A revised mechanism for the activation of complement C3 to C3b: a molecular explanation of a disease-associated polymorphism [Molecular Bases of Disease]

December 8th, 2014 by Rodriguez, E., Nan, R., Li, K., Gor, J., Perkins, S. J.

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 analogue of C3b. C3b cleavage results in C3c and C3d (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 R102. In 50 mM NaCl buffer, atomistic scattering modelling showed that both C3b and C3u adopted a compact structure, similar to the C3b crystal structure in which its TED and MG1 domains (MG: macroglobulin) were connected through the R102-E1032 salt-bridge. In physiological 137 mM NaCl, scattering modelling 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 R102-E1032 salt-bridge was determined using surface plasmon resonance to monitor the binding of wild-type C3d(E1032) and mutant C3d(A1032) to immobilised C3c. The mutant did not bind while 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 R102-E1032 salt-bridge is essential for the C3b-Factor H complex during the regulatory control of C3b, the known clinical associations of the major C3S (R102) and disease-linked C3F (G102) allotypes of C3b were experimentally explained for the first time.
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