Piotr Drączkowski, Szu-Ni Chen, Ting Chen, Yong‐Sheng Wang, Hsin-An Shih, Jessica Y. C. Huang, Ming-Chieh Tsai, Shu‐Yu Lin, Steven Lin, Rosa Viner, Yuan‐Chih Chang, Kuen‐Phon Wu, Shang‐Te Danny Hsu
Beyond the canonical K48-linked homotypic polyubiquitination for proteasome-targeted proteolysis, K11/K48-branched ubiquitin (Ub) chains are involved in fast-tracking protein turnover during cell cycle progression and proteotoxic stress. Here, we report cryo-EM structures of human 26S proteasome in a complex with a K11/K48-branched Ub chain. The structures revealed a multivalent substrate recognition mechanism involving a hitherto unknown K11-linked Ub binding site at the groove formed by RPN2 and RPN10 in addition to the canonical K48-linkage binding site formed by RPN10 and RPT4/5 coiled-coil. Additionally, RPN2 recognizes an alternating K11-K48-linkage through a conserved motif similar to the K48-specific T1 binding site of RPN1. The insights gleaned from these structures explain the molecular mechanism underlying the recognition of the K11/K48-branched Ub as a priority signal in the ubiquitin-mediated proteasomal degradation. K11/K48 branched ubiquitin chains regulate protein degradation and cell cycle progression. Here, the authors report the structural basis of how such a branched ubiquitin chain is recognized by the human 26S proteasome, revealing a multivalent binding mode that underlies selective recognition.