The lack of consensus for I-Ag7-peptide binding motifs:: Is there a requirement for anchor amino acid side chains?

We discuss here the problems in identifying sequence motifs of peptides that bind to I-Ag-7, the class II histocompatibility molecule of NOD diabetic mice. We present studies that indicate a minor contribution of amino acid side chains for binding. A peptide from the E alpha chain binds to I-A(g7) molecules and is recognized by CD4 T cells. By producing single-residue mutations we identified four residues that were considered to contact the T cell receptor. No residue was found to be essential for binding to I-A(g7): a peptide that contained the T cell contact residues, on a backbone of alanines, bound to I-A(g7) and stimulated the T cells. We conclude that peptides can bind to I-A(g7) without the requirement for residues with prominent side chains to anchor them.