Structural analysis of chicken factor B-like protease and comparison with mammalian complement proteins factor B and C2.

Chicken complement factor B-like protease is a glycoprotein of 95 kDa. Activation of chicken serum complement with inulin cleaved the B-like protease into an N-terminal Ba fragment of 37 kDa and a C-terminal Bb fragment of 60 kDa. The whole protein and the two fragments were purified by affinity chromatography using mAb to chicken Ba or Bb followed by ion exchange chromatography. Amino acid sequencing showed that chicken B-like protease 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. More than 200 residues of the Ba sequence and two N-linked glycosylation sites were established by amino acid sequencing of peptides derived by digestion with four proteases. Comparison of human and mouse C2 and B sequences indicated a slower evolutionary rate for B (85% sequence identity) than for C2 (74% sequence identity). 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. Taking the slower evolutionary rate of factor B into account, chicken factor B-like protease seems to be equally related to mammalian complement components B and C2, and the B-like protease most likely represents the present-day descendant of a common ancestral protein for mammalian B and C2. This conclusion is in agreement with the requirement for the B-like protease in both classical and alternative activation pathways for chicken complement, and with the apparent lack of a chicken serum protein with exclusive C2 activity.