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Structure of Ion Channels

Structure of Ion Channels. 8 February 2012. Problem Set 1. New Due Date: Monday, February 13. Take out your clickers!. The Nernst Equation predicts…. The Resting Membrane Potential The Neuron Doctrine for a given cell The Threshold Voltage for a given ion

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Structure of Ion Channels

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  1. Structure of Ion Channels 8 February 2012

  2. Problem Set 1 New Due Date: Monday, February 13

  3. Take out your clickers!

  4. The Nernst Equation predicts… • The Resting Membrane Potential • The Neuron Doctrine for a given cell • The Threshold Voltage for a given ion • The Equilibrium potential for a given ion • The Number of Ions in a given cell

  5. What does the patch clamp measure? • Ionic Current • Membrane Voltage • Ionic Concentration • The Equilibrium Potential • Feedback Distortion

  6. Neurons • Detect physical stimuli • Interpret meaning • Initiate action • All of the above • None of the above

  7. The Equilibrium potential is also known as the • Fick-Ohms Potential • Equilectrochemical Potential • Reversal Potential • Current-Loss Potential • Zero Amp Potential

  8. Structure of Ion Channels

  9. Structure of Ion Channels • Nicotinic Acetylcholine Receptor (nAChR) • Located in many tissues • Most notably at the “neuromuscular junction” • Found in “electric organ” of Torpedo sp.

  10. Elucidating Ion Channel Structure • Find a rich source of the protein • Purify the protein • Sequence small parts of the protein • Synthesize corresponding DNA probes • Identify mRNA coding for the protein • Determine sequence of mRNA • Predict protein structure

  11. Sequencing Ion Channels

  12. Sequencing Ion Channels

  13. AChR One Subunit One Receptor (five subunits)

  14. AChR • A single nicotinic ACh receptor is made up of 5 subunits: • a2bgd • Subunits are not alike! • a subunit contains the binding site for ACh molecules

  15. Nicotinic AChR • Pentameric structure (consists of 5 subunits) • Ligand-gated ion channel (Na+) • Transmembrane regions consist of hydrophobic portions of each subunit • Acetylcholine binding to the a subunit leads to a conformational change in the channel shape, opening the pore

  16. The Superfamily of Ligand-Gated Channels • g-aminobutyric acid (GABA) receptor • Opens an anion-selective channel • Serotonin 5-HT3 receptor • Opens a cation-selective channel • Glycine receptor • Opens an anion-selective channel • ACh receptor • Opens a cation-selective channel • The similarity of the genes for these proteins suggests a common origin.

  17. Voltage-gated ion channels • Ion channels that open when Vm changes – Why? • 1. Find a rich source of the protein

  18. Voltage-gated Na+ channels • Consist of a single, large protein (approx 1830 AA) • Protein has four Domains (I-IV) reminiscent of subunits • Each domain consists of 7 hydrophobic regions • Each S4 region contains several charged AAs

  19. Voltage-gated Na+ channels • 3-D structure • Slightly different in mammals • Two additional b subunits (1 and 2) can be co-expressed; these augment aspects of channel kinetics • Several subtypes; varies by tissue (nerve, muscle, heart)

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