Is a ferroxidase involved in the high-affinity iron uptake in Chlamydomonas reinhardtii?

In contrast to the halotolerant algae Dunaliella salina, Fe assimilation in the unicellular green algae Chlamydomonas reinhardtii involves a reductive step. After fractionation of proteins from Fe-sufficient and Fe-deficient cells, a distinct increase of a 150 kDa protein was observed in the Fe-deficient plasma membrane. The induction of the Fe3+ reductase activity and the 150 kDa protein under Fe deficiency was completely blocked by cycloheximide, indicating transcriptional regulation. After tryptic digestion, internal peptide sequences were obtained using MALDI-TOF MS. Based on the homology of these sequences to human hephaestin, a multicopper oxidase, which is involved in Fe homeostasis, the 150 kDa protein was named: ferroxidase-like protein (FLP). An oxidase activity measured as p-phenylenediamine oxidation, was also increased under Fe-deficiency, and this oxidase was inhibited by the inhibitor for multicopper oxidases, tetrathiomolybdate. Based on the physiological experiments and the homology in the primary structure of the ferroxidase-like protein to multicopper oxidases, we suggest that as in yeast and humans an oxidase is involved in the Fe uptake in C. reinhardtii.