Svensson L A, Surin B P, Dixon N E, Spangfort M D
Department of Molecular Biophysics, University of Lund, Sweden.
J Mol Biol. 1994 Jan 7;235(1):47-52. doi: 10.1016/s0022-2836(05)80011-7.
The internal symmetries of the Escherichia coli molecular chaperone cpn60 oligomer, also called GroEL, have been examined by X-ray crystallography and self-rotation functions calculated at a resolution of 8.9 A. The oligomer ([cpn60]14) has one 7-fold symmetry axis and seven 2-fold axes that are all perpendicular to the 7-fold. The symmetry can be explained if oligomeric cpn60 is arranged as two heptamers stacked on top of each other, where the heptameric arrangement generates the 7-fold symmetry axis and the head-to-head assembly of two heptamers results in the seven 2-fold axes. This is an agreement with interpretations of electron microscopy data. However, the experimental determination of the symmetries reported here are made with an independent technique and at higher resolution. In addition self-rotation function calculations show that the symmetries observed are valid also for the internal parts of GroEL and not only for surface views. The orientations of the symmetry axes of the two independent cpn60 oligomers in the triclinic unit cell have been determined relative to the crystallographic axes. The planes formed by the 2-fold axes in the two oligomers deviate by about 2 degrees from the plane formed by the crystallographic a and c axes, while the 7-fold axes form angles of about 16 degrees with the b-axis. The two oligomers in the unit cell are arranged with their 7-fold axis parallel, but the second oligomer is rotated 26 degrees around the 7-fold axis relative to the first oligomer. Knowledge of the symmetry and orientation of the oligomers in the unit cell will be of great help in further crystallographic work.
通过X射线晶体学以及在8.9埃分辨率下计算的自旋转函数，对大肠杆菌分子伴侣cpn60寡聚体（也称为GroEL）的内部对称性进行了研究。该寡聚体（[cpn60]14）有一条7重对称轴和七条2重对称轴，它们都垂直于7重轴。如果寡聚cpn60排列为两个七聚体相互堆叠，其中七聚体排列产生7重对称轴，两个七聚体的头对头组装产生七条2重对称轴，那么这种对称性就可以得到解释。这与电子显微镜数据的解释一致。然而，这里报道的对称性的实验测定是用一种独立技术且在更高分辨率下进行的。此外，自旋转函数计算表明，观察到的对称性不仅对于GroEL的表面视图有效，对于其内部部分也有效。已经确定了三斜晶胞中两个独立cpn60寡聚体的对称轴相对于结晶轴的取向。两个寡聚体中由2重轴形成的平面与由结晶学a轴和c轴形成的平面相差约2度，而7重轴与b轴形成约16度的角。晶胞中的两个寡聚体以其7重轴平行排列，但第二个寡聚体相对于第一个寡聚体绕7重轴旋转了26度。了解晶胞中寡聚体的对称性和取向将对进一步的晶体学工作有很大帮助。
