Identity of the putative serine-proteinase fold in proteins of the complement system with nine relevant crystal structures.

The serine-proteinase domain is responsible for the proteolytic events that occur during complement activation. The sequences of nine serine proteinases of known crystal structure were compared with the serine-proteinase sequences in the six complement proteins C1r, C1s, C2, factor B, factor I and factor D to assess the degree of structural homology of the latter with the crystal structures. All sequence insertions and deletions were readily located at the protein surface. The internal location of disulphide bridges and the surface location of putative glycosylation sites are compatible with this structure. Secondary-structure predictions for the sequences were fully consistent with the crystal structures. It is concluded that the double subdomain beta-sheet motif is retained in the complement sequences, but that localized differences are observed for factor I, C2 and factor B.