Figure 6.

Fast and slow movements of the S4 in Z direction and pore opening. (Left) The steady-state structure cluster KDE for the indicated $V_m$ when depolarized from the holding potential of −80 mV is shown. These panels represent transitional structure distributions when a step voltage of 60 mV is applied from a holding potential of −80 mV. The comparison of the temporal KDE plots (Fig. 5, top) with the steady-state KDE (left) indicates that at negative $V_m$, only the first fast transition (I) occurred, whereas at positive $V_m$, the first transition followed by a slower second transition (II) was observed. The first transition captures the major movement of the S4 in the Z direction, after which the second transition only shows a slight increase in the S4 Z (∼1.4 Å) and is consistent with the gating charge calculations shown in Fig. 4. The slowest transition (III) contributes very little to the current and conformational changes observed in the simulation time course. (Right) Cartoon schema of the fast, slow, and extremely slow conformational changes. The pink arrows represent the initial fast conformational transition because of S4 movement in the Z direction, whereas the green arrow shows the slower transition from medium to large pore and the further small S4 Z movement. The blue arrow represents the extremely slow transition from a very small pore to a large pore. Note that the observed transitions are not sequential and happen simultaneously.