Covalent post-translational modifications (PTMs1
) 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
). 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 phosphorylate both histones H3 and H2A. Yet, while the phosphorylation of H3 leads to gene activation (5
) 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 intra- and 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
) and HMGN (10
), which bind to nucleosomes and alter the compactness of the chromatin fiber, have also been shown to affect the levels of specific modifications in the tail of histone H3. HMGN proteins bind specifically to the 147 base pair nucleosomal core particle, the building block of the chromatin (12
). The binding of these proteins to nucleosomes reduces the compaction of the chromatin fiber and alters the transcription, replication, and repair potential of chromatin templates (12
). HMGN proteins have a modular structure and contact both the nucleosomal DNA and the histone through multiple interaction sites (15
). A central, positively charged region contains the major HMGN-chromatin contacts (16
); however, additional sites of interactions have been identified. Thus, site-directed cross-linking of HMGN-nucleosome complexes indicates that the C-terminal region of HMGN1 is located near the amino terminal tail of histone H3 (17
). Significantly, HMGN1 enhances the levels of acetylation in H3K14, reduces the level of phosphorylation of H3S10 and H3S28, and also changes the acetylation and methylation state of H3K9 (10
). Taken together with the structural data, the results suggest that the close proximity of HMGN1 to the H3 tail in the HMGN1-nucleosome complex alters the ability of histone modifiers to access and modify residues in H3. So far it has not been studied in detail whether HMGN1 affects the modification levels in the tail of core histones other than H3.
Here we use Hmgn1-/- mouse embryonic fibroblasts (MEFs) and in vitro reconstitution experiments to demonstrate that HMGN1 modulates the phosphorylation of serine 1 in histone H2A. We find that loss of HMGN1 alters the steady-state phosphorylation levels of H2AS1 and demonstrate that the binding of HMGN1 to nucleosomes inhibits the phosphorylation of H2A. Our findings extend the known range of histone modifications affected by HMGN1 and strengthen the possibility that HMGNs, and similar architectural chromatin binding proteins, are part of the mechanism that modulates the cellular levels of these epigenetic markers.