METAL-BINDING IN SUPERANTIGENS. E.M. Schad¹, M.Dohlsten², Per Björk², L.A. Svensson¹. Molecular Biophysics, Chemical Center, Lund University, P.O. Box 124, 221 00 Lund, Sweden¹, Pharmacia Oncology Immunology, Lund, Sweden².

One of the most striking aspects of the structure of SEA is the unusual octahedral metal-coordination geometry. In addition, SEA displays a N-terminal coordination to the metal-ion (Schad et al., 1995). The coordinating residues of Zn²⁺ are the same ligands as those found for Cd²⁺ including the N-terminal coordination. The unusual ligand coordination by the N-terminal serine residue observed is comparable to the coordination found in the structure of phospholipase C (Hough et al., 1989; Hansen et al., 1992). In SEA, the metal-ion is coordinated by a primary bidentate formed by ligands His 225 and Asp 227. These two ligands are separated by a short spacer that according to Vaillee & Auld (1990) provides localized and overall stabilization to the protein. A longer spacer provided by His 187 donates flexibility to the coordination site. It should be noted that zinc binding proteins commonly have a tetrahedral geometry with this short spacer-long spacer ligand composition (Vallee & Auld, 1990). Alanine substitution of His 225 and Asp 227 resulted in a more than 1000-fold reduced MHC class II binding affinity, whereas the His 187 mutation displayed only a 100-fold reduced binding affinity (Abrahmsen et al., 1995). This suggests that modification of the short spacer bidentate formed by His 225 and Asp 227 severely affects the MHC class II binding to domain II. In contrast, mutations of the longer spacer His 187 have less of an effect on the other metal ligands in retaining significant MHC class II binding in this region. This is further supported by the varying temperature factors observed in the metal coordination site mentioned previously.