Mapping the energy of superantigen Staphylococcus enterotoxin C3 recognition of an alpha/beta T cell receptor using alanine scanning mutagenesis

Binding of the T cell receptor (TCR) to a bacterial superantigen (SAG) resu lts in stimulation of a large population of T cells and subsequent inflamma tory reactions. To define the functional contribution of TCR residues to SA G recognition, binding by 24 single-site alanine substitutions in the TCR V beta domain to Staphylococcus aureus enterotoxin (SE) C3 was measured, pro ducing an energy map of the TCR-SAG interaction. The results showed that co mplementarity determining region 2 (CDR2) of the V beta contributed the maj ority of binding energy, whereas hypervariable region 4 (HV4) and framework region 3 (FR3) contributed a minimal amount of energy. The crystal structu re of the V beta 8.2-SEC3 complex suggests that the CDR2 mutations act by d isrupting V beta main chain interactions with SEC3, perhaps by affecting th e conformation of CDR2. The finding that single V beta side chain substitut ions had significant effects on binding and that other SECS-reactive V beta are diverse at these same positions indicates that SECS binds to other TCR s through compensatory mechanisms. Thus, there appears to be strong selecti ve pressure on SAGs to maintain binding to diverse T cells.