Crystal structures of a group II chaperonin reveal the open and closed states associated with the protein folding cycle.

Chaperonins are large protein complexes consisting of two stacked multisubunit rings, which open and close in an ATP-dependent manner to create a protected environment for protein folding. Here, we describe the first crystal structure of a group II chaperonin in an open conformation. We have obtained structures of the archaeal chaperonin from Methanococcus maripaludis in both a peptide acceptor (open) state and a protein folding (closed) state. In contrast with group I chaperonins, in which the equatorial domains share a similar conformation between the open and closed states and the largest motions occurs at the intermediate and apical domains, the three domains of the archaeal chaperonin subunit reorient as a single rigid body. The large rotation observed from the open state to the closed state results in a 65% decrease of the folding chamber volume and creates a highly hydrophilic surface inside the cage. These results suggest a completely distinct closing mechanism in the group II chaperonins as compared with the group I chaperonins.