INTERACTIONS OF HUMAN-COMPLEMENT COMPONENT C3 WITH FACTOR AND AND WITH COMPLEMENT RECEPTORS TYPE-1 (CR-1, 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 wi th many different ligands and receptors. Several experimental approach es involving blocking Abs, proteolytic fragmentation, and synthetic pe ptides 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 be en 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 o f 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 t ested for its ability to interact with these molecules. This study cle arly demonstrates that a segment near the N-terminus of the alpha'-cha in, which contains the negatively charged residues (730)DE and (736)EE , is involved in the interactions of C3 proteolytic fragments with fac tor B and complement receptors type 1 and 3, but not type 2. Factor H cofactor activity was also partially affected by these mutations. In c ontrast, mutation of the (937)KED triplet in the C3 933-942 segment ha d little or no effect on any of these activities.