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X-ray structures and mechanism of the human serotonin transporter

Nature volume 532, pages 334339 (21 April 2016) | Download Citation

Abstract

The serotonin transporter (SERT) terminates serotonergic signalling through the sodium- and chloride-dependent reuptake of neurotransmitter into presynaptic neurons. SERT is a target for antidepressant and psychostimulant drugs, which block reuptake and prolong neurotransmitter signalling. Here we report X-ray crystallographic structures of human SERT at 3.15 Å resolution bound to the antidepressants (S)-citalopram or paroxetine. Antidepressants lock SERT in an outward-open conformation by lodging in the central binding site, located between transmembrane helices 1, 3, 6, 8 and 10, directly blocking serotonin binding. We further identify the location of an allosteric site in the complex as residing at the periphery of the extracellular vestibule, interposed between extracellular loops 4 and 6 and transmembrane helices 1, 6, 10 and 11. Occupancy of the allosteric site sterically hinders ligand unbinding from the central site, providing an explanation for the action of (S)-citalopram as an allosteric ligand. These structures define the mechanism of antidepressant action in SERT, and provide blueprints for future drug design.

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Accessions

Data deposits

The atomic coordinates and structure factors have been deposited in the Protein Data Bank (PDB) under the following accession codes: ts3 paroxetine (5I6X), ts2 paroxetine (5I6Z), ts3 (S)-citalopram (5I71), ts3 (S)-citalopram (soaked) (5I73), ts3 Br-citalopram (5I74), ts3 Br-citalopram (soaked) (5I75), and 8B6 Fab (5I66).

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Acknowledgements

We thank D. Cawley for generating monoclonal antibodies and Lundbeck for Br-citalopram. We thank A. Penmatsa and K. Wang for assistance with initial crystal screening, K. Dürr and W. Lü for help with Fab crystallization and structure refinement, respectively, L. Vaskalis for assistance with figures, H. Owen for help with manuscript preparation and other Gouaux laboratory members for discussions. We acknowledge the staff of the Berkeley Center for Structural Biology at the Advanced Light Source and the Northeastern Collaborative Access Team at the Advanced Photon Source for assistance with data collection. J.A.C. has support from a Banting postdoctoral fellowship from the Canadian Institutes of Health Research. We are particularly grateful to Bernie and Jennifer LaCroute for their generous support, as well as for funding from the National Institutes of Health (NIH) (5R37MH070039). E.G. is an Investigator with the Howard Hughes Medical Institute.

Author information

Author notes

    • Evan M. Green

    Present address: Graduate Group in Biophysics, University of California, San Francisco, San Francisco, California 94158, USA.

Affiliations

  1. Vollum Institute, Oregon Health & Science University, Portland, Oregon 97239, USA

    • Jonathan A. Coleman
    • , Evan M. Green
    •  & Eric Gouaux
  2. Howard Hughes Medical Institute, Oregon Health & Science University, Portland, Oregon 97239, USA

    • Eric Gouaux

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Contributions

J.A.C., E.M.G. and E.G. designed the project. E.M.G. and J.A.C. developed thermostable constructs for crystallization. J.A.C. performed protein purification, crystallography and biochemical analysis. J.A.C., E.M.G. and E.G. wrote the manuscript.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Eric Gouaux.

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DOI

https://doi.org/10.1038/nature17629

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