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

Sci Rep. 2017 May 31;7(1):2548. doi: 10.1038/s41598-017-02794-x.


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° rigid-body rotation and a ~17 Å 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.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Amino Acid Sequence
  • Anaerobiosis / physiology
  • Bacterial Proteins / chemistry*
  • Bacterial Proteins / genetics
  • Bacterial Proteins / metabolism
  • Binding Sites
  • Biological Transport
  • Carrier Proteins / chemistry*
  • Carrier Proteins / genetics
  • Carrier Proteins / metabolism
  • Citric Acid / chemistry*
  • Citric Acid / metabolism
  • Crystallography, X-Ray
  • Gene Expression
  • Kinetics
  • Klebsiella pneumoniae / chemistry*
  • Klebsiella pneumoniae / genetics
  • Klebsiella pneumoniae / metabolism
  • Models, Molecular
  • Protein Binding
  • Protein Conformation, alpha-Helical
  • Protein Conformation, beta-Strand
  • Protein Interaction Domains and Motifs
  • Protein Multimerization
  • Recombinant Proteins / chemistry
  • Recombinant Proteins / genetics
  • Recombinant Proteins / metabolism
  • Salmonella enterica / chemistry*
  • Salmonella enterica / genetics
  • Salmonella enterica / metabolism
  • Sodium / chemistry*
  • Sodium / metabolism
  • Structural Homology, Protein
  • Substrate Specificity


  • Bacterial Proteins
  • Carrier Proteins
  • CitS protein, bacteria
  • Recombinant Proteins
  • Citric Acid
  • Sodium