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S4-based voltage sensors have three major conformations

Voltage sensors containing the charged S4 membrane segment display a gating charge vs. voltage (Q–V) curve that depends on the initial voltage. The voltage-dependent phosphatase (Ci-VSP), which does not have a conducting pore, shows the same phenomenon and the Q–V recorded with a depolarized initial voltage is more stable by at least 3RT. The leftward shift of the Q–V curve under prolonged depolarization was studied in the Ci-VSP by using electrophysiological and site-directed fluorescence measurements. The fluorescence shows two components: one that traces the time course of the charge movement between the resting and active states and a slower component that traces the transition between the active state and a more stable state we call the relaxed state. Temperature dependence shows a large negative enthalpic change when going from the active to the relaxed state that is almost compensated by a large negative entropic change. The Q–V curve midpoint measured for pulses that move the sensor between the resting and active states, but not long enough to evolve into the relaxed states, show a periodicity of 120°, indicating a 310 secondary structure of the S4 segment when determined under histidine scanning. We hypothesize that the S4 segment moves as a 310 helix between the resting and active states and that it converts to an α-helix when evolving into the relaxed state, which is most likely to be the state captured in the crystal structures.