Structural and Functional Role of Threonine 112 in a SuperantigenStaphylococcus aureus Enterotoxin B*

Bacterial superantigens are potent T-cell stimulatory protein molecules produced by Staphylococcus aureus and Streptococcus pyogenes. Their superantigenic activity can be attributed to their ability to cross-link major histocompatibility complex class II molecules with T-cell receptors (TCRs) to form a tri-molecular complex. Each superantigen is known to interact with a specific Vβelement of TCR. Staphylococcal enterotoxin B (SEB, a superantigen), a primary cause of food poisoning, is also responsible for a significant percentage of non-menstrual associated toxic shock syndrome in patients with a variety of staphylococcal infections. Structural studies have elucidated a binding cavity on the toxin molecule essential for TCR binding. To understand the crucial residues involved in binding, mutagenesis analysis was performed. Our analysis suggest that mutation of a conserved residue Thr112 to Ser (T112S) in the binding cavity induces a selective reduction in the affinity for binding one TCR Vβ family and can be attributed to the structural differences in the native and mutant toxins. We present a detailed comparison of the mutant structure determined at 2.0 Å with the previously reported native SEB and SEB-TCR Vβ complex structures.