Constraints on voltage sensor movement in the shaker K+ channel

Area of Science:

• Molecular and Cellular Neuroscience
• Biophysics
• Ion Channel Physiology

Background:

• Voltage-gated ion channels control cellular excitability through charge movement across the membrane.
• The precise physical mechanism of charge translocation during gating, particularly the movement of the S4 helix, remains debated.
• Models propose either small-scale motions or large-scale translocation of a 'voltage-sensor paddle' (S3-S4 helices).

Purpose of the Study:

• To investigate the physical movement of the S3-S4 voltage-sensor paddle in the Shaker potassium channel.
• To test hypotheses suggesting large-scale translocations of the voltage-sensor paddle during channel gating.

Main Methods:

• Utilized two experimental approaches to assess aqueous exposure of residues near the S3 helix in the Shaker channel.
• Method 1: Employed a pore-blocking maleimide reagent to detect state-dependent changes in reactivity of substituted cysteines.
• Method 2: Assessed state-dependent accessibility of a tethered biotin to external streptavidin.

Main Results:

• Residues predicted to be near the top of the S3 helix showed no change in aqueous exposure during the gating cycle.
• Both experimental methods yielded consistent results, indicating a lack of significant state-dependent exposure changes.
• These findings directly contradict models predicting large-scale, translocating movements of the S3-S4 voltage-sensor paddle.

Conclusions:

• The results argue against large-scale axial or radial movements of the Shaker channel's S3-S4 voltage-sensor paddle.
• The study provides evidence supporting models involving smaller-scale motions for charge displacement during Shaker channel gating.
• This work contributes to understanding the fundamental mechanisms of voltage sensing in ion channels.