A novel loop domain in superantigens extends their T cell receptor recognition site.

Superantigens (SAGs) interact with host immune receptors to induce a massive release of inflammatory cytokines that can lead to toxic shock syndrome and death. Bacterial SAGs can be classified into five distinct evolutionary groups. Group V SAGs are characterized by the α3-β8 loop, a unique ∼15 amino acid residue extension that is required for optimal T cell activation. Here, we report the X-ray crystal structures of the group V SAG staphylococcal enterotoxin K (SEK) alone and in complex with the TCR hVβ5.1 domain. SEK adopts a unique TCR binding orientation relative to other SAG–TCR complexes, which results in the α3-β8 loop contacting the apical loop of framework region 4, thereby extending the known TCR recognition site of SAGs. These interactions are absolutely required for TCR binding and T cell activation by SEK, and dictate the TCR Vβ domain specificity of SEK and other group V SAGs.