Jonathan S. Weissman
A COHERENT MODEL FOR A GROEL-GROES MEDIATED PROTEIN FOLDING REACTION
Chaperonins are large ring-shaped protein complexes which use the energy of ATP hydrolysis to increase the efficiency of protein folding in the cell. The best characterized chaperonin is the GroEL molecule of E. coli. Full function of GroEL requires the presence of the co-chaperonin molecule GroES which is also a ring structure. We present here a model for a GroEL-GroES mediated folding reaction based on the available experimental data: First, an asymmetric GroEL-GroES complex, in which GroES is bound to one end of the GroEL cylinder, acts as the polypeptide acceptor state. Polypeptide binding occurs exclusively to the GroEL ring not occupied by GroES, resulting in the formation of a trans complex. While this binding might reduce the affinity of GroEL for nucleotide and GroES, order of addition experiments indicate that polypeptide binding alone does not obligatorily result in the release of GroES. Second, a single round of ATP hydrolysis in the GroEL ring occupied by polypeptide induces the release of GroES. Third, rebinding of GroES can now occur on the same GroEL ring as polypeptide, yielding a cis complex in which the polypeptide is sequestered under GroES in an enlarged GroEL central cavity. Fourth, 7 ATP molecules are bound by the unoccupied GroEL ring. Nucleotide binding has the dual effect of initiating the folding reaction in the sequestered polypeptide and starting a "timer" for the release of GroES. Fifth, after 15-30 seconds the "timer" goes off resulting in the release of GroES driven by hydrolysis of ATP. Finally, the release of GroES allows the polypeptide to exit the GroEL central cavity. The fraction of substrate molecules that have achieved sufficient native structure prior to release go on to complete folding and/or oligomerization in solution. The remaining molecules are rapidly rebound by the same or different GroEL complexes. This rebinding acts to unfold any incorrect structure in kinetically trapped intermediates and to reinitiate the folding cycle.