The relative energetic contributions of dominant P1 pocket versus hydrogen bonding interactions to peptide:class II stability: Implications for the mechanism of DM function

Peptides are bound to MHC class II molecules by an array of hydrogen bonds between conserved MHC class II protein side-chains and the peptide backbone and through interactions between MHC protein pockets and peptide side-chain anchors. The crystal structure of murine I-Ak protein with peptide shows a network of electrostatic interactions with the P1 aspartic acid anchor and an arginine in the P1 pocket that are thought to constitute the major stabilizing interaction between peptide and MHC. In this paper, have explored the relative energetic contribution of this dominant P1 pocket interaction with that made by a genetically conserved hydrogen bond which is formed by the beta 81 histidine residue and the main chain of the bound peptide. We have performed peptide dissociation experiments using antigenic peptides or variants that have altered side-chain interactions with the I-Ak P1 pocket using either native I-Ak or I-Ak proteins mutated to disrupt the N-terminal hydrogen bond. The results demonstrate that the N-terminal hydrogen bonds in I-Ak complexes make highly significant energetic contributions to the kinetic stabilities comparable to or greater than the energetic contribution of highly favorable P1 pocket interactions. Hence, we conclude that the kinetic stability of MHC class II:peptide complexes critically depends on two quite distinct molecular interactions between peptide and MHC located at the peptide's amino terminus. We discuss these results in light of the proposed mechanism for DM function.