THE CO-CRYSTAL STRUCTURE OF STAPHYLOCOCCAL-ENTEROTOXIN TYPE-A WITH ZN2- IMPLICATIONS FOR MAJOR HISTOCOMPATIBILITY COMPLEX CLASS-II BINDING(AT 2.7 ANGSTROM RESOLUTION )

Superantigens form complexes with major histocompatibility complex (MH C) class II molecules and T-cell receptors resulting in extremely stro ng immunostimulatory properties. Staphylococcus aureus enterotoxin A ( SEA) belongs to a subgroup of the staphylococcal superantigens that ut ilizes Zn2+ in the high affinity interaction with MHC class II molecul es. A high affinity metal binding site was described previously in SEA cocrystallized with Cd2+ in which the metal ion was octahedrally co-o rdinated, involving the N-terminal serine. We have now co-crystallized SEA with its native cofactor Zn2+ and determined its crystal structur e at 2.1 Angstrom resolution. As expected for a Zn2+ ion, the co ordin ation was found to be tetrahedral. Three of the ligands are located on the SEA surface on a C-terminal domain beta-sheet, while the fourth v aries with the conditions. Further analysis of the zinc binding event was performed using titration microcalorimetry, which showed that SEA binds Zn2+ with an affinity of K-D = 0.3 mu M in an entropy driven pro cess. The differential Zn2+ co-ordination observed here has implicatio ns for the mechanism of the SEA-MHC class II interaction.