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J. Biol. Chem., Vol. 283, Issue 18, 12520-12527, May 2, 2008

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FutA2 Is a Ferric Binding Protein from Synechocystis PCC 6803^*

From the Institute for Cell and Molecular Biosciences, Medical School, Newcastle University, Newcastle upon Tyne NE2 4HH, United Kingdom

Synechocystis PCC 6803 has a high demand for iron (10 times greater than Escherichia coli) to sustain photosynthesis and is unusual in possessing at least two putative iron-binding proteins of a type normally associated with ATP-binding cassette-type importers. It has been suggested that one of these, FutA2, binds ferrous iron, but herein we clearly demonstrate that this protein avidly binds Fe(III), the oxidation state preference of periplasmic iron-binding proteins. Structures of apo-FutA2 and Fe-FutA2 have been determined at 1.7 and 2.7Å_, respectively. The metal ion is bound in a distorted trigonal bipyramidal arrangement with no exogenous anions as ligands. The metal-binding environment, including the second coordination sphere and charge properties, is consistent with a preference for Fe(III). Atypically, FutA2 has a Tat signal peptide, and its inability to coordinate divalent cations may be crucial to prevent metals from binding to the folded protein prior to export from the cytosol. A loop containing the His⁴³ ligand undergoes considerable movement in apo-versus Fe-FutA2 and may control metal release to the importer. Although these data are consistent with FutA2 being the periplasmic component involved in iron uptake, deletion of another putative ferric binding protein, FutA1, has a greater effect on the accumulation of iron and is more analogous to a futA1futA2 double mutant than futA2. Here, we also discover that there is a reduced level of ferric FutA2 in the periplasm of the futA1 mutant providing an explanation for its severe iron-uptake phenotype.

Received for publication, December 4, 2007 , and in revised form, January 11, 2008.

The atomic coordinates and structure factors (codes 2voz and 2vp1) have been deposited in the Protein Data Bank, Research Collaboratory for Structural Bioinformatics, Rutgers University, New Brunswick, NJ (http://www.rcsb.org/).

^* This work was supported in part by Newcastle University and Biotechnology and Biological Sciences Research Council Grants BB/E016529 and BB/E001688/1. 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.

¹ Both authors contributed equally to this work.

² Supported by a Royal Society (UK) University Research Fellowship. To whom correspondence may be addressed: Dept. of Biological Chemistry, John Innes Centre, Norwich NR4 7UH, UK. Tel.: 44-1603-450-742; Fax: 44-1603-450-018; E-mail: Mark.Banfield{at}bbsrc.ac.uk.

³ To whom correspondence may be addressed. Tel.: 44-191-222-7127; Fax: 44-191-222-7424; E-mail: christopher.dennison{at}ncl.ac.uk.

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