Abstract
Chaperonins are biomolecular complexes that assist in protein folding. Thermophilic factor 55 (TF55) is a group II chaperonin found in the archaeal genus Sulfolobus that has α, β and γ subunits. Using cryo-electron microscopy, structures of the β-only complex of S. solfataricus TF55 (TF55β) were determined to 3.6-4.2 Å resolution. The structures of the TF55β complexes formed in the presence of ADP or ATP highlighted an open state in which nucleotide exchange can occur before progressing in the refolding cycle.