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      Crystal structure of HLA-G: a nonclassical MHC class I molecule expressed at the fetal-maternal interface.

      Proceedings of the National Academy of Sciences of the United States of America

      Sequence Homology, Amino Acid, Amino Acid Sequence, Animals, Crystallography, X-Ray, Female, HLA Antigens, chemistry, immunology, HLA-G Antigens, Histocompatibility Antigens Class I, Humans, Maternal-Fetal Exchange, Mice, Models, Molecular, Molecular Sequence Data, Pregnancy, Protein Conformation

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          HLA-G is a nonclassical major histocompatibility complex class I (MHC-I) molecule that is primarily expressed at the fetal-maternal interface, where it is thought to play a role in protecting the fetus from the maternal immune response. HLA-G binds a limited repertoire of peptides and interacts with the inhibitory leukocyte Ig-like receptors LIR-1 and LIR-2 and possibly with certain natural killer cell receptors. To gain further insights into HLA-G function, we determined the 1.9-A structure of a monomeric HLA-G complexed to a natural endogenous peptide ligand from histone H2A (RIIPRHLQL). An extensive network of contacts between the peptide and the antigen-binding cleft reveal a constrained mode of binding reminiscent of the nonclassical HLA-E molecule, thereby providing a structural basis for the limited peptide repertoire of HLA-G. The alpha3 domain of HLA-G, a candidate binding site for the LIR-1 and -2 inhibitory receptors, is structurally distinct from the alpha3 domains of classical MHC-I molecules, providing a rationale for the observed affinity differences for these ligands. The structural data suggest a head-to-tail mode of dimerization, mediated by an intermolecular disulfide bond, that is consistent with the observation of HLA-G dimers on the cell surface.

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