Two human T cell receptors bind in a similar diagonal mode to the HLA-A2/Tax peptide complex using different TCR amino acids.

The three-dimensional structure of a human alphabeta T cell receptor (TCR), B7, bound to the HLA-A2 molecule/HTLV-1 Tax peptide complex was determined by x-ray crystallography. Although different from the A6 TCR, previously studied, in 16 of the 17 residues that contact HLA-A2/Tax, the B7 TCR binds in a similar diagonal manner, only slightly tipped and rotated, relative to the A6 TCR. The structure explains data from functional assays on the specificity differences between the B7 and A6 TCRs for agonist, partial agonist, and null peptides. The existence of a structurally similar diagonal binding mode for TCRs favors mechanisms based on the formation of geometrically defined supramolecular assemblies for initiating signaling.