H2B ubiquitylation: the end is in sight.

Historically, the first eukaryotic protein found to be modified by ubiquitin was H2A, originally isolated from HeLa cells in 1975 by Harrison Busch and coworkers as a histone-like, nonhistone chromosomal protein called A24. Ubiquitylated histones have subsequently been found in many eukaryotic species, and to date, the core histones H2A, H2B, H3, the linker histone H1, and the histone variant H2A.Z are known to carry this modification. Although first on the scene, it was only recently that studies on histone ubiquitylation have enjoyed a renaissance. Part of the reason for the relatively slow pace of research on this fascinating histone modification was the absence of a good genetic system with which to study its cellular roles. This changed in 2000, when histone H2B was found to be ubiquitylated in the budding yeast S. cerevisiae, an organism with a low histone gene copy number and highly tractable genetics. Another factor was the almost exclusive focus of research on the role of polyubiquitylation in protein turnover. Because histones are generally monoubiquitylated, a form of the modification that is not associated with protein degradation, the significance of this minimalist ubiquitin conjugation was not heavily pursued. But perhaps the key reason for the renewed interest in histone ubiquitylation was the unexpected discovery of the past year that ubiquitylated H2B plays an important role in the trans-histone methylation of histone H3, a modification with close ties to the regulation of gene expression. This review will highlight some of the recent findings on the regulation and cellular roles of H2B ubiquitylation in yeast.