Figure 6. Schematic model for the role of the CTD in gating.

The helices in the TM domain are depicted as cylinders, with the break in the middle of the S6 cylinder located at the threonine hinge that produces the difference between open and closed pore structures. Only two monomers of the tetramer are shown for clarity. As an aid to visualisation of extended and relaxed states, the tetrameric coiled-coil at the distal end of the CTD is depicted as a mass on the end of a ‘spring’ formed by the flexible linker region, which can change conformation (without breaking hydrogen bonds) to accommodate the differences in spacing of the ends of the open and closed states.