Complement B/C2 (IPR011360)

Short name: Compl_C2_B

Overlapping homologous superfamilies

Family relationships


These proteins belong to MEROPS peptidase family S1 (chymotrypsin family, clan PA(S)), subfamily S1A.

This family contains two mammalian proteins, complement C2 and complement factor B, which, respectively, have analogous roles in the classical and alternative pathways of complement activation. These proteins are composed of three regions, an N-terminal three-module complement control protein domain, a von Willebrand factor A domain, and a C-terminal serine protease domain. Briefly, they are activated by cleavage and function as the serine protease components of the C3/C5 convertases, which play similar roles in these pathways although composed of different proteins. Homologs in non-mammalian species are often more or less equally related to mammalian C2 and B and may be designated as complement B/C2. Strongylocentrotus purpuratus (Purple sea urchin) has an atypical factor B with a five-module complement control protein domain.

The structures of the von Willebrand factor A and serine protease domains from human complement factor B (P00751) have been analysed [PMID: 15016353, PMID: 10637221]. The A domain forms the classical vWF A domain fold, which consists of a central beta-sheet flanked on both sides by amphipathic alpha helices. It contains an integrin-like MIDAS (metal ion-dependent adhesion site) motif that adopts the open conformation typical of integrin-ligand complexes, with an acidic residue from another A domain (provided by a fortuitous crystal contact) completing the coordination of the metal ion. Although a closed conformation was not observed, modelling studies suggest that the A domain could adopt this conformation, implying that as with integrins, ligand-binding may induce conformational changes which transduce a signal to other domains in the protein [PMID: 15016353]. The serine protease domain forms a chymotrypsin fold with several novel features [PMID: 10637221]. Like other serine proteases it forms two beta-sheets, composed of six beta-strands each, surrounded by surface helices and loops. However, several novel deletions and insertions occur within these surface helices and loops, and differences in active site conformation also exist.

GO terms

Biological Process

GO:0006956 complement activation

Molecular Function

GO:0004252 serine-type endopeptidase activity

Cellular Component

GO:0005576 extracellular region

Contributing signatures

Signatures from InterPro member databases are used to construct an entry.