Structure of Calcium Channel Protein May Lead to New CF Drugs

Cryo-EM allows the observation of specimens that have not been stained or fixed in any way, showing them in their native environment while integrating multiple images to form a three-dimensional model of the sample.

The calcium-activated chloride channel TMEM16A is a ligand-gated anion channel that opens in response to an increase in intracellular Ca2+ concentration. The protein is broadly expressed and contributes to diverse physiological processes, including transepithelial chloride transport and the control of electrical signaling in smooth muscles and certain neurons. As a member of the TMEM16 (or anoctamin) family of membrane proteins, TMEM16A (Anoctamin-1 or ANO1) is closely related to similar proteins in other organisms that function as scramblases, which are enzymes that 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.

Investigators at the University of Zurich (Switzerland) used advanced cryo-EM technology to establish the structures of mouse TMEM16A at high resolution in the presence and absence of Ca2+.

They reported in the December 13, 2017, online edition of the journal Nature that these structures revealed the differences between ligand-bound and ligand-free states of the calcium-activated chloride channel, and when combined with functional experiments suggested a mechanism for gating. During activation, the binding of Ca2+ to a site located within the transmembrane domain, in the vicinity of the pore, altered the electrostatic properties of the ion conduction path and triggered a conformational rearrangement of an alpha-helix that came into physical contact with the bound ligand, and thereby directly coupled ligand binding and pore opening. This was process unique among channel proteins, but one that was presumably general for both functional branches of the TMEM16 family.

These findings described the underlying structures and functions of this channel protein and provided promising insights for developing drugs for the treatment of cystic fibrosis, which is an inherited, autosomal recessive disorder of the lungs caused by mutations in the chloride channel gene CTFR.

"The molecular architecture of this membrane protein is crucial for the targeted development of drugs for treating cystic fibrosis," said senior author Dr. Raimund Dutzler, professor of biochemistry at the University of Zurich. "Substances leading to the activation of the TMEM16A would compensate the defect in the secretion of chloride ions in the lung.”

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