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Published online on November 16, 2007, 10.1073/pnas.0709387104
PNAS | November 27, 2007 | vol. 104 | no. 48 | 18999-19004


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BIOLOGICAL SCIENCES / BIOPHYSICS
X-ray structure of EmrE supports dual topology model

Yen-Ju Chen, Owen Pornillos{dagger}, Samantha Lieu, Che Ma{ddagger}, Andy P. Chen, and Geoffrey Chang§

Department of Molecular Biology, The Scripps Research Institute, 10550 North Torrey Pines Road, CB-105, La Jolla, CA 92037

Communicated by K. Barry Sharpless, The Scripps Research Institute, La Jolla, CA, October 2, 2007 (received for review June 19, 2007)

EmrE, a multidrug transporter from Escherichia coli, functions as a homodimer of a small four-transmembrane protein. The membrane insertion topology of the two monomers is controversial. Although the EmrE protein was reported to have a unique orientation in the membrane, models based on electron microscopy and now defunct x-ray structures, as well as recent biochemical studies, posit an antiparallel dimer. We have now reanalyzed our x-ray data on EmrE. The corrected structures in complex with a transport substrate are highly similar to the electron microscopy structure. The first three transmembrane helices from each monomer surround the substrate binding chamber, whereas the fourth helices participate only in dimer formation. Selenomethionine markers clearly indicate an antiparallel orientation for the monomers, supporting a "dual topology" model.

membrane protein structure | multidrug transport | SMR family

Author contributions: Y.-J.C. and O.P. contributed equally to this work; Y.-J.C., O.P., C.M., and G.C. designed research; Y.-J.C., O.P., S.L., C.M., A.P.C., and G.C. performed research; Y.-J.C., O.P., and G.C. contributed new reagents/analytic tools; Y.-J.C., O.P., C.M., and G.C. analyzed data; and Y.-J.C., O.P., C.M., and G.C. wrote the paper.

{dagger}Present address: Celgene, 4550 Towne Centre Court, San Diego, CA 92121.

{ddagger}Present address: Academia Sinica, 128 Academia Road, Nankang, Taipei 115, Taiwan.

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 codes 3B5D, 3B61, and 3B62).

This article contains supporting information online at www.pnas.org/cgi/content/full/0709387104/DC1.

§To whom correspondence should be addressed. E-mail: gchang{at}scripps.edu

© 2007 by The National Academy of Sciences of the USA


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