Tracking voltage-dependent conformational changes in skeletal muscle sodium channel during activation

Area of Science:

• Molecular Biology
• Biophysics
• Neuroscience

Background:

• Sodium channels are crucial for electrical signaling in excitable cells.
• The voltage sensor, primarily composed of S4 segments, controls channel gating.
• Differential contributions of S4 segments to gating kinetics are not fully understood.

Purpose of the Study:

• To investigate the kinetic and steady-state behavior of individual S4 segments during sodium channel activation.
• To correlate fluorescence signals from S4 segments with gating charge movement.
• To map the sequence and timing of S4 domain movements during channel gating.

Main Methods:

• Utilized site-directed fluorescence labeling on a rat skeletal muscle sodium channel.
• Simultaneously monitored fluorescence signals from S4 segments and gating/ionic currents.
• Analyzed fluorescence kinetics and equilibrium properties across a range of activation voltages.

Main Results:

• Fluorescence changes from S4 domains I, II, and III correlate with fast gating currents and initiate simultaneously.
• Fluorescence from S4 domain IV shows a lag phase and correlates with slower gating components.
• S4 domain III moves at hyperpolarized potentials; S4 domains I and II move at depolarized potentials.
• S4 domain IV movement kinetics are slower than activation time constants, suggesting it's not essential for initial opening.

Conclusions:

• Voltage-dependent movement of S4 domain IV is a later step in the sodium channel activation sequence.
• The S4 segments exhibit distinct kinetic and voltage-dependent behaviors, contributing sequentially to channel gating.
• These findings provide a detailed kinetic map of sodium channel activation at the level of individual S4 domains.