Quickly evolving histones, nucleosome stability and chromatin folding: all about histone H2A.Bbd

Histone H2A.Bbd (Barr body-deficient) is a novel histone variant which is largely excluded from the inactive X chromosome of mammals. Discovered only 6 years ago, H2A.Bbd displays very unusual structural and functional properties, for instance, it is relatively shorter and only 48% identical compared to H2A, lacking both the typical C-terminal tail of the H2A family and the very last sequence of the docking domain, making it the most specialized among all histone variants known to date. Indeed, molecular evolutionary analyses have shown that H2A.Bbd is a highly hypervariable and quickly evolving protein exclusive to mammalian lineages, in striking contrast to all other histones. Different studies have described a deposition pattern of H2A.Bbd in the chromatin that overlaps with regions of histone H4 acetylation suggesting its association with transcriptionally active euchromatic regions of the genome. In this regard, it is believed that this histone variant plays an important role in determining such regions by destabilizing the nucleosome and locally unfolding the chromatin fiber. This review provides a concise, comprehensive and timely summary of the work published on H2A.Bbd structure and function. Special emphasis is placed on its chromatin deposition patterns in relation to gene expression profiles and its evolutionary history, as well as on the dynamics of H2A.Bbd-containing nucleosomes.