A periplasmic iron-binding protein contributes toward inward copper supply

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 DeltafutA2 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 DeltafutA2. 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 DeltafutA2 relative to wild type. Changes in periplasm protein and small-molecule complexes for other metals are also observed in DeltafutA2. 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.