title: Projection structure of the secondary citrate/sodium symporter CitS at 6 Å resolution by electron crystallography.
abstract: 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.
authors.affiliation: Center for Cellular Imaging and NanoAnalytics (C-CINA), Biozentrum, University Basel, Mattenstrasse 26, CH-4058 Basel, Switzerland.
authors.forename: Fabian
authors.initials: F
authors.lastname: Kebbel
authors.forename: Mareike
authors.initials: M
authors.lastname: Kurz
authors.forename: Markus G
authors.initials: MG
authors.lastname: Grütter
authors.forename: Henning
authors.initials: H
authors.lastname: Stahlberg
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry*; D002352:Carrier Proteins / Q000737:chemistry*; D020285:Cryoelectron Microscopy; D003461:Crystallography / Q000379:methods*; D007711:Klebsiella pneumoniae / Q000378:metabolism*; D011487:Protein Conformation
chemical_list: D001426:Bacterial Proteins; D002352:Carrier Proteins; C086246:CitS protein, bacteria