Triggering of protein folding by the GroEL-GroES chaperonin system

Productive folding of many cellular proteins requires help from large, ring-shaped folding machines known as chaperonins. GroEL, the Escherichia coli chaperonin, is perhaps the best-studied member of this essential molecular chaperone family. GroEL mediated folding of many proteins involves the brief, ATP-dependent encapsulation of non-native substrate proteins inside an enclosed chamber formed when the cochaperonin GroES binds to GroEL. Precisely how the capture and encapsulation of a non-native protein within the GroEL-GroES cavity leads to stimulated protein folding remains poorly understood. Also unclear is specifically how the binding of non-native substrate protein, ATP and GroES are coordinated to allow for efficient protein encapsulation underneath the GroES lid.;The work in this thesis addresses both of these questions fundamental to understanding how GroEL functions as a folding machine. The mechanism of GroEL-mediated protein folding is shown to involve unfolding of misfolded intermediates. This work further shows that a GroEL ring transits from a substrate bound state to a conformation where the substrate is released into the GroEL cavity via a transient allosteric state that simultaneously binds GroES and the substrate protein. The ATP-driven formation of this conformation of the GroEL ring is necessary not only for ensuring that the substrate protein does not escape into the free solution before GroES binds, but also for the forced unfolding of the non-native substrate protein.