Identification of the Docking Site for CD3 on the T Cell Receptor β Chain by Solution NMR

The T cell receptor (TCR)-CD3 complex is composed of a genetically diverse alpha beta TCR heterodimer associated noncovalently with the invariant CD3 dimers CD3 epsilon gamma, CD3 epsilon delta, and CD3 zeta zeta. The TCR mediates peptide-MHC recognition, whereas the CD3 molecules transduce activation signals to the T cell. Although much is known about downstream T cell signaling pathways, the mechanism whereby TCR engagement by peptide-MHC initiates signaling is poorly understood. A key to solving this problem is defining the spatial organization of the TCR-CD3 complex and the interactions between its subunits. We have applied solution NMR methods to identify the docking site for CD3 on the beta chain of a human autoimmune TCR. We demonstrate a low affinity but highly specific interaction between the extracellular domains of CD3 and the TCR constant beta (C beta) domain that requires both CD3 epsilon gamma and CD3 epsilon delta subunits. The mainly hydrophilic docking site, comprising 9-11 solvent-accessible C beta residues, is relatively small (similar to 400 angstrom(2)), consistent with the weak interaction between TCR and CD3 extracellular domains, and devoid of glycosylation sites. The docking site is centered on the alpha A and alpha B helices of C beta, which are located at the base of the TCR. This positions CD3 epsilon gamma and CD3 epsilon delta between the TCR and the T cell membrane, permitting us to distinguish among several possible models of TCR-CD3 association. We further correlate structural results from NMR with mutational data on TCR-CD3 interactions from cell-based assays.