Internal symmetry of the molecular chaperone cpn60 (GroEL) determined by X-ray crystallography

The Escherichia coli molecular chaperone cpn60 oligomer, [cpn60]₁₄, also called GroEL, has been crystallized and examined by X-ray crystallography and self-rotation function calculations. The crystals show unit-cell dimensions a = 143.3, b = 154.6 and c = 265 Å, with α = 82, β = 95 and γ = 107°. The space group is P1 and crystals diffract to 7 Å. X-ray analysis shows that the oligomer has one sevenfold symmetry axis and seven twofold axes that are all perpendicular to the sevenfold. The symmetry suggests that [cpn60]₂₄ consists of two heptamers, [cpn60]₇, stacked on top of each other. The orientations of the symmetry axes of the two independent [cpn60]₁₄ oligomers in the triclinic unit cell have been determined relative to the crystallographic axes. The two oligomers in the unit cell are arranged side-by- side, but the second oligomer is rotated 26° around the sevenfold axis relative to the first oligomer.