The structure of bovine interferon-gamma (IFN-gamma) was determined by multiple isomorphous replacement at 2.0 Angstrom resolution. Bovine IFN-gamma crystallizes in two related crystal forms. Crystal form 1 diffracts to 2.9 Angstrom resolution and is reproducible and stable to derivatization. Crystal form 2 diffracts to 2.0 Angstrom resolution, but shows significant non-isomorphism from crystal to crystal. The previously determined structures of several different species of INF-gamma were either at too low a resolution [human, 1hig; Ealick et nl. (1991), Science, 252, 698-702] or were too inaccurate [bovine, 1rfb; Samudzi & Rubin (1993), Acta Cryst. D49(6), 505-512; rabbit, 2rig; Samudzi ct al. (1991), J. Biol. Chem. 266(32), 21791-21797] for the structure to be solved by molecular replacement. The structure was solved in crystal form 1 using two derivatives produced by chemically modifying two free cysteine residues that were introduced by site-directed mutagenesis (Ser30Cys, Asn59Cys). After model building and refinement, the final R value was 21.8% (R-free = 30.9%) for all data in the resolution range 8.0-2.9 Angstrom. The crystal form 1 structure was then used as a molecular-replacement model for crystal form 2 data collected from a flash-cooled crystal. Subsequent model building and refinement, using all data in the resolution range 15.0-2.0 Angstrom, gave an R value of 19.7% and an R-free of 27.5%. Painwise comparison of C-alpha positions of bovine IFN-gamma (BOV) and the previously determined 1rfb and 2rig structures indicated some significant differences in the models (r.m.s.d. values for BOV to 1rfb, 4.3 Angstrom; BOV to 2rig, 4.0 Angstrom). An assessment of the quality of the structures was made using the 3D-1D algorithm [Eisenberg fr al. (1992), Faraday Discuss. 93, 25-34]. The resulting statistical scoring indicated that BOV was consistent with expected criteria for a 2.0 Angstrom structure, whereas both 1rfb and 2rig fell below acceptable criteria.
