Mammalian C2 binds to surface-bound C4b to form the C4b:C2 complex, which is subsequently cleaved by C1s or MASPs to form the classical C3 convertase - C4b:C2b (Laich A & Sim RB 2001; Wallis R et al 2007).
The chicken event was inferred from the curated human reaction. A homologue for C2 was not found in chickens (Barta O & Hubbert NL 1978). It was assumed that the role of C2 may be fulfilled by the chicken complement factor B-like protease (factor B), an evolutionary remnant of a common C2/factor B ancestor (Kjalke M et al. 1993; Salter-Cid L & Flajnik MF 1995). Chicken factor B-like protease (factor B) is a glycoprotein of 95 kDa. Activation of chicken serum complement with inulin cleaved factor B into an N-terminal Ba fragment of 37 kDa and a C-terminal Bb fragment of 60 kDa (Kjalke M et al. 1993). The whole protein and the two fragments were purified by affinity chromatography using mAb to chicken Ba or Bb followed by ion exchange chromatography (Kjalke M et al. 1993). Amino acid sequencing showed that the chicken factor B was cleaved at a site homologous to that cleaved in mammalian complement components B and C2 on activation. Limited tryptic digestion of the B-like protease generated fragments similar to Ba and Bb (Kjalke M et al. 1993). Comparison of chicken Ba to human and mouse C2b and Ba showed 42 to 45% sequence identity with respect to C2b fragments, and 46 to 49% sequence identity with respect to Ba fragments (Kjalke M et al. 1993). Thus, chicken factor B-like protease seemed to be equally related to mammalian complement components B and C2, and the B-like protease is thought to represent the present-day descendant of a common ancestral protein for mammalian B and C2 (Kjalke M et al. 1993).