Structural studies on HLA-G: implications for ligand and receptor binding.

Area of Science:

• Immunology
• Structural Biology
• Molecular Genetics

Background:

• Human leukocyte antigen-G (HLA-G) is a unique class Ib major histocompatibility complex (MHC) molecule.
• It is predominantly expressed in immune-privileged tissues, playing a crucial role in immune regulation.
• Its structure resembles other class I MHC molecules, comprising a heavy chain and beta(2)microglobulin (beta(2)m).

Purpose of the Study:

• To elucidate the structural basis of HLA-G's function and interactions with immune receptors.
• To investigate the binding characteristics of peptides within the HLA-G molecule.
• To analyze the structural implications of HLA-G dimerization on receptor binding.

Main Methods:

• X-ray crystallography to determine the three-dimensional structure of HLA-G.
• Analysis of peptide binding within the MHC cleft.
• Comparative structural analysis with other MHC class I molecules and their known ligands.
• Investigation of HLA-G dimer conformation and potential receptor stoichiometry.

Main Results:

• HLA-G binds a nine-residue self-peptide in a constrained manner, similar to HLA-E.
• The alpha3 domain of HLA-G exhibits structural distinctions from class Ia MHC molecules, affecting leukocyte immunoglobulinlike receptor (LIR) binding affinity.
• Disulfide-bonded HLA-G dimers adopt an oblique conformation, suggesting a potential 1:2 dimer:receptor stoichiometry.
• The orientation of protomers in the dimer makes binding to killer immunoglobulinlike receptor 2DL4 (KIR2DL4) unlikely.

Conclusions:

• The structural features of HLA-G, including its peptide-binding cleft and alpha3 domain, dictate its specific interactions with immune receptors.
• HLA-G's unique structure supports its role in immune tolerance within immune-privileged sites.
• Dimerization of HLA-G influences its interaction with receptors, but likely not KIR2DL4.