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

Abstract

The structure of a mutant form of staphylococcal enterotoxin A (SEA) has been determined to 2.1 Å resolution. The studied SEA substitution H187→A187 (SEA_H187A) leads to an almost 10-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 low zinc concentration (no Zn²⁺ added). Since a water molecule replaces the missing H187, H44 binds Zn²⁺ at the position where βH81 from MHC class II probably will bind. Dynamic light scattering analysis reveals that in solution as well as in the crystal lattice the SEA_H187A mutant forms aggregates. The substitution per se does not cause aggregation since wild-type SEA also forms aggregates. Addition of EDTA reduces the size of the aggregates, indicating a cross-linking function of Zn²⁺. In agreement with the biological function, the aggregation is weak (i.e. not revealed by gel filtration) and non-specific.