Middle ferritin genes from the icefish Chionodraco rastrospinosus: Comparative analysis and evolution of fish ferritins

Ferritin is a major intracellular iron storage protein in higher vertebrates and plays an important role in iron metabolism. This study reports the identification from the Antarctic icefish Chionodraco rastrospinosus of a complete mRNA sequence and four partial mRNA sequences, all encoding the ferritin M subunit and sharing a clear homology with the ferritin M-chain of other fish species. The open reading frame of the complete ferritin M transcript is of 528 base pairs and encodes a protein of 176 amino acids that retains the residues involved in the ferroxidase diiron center and in the ferrihydrite nucleation center. Despite the absence of hemoglobin and of any appreciable amount of iron in the icefish blood, RT-PCR analysis shows that H and M ferritin subunits are expressed both in blood and in other tissues, such as spleen, head kidney, liver and kidney. Phylogenetic analysis shows that the H and M subunits form two well separated clusters. Basal to these two clusters emerges a heterogeneous cluster, formed by two Danio rerio M, a Salmo salar M and an Orechromis niloticus H isoforms; these forms maybe represent the heritage of ancestral forms from which arose the two major H and M subunits of the fishes.