FACT frees Ubp10 from nucleosome sequestration after rapid H2B deubiquitination

Researchers have shed light on the mechanism behind the turnover of a chromatin-modifying enzyme called Ubp10. Utilizing a novel technique called three-color single-molecule fluorescence, they were able to directly observe Ubp10 removing ubiquitin from H2B on individual nucleosomes within seconds of binding. Despite this rapid action, Ubp10 did not release the nucleosome, with approximately 80% of them remaining bound after 45 minutes.

Additionally, unmodified nucleosomes formed similarly long-lasting complexes. This indicates that Ubp10 turnover is primarily limited by its release from the nucleosome rather than the catalytic step itself.

Further investigation revealed that a histone chaperone called FACT played a crucial role in facilitating Ubp10-nucleosome dissociation and overcoming this bottleneck. In living yeast cells, the nuclear depletion of the FACT subunit Spt16 led to an increase in the nucleoplasmic fraction of Ubp10, which is bound to chromatin and moves more slowly.

These findings suggest that FACT-dependent release of Ubp10 on chromatin could be a key mechanism for the recycling of this enzyme. The broader implication of this research is that the sequestration of enzymes by nucleosomes can act as a regulatory control point for chromatin modification processes.