Chaperonins are large ring assemblies that assist protein folding to the native state by
binding nonnative proteins in their central cavities and then, upon binding ATP, release the
substrate protein into a now-encapsulated cavity to fold productively. Two families of such
components have been identified: type I in mitochondria, chloroplasts, and the bacterial
cytosol, which rely on a detachable “lid” structure for encapsulation, and type II in archaea
and the eukaryotic cytosol, which contain a built-in protrusion structure. We discuss here a …