Proteasome Cap Targeting Chimeras for Ubiquitination‐Independent Targeted Protein Degradation

Selective degradation of a disease-associated protein of interest (POI) is a powerful therapeutic strategy. FDA-approved and investigational glue and degrader drugs function by recruiting a POI to an E3 ubiquitin ligase that mediates POI polyubiquitination and triggers proteasomal degradation. However, using E3 ligases as an intermediary and requiring POI polyubiquitination makes this mechanism of action complex and difficult to rationalize and optimize. These issues have led to interest in evaluating whether direct recruitment of non-ubiquitinated POIs to the proteasome might achieve the same pharmacological outcome. Here, we examined the potential of direct-to-proteasome non-ubiquitinated POI recruitment. Using a tag strategy, we first demonstrated that the proteasomal 19S cap region proteins, RPN13 and RPN1, can recruit non-ubiquitinated POI model proteins, such as BRD4, to the proteasome and induce degradation. Subsequently, we developed small molecule-based bifunctional recruiter molecules (Proteasome Cap Targeting Chimeras, CAP-TACs) and showed that they recruit several distinct POIs, including BRD4, PRMT5, and FKBP12, to specific subunits in the 19S cap region and induce their ubiquitination-independent, proteasome-dependent degradation. This study provides further evidence that bifunctional small molecules can re-localize POIs to the proteasome and induce their degradation in the absence of ubiquitination, which broadens the capabilities of targeted protein degradation.