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How Ion Channels Move to Create Action Potentials

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?.

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How Ion Channels Move to Create Action Potentials

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  1. How Ion Channels Move to Create Action Potentials

  2. 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!

  3. How does gate spontaneously shut-off? How fast?

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

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

  6. Crystal Structure of S1-S6 Ion Channel Nobel Prize for Rod MacKinnon, 2006 S1-S4 Voltage Sensor Lies on the Outside of S5/S6 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.

  7. 3 Models for how S4 moves c) Helix: Twist and Rotation Rotation plus small rotation Translation

  8. What can fluorescence tell us about Shaker K+ channelopening/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). www.mpibp-frankfurt.mpg.de/schwarz/oocytes.html

  9. (review)

  10. How to measure? Where to put probes?

  11. Problems with oocytes • Tremendous amount of autofluorescence • 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.

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

  13. Fluor-escein Example of LRET

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

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

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

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

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

  19. Class evaluation • What was the most interesting thing you learned in class today? • 2. What are you confused about? • 3. Related to today’s subject, what would you like to know more about? • 4. Any helpful comments. Answer, and turn in at the end of class.

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