5oyg
Structure of calcium-free mTMEM16A chloride channel at 4.06 A resolutionStructure of calcium-free mTMEM16A chloride channel at 4.06 A resolution
Structural highlights
Function[ANO1_MOUSE] Calcium-activated chloride channel (CaCC) which plays an important role in transepithelial anion transport and smooth muscle contraction. Required for the normal functioning of the interstitial cells of Cajal (ICCs) which generate electrical pacemaker activity in gastrointestinal smooth muscles. Acts as a major contributor to basal and stimulated chloride conductance in airway epithelial cells and plays an important role in tracheal cartilage development.[1] [2] [3] [4] [5] Publication Abstract from PubMedThe calcium-activated chloride channel TMEM16A is a ligand-gated anion channel that opens in response to an increase in intracellular Ca(2+) concentration. The protein is broadly expressed and contributes to diverse physiological processes, including transepithelial chloride transport and the control of electrical signalling in smooth muscles and certain neurons. 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. 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), 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 Ca(2+). Here we describe the structures of mouse TMEM16A at high resolution in the presence and absence of Ca(2+). 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 Ca(2+) 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 alpha-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. Activation mechanism of the calcium-activated chloride channel TMEM16A revealed by cryo-EM.,Paulino C, Kalienkova V, Lam AKM, Neldner Y, Dutzler R Nature. 2017 Dec 13. pii: nature24652. doi: 10.1038/nature24652. PMID:29236691[6] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. References
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