BRD4 degradation via stabilization of a native interaction with DCAF16

Targeted protein degradation is a modality based on small molecules that induce proximity between a protein of interest (POI) and an E3 ubiquitin ligase, thus prompting POI ubiquitination and degradation. To expand the reach of TPD, current research is geared to unlock novel E3s. To that end, the transcriptional co-activator protein BRD4 has emerged as a frequently used POI. Derivatization of known BRD4 ligands with structurally diverse substituents has yielded a suite of potent BRD4 degraders, thus generating the notion that BRD4 is particularly amenable to TPD. Here, we mechanistically characterize two BRD4 degraders via orthogonal CRISPR screens. Despite their structural dissimilarity, both degraders functionally converge on a unifying mechanism of action that involves the CRL4DCAF16 ligase complex. Using recombinant reconstitution, as well as biophysical and structural elucidation, we demonstrate that BRD4 has an intrinsic activity for DCAF16, which is further enhanced by both compounds. We uncover a novel mode of molecular recognition based on multivalent "gluing" of BRD4 onto DCAF16 that requires both bromodomains of BRD4. Our data thus imply a substantial conceptual overlap between PROTACs and molecular glue degraders, and highlight multivalency as a novel concept in the design of proximity-inducing drugs.