title: Functional activity of MD-2 polymorphic variant is significantly different in soluble and TLR4-bound forms: decreased endotoxin binding by G56R MD-2 and its rescue by TLR4 ectodomain.
abstract: MD-2 is an essential component of endotoxin (LPS) sensing, binding LPS independently and when bound to the ectodomain of the membrane receptor TLR4. Natural variation of proteins involved in the LPS-recognition cascade such as the LPS-binding protein, CD14, and TLR4, as well as proteins involved in intracellular signaling downstream of LPS binding, affect the cellular response to endotoxin and host defense against bacterial infections. We now describe the functional properties of two nonsynonymous coding polymorphisms of MD-2, G56R and P157S, documented in HapMap. As predicted from the MD-2 structure, the P157S mutation had little or no effect on MD-2 function. In contrast, the G56R mutation, located close to the LPS-binding pocket, significantly decreased cellular responsiveness to LPS. Soluble G56R MD-2 showed markedly reduced LPS binding that was to a large degree rescued by TLR4 coexpression or presence of TLR4 ectodomain. Thus, cells that express TLR4 without MD-2 and whose response to LPS depends on ectopically produced MD-2 were most affected by expression of the G56R variant of MD-2. Coexpression of wild-type and G56R MD-2 yielded an intermediate phenotype with responses to LPS diminished to a greater extent than that resulting from expression of the D299G TLR4 polymorphic variant.
authors.affiliation: Department of Biotechnology, National Institute of Chemistry, Hajdrihova 19, Ljubljana, Slovenia.
authors.forename: Jozica
authors.initials: J
authors.lastname: Vasl
authors.forename: Polonca
authors.initials: P
authors.lastname: Prohinar
authors.forename: Theresa L
authors.initials: TL
authors.lastname: Gioannini
authors.forename: Jerrold P
authors.initials: JP
authors.lastname: Weiss
authors.forename: Roman
authors.initials: R
authors.lastname: Jerala
mesh_terms: D001424:Bacterial Infections / Q000235:genetics / Q000276:immunology* / Q000378:metabolism; D001665:Binding Sites / Q000235:genetics / Q000276:immunology; D002460:Cell Line; D015870:Gene Expression; D006801:Humans; D008070:Lipopolysaccharides / Q000276:immunology* / Q000378:metabolism / Q000494:pharmacology; D051218:Lymphocyte Antigen 96 / Q000235:genetics / Q000276:immunology* / Q000378:metabolism; D011110:Polymorphism, Genetic / Q000276:immunology*; D011485:Protein Binding / Q000187:drug effects / Q000235:genetics / Q000276:immunology; D017434:Protein Structure, Tertiary / Q000235:genetics; D013329:Structure-Activity Relationship; D051197:Toll-Like Receptor 4 / Q000235:genetics / Q000276:immunology* / Q000378:metabolism
chemical_list: C495377:LY96 protein, human; D008070:Lipopolysaccharides; D051218:Lymphocyte Antigen 96; C495345:TLR4 protein, human; D051197:Toll-Like Receptor 4