Bi / CNS 150 Lecture 2 Friday, October 4, 2013 Voltage-gated channels (no action potentials today) Henry Lester. The Bi / CNS 150 2013 Home Page. http://www.cns.caltech.edu/bi150/. Please note: Henry Lester’s office hours Read the book. If you drop the course, or if you register late,
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Friday, October 4, 2013
Voltage-gated channels (no action potentials today)
Henry Lester’s office hours
Read the book
or if you register late,
please email Teagan Wall
(in addition to the Registrar’s cards).
Also, if you want to change sections,
please email Teagan
In the “selectivity filter” of most K+ channels,
K+ ions lose their waters of hydration and are co-ordinated by backbone carbonyl groups
From Lecture 1
Ion selectivity filter
(Like Kandel Figure 5-15)
chemical transmission at
electrical transmission in
Major Roles for Ion Channels
compared with other electric fields in the modern world
1. A “high-voltage” transmission line
1 megavolt = 106 V.
The ceramic insulators have a length of ~ 1 m.
The field is ~ 106 V/m.
2. A biological membrane
The “resting potential” ~ the Nernst potential for K+, -60 mV.
The membrane thickness is ~ 3 nm = 30 Å.
The field is (6 x 10-2 V) / (3 x 10-9 m) = 2 x 107 V/m !!!
open channel = conductor
H. A. L
Intracellular recording with sharp glass electrodes
A better way: record the current from channels directly?
5 pA = 104 ions/ms
A single voltage-gated Na+ channel
10 ms to 20 min : 108
2 pA to 100 nA
Francisco Bezanilla's simulation program at the Univ. of Chicago.
“Shaker”, a well-studied voltage-gated K+ channel
“Shaker”, a Drosophila mutant first studied in (the late) Seymour Benzer’s lab
by graduate students Lily & Yuh-Nung Jan (now at UCSF);
Gene isolated simultaneously by L & Y-N Jan lab
& by Mark Tanouye (Benzer postdoc, then Caltech prof, now at UC Berkeley).
Today we emphasize H & H’s description of channel gating
(although they never mentioned channels, or measured a single channel)
Channel opening and closing rate constants are functions of voltage--not of time:
The conformational changes are “Markov processes”.
The rate constants depend instantaneously on the voltage--not on the history of the voltage.
These same rate constants govern both the macroscopic (summed) behavior and the single-molecule behavior.
This channel is actually Shaker with inactivation removed (Shaker-IR).
Based on biochemistry, electrophys, site-directed mutagenesis, X-ray crystallography, fluorescence.
Two of 4 subunits. Outside is always above (show membrane). Green arrows = K+.
C1 and C2 are closed states, A is “active” = open.
6 helices (S1-S6) + P region, total / subunit.
Structure corresponds roughly to slide 7,
The two green helices (S5, S6 + P) correspond to the entire Xtal structure on slide 4.
First use manual opening. Channel opens when all 4 subunits are “A”.
Note the charges in S4 (5/subunit, but measurements give ~ 13 total). Alpha-helix with Lys, Arg every 3 rd residue.
Countercharges are in other helices.
Note the S4 charge movement, “shots”. Where is the field, precisely? Near the top.
Note the “hinge” in S6, usually a glycine.
Read the explanation on the simulation.
Show plot. Manual. Then Voltage (start at default, 0 mV ““delayed rectifier”.
Although we simulate sequentially, the cell adds many channels in parallel.
Not an action potential; this is a “voltage jump” or “voltage clamp” experiment.
Describe shots (measure with fluorescence, very approximately).
I = current. Note three types of I.
Describe gating current (average = I(gate); its waveform does not equal the I(average).
Show -30 mV (delayed openings,) -50 mV (no openings), 0 (default).
Note tail current.
There are many V-gated K channels, each with its own V-sens and kinetics.
Some voltage-gated K+ channels
See also Appendix A in Kandel