title: Analysis of the interaction between properdin and factor B, components of the alternative-pathway C3 convertase of complement.
abstract: The interactions between Factor B (B), its activation products Ba and Bb, properdin (P) and C3i or C3b, components that together form the alternative-pathway C3 convertase enzyme of human complement, have been analysed. Fluid-phase complexes of the purified components C3i, B and P were probed with the homobifunctional cross-linking reagent disuccinimidyl tartarate, and efficient cross-linking of B to P was observed. The 140 kDa B-P conjugate formed was cleaved by Factor D to yield a single product of 85 kDa. This is consistent with a Ba-P heterodimer, and suggests that the initial interaction of B and P includes an interaction of P with the Ba domain of intact B. (The Ba fragment is not retained in the active P-stabilized complex, C3bBbP). By contrast, no cross-linking of P to the Bb domain of B could be demonstrated. Binding studies on cellular intermediates also provided evidence for a site of interaction between B and P, with high concentrations of B inhibiting P binding to EAC3b (sheep erythrocytes coated with antibody and C3b). Neither isolated Ba nor Bb had any effect on the P-EAC3b interaction. High concentrations of B also accelerated the decay of the functional EAC3bBbP complex. These data indicate that the positive co-operativity of binding to C3i or to C3b between B and P is mediated, at least in part, through a direct interaction between B and P.
mesh_terms: D003166:Complement Activating Enzymes / Q000378:metabolism*; D003167:Complement Activation; D003176:Complement C3 / Q000378:metabolism; D050577:Complement C3-C5 Convertases / Q000378:metabolism*; D003179:Complement C3b / Q000378:metabolism; D011415:Complement Factor B / Q000378:metabolism*; D003170:Complement Pathway, Alternative; D003432:Cross-Linking Reagents; D004492:Edetic Acid / Q000494:pharmacology; D004591:Electrophoresis, Polyacrylamide Gel; D004792:Enzyme Precursors / Q000378:metabolism*; D006801:Humans; D046911:Macromolecular Substances; D008970:Molecular Weight; D010446:Peptide Fragments / Q000378:metabolism; D011414:Properdin / Q000378:metabolism*; D011485:Protein Binding; D013388:Succinimides; D014357:Trypsin / Q000494:pharmacology
chemical_list: D003176:Complement C3; D003432:Cross-Linking Reagents; D004792:Enzyme Precursors; D046911:Macromolecular Substances; D010446:Peptide Fragments; D013388:Succinimides; C051367:complement C3bBa complex; D011414:Properdin; C018052:disuccinimidyl tartarate; D003179:Complement C3b; D004492:Edetic Acid; D003166:Complement Activating Enzymes; D050577:Complement C3-C5 Convertases; D014357:Trypsin; D011415:Complement Factor B