Channel Structure
Single Channel Function
February 4, 2010
Ionic Channels
• Existence hypothesized by Hodgkin and
Huxley
Channels allow ions to move across
membranes
Channels discriminate between
different ions
Channels sense voltage across the
membrane
Current flow through channels
• Voltage clamp measure current through
thousands of channels
Invented by Kenneth Cole
• Patch clamp
Invented by Neher and Sakman in 1976
Allows current measurement through
single channel
Confirmed existence of ionic channels
Ionic Channels
• Channels are proteins
Portions form pore through membrane
Portions are sensitive to membrane potential
Portions have ligand binding sites
• Channels have distinct functional states
Channel pores are either open or closed
Transition between states is abrupt and
random
Transitions depend on potential and ligand
binding
Single Channel Current
• Flow of current through single channels is
tiny
Microscopic current
1-2 pAmp = thousands of ions per ms
• Single channel current = single channel
conductance * driving potential
i g (VM ER )
• At any time, each channel is either open
(conductance=g) or closed (conductance=0)
Currents through
single Na+ channels
• Each depolarization
produces brief current
flow
Timing of channel
opening is random
Currents reverse at
sodium reversal
potential
Macroscopic current
• Equals the sum of
microscopic currents
One channel: randomly
open or closed
Three channels: up to
three open at once
Many channels:
individual channel
openings difficult
(impossible) to discern
The Neuron, Levitan and Kaczmarek
Na+ channel Current
• Sum of microscopic currents
looks like macroscopic
current
• Probability of single channel
opening has voltage
dependence of macroscopic
current
Currents through
single K+ channels
• Each depolarization
produces prolonged
current flow
Timing of channel
opening is random
Channel closings
are infrequent
K+ channel Current
• Sum of microscopic currents
looks like macroscopic
current
• Probability of single channel
opening has voltage
dependence of macroscopic
current
Channel Function
• Macroscopic Current, I = G(VM-ER)
• Sum of microscopic (single channel)
current, i
• Sum of currents through N channels
• Usually, only a fraction (0 to 1) of channels
open = Fo
• I = N Fo i
• Conductance is sum of single channel
conductances
• G = N Fo g
Functional states of voltage-gated
channels
• Voltage dependence mediated by gates
• Activation gates open with depolarization
And close with hyperpolarization
• Inactivation gates close with
depolarization
And open with hyperpolarization
Not all channels have inactivation gates
Functional states of voltage-gated
Na+ and K+ channels
• Response to
depolarization
Potassium Channels
• Squid Potassium channel has one type of
gate
Gate opens with depolarization
Gate closes with hyperpolarization
• Current continues to flow with sustained
depolarization
Sodium Channels
• Sodium channel has two types of gates
Activation gate
Opens with depolarization
Closes with hyperpolarization
Inactivation gate
Closes with depolarization
Opens with hyperpolarization
• Current flows when both gates open
Sodium Channels
• Depolarization causes activation gate to
open quickly
Current flows
• Depolarization causes inactivation gate to
close slowly
Current stops flowing
• Repolarization required to de-inactivate
channel, allow subsequent current flow
Sodium Current States
Diversity of Ion Channels
• Multitude of ion channel genes
• Many genes have alternative splice
variants
• RNA can be edited prior to translation
• Channel proteins modified post-
translationally
Classification of Ion Channels
• Single channel conductance
• Ion Selectivity
• Which ions permeate channel
• Gating
• What causes channel to open or close
• Pharmacology
• What drugs block or inactivate the
channel
Types of voltage-gated ion channels
> 10 types >16 types Dozens
Types of voltage-gated ion channels
Ligand-Gated Ion Channels
Extracellular ligand Intracellular ligand
Types of Ca2+ ion channels
• Action Potential Generating
• Modify Action Potential Shape
• Regulate release of neurotransmitters
• Regulate other cell processes
Channel Function versus Structure
• What parts of the channel produce the
pore?
How is selectivity achieved?
• What parts of the channel sense
membrane potential?
• What parts of the channel bind to ligands?
Channel Function versus Structure
• Gene mutation
• Expression of gene in cells
Frog Oocytes – few other channels
Mammalian cells
• Measure properties of ion channel
Expression of Ion Channels in
Xenopus Oocytes
Structure of Voltage-Gated Channels
• Sodium and Calcium channels
Four homologous domains
Each domain with 6 transmembrane
segments, S1 through S6
Total of 24 transmembrane regions
Accessory subunits can regulate
function, but are not essential for
functional channel
Structure of Voltage-Gated Channels
Structure of Voltage-Gated Channels
Structure of Voltage-Gated Channels
• Potassium channels
Most subunits have 6 transmembrane
segments
Four subunits are required to form
channel
Total of 24 transmembrane regions
produce function channel
• Different types of potassium channels
have subunits with 2, 4 or 7
transmembrane domains
Structure of Voltage-Gated Channels
Structure of
Voltage-Gated
Channels
• Chloride Channels
Structurally
distinct from
cation channels
S4: Voltage Sensor
• One of the transmembrane domains has
many charged amino acids
• Movement of voltage sensor opens the pore
Voltage Sensor
• Voltage sensor amino
acids
• Highly conserved
• Every third one has
charge
• Charges spiral
around helix
Structure of a bacterial K+ channel
• Two transmembrane domains and
connecting loop form the pore
• Four subunits create functional channel
Structure of a bacterial K+ channel
• Only non hydrated
K+ can fit through
selectivity filter
Cs+ is too big
Na+ is too small
to be stabilized
by amino acids
Structure of a bacterial K+ channel
• Hydrated K+ are
collected, and
then dehydrated
in central cavity
• Selectivity filter
contains four K+
binding sites
• Electrostatic
repulsion helps to
speed transit
Structure of a mammalian voltage-
gated K+ channel
• Parts of T1 domain or b subunit participate
in inactivation
SEM of mammlian
K+ channels
• Pore region is
similar to bacterial
K+ channel
• Bacterial channel
corresponds to S5,
S6 and the
intracellular loop
connecting them
Structure of a mammalian voltage-gated
K+ channel
• Additional voltage sensors (S4) exert force on
S4-S5 linker to open or close pore
Structure of a mammalian voltage-
gated K+ channel
• Paddle like movement of S4-S5 linker
From extracellular exposure
To intracellular membrane surface