Projection Structure of the Secondary Citrate/Sodium Symporter CitS at 6 angstrom Resolution by Electron Crystallography

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-angstrom-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 alpha-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. (C) 2012 Elsevier Ltd. All rights reserved.