Structural and functional role of threonine 112 in a superantigen Staphylococcus 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 11 molecules with T-cell receptors (TCRs) to form a tri-molecular complex. Each superantigen is known to interact with a specific V-beta element of TCR. Staphylococcal enterotoxin B (SEB, a superantigen), a primary cause of food poisoning, is also responsible for a significant percentage of nonmenstrual 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 Thr(112) to Ser (T112S) in the binding cavity induces a selective reduction in the affinity for binding one TCR V-beta 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 Angstrom with the previously reported native SEB and SEB-TCR V-beta complex structures.