Middle-Down MS is Ready to Answer Complex Questions in Chromatin Biology.

Histones are the most abundant protein family in the cells of complex organisms such as mammals and, together with DNA, they define the backbone of chromatin. Histone PTMs are key players of chromatin biology, as they function as anchors for proteins that bind and modulate chromatin readout, including gene expression. Middle-down mass spectrometry (MS) has been optimized for about 10 years to study histone N-terminal tails, but it has been rarely applied to identify the role of coexisting histone marks in biology. In this work, Jiang et al. used middle-down MS to study the dynamics of coexisting PTMs on histone H4 in two breast cancer cell lines. They found that overall serine 1 phosphorylation (S1ph) is mildly regulated during the cell cycle, but S1ph coexistence frequency with acetylations and methylations on the lysine residues of the N-terminal tail is remarkably tuned during S phase and G2/M phase. Together, the team placed another benchmark proving that MS analysis of combinatorial histone PTMs provides a more comprehensive view on chromatin state than studying individual marks. We should then constantly question ourselves regarding the limitations of analyzing single PTMs when we attempt to define their effect on protein functions.