Mapping the Energy of Superantigen Staphylococcus Enterotoxin C3 Recognition of an α/β T Cell Receptor Using Alanine Scanning Mutagenesis

Binding of the T cell receptor (TCR) to a bacterial superantigen (SAG) results in stimulation of a large population of T cells and subsequent inflammatory reactions. To define the functional contribution of TCR residues to SAG recognition, binding by 24 single-site alanine substitutions in the TCR Vβ domain to Staphylococcus aureus enterotoxin (SE) C3 was measured, producing an energy map of the TCR–SAG interaction. The results showed that complementarity determining region 2 (CDR2) of the Vβ contributed the majority of binding energy, whereas hypervariable region 4 (HV4) and framework region 3 (FR3) contributed a minimal amount of energy. The crystal structure of the Vβ8.2–SEC3 complex suggests that the CDR2 mutations act by disrupting Vβ main chain interactions with SEC3, perhaps by affecting the conformation of CDR2. The finding that single Vβ side chain substitutions had significant effects on binding and that other SEC3-reactive Vβ are diverse at these same positions indicates that SEC3 binds to other TCRs through compensatory mechanisms. Thus, there appears to be strong selective pressure on SAGs to maintain binding to diverse T cells.