Three conformations of an archaeal chaperonin, TF55 from Sulfolobus shibatae

Chaperonins are cylindrical, oligomeric complexes, essential for viability and required for the folding of other proteins. The GroE (group I) subfamil y, found in eubacteria, mitochondria and chloroplasts, have 7-fold symmetry and provide an enclosed chamber for protein subunit folding. The central c avity is transiently closed by interaction with the co-protein, GroES. The most prominent feature specific to the group II subfamily, found in archaea and in the eukaryotic cytosol, is a long insertion in the substrate-bindin g region. In the archaeal complex, this forms an extended structure acting as a built-in lid, obviating the need for a GroES-like cofactor. This exten sion occludes a site known to bind non-native polypeptides in GroEL. The si te and nature of substrate interaction are not known for the group II subfa mily. The atomic structure of the thermosome, an archaeal group II chaperon in, has been determined in a fully closed form, but the entry and exit of p rotein substrates requires transient opening. Although an open form has bee n investigated by electron microscopy, conformational changes in group II c haperonins are not well characterized. Using electron cryo-microscopy and t hree-dimensional reconstruction, we describe three conformations of a group ii chaperonin, including an asymmetric, bullet-shaped form, revealing the range of domain movements in this subfamily. (C) 2000 Academic Press.