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Science 322 (5902): 709-713

Copyright © 2008 by the American Association for the Advancement of Science

Structure and Molecular Mechanism of a Nucleobase–Cation–Symport-1 Family Transporter

Simone Weyand,1,2,3* Tatsuro Shimamura,2,3,4* Shunsuke Yajima,2,3*{dagger} Shun'ichi Suzuki,5*{ddagger} Osman Mirza,2*§ Kuakarun Krusong,2|| Elisabeth P. Carpenter,1,2 Nicholas G. Rutherford,5 Jonathan M. Hadden,5 John O'Reilly,5 Pikyee Ma,5 Massoud Saidijam,5,6 Simon G. Patching,5 Ryan J. Hope,5 Halina T. Norbertczak,5 Peter C. J. Roach,5 So Iwata,1,2,3,4,7 Peter J. F. Henderson,5 Alexander D. Cameron1,2,3

Abstract: The nucleobase–cation–symport-1 (NCS1) transporters are essential components of salvage pathways for nucleobases and related metabolites. Here, we report the 2.85-angstrom resolution structure of the NCS1 benzyl-hydantoin transporter, Mhp1, from Microbacterium liquefaciens. Mhp1 contains 12 transmembrane helices, 10 of which are arranged in two inverted repeats of five helices. The structures of the outward-facing open and substrate-bound occluded conformations were solved, showing how the outward-facing cavity closes upon binding of substrate. Comparisons with the leucine transporter LeuTAa and the galactose transporter vSGLT reveal that the outward- and inward-facing cavities are symmetrically arranged on opposite sides of the membrane. The reciprocal opening and closing of these cavities is synchronized by the inverted repeat helices 3 and 8, providing the structural basis of the alternating access model for membrane transport.

1 Membrane Protein Laboratory, Diamond Light Source Limited, Harwell Science and Innovation Campus, Chilton, Didcot, Oxfordshire OX11 0DE, UK.
2 Division of Molecular Biosciences, Membrane Protein Crystallography Group, Imperial College, London SW7 2AZ, UK.
3 Human Receptor Crystallography Project, Exploratory Research for Advanced Technology (ERATO), Japan Science and Technology Agency, Yoshidakonoe-cho, Sakyo-ku, Kyoto 606-8501, Japan.
4 Department of Cell Biology, Graduate School of Medicine, Kyoto University, Yoshida-Konoe, Sakyo-Ku, Kyoto 606-8501, Japan.
5 Astbury Centre for Structural Molecular Biology, Institute for Membrane and Systems Biology, University of Leeds, Leeds LS2 9JT, UK.
6 School of Medicine, Hamedan University of Medical Sciences, Hamedan, Iran.
7 Systems and Structural Biology Center, RIKEN, 1-7-22 Suehiro-cho Tsurumi-ku, Yokohama 230-0045 Japan.

* These authors contributed equally to this work

{dagger} Present address: Department of Bioscience, Tokyo University of Agriculture, Sakuragaoka 1-1-1, Setagaya-ku, Tokyo 156-8502, Japan.

{ddagger} Present address: Aminosciences Laboratories, Ajinomoto Company Incorporated, 1-1 Suzuki-cho, Kawasaki-ku, Kawasaki-shi, Kanagawa 210-8681, Japan.

§ Present address: Department of Medicinal Chemistry, Faculty of Pharmaceutical Sciences, University of Copenhagen, Universitetsparken 2, DK-2100, Denmark.

|| Present address: Department of Biochemistry, Faculty of Science, Chulalongkorn University, Phyathai Road, Patumwan, Bangkok 10330, Thailand.

To whom correspondence should be addressed. E-mail: s.iwata{at} (S.I.); p.j.f.henderson{at} (P.J.F.H.)

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