title: Structural basis for activation of the complement system by component C4 cleavage.
abstract: An essential aspect of innate immunity is recognition of molecular patterns on the surface of pathogens or altered self through the lectin and classical pathways, two of the three well-established activation pathways of the complement system. This recognition causes activation of the MASP-2 or the C1s serine proteases followed by cleavage of the protein C4. Here we present the crystal structures of the 203-kDa human C4 and the 245-kDa C4·MASP-2 substrate·enzyme complex. When C4 binds to MASP-2, substantial conformational changes in C4 are induced, and its scissile bond region becomes ordered and inserted into the protease catalytic site in a manner canonical to serine proteases. In MASP-2, an exosite located within the CCP domains recognizes the C4 C345C domain 60 Å from the scissile bond. Mutations in C4 and MASP-2 residues at the C345C-CCP interface inhibit the intermolecular interaction and C4 cleavage. The possible assembly of the huge in vivo enzyme-substrate complex consisting of glycan-bound mannan-binding lectin, MASP-2, and C4 is discussed. Our own and prior functional data suggest that C1s in the classical pathway of complement activated by, e.g., antigen-antibody complexes, also recognizes the C4 C345C domain through a CCP exosite. Our results provide a unified structural framework for understanding the early and essential step of C4 cleavage in the elimination of pathogens and altered self through two major pathways of complement activation.
authors.affiliation: Department of Molecular Biology and Genetics, Aarhus University, Aarhus, Denmark.
authors.forename: Rune T
authors.initials: RT
authors.lastname: Kidmose
authors.forename: Nick S
authors.initials: NS
authors.lastname: Laursen
authors.forename: József
authors.initials: J
authors.lastname: Dobó
authors.forename: Troels R
authors.initials: TR
authors.lastname: Kjaer
authors.forename: Sofia
authors.initials: S
authors.lastname: Sirotkina
authors.forename: Laure
authors.initials: L
authors.lastname: Yatime
authors.forename: Lars
authors.initials: L
authors.lastname: Sottrup-Jensen
authors.forename: Steffen
authors.initials: S
authors.lastname: Thiel
authors.forename: Péter
authors.initials: P
authors.lastname: Gál
authors.forename: Gregers R
authors.initials: GR
authors.lastname: Andersen
mesh_terms: D001665:Binding Sites; D003181:Complement C4 / Q000737:chemistry*; D003461:Crystallography / Q000379:methods; D057809:HEK293 Cells; D006801:Humans; D007113:Immunity, Innate; D008351:Mannans / Q000737:chemistry; D050606:Mannose-Binding Protein-Associated Serine Proteases / Q000737:chemistry; D008968:Molecular Conformation; D009154:Mutation; D011485:Protein Binding; D011487:Protein Conformation; D017434:Protein Structure, Tertiary; D011506:Proteins / Q000737:chemistry; D059748:Proteolysis; D011994:Recombinant Proteins / Q000737:chemistry; D055672:Static Electricity; D013379:Substrate Specificity
chemical_list: D003181:Complement C4; D008351:Mannans; D011506:Proteins; D011994:Recombinant Proteins; C105296:MASP2 protein, human; D050606:Mannose-Binding Protein-Associated Serine Proteases