Soluble T cell receptors (TCRs) have been engineered for secretion by Lec3.2.8.1 CHO cells which synthesize homogeneous gylcans; a leucine zipper is used to facilitate a-b chain association. Two mouse TCRs, N15 and N26, specific for vesicular stomatitis virus octapeptide (VSV8) in the context of the Kb class I molecule, have been expressed by this method. Crystals of N15 and N26 complexed with the Fab of an anti-Cb monoclonal antibody (FabH57) and of the N15-VSV8/Kb and N26-VSV8/Kb complexes have been obtained. The structure of the N15-FabN15 complex has been solved to 2.8 A resolution by molecular replacement in conjunction with selenomethionine/MAD phasing. The structure of the N15 TCR will be compared with that of other Ig superfamily members, including antibodies, CD4, VCAM-1 and CD2. By comparing each of two complexes in the asymmetric unit of N15-FabH57 crystals, it may be possible to ascertain whether there is intrinsic mobility between V-V, V-C and/or C-C domains of the TCR. The structure of the N15-VSV8/Kb complex will be solved by molecular replacement using crystals which diffract to 3 A resolution. The role of individual CDRs in peptide/MHC recognition will be determined and any induced fit upon antigen binding analyzed by comparing the structure of the CDRs in the TCR-VSV8/Kb versus TCR-Fab complex. Structure analysis of the N26 TCR complexed with VSV8/Kb will also be carried out in order to determine how two different TCRs bind the same peptide/MHC ligand. This should establish whether the docking mode is fixed or variable and whether there are restricted interactions between specific CDRs and MHC versus peptide residues.
