Functional analysis of the second and third complement component (C3) in carp using anti-peptide antibodies.

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We describe two diverged B/C2-like cDNA clones (B/C2-A and B/C2-B) isolated from a bony fish, common carp (Cyprinus carpio). B/C2-A shares the same domain structure with B and C2 of the vertebrate reported so far, and shows a close similarity to zebrafish B and medaka fish B/C2. These teleost sequences show almost the same similarity to C2 and B of higher vertebrates. In contrast, B/C2-B has a novel structural feature in that it contains four SCR modules and does not have a close relative upon phylogenetic analysis. Northern blotting revealed the presence of two transcripts with different size for each of B/C2-A and B/C2-B in the hepatopancreas of carp. A possible phylogenetic relationship between two carp sequences, and mammalian C2 and B is discussed. We have also isolated eight distinct cDNA clones encoding C3 from a single carp (Cyprinus carpio). The eight sequences were grouped into five C3 types, designated C3-H1, C-H2, C3-S, C3-Q1, and C3-Q2. On the other hand, five distinct C3 forms, named C3-1 to C3-5, were purified from serum of a single carp. N-terminal sequencing and covalent binding to [3H] glycine identified C3-1as the translated product of C3-S, while C3-2 was that of C3-H1, and C3-5 that of C3-H2. C3-1 showed a hemolytic activity three-fold higher than that of C3-2, whereas C3-5 was inactive, suggesting that the thioester-catalytic mechanism is not a necessary determinant for the C3 activity and that C3 lacking the catalytic histidine plays a significant role in the complement system of carp and probably other bone fish.