The gamma subunit of the eighth complement component (C8) in rainbow trout

Of the 35 proteins, enzymes, receptors and regulatory components of the complement system, C8γ is unique in that it is the only lipocalin. C8γ is a subunit of the C8 molecule, which is one of the five components (C5b, C6, C7, C8 and C9) that interact as a consequence of complement activation to form the membrane attack complex. Until now, C8γ has been characterized only in mammalian species. In order to elucidate the phylogeny of this molecule, we have cloned the C8γ subunit in rainbow trout (Oncorhynchus mykiss), a teleost fish representing a critical point in the evolutionary divergence of the complement system. The deduced amino acid sequence of trout C8γ shows significant identity (37%) to the human C8γ homolog and much lower to the other known lipocalins. The lipocalin domain is present and all the cysteine residues are conserved. The trout C8γ gene is probably present as a single copy in the trout genome showing a differential expression pattern among tissues investigated.