STRUCTURE OF MACROPHAGE MIGRATION INHIBITORY FACTOR FROM HUMAN LYMPHOCYTES. A.Nakagawa^*, H.Sugimoto^*, M.Suzuki^[[section]], I.Tanaka^* and J.Nishihira^{+ *}Division of Biological Scieces, Graduate School of Science, Hokkaido Univ., Sapporo 060, Japan, ^[[section]]Photon Factory, National Laboratory for High Energy Physics, Tsukuba 305, Japan, ⁺Central Research Institute, School of Medicine, Hokkaido University, Sapporo 060, Japan

The three-dimensional sructure of macrophage migration inhibitory factor (MIF) from human lymphocytes was solved at 2.1Å resolution by the molecular replacement. MIF was suggested to concentrate macrophages at the infection site and make them function in antigen processing and phagocytosis. MIF is also a potent activator of macrophages and is likely to be critical in cell-mediated immune host defenses. However, actual function and mechanism of macrophage migration inhibitaion of MIF is still unknown. We have reported the structure of MIF from rat liver and unexpected simularites between MIF and two isomerases (Suzuki et al., Nature Struct. Biol., in press, 1996). Crystal of rat-MIF contains one monomer, which consists of two [[beta]][[alpha]][[beta]] motifes aligned quasi two-fold symmetry, in an asymmetric unit. However, the crystal structure shows that the protein forms a trimer by inter-subunits [[beta]]-sheets. The MIF trimer, which we call trimeric [[beta]]-cage, is formed by three five-stranded (one [[beta]]-strand comes from a next monomer) [[beta]]-sheets, which surrounded by six [[alpha]]-helices. The electron density of MIF from rat liver does not show clear electron density at the carboxyl terminal, and the model of rat-MIF doesn't have C-terminal eleven residues.

The space group of crystal of the MIF from human lymphocytes is P2₁2₁2₁(a=68.4,b=68.8,c/i>=86.8Å). This crystal contains one MIF trimer in an asymmetric unit, and we used rat-MIF trimer structure as a starting model for molecular replacement. Rotation and translation searches were calculated by X-PLOR (Brünger, X-PLOR version 3.1 Manual, 1993). After the first cycle of simulated anneaing refinent, the (2Fo-Fc)-map showed a clear electron density at the C-terminal region. The structure of human-MIF has additional two [[beta]]-strands, those make [[beta]]-sheet structure with neighbouring MIF monomer. Thus, the structure of human-MIF has seven-stranded [[beta]]-sheets, which consist from three monomers. The crystallographic R-factor of the current structure is 24% (R_Free=29%) at 2.1Å, and refinement is in progress.