DIFFERENTIAL ROLE OF CONSERVED AND POLYMORPHIC RESIDUES OF THE BINDING GROOVE OF MHC CLASS-I MOLECULES IN THE SELECTION OF PEPTIDES

A set of 17-point mutants of H-2K(d) was produced as secreted single-c hain MHC molecules in the yeast Kluyveromyces lactis. Using a library of 648 synthetic peptides displaying the K(d)-specific motif, the repe rtoire of peptides selected by each mutant was compared by a two-dimen sional analysis technique. When conserved residues of K(d) were substi tuted by an alanine, no binding was observed. When polymorphic residue s were changed, two outcomes were observed. For residues 95, 99, and 1 1 6, no binding was observed, implying that the side chains of these r esidues contribute directly to the interaction with the peptide. When positions 9, 45, 97, and 114 were changed to alanine, the repertoire o f selected peptides was enlarged. Position 97 was also changed to all four possible residues found in natural variants of mouse MHC class I molecules. The repertoires of selected peptides were analyzed and appe ared to be included one in another. Their dimension was inversely corr elated with the size of the side-chain of residue 97. We conclude that during the course of evolution some polymorphic residues may have bee n selected for their capacity to reduce the size of the peptide repert oire by preventing certain peptides from binding.