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Amino-Terminal Flanking Residues Determine the Conformation of a Peptide–Class II MHC Complex¹

Department of Pathology and Immunology, Washington University School of Medicine, St. Louis, MO 63110

The peptide spanning residues 48–62 of hen egg white lysozyme presented by I-A^k molecules gives rise to two T cell populations, types A and B, that recognize distinct conformers of the complex generated in late and recycling endosomes. The class II–like accessory molecule H2-DM functions as a conformational editor, eliminating the type B conformer in late endosomes. Here, we show that the conformation of the complex, and its susceptibility to editing by H2-DM, are determined by peptide amino-terminal flanking residues. Elimination of these residues abolished editing, permitting formation of the type B conformer in late endosomes. Substitutions at P(–2) affected the stability of the type B conformer, preventing its formation and/or editing, without hindering peptide binding or formation of the type A conformer of the complex. We conclude that interactions involving amino-terminal flanking residues stabilize peptide-MHC conformers and confer resistance to editing by H2-DM, influencing the nature of the T cell repertoire.

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