Mapping of the properdin-binding site in the third component of complement

Abstract

The properdin-binding site in the human third complement component (C3) was mapped by using isolated C3b, C3c, α- and β-chains of C3 and C3 polypeptide fragments and an enzyme-linked-immunosorbent-assay procedure. The C3 chains and the polypeptide fragments were purified to homogeneity by preparative sodium dodecyl sulphate/polyacrylamide-gel electrophoresis. The α-chain polypeptides included a 68 kDa polypeptide, which were generated by cleavage of C3b with factors I and H, and a 40 kDa, 33 kDa (C3d) and 27 kDa polypeptide, which were generated by cleavage of C3b with porcine elastase. It was shown that properdin binds to C3b, C3c, α-chain, and to the 43 kDa (factor-I + H-derived), as well as to 40 kDa (elastase-derived) α-chain fragment, but not to the β-chain 68 kDa, 33 kDa (C3d) and 27 kDa α-chain fragments. Thus the binding site for properdin resides on the 40-43 kDa C-terminal α-chain fragment of C3.