Structures of the 26S proteasome in complex with the Hsp70 cochaperone Bag1 reveal a novel mechanism of ubiquitin-independent proteasomal degradation

The 26S proteasome primarily degrades proteins marked by polyubiquitin chains. Although ubiquitin-independent pathways for proteasomal degradation exist, the mechanisms involved remain poorly understood. Bag1 links Hsp70 chaperone to the 26S proteasome, recruiting Hsp70-bound aberrant proteins for degradation. Here, we show that Bag1 interacts with the 26S proteasome through its subunit Rpn1. Using cryo-electron microscopy, we present high-resolution structures of the Bag1-bound 26S proteasome, which reveal unprecedented conformational changes within the 19S regulatory particle. Bag1 binding to the Rpn1 subunit induces a dramatic reconfiguration of AAA+ ATPase subunits, disrupting the canonical spiral staircase conformation and remodeling the central channel architecture. This creates a large cavity above the substrate entry gate of the 20S catalytic chamber, enabling the direct entry of Hsp70 clients into the proteolytic chamber. Thus, in this ubiquitin-independent degradation pathway, unfolded proteins skip the need for both client ubiquitination and ATP hydrolysis for degradation.