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

The formation of two-dimensional crystals of the membrane-bound enzyme micr osomal glutathione transferase is sensitive to fractional changes in the li pid-to-protein ratio. Variation of this parameter results in crystal polymo rphism. The projection structure of a p6 crystal form of the enzyme has bee n determined by the use of electron crystallography. The unit cell at 3 Ang strom resolution is comprised of two trimers. The hexagonal p6 and the orth orhombic p2(1)2(1)2 crystal types have common elements in the packing arran gement which imply dominant crystal contacts. An overall structural similar ity 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 pro jection maps identifies additional corresponding protein densities which co uld not be assigned to the microsomal glutathione transferase trimer previo usly. Surprisingly, an ambiguity of the rotational orientation was found fo r trimers interspersed at certain positions within the crystal lattice. (C) 1999 Academic Press.