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Projection structure of the secondary citrate/sodium symporter CitS at 6 Å resolution by electron crystallography.

Type of publication Peer-reviewed
Publikationsform Original article (peer-reviewed)
Publication date 2012
Author Kebbel Fabian, Kurz Mareike, Grütter Markus G, Stahlberg Henning,
Project Software for Electron Microscopy of Membrane Proteins
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Original article (peer-reviewed)

Journal Journal of molecular biology
Volume (Issue) 418(1-2)
Page(s) 117 - 26
Title of proceedings Journal of molecular biology
DOI 10.1016/j.jmb.2012.02.016


CitS from Klebsiella pneumoniae acts as a secondary symporter of citrate and sodium ions across the inner membrane of the host. The protein is the best characterized member of the 2-hydroxycarboxylate transporter family, while no experimental structural information at sub-nanometer resolution is available on this class of membrane proteins. Here, we applied electron crystallography to two-dimensional crystals of CitS. Carbon-film-adsorbed tubular two-dimensional crystals were studied by cryo-electron microscopy, producing the 6-Å-resolution projection structure of the membrane-embedded protein. In the p22(1)2(1)-symmetrized projection map, the predicted dimeric structure is clearly visible. Each monomeric unit can tentatively be interpreted as being composed of 11 transmembrane α-helices. In projection, CitS shows a high degree of structural similarity to NhaP1, the Na(+)/H(+) antiporter of Methanococcus jannaschii. We discuss possible locations for the dimer interface and models for the helical arrangements and domain organizations of the symporter based on existing models.