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Originally published in Science Express on 2 August 2007
Science 7 September 2007:
Vol. 317. no. 5843, pp. 1387 - 1390
DOI: 10.1126/science.1145950
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Reports

Asymmetry in the Structure of the ABC Transporter-Binding Protein Complex BtuCD-BtuF

Rikki N. Hvorup,1 Birke A. Goetz,1 Martina Niederer,1 Kaspar Hollenstein,1 Eduardo Perozo,2 Kaspar P. Locher1*

BtuCD is an adenosine triphosphate–binding cassette (ABC) transporter that translocates vitamin B12 from the periplasmic binding protein BtuF into the cytoplasm of Escherichia coli. The 2.6 angstrom crystal structure of a complex BtuCD-F reveals substantial conformational changes as compared with the previously reported structures of BtuCD and BtuF. The lobes of BtuF are spread apart, and B12 is displaced from the binding pocket. The transmembrane BtuC subunits reveal two distinct conformations, and the translocation pathway is closed to both sides of the membrane. Electron paramagnetic resonance spectra of spin-labeled cysteine mutants reconstituted in proteoliposomes are consistent with the conformation of BtuCD-F that was observed in the crystal structure. A comparison with BtuCD and the homologous HI1470/71 protein suggests that the structure of BtuCD-F may reflect a posttranslocation intermediate.

1 Institute of Molecular Biology and Biophysics, ETH Zurich, HPK D14.3, 8093 Zurich, Switzerland.
2 Institute of Molecular Pediatric Science, Institute for Biophysical Dynamics, and Department of Biochemistry and Molecular Biology, University of Chicago, 929 East 57th Street, Gordon Center for Integrative Science W206, Chicago, IL 60637, USA.

* To whom correspondence should be addressed. E-mail: kaspar.locher{at}mol.biol.ethz.ch

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Science. ISSN 0036-8075 (print), 1095-9203 (online)