TCR Scanning of Peptide/MHC through Complementary Matching of Receptor and Ligand Molecular Flexibility

Although conformational changes in TCRs and peptide Ags presented by MHC protein (pMHC) molecules often occur upon binding, their relationship to intrinsic flexibility and role in ligand selectivity are poorly understood. In this study, we used nuclear magnetic resonance to study TCR-pMHC binding, examining recognition of the QL9/H-2L(d) complex by the 2C TCR. Although the majority of the CDR loops of the 2C TCR rigidify upon binding, the CDR3 beta loop remains mobile within the TCR-pMHC interface. Remarkably, the region of the QL9 peptide that interfaces with CDR3 beta is also mobile in the free pMHC and in the TCR-pMHC complex. Determination of conformational exchange kinetics revealed that the motions of CDR3 beta and QL9 are closely matched. The matching of conformational exchange in the free proteins and its persistence in the complex enhances the thermodynamic and kinetic stability of the TCR-pMHC complex and provides a mechanism for facile binding. We thus propose that matching of structural fluctuations is a component of how TCRs scan among potential ligands for those that can bind with sufficient stability to enable T cell signaling.