Cloning and purification of the Rainbow Trout fifth component of complement (C5).

To gain further insight into the evolutionary history of the complement proteins C3, C4, and C5 we have now cloned the fifthcomponent of complement from a rainbow trout (Oncorhynchus mykiss) liver cDNA library; this is the ®rst report of C5 cloningin a species other than human and mouse. The deduced amino acid sequence of a partial cDNA clone (2.25 kb), representingapproximately 44% of the coding sequence, showed 60 and 58% similarity to human and mouse C5, respectively. To validatethe molecular information derived from the cloning we developed an improved puri®cation protocol. Mass spectrometricanalysis of C5 tryptic digests yielded peptide signals that matched theoretical protein sequence derived from the partialcDNA. Northern blot analysis of RNA from various tissues showed the presence of a single mRNA transcript in trout liverand Southern blot analysis indicated that the gene coding for C5 is present as a single copy in the trout genome. The presence ofC5 in trout suggests that C3, C4, and C5 must have diverged before the appearance of teleost fish.