Interaction of C3b(2)--IgG complexes with complement proteins properdin, factor B and factor H: implications for amplification.
Nascent C3b can form ester bonds with various target molecules on the cell surface and in the fluid phase. Previously, we showed that C3b(2)--IgG complexes represent the major covalent product of C3 activation in serum [Lutz, Stammler, Jelezarova, Nater and Späth (1996) Blood 88, 184--193]. In the present report, binding of alternative pathway proteins to purified C3b(2)--IgG complexes was studied in the fluid phase by using biotinylated IgG for C3b(2)--IgG generation and avidin-coated plates to capture complexes. Up to seven moles of properdin 'monomer' bound per mole of C3b(2)--IgG at physiological conditions in the absence of any other complement protein. At low properdin/C3b(2)--IgG ratios bivalent binding was preferred. Neither factor H nor factor B affected properdin binding. On the other hand, properdin strongly stimulated factor B binding. Interactions of all three proteins with C3b(2)--IgG exhibited pH optima. An ionic strength optimum was most pronounced for properdin, while factor B binding was largely independent of the salt concentration. C3b(2)--IgG complexes were powerful precursors of the alternative pathway C3 convertase. In the presence of properdin, C3 convertase generated from C3b(2)--IgG cleaved about sevenfold more C3 than the enzyme generated on C3b. C3b(2)--IgG complexes could therefore maintain the amplification loop of complement longer than free C3b.
Jelezarova
E
E
Institute of Biochemistry, Swiss Federal Institute of Technology, ETH-Zentrum, CH 8092, Zurich, Switzerland.
Vogt
A
A
Lutz
H U
HU
D050577:Complement C3-C5 Convertases / Q000378:metabolism; D003179:Complement C3b / Q000737:chemistry / Q000378:metabolism*; D011415:Complement Factor B / Q000737:chemistry / Q000378:metabolism*; D017242:Complement Factor H / Q000737:chemistry / Q000378:metabolism*; D019281:Dimerization; D004305:Dose-Response Relationship, Drug; D004591:Electrophoresis, Polyacrylamide Gel; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007074:Immunoglobulin G / Q000737:chemistry / Q000378:metabolism*; D007477:Ions; D007700:Kinetics; D011414:Properdin / Q000737:chemistry / Q000378:metabolism*; D011485:Protein Binding; D011863:Radioimmunoassay; D012965:Sodium Chloride / Q000494:pharmacology
C493495:CFH protein, human; D007074:Immunoglobulin G; D007477:Ions; D011414:Properdin; D012965:Sodium Chloride; D003179:Complement C3b; D017242:Complement Factor H; D050577:Complement C3-C5 Convertases; D011415:Complement Factor B