Cellular protein painting for structural and binding sites analysis via lysine reactivity profiling with o-phthalaldehyde

The three-dimensional structure and the molecular interaction of proteins determine their roles in many cellular processes. Chemical protein painting with protein mass spectrometry can identify changes in structural conformations and molecular interactions of proteins including their binding sites. Nevertheless, most current protein painting techniques identified protein targets and binding sites of drugs in vitro using cell lysate or purified protein. Here, we screened 11 membrane-permeable lysine-reactive chemical probes for intracellular covalent labeling of endogenous proteins, which reveals ortho-phthalaldehyde (OPA) as the most reactive probe in intracellular environment. An MS workflow was developed and coupled with a new data analysis strategy termed RAPID (Reactive Amino acid Profiling by Inverse Detection) to enhance detection sensitivity. RAPID-OPA successfully identified structural change induced by allosteric drug TEPP-46 on its target protein PKM2, and was applied to profile conformation change of the proteome occurring in cells during thermal denaturation. Application of RAPID-OPA on cells treated with geldanamycin, selumetinib, and staurosporine successfully revealed their binding sites on target proteins. Thus, RAPID-OPA for cellular protein painting permits the identification of ligand-binding sites and detection of protein structural changes occurring in cells. Significance StatementProtein painting can be used to identify changes in the three-dimensional structure and molecular interaction of proteins that govern many cellular processes but are mostly applied to cell lysate or purified protein. We identified lysine reactive probes for the intracellular labeling of endogenous proteins, and developed an MS procedure with new data analysis strategy termed RAPID-OPA to characterize the intracellular conformation change of the proteome during thermal denaturation, and identified structural change mediated by allosteric regulator TEPP-46 on target protein PKM2. Furthermore, the approach could identify ligand binding sites exemplified by labeling of target proteins in cells treated with geldanamycin, selumetinib and staurosporine. Overall, RAPID-OPA for cellular protein painting enables the detection of protein structural changes happening in cells as well as the identification of ligand-binding sites.