Projection structure of the secondary citrate/sodium symporter CitS at 6 Å 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-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.