The 3.0 A projection structure of microsomal glutathione transferase as determined by electron crystallography of p 21212 two-dimensional crystals

Abstract

Two-dimensional crystals of rat microsomal glutathione transferase were grown during dialysis of detergent-solubilized enzyme after addition of a small amount of phospholipid. The crystals had two-sided plane group symmetry p21212 with a calibrated unit cell size of a=91.90 A, b=90.83 A. Electron diffraction patterns were recorded showing significant reflections extending to 3.0 A. A combination of these structure factor amplitudes with phases from high-resolution images following image processing was used to calculate a projection map of the protein. The asymmetric unit of the structure consists of three microsomal glutathione transferase molecules. The local 3-fold axis at the center of the trimer is delineated by six parallel alpha-helices, two from each monomer. The two helices differ significantly in their respective projection structure. The inner helical core of the trimer is partly surrounded by elongated domains with extensions towards the helices and which contain resolved density maxima at a spacing of 4 to 5 A. A well-defined strong peak is localized close to the elongated domain and at a distance of about 9.5 A from two of the inner helices.