The Involvement of a Multicopper Oxidase in Iron Uptake by the Green Algae Chlamydomonas reinhardtii

In the unicellular green algae Chlamydomonas reinhardtii, high-affinity uptake of iron (Fe) requires an Fe(3+)-chelate reductase and an Fe transporter. Neither of these proteins nor their corresponding genes have been isolated. We previously identified, by analysis of differentially expressed plasma membrane proteins, an approximately 150-kD protein whose synthesis was induced under conditions of Fe-deficient growth. Based on homology of internal peptide sequences to the multicopper oxidase hephaestin, this protein was proposed to be a ferroxidase. A nucleotide sequence to the full-length cDNA clone for this ferroxidase-like protein has been obtained. Analysis of the primary amino acid sequence revealed a putative transmembrane domain near the amino terminus of the protein and signature sequences for two multicopper oxidase I motifs and one multicopper oxidase II motif. The ferroxidase-like gene was transcribed under conditions of Fe deficiency. Consistent with the role of a copper (Cu)-containing protein in Fe homeostasis, growth of cells in Cu-depleted media eliminated high-affinity Fe uptake, and Cu-deficient cells that were grown in optimal Fe showed greatly reduced Fe accumulation compared with control, Cu-sufficient cells. Reapplication of Cu resulted in the recovery of Fe transport activity. Together, these results were consistent with the participation of a ferroxidase in high-affinity Fe uptake in C. reinhardtii.