Purification of a major serum protein of rainbow trout (Salmo gairdneri) homologous to the third component of mammalian complement.

The present paper describes the purification and characterization of a major serum protein of the rainbow trout (Salmo gairdneri) required for complement-related functions (target cell lysis and opsonization). This protein, termed C3-1, was identified as the third component (C3) of rainbow trout complement. It is homologous to component C3 of human and other mammalian complements based on its structural and functional characteristics. Rainbow trout C3-1 is a beta-globulin of Mr = 190,000 composed of two polypeptide chains (Mr = 128,000 alpha-chain and Mr = 74,000 beta-chain) linked by disulfide bonds. The chain structure of C3-1 is, therefore, very similar to those of the third and fifth components of mammalian complement. C3-1 retains, on its alpha-chain, a unique hidden thiol site that can be exposed upon attachment of methylamine in the same way as human C3 and C4. The amino acid composition and NH2-terminal sequence of C3-1 show significant homology to mammalian C3 and to cobra venom factor (cobra C3).