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J. Biol. Chem., Vol. 282, Issue 12, 8658-8666, March 23, 2007

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

From the Department of Biological Chemistry, The Weizmann Institute of Science, Rehovot 76100, Israel

The halotolerant alga Dunaliella salina is unique among plants in that it utilizes a transferrin (TTf) to mediate iron acquisition (Fisher, M., Zamir, A., and Pick, U. (1998) J. Biol. Chem. 273, 17553–17558). Two new proteins that are induced by iron deprivation were identified in plasma membranes of D. salina as follows: a multicopper ferroxidase termed D-Fox and an internally duplicated glycoprotein (p130B). D-Fox and p130B are accessible to glycolytic, proteolytic, and biotin surface tagging treatments, suggesting that they are surface-exposed glycoproteins. Induction of D-Fox was also manifested by ferroxidase activity in plasma membrane preparations. These results are puzzling because ferroxidases in yeast and in Chlamydomonas reinhardtii function in redox-mediated iron uptake, a mechanism that is not known to operate in D. salina. Two lines of evidence suggest that D-Fox and p130B interact with D. salina triplicated transferrin (TTf). First, chemical cross-linking combined with mass spectroscopy analysis showed that D-Fox and p130B associate with TTf and with another plasma membrane transferrin. Second, detergent-solubilized D-Fox and p130B comigrated on blue native gels with plasma membrane transferrins. ⁵⁹Fe autoradiography indicated that this complex binds Fe³⁺ ions. Also, the induction of D-Fox and p130B is kinetically correlated with enhanced iron binding and uptake activities. These results suggest that D-Fox and p130B associate with plasma membrane transferrins forming a complex that enhances iron binding and iron uptake. We propose that the function of D-Fox in D. salina has been modified during evolution from redox-mediated to transferrin-mediated iron uptake, following a gene transfer event of transferrins from an ancestral animal cell.

Received for publication, October 17, 2006 , and in revised form, January 9, 2007.

^* This work was supported by grants from The Israel Science Foundation and by The Charles and Louise Gartner fund (to U. P.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

The nucleotide sequence(s) reported in this paper has been submitted to the GenBank^TM/EBI Data Bank with accession number(s) AY987035 [GenBank] and AF450137 [GenBank] .

¹ To whom correspondence should be addressed. Tel.: 972-8-9342732; Fax: 972-8-9344118; E-mail: uri.pick{at}weizmann.ac.il.

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