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J. Biol. Chem., Vol. 282, Issue 6, 3837-3846, February 9, 2007

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A Periplasmic Iron-binding Protein Contributes toward Inward Copper Supply^*

From the Institute for Cell and Molecular Biosciences, Medical School, Newcastle University, Newcastle NE4 2HH, United Kingdom

Periplasmic substrate binding proteins are known for iron, zinc, manganese, nickel, and molybdenum but not copper. Synechocystis PCC 6803 requires copper for thylakoid-localized plastocyanin and cytochrome oxidase. Here we show that mutants deficient in a periplasmic substrate binding protein FutA2 have low cytochrome oxidase activity and produce cytochrome c₆ when grown under copper conditions (150 nM) in which wild-type cells use plastocyanin rather than cytochrome c₆. Anaerobic separation of extracts by two-dimensional native liquid chromatography followed by metal analysis and peptide mass-fingerprinting establish that accumulation of copper-plastocyanin is impaired, but iron-ferredoxin is unaffected in futA2 grown in 150 nM copper. However, recombinant FutA2 binds iron in preference to copper in vitro with an apparent Fe(III) affinity similar to that of its paralog FutA1, the principal substrate binding protein for iron import. FutA2 is also associated with iron and not copper in periplasm extracts, and this Fe(III)-protein complex is absent in futA2. There are differences in the soluble protein and small-molecule complexes of copper and iron, and the total amount of both elements increases in periplasm extracts of futA2 relative to wild type. Changes in periplasm protein and small-molecule complexes for other metals are also observed in futA2. It is proposed that FutA2 contributes to metal partitioning in the periplasm by sequestering Fe(III), which limits aberrant Fe(III) associations with vital binding sites for other metals, including copper.

Received for publication, October 23, 2006 , and in revised form, November 29, 2006.

^* 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 on-line version of this article (available at http://www.jbc.org) contains supplemental Figs. S1–S3.

¹ These authors contributed equally to this work.

² To whom correspondence should be addressed: ICaMB, Medical School, Newcastle, NE2 4HH, UK. Tel.: 191-222-7695; Fax: 191-222-7424; E-mail: n.j.robinson{at}ncl.ac.uk.

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