title: Structure of Complement C3(H2O) Revealed By Quantitative Cross-Linking/Mass Spectrometry And Modeling.
abstract: The slow but spontaneous and ubiquitous formation of C3(H2O), the hydrolytic and conformationally rearranged product of C3, initiates antibody-independent activation of the complement system that is a key first line of antimicrobial defense. The structure of C3(H2O) has not been determined. Here we subjected C3(H2O) to quantitative cross-linking/mass spectrometry (QCLMS). This revealed details of the structural differences and similarities between C3(H2O) and C3, as well as between C3(H2O) and its pivotal proteolytic cleavage product, C3b, which shares functionally similarity with C3(H2O). Considered in combination with the crystal structures of C3 and C3b, the QCMLS data suggest that C3(H2O) generation is accompanied by the migration of the thioester-containing domain of C3 from one end of the molecule to the other. This creates a stable C3b-like platform able to bind the zymogen, factor B, or the regulator, factor H. Integration of available crystallographic and QCLMS data allowed the determination of a 3D model of the C3(H2O) domain architecture. The unique arrangement of domains thus observed in C3(H2O), which retains the anaphylatoxin domain (that is excised when C3 is enzymatically activated to C3b), can be used to rationalize observed differences between C3(H2O) and C3b in terms of complement activation and regulation.
authors.affiliation: From the ‡Wellcome Trust Centre for Cell Biology, Institute of Cell Biology, School of Biological Sciences, University of Edinburgh, Edinburgh EH9 3BF, UK;
authors.forename: Zhuo A
authors.identifier: 0000-0003-0811-8348
authors.initials: ZA
authors.lastname: Chen
authors.affiliation: §Unité de Bioinformatique Structurale, CNRS UMR 3528, Institut Pasteur, 75015 Paris, France; ¶Department of Bioengineering and Therapeutic Sciences, Department of Pharmaceutical Sciences, and California Institute for Quantitative Biomedical Sciences, University of California, San Francisco, California 94158, United States;
authors.forename: Riccardo
authors.initials: R
authors.lastname: Pellarin
authors.affiliation: From the ‡Wellcome Trust Centre for Cell Biology, Institute of Cell Biology, School of Biological Sciences, University of Edinburgh, Edinburgh EH9 3BF, UK;
authors.forename: Lutz
authors.identifier: 0000-0003-4978-0864
authors.initials: L
authors.lastname: Fischer
authors.affiliation: ¶Department of Bioengineering and Therapeutic Sciences, Department of Pharmaceutical Sciences, and California Institute for Quantitative Biomedical Sciences, University of California, San Francisco, California 94158, United States;
authors.forename: Andrej
authors.initials: A
authors.lastname: Sali
authors.affiliation: §Unité de Bioinformatique Structurale, CNRS UMR 3528, Institut Pasteur, 75015 Paris, France;
authors.forename: Michael
authors.identifier: 0000-0002-1451-8092
authors.initials: M
authors.lastname: Nilges
authors.affiliation: ‖Schools of Chemistry and Biological Sciences, University of Edinburgh, Edinburgh EH9 3JJ, UK; juri.rappsilber@ed.ac.uk paul.barlow@ed.ac.uk.
authors.forename: Paul N
authors.initials: PN
authors.lastname: Barlow
authors.affiliation: From the ‡Wellcome Trust Centre for Cell Biology, Institute of Cell Biology, School of Biological Sciences, University of Edinburgh, Edinburgh EH9 3BF, UK; **Chair of Bioanalytics, Institute of Biotechnology, Technische Universität Berlin, 13355 Berlin, Germany juri.rappsilber@ed.ac.uk paul.barlow@ed.ac.uk.
authors.forename: Juri
authors.identifier: 0000-0001-5999-1310
authors.initials: J
authors.lastname: Rappsilber
mesh_terms: D003176:Complement C3 / Q000737:chemistry*; D003179:Complement C3b / Q000737:chemistry*; D003432:Cross-Linking Reagents; D018360:Crystallography, X-Ray; D006801:Humans; D013058:Mass Spectrometry / Q000379:methods*; D008958:Models, Molecular; D011487:Protein Conformation; D000072417:Protein Domains
chemical_list: C038245:C3 protein, human; D003176:Complement C3; D003432:Cross-Linking Reagents; D003179:Complement C3b