(A) The bacterial chaperonin complex, based on X-ray crystallography. The GroEL rings (gold) are capped by another complex component, GroES (white). The open ring of the bottom cavity exposes at its terminal hydrophobic amino acid side chains — these capture non-native protein through their own exposed hydrophobic side chains. Such binding prevents non-native protein from aggregating. The top cavity is the site of protein folding. A protein released after initial binding in an open ring can fold in this space, which has hydrophilic walls, in solitary confinement, without the possibility of aggregation. This cage-like structure has been termed the “Anfinsen cage.” Figure reproduced with permission from Nature (15). (B) Schematic of protein folding within the GroEL-GroES complex. Image courtesy of F.-U. Hartl.