Jo. Sunyer et al., STRUCTURAL C3 DIVERSITY IN FISH - CHARACTERIZATION OF 5 FORMS OF C3 IN THE DIPLOID FISH SPANS AURATA, The Journal of immunology, 158(6), 1997, pp. 2813-2821
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.