Crystal structure of E. coli GroEL and GroES complex. (a) The overall structure of GroEL/GroES complex reported by Xu et al. (PDB ID: 1AON) [42]. The GroES molecule is represented by orange surface. One GroEL monomer in the GroEL top (cis) ring is displayed as ribbon, with the apical, intermediate, and equatorial domains colored in blue, green, and red, respectively. The rest of the top (cis) ring and the entire bottom (trans) ring are shown as grey and cyan surface representations, respectively. (b) Ribbon representation of the GroEL heptameric bottom (trans) ring. Each GroEL monomers is shown in a different color.