Abstract
Transferrin (TF) is a protein that plays a central role in iron metabolism. This protein is associated with the innate immune system, which is responsible for disease defense responses after bacterial infection. The clear link between TF and the immune defense mechanism has led researchers to consider TF as a candidate gene for disease resistance. In this study, the Miichthys miiuy (miiuy croaker) TF gene (MIMI-TF) was cloned and characterized. The gene structure consisted of a coding region of 2070 nucleotides divided into 17 exons, as well as a non-coding region that included 16 introns and spans 6757 nucleotides. The deduced MIMI-TF protein consisted of 689 amino acids that comprised a signal peptide and two lobes (N- and C-lobes). MIMI-TF expression was significantly up-regulated after infection with Vibrio anguillarum. A series of model tests implemented in the CODEML program showed that TF underwent a complex evolutionary process. Branch-site models revealed that vertebrate TF was vastly different from that of invertebrates, and that the TF of the ancestors of aquatic and terrestrial organisms underwent different selection pressures. The site models detected 10 positively selected sites in extant TF genes. One site was located in the cleft between the N1 and N2 domains and was expected to affect the capability of TF to bind to or release iron indirectly. In addition, eight sites were found near the TF exterior. Two of these sites, which could have evolved from the competition for iron between pathogenic bacteria and TF, were located in potential pathogen-binding domains. Our results could be used to further investigate the function of TF and the selective mechanisms involved.
Highlights
As a main transporter in the blood stream of vertebrates, transferrin (TF) is vital to iron metabolism and maintains the iron requirement of cells through its binding and transport of iron [1]
The miiuy (miiuy croaker) TF gene (MIMI-TF) gene structure was determined from the alignments of the genomic and cDNA sequences
The MIMI-TF genomic DNA sequence described in this study consisted of 17 exons and 16 introns (Fig. 1)
Summary
As a main transporter in the blood stream of vertebrates, transferrin (TF) is vital to iron metabolism and maintains the iron requirement of cells through its binding and transport of iron [1]. TF absorbs iron in the gut, between circumjacent sites of storage and usage, maintaining an appropriate iron balance in the body [1,8,9]. This protein is vital to metal transport. The TF gene comprises two functional domains: the N-terminal half domain (N-lobe) and the C-terminal half domain (C-lobe). Each of these two domains consists of ,335 amino acids, with a highly conserved iron-binding site in each lobe [10,11]. The N-lobe is apparently more important to iron binding, whereas the C-lobe may be the primary binding site in the TF receptor [12]
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