Posttranslational modifications remodel proteome-wide ligandability.

Posttranslational modifications (PTMs) vastly expand the diversity of the human proteome, dynamically reshaping protein activity, interactions and localization in response to environmental, pharmacologic and disease-associated cues. However, their proteome-wide impact on small-molecule recognition-and, thus, druggability-remains largely unexplored. Here we present a chemical proteomic strategy to delineate how PTM states remodel protein ligandability in human cells. Using broad-spectrum photoaffinity probes, we identified more than 400 functionally diverse proteins whose ability to engage small molecules is impacted by phosphorylation or N-linked glycosylation status. Integrating binding site mapping with structural analyses reveals a diverse array of PTM-dependent pockets. Among these, we discovered that the phosphorylation status of common oncogenic KRAS mutants impacts the action of small molecules, including clinically approved inhibitors. These findings illuminate a previously underappreciated layer of proteome plasticity governed by PTMs and highlight opportunities to develop chemical probes that selectively target proteins in defined modification states.