterations in the HMGN levels by either microinjection of recombinant proteins or by morpholino-antisense oligo treatments produced embryos with imperfectly closed blastopore, distorted body axis and showed abnormal head structures. Analyses of animal cap explants indicated that HMGN proteins are involved in the regulation of mesoderm specific genes. In addition, HMGN misexpression caused altered expression of specific genes at MBT rather than global changes of transcription rates. Our results demonstrate that proper embryonic development of Xenopus laevis requires precisely regulated levels of HMGN proteins and suggest that these nucleosomal binding proteins modulate the expression of specific genes.

Activation of transiently transfected reporter genes in 3T3 Swiss cells by the inducers of differentiation/apoptosis - dimethylsulfoxide, hexamethylene bisacetamide and trichostatin A

Despite decades of investigation, the primary site of action of the prototypical inducers of differentiation, dimethylsulfoxide and hexamethylene bisacetamide (HMBA), has not been delineated. During studies designed to analyze cis-acting elements responsible for induction of stage-specific globin genes, we discovered the capacity of HMBA and dimethylsulfoxide to enhance the expression of transiently transfected reporter genes derived from globin and nonglobin gene promoters, prominently in nonerythroid 3T3 Swiss cells. The action of HMBA and dimethylsulfoxide in the transient transfection system resembled that of the inhibitor of histone deacetylases (HDACs), trichostatin A (TSA), in that the three agents enhanced reporter gene expression (a) regardless of the promoter employed, (b) with similar kinetics and (c) with an increase in the steady-state level of reporter mRNA. Transiently transfected DNA was assembled rapidly into a chromatinized structure in 3T3 cells, suggesting that transcription of reporter genes was at least in part repressed by chromatin organization. Nuclear run-on analyses indicated that dimethylsulfoxide and HMBA enhanced transcriptional initiation of the reporter and p21/WAF1/Cip1 genes. In contrast, TSA produced negligible effects on nuclear run-on transcription of these genes. HMBA and dimethylsulfoxide did not change the acetylation, phosphorylation, or methylation status of histones and did not activate stably transfected genes. Despite these differences, the three agents modulated the expression of common sets of cellular genes and induced differentiation or apoptosis in intact cells. The findings imply that HMBA and dimethylsulfoxide modulate transcription by a mechanism independent of histone acetylation.

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.

Competition between histone H1 and HMGN proteins for chromatin binding sites

The ability of regulatory factors to access their nucleosomal targets is modulated by nuclear proteins such as histone H1 and HMGN (previously named HMG-14/-17 family) that bind to nucleosomes and either stabilize or destabilize the higher-order chromatin structure. We tested whether HMGN proteins affect the interaction of histone H1 with chromatin. Using microinjection into living cells expressing H1-GFP and photobleaching techniques, we found that wild-type HMGN, but not HMGN point mutants that do not bind to nucleosomes, inhibits the binding of H1 to nucleosomes. HMGN proteins compete with H1 for nucleosome sites but do not displace statically bound H1 from chromatin. Our results provide evidence for in vivo competition among chromosomal proteins for binding sites on chromatin and suggest that the local structure of the chromatin fiber is modulated by a dynamic interplay between nucleosomal binding proteins.

Mitotic phosphorylation prevents the binding of HMGN proteins to chromatin

Condensation of the chromatin fiber and transcriptional inhibition during mitosis is associated with the redistribution of many DNA- and chromatin-binding proteins, including members of the high-mobility-group N (HMGN) family. Here we study the mechanism governing the organization of HMGN proteins in mitosis. Using site-specific antibodies and quantitative gel analysis with proteins extracted from synchronized HeLa cells, we demonstrate that, during mitosis, the conserved serine residues in the nucleosomal binding domain (NBD) of this protein family are highly and specifically phosphorylated. Nucleosome mobility shift assays with both in vitro-phosphorylated proteins and with point mutants bearing negative charges in the NBD demonstrate that the negative charge abolishes the ability of the proteins to bind to nucleosomes. Fluorescence loss of photobleaching demonstrates that, in living cells, the negative charge in the NBD increases the intranuclear mobility of the protein and significantly decreases the relative time that it is bound to chromatin. Expression of wild-type and mutant proteins in HmgN1(-/-) cells indicates that the negatively charged protein is not bound to chromosomes. We conclude that during mitosis the NBD of HMGN proteins is highly phosphorylated and that this modification regulates the interaction of the proteins with chromatin.

Distinct importin recognition properties of histones and chromatin assembly factors

Synthesis of the protein components of nuclear chromatin occurs in the cytoplasm, necessitating specific import into the nucleus. Here, we report the binding affinities of the nuclear localisation sequence (NLS)-binding importin subunits for a range of histones and chromatin assembly factors. The results suggest that import of histones to the nucleus may be mediated predominantly by importin beta1, whereas the import of the other components probably relies on the conventional alpha/beta1 import pathway. Differences in recognition by importin beta1 were observed between histone H2A and the variant H2AZ, as well as between histone H3/4 with or without acetylation. The results imply that different histone variants may possess distinct nuclear import properties, with acetylation possibly playing an inhibitory role through NLS masking.

Chromosomal proteins HMG-14 and HMG-17 are released from mitotic chromosomes and imported into the nucleus by active transport

The high mobility group 14/17 (HMG-14/-17) proteins form specific complexes with nucleosome core particles and produce distinct footprints on nucleosomal DNA. Therefore, they could be an integral part of the chromatin fiber. Here we show that during the cell cycle these proteins are transiently dissociated from chromatin. They colocalize with the nuclear DNA in interphase and prophase but not in metaphase and anaphase. They relocate into the nucleus and colocalize again with the DNA in late telophase, concomitantly with the appearance of the nuclear envelope. Thus, these nucleosomal binding proteins are not always associated with chromatin. Using reconstituted nuclei and permeabilized cells, we demonstrate that these two small proteins, with a molecular mass <10 kD, are actively imported into the nucleus. We identify the major elements involved in the nuclear import of these chromosomal proteins: HMG-14/-17 proteins contain an intrinsic bipartite nuclear localization signal, and their entry into the nucleus through nuclear pores requires energy and the participation of importin alpha. These findings suggest that the cell cycle-related association of HMG-14/-17 with chromatin is dependent on, and perhaps regulated by, nuclear import processes.

Isolation and separation of HMG proteins and histones H1 and H5 and core histones by column chromatography on phosphocellulose

A single-step method to dissociate histones as well as nonhistone chromosomal proteins from chicken erythrocyte chromatin and their separation into histones H1, H5, core histones, and high mobility group proteins by column chromatography on phosphocellulose is presented. NaCl at 2.0 M is effective in dissociating both histones and nonhistone proteins. The core histones elute as a complex. The pH is a critical factor in separating H5 from the core histones.

The chromatin unfolding domain of chromosomal protein HMG-14 targets the N-terminal tail of histone H3 in nucleosomes

Nonhistone chromosomal protein HMG-14 is a nucleosomal binding protein that unfolds the higher-order chromatin structure and enhances the transcriptional potential of chromatin, but not that of DNA. Both the transcriptional enhancement and the chromatin unfolding activities of HMG-14 are mediated through the C-terminal region of the protein. Here we study the molecular interactions of both this region and the N-terminal region of HMG-14 with nucleosome cores. By protein photocrosslinking we demonstrate that the N-terminal domain of HMG-14 targets a restricted region in histone H2B, whereas the C-terminal chromatin unfolding domain of HMG-14 targets a restricted region in the N terminus of histone H3. The N-terminal regions of the core histones are involved in the folding of oligonucleosomes and are the target of various activities associated with chromatin unfolding and transcriptional activation. We suggest that specific interactions between the C-terminal domain of HMG-14 and the N-terminal tail of histone H3 reduce the compaction of chromatin. These findings provide insights into the molecular mechanism whereby HMG-14/-17 proteins reduce the repressive effect of chromatin, and they also broaden the scope of the molecular interactions involving the N termini of the core histones in nucleosomes.

Stimulation of replication efficiency of a chromatin template by chromosomal protein HMG-17

The effect of chromosomal protein HMG-17 on the replication of a chromatin template was studied with minichromosomes containing the SV40 origin of replication. The minichromosomes were assembled from M13 DNA in Xenopus egg extracts in either the absence or presence of HMG-17. Structural data show that HMG-17 was efficiently incorporated into the chromatin and induced an extended chromatin structure. Using an in vitro SV40 replication system, we find that minichromosomes containing HMG-17 replicate with higher efficiency than minichromosomes deficient of HMG-17. The replicational potential of chromatin was enhanced only when HMG-17 was incorporated into the template during, but not after, chromatin assembly. HMG-17 stimulated replication only from a chromatin template, but not from protein-free DNA. Thus, HMG-17 protein enhances the rate of replication of a chromatin template by unfolding the higher order chromatin structure and increasing the accessibility of target sequences to components of the replication machinery.

Enhancement of the transcription potential of nascent chromatin by chromosomal proteins HMG-14/-17 is coupled to nucleosome assembly and not DNA synthesis

We have previously demonstrated that in Xenopus egg extracts, which support DNA strand synthesis and chromatin assembly, incorporation of chromosomal proteins HMG-14/-17 into nascent nucleosomes increases the transcriptional potential of a chromatin template carrying the Xenopus 5S RNA gene. Here we use the single-stranded and double-stranded forms of a plasmid carrying a 5S RNA maxigene, to test whether the effect of HMG-14/-17 on transcription requires DNA synthesis and whether these proteins will affect transcription through a region containing nucleosomes. We find that most of the transcripts were about 350 nucleotides long, suggesting that HMG-14/-17 enhance transcription through a region that could contain nucleosomes. HMG-14/-17 enhance transcription of chromatin templates assembled onto double-stranded DNA, in the absence of DNA synthesis. Single-round transcription assays suggest that HMG-14/-17 increase transcription from templates assembled onto both single- and double-stranded DNA by increasing the specific activity, and not the number, of transcriptionally active templates. We conclude that the effect of HMG-14/-17 on the transcriptional potential of chromatin is linked to nucleosome assembly and is not linked to DNA synthesis.

Alleviation of histone H1-mediated transcriptional repression and chromatin compaction by the acidic activation region in chromosomal protein HMG-14

Histone H1 promotes the generation of a condensed, transcriptionally inactive, higher-order chromatin structure. Consequently, histone H1 activity must be antagonized in order to convert chromatin to a transcriptionally competent, more extended structure. Using simian virus 40 minichromosomes as a model system, we now demonstrate that the nonhistone chromosomal protein HMG-14, which is known to preferentially associate with active chromatin, completely alleviates histone H1-mediated inhibition of transcription by RNA polymerase II. HMG-14 also partially disrupts histone H1-dependent compaction of chromatin. Both the transcriptional enhancement and chromatin-unfolding activities of HMG-14 are mediated through its acidic, C-terminal region. Strikingly, transcriptional and structural activities of HMG-14 are maintained upon replacement of the C-terminal fragment by acidic regions from either GAL4 or HMG-2. These data support the model that the acidic C terminus of HMG-14 is involved in unfolding higher-order chromatin structure to facilitate transcriptional activation of mammalian genes.

Modular structure of chromosomal proteins HMG-14 and HMG-17: definition of a transcriptional enhancement domain distinct from the nucleosomal binding domain

Chromosomal proteins HMG-14 and HMG-17 are the only known nuclear proteins which specifically bind to the nucleosome core particle and are implicated in the generation and/or maintenance of structural features specific to active chromatin. The two proteins facilitate polymerase II and III transcription from in vitro- and in vivo-assembled circular chromatin templates. Here we used deletion mutants and specific peptides to identify the transcriptional enhancement domain and delineate the nucleosomal binding domain of the HMG-14 and -17 proteins. Deletion of the 22 C-terminal amino acids of HMG-17 or 26 C-terminal amino acids of HMG-14 reduces significantly the ability of the proteins to enhance transcription from chromatin templates. In contrast, N-terminal truncation mutants had the same transcriptional enhancement activity as the full-length proteins. We conclude that the negatively charged C-terminal region of the proteins is required for transcriptional enhancement. Chromatin transcription enhancement assays, which involve binding competition between the full-length proteins and peptides derived from their nucleosomal binding regions, indicate that the minimal nucleosomal binding domain of human HMG-17 is 24 amino acids long and spans residues 17 to 40. The results suggest that HMG-14 and -17 proteins have a modular structure and contain distinct functional domains.

The chicken HMG-17 gene is dispensable for cell growth in vitro

HMG-17 is a highly conserved and ubiquitous nonhistone chromosomal protein that binds to nucleosome core particles. HMG-17 and HMG-14 form a family of chromosomal proteins that have been reported to bind preferentially to regions of active chromatin structure. To study the functional role of the single-copy chicken HMG-17 gene, null mutants were generated by targeted gene disruption in a chicken lymphoid cell line, DT40. Heterozygous and homozygous null mutant cell lines were generated by two independent selection strategies. Heterozygous null mutant lines produced about half the normal level of HMG-17 protein, and homozygous null lines produced no detectable HMG-17. No significant changes in cell phenotype were observed in cells harboring either singly or doubly disrupted HMG-17 genes, and no compensatory changes in HMG-14 or histone protein levels were observed. It is concluded that HMG-17 protein is not required for normal growth of avian cell lines in vitro, nor does the absence of HMG-17 protein lead to any major changes in cellular phenotype, at least in lymphoid cells.

The cooperative binding of chromosomal protein HMG-14 to nucleosome cores is reduced by single point mutations in the nucleosomal binding domain

Mutants of human chromosomal protein HMG-14 were generated by site directed mutagenesis and used to study functional domains in this protein. A replacement of serine by cysteine at position 7 did not affect the binding of the protein to nucleosome cores. The sulfhydryl group in the nucleosome-bound protein is accessible to modifying agents suggesting that position 7 in the protein is not in close contact with either the DNA or the histones in the core particles. Under cooperative binding conditions, replacements of alanine by proline at position 21, or of lysine by cysteine at position 26, decreased the affinity of the protein for nucleosome cores 6.7- and 3-fold respectively. In contrast, the non-cooperative mode of binding was only minimally affected. A replacement of glutamic acid by glutamine at position 76 caused only minor changes in the binding of the protein to the cores. The results indicate that single point mutations, which change either the conformation or change in the nucleosomal binding domain of the protein, significantly reduce the ability of the HMG-14 protein to bind to nucleosome cores. We suggest that in chromatin the protein binds to nucleosomes in a cooperative manner and that upon binding to nucleosomes the protein acquires a distinct conformation.

Stimulation of RNA polymerase II elongation by chromosomal protein HMG-14

The high-mobility group protein 14 (HMG-14) is a non-histone chromosomal protein that is preferentially associated with transcriptionally active chromatin. To assess the effect of HMG-14 on transcription by RNA polymerase II, in vivo-assembled chromatin with elevated amounts of HMG-14 was obtained. Here it is shown that HMG-14 enhanced transcription on chromatin templates but not on DNA templates. This protein stimulated the rate of elongation by RNA polymerase II but not the level of initiation of transcription. These findings suggest that the association of HMG-14 with nucleosomes is part of the cellular process involved in the generation of transcriptionally active chromatin.

Sera from JRA patients contain antibodies against a defined epitope in chromosomal protein HMG-17

Autoantibodies against the nonhistone nucleosomal protein HMG-17 have been detected in a high percentage of ANA-positive patients with pauciarticular-onset JRA4. Here we report on the epitope mapping of the HMG-17 autoantigen with a set of overlapping and nested synthetic peptides spanning the entire amino acid sequence of the human HMG-17 protein. Competition ELISA experiments defined a proline and lysine rich octapeptide PKPEPKPK as the major epitope recognized by more than 70% of the HMG-17 positive JRA sera. Point mutations introduced in the autoimmune peptide determined the amino acid residues important for autoantibody recognition. Computer based sequence comparison shows close homology between the HMG-17 autoimmune epitope and certain infectious organisms, supporting the possibility that molecular mimicry is an important factor in the etiology of JRA.

Nucleosome core binding region of chromosomal protein HMG-17 acts as an independent functional domain

Chromosomal proteins HMG-14 and HMG-17 have a modular structure. Here we examine whether the putative nucleosome-binding domain in these proteins can function as an independent module. Mobility shift assays with recombinant HMG-17 indicate that synthetic molecules can be used to analyze the interaction of this protein with the nucleosome core. Peptides corresponding to various regions of the protein have been synthesized and their interaction with nucleosome cores analyzed by mobility shift, thermal denaturation and DNase I digestion. A 30 amino acid long peptide, corresponding to the putative nucleosome-binding domain of HMG-17, specifically shifts the mobility of cores as compared to free DNA, elevates the tm of both the premelt and main melt of the cores and protects from DNase I digestion the same nucleosomal DNA sites as the intact protein. The binding of both the peptide and the intact protein is lost upon digestion of the histone tails by trypsin. The nucleosomal binding sites of the peptide appear identical to those of the intact protein. Thus, a region of the protein can acts as an independent functional domain. This supports the notion that HMG-14 and HMG-17 are modular proteins. This finding is relevant to the understanding of the function and evolution of HMG-14/-17, the only nucleosome core particle binding proteins known to date.

Autoantibodies to the chromosomal protein HMG-17 in juvenile rheumatoid arthritis

OBJECTIVE

To determine the antibody profiles in sera from patients with juvenile rheumatoid arthritis (JRA).

METHODS

Immunoblotting using nuclear extracts and recombinant high-mobility group (HMG) nonhistone chromosomal proteins.

RESULTS

Antibodies directed against HMG-17 were found in 47% of antinuclear antibody (ANA)-positive patients with pauciarticular-onset JRA and in 16% of ANA-positive patients with polyarticular-onset JRA. HMG-17 values of 6% and 8%, respectively, were detected in ANA-negative patients with JRA and in those with nonrheumatic diseases.

CONCLUSION

There is evidence for a high prevalence of anti-HMG-17 antibodies in sera of patients with pauciarticular-onset JRA.