HLA–DM senses peptide-MHC class II interactions throughout the peptide binding groove.

Abstract

Abstract HLA-DM is an enzyme-like molecule that edits the MHC class II peptide repertoire by catalyzing multiple rounds of peptide exchange on MHCII molecules. It differentially edits peptide-MHCII complexes and thereby skews the foreign and self-peptide repertoire available for activation or tolerance induction of CD4 T-cells. Though it has important implications for the adaptive immune response, the determinants of susceptibility to DM mediated editing remain undefined. It has been suggested that unstable interactions at the N-terminus of a bound peptide is a key determinant for DM editing and that partial dissociation of the peptides N-terminus is necessary for this sensitivity. Herein, we assessed individual components of the catalytic mechanism in real-time kinetic measurements of DM activity. Using a series of amino acid substituted peptides, we analyzed the effects of disfavored interactions on the catalytic turnover and affinity of DM editing and find that disruptions throughout the peptide binding groove contribute to both the affinity of DM for a given peptide complex and to the rate of peptide dissociation from the catalytic complex. Moreover, disruptions in the conserved hydrogen bond network or disfavored amino acid residues near a peptides C-terminus result in increased DM affinity and catalytic turnover. Furthermore, complexes with N-terminal truncated peptides show increased catalytic turnover when compared to parent complexes, indicating that the functionally active DM-peptide-MHCII catalytic complex can operate on a peptide-MHCII complex with full occupancy. Together, this implies that interactions at the DM-MHCII interface are intimately linked to unstable interactions throughout the peptide-binding groove.