Chromosomal Protein HMGN1 Modulates the Phosphorylation of Serine 1 in Histone H2A

Abstract

Here we demonstrate that HMGN1, a nuclear protein that binds specifically to nucleosomes, modulates the level of histone H2A phosphorylation. In Hmgn1 -/-cells, loss of HMGN1 elevates the steady-state levels of H2AS1ph throughout the cell cycle. In vitro, HMGN1 reduces the rate of Rsk2-and Msk1-mediated phosphorylation of nucleosomal, but not free, histone H2A. HMGN1 inhibits H2A phosphorylation by binding to nucleosomes since an HMGN mutant, which cannot bind to chromatin, does not inhibit the Rsk2-mediated H2A phosphorylation. HMGN2 also inhibits H2A phosphorylation, suggesting that the inhibition of H2A phosphorylation is not specific to only one member of this protein family. Thus, the present data add modifications of histone H2A to the list of histone modifications affected by HMGN proteins. It supports the suggestion that structural chromatin binding proteins can modify the whole profile of post-translational modifications of core histones.Covalent post-translational modifications (PTMs 1 ) of histones play a role in the epigenetic regulation of gene expression and are often associated with nuclear processes occurring in the context of chromatin. For example, a transient amplification of serine 10 phosphorylation of core histone H3 (H3S10ph) is a characteristic feature of mitosis and meiosis in vertebrates (1,2). Likewise, serine 1 in histone H2A (H2AS1ph), one of the most abundant PTMs in histone H2A (3), is noticeably phosphorylated during mitosis in Caenorhabditis elegans, Drosophila melanogaster, and mammals (4). The timing and localization of H3S10ph and H2AS1ph during mitosis are similar, and both modifications peak in mitotic chromosomes, when the chromatin fiber is the most condensed (4). Although these modifications often serve as markers for mitotic chromosomes, it is not clear if they are in fact required for chromatin condensation since H3 phosphorylation is not essential for mitosis (2). Among various mitotic phosphorylation sites the functional redundancy may be one explanation for this finding. H3S10 and H2AS1 phosphorylation occurs not only in mitosis but also during other stages in the cell cycle; however in contrast to mitosis, in the other cell cycle stages the timing and location of these two histone phosphorylations differ (4). Interestingly, upon cellular stress, the effector kinases NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript phosphorylate both histones H3 and H2A. Yet, while the phosphorylation of H3 leads to gene activation (5,6) the phosphorylation of H2AS1 seems to inhibit transcription (7). Thus, as already amply documented, changes in PTM levels occur in response to wide and diverse intraand extracellular signals.The cellular levels of histone modification are not fixed; they are in a constant state of flux and reflect the equilibrium between the activities of the enzymes that modify and demodify specific sites. In addition, structural proteins such as histone H1 (8,9) and HMGN (10,11), which bind to nucleosomes and alter the compactness ...

HMGN1 Modulates Estrogen-Mediated Transcriptional Activation through Interactions with Specific DNA-Binding Transcription Factors

“…Previous studies identified two general mechanisms that underlie the ability of HMGN1 to affect transcription by affecting nucleosomal histone modification (42,43,57,77) and by counteracting the ability of histone H1 to compact the chromatin fiber, mediated through the C-terminal CHUD domain (20,74). Our finding that the abundant HMGN1 interacts with specific DNA-binding transcription factors raised another possibility, that HMGN1 may squelch transcription factor activities.…”

High mobility group proteins and their post-translational modifications

“…The major members, HMGN1 and HMGN2, are present in the nuclei of all mammalian and most vertebrate cells, and have been studied for more than 30 years. It was demonstrated very recently that HMGN proteins can affect the PTM profile of core histones, indicating that HMGN may play a role in epigenetic regulatory mechanisms [60][61][62].…”

Delineation of the Protein Module That Anchors HMGN Proteins to Nucleosomes in the Chromatin of Living Cells

“…The interaction of HMGN proteins with nucleosomes stabilizes the structure of the isolated CP, reduces the "compaction" of the higher-order chromatin structure (10), and alters the levels of posttranslational modifications in the tail of the nucleosomal histones (22,23,29,38).…”