H2A.Z stabilizes chromatin in a way that is dependent on core histone acetylation

The functional and structural chromatin roles of H2A. Z are still controversial. This work represents a further attempt to resolve the current functional and structural dichotomy by characterizing chromatin structures containing native H2A. Z. We have analyzed the role of this variant in mediating the stability of the histone octamer in solution using gel-filtration chromatography at different pH. It was found that decreasing the pH from neutral to acidic conditions destabilized the histone complex. Furthermore, it was shown that the H2A. Z-H2B dimer had a reduced stability. Sedimentation velocity analysis of nucleosome core particles (NCPs) reconstituted from native H2A. Z-containing octamers indicated that these particles exhibit a very similar behavior to that of native NCPs consisting of canonical H2A. Sucrose gradient fractionation of native NCPs under different ionic strengths indicated that H2A. Z had a subtle tendency to fractionate with more stabilized populations. Anextensive analysis of the salt-dependent dissociation of histones from hydroxyapatite-adsorbed chromatin revealed that, whereas H2A. Z co-elutes with H3-H4, hyperacetylation of histones (by treatment of chicken MSB cells with sodium butyrate) resulted in a significant fraction of this variant eluting with the canonical H2A. These studies also showed that the late elution of this variant (correlated to enhanced binding stability) was independent of the chromatin size and of the presence or absence of linker histones.