Ubiquitin receptor–mediated, ubiquitin-independent targeted protein degradation via 26 <i>S</i> proteasomes

The 26 S proteasome engages with ubiquitinated substrates primarily through its constituent ubiquitin (Ub) receptors, which initiates a cascade of proteolytic processes. Leveraging this recognition mechanism, we developed a targeted protein degradation (TPD) strategy that recruits substrates directly to the proteasome, thereby bypassing the ubiquitination step. Our proteasome-targeting chimera, Protea-Tac, is a heterobifunctional protein degrader composed of a Ub receptor and an intracellular antibody. This chimera integrates into 26 S proteasomes without altering their structural or functional integrity. Protea-Tac with cognate antibodies degraded various target proteins, including c-Fos, BRD4, Flag TDP43, HA tau, and GFP ODC. We mechanistically demonstrated that this platform is (i) modular, allowing facile target switching, (ii) Ub independent, and (iii) highly target specific. Furthermore, Protea-Tac exhibited potent in vivo antitumor efficacy, posttranslationally inducing c-Fos degradation and substantially delaying tumor progression through both viral and nonviral delivery systems. These findings identify Protea-Tac as a distinct TPD platform capable of directly degrading intracellular proteins via engineered 26 S proteasomes.