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(NAS Colloquium) Vision From Photon to Perception (1999) / Chapter Skim
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Gain and kinetics of activation in the G-protein cascade of phototransduction
Pages 10-14

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From page 10...
... Now that a quantitative description of the activation steps in transduction has been obtained, perhaps the greatest challenge for the future is to provide a comprehensive description of the shutoff reactions, so that a complete account of the photoreceptor's response to light can be achieved. The transduction of light into a neural signal in rod and cone photoreceptors provides an excellent example of signaling by a guanine nucleotide binding protein (G protein)
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From page 11...
... Although qualitative information is available on rhodopsin phosphorylation, on the Gprotein's GTPase activity, on the decline in cytoplasmic Ca2+ concentration, and on calcium's role in activating guanylyl cyclase, there is not as yet a quantitative understanding of the way in which all these factors fit together. For this reason, the following analysis is restricted to the activation steps in phototransduction, and it therefore provides a description only of the onset phase of the response to light.
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From page 12...
... As the quantity of activated PDE increases, the soluble cytoplasmic messenger cGMP will be hydrolyzed more rapidly, and the free cGMP concentration will decline (Fig.
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From page 13...
... This parameter, A, specifies the amplification constant of transduction and is given by r (kcat/2Km~ L vcy~O NAVBp ~ where kept and Km are the turnover number and Michaelis constant for the fully activated PDE, Vcy~o is the cytoplasmic volume of the outer segment, NAV is Avogadro's number, and BP is the cytoplasm's buffering power for cGMP (74. Comparison with Experiment Amphibian Rods.
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From page 14...
... Now that activation of the electrical response can be described quantitatively, a major challenge for the future will be to provide a comprehensive description of the shutoff reactions. Once the inactivation steps can be described quantitatively at a molecular level, it should be straightforward to incorporate them into the stochastic model WALK.
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Key Terms

  • time course
  • amplification constant
  • circulating current
  • lateral diffusion
  • activated pde

  • rising phase
  • activated molecules
  • nucleotide binding
  • outer segment
  • flash intensities

  • fractional circulating
  • cgmp concentration
  • plasma membrane
  • ion channels
  • protein activation

  • guanine nucleotide
  • activation steps
  • binding protein
  • single molecule
  • onset phase

  • diffusion limit
  • chemical ligand
  • activation kinetics
  • cytoplasmic messenger
  • diffusion coefficients

  • messenger cgmp
  • cone photoreceptors
  • underlying activation
  • pipette recordings
  • undergoing lateral


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