Zymosan, a yeast cell wall preparation that binds activated forms of complement C3, is a useful model target to activate the complement system. In our trial to analyze C3 diversity in Nile tilapia at the protein level using zymosan, we found that a novel 240-kDa serum protein (ZBP-240) also bound to zymosan in addition to C3-derived fragments. In the present study, we aimed to characterize tilapia C3 and ZBP-240, focusing on their immune-related functions. Four distinct C3 isoforms were purified from tilapia serum and shown to possess an intrachain thioester bond. ZBP-240 was also isolated from tilapia serum and examined for its binding properties to various microbial targets. As a result, ZBP-240 showed a wide spectrum of binding to Gram-positive and Gram-negative bacteria and yeasts. Amino acid sequence analysis of CNBr fragments of ZBP-240 suggested that this is a novel protein with no homologous sequence in protein databases. It was also suggested that the binding of ZBP-240 to microbes largely depends on hydrophobic interactions in a divalent-cation-independent manner, and that there may be a divalent-cation-dependent factor that enhances the binding of ZBP-240 in tilapia serum. Interestingly, ZBP-240 showed opsonic activity for tilapia kidney phagocytes at a level comparable to that of C3, implying that ZBP-240 is a novel teleost opsonic serum protein.
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