Protein-Labeling Reagents Selectively Activated by Copper(I).

Copper is an essential trace element that participates in many biological processes through its unique redox cycling between cuprous (Cu) and cupric (Cu) oxidation states. To elucidate the biological functions of copper, chemical biology tools that enable selective visualization and detection of copper ions and proteins in copper-rich environments are required. Herein, we describe the design of Cu-responsive reagents based on a conditional protein labeling strategy. Upon binding Cu, the probes generated quinone methide via oxidative bond cleavage, which allowed covalent labeling of surrounding proteins with high Cu selectivity. Using gel- and imaging-based analyses, the best-performing probe successfully detected changes in the concentration of labile Cu in living cells. Moreover, conditional proteomics analysis suggested intramitochondrial Cu accumulation in cells undergoing cuproptosis. Our results highlight the power of Cu-responsive protein labeling in providing insights into the molecular mechanisms of Cu metabolism and homeostasis.