CHAPERONIN FILAMENTS - THE ARCHAEAL CYTOSKELETON

Chaperonins are high molecular mass double-ring structures composed of 60-kDa protein subunits. In the hyperthermophilic archaeon Sulfolobus shibatae the two chaperonin proteins represent approximate to 4% of i ts total protein and have a combined intracellular concentration of >3 0 mg/ml. At concentrations greater than or equal to 0.5 mg/ml purified chaperonins form filaments in the presence of Mg2+ and nucleotides. F ilament formation requires nucleotide binding (not hydrolysis), and oc curs at physiological temperatures in biologically relevant buffers, i ncluding a buffer made from cell extracts. These observations suggest that chaperonin filaments may exist in vivo and the estimated 4600 cha peronins per cell suggest that such filaments could form an extensive cytostructure. We observed filamentous structures in unfixed, uranyl-a cetate-stained S. shibatae cells, which resemble the chaperonin filame nts in size and appearance. ImmunoGold (Janssen) labeling using chaper onin antibodies indicated that many chaperonins are associated with in soluble cellular structures and these structures appear to be filament ous in some areas, although they could not be uranyl-acetate-stained. The existence of chaperonin filaments in vivo suggests a mechanism whe reby their protein-folding activities can be regulated. More generally , the filaments themselves may play a cytoskeletal role in Archaea.