Location of a folding protein and shape changes in GroEL–GroES complexes imaged by cryo-electron microscopy

Abstract

PROTEIN folding mediated by the molecular chaperone GroEL occurs by its binding to non-native polypeptide substrates and is driven by ATP hydrolysis¹. Both of these processes are influenced by the reversible association of the co-protein, GroES (refs 2–4). GroEL and other chaperonin 60 molecules⁵ are large, cylindrical oligomers consisting of two stacked heptameric rings of subunits^6,7; each ring forms a cage-like structure⁸ thought to bind polypeptides in a central cavity^8–10. Chaperonins play a passive role in folding by binding or sequestering folding proteins to prevent their aggregation^11–13, but they may also actively unfold substrate proteins trapped in misfolded forms, enabling them to assume productive folding conformations^14–16. Biochemical studies show that GroES improves the efficiency of GroEL function^2,3,17, but the structural basis for this is unknown. Here we report the first direct visualization, by cryo-electron microscopy, of a non-native protein substrate (malate dehydrogenase) bound to the mobile, outer domains at one end of GroEL. Addition of GroES to GroEL in the presence of ATP causes a dramatic hinge opening of about 60°. GroES binds to the equivalent surface of the GroEL outer domains, but on the opposite end of the GroEL oligomer to the protein substrate.