Flexible interwoven termini determine the thermal stability of thermosomes ESEARCH ARTICLE R

Group II chaperonins, which assemble as double-ring complexes, assist in the refolding of nascent peptides or denatured proteins in an ATP-dependent manner. The mo- lecular mechanism of group II chaperonin assembly and thermal stability is yet to be elucidated. Here, we selected the group II chaperonins (cpn-α and cpn-β), also called thermosomes, from Acidianus tengchongensis and in- vestigated their assembly and thermal stability. We found that the binding of ATP or its analogs contributed to the successful assembly of thermosomes and enhanced their thermal stabilities. Cpn-β is more thermally stable than cpn-α, while the thermal stability of the hetero thermo- some cpn-αβ is intermediate. Cryo-electron microscopy reconstructions of cpn-α and cpn-β revealed the interwo- ven densities of their non-conserved fl exible N/C-termini around the equatorial planes. The deletion or swapping of their termini and pH-dependent thermal stability assays revealed the key role of the termini electrostatic interac- tions in the assembly and thermal stability of the ther- mosomes.