The crystal structure of Escherichia coli GroEL shows a porous cylinder of 14 subunits made of two nearly 7-fold rotationally symmetrical rings stacked back-to-back with dyad symmetry. The subunits consist of three domains : a targe equatorial domain that forms the foundation of the assembly at its waist and holds the rings together; a large loosely structured apical domain that forms the ends of the cylinder; and a small slender intermediate domain that connects the two, creating side windows. The three-dimensional structure places most of the mutationally defined functional sites on the channel walls and its outward invaginations, and at the ends of the cylinder.