cpn60 chaperonin family. Chaperonins are involved in productive folding of proteins. They share a common general morphology, a double toroid of 2 stacked rings. Archaeal cpn60 (thermosome), together with TF55 from thermophilic bacteria and the eukaryotic cytosol chaperonin (CTT), belong to the type II group of chaperonins. Cpn60 consists of two stacked octameric rings, which are composed of one or two different subunits. Their common function is to sequester nonnative proteins inside their central cavity and promote folding by using energy derived from ATP hydrolysis.