The Structure and Function of Calcium Activated TMEM16 Channels and Scramblases

The TMEM16/Anoctamin family of membrane proteins is broadly expressed in eukaryotes and features a remarkable functional diversity. The family contains the long sought-after Ca2+-activated chloride channels but also lipid scramblases or cation channels. We have determined the crystal structure of nhTMEM16, a fungal family member that operates as a Ca2+-activated lipid scramblase [1]. Each subunit of the homodimeric protein encompasses ten transmembrane helices and structured cytosolic domains at the N- and C-terminus. The ‘subunit cavity’, a hydrophilic membrane-traversing cavity contained within each subunit that is exposed to the lipid bilayer acts as potential site of catalysis in both functional branches of the family. This cavity is sufficiently wide to accommodate the polar head groups of lipids and it contains residues that, upon mutation in the Cl--channel TMEM16A, affect its ion selectivity. The ‘subunit cavity’ harbors a Ca2+-binding site located within the hydrophobic core of the membrane. In this site two Ca2+ ions are coordinated by six conserved residues, five of which carry a negative charge. Mutations of these residues decrease the potency of Ca2+ in the activation of lipid scrambling in nhTMEM16 and ion conduction in TMEM16A. The nhTMEM16 structure thus reveals the general architecture of the family and its mode of Ca2+-activation. It also provides insight into potential scrambling mechanisms and serves as a framework to unravel the conduction of ions in certain TMEM16 proteins acting as ion channels.[1] Brunner, J.D., et al., X-ray structure of a calcium-activated TMEM16 lipid scramblase. Nature, 2014. 516(7530): p. 207-12.