Two different, highly exposed, bulged structures for an unusually long peptide bound to rat MHC class I RT1-A(a)

The rat MHC class la molecule RT1-A(a) has the unusual capacity to bind lon g peptides ending in arginine, such as MTF-E, a thirteen-residue, maternall y transmitted minor histocompatibility antigen. The antigenic structure of MTF-E was unpredictable due to its extraordinary length and two arginines t hat could serve as potential anchor residues. The crystal structure of RT1- A(a)-MTF-E at 2.55 Angstrom shows that both peptide termini are anchored, a s in other class I molecules, but the central residues in two independent p MHC complexes adopt completely different bulged conformations based on loca l environment. The MTF-E epitope is fully exposed within the putative T cel l receptor (TCR) footprint. The flexibility demonstrated by the MTF-E struc tures illustrates how different TCRs may be raised against chemically ident ical, but structurally dissimilar, pMHC complexes.