Destabilization of nucleosomes by HMGB proteins and FACT complexes

Nucleosome disruption plays a key role in many nuclear processes including transcription, DNA repair and recombination. We combine atomic force microscopy (AFM) and optical tweezers (OT) experiments to probe the effects of various motifs of High Mobility Group B (HMGB) proteins on nucleosome stability. We find that the double box Hmo1 and the single box Nhp6A from S. cerevisiae destabilize and unwind DNA from the H2A-H2B dimers that are part of the histone octamer. Unlike Nhp6A, Hmo1 also releases half of the DNA held by the (H3-H4)₂ tetramer. Despite this destabilization, the octamers appear intact, and the remaining (H3-H4)₂ tetramer interactions with the DNA are also destabilized. We also probe the human histone chaperone protein FACT (facilitates chromatin transcription), which also contains an HMGB box. We find that FACT (including the subunits Spt16 and SSRP1) binds directly to the nucleosome, specifically disrupting the H2A-H2B dimer interaction with DNA. Disruption leads to the release of that DNA from the nucleosome and the ejection of these dimers from the octamer, revealing a mechanism for regulation of chromatin assembly. Differences in nucleosome destabilization point to complementary roles that HMGB proteins play in chromatin remodeling.