Crystallization and preliminary X-ray investigation of the Escherichia coli molecular chaperone cpn60 (GroEL)

The Escherichia coli molecular chaperone, cpn60 (GroEL), has been purified from an overproducing E. coli strain and crystallized. Of the two crystal forms that were obtained, one was found to be suitable for crystallographic and structural studies at low resolution. Preliminary X-ray investigation of the crystals show unit cell dimensions: a = 143.3, b = 154.6 and c = 265 A, with alpha = 82 degrees, beta = 95 degrees and gamma = 107 degrees. The space group is P1 and the crystals diffract to a maximum of 7 A when using CuK alpha X-rays from a rotating anode. Both electron microscopy and non-denaturing electrophoretic analysis of redissolved cpn60 crystals show that cpn60 crystallizes in the native oligomeric form. Comparison between the dimensions of oligomeric cpn60 and the crystallographic unit cell volume suggests that the unit cell contains two oligomeric cpn60 molecules. The VM value for two cpn60 molecules per unit cell is 3.5 A3/Da, corresponding to a water content of 65%. Electrophoretic analysis under denaturing conditions shows that the cpn60 in crystals is heterogeneous, and this probably explains the limited resolution of the diffraction data.