title: Access to the complement factor B scissile bond is facilitated by association of factor B with C3b protein.
abstract: Factor B is a zymogen that carries the catalytic site of the complement alternative pathway C3 convertase. During convertase assembly, factor B associates with C3b and Mg(2+) forming a pro-convertase C3bB(Mg(2+)) that is cleaved at a single factor B site by factor D. In free factor B, a pair of salt bridges binds the Arg(234) side chain to Glu(446) and to Glu(207), forming a double latch structure that sequesters the scissile bond (between Arg(234) and Lys(235)) and minimizes its unproductive cleavage. It is unknown how the double latch is released in the pro-convertase. Here, we introduce single amino acid substitutions into factor B that preclude one or both of the Arg(234) salt bridges, and we examine their impact on several different pro-convertase complexes. Our results indicate that loss of the Arg(234)-Glu(446) salt bridge partially stabilizes C3bB(Mg(2+)). Loss of the Arg(234)-Glu(207) salt bridge has lesser effects. We propose that when factor B first associates with C3b, it bears two intact Arg(234) salt bridges. The complex rapidly dissociates unless the Arg(234)-Glu(446) salt bridge is released whereupon conformational changes occur that activate the metal ion-dependent adhesion site and partially stabilize the complex. The remaining salt bridge is then released, exposing the scissile bond and permitting factor D cleavage.
authors.affiliation: Department of Medicine, Division of Rheumatology, Washington University School of Medicine, St. Louis, Missouri 63110. Electronic address: dhourcad@dom.wustl.edu.
authors.forename: Dennis E
authors.initials: DE
authors.lastname: Hourcade
authors.affiliation: Department of Medicine, Division of Rheumatology, Washington University School of Medicine, St. Louis, Missouri 63110.
authors.forename: Lynne M
authors.initials: LM
authors.lastname: Mitchell
mesh_terms: D019943:Amino Acid Substitution; D003179:Complement C3b / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D011415:Complement Factor B / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D011416:Complement Factor D / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D006801:Humans; D009097:Multienzyme Complexes / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D020125:Mutation, Missense; D011485:Protein Binding
chemical_list: D009097:Multienzyme Complexes; D003179:Complement C3b; D011416:Complement Factor D; D011415:Complement Factor B