Figure 3. The S6 helices remain kinked when the pore is closed.

(a) The height of energetic barrier faced by a potassium ion is extracted from a series of fifteen 1D potential of mean forces (PMF) (Supplementary Figure S8) and then plotted against the average pore radius. The relationship is well fitted by an exponential and shows that as the pore radius decreases, the free energy required to move a potassium ion past the constriction point increases. (b) Using this relationship, the height of the energetic barrier is predicted along the minimum free energy path (MFEP). The channel is empirically defined as open (green) where the barrier is <2.7 kcal mol⁻¹, closed (red) where the barrier has been increased by between 2.8 and 7.1 kcal mol⁻¹ and inaccessible (grey) where the barrier has increased by >7.1 kcal mol. This colour scheme is used in the remaining panels. (c) The 2D PMF in Figure 2b can be divided into open, closed and inaccessible regions. (d) To illustrate this, the accessible pore surfaces taken from six points along the MFEP are shown. For clarity, only two of the four monomers are drawn and the S6 helices are colored dark grey.