Project Summary Proteolysis targeting chimeras (PROTACs) represent an exciting new modality to inhibit proteins. These reagents are chemical dimerizers that recruit an E3 ubiquitin ligase to a target protein of interest, resulting in poly- ubiquitylation of the target protein and its subsequent degradation by the proteasome. Recently, it has become clear that the E3 ligase must make productive contacts with the substrate. In other words, not all target proteins can be turned over by a single E3 Ub ligase. This project will develop a cell-based screen for degradation of a GFP-target protein fusion that employs a known ligand for the target protein fused to a complex library of potential E3 ligase ligands, thus allowing the cell to “tell us” which of the ≈ 500 E3 Ub ligases in the cell are best suited for targeting that protein for destruction.