Summary HLA-DR molecules bind microbial peptides in an endosomal compartment and present them on the cell surface for CD4 T cell surveillance. HLA-DM plays a critical role in the endosomal peptide selection process. The structure of the HLA-DM – HLA-DR complex shows major rearrangements of the HLA-DR peptide binding groove. Flipping of a tryptophan away from the HLA-DR1 P1 pocket enables major conformational changes that position hydrophobic HLA-DR residues into the P1 pocket. These conformational changes accelerate peptide dissociation and stabilize the empty HLA-DR peptide binding groove. Initially, incoming peptides have access to only part of the HLA-DR groove and need to compete with HLA-DR residues for access to the P2 site and the hydrophobic P1 pocket. This energetic barrier creates a rapid and stringent selection process for the highest-affinity binders. Insertion of peptide residues into the P2 and P1 sites reverses the conformational changes, terminating selection through DM dissociation.

The mechanisms of HLA-DM catalyzed peptide exchange remain uncertain. We found that all stages of the interaction of DM with HLA-DR were dependent on the occupancy state of the peptide binding groove. High-affinity peptides were protected from removal by DM through two mechanisms: peptide binding induced dissociation of a long-lived complex of empty DR and DM, and high-affinity DR-peptide complexes bound DM only very slowly. Non-binding covalent DR-peptide complexes were converted to efficient DM binders upon truncation of an N-terminal peptide segment that emptied the P1 pocket and disrupted conserved hydrogen bonds to MHC. DM thus only binds to DR conformers in which a critical part of the binding site is vacant, due to spontaneous peptide motion.