Structural insights into the ubiquitin-independent midnolin-proteasome pathway
Nagesh Peddada, Xue Zhong, Yan Yin, Danielle Renee Lazaro, Jianhui Wang, Stephen Lyon, Jin Huk Choi, Xiao‐chen Bai, Eva Marie Y. Moresco, Bruce Beutler
Abstract
The protein midnolin (MIDN) augments proteasome activity in lymphocytes and dramatically facilitates the survival and proliferation of B-lymphoid malignancies. MIDN binds both to proteasomes and to substrates, but the mode of interaction with the proteasome is unknown, and the mechanism by which MIDN facilitates substrate degradation in a ubiquitin-independent manner is incompletely understood. Here, we present cryoelectron microscopy (cryo-EM) structures of the substrate-engaged, MIDN-bound human proteasome in two conformational states. MIDN induces proteasome conformations similarly to ubiquitinated substrates by using its ubiquitin-like domain to bind to the deubiquitinase RPN11 (PSMD14). By simultaneously binding to RPN1 (PSMD2) with its C-terminal α-helix, MIDN positions its substrate-carrying Catch domain above the proteasome ATPase channel through which substrates are translocated before degradation. Our findings suggest that both ubiquitin-like domain and C-terminal α-helix must bind to the proteasome for MIDN to stimulate proteasome activity.