Analysis of histone ubiquitylation by MSL1/MSL2 proteins in vitro

Histone posttranslational modifications (PTM) control gene activity by targeting chromatin-regulatory proteins. By altering histone charges PTMs could also modulate inter- and intra-nucleosomal interactions, and thus affect chromatin high-order compaction and nucleosome stochastic folding, respectively. However, recently it has been shown that histone H2BK34- ubiquitylation (which is deposited in vivo by MOF-MSL) can destabilize one of the nucleosomal H2A-H2B dimers in symmetrically and (albeit to a lesser extend) asymmetrically modified nucleosomes, and thus promote formation of a hexasome particle. Here we have studied ubiquitylation patterns by purified MSL1/MSL2 using nucleosomes and different histone substrates. We have shown that H2B-ubiquitylation by MSL1/2 depends on substrate configuration. In addition, MSL1/2 efficiently ubiquitylate histone substrates but very poorly modify nucleosomes, which implies a requirement for nucleosome structural alteration for efficient ubiquitylation of H2BK34. Nucleosome modification by MSL1/MSL2 in vitro was analyzed directly using nucleosome gel-mobility shift assay, which suggested that MSL1/2 can deposit two ubiquitin moieties in one nucleosome.