title: A revised mechanism for the activation of complement C3 to C3b: a molecular explanation of a disease-associated polymorphism.
abstract: The solution structure of complement C3b is crucial for the understanding of complement activation and regulation. C3b is generated by the removal of C3a from C3. Hydrolysis of the C3 thioester produces C3u, an analog of C3b. C3b cleavage results in C3c and C3d (thioester-containing domain; TED). To resolve functional questions in relation to C3b and C3u, analytical ultracentrifugation and x-ray and neutron scattering studies were used with C3, C3b, C3u, C3c, and C3d, using the wild-type allotype with Arg(102). In 50 mm NaCl buffer, atomistic scattering modeling showed that both C3b and C3u adopted a compact structure, similar to the C3b crystal structure in which its TED and macroglobulin 1 (MG1) domains were connected through the Arg(102)-Glu(1032) salt bridge. In physiological 137 mm NaCl, scattering modeling showed that C3b and C3u were both extended in structure, with the TED and MG1 domains now separated by up to 6 nm. The importance of the Arg(102)-Glu(1032) salt bridge was determined using surface plasmon resonance to monitor the binding of wild-type C3d(E1032) and mutant C3d(A1032) to immobilized C3c. The mutant did not bind, whereas the wild-type form did. The high conformational variability of TED in C3b in physiological buffer showed that C3b is more reactive than previously thought. Because the Arg(102)-Glu(1032) salt bridge is essential for the C3b-Factor H complex during the regulatory control of C3b, the known clinical associations of the major C3S (Arg(102)) and disease-linked C3F (Gly(102)) allotypes of C3b were experimentally explained for the first time.
authors.affiliation: From the Department of Structural and Molecular Biology, Division of Biosciences, Darwin Building, University College London, Gower Street, London WC1E 6BT, United Kingdom.
authors.forename: Elizabeth
authors.initials: E
authors.lastname: Rodriguez
authors.affiliation: From the Department of Structural and Molecular Biology, Division of Biosciences, Darwin Building, University College London, Gower Street, London WC1E 6BT, United Kingdom.
authors.forename: Ruodan
authors.initials: R
authors.lastname: Nan
authors.affiliation: From the Department of Structural and Molecular Biology, Division of Biosciences, Darwin Building, University College London, Gower Street, London WC1E 6BT, United Kingdom.
authors.forename: Keying
authors.initials: K
authors.lastname: Li
authors.affiliation: From the Department of Structural and Molecular Biology, Division of Biosciences, Darwin Building, University College London, Gower Street, London WC1E 6BT, United Kingdom.
authors.forename: Jayesh
authors.initials: J
authors.lastname: Gor
authors.affiliation: From the Department of Structural and Molecular Biology, Division of Biosciences, Darwin Building, University College London, Gower Street, London WC1E 6BT, United Kingdom s.perkins@ucl.ac.uk.
authors.forename: Stephen J
authors.initials: SJ
authors.lastname: Perkins
mesh_terms: D001120:Arginine / Q000737:chemistry; D003167:Complement Activation; D003176:Complement C3 / Q000378:metabolism*; D003179:Complement C3b / Q000378:metabolism*; D015932:Complement C3c / Q000378:metabolism*; D015933:Complement C3d / Q000378:metabolism*; D018360:Crystallography, X-Ray; D006801:Humans; D008259:Macroglobulins / Q000378:metabolism; D016296:Mutagenesis; D009154:Mutation; D011487:Protein Conformation; D055503:Protein Multimerization; D012542:Scattering, Radiation; D020349:Surface Plasmon Resonance; D014461:Ultracentrifugation
chemical_list: D003176:Complement C3; D008259:Macroglobulins; D003179:Complement C3b; D015932:Complement C3c; D015933:Complement C3d; D001120:Arginine
keywords: Analytical Ultracentrifugation; Complement C3; Inflammation; Molecular Modeling; Neutron Scattering; Surface Plasmon Resonance (SPR); X-ray Scattering