Pore-lining residues along TM5 located spatially close to R352 on TM6. (A) A cross-eye stereo lateral view of CFTR’s two TMDs based on the cryo-EM structure of CFTR (Zhang and Chen, 2016). The whole TMD is colored in wheat. TM6 is colored in cyan, TM9 in magenta, and TM5 in black. The pore-lining residues R352 and M348 in TM6 are colored in blue and pink, respectively, the pore-lining residues R303 and N306 along TM5 are colored in orange and yellow, respectively, and A299, S307, F310, and F311 along TM5 are colored in black. D993 in TM9 is colored in green. The surface of the internal vestibule is shown with its aqueous side in light blue and the back side in light blue shadow. From this lateral view, R352 and D993 reside at a critical location where the internal vestibule tapers down to the narrow region. (B) Enlarged view of the boxed region in A. If we use R352/D993 as a reference plane, pore-lining M348, F310, and F311 lie slightly above, whereas N306 lies below, this plane. In contrast, R303 and A299 reside at a more internal and spacious region of the internal vestibule. (C) Top view of TM5, 6, and 9. Distances between the residues were measured in angstroms.