Nature Structural Biology
6, 56 - 63 (1999)
doi:10.1038/4931
Crystal structure of the outer membrane active transporter FepA from Escherichia coliSusan K. Buchanan1, 3, Barbara S. Smith1, Lalitha Venkatramani2, Di Xia1, Lothar Esser1, Maya Palnitkar1, Ranjan Chakraborty2, Dick van der Helm2
& Johann Deisenhofer11
Howard Hughes Medical Institute, and Department of Biochemistry, University of Texas Southwestern Medical Center, 5323 Harry Hines Blvd, Dallas, Texas 75235−9050, USA. 2
Department of Chemistry and Biochemistry, The University of Oklahoma, 620 Parrington Oval, Norman, Oklahoma 73019−0370, USA. 3
Present address: Department of Crystallography, Birkbeck College, University of London, Malet Street, London WC1E 7HX, UK.
Correspondence should be addressed to Johann Deisenhofer Johann.Deisenhofer@email.swmed.edu
Integral outer membrane receptors for iron chelates and vitamin B
12 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 Å resolution. Two distinct
functional domains are revealed: (i) a 22−stranded  −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.
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