Superantigen Natural Affinity Maturation Revealed by the Crystal Structure of Staphylococcal Enterotoxin G and Its Binding to T-cell Receptor Vbeta8.2

MM FERNÁNDEZ, S BHATTACHARYA, MC DE MARZI, PH BROWN, M KERZIC, P SCHUCK, RA MARIUZZA, EL MALCHIODI

PROTEINS: STRUCTURE, FUNCTION AND GENETICS

Wiley-Liss

Lugar: United States; Año: 2006

0887-3585

The illnesses associated with bacterial superantigens (SAgs) such as food poisoning and toxic shock syndrome, as well as the emerging threat of purpura fulminans and community-associated methicillin-resistant S. aureus producer of SAgs, emphasize the importance of a better characterization of SAg binding to their natural ligands, which would allow the development of drugs or biological reagents able to neutralize their action. SAgs are toxins that bind major histocompatibility complex class II molecules (MHC-II) and T-cell receptors (TCR), in a non-conventional manner, inducing Tcell activation that leads to production of cytokines such as tumor necrosis factor and interleukin-2, which may result in acute toxic shock. Previously, we cloned and expressed a new natural variant of staphylococcal enterotoxin G (SEG) and evaluated its ability to stimulate in vivo murine T-cell subpopulations. We found an early, strong and widespread stimulation of mouse Vb8.2 T-cells when compared with other SAgs member of the SEB subfamily. In search for the reason of the strong mitogenic potency, we determined the SEG crystal structure by X-ray crystallography to 2.2 A resolution and analyzed SEG binding to mVb8.2 and MHC-II. Calorimetry and SPR analysis showed that SEG has an affinity for mVb8.2 40 to 100-fold higher than that reported for other members of SEB subfamily, and the highest reported for a wild type SAg-TCR couple. We also found that mutations introduced in mVb8.2 to produce a high affinity mutant for other members of the SEB subfamily do not greatly affect binding to SEG. Crystallographic analysis and docking into mVb8.2 in complex with SEB, SEC3 and SPEA showed that the deletions and substitution of key amino acids remodeled the putative surface of the mVb8.2 binding site without affecting the binding to MHC-II. This results in a SAg with improved binding to its natural ligands, which may confer a possible evolutionary advantage for bacterial strains expressing SEG.