ByeTAC: Bypassing an E3 Ligase for Targeted Protein Degradation

Targeted protein degradation utilizing a bifunctional molecule to initiate ubiquitination and subsequent degradation by the 26S proteasome has been shown to be a powerful therapeutic intervention. Many bifunctional molecules, including covalent and non-covalent ligands to proteins of interest, have been developed. The traditional target protein degradation methodology targets the protein of interest in both healthy and diseased cell populations, and a therapeutic window is obtained based on the overexpression of the targeted protein. We report here a series of bifunctional degraders that do not rely on interacting with an E3 ligase, but rather a 26S proteasome subunit, which we have named ByeTACs: Bypassing E3 Targeting Chimeras. Rpn-13 is a non-essential ubiquitin receptor for the 26S proteasome. Cells under significant stress or require significant ubiquitin-dependent degradation of proteins for survival, incorporate Rpn-13 in the 26S to increase protein degradation rates. The targeted protein degraders reported here are bifunctional molecules that include a ligand to Rpn-13 and BRD4, the protein of interest we wish to degrade. We synthesized a suite of degraders with varying PEG chain lengths and showed that bifunctional molecules that incorporate a Rpn-13 binder (TCL1) and a BRD4 binder (JQ1) with a PEG linker of 3 or 4 units are the most effective to induce BRD4 degradation. We also demonstrate that our new targeted protein degraders are dependent upon proteasome activity and Rpn-13 expression levels. This establishes a new mechanism of action for our ByeTACs that can be employed for the targeted degradation of a wide variety of protein substrates.