LOCATION OF A FOLDING PROTEIN AND SHAPE CHANGES IN GROEL-GROES COMPLEXES IMAGED BY CRYOELECTRON MICROSCOPY

PROTEIN folding mediated by the molecular chaperone GroEL occurs by it s binding to non-native polypeptide substrates and is driven by ATP hy drolysis(1). Both of these processes are influenced by the reversible association of the co-protein, GroES (refs 2-4). GroEL and other chape ronin 60 molecules(5) are large, cylindrical oligomers consisting of t wo stacked heptameric rings of subunits(6,7); each ring forms a cage-l ike structure(8) thought to bind polypeptides in a central cavity(8-10 ). Chaperonins play a passive role in folding by binding or sequesteri ng folding proteins to prevent their aggregation(11-13), but they may also actively unfold substrate proteins trapped in misfolded forms, en abling them to assume productive folding conformations(14-16). Biochem ical 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 p resence of ATP causes a dramatic hinge opening of about 60 degrees. Gr oES binds to the equivalent surface of the GroEL outer domains, but on the opposite end of the GroEL oligomer to the protein substrate.