Research Article

Siderophore-Mediated Iron Transport: Crystal Structure of FhuA with Bound Lipopolysaccharide

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Science  18 Dec 1998:
Vol. 282, Issue 5397, pp. 2215-2220
DOI: 10.1126/science.282.5397.2215

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FhuA, the receptor for ferrichrome-iron in Escherichia coli, is a member of a family of integral outer membrane proteins, which, together with the energy-transducing protein TonB, mediate the active transport of ferric siderophores across the outer membrane of Gram-negative bacteria. The three-dimensional structure of FhuA is presented here in two conformations: with and without ferrichrome-iron at resolutions of 2.7 and 2.5 angstroms, respectively. FhuA is a β barrel composed of 22 antiparallel β strands. In contrast to the typical trimeric arrangement found in porins, FhuA is monomeric. Located within the β barrel is a structurally distinct domain, the “cork,” which mainly consists of a four-stranded β sheet and four short α helices. A single lipopolysaccharide molecule is noncovalently associated with the membrane-embedded region of the protein. Upon binding of ferrichrome-iron, conformational changes are transduced to the periplasmic pocket of FhuA, signaling the ligand-loaded status of the receptor. Sequence homologies and mutagenesis data are used to propose a structural mechanism for TonB-dependent siderophore-mediated transport across the outer membrane.

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