THE OLIGOMERIC STRUCTURE OF GROEL GROES IS REQUIRED FOR BIOLOGICALLY SIGNIFICANT CHAPERONIN FUNCTION IN PROTEIN-FOLDING/

Two models are being considered for the mechanism of chaperonin-assist ed protein folding in E. coil: (i) GroEL/GroES act primarily by enclos ing substrate polypeptide in a folding cage in which aggregation is pr evented during folding. (ii) GroEL mediates the repetitive unfolding o f misfolded polypeptides, returning them onto a productive folding tra ck. Both models are not mutually exclusive, but studies with the polyp eptide-binding domain of GroEL have suggested that unfolding is the pr imary mechanism, enclosure being unnecessary. Here we investigate the capacity of the isolated apical polypeptide-binding domain to function ally replace the complete GroEL/GroES system. We show that the apical domain binds aggregation-sensitive polypeptides but cannot significant ly assist their refolding in vitro and fails to replace the groEL gene or to complement defects of groEL mutants in vivo. A single-ring vers ion of GroEL cannot substitute for GroEL. These results strongly suppo rt the view that sequestration of aggregation-prone intermediates in a folding cage is an important element of the chaperonin mechanism.