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J Biol Chem. 2017 May 5;292(18):7372-7384. doi: 10.1074/jbc.M116.757153. Epub 2017 Mar 20.

The role of transmembrane segment 5 (TM5) in Na2 release and the conformational transition of neurotransmitter:sodium symporters toward the inward-open state.

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From the Computational Molecular Biology Group, Institute for Mathematics, Freie Universität Berlin, 14195 Berlin, Germany.
the Department of Physiology and Biophysics and.
the Department of Physics, Cornell University, Ithaca, New York, 14850.
the Departments of Psychiatry and.
the Division of Molecular Therapeutics, New York State Psychiatric Institute, New York, New York, 10032.
the Danish Research Institute of Translational Neuroscience-DANDRITE, Nordic-EMBL Partnership for Molecular Medicine, Department of Molecular Biology and Genetics, Aarhus University, Aarhus C DK-8000, Denmark, and.
the Institute for Computational Biomedicine, Weill Medical College of Cornell University, New York, New York, 10065.
Pharmacology, Columbia University College of Physicians and Surgeons, New York, New York, 10032.
the Department of Physiology and Biophysics and
the Computational Chemistry and Molecular Biophysics Unit, Intramural Research Program, National Institute on Drug Abuse, National Institutes of Health, Baltimore, Maryland 21224.


Neurotransmitter:sodium symporters (NSSs) terminate neurotransmission by the reuptake of released neurotransmitters. This active accumulation of substrate against its concentration gradient is driven by the transmembrane Na+ gradient and requires that the transporter traverses several conformational states. LeuT, a prokaryotic NSS homolog, has been crystallized in outward-open, outward-occluded, and inward-open states. Two crystal structures of another prokaryotic NSS homolog, the multihydrophobic amino acid transporter (MhsT) from Bacillus halodurans, have been resolved in novel inward-occluded states, with the extracellular vestibule closed and the intracellular portion of transmembrane segment 5 (TM5i) in either an unwound or a helical conformation. We have investigated the potential involvement of TM5i in binding and unbinding of Na2, i.e. the Na+ bound in the Na2 site, by carrying out comparative molecular dynamics simulations of the models derived from the two MhsT structures. We find that the helical TM5i conformation is associated with a higher propensity for Na2 release, which leads to the repositioning of the N terminus and transition to an inward-open state. By using comparative interaction network analysis, we also identify allosteric pathways connecting TM5i and the Na2 binding site to the extracellular and intracellular regions. Based on our combined computational and mutagenesis studies of MhsT and LeuT, we propose that TM5i plays a key role in Na2 binding and release associated with the conformational transition toward the inward-open state, a role that is likely to be shared across the NSS family.


amino acid transport; conformational change; metal ion-protein interaction; molecular dynamics; neurotransmitter transport; transporter

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