Voltage-Gated Ion Channels
• Voltage-dependent ion channels play a fundamental role in the generation and propagation of the nerve impulse and in cell homeostasis.
•
The voltage sensor is a region of the protein bearing charged amino acids that relocate upon changes in the membrane electric field
.
Segments (S5 and S6) and the pore loop were found to be responsible for ion conduction.
Lipid bilayer
Transmembrane segment (cylinder)
Voltage sensor part of the channel
Pore
General architecture of voltage-gated channels (Na + and Ca 2+ ).The “+” or “-“ signs indicate charges that have been implicated in voltage sensing.

Voltage-Gated Ion Channels
The Gate The ion conduction through the pore may be interrupted by closing a gate
Lipid bilayer
Closed states voltage gated ion channel field-effect transistor
Open states voltage gated ion channel field-effect transistor
Comparison between a field-effect transistor (FET) and a voltage gated ion channel. The FET transistor is represented as a p-channel device to make a closer analogy to a cation selective voltage-gated channel. In the
FET, D is the drain, S is the source, and G is the gate.

Voltage-Gated Ion Channels
Ionic and gating currents in Shaker-IR K + channel
Time course of ionic currents for pulses to the indicated potentials starting and returning to –- 90 mV
The voltage dependence of the open probability and the charge moved per channel.
Time course of the gating currents for the pulses indicated
.

Voltage-Gated Ion Channels
Typical gating current trace
Gating current recorded at 10-kHz bandwidth.
When the bandwidth is increased to
200 kHz, the first surge of current is the predominant amplitude
The time course of the charging of the membrane capacitance for the experiment in (b).

Voltage-Gated Ion Channels
Current fluctuations in gating currents
Top trace is the time course of a pulse to
+10 mV; middle trace is the variance and bottom trace is the mean computed from several hundred traces.
Top trace is the time course of a smaller pulse to
- 40 mV; middle trace is the variance; lower trace is the mean computed from hundreds of traces.

Voltage-Gated Ion Channels
Three models of the voltage sensor
Helical screw model Transporter model. Paddle model.
Positively charged residues