The GroEL/GroES chaperonin system of E. coli facilitates nucleotide dependent folding of select proteins. The structure of GroEL has been described by three-dimensional electron microscopy and at higher resolution by X-ray crystallography. The GroEL oligomer is a cylindrical tetradecamer composed of two heptameric rings of 57 kDa protein subunits, stacked back to back. Each subunit is comprised of two large domains, equatorial and apical, connected by a smaller intermediate domain. The intermediate “hinge” domain links the apical and equatorial domain and provides flexibility for en bloc rearrangement associated with nucleotide and GroES binding. Equatorial domains are responsible for interactions between the two heptamers and contain the ATPase activity of GroEL. Each ring of GroEL has a central cavity that is the binding site for denatured protein substrate. GroES and denatured substrate binding sites are located on the apical domains facing the central cavity.