Unliganded GroEL at 2.8 A: structure and functional implications.

The three-dimensional structure of the E. coli chaperonin, GroEL, has been determined crystallographically and refined to 2.7 A in two crystal forms: an orthorhombic form from high salt and a monoclinic form from polyethylene glycol. The former is ligand free, the latter is both liganded with ATP analogues and ligand free. These structures provide a structural scaffold upon which to interpret extensive mutagenesis and biochemical studies. GroEL contains two sevenfold rotationally symmetric rings of identical 547-amino acid subunits. The rings are arranged 'back-to-back' with exact dyad symmetry to form a stubby cylinder that is 146 A high with an outer diameter of about 143 A. The cylinder has a substantial central channel that is unobstructed for the entire length of the cylinder and has a diameter of about 45 A except for large bulges that lead into a sevenfold symmetric array of elliptical side windows in each ring. Each subunit is composed of three distinct domains: (i) an 'equatorial' domain that contains the N- and C-terminus and the ATP-binding pocket, (ii) an 'apical domain' that forms the opening of the central channel and contains poorly ordered segments that mutational studies implicate in binding unfolded polypeptides and GroES, and (iii) an intermediate domain tht connects the other two domains and may serve to transmit allosteric adjustments.