Middle‐Down and Top‐Down Mass Spectrometric Analysis of Co‐occurring Histone Modifications

Histones are chromatin proteins that are highly modified with many different types of post-translational modifications. These modifications act in concert to regulate a number of chromatin-related processes. However, identification and quantification of co-occurring histone post-translational modifications is challenging because there are many potential combinations of modifications and because the commonly used strategy of fragmenting proteins using trypsin or an alternative protease prior to LC-MS/MS analysis results in the loss of connectivity between modifications on different peptides. In this unit, mass spectrometric methods to analyze combinatorial histone modifications on histone tails (middle-down mass spectrometry) and on intact histones (top-down mass spectrometry) are described.