Cloning, structure, and function of two rainbow trout Bf molecules.

The factor B (Bf) and C2 complement genes are closely linked within the MHC class III region and are thought to have arisen by gene duplication from a single gene encoding an ancestral molecule; the animal phyla in which this duplication event took 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 mammalian Bf and C2. In contrast, here we present the characterization of 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 than C2, yet play a role in both alternative and classical pathways of complement activation. The full lengths of Bf-1 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 in liver and appear to be single-copy genes. The isolated Bf-1 and Bf-2 proteins are able to form the alternative pathway C3 convertase and are cleaved (in the presence of purified trout C3, trout factor D, and Mg2+ EGTA) into Ba- and Bb-like fragments in 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 through both the alternative and classical pathways; whether Bf-1 possess similar activity is unclear at present.