title: A conserved element in the serine protease domain of complement factor B.
abstract: Factor B and C2 are serine proteases that carry the catalytic sites of the complement C3 and C5 convertases. Their protease domains are activated by conformational changes that occur during convertase assembly and are deactivated upon convertase dissociation. Factor B and C2 share an 8-amino acid conserved sequence near their serine protease termini that is not seen in other serine proteases. To determine its importance, 24 factor B mutants were generated, each with a single amino acid substitution in this region. Whereas most mutants were functionally neutral, all five different substitutions of aspartic acid 715 and one phenylalanine 716 substitution severely reduced hemolytic activity. Several aspartic acid 715 mutants permitted the steps of convertase assembly including C3b-dependent factor D-mediated cleavage and activation of the high affinity C3b-binding site, but the resulting complexes did not cleave C3. Given that factor B and C2 share the same biological substrates and that part of the trypsin-like substrate specificity region is not apparent in either protein, we propose that the conserved region plays a critical role in the conformational regulation of the catalytic site and could offer a highly specific target for the therapeutic inhibition of complement.
authors.affiliation: Department of Medicine, Washington University School of Medicine, St. Louis, Missouri 63110, USA. dhourcad@im.wustl.edu
authors.forename: D E
authors.initials: DE
authors.lastname: Hourcade
authors.forename: L M
authors.initials: LM
authors.lastname: Mitchell
authors.forename: T J
authors.initials: TJ
authors.lastname: Oglesby
mesh_terms: D000595:Amino Acid Sequence; D001665:Binding Sites / Q000235:genetics; D003175:Complement C2 / Q000378:metabolism; D003179:Complement C3b / Q000378:metabolism; D011415:Complement Factor B / Q000737:chemistry* / Q000235:genetics; D011416:Complement Factor D / Q000378:metabolism; D017124:Conserved Sequence / Q000235:genetics*; D006461:Hemolysis / Q000235:genetics; D006801:Humans; D008968:Molecular Conformation; D008969:Molecular Sequence Data; D017354:Point Mutation / Q000235:genetics; D011414:Properdin / Q000494:pharmacology; D011485:Protein Binding / Q000235:genetics; D011994:Recombinant Proteins / Q000737:chemistry; D017386:Sequence Homology, Amino Acid; D012697:Serine Endopeptidases / Q000737:chemistry; D013379:Substrate Specificity
chemical_list: D003175:Complement C2; D011994:Recombinant Proteins; D011414:Properdin; D003179:Complement C3b; D012697:Serine Endopeptidases; C493488:CFD protein, human; D011416:Complement Factor D; D011415:Complement Factor B