title: Mutations in alpha-chain of C4BP that selectively affect its factor I cofactor function.
abstract: C4b-binding protein (C4BP) inhibits all pathways of complement activation, acting as a cofactor to the serine protease factor I (FI) in the degradation of activated complement factors C4b and C3b. C4BP is a disulfide-linked polymer of seven alpha-chains and a unique beta-chain, the alpha- and beta-chains being composed of eight and three complement control protein (CCP) domains, respectively. In previous studies we have localized cofactor activity and binding of C4b to alpha-chain CCP1-3 of C4BP, whereas the binding of C3b required additionally CCP4. Likewise, introduced point mutations that decreased binding of C4b/C3b caused a decrease in cofactor activity. In the present study, we describe two mutants of C4BP, K126Q/K128Q and F144S/F149S, clustered on alpha-chain CCP3, which selectively lost their ability to act as cofactors in the cleavage of both C4b and C3b. Both mutants show the same binding affinity for C4b/C3b as measured by surface plasmon resonance and have the same inhibitory effect on formation and decay of the classical pathway C3-convertase as the wild type C4BP. It appears that C4b and C3b do not undergo the same conformational changes upon binding to the C4BP mutants as during the interaction with the wild type C4BP, which then results in the observed loss of the cofactor activity.
authors.affiliation: Lund University Department of Clinical Chemistry, University Hospital Malmö, The Wallenberg Laboratory, S-205 02 Malmö, Sweden and INSERM U428, Université Paris V, Paris 75006, France. Anna.Blom@klkemi.mas.lu.se
authors.forename: Anna M
authors.initials: AM
authors.lastname: Blom
authors.forename: Bruno O
authors.initials: BO
authors.lastname: Villoutreix
authors.forename: Björn
authors.initials: B
authors.lastname: Dahlbäck
mesh_terms: D000817:Anilino Naphthalenesulfonates / Q000494:pharmacology; D001667:Binding, Competitive; D002942:Circular Dichroism; D003167:Complement Activation; D003179:Complement C3b / Q000737:chemistry; D015935:Complement C4b / Q000378:metabolism*; D003169:Complement Inactivator Proteins; D018076:DNA, Complementary / Q000378:metabolism; D004220:Disulfides; D004305:Dose-Response Relationship, Drug; D004591:Electrophoresis, Polyacrylamide Gel; D005456:Fluorescent Dyes / Q000494:pharmacology; D006023:Glycoproteins / Q000737:chemistry* / Q000235:genetics* / Q000378:metabolism; D006801:Humans; D007700:Kinetics; D016297:Mutagenesis, Site-Directed; D009154:Mutation; D017354:Point Mutation; D011485:Protein Binding; D011487:Protein Conformation; D017433:Protein Structure, Secondary; D011994:Recombinant Proteins / Q000378:metabolism; D020349:Surface Plasmon Resonance
chemical_list: D000817:Anilino Naphthalenesulfonates; D003169:Complement Inactivator Proteins; D018076:DNA, Complementary; D004220:Disulfides; D005456:Fluorescent Dyes; D006023:Glycoproteins; D011994:Recombinant Proteins; C027132:1-anilino-8-naphthalenesulfonate; D003179:Complement C3b; D015935:Complement C4b