Crassostrea gigas ferritin: cDNA sequence analysis for two heavy chain type subunits and protein purification

Ferritin has been shown as being the principal iron storage in the majority of living organisms. In marine species, ferritin is also involved in high-level accumulation of (210)Po. As part of our work on the investigation of these radionuclides' concentration in natural environment, ferritin was searched at the gene and protein level. Ferritin was purified from the visceral mass of the oyster Crassostrea gigas by ion-exchange chromatography and HPLC. SDS-PAGE revealed one band of 20 kDa. An Expressed Sequence Tag (EST) library was screened and led to the identification of two complementary DNA (cDNA) involved in ferritin subunit expression. The complete coding sequences and the untranslated regions (UTRs) of the two genes were obtained and a 5' Rapid Amplification of cDNA Ends (RACE) was used to obtain the two iron-responsive elements (IREs) with the predicted stem-loop structures usually present in the 5'-UTR of ferritin mRNA. Sequence alignment in amino acid of the two new cDNA showed an identity with Pinctada fucata (85.4-88.3%), Lymnaea stagnalis (79.3-82.2%) and Helix pomatia (79.1-79.1%). The residues responsible for the ferroxidase center, conserved in all vertebrate H-ferritins, are present in the two oyster ferritin subunits. Oyster ferritins do not present the special characteristics of other invertebrate ferritins like insect ferritins but have some functional similarities with the vertebrate H chains ferritin.