Structural dynamics of the midnolin-proteasome during ubiquitin-independent substrate turnover

Area of Science:

• Molecular Biology
• Structural Biology
• Biochemistry

Background:

• The 26S proteasome primarily degrades ubiquitinated proteins.
• A ubiquitin-independent pathway for nuclear protein degradation mediated by midnolin is known but poorly understood.

Purpose of the Study:

• To elucidate the molecular mechanism of midnolin-mediated ubiquitin-independent protein degradation by the 26S proteasome.
• To provide structural insights into the interaction between midnolin and the 26S proteasome.

Main Methods:

• Cryo-electron microscopy (cryo-EM) to determine structures of the human 26S proteasome in complex with midnolin.
• Structural analysis of nine distinct conformations.

Main Results:

• Nine cryo-EM structures reveal midnolin binding to the RPN1 subunit via its C-terminal helix.
• Midnolin's ubiquitin-like domain interacts non-catalytically with RPN11, positioning the substrate-binding Catch domain.
• Observed spiral staircase conformations of the AAA+ ATPase hexamer during substrate translocation.

Conclusions:

• The study provides a near-complete catalytic cycle of midnolin-mediated degradation.
• Findings offer mechanistic insights into ubiquitin-independent nuclear protein degradation.
• Potential strategies for targeting nuclear proteins through direct proteasomal degradation can be developed.