The projection structure of the membrane protein microsomal glutathione transferase at 3 Å resolution as determined from two-dimensional hexagonal crystals

The formation of two-dimensional crystals of the membrane-bound enzyme microsomal glutathione transferase is sensitive to fractional changes in the lipid-to-protein ratio. Variation of this parameter results in crystal polymorphism. The projection structure of a p6 crystal form of the enzyme has been determined by the use of electron crystallography. The unit cell at 3 Angstrom resolution is comprised of two trimers. The hexagonal p6 and the orthorhombic p2(1)2(1)2 crystal types have common elements in the packing arrangement which imply dominant crystal contacts. An overall structural similarity between the protein molecules in the two crystal forms is suggested by the projection maps. Furthermore, a comparison of the p6 and p2(1)2(1)2 projection maps identifies additional corresponding protein densities which could not be assigned to the microsomal glutathione transferase trimer previously. Surprisingly, an ambiguity of the rotational orientation was found for trimers interspersed at certain positions within the crystal lattice. (C) 1999 Academic Press.