The kinetics of peptide binding to HLA-A2 and the conformation of the peptide-A2 complex can be determined by amino acid side chains on the floor of the peptide binding groove

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Abstract
The ability of amino acid side chains in the floor of the peptide binding groove of HLA-A2 to affect the presentation of a viral peptide to peptide-speclfic cytotoxic T lymphocytes (CTL) has been examined. HLA-A2 molecules with naturally occurring single amino acid substitutions of Phe to Tyr at position 9 (HLA-A2.4a, Tyr9) and Tyr to Cys at position 99 (HLA-A2.4b, Cys99) and a site directed mutant with a Val to Leu substitution at position 95 (Leu95) were examined for their ability to present the Influenza virus matrix M1 55–73 peptide and several sequence variants of the M1 peptide to a panel of 36 M1 55–73-speclfic HLA-A2.1-restricted CTL lines. The Leu95 molecule demonstrated enhanced kinetics of M1 peptide presentation and the ability to be sensitized by lower concentrations of the M1 peptide than the A2.1 molecule. The Tyr9 and Cys99 molecules exposed to M1 peptide were not recognized by 33 out of 36 CTL lines. The Tyr9 and Cys99 HLA-A2 molecules could bind the M1 55–73 peptide because at least one CTL line was found that could recognize each of these molecules that were exposed to the M1 peptide. CTL recognition patterns of variant M1 peptides presented by the Tyr9 molecule demonstrated that the amino acid at position 9 can be a critical determinant of the conformation of the peptide-A2 complex, and indicated that a particular peptide can bind in the HLA-A2 peptide binding groove In more than one conformation. These results demonstrate that amino acid side chains in the floor of the peptide binding groove can have a major functional role in determining the formation and conformation of the peptide-A2 complex that is recognized by peptide-speclfic CTL.

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