Mass spectrometric mapping of linker histone H1 variants reveals multiple acetylations, methylations, and phosphorylation as well as differences between cell culture and tissue

Posttranslational modifications of histones are involved in regulation of chromatin structure and gene activity. Whereas the modifications of the core histones H2A, H2B, H3, and H4 have been extensively studied, our knowledge of H1 modifications remained mainly limited to its phosphorylation. Here we analyzed the composition of histone H1 variants and their modifications in two human cell lines and nine mouse tissues. Use of a hybrid linear ion trap-orbitrap mass spectrometer facilitated assignment of modifications by high resolution and low ppm mass accuracy for both the precursor and product mass spectra. Across different tissues we identified a range of phosphorylation, acetylation, and methylation sites. We also mapped sites of ubiquitination and report identification of formylated lysine residues. Interestingly many of the mapped modifications are located within the globular domain of the histones at sites that are thought to be involved in binding to nucleosomal DNA. Investigation of mouse tissue in addition to cell lines uncovered a number of interesting differences. For example, whereas methylation sites are frequent in tissues, this type of modification was much less abundant in cultured cells and escaped detection. Our study significantly extends the known spectrum of linker histone variability.