Molecular cloning and protein analysis of divergent forms of the complement component C3 from a bony fish, the common carp (Cyprinus carpio): presence of variants lacking the catalytic histidine.

Unlike mammals, some bony fish species have been reported to possess multiple forms of the complement component C3. To explore the structural and functional diversity of bony fish C3, we have 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, C3-H2, C3-S, C3-Q1 and C3-Q2, each sharing 80-86 % amino acid sequence identity with the others. A striking amino acid substitution was noted at the position corresponding to the catalytic histidine, which is conserved in C3 from all the animals analyzed to date and provides the thioester with the ability to bind covalently to hydroxy groups on the target cells or to be hydrolyzed quickly; C3-S, C3-Q1 and C3-Q2 have serine, glutamine and glutamine residues, respectively, in place of the histidine which is conserved in C3-H1 and C3-H2. On the other hand, five distinct C3 forms, named C3-1 to C3-5, were purified from the serum of a single carp. N-terminal sequencing and covalent binding to [3H]glycine identified C3-1 as 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 threefold higher than that of C3-2, whereas C3-5 was inactive, suggesting that the thioester catalytic mechanism is not a necessary determinant for C3 activity and that C3 lacking the catalytic histidine plays a significant role in the complement system of carp and probably other bony fish.