Structural insights into the elevator-like mechanism of the sodium/citrate symporter CitS

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The sodium-dependent citrate transporter of Klebsiella pneumoniae (KpCitS) belongs to the 2-hydroxycarboxylate transporter (2-HCT) family and allows the cell to use citrate as sole carbon and energy source in anaerobic conditions. Here we present crystal structures of KpCitS in citrate-bound outward-facing, citrate-bound asymmetric, and citrate-free inward-facing state. The structures reveal that the KpCitS dimerization domain remains stationary throughout the transport cycle due to a hydrogen bond network as well as extensive hydrophobic interactions. In contrast, its transport domain undergoes a -35(omicron) rigid-body rotation and a -17 angstrom translocation perpendicular to the membrane to expose the substrate-binding site alternately to either side of the membrane. Furthermore, homology models of two other 2-HCT proteins based on the KpCitS structure offer structural insights into their differences in substrate specificity at a molecular level. On the basis of our results and previous biochemical data, we propose that the activity of the 2-HCT CitS involves an elevator-like movement in which the transport domain itself traverses the lipid bilayer, carrying the substrate into the cell in a sodium-dependent manner.
Publisher
NATURE PUBLISHING GROUP
Issue Date
2017-05
Language
English
Article Type
Article
Keywords

DEPENDENT CITRATE CARRIER; LACTIC-ACID BACTERIA; NA+/CITRATE TRANSPORTER CITS; INWARD-FACING STATE; KLEBSIELLA-PNEUMONIAE; DICARBOXYLATE TRANSPORTER; ELECTRON CRYSTALLOGRAPHY; ASPARTATE TRANSPORTER; FUNCTIONAL-PROPERTIES; MEMBRANE TOPOLOGY

Citation

SCIENTIFIC REPORTS, v.7, no.2548

ISSN
2045-2322
DOI
10.1038/s41598-017-02794-x
URI
http://hdl.handle.net/10203/224745
Appears in Collection
CH-Journal Papers(저널논문)
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