A Multicopper Ferroxidase Involved in Iron Binding to Transferrins in Dunaliella salina Plasma Membranes

Abstract

Iron is an essential element for all photosynthetic organisms because it is a cofactor of multiple components in photosynthetic electron transport. Iron deficiency is a common limitation for plant and algae proliferation because of its low availability in aerobic aqueous solutions. On the other hand, an excess of iron is highly toxic because it can elicit the formation of reactive oxygen species causing oxidative damage. Therefore, all photosynthetic organisms had to develop efficient regulatory mechanisms to control the uptake and storage of iron in order to maintain optimal iron metabolism.Two major strategies of high affinity iron uptake have been demonstrated in plants and in algae as follows: siderophoremediated Fe 3ϩ uptake and a redox-mediated mechanism. The siderophore-mediated Fe 3ϩ uptake was first described in bacteria and is characteristic to several algae (1, 2) and to grasses (strategy II plants). Redox-mediated iron acquisition, initiated by reduction of ferric to ferrous ions by a plasma membrane ferrireductase, is characteristic to strategy I plants, yeast, and algae (3-6). The mechanism of redox-mediated iron uptake in plants differs from that in yeast and in the alga Chlamydomonas reinhardtii in that plants transport the reduced ferrous ions directly via a divalent metal transporter, whereas in yeast and in C. reinhardtii ferrous ions are reoxidized to ferric ions. Reoxidation is mediated by a membranal multicopper ferroxidase, Fet3 or Fox1, belonging to the multicopper oxidase (MCO) 2 superfamily (7,8). Ferric ions are next transported through a coupled ferric-specific transporter. The MCO Fox1 and the iron permease Ftr1 are believed to form a complex at the plasma membrane as demonstrated in yeast (9, 10). Both siderophore and redox-mediated iron uptake are up-regulated under iron deprivation, thus compensating for the decreased iron availability by a more efficient high affinity iron uptake.The halotolerant alga Dunaliella salina is well adapted to iron deprivation, as manifested by its proliferation and by maintenance of photosynthetic activity under iron deprivation (11). We found that D. salina has an unusual mechanism of iron uptake, mediated by binding and internalization of Fe 3ϩ to a surface transferrin-like protein, TTf (12, 13). TTf serves as a housekeeping mechanism for iron uptake, but its expression level and activity are enhanced under iron deprivation or at high salinity, which limits iron availability. The origin of transferrins in Dunaliella is not clear, because no other transferrins have been described so far in other plants or in related organisms. More recently we identified another transferrin, DTf, that is induced under iron deprivation in D. salina plasma membrane. The function of DTf has not been elucidated yet (14).

Nonreductive Iron Uptake Mechanism in the Marine Alveolate Chromera velia

“…Another nonreductive iron uptake system involving binding of iron by a surface transferrin-like protein was described in the halotolerant alga Dunaniella salina (Paz et al, 2007a(Paz et al, , 2007b. In this alga, iron deficiency induces primarily iron binding to the cell surface rather than iron internalization (Paz et al, 2007b), which is the opposite of what we observed in C. velia.…”

Whole-cell response of the pennate diatom Phaeodactylum tricornutum to iron starvation

“…Fe-responsive gene in Dunaliella salina encodes a protein thought to associate with plasma membrane transferrins where it functions, along with a multicopper ferroxidase, as part of a complex that enhances binding and uptake of ferric ions (41). However, there is no evidence for the occurrence of genes that encode transferrin in the genome of P. tricornutum.…”

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

“…To test whether iron internalization indeed results from endocytosis of iron-binding proteins, we tagged membrane surface proteins in irondeficient cells with a membrane-impermeable biotin derivative and monitored biotin-tagged proteins labeled with the fluorescein avidin with a confocal microscope. We found that, in iron-deficient cells, biotin tags almost exclusively three protein bands that were identified as the major iron deprivation-induced plasma membrane proteins (Paz et al, 2007): two transferrin-like proteins, TTf (Fisher et al, 1997(Fisher et al, , 1998 and DTf (Schwarz et al, 2003a), and a 130-kD band that was found to contain two proteins, a multicopper ferroxidase, termed DFox, and a second unknown protein. In stained cells that were kept at 4°C, biotin-tagged label was confined to the outer cell surface (Fig.…”

“…However, the kinetic mechanism has not been investigated and the site of iron internalization was not identified. In a recent study, we found that iron deprivation induces in D. salina a large increase in ironbinding capacity and that this enhanced binding was correlated with accumulation of three additional plasma membrane proteins, which were found to associate with TTf: a second transferrin, termed DTf, a multicopper ferroxidase, termed DFox, and another 130-kD protein (Paz et al, 2007).…”