Competition for binding sites on C3b by CR1, CR2, MCP, factor B and factor H.
The reaction of radiolabeled C3b-binding proteins with C3b-coated particles has been investigated. CR1 binding was inhibited by factor H and factor B (in the presence of properdin), but not by properdin alone. CR2 and MCP binding were also inhibited by factor H. Therefore factor H, factor B, CR1, CR2 and MCP probably comprise a group of mutually competitive proteins with similar or overlapping binding sites on C3b. These results correlate with their structural homology and suggest that they all evolved from a single C3b-binding molecule. Factor H, CR1 and MCP are also cofactors for the factor-I-mediated cleavage of C3b. A species incompatibility between rat factor I and human CR1 for the cleavage of human C3b suggests the possibility that cofactors may also function by interacting directly with factor I.
Farries
T C
TC
Howard Hughes Medical Institute Laboratories, Washington University School of Medicine, St. Louis, Mo.
Seya
T
T
Harrison
R A
RA
Atkinson
J P
JP
D000818:Animals; D015703:Antigens, CD; D001665:Binding Sites; D001667:Binding, Competitive; D003179:Complement C3b / Q000378:metabolism*; D003180:Complement C3b Inactivator Proteins / Q000378:metabolism* / Q000494:pharmacology; D011415:Complement Factor B / Q000378:metabolism* / Q000494:pharmacology; D017242:Complement Factor H; D004792:Enzyme Precursors / Q000378:metabolism*; D006801:Humans; D051916:Membrane Cofactor Protein; D008562:Membrane Glycoproteins / Q000378:metabolism*; D008565:Membrane Proteins / Q000378:metabolism*; D011414:Properdin / Q000494:pharmacology; D051381:Rats; D012756:Sheep
D015703:Antigens, CD; C497857:CD46 protein, human; C493495:CFH protein, human; C497860:Cd46 protein, rat; D003180:Complement C3b Inactivator Proteins; D004792:Enzyme Precursors; D051916:Membrane Cofactor Protein; D008562:Membrane Glycoproteins; D008565:Membrane Proteins; D011414:Properdin; D003179:Complement C3b; D017242:Complement Factor H; D011415:Complement Factor B