Cloning and phylogenetic analysis of the alpha subunit of the eighth complement component (C8) in rainbow trout

The alpha subunit of the eighth complement component (C8) is a single-chain plasma glycoprotein which functions in the cytolytic process mediated by the complement system through a sequence of polymerization reactions with other terminal components. We have previously isolated and characterized the C8β and C8γ subunits of the eighth complement component in rainbow trout (Oncorhynchus mykiss). Here, we report the primary sequence, the tissue expression profile, the domain architecture and the phylogenetic analysis of the trout C8α gene. The deduced amino acid sequence of the trout C8α gene exhibits 44 and 43% identity with human and frog orthologs, respectively. The domain architecture of the trout C8α resembles that of mammalian orthologs, and the cysteine backbone shows a high degree of conservation. The trout C8α shows a similar expression profile with that of trout C8β and C8γ, pointing to the liver as the main source of the C8 genes expression. Although the presence of a fully developed lytic pathway of complement system is expected in teleost, this is the first report of the C8α gene in an organism other than mammalian.