Atomic structures of TMEM16 proteins. (A) Structures of TMEM16A (red) and nhTMEM16 (purple). In each structure, the right subunit of the molecule rotated by 90° along the twofold symmetry axis is depicted on the right. Black spheres represent Ca²⁺ ions. (B) Stereo views of single TMEM16A (left) and nhTMEM16 (right) subunits highlighting the arrangement of helices 3–7. The view angle is the same as that of viewing the single subunit in A. Helix 3 and helices 5–7 of TMEM16A and TMEM16F are colored in red and purple, respectively, while helix 4 is in green in both molecules. (C) Sequence alignment of helix 5 of TMEM16A, TMEM16F, and nhTMEM16. K584_16A, Q559_16F, and the corresponding residue N378 in nhTMEM16 (highlighted in yellow) are located in this helix. K584_16A and N378 of nhTMEM16 are also depicted as a space-filled residue (colored in yellow) in the structures shown in A and B.