title: Structural elucidation of the m157 mouse cytomegalovirus ligand for Ly49 natural killer cell receptors.
abstract: Natural killer (NK) cells express activating and inhibitory receptors that, in concert, survey cells for proper expression of cell surface major histocompatibility complex (MHC) class I molecules. The mouse cytomegalovirus encodes an MHC-like protein, m157, which is the only known viral antigen to date capable of engaging both activating (Ly49H) and inhibitory (Ly49I) NK cell receptors. We have determined the 3D structure of m157 and studied its biochemical and cellular interactions with the Ly49H and Ly49I receptors. m157 has a characteristic MHC-fold, yet possesses several unique structural features not found in other MHC class I-like molecules. m157 does not bind peptides or other small ligands, nor does it associate with beta(2)-microglobulin. Instead, m157 engages in extensive intra- and intermolecular interactions within and between its domains to generate a compact minimal MHC-like molecule. m157's binding affinity for Ly49I (K(d) approximately 0.2 microM) is significantly higher than that of classical inhibitory Ly49-MHC interactions. Analysis of viral escape mutations on m157 that render it resistant to NK killing reveals that it is likely to be recognized by Ly49H in a binding mode that differs from Ly49/MHC-I. In addition, Ly49H+ NK cells can efficiently lyse RMA cells expressing m157, despite the presence of native MHC class I. Collectively, our results show that m157 represents a structurally divergent form of MHC class I-like proteins that directly engage Ly49 receptors with appreciable affinity in a noncanonical fashion.
authors.affiliation: Departments of Molecular and Cellular Physiology and Structural Biology, Howard Hughes Medical Institute, Stanford University School of Medicine, Stanford, CA 94305, USA.
authors.forename: Erin J
authors.initials: EJ
authors.lastname: Adams
authors.forename: Z Sean
authors.initials: ZS
authors.lastname: Juo
authors.forename: Rayna Takaki
authors.initials: RT
authors.lastname: Venook
authors.forename: Martin J
authors.initials: MJ
authors.lastname: Boulanger
authors.forename: Hisashi
authors.initials: H
authors.lastname: Arase
authors.forename: Lewis L
authors.initials: LL
authors.lastname: Lanier
authors.forename: K Christopher
authors.initials: KC
authors.lastname: Garcia
mesh_terms: D000818:Animals; D000950:Antigens, Ly / Q000737:chemistry*; D016367:Baculoviridae / Q000235:genetics; D001665:Binding Sites; D045744:Cell Line, Tumor; D018360:Crystallography, X-Ray; D004220:Disulfides / Q000737:chemistry; D015395:Histocompatibility Antigens Class I / Q000276:immunology; D006860:Hydrogen Bonding; D007694:Killer Cells, Natural / Q000276:immunology*; D037181:Lectins, C-Type / Q000737:chemistry*; D008024:Ligands; D016399:Lymphoma, T-Cell / Q000473:pathology; D051379:Mice; D008958:Models, Molecular; D018146:Muromegalovirus / Q000276:immunology*; D055650:NK Cell Lectin-Like Receptor Subfamily A; D011485:Protein Binding; D011487:Protein Conformation; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D011971:Receptors, Immunologic / Q000737:chemistry* / Q000235:genetics / Q000276:immunology*; D055630:Receptors, NK Cell Lectin-Like
chemical_list: D000950:Antigens, Ly; D004220:Disulfides; D015395:Histocompatibility Antigens Class I; C527096:Klra8 protein, mouse; D037181:Lectins, C-Type; D008024:Ligands; D055650:NK Cell Lectin-Like Receptor Subfamily A; D011971:Receptors, Immunologic; D055630:Receptors, NK Cell Lectin-Like