Complement factors B and C2 are two critical proteases for complement activation. They both contain three CCP or Sushi domains, a trypsin-type serine protease domain and a single VWA domain with a conserved metal ion dependent adhesion site referred commonly as the MIDAS motif. Orthologues of these molecules are found from echinoderms to chordates. During complement activation, the CCP domains are cleaved off, resulting in the formation of an active protease that cleaves and activates complement C3. Complement C2 is in the classical pathway and complement B is in the alternative pathway. The interaction of C2 with C4 and of factor B with C3b are both dependent on Mg2+ binding sites within the VWA domains and the VWA domain of factor B has been shown to mediate the binding of C3. This is consistent with the common inferred function of VWA domains as magnesium-dependent protein interaction domains.