STRUCTURAL C3 DIVERSITY IN FISH - CHARACTERIZATION OF 5 FORMS OF C3 IN THE DIPLOID FISH SPANS AURATA

In virtually all species examined to date, the functionally active thi rd component of complement (C3) is encoded by a single gene. We have r ecently demonstrated, however, that trout possess three structurally a nd functionally distinct active C3 that represent the products of at l east two different C3 genes. In the present study, we provide evidence that multiple forms of functional C3 occur not only in the rainbow tr out (Salmo gairdneri), a quasi-tetraploid old teleost fish, but also i n the diploid gilthead sea bream (Sparus aurata), a modern teleost fis h. In the gilthead sea bream, we have characterized five different for ms of C3 (C3-1, C3-2, C3-3, C3-4, and C3-5); in addition, we have iden tified and isolated a C5-like molecule. Each of the six proteins was c omposed of an alpha-chain and a beta-chain; however, only the C3 isofo rms contained a thioester bond in their alpha-chains. These proteins a ll differed in the molecular masses of their alpha- and beta-chains an d in their glycosylation patterns, reactivity with various Abs, trypti c peptide maps, and NH2-terminal sequences of their chains. These obse rvations together with the fact that each of the six proteins were als o purified from a single fish suggest that the C3 isoforms represent t he products of several genes. The presence of multiple forms of C3 in a modern diploid fish, very distant in evolutionary time from the trou t, strongly suggests that the C3 isoforms generated once from a single C3 gene have remained functional, in the genomes of these animals. Th ese findings not only have important consequences for our understandin g of the evolution of the C3 protein, but also provide evidence for th e formation and generation of a new C3-related gene family.