The factor B (Bf) and C2 complement genes are closely linked within th
e MHC class III region and are thought to have arisen by gene duplicat
ion from a single gene encoding an ancestral molecule; the animal phyl
a in which this duplication event tool; place is unknown. Two teleost
fish, (zebrafish and medaka fish) have each been shown to possess only
a single molecule that shows an equivalent degree of similarity to ma
mmalian Bf and CZ. In contrast, here we present the characterization o
f two factor B molecules (Bf-1 and Bf-2) in another teleost fish (the
rainbow trout) that are about 9% more similar to mammalian factor B th
an C2, yet play a role in both alternative and classical pathways of c
omplement activation. The full lengths of Bf-l and Bf-2 cDNAs are 2509
and 2560 bp, respectively, and their deduced amino acid sequences are
75% identical, Both trout Bf genes are mainly expressed ire lives and
appear to be single-copy genes. The isolated Bf-1 and Bf-2 proteins a
re able to form the alternative pathway C3 convertase and are cleaved
(in the presence of purified trout C3, trout factor D, and Mg(2+)EGTA)
into Ba- and Bib-like fragments ire a manner similar to that seen for
mammalian factor B, The most remarkable feature of trout Bf-2 is its
ability to restore the hemolytic activity of trout Bf-depleted serum t
hrough both the alternative and classical pathways; whether Bf-1 posse
ss similar activity is unclear at present.