S4-based voltage sensors have three major conformations.

Area of Science:

• Biophysics
• Molecular Biology
• Membrane Protein Dynamics

Background:

• Voltage sensors, crucial for cellular electrical signaling, possess charged S4 transmembrane segments.
• The gating charge versus voltage (Q-V) curve of these sensors is known to be voltage-dependent.
• The voltage-dependent phosphatase (Ci-VSP) lacks a conducting pore but exhibits similar voltage-dependent phenomena.

Purpose of the Study:

• To investigate the leftward shift of the Q-V curve in Ci-VSP under prolonged depolarization.
• To characterize the distinct conformational states of the voltage sensor.
• To elucidate the structural dynamics of the S4 segment during voltage sensing.

Main Methods:

• Electrophysiological recordings to measure charge movement.
• Site-directed fluorescence measurements to track conformational changes.
• Temperature dependence studies to analyze thermodynamic properties.

Main Results:

• Prolonged depolarization of Ci-VSP leads to a more stable Q-V curve.
• Fluorescence data revealed two kinetic components: charge movement and a slower transition to a relaxed state.
• Thermodynamic analysis indicated significant enthalpic and entropic changes during the active-to-relaxed state transition.
• Q-V curve midpoint periodicity suggests a 3(10) helical structure for S4 in the active state.

Conclusions:

• The S4 segment undergoes distinct structural transitions, potentially moving as a 3(10) helix between resting and active states.
• A further transition to a more stable relaxed state involves a structural conversion, likely to an alpha-helix.
• These findings provide insights into the molecular mechanisms of voltage sensing and conformational changes in membrane proteins.