Linker Histone H1.2 cooperates with Cul4A and PAF1 to drive H4K31 ubiquitylation-mediated transactivation

Genome distribution of replication-independent histone H1 variants shows H1.0 associated with nucleolar domains and H1X associated with RNA polymerase II-enriched regions

Unlike core histones, the linker histone H1 family is more evolutionarily diverse, and many organisms have multiple H1 variants or subtypes. In mammals, the H1 family includes seven somatic H1 variants; H1.1 to H1.5 are expressed in a replication-dependent manner, whereas H1.0 and H1X are replication-independent. Using ChIP-sequencing data and cell fractionation, we have compared the genomic distribution of H1.0 and H1X in human breast cancer cells, in which we previously observed differential distribution of H1.2 compared with the other subtypes. We have found H1.0 to be enriched at nucleolus-associated DNA repeats and chromatin domains, whereas H1X is associated with coding regions, RNA polymerase II-enriched regions, and hypomethylated CpG islands. Further, H1X accumulates within constitutive or included exons and retained introns and toward the 3' end of expressed genes. Inducible H1X knockdown does not affect cell proliferation but dysregulates a subset of genes related to cell movement and transport. In H1X-depleted cells, the promoters of up-regulated genes are not occupied specifically by this variant, have a lower than average H1 content, and, unexpectedly, do not form an H1 valley upon induction. We conclude that H1 variants are not distributed evenly across the genome and may participate with some specificity in chromatin domain organization or gene regulation.

In vitro and in vivo assays for studying histone ubiquitination and deubiquitination

Posttranslational histone modifications play important roles in regulating chromatin structure and function (Rando, Curr Opin Genet Dev 22:148-155, 2012; Zentner and Henikoff, Nat Struct Mol Biol 20:259-266, 2013). One example of such modifications is histone ubiquitination, which occurs predominately on H2A and H2B. Recent studies have highlighted important regulatory roles of H2A ubiquitination in Polycomb group protein-mediated gene silencing and DNA damage repair (de Napoles et al., Dev Cell 7:663-676, 2004; Wang et al., Nature 431:873-878, 2004; Doil et al., Cell 136:435-446, 2009; Gatti et al., Cell Cycle 11:2538-2544, 2012; Mattiroli et al., Cell 150:1182-1195, 2012; Stewart et al., Cell 136:420-434, 2009; Bergink et al., Genes Dev 20:1343-1352, 2006; Facchino et al., J Neurosci 30:10096-10111, 2010; Ginjala et al., Mol Cell Biol 31:1972-1982, 2011; Ismail et al., J Cell Biol 191:45-60, 2010), H2B ubiquitination in transcription initiation and elongation (Xiao et al., Mol Cell Biol 25:637-651, 2005; Kao et al., Genes Dev 18:184-195, 2004; Pavri et al., Cell 125:703-717, 2006; Kim et al., Cell 137:459-471, 2009), pre-mRNA splicing (Jung et al. Genome Res 22:1026-1035, 2012; Shieh et al., BMC Genomics 12:627, 2011; Zhang et al., Genes Dev 27:1581-1595, 2013), nucleosome stabilities (Fleming et al., Mol Cell 31:57-66, 2008; Chandrasekharan et al., Proc Natl Acad Sci U S A 106:16686-16691, 2009), H3 methylation (Sun and Allis, Nature 418:104-108, 2002; Briggs et al., Nature 418:498, 2002; Dover et al., J Biol Chem 277:28368-28371, 2002; Ng et al., J Biol Chem 277:34655-34657, 2002), and DNA methylation (Sridhar et al., Nature 447:735-738, 2007). Here we describe methods for in vitro histone ubiquitination and deubiquitination assays. We also describe approaches to investigate the in vivo function of putative histone ubiquitin ligase(s) and deubiquitinase(s). These experimental procedures are largely based on our studies in mammalian cells. These methods should provide useful tools for studying this bulky histone modification.

Generation of nonhydrolyzable ubiquitin-histone mimics

Histone proteins undergo various types of post-translational modifications (PTMs) to regulate dynamic processes in the cell, including replication, transcription and DNA damage repair. One type of histone PTM is the attachment of a small protein, ubiquitin (Ub). In eukaryotic organisms, a single Ub is attached to specific lysine residues of histones H2A and H2B in a modification that, unlike many other forms of ubiquitination in the cell, does not signal degradation. Instead, both attachment and removal of Ub to these histones has been shown to affect gene transcription, pre-mRNA splicing, and DNA damage repair, but the mechanisms by which histone ubiquitination governs these processes are not well understood. In an effort to identify "readers" of Ub-histones, we developed a straightforward crosslinking strategy to generate nonhydrolyzable Ub-histone mimics. These mimics were assembled into Ub-histone-containing dimers or nucleosomes. We demonstrate that they can be used in pulldown assays to identify proteins that differentiate unmodified and ubiquitinated histones.