Iron uptake by the halotolerant alga Dunaliella is mediated by a plasma membrane transferrin

A 150-kDa transferrin-like protein (Ttf) is associated with the plasma membrane of the halotolerant unicellular alga Dunaliella salina (Fisher, M., Gokhman, I., pick, U., and Zamir, A. (1997) J. Biol. Chem. 272, 1565-1570), The Ttf level rises with medium salinity or upon iron depletion. Evidence that Ttf is involved in iron uptake by Dunaliella is presented here. Algal iron uptake exhibits characteristics resembling those of animal transferrins: high specificity and affinity for Fe3+ ions, strict dependence on carbonate/bicarbonate ions, and very low activity in acidic pH. Reducing the level of Ttf by mild proteolysis of whole cells is accompanied by lowered uptake activity. Conversely, accumulation of high levels of Ttf is correlated with an enhancement of iron uptake. Kinetically, iron uptake consists of two steps: an energy-independent binding of iron to the cell surface and an energy-dependent internalization. Salinities as high as 3.5 M NaCl do not inhibit iron uptake or decrease the apparent affinity for Fe3+ ions, implying that Ttf activity is not affected by high salt, These results indicate that transferrins, hitherto identified only in animals, are present and function in iron transport also in plant systems.