A molecular plugin rescues GroEL/ES substrates from pre-folding oxidation

Abstract

Hsp60 chaperonins and their Hsp10 cofactors assist protein folding in all living cells, constituting the paradigmatic example of molecular chaperones. Despite extensive investigations of their structure and mechanism, crucial questions regarding how these chaperonins promote folding remain unsolved. Here, we report that the bacterial Hsp60 chaperonin GroEL forms a stable, functionally relevant complex with the chaperedoxin CnoX, a protein combining a chaperone and a redox function. Binding of GroES (Hsp10) to GroEL induces CnoX release. Cryo-electron microscopy provided crucial structural information on the GroEL-CnoX complex, showing that CnoX binds GroEL outside the substrate-binding site via a highly conserved C-terminal α-helix. Furthermore, the identification of complexes in which CnoX, bound to GroEL, forms mixed-disulfides with GroEL substrates indicates that CnoX likely functions as a redox quality-control plugin for GroEL. Proteins sharing structural features with CnoX exist in eukaryotes, which suggests that Hsp60 molecular plugins have been conserved through evolution.