Modulation of chromatin function through linker histone H1 variants

In this review, the structural aspects of linker H1 histones are presented as a background for characterization of the factors influencing their function in animal and human chromatin. The action of H1 histone variants is largely determined by dynamic alterations of their intrinsically disordered tail domains, posttranslational modifications and allelic diversification. The interdependent effects of these factors can establish dynamic histone H1 states that may affect the organization and function of chromatin regions.

Dynamics and dispensability of variant-specific histone H1 Lys-26/Ser-27 and Thr-165 post-translational modifications

In mammals, the linker histone H1, involved in DNA packaging into chromatin, is represented by a family of variants. H1 tails undergo post-translational modifications (PTMs) that can be detected by mass spectrometry. We developed antibodies to analyze several of these as yet unexplored PTMs including the combination of H1.4 K26 acetylation or trimethylation and S27 phosphorylation. H1.2-T165 phosphorylation was detected at S and G2/M phases of the cell cycle and was dispensable for chromatin binding and cell proliferation; while the H1.4-K26 residue was essential for proper cell cycle progression. We conclude that histone H1 PTMs are dynamic over the cell cycle and that the recognition of modified lysines may be affected by phosphorylation of adjacent residues.

Linker histones in hormonal gene regulation

In the present review, we summarize advances in our knowledge on the role of the histone H1 family of proteins in breast cancer cells, focusing on their response to progestins. Histone H1 plays a dual role in gene regulation by hormones, both as a structural component of chromatin and as a dynamic modulator of transcription. It contributes to hormonal regulation of the MMTV promoter by stabilizing a homogeneous nucleosome positioning, which reduces basal transcription whereas at the same time promoting progesterone receptor binding and nucleosome remodeling. These combined effects enhance hormone dependent gene transcription, which eventually requires H1 phosphorylation and displacement. Various isoforms of histone H1 have specific functions in differentiated breast cancer cells and compact nucleosomal arrays to different extents in vitro. Genome-wide studies show that histone H1 has a key role in chromatin dynamics of hormone regulated genes. A complex sequence of enzymatic events, including phosphorylation by CDK2, PARylation by PARP1 and the ATP-dependent activity of NURF, are required for H1 displacement and gene de-repression, as a prerequisite for further nucleosome remodeling. Similarly, during hormone-dependent gene repression a dedicated enzymatic mechanism controls H1 deposition at promoters by a complex containing HP1γ, LSD1 and BRG1, the ATPase of the BAF complex. Thus, a broader vision of the histone code should include histone H1, as the linker histone variants actively participate in the regulation of the chromatin structure. How modifications of the core histones tails affect H1 modifications and vice versa is one of the many questions that remains to be addressed to provide a more comprehensive view of the histone cross-talk mechanisms.