Structural and biochemical analyses of monoubiquitinated human histones H2B and H4

Monoubiquitination is a major histone post-translational modification. In humans, the histone H2B K120 and histone H4 K31 residues are monoubiquitinated and may form transcriptionally active chromatin. In this study, we reconstituted nucleosomes containing H2B monoubiquitinated at position 120 (H2Bub(120)) and/or H4 monoubiquitinated at position 31 (H4ub(31)). We found that the H2Bub(120) and H4ub(31) monoubiquitinations differently affect nucleosome stability: the H2Bub(120) monoubiquitination enhances the H2A–H2B association with the nucleosome, while the H4ub(31) monoubiquitination decreases the H3–H4 stability in the nucleosome, when compared with the unmodified nucleosome. The H2Bub(120) and H4ub(31) monoubiquitinations both antagonize the Mg(2+)-dependent compaction of a poly-nucleosome, suggesting that these monoubiquitinations maintain more relaxed conformations of chromatin. In the crystal structure, the H2Bub(120) and H4ub(31) monoubiquitinations do not change the structure of the nucleosome core particle and the ubiquitin molecules were flexibly disordered in the H2Bub(120)/H4ub(31) nucleosome structure. These results revealed the differences and similarities of the H2Bub(120) and H4ub(31) monoubiquitinations at the mono- and poly-nucleosome levels and provide novel information to clarify the roles of monoubiquitination in chromatin.