Binding of HMG14 non-histone protein to histones H2A, H2B, H1 and DNA in reconstituted chromatin

Regulation of chromatin structure and function by HMGN proteins

High mobility group nucleosome-binding (HMGN) proteins are architectural non-histone chromosomal proteins that bind to nucleosomes and modulate the structure and function of chromatin. The interaction of HMGN proteins with nucleosomes is dynamic and the proteins compete with the linker histone H1 chromatin-binding sites. HMGNs reduce the H1-mediated compaction of the chromatin fiber and facilitate the targeting of regulatory factors to chromatin. They modulate the cellular epigenetic profile, affect gene expression and impact the biological processes such as development and the cellular response to environmental and hormonal signals. Here we review the role of HMGN in chromatin structure, the link between HMGN proteins and histone modifications, and discuss the consequence of this link on nuclear processes and cellular phenotype.

HMGN1 modulates estrogen-mediated transcriptional activation through interactions with specific DNA-binding transcription factors

HMGN1, an abundant nucleosomal binding protein, can affect both the chromatin higher order structure and the modification of nucleosomal histones, but it alters the expression of only a subset of genes. We investigated specific gene targeting by HMGN1 in the context of estrogen induction of gene expression. Knockdown and overexpression experiments indicated that HMGN1 limits the induction of several estrogen-regulated genes, including TFF1 and FOS, which are induced by estrogen through entirely distinct mechanisms. HMGN1 specifically interacts with estrogen receptor alpha (ER alpha), both in vitro and in vivo. At the TFF1 promoter, estrogen increases HMGN1 association through recruitment by the ER alpha. HMGN1 S20E/S24E, although deficient in binding nucleosomal DNA, still interacts with ER alpha and, strikingly, still represses estrogen-driven activation of the TFF1 gene. On the FOS promoter, which lacks the ER alpha binding sites, constitutively bound serum response factor (SRF) mediates estrogen stimulation. HMGN1 also interacts specifically with SRF, but HMGN1 S20E/S24E does not. Consistent with the protein interactions, only wild-type HMGN1 significantly inhibits the estrogen-driven activation of the FOS gene. Mechanistically, the inhibition of estrogen induction of several ER alpha-associated genes, including TFF1, by HMGN1 correlates with decreased levels of acetylation of Lys9 on histone H3. Together, these findings indicate that HMGN1 regulates the expression of particular genes via specific protein-protein interactions with transcription factors at target gene regulatory regions.

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.

HMG proteins 14 and 17 become cross-linked to the globular domain of histone H3 near the nucleosome core particle dyad

HMG proteins were derivatized with the photoactivatable cross-linker N-succinimidyl 3-((4-azidophenyl)dithio)propionate and then allowed to associate with nucleosome core particles. Following photolysis, peptide mapping of the principal dimeric adducts was carried out. Cross-linking occurred primarily from a central location in the HMGs to a central location in H3. The positions of these cross-links, considered along with other data from the literature, show that HMG proteins 14 and 17 make important contacts to H3 near the front face of the nucleosome. This raises the possibility that HMGs 14 and 17 participate in the reported conformational transition which exposes the H3 sulfhydryls of active nucleosomes.

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

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

A one-step preparative method for separating SER 6-phosphorylated HMG 14 from unphosphorylated HMG 14 and in vitro phosphorylation reaction components

While clear evidence exists for the regulation of the phosphorylation of the very basic high mobility group (HMG) and histone chromatin proteins, the physiological role of their phosphorylation remains poorly understood. Elucidation of these roles has been difficult, in part, because of the inability to obtain sufficient quantities of purified phosphorylated derivatives. We have used Mono S cation-exchange chromatography to prepare milligram quantities of pure Ser 6-phosphorylated HMG 14 (Ser 6-PO4-HMG) from unphosphorylated Mono S-purified calf thymus HMG 14 following in vitro phosphorylation with cAMP-dependent protein kinase (A-kinase). In one step, this technique separates the phosphorylated derivative from A-kinase, ATP, unphosphorylated HMG 14, and a minor phosphorylated by-product which evidence suggests may be the previously reported Ser 6, 24-diphospho-HMG 14. Mono S chromatography also enhances the purity of calf thymus HMG 14 prepared by perchloric acid extraction, acetone and ethanol precipitations, and CM-Sephadex chromatography. In addition, it permits the detection of apparent microheterogenous forms of both unphosphorylated and Ser 6-PO4-HMG 14. The significant reductions in binding affinity resulting from the incorporation of phosphate groups into HMG 14 suggest that Mono S chromatography could have more general application in the isolation of phosphorylated derivatives of other basic proteins, including other chromatin-associated DNA-binding proteins which are known to undergo specific phosphorylation. It would especially be useful when the proteins and their phosphorylated derivatives bind more tightly to Mono S than the kinases used for their phosphorylation.

Binding of high-mobility-group proteins HMG 14 and HMG 17 to DNA and histone H1 as influenced by phosphorylation

We have used affinity chromatography to study the effects of phosphorylation of calf thymus high-mobility-group proteins HMG 14 and HMG 17 on their binding properties towards calf thymus single- and double-stranded DNA and histone H1. Without in vitro phosphorylation, HMG 14 and HMG 17 eluted from double-stranded DNA-columns at 200 mM NaCl. HMG 14 was released from single-stranded DNA-column at 300 mM NaCl and from H1-column at 130 mM NaCl, whereas the corresponding values for HMG 17 were 230 mM and 20 mM, respectively. Phosphorylation of HMG 14 and HMG 17 by cAMP-dependent protein kinase (A-kinase) decreased markedly their affinity (270 mM and 200 mM NaCl, respectively) for single-stranded DNA, whereas HMG 14 phosphorylated by nuclear protein kinase II (NII-kinase) eluted only slightly (290 mM NaCl) ahead of the unphosphorylated protein. HMG 14 phosphorylated by both A-kinase and NII-kinase eluted from double-stranded DNA-columns almost identically (190 mM NaCl) with the unphosphorylated protein. Interestingly, phosphorylation of HMG 14 by NII-kinase increased considerably its affinity for histone H1 and the phosphorylated protein eluted at 200 mM NaCl. Phosphorylation of HMG 14 by A-kinase did not alter its interaction towards histone H1. These results indicate that modification of HMG 14 by phosphorylation at specific sites may have profound effects on its binding properties towards DNA and histone H1, and that HMG 17 has much weaker affinity for single-stranded DNA and histone H1 than HMG 14.

Phosphorylation of high-mobility-group protein 14 by two specific kinases modifies its interaction with histone oligomers in free solution

Chromosomal protein HMG14 can be specifically phosphorylated by the cyclic AMP-dependent protein kinase at the N-terminus and by casein kinase 2 at the acidic C-terminus. Under the same conditions used for HMG14, HMG17 is not significantly phosphorylated by either of the two kinases. Further, we have studied the effect of phosphorylation by these kinases on the interaction of HMG14 with histone oligomers, using chemical cross-linking. Our results indicate that the phosphorylation of HMG14 by casein kinase 2 enhances its interaction with histone oligomers in free solution, whereas a minor effect was observed by phosphorylation with cyclic AMP-dependent protein kinase. In contrast, HMG17 does not interact at all with any histone oligomer in free solution under the conditions used. To gain insight into the possible effect that phosphorylation may play in vivo, the pattern of distribution among different chromatin fractions was analysed. It was found that, although phosphorylation of HMG14 by both kinases allowed reconstitution of HMG14 to chromatin, the patterns obtained showed some slight differences.