The APEXTAC System for Ligand‐Guided Proximity Labeling

Unbiased identification of drug-targets in live cells is essential for understanding the mechanism-of-action and potential off-target effects of drugs. The BioTAC system to measure these effects is previously developed. However, BioTAC has limitations, including lysine-directed biotinylation chemistry, and relatively long biotin labeling times. Herein, the development of the APEXTAC system, a small molecule guided proximity labeling platform based on the APEX2 peroxidase proximity labeling enzyme, is described as a complementary tool. A head-to-head comparison is performed between the APEXTAC system and the BioTAC system for (+)-JQ1 target-ID and demonstrated that APEXTAC can label E3-ligases via their ligands without requiring a proteasome inhibitor which can significantly perturb cell state. The APEXTAC system supports live cell target-ID across numerous molecules and targets, including components of the protein homeostasis machinery, without degradation of the labeling system, highlighting its potential application to the targeted protein degradation field.