Two components of a large (20 nm) dustbin shaped complex that helps to fold proteins. It is a chaperonin with two heptameric rings and a sevenfold lid (for 14-28 parts in all). The barrels have a space inside to hold an unfolded protein, although only one at a time - despite the two cavities. The whole machine goes through a cycle of binding, release, binding until the protein substrate is folded correctly.

Exactly how the complex catalyses folding is not 100% clear, but it is thought to involve a 'pulling apart' of the misfolded substrate. Instead of guiding the fold, it provides a microenvironment for the protein to do its thing. Sort of infinite dilution or affinsen's cage. To achieve this, the internal facets of the subunits grip the backbone and wrench the mess apart and then turn through 90° to form a smooth walled cavity. This turning is activated by ATP and the cap binding. See also: