Figure 12.

An extracellular appendage above the selectivity filter in K2P1channel. (A) The X-ray structure (PDB code 3UKM) shows an extracellular cap extending 35 Å above the plane membrane, with inter-subunit disulfide bond at the apex. The cap limits drug access to the pore. Ions move through side portals, which serve as a pre-filter. Reproduced with permission from Miller and Long (2012). (B) Surface image of the of the K2P1 channels generated with PyMoL from the PDB coordinates 3UKM. (C) Single particle cryo-electron microscopy image of the electric eel Na_v1 channel at 19 Å resolution reproduced with permission from Sato et al. (2001). Horizontal lines demarcate the boundaries of trans-membrane helices. The large extracellular bell-shape cap above the membrane, which is reminiscent of the extracellular cap in K2P channels, can be formed by pairing of long extracellular turrets. Note different scale of images (B,C).