title: Analogous interactions in initiating complexes of the classical and lectin pathways of complement.
abstract: The classical and lectin pathways of complement activation neutralize pathogens and stimulate key immunological processes. Both pathways are initiated by collagen-containing, soluble pattern recognition molecules associated with specific serine proteases. In the classical pathway, C1q binds to Ab-Ag complexes or bacterial surfaces to activate C1r and C1s. In the lectin pathway, mannan-binding lectin and ficolins bind to carbohydrates on pathogens to activate mannan-binding lectin-associated serine protease 2. To characterize the interactions leading to classical pathway activation, we have analyzed binding between human C1q, C1r, and C1s, which associate to form C1, using full-length and truncated protease components. We show that C1r and C1s bind to C1q independently. The CUB1-epidermal growth factor fragments contribute most toward binding, but CUB2 of C1r, but not of C1s, is also important. Each C1rs tetramer presents a total of six binding sites, one for each of the collagenous domains of C1q. We also demonstrate that subcomponents of the lectin and classical pathways cross-interact. Thus, although the stoichiometries of complexes differ, interactions are analogous, with equivalent contacts between recognition and protease subcomponents. Importantly, these new data are contrary to existing models of C1 and enable us to propose a new model using mannan-binding lectin-mannan-binding lectin-associated serine protease interactions as a template.
authors.affiliation: Department of Infection, University of Leicester, Leicester, United Kingdom.
authors.forename: Anna E
authors.initials: AE
authors.lastname: Phillips
authors.forename: Julia
authors.initials: J
authors.lastname: Toth
authors.forename: Alister W
authors.initials: AW
authors.lastname: Dodds
authors.forename: Umakhanth Venkatraman
authors.initials: UV
authors.lastname: Girija
authors.forename: Christopher M
authors.initials: CM
authors.lastname: Furze
authors.forename: Eleni
authors.initials: E
authors.lastname: Pala
authors.forename: Robert B
authors.initials: RB
authors.lastname: Sim
authors.forename: Kenneth B M
authors.initials: KB
authors.lastname: Reid
authors.forename: Wilhelm J
authors.initials: WJ
authors.lastname: Schwaeble
authors.forename: Ralf
authors.initials: R
authors.lastname: Schmid
authors.forename: Anthony H
authors.initials: AH
authors.lastname: Keeble
authors.forename: Russell
authors.initials: R
authors.lastname: Wallis
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D003165:Complement System Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D007700:Kinetics; D037102:Lectins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D008958:Models, Molecular; D008969:Molecular Sequence Data; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D011994:Recombinant Proteins / Q000235:genetics / Q000378:metabolism; D016415:Sequence Alignment
chemical_list: D037102:Lectins; D011994:Recombinant Proteins; D003165:Complement System Proteins