Transcript Femtosecond pulses help to study voltage

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 Ca2+).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
Time course of the gating currents for the
pulses indicated.
The voltage dependence of the open probability
and the charge moved per channel.
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.
Positively charged residues
Paddle model.