Structure of Nonclassical MHC I (HLA-E, HLA-F, HLA-G, and Orthologs)
© 2016 Elsevier Ltd All rights reserved. The nonclassical HLA molecules HLA-E, HLA-F, and HLA-G are relatively nonpolymorphic MHC molecules and crystallographic structures are available for HLA-E and HLA-G, but not HLA-F. HLA-E has a hydrophobic peptide-binding groove with five pockets to accommodate peptide side chains. Three of these pockets at peptide positions 2, 7, and 9 are deep anchor pockets. Interactions with the multiple pockets, combined with a network of hydrogen bonds to the peptide main chain contribute to the high specificity of the HLA-E groove for a limited range of hydrophobic peptides. Structures of HLA-E bound to the CD94-NKG2A receptor or to a T cell receptor demonstrate that both receptors straddle the peptide-binding groove and interact with exposed peptide residues. HLA-G also has five pockets, three of which are deep anchor pockets at the 2, 7, and 9 peptide positions and the HLA-G peptide-binding groove also provides a strong network of hydrogen bonds to the peptide main chain. HLA-G can exist in a dimeric disulfide-bonded form that is otherwise structurally similar to the standard monomeric form. The LIR-1 receptor interacts with HLA-G primarily through a loop in the HLA-G alpha-3 domain. Both HLA-E and HLA-G display structural features consistent with their peptide selectivity.