CONFORMATIONAL CYCLE OF THE ARCHAEOSOME, A TCP1-LIKE CHAPERONIN FROM SULFOLOBUS-SHIBATAE

The major heat shock proteins in the archaeon Sulfolobus shibatae are similar to the cytosolic eukaryotic chaperonin and form an 18-subunit bitoroidal complex. Two sequence-related subunits constitute a functio nal complex, named the archaeosome. The archaeosome exists in two dist inct conformational states that are part of chaperonin functional cycl e. The closed archaeosome complex binds ATP and forms an open complex. Upon ATP hydrolysis, the open complex dissociates into subunits, Free subunits reassemble into a two-ring structure. The equilibrium betwee n the complexes and free subunits is affected by ATP and temperature. Denatured proteins associate with both conformational states as well a s with free subunits that form an intermediate complex. These unexpect ed observations suggest a new mechanism of archaeosome-mediated thermo tolerance and protein folding.