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Published on May 18, 2007; 10.1104/pp.107.100644

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Received April 10, 2007
Accepted May 11, 2007

Effects of Iron Deficiency on Iron Binding and Internalization into Acidic Vacuoles in Dunaliella salina

Yakov Paz , Eyal Shimoni , Meira Weiss , and Uri Pick *

The Department of Biological Chemistry and Electron Microscopy Unit, The Weizmann Institute of Science, Rehovot 76100, Israel

* Corresponding author; email: uri.pick{at}weizmann.ac.il.

Uptake of iron in the halotolerant alga Dunaliella salina is mediated by a transferrin-like protein, TTf, which binds and internalizes Fe3+ ions (Fisher M, Zamir A, Pick U [1998] J Biol Chem 273:17553-17558). Recently we found that iron deficiency induces a large enhancement of iron binding which is associated with accumulation of three other plasma membrane proteins that associate with TTf (Paz J, Katz A, Pick U [2007] J Biol Chem 282:8658-8666). In the present study we characterized the kinetic properties of iron binding and internalization and identified the site of iron internalization. Iron deficiency induces a 4-fold increase in Fe binding but only 50% enhancement in the rate of iron uptake, and also increases the affinity for iron and for bicarbonate, a co-ligand for iron binding. These results indicate that iron deprivation leads to accumulation and to modification of iron binding sites. Iron uptake in iron-sufficient cells is preceded by an apparent time lag, resulting from pre-bound iron, which can be eliminated by unloading iron binding sites. Iron is tightly bound to surface-exposed sites and hardly exchanges with medium iron. All bound iron is subsequently internalized. Accumulation of iron inhibits further iron binding and internalization. The vacuolar inhibitor bafilomycin inhibits iron uptake and internalization. Internalized Fe was localized by electron microscopy within vacuolar structures that were identified as acidic vacuoles. Fe internalization is accompanied by endocytosis of surface proteins into these acidic vacuoles.

A novel kinetic mechanism for iron uptake is proposed, that includes two pools of bound/compartmentalized iron separated by a rate-limiting internalization stage. The major parameter that is modulated by iron deficiency is the iron binding capacity. We propose that the excessive iron binding in iron-deficient cells serves as a temporary reservoir for iron that is subsequently internalized. This mechanism is particularly suitable for organisms that are exposed to large fluctuations in iron availability.






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