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Domain structure of human complement C4b extends with increasing NaCl concentration: implications for its regulatory mechanism

Fung, KW; Wright, DW; Gor, J; Swann, MJ; Perkins, SJ; (2016) Domain structure of human complement C4b extends with increasing NaCl concentration: implications for its regulatory mechanism. Biochemical Journal , 473 (23) pp. 4473-4491. 10.1042/BCJ20160744. Green open access

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Abstract

During the activation of complement C4 to C4b, the exposure of its thioester domain (TED) is crucial for the attachment of C4b to activator surfaces. In the C4b crystal structure, TED forms an Arg(104)-Glu(1032) salt bridge to tether its neighbouring macroglobulin (MG1) domain. Here, we examined the C4b domain structure to test whether this salt bridge affects its conformation. Dual polarisation interferometry of C4b immobilised at a sensor surface showed that the maximum thickness of C4b increased by 0.46 nm with increase in NaCl concentration from 50 mM to 175 mM NaCl. Analytical ultracentrifugation showed that the sedimentation coefficient s20, w of monomeric C4b of 8.41 S in 50 mM NaCl buffer decreased to 7.98 S in 137 mM NaCl buffer, indicating that C4b became more extended. Small angle X-ray scattering reported similar RG values of 4.89-4.90 nm for C4b in 137-250 mM NaCl. Atomistic scattering modelling of the C4b conformation showed that TED and the MG1 domain were separated by 4.7 nm in 137-250 mM NaCl, this being greater than that of 4.0 nm in the C4b crystal structure. Our data reveal that in low NaCl concentrations, both at surfaces and in solution, C4b forms compact TED-MG1 structures. In solution, physiologically-relevant NaCl concentrations lead to the separation of the TED and MG1 domain, making C4b less able to bind to its complement regulators. These conformational changes are similar to those seen previously for complement C3b, confirming the importance of this salt bridge for regulating both C4b and C3b.

Type: Article
Title: Domain structure of human complement C4b extends with increasing NaCl concentration: implications for its regulatory mechanism
Location: England
Open access status: An open access version is available from UCL Discovery
DOI: 10.1042/BCJ20160744
Publisher version: http://dx.doi.org/10.1042/BCJ20160744
Language: English
Additional information: © 2016 The Author(s) This is an open access article published by Portland Press Limited on behalf of the Biochemical Society and distributed under the Creative Commons Attribution License 4.0 (CC BY).
Keywords: Complement C4b, dual polarisation interferometry, molecular modelling, small angle X-ray scattering, ultracentrifugation
UCL classification: UCL > Provost and Vice Provost Offices
UCL > Provost and Vice Provost Offices > School of Life and Medical Sciences
UCL > Provost and Vice Provost Offices > School of Life and Medical Sciences > Faculty of Life Sciences
UCL > Provost and Vice Provost Offices > School of Life and Medical Sciences > Faculty of Life Sciences > Div of Biosciences
UCL > Provost and Vice Provost Offices > School of Life and Medical Sciences > Faculty of Life Sciences > Div of Biosciences > Structural and Molecular Biology
UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences > Dept of Chemistry
URI: http://discovery.ucl.ac.uk/id/eprint/1522471
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