Elucidating the influence of linker histone variants on chromatosome dynamics and energetics

Linker histones are epigenetic regulators that bind to nucleosomes and alter chromatin structures and dynamics. Biophysical studies have revealed two binding modes in the linker histone/nucleosome complex, the chromatosome, where the linker histone is either centered on or askew from the dyad axis. Each has been posited to have distinct effects on chromatin, however the molecular and thermodynamic mechanisms that drive them and their dependence on linker histone compositions remain poorly understood. We present molecular dynamics simulations of chromatosomes with the globular domain of two linker histone variants, generic H1 (genGH1) and H1.0 (GH1.0), to determine how their differences influence chromatosome structures, energetics and dynamics. Results show that both unbound linker histones adopt a single compact conformation. Upon binding, DNA flexibility is reduced, resulting in increased chromatosome compaction. While both variants enthalpically favor on-dyad binding, energetic benefits are significantly higher for GH1.0, suggesting that GH1.0 is more capable than genGH1 of overcoming the large entropic reduction required for on-dyad binding which helps rationalize experiments that have consistently demonstrated GH1.0 in on-dyad states but that show genGH1 in both locations. These simulations highlight the thermodynamic basis for different linker histone binding motifs, and details their physical and chemical effects on chromatosomes.

Elucidating the Influence of Linker Histone Variants on Chromatosome Dynamics and Energetics

Linker histones are epigenetic regulators that bind to nucleosomes and alter chromatin structures and dynamics. Biophysical studies have revealed two binding modes in the linker histone/nucleosome complex, the chromatosome, where the linker histone is either centered on or askew from the dyad axis. Each has been posited to have distinct effects on chromatin, however the molecular and thermodynamic mechanisms that drive them and their dependence on linker histone compositions remain poorly understood. We present molecular dynamics simulations of chromatosomes with the globular domain of two linker histone variants, generic H1 (genGH1) and H1.0 (GH1.0), to determine how their differences influence chromatosome structures, energetics, and dynamics. Results show that both unbound linker histones adopt a single compact conformation. Upon binding, DNA flexibility is reduced, resulting in increased chromatosome compaction. While both variants enthalpically favor on-dyad binding, energetic benefits are significantly higher for GH1.0, suggesting that GH1.0 is more capable than genGH1 of overcoming the large entropic reduction required for on-dyad binding which helps rationalize experiments that have consistently demonstrated GH1.0 in on-dyad states but that show genGH1 in both locations. These simulations highlight the thermodynamic basis for different linker histone binding motifs, and details their physical and chemical effects on chromatosomes.

Regulation of Cellular Dynamics and Chromosomal Binding Site Preference of Linker Histones H1.0 and H1.X

Linker histones play important roles in the genomic organization of mammalian cells. Of the linker histone variants, H1.X shows the most dynamic behavior in the nucleus. Recent research has suggested that the linker histone variants H1.X and H1.0 have different chromosomal binding site preferences. However, it remains unclear how the dynamics and binding site preferences of linker histones are determined. Here, we biochemically demonstrated that the DNA/nucleosome and histone chaperone binding activities of H1.X are significantly lower than those of other linker histones. This explains why H1.X moves more rapidly than other linker histonesin vivo. Domain swapping between H1.0 and H1.X suggests that the globular domain (GD) and C-terminal domain (CTD) of H1.X independently contribute to the dynamic behavior of H1.X. Our results also suggest that the N-terminal domain (NTD), GD, and CTD cooperatively determine the preferential binding sites, and the contribution of each domain for this determination is different depending on the target genes. We also found that linker histones accumulate in the nucleoli when the nucleosome binding activities of the GDs are weak. Our results contribute to understanding the molecular mechanisms of dynamic behaviors, binding site selection, and localization of linker histones.