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

Batista Paulino, C., Kalienkova, V., Lam, A. KM., Neldner, Y. & Dutzler, R., 21-Dec-2017, In : Nature. 552, 7685, p. 421-425 5 p., 24652.

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DOI

The calcium-activated chloride channel TMEM16A is a ligand-gated anion channel that opens in response to an increase in intracellular Ca2+ concentration(1-3). The protein is broadly expressed(4) and contributes to diverse physiological processes, including transepithelial chloride transport and the control of electrical signalling in smooth muscles and certain neurons(5-7). 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 membranes(8-11). 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 A (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 a-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.
Original languageEnglish
Article number24652
Pages (from-to)421-425
Number of pages5
JournalNature
Volume552
Issue number7685
Publication statusPublished - 21-Dec-2017
Externally publishedYes

    Keywords

  • Chloride channels, Cryo-EM, Ion transport, Single particle analysis
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