Structural insights into the ubiquitin independent MIDN-proteasome pathway

The protein midnolin (MIDN) augments proteasome activity in lymphocytes, where it is predominantly expressed in adult mice. MIDN binds both to proteasomes and to substrates, but the mechanism by which MIDN facilitates substrate degradation in a ubiquitin-independent manner is unknown. Here, we present cryo-electron microscopy 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. By simultaneously binding to RPN1 with its C-terminal -helix, MIDN positions its substrate-carrying Catch domain above the proteasome channel through which substrates are translocated before degradation. Our findings suggest that both ubiquitin-like domain and C-terminal -helix must bind to the proteasome for MIDN to stimulate proteasome activity.