A transferrin-like homolog in amphioxus Branchiostoma belcheri: Identification, expression and functional characterization.

Previous studies have shown the universal presence of transferrin (Tf) in both invertebrates and vertebrates, but little information is available regarding Tf in amphioxus, a protochordate on the evolutionary boundary between invertebrates and vertebrates. Here we isolated a Tf-like homolog from Branchiostoma belcheri, which encodes a deduced protein, BbTfl, of 1256 amino acids containing a N-terminal signal peptide, a conserved transferrin domain in its N-terminal lobe, with a putative iron-binding site consisting of Asp63 and Try188 and another transferrin domain in its C-terminal lobe with an long intervening sequence of 305 amino acids. Phylogenetic analysis shows BbTfl is grouped together with all the invertebrate Tfs and located at the base of melanotransferrins and other Tfs. Quantitative PCR analysis reveals that exposure to Escherichia coli and Vibrio anguillarum causes a significant increase in BbTfl expression mainly in the gut within 12-24h, suggesting that BbTfl is a positive acute phase reactant involved in the immune defense of B. belcheri. The recombinant N-terminal lobe, BbTflN, is able to bind iron and to inhibit E. coli and Staphylococcus aureus growth. Moreover, the antibacterial activity of BbTflN markedly decreases in the presence of excess iron. All these results provide a direct empirical evidence establishing a definitive link between binding to iron and bacterial growth-inhibiting activity. It is also shown that BbTfl is expressed in a tissue-specific manner, with the most abundant expression in the hepatic caecum, hind-gut and ovary, supporting the idea that the digestive system including the hepatic caecum of amphioxus is the primary tissue involved in acute phase response.