Activation mechanism of the calcium-activated chloride channel TMEM16A revealed by cryo-EM

  • Nature volume 552, pages 421425 (21 December 2017)
  • doi:10.1038/nature24652
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The calcium-activated chloride channel TMEM16A is a ligand-gated anion channel that opens in response to an increase in intracellular Ca2+ concentration1,2,3. The protein is broadly expressed4 and contributes to diverse physiological processes, including transepithelial chloride transport and the control of electrical signalling in smooth muscles and certain neurons5,6,7. As a member of the TMEM16 (or anoctamin) family of membrane proteins, TMEM16A is closely related to paralogues that function as scramblases, which facilitate the bidirectional movement of lipids across membranes8,9,10,11. The unusual functional diversity of the TMEM16 family and the relationship between two seemingly incompatible transport mechanisms has been the focus of recent investigations. Previous breakthroughs were obtained from the X-ray structure of the lipid scramblase of the fungus Nectria haematococca (nhTMEM16)12,13, and from the cryo-electron microscopy structure of mouse TMEM16A at 6.6 Å (ref. 14). Although the latter structure disclosed the architectural differences that distinguish ion channels from lipid scramblases, its low resolution did not permit a detailed molecular description of the protein or provide any insight into its activation by Ca2+. Here we describe the structures of mouse TMEM16A at high resolution in the presence and absence of Ca2+. These structures reveal the differences between ligand-bound and ligand-free states of a calcium-activated chloride channel, and when combined with functional experiments suggest a mechanism for gating. During activation, the binding of Ca2+ to a site located within the transmembrane domain, in the vicinity of the pore, alters the electrostatic properties of the ion conduction path and triggers a conformational rearrangement of an α-helix that comes into physical contact with the bound ligand, and thereby directly couples ligand binding and pore opening. Our study describes a process that is unique among channel proteins, but one that is presumably general for both functional branches of the TMEM16 family.

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We thank O. Medalia and M. Eibauer, the Center for Microscopy and Image Analysis (ZMB) of the University of Zurich, and the Mäxi foundation for support and access to electron microscopes. J. D. Walter is acknowledged for comments on the manuscript and all members of the Dutzler laboratory for help at various stages of the project. This research was supported by a grant from the European Research Council (number 339116, AnoBest). C.P. was supported by a postdoctoral fellowship (Forschungskredit) of the University of Zurich.

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Author notes

    • Cristina Paulino

    Present address: Department of Structural Biology at the Groningen Biomolecular Sciences and Biotechnology Institute, University of Groningen, Nijenborgh 7, 9747 AG Groningen, The Netherlands.


  1. Department of Biochemistry, University of Zurich, Winterthurerstrasse 190, CH-8057 Zurich, Switzerland

    • Cristina Paulino
    • , Valeria Kalienkova
    • , Andy K. M. Lam
    • , Yvonne Neldner
    •  & Raimund Dutzler


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V.K. and Y.N. expressed and purified the protein for cryo-EM and functional reconstitution and performed the flux assay. C.P. prepared the sample for cryo-EM, collected electron microscopy data and proceeded with structure determination. A.K.M.L. generated mutants, performed electrophysiological recordings and fitted data. C.P., V.K., A.K.M.L., Y.N. and R.D. jointly planned experiments, analysed the data and wrote the manuscript.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Raimund Dutzler.

Reviewer Information Nature thanks C. Hartzell and the other anonymous reviewer(s) for their contribution to the peer review of this work.

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  1. 1.

    Ligand-bound mTMEM16A structure.

    cryo-EM density map of the mTMEM16A ion channel obtained in presence of calcium ions with the modelled structure superimposed. Only one subunit is shown and the two bound Ca2+ are coloured in blue.

  2. 2.

    Ligand-free mTMEM16A structure.

    cryo-EM density map of the mTMEM16A ion channel obtained in absence of calcium ions with the modelled structure superimposed. Only one subunit is shown.

  3. 3.

    Comparison of the Ca2+-bound and Ca2+-free state.

    Superimposition of the mTMEM16A ion channel structure in the Ca2+-bound (green) and the Ca2+-free (violet) state.

  4. 4.

    Ca2+-induced conformational changes.

    Morph between the Ca2+-bound and the Ca2+-free state (violet) superimposed on the structure in presence of calcium (green, Ca2+ shown as blue spheres). The structure is viewed from within the membrane rotated by approximately 70 degrees about the y axis compared to Figure 1.


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