CLONING, STRUCTURE, AND FUNCTION OF 2 RAINBOW-TROUT BF MOLECULES

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.