Interactions of human complement component C3 with factor B and with complement receptors type 1 (CR1, CD35) and type 3 (CR3, CD11b/CD18) involve an acidic sequence at the N-terminus of C3 alpha'-chain.

Complement component C3 is a multifunctional protein that interacts with many different ligands and receptors. Several experimental approaches involving blocking Abs, proteolytic fragmentation, and synthetic peptides have been used to predict which regions in C3 are required for its various functions. We have used site-directed mutagenesis to alter specific residues in the C3 segments 730-739 and 933-942 that have been proposed to be required for the binding of factor B by C3, and have examined, within the context of the intact C3b molecule, the effect of these substitutions on the C3b-factor B interaction. Because it has been suggested that factor H and complement receptors type 1, 2, and 3 may recognize sites in C3 that partially or completely overlap those of factor B, the relevant proteolytic fragment of each mutant C3 was tested for its ability to interact with these molecules. This study clearly demonstrates that a segment near the N-terminus of the alpha'-chain, which contains the negatively charged residues 730DE and 736EE, is involved in the interactions of C3 proteolytic fragments with factor B and complement receptors type 1 and 3, but not type 2. Factor H cofactor activity was also partially affected by these mutations. In contrast, mutation of the 937KED triplet in the C3 933-942 segment had little or no effect on any of these activities.