STRUCTURE OF A MOLECULAR CHAPERONE FROM A THERMOPHILIC ARCHAEBACTERIUM

WE recently reported that a wide range of thermophilic archaebacteria have a novel ATPase complex which accumulates to high levels upon heat shock and may be a new type of molecular chaperone related to the chaperonins1. Striking similarities between the complex, referred to here as the 'thermosome', and the subsequently characterized chaperones TF55 of the thermophilic archaebacterium Sulfolobus shibatae2 and TCP-1 from eukaryotic cytosol2-5 suggest that these proteins belong to a single family. Here we determine the three-dimensional structure of the thermosome from Pyrodictium occultum by random conical tilt reconstruction from electron micrographs. The reconstruction reveals a complex consisting of two rings of eight subunits each, stacked face-to-face. The subunits are kidney-shaped and composed of at least two domains. In the centre of the thermosome is a large cavity of about 6.7 nm diameter; the opening of this cavity on each face of the complex is partially blocked by a mass which appears to be weakly connected to the eight-membered ring.