We have determined the crystal structure of apo-CusF, a periplasmic protein involved in copper and silver resistance in Escherichia coli. The protein forms a five-stranded β-barrel, classified as an OB-fold, which is a unique topology for a copper-binding protein. NMR chemical shift mapping experiments suggest that Cu(I) is bound by conserved residues H36, M47, and M49 located in β-strands 2 and 3. These residues are clustered at one end of the β-barrel, and their side chains are oriented toward the interior of the barrel. Cu(I) can be modeled into the apo-CusF structure with only minimal structural changes using H36, M47, and M49 as ligands. The unique structure and metal binding site of CusF are distinct from those of previously characterized copper-binding proteins.
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