How Ion Channels Move to Create Action Potentials.

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Presentation transcript:

How Ion Channels Move to Create Action Potentials

Suggests model where 2 states that differ in energy by qV Where q is about 13e, or 13e/4 per S1-S4 sub-unit; V= -80mV. q is part of channel—gating current, not ionic current!

How does gate spontaneously shut-off? How fast?

Nerve Impulse propagate, not spread, because Na + spontaneously shut-off.

Structure of Pore-Domain (S5-S6) is known (KvAP, Kv1.2… all yield the same structure) But how S4 (and S1-S3) move, remain controversial. Explains ion selectivity (K + > Na + ) and rapid ion flux. Excellent agreement between LRET and Crystallography

S1-S4 Voltage Sensor Lies on the Outside of S5/S6 Crystal Structure of S1-S6 Ion Channel Nobel Prize for Rod MacKinnon, 2006 But… Channel has been crystallized in only one state. There is no crystal structure of a channel in the open and closed state. Also, there were some serious problems with some (all?) states. Need alternative techniques…lower resolution but can tell about channel in a more realistic setting.

3 Models for how S4 moves c) Helix: Twist and Rotation Translation Rotation plus small rotation

What can fluorescence tell us about Shaker K + channel opening/closing? Shaker Channel– Can measure both Open & Closed States. Inject mRNA in oocytes; wait 2-5 days; protein in membrane. (Use mRNA where ionic current is blocked, if necessary).

(review)

How to measure? Where to put probes?

Problems with oocytes 1.Tremendous amount of autofluorescence 2.Donor only and acceptor only plus desired donor-acceptor only Answer: Luminescence Resoance Energy Transfer (LRET). 1. Donor has long lifetime– gets away from autofluorescence 2. Can isolate donor-acceptor complex.

Luminescent Chelates Ln Luminescence O O - O C O O - 3 CH O N CO 2 - NH C C N N N O CO O Tb 3+ in out Energy transfer Donor Acceptor Comparatively short lifetime. Emits where donor is dark. Long lifetime Sharply spiked in. Can see D-A even if incomplete labeling: D- only, A-only.

Example of LRET Fluor- escein

O O - O C O O - 3 CH O N CO 2 - NH C C N N N O CO O Tb 3+ Low [KCl] Inside Outside High [KCl] -60 mV (resting, closed) 0 mV (active, open) in out K+K+K+K+K+K+ gate Energy transfer Shaker Potassium Channel

S4: Geometry & data of Shaker channel Cha, Nature, 1999 Two exponential= two-distances

b.b. c. Voltage dependent movement LRET is tracking Gating charge movement

Three neighboring residues, 351, 352, 352 Move in different directions farther Shaker voltage sensor twists, does not translate too much.

How it all adds up: Shaker voltage sensor twists, does not translate too much. S4 Resting S4 Activated Cha, Nature, 1999

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