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Structures of the 26S proteasome in complex with the Hsp70 cochaperone Bag1 reveal a novel mechanism for direct transfer of substrates to the catalytic chamber

ABSTRACT: Coupling of the degradation and chaperone systems, particularly under cellular stress, is essential for eliminating unfolded proteins. The co-chaperone Bag1 links the Hsp70 chaperone to the 26S proteasome, recruiting Hsp70-bound unfolded proteins for proteasomal degradation. Here, we present cryo-EM 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 reconfiguration generates a large cavity above the substrate entry gate of the 20S core particle (CP). Additionally, our structures highlight key roles for the conserved pore-2 loops of ATPases Rpt2 and Rpt5 in opening the gate of the 20S CP, enabling direct translocation of Hsp70 clients into the proteolytic chamber independently of ubiquitination. Thus, these findings suggest a new mechanism of the proteasomal degradation, by which remodeling the central cavity and 20S gate in the presence of Bag1, possibly bypassing the need for ubiquitination and ATPase motor function.

INSTRUMENT(S):

ORGANISM(S): Homo Sapiens (human) Saccharomyces Cerevisiae (baker's Yeast)

SUBMITTER: Jimena Muntaner

LAB HEAD: Jorge Cuellar

PROVIDER: PXD069071 | Pride | 2025-12-23

REPOSITORIES: Pride

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	PROT2385_JM06_B01.raw	Raw
	PROT2385_JM06_B01_MeroX.xlsx	Xlsx
	PROT2385_JM06_B01_XlinkX.xlsx	Xlsx
	PROT2385_JM06_B02.raw	Raw
	PROT2385_JM06_B02_MeroX.xlsm	Xls

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

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