Allelic discrimination, three-dimensional analysis and gene expression of multiple transferrin alleles of common carp (Cyprinus carpio L.).

We cloned and sequenced four different transferrin (Tf) alleles (C, D, F and G) of European common carp (Cyprinus carpio carpio L.) and studied allelic diversity with respect to differences in sequence, constitutive transcription and three-dimensional structure. Most of the disulfide bonds were conserved between human and carp Tf, and modeling confirmed the overall conservation of the three-dimensional structure of carp Tf. While the iron-binding sites in the C-lobe of carp Tf were completely conserved, in the N-lobe the majority of iron-coordinating residues were not conserved. This may have a serious impact on the ability of carp Tf to bind iron with both the C- and N-lobe. In contrast to human Tf, we could not detect potential N-glycosylation sites in carp Tf, which does not seem to be a glycoprotein. Comparison of the cDNA of the four Tf alleles of carp indicated 21 polymorphic sites of which 13 resulted in non-synonymous changes. Allelic diversity did not seem to influence the overall conservation of carp Tf. Neither the iron binding sites nor the receptor binding of carp Tf seemed influenced by allelic diversity. Possibly, interaction with pathogen-associated receptors for Tf could be influenced by allelic diversity. Basal gene expression of Tf alleles D and G was especially high in carp liver. Although we could detect a higher transcription level of allele D than of Tf allele G in head kidney, thymus and spleen, the differences seem minor with respect to the very high transcription level in liver. Preliminary results with Tf-typed serum suggest a difference in the ability of Tf alleles D and G to modulate LPS-induced NO production in carp macrophages.