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

Cryo-electron microscopy mapping of the calcium-activated chloride channel TMEM16A combined with functional experiments reveals that calcium ions interact directly with the pore to activate the channel. The diverse TMEM16 membrane protein family contains Ca(II)-activated chloride channels, lipid scramblases and cation channels. TMEM16A mediates chloride-ion permeation, which controls neuronal signalling, muscle contraction and numerous other physiological functions. In this issue of Nature, two groups have solved the structure of TMEM16A by using cryo-electron microscopy, providing insights into the function of this channel. Unlike other ligand-gated ion channels, the Ca(II) ion interacts with the pore directly, where a glycine residue acts as a flexible hinge to adjust calcium sensitivity. Raimund Dutzler and colleagues report the structure of the protein in both Ca(II)-free and Ca(II)-bound states, which shows how calcium binding facilitates the structural rearrangements involved in channel activation. In the second Letter, Lily Jan and colleagues present two functional states of TMEM16A in the glycolipid LMNG and in nanodiscs, with one and two Ca(II) ions bound, respectively. The closed conformation observed in nanodiscs is proposed to show channel rundown after prolonged Ca(II) activation. 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.