title: Crystal structure of the outer membrane active transporter FepA from Escherichia coli.
abstract: Integral outer membrane receptors for iron chelates and vitamin B12 carry out specific ligand transport against a concentration gradient. Energy for active transport is obtained from the proton-motive force of the inner membrane through physical interaction with TonB-ExbB-ExbD, an inner membrane complex. Here we report the crystal structure of an active transport, outer membrane receptor at 2.4 A resolution. Two distinct functional domains are revealed: (i) a 22-stranded beta-barrel that spans the outer membrane and contains large extracellular loops which appear to function in ligand binding; and (ii) a globular N-terminal domain that folds into the barrel pore, inhibiting access to the periplasm and contributing two additional loops for potential ligand binding. These loops could provide a signaling pathway between the processes of ligand recognition and TonB-mediated transport. The blockage of the pore suggests that the N-terminal domain must undergo a conformational rearrangement to allow ligand transport into the periplasm.
authors.affiliation: Howard Hughes Medical Institute, and Department of Biochemistry, University of Texas Southwestern Medical Center, Dallas 75235-9050, USA.
authors.forename: S K
authors.initials: SK
authors.lastname: Buchanan
authors.forename: B S
authors.initials: BS
authors.lastname: Smith
authors.forename: L
authors.initials: L
authors.lastname: Venkatramani
authors.forename: D
authors.initials: D
authors.lastname: Xia
authors.forename: L
authors.initials: L
authors.lastname: Esser
authors.forename: M
authors.initials: M
authors.lastname: Palnitkar
authors.forename: R
authors.initials: R
authors.lastname: Chakraborty
authors.forename: D
authors.initials: D
authors.lastname: van der Helm
authors.forename: J
authors.initials: J
authors.lastname: Deisenhofer
mesh_terms: D000595:Amino Acid Sequence; D001425:Bacterial Outer Membrane Proteins / Q000737:chemistry; D002352:Carrier Proteins / Q000737:chemistry*; D018360:Crystallography, X-Ray; D004926:Escherichia coli / Q000737:chemistry*; D008969:Molecular Sequence Data; D011487:Protein Conformation; D011956:Receptors, Cell Surface
chemical_list: D001425:Bacterial Outer Membrane Proteins; D002352:Carrier Proteins; D011956:Receptors, Cell Surface; C029489:enterobactin receptor