Structural Analysis of a Filamentous Chaperonin from Sulfolobus solfataricus

Chaperonins are biomolecular complexes that assist protein folding. Thermophilic Factor 55 (TF55) is a group II chaperonin found in the archaeal genus and which undergoes changes in modular subunit composition in a temperature-dependent manner. TF55 can form filamentous assemblies that may be a component of the archaeal cytoskeleton or sequester inactive chaperonin. Using cryo-electron microscopy, we have determined the structure of the β-only complex of TF55 complexes to 3.6 Å resolution and its filamentous form to 5.2 Å resolution. Filament formation can be induced when the protein is enriched in solution or in the presence of the detergent dodecyl maltoside. Helical protrusions in the apical domain facilitate end-on-end interactions in the filamentous state. Our findings establish the molecular basis for forming chaperonin filaments in and may suggest how filament formation could function as a cold-shock response and provides a background for generating tuneable protein nanowires.