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

Abstract
The properdin-binding site in the human 3rd complement component (C3) was mapped by using isolated C3b, C3c, .alpha.- and .beta.-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 sulfate/polyacrylamide-gel electrophoresis. The .alpha.-chain polypeptides included a 68kDa [Kilodalton] and a 43 kDA polypeptide, which were generated by cleavage of C3b with factors I and H, and a 40kDa, 33kDa (C3d) and 27 kDa polypeptide, which were generated by cleavage of C3b with porcine elastase. Properdin binds to C3b, C3c, .alpha.-chain, and to the 43 kDa (factor I + H-derived), as well as to 40 kDa (elastase-derived) .alpha.-chain fragment, but not to the .beta.-chain 68 kDa, 33kDa (C3d) and 27 kDa .alpha.-chain fragments. The binding site for properdin resides on the 40-43 kDa C-terminal .alpha.-chain fragment of C3.