Chemically Synthesized Phosphorylated Ubiquitin Unlocks Parkin for Activation-Controlled PROTAC Platforms

Proteolysis targeting chimeras (PROTACs) have been considered a promising therapeutic strategy, but their application is restricted by the limited number of available E3 ligases. Here, we expanded Parkin, an autoinhibitory E3 whose activity can be triggered by binding to its activating ligand, phosphorylated ubiquitin (UbP), into the repertoire of E3 ligases for targeted protein degradation (TPD). Specifically, we rationally design a Parkin-activating ligand UbP-M based on UbP and conjugated it with small-molecule binders against target proteins such as Brd4 and SF3B1 using total chemical synthesis. These de-graders successfully induced ubiquitination and degradation of target proteins. Notably, conjugating small-molecule binders of Brd4 and SF3B1 together to UbP-M simultaneously targets both proteins for degradation. As proof-of-concept, our studies demonstrate the potential of Parkin as a potential E3 ligase for PROTACs, providing a new framework for developing tar-geted degraders via dynamic activation mechanisms.