PROTAC repurposing uncovers a noncanonical binding surface that mediates chemical degradation of nuclear receptors

Proteolysis-targeting chimeras (PROTACs) containing a target protein ligand linked to an E3 ubiquitin ligase ligand induce target protein degradation through E3 recruitment. Most PROTACs bind a surface cleft of the protein of interest rather than a buried pocket. Using the nuclear receptor PXR, we previously described the inherent difficulties of PROTAC targeting via a deep solvent-inaccessible ligand binding pocket. Here, we discover that the CRBN-dependent MDM2 PROTAC MD-224 is a potent PXR degrader that achieves its activity from binding adjacent to the ligand-binding pocket. Furthermore, because the proximal region is a structural feature common among nuclear receptors, MD-224 also targets additional receptors for proteasomal degradation. Using structure- and activity-guided medicinal chemistry, we ablated MDM2 degradation and generated MD-224 analogs with activities skewed toward different receptors. Thus, we describe (1) PROTAC repurposing as a potential route of degrader discovery and (2) nuclear receptor-targeted degradation through a noncanonical binding site. Chemically induced protein degradation is a powerful alternative to classical inhibition, but some proteins have deeply masked binding pockets that make the development of degrader molecules difficult. Here, the authors discover an alternate site on nuclear receptors that can be targeted by degraders.