A MOLECULAR CHAPERONE FROM A THERMOPHILIC ARCHAEBACTERIUM IS RELATED TO THE EUKARYOTIC PROTEIN T-COMPLEX POLYPEPTIDE-1

THERE is evidence to suggest that components of archaebacteria are evolutionarily related to cognates in the eukaryotic cytosol 1-7. We postulated that the major heat-shock protein of the thermophilic archaebacterium, Sulfolobus shibatae, is a molecular chaperone and that it is related to an as-yet unidentified chaperone component in the eukaryotic cytosol. Acquired thermotolerance in S. shibatae correlates with the predominant synthesis of this already abundant protein, referred to as thermophilic factor 55 (TF55; ref. 8). TF55 is a homo-oligomeric complex of two stacked 9-membered rings, closely resembling the 7-membered-ring complexes of the chaperonins, groEL, hsp60 and Rubisco-binding protein 9-15. The TF55 complex binds unfolded polypeptides in vitro and has ATPase activity-features consistent with its being a molecular chaperone 16,17. The primary structure of TF55, however, is not significantly related to the chaperonins. On the other hand, it is highly homologous (36-40% identity) to a ubiquitous eukaryotic protein, t-complex polypeptide-1 (TCP1; refs 18-20). In Saccharomyces cerevisiae, TCP1 is an essential protein that may play a part in mitotic spindle formation 21. We suggest that TF55 in archaebacteria and TCP1 in the eukaryotic cytosol are members of a new class of molecular chaperones.