Targeted protein degradation has arisen as a powerful strategy for drug discovery allowing the targeting of undruggable proteins for proteasomal degradation. This approach most often employs heterobifunctional degraders consisting of a protein-targeting ligand linked to an E3 ligase recruiter to ubiquitinate and mark proteins of interest for proteasomal degradation. One challenge with this approach， however， is that only a few E3 ligase recruiters currently exist for targeted protein degradation applications， despite the hundreds of known E3 ligases in the human genome. Here， we utilized activity-based protein profiling (ABPP)-based covalent ligand screening approaches to identify cysteine-reactive smal... More
Targeted protein degradation has arisen as a powerful strategy for drug discovery allowing the targeting of undruggable proteins for proteasomal degradation. This approach most often employs heterobifunctional degraders consisting of a protein-targeting ligand linked to an E3 ligase recruiter to ubiquitinate and mark proteins of interest for proteasomal degradation. One challenge with this approach， however， is that only a few E3 ligase recruiters currently exist for targeted protein degradation applications， despite the hundreds of known E3 ligases in the human genome. Here， we utilized activity-based protein profiling (ABPP)-based covalent ligand screening approaches to identify cysteine-reactive small-molecules that react with the E3 ubiquitin ligase RNF4 and provide chemical starting points for the design of RNF4-based degraders. The hit covalent ligand from this screen reacted with either of two zinc-coordinating cysteines in the RING domain， C132 and C135， with no effect on RNF4 activity. We further optimized the potency of this hit and incorporated this potential RNF4 recruiter into a bifunctional degrader linked to JQ1， an inhibitor of the BET family of bromodomain proteins. We demonstrate that the resulting compound CCW 28-3 is capable of degrading BRD4 in a proteasome- and RNF4-dependent manner. In this study， we have shown the feasibility of using chemoproteomics-enabled covalent ligand screening platforms to expand the scope of E3 ligase recruiters that can be exploited for targeted protein degradation applications.
