Cooperative zinc binding in a staphylococcal enterotoxin A mutant mimics the SEA-MHC class II interaction

The structure of a mutant form of staphylococcal enterotoxin A (SEA) has be en determined to 2.1 Angstrom resolution. The studied SEA substitution H187 --> A187 (SEA(H187A)) leads to an almost 110-fold reduction of the binding to major histocompatibility complex (MHC) class II. H187 is important for this interaction since it coordinates Zn2+. The zinc ion is thought to hold MHC class II and SEA together in a complex. Interestingly, only one of two molecules in the asymmetric unit binds Zn2+. H225, D227, a water molecule, and H44 from a symmetry-related molecule ligate Zn2+. The symmetry-related histidine is necessary for this substituted Zn2+ site to bind to Zn2+ at l ow zinc concentration (no Zn2+ added). Since a water Molecule replaces the missing H187, H44 binds Zn2+ at the position where beta H81 frorm MHC class II probably will bind. Dynamic light scattering analysis reveals that in s olution as well as in the crystal lattice the SEA(H187A) mutant forms aggre gates. The substitution per se does not cause aggregation since wild-type S EA also forms aggregates. Addition of EDTA reduces the size of the aggregat es, indicating a cross-linking function of Zn2+. In agreement with the biol ogical function, the aggregation is weak (i.e. not revealed by gel filtrati on,) and non-specific.