Thermosome: A Group II Chaperonin of Archaea

Abstract

Chaperonin, heat shock protein 60, plays an important role in the proteostasis of cytosol. Chaperonins are divided into two groups: group I and group II. Group II chaperonins exist in eukaryotes and archaea, and these chaperonins are named CCT/TRiC and Thermosome, respectively. Group II chaperonins have almost the same structure as group I chaperonins. The main difference is the existence of a helical protrusion, which constitutes a built‐in lid of the cavity. Group II chaperonin captures an unfolded protein in the cavity in the open conformation and changes to the closed conformation in an ATP‐dependent manner, which triggers folding of the captured protein. As Thermosome is relatively stable and simple compared with CCT, the conformational change mechanism and also the interaction with co‐chaperone, Prefoldin, have been studied in detail using Thermosome. The conformational change procedure of Thermosome will give insights on protein folding mechanism by group II chaperonins. Key Concepts Group II chaperonin is an essential cytosolic molecular chaperone in eukaryotes and archaea. The archaeal group II chaperonin is named Thermosome. Group II chaperonin has a built‐in lid for the central cavity. As ATP induces a conformational change from the open to the closed conformation, twisting of the ring occurs. Group II chaperonin cooperates with a co‐chaperone, Prefoldin, which captures an unfolded protein and transfers it to the central cavity of the group II chaperonin.