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J. Biol. Chem., Vol. 282, Issue 37, 27468-27477, September 14, 2007

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The Structure of the Iron-binding Protein, FutA1, from Synechocystis 6803^*

Nicole Koropatkin, Amelia M. Randich, Maitrayee Bhattacharyya-Pakrasi, Himadri B. Pakrasi, and Thomas J. Smith¹

From the Donald Danforth Plant Science Center, Saint Louis, Missouri 63132 and Department of Biology, Washington University, St. Louis, Missouri 63130

Cyanobacteria account for a significant percentage of aquatic primary productivity even in areas where the concentrations of essential micronutrients are extremely low. To better understand the mechanism of iron selectivity and transport, the structure of the solute binding domain of an ATP binding cassette iron transporter, FutA1, was determined in the presence and absence of iron. The iron ion is bound within the "C-clamp" structure via four tyrosine and one histidine residues. There are extensive interactions between these ligating residues and the rest of the protein such that the conformations of the side chains remain relatively unchanged as the iron is released by the opening of the metal binding cleft. This is in stark contrast to the zinc-binding protein, ZnuA, where the domains of the metal-binding protein remain relatively fixed, whereas the ligating residues rotate out of the binding pocket upon metal release. The rotation of the domains in FutA1 is facilitated by two flexible -strands running along the back of the protein that act like a hinge during domain motion. This motion may require relatively little energy since total contact area between the domains is the same whether the protein is in the open or closed conformation. Consistent with the pH dependence of iron binding, the main trigger for iron release is likely the histidine in the iron-binding site. Finally, neither FutA1 nor FutA2 binds iron as a siderophore complex or in the presence of anions, and both preferentially bind ferrous over ferric ions.

Received for publication, May 18, 2007 , and in revised form, June 29, 2007.

The atomic coordinates and structure factors (code 2PT2 and 2PT1) 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 by National Institutes of Health Grant GM078800 and the Membrane Biology EMSL Scientific Grand Challenge project at the W. R. Wiley Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the United States Dept. of Energy Office of Biological and Environmental Research program located at Pacific North-west National Laboratory, and by National Science Foundation Grant EF0425749 (to M. B.-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.

¹ To whom correspondence should be addressed: Donald Danforth Plant Science Center, 975 North Warson Rd., Saint Louis, MO 63132. Tel.: 314-587-1451; Fax: 314-587-1551; E-mail: tsmith{at}danforthcenter.org.

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