Antigen Ligation Triggers a Conformational Change within the Constant Domain of the αβ T Cell Receptor

Ligation of the alpha beta T cell receptor (TCR) by a specific peptide-loaded major histocompatibility complex (pMHC) molecule initiates T cell signaling via the CD3 complex. However, the initial events that link antigen recognition to T cell signal transduction remain unclear. Here we show, via fluorescence-based experiments and structural analyses, that MHC-restricted antigen recognition by the alpha beta TCR results in a specific conformational change confined to the A-B loop within the alpha chain of the constant domain (Cot). The apparent affinity constant of this A-B loop movement mirrored that of alpha beta TCR-pMHC ligation and was observed in two alpha beta TCRs with distinct pMHC specificities. The Ag-induced A-B loop conformational change could be inhibited by fixing the juxtapositioning of the constant domains and was shown to be reversible upon pMHC disassociation. Notably, the loop movement within the C alpha domain, although specific for an agonist pMHC ligand, was not observed with a pMHC antagonist. Moreover, mutagenesis of residues within the A-B loop impaired T cell signaling in an in vitro system of antigen-specific TCR stimulation. Collectively, our findings provide a basis for the earliest molecular events that underlie Ag-induced T cell triggering.