Structural characterization of the TCR complex by electron microscopy

Structural information on how the TCR transmits signals upon binding of its antigen peptide MHC molecule ligand is still lacking. The ectodomains of the TCR alpha/beta, CD3 epsilon gamma and CD3 epsilon delta dimers, as well as the transmembrane domain of CD3 zeta, have been characterized by X-ray crystallography and nuclear magnetic resonance (NMR). However, no structural data have been obtained for the entire TCR complex. In this study, we have purified the TCR from T cells under native conditions and used electron microscopy to derive a three-dimensional structure. The TCR complex appears as a pear-shaped structure of 180 x 120 x 65 angstrom. Furthermore, the use of mAbs has allowed to determine the orientation of the TCR alpha/beta and CD3 subunits and to suggest a model of interactions. Interestingly, the reconstructed TCR is larger than expected for a complex with a alpha beta gamma epsilon delta epsilon zeta zeta stoichiometry. The accommodation of a second TCR alpha beta to fill in the extra volume is discussed.