A map of general and specialized chromatin readers in mouse tissues generated by label-free interaction proteomics.

Posttranslational modifications on core histones can serve as binding scaffolds for chromatin-associated proteins. Proteins that specifically bind to or "read" these modifications were previously identified in mass spectrometry-based proteomics screens based on stable isotope-labeling in cell lines. Here we describe a sensitive, label-free histone peptide pull-down technology with extracts of different mouse tissues. Applying this workflow to the classical activating and repressive epigenetic marks on histone H3, H3K4me3, and H3K9me3, we identified known and putative readers in extracts from brain, liver, kidney, and testis. A large class of proteins were specifically repelled by H3K4me3. Our screen reached near-saturation of direct interactors, most of which are ubiquitously expressed. In addition, it revealed a number of specialized readers in tissues such as testis. Apart from defining the chromatin interaction landscape in mouse tissues, our workflow can be used for peptides with different modifications and cell types of any organism.