title: The regulatory SCR-1/5 and cell surface-binding SCR-16/20 fragments of factor H reveal partially folded-back solution structures and different self-associative properties.
abstract: Factor H (FH) is a plasma glycoprotein that plays a central role in regulation of the alternative pathway of complement. It is composed of 20 short complement regulator (SCR) domains. The SCR-1/5 fragment is required for decay acceleration and cofactor activity, while the SCR-16/20 fragment possesses binding sites for complement C3d and heparin. X-ray scattering and analytical ultracentrifugation showed that SCR-1/5 was monomeric, while SCR-16/20 formed dimers. The Guinier radius of gyration R(G) of 4.3 nm for SCR-1/5 and those of 4.7 nm and about 7.8 nm for monomeric and dimeric SCR-16/20, respectively, showed that their structures are partially folded back and bent. The distance distribution function P(r) showed that SCR-1/5 has a maximum dimension of 15 nm while monomeric and dimeric SCR-16/20 are 17 nm and about 27 nm long, respectively. The sedimentation coefficient of 2.4 S for SCR-1/5 showed no concentration-dependence, while that for SCR-16/20 was 2.8 S for the monomer and 3.9 S for the dimer. Sedimentation equilibrium data showed that SCR-1/5 is monomeric while SCR-16/20 exhibited a weak monomer-dimer equilibrium with a dissociation constant of 16 microM. The constrained scattering and sedimentation modelling of SCR-1/5 and SCR-16/20 showed that partially folded-back and bent flexible SCR arrangements fitted both data sets better than extended linear arrangements, and that the dimer was best modelled in the SCR-16/20 model by an end-to-end association of two SCR-20 domains. The SCR-1/5 and SCR-16/20 models were conformationally similar to the previously determined partially folded-back structure for intact wild-type FH, hence suggesting a partial explanation of the intact FH structure. Comparison of the SCR-16/20 model with the crystal structure of C3b clarified reasons for the distribution of mutations leading to atypical haemolytic uraemic syndrome.
authors.affiliation: Department of Biochemistry and Molecular Biology, Darwin Building, University College London, Gower Street, London WC1E 6BT, UK.
authors.forename: Azubuike I
authors.initials: AI
authors.lastname: Okemefuna
authors.forename: Hannah E
authors.initials: HE
authors.lastname: Gilbert
authors.forename: Kim M
authors.initials: KM
authors.lastname: Griggs
authors.forename: Rebecca J
authors.initials: RJ
authors.lastname: Ormsby
authors.forename: David L
authors.initials: DL
authors.lastname: Gordon
authors.forename: Stephen J
authors.initials: SJ
authors.lastname: Perkins
mesh_terms: D000595:Amino Acid Sequence; D001665:Binding Sites; D003176:Complement C3 / Q000378:metabolism; D017242:Complement Factor H / Q000737:chemistry* / Q000378:metabolism*; D016384:Consensus Sequence; D017124:Conserved Sequence; D019281:Dimerization; D006493:Heparin / Q000378:metabolism; D006801:Humans; D008958:Models, Molecular; D008969:Molecular Sequence Data; D010446:Peptide Fragments / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D011485:Protein Binding; D011487:Protein Conformation; D017510:Protein Folding; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism; D012542:Scattering, Radiation; D055672:Static Electricity; D014461:Ultracentrifugation; D014965:X-Rays
chemical_list: D003176:Complement C3; D010446:Peptide Fragments; D011994:Recombinant Proteins; D017242:Complement Factor H; D006493:Heparin