Segment spanning residues 727-768 of the complement C3 sequence contains a neoantigenic site and accommodates the binding of CR1, factor H, and factor B.
CR1, CR2, DAF, MCP, factor H, C4bp, factor B, and C3 are members of a family of structurally related molecules, the majority of which belong to the complement system. Several of these molecules also share functional features such as cofactor and decay/dissociation activity and compete with one another in binding to C3b. Since factor H appears to bind to multiple sites in C3, we investigated the relationship between the factor H- and CR1-binding sites in C3b. Factor H binding to C3b is inhibited by either the C3c or C3d fragments, and addition of both fragments together augments this inhibition. One monoclonal anti-C3c antibody, anti-C3-9, which recognizes a neoantigenic epitope expressed upon cleavage to C3 to C3b, inhibited both factor H and CR1 binding to EC3b cells. This monoclonal antibody (MoAb) also inhibited factor B binding to EC3b. Two observations further supported our hypothesis that these molecules bind to proximal sites in C3b. First, a synthetic peptide spanning this region of C3b (C3(727-768)) inhibited factor H binding. Second, antibodies raised against this peptide inhibited binding to CR1, factor H, and factor B to C3b. These data show that H binds to at least two sites in C3b: the site in the C3c fragment is within the identified CR1-binding domain while the site in the C3d fragment surrounds the CR2-binding site.(ABSTRACT TRUNCATED AT 250 WORDS)
Becherer
J D
JD
Basel Institute for Immunology, Switzerland.
Alsenz
J
J
Esparza
I
I
Hack
C E
CE
Lambris
J D
JD
D000595:Amino Acid Sequence; D000911:Antibodies, Monoclonal; D001665:Binding Sites; D001667:Binding, Competitive; D050577:Complement C3-C5 Convertases / Q000378:metabolism*; D003180:Complement C3b Inactivator Proteins / Q000378:metabolism*; D015932:Complement C3c / Q000737:chemistry* / Q000276:immunology / Q000378:metabolism; D011415:Complement Factor B / Q000378:metabolism*; D017242:Complement Factor H; D000939:Epitopes / Q000737:chemistry* / Q000276:immunology; D008969:Molecular Sequence Data; D010446:Peptide Fragments / Q000737:chemistry* / Q000378:metabolism
D000911:Antibodies, Monoclonal; D003180:Complement C3b Inactivator Proteins; D000939:Epitopes; D010446:Peptide Fragments; D015932:Complement C3c; D017242:Complement Factor H; D050577:Complement C3-C5 Convertases; D011415:Complement Factor B