A mechanism for intracellular release of Na+ by neurotransmitter/sodium symporters

Nat Struct Mol Biol. 2014 Nov;21(11):1006-12. doi: 10.1038/nsmb.2894. Epub 2014 Oct 5.


Neurotransmitter/sodium symporters (NSSs) terminate synaptic signal transmission by Na+-dependent reuptake of released neurotransmitters. Key conformational states have been reported for the bacterial homolog LeuT and an inhibitor-bound Drosophila dopamine transporter. However, a coherent mechanism of Na+-driven transport has not been described. Here, we present two crystal structures of MhsT, an NSS member from Bacillus halodurans, in occluded inward-facing states with bound Na+ ions and L-tryptophan, providing insight into the cytoplasmic release of Na+. The switch from outward- to inward-oriented states is centered on the partial unwinding of transmembrane helix 5, facilitated by a conserved GlyX9Pro motif that opens an intracellular pathway for water to access the Na2 site. We propose a mechanism, based on our structural and functional findings, in which solvation through the TM5 pathway facilitates Na+ release from Na2 and the transition to an inward-open state.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Amino Acid Sequence
  • Bacillus / chemistry*
  • Bacillus / metabolism
  • Bacterial Proteins / chemistry*
  • Bacterial Proteins / genetics
  • Binding Sites
  • Biological Transport
  • Cations, Monovalent
  • Conserved Sequence
  • Crystallography, X-Ray
  • Gene Expression
  • Kinetics
  • Lactococcus lactis / genetics
  • Lactococcus lactis / metabolism
  • Models, Molecular
  • Molecular Sequence Data
  • Protein Binding
  • Protein Conformation
  • Recombinant Proteins / chemistry
  • Recombinant Proteins / genetics
  • Sodium / chemistry*
  • Structural Homology, Protein
  • Substrate Specificity
  • Tryptophan / chemistry*


  • Bacterial Proteins
  • Cations, Monovalent
  • Recombinant Proteins
  • Tryptophan
  • Sodium

Associated data

  • PDB/4US3
  • PDB/4US4