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BIOLOGICAL SCIENCES / BIOCHEMISTRY
Structural determination of wild-type lactose permease

Lan Guan^*, Osman Mirza^,, Gillian Verner^*, So Iwata^,,¶, and H. Ronald Kaback^*,||

*Department of Physiology and Department of Microbiology, Immunology, and Molecular Genetics, Molecular Biology Institute, University of California, Los Angeles, CA 90095-1662; Department of Medicinal Chemistry, Faculty of Pharmaceutical Sciences, University of Copenhagen, Universitetsparken 2, 2100 Copenhagen, Denmark; Division of Molecular Biosciences, Imperial College London, London SW7 2AZ, United Kingdom; Exploratory Research for Advanced Technology Human Receptor Crystallography Project, Kawasaki, 210-0855 Kanagawa, Japan; and ^¶RIKEN Genomics Sciences Center, 1-7-22 Suchiro-cho, Tsumi, Yokohama 230-0045, Japan

Contributed by H. Ronald Kaback, August 15, 2007 (received for review August 3, 2007)

Here we describe an x-ray structure of wild-type lactose permease (LacY) from Escherichia coli determined by manipulating phospholipid content during crystallization. The structure exhibits the same global fold as the previous x-ray structures of a mutant that binds sugar but cannot catalyze translocation across the membrane. LacY is organized into two six-helix bundles with twofold pseudosymmetry separated by a large interior hydrophilic cavity open only to the cytoplasmic side and containing the side chains important for sugar and H⁺ binding. To initiate transport, binding of sugar and/or an H⁺ electrochemical gradient increases the probability of opening on the periplasmic side. Because the inward-facing conformation represents the lowest free-energy state, the rate-limiting step for transport may be the conformational change leading to the outward-facing conformation.

conformation | mechanism | membrane protein | transport | x-ray structure

The authors declare no conflict of interest.

Data deposition: The coordinates and structure factors have been deposited in the Protein Data Bank, www.pdb.org (PDB ID code 2v8n).

This article contains supporting information online at www.pnas.org/cgi/content/full/0707688104/DC1.

^||To whom correspondence should be addressed. E-mail: rkaback{at}mednet.ucla.edu

© 2007 by The National Academy of Sciences of the USA

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