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Efficiencies of activation of transducin by cone and rod visual pigments

Yasushi Imamoto1, Ichirota Seki, Takahiro Yamashita

  • 1Department of Biophysics, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan.

Biochemistry
|April 11, 2013
PubMed

Insights

Cone and rod visual pigments activate transducin (Gt) differently. Cone pigment

Area of Science:

  • Biochemistry
  • Molecular Biology
  • Vision Science

Background:

  • The biochemical differences in transducin (Gt) activation between cone and rod visual pigments are not well understood.
  • Cone visual pigment's Gt-activating state (Meta-II) decays rapidly, hindering precise biochemical analysis.
  • Conventional methods like GTPγS binding assays are unsuitable for measuring fast-decaying cone pigment activation.

Purpose of the Study:

  • To investigate and compare the real-time Gt activation efficiencies of cone visual pigment and rhodopsin.
  • To elucidate the biochemical mechanisms underlying the differences in Gt activation between cone and rod pigments.
  • To identify the role of specific amino acid residues in pigment activation and stability.

Main Methods:

  • Real-time measurement of Gt activation efficiency using intrinsic fluorescence of tryptophan residues in pigments and Gt.
  • Michaelis-Menten kinetic analysis to determine activation velocities and constants.
  • Site-directed mutagenesis of cone pigment and rhodopsin at positions 122 and 189 to assess chromophore hydrolysis rates and Gt activation.

Main Results:

  • Chicken green-sensitive cone visual pigment (cG) exhibited approximately half the initial Gt activation velocity of bovine rhodopsin (bRh), with comparable Michaelis constants.
  • Gt activation by cG was rapidly attenuated due to fast retinal Schiff base hydrolysis in Meta-II, though complex formation with Gt suppressed this hydrolysis.
  • Mutant studies revealed a negative correlation between initial Gt activation velocity and the rate of chromophore hydrolysis, implicating residues 122 and 189.

Conclusions:

  • Amino acid residues at positions 122 and 189 influence both the stability of the Meta-II state against chromophore hydrolysis and its conformation for efficient Gt activation.
  • The faster decay of cone pigment's Meta-II state, linked to chromophore hydrolysis, contributes to its distinct Gt activation kinetics compared to rhodopsin.
  • The longer lifetime of rhodopsin's Gt-activating state is crucial for sensitive single-photon detection in rods by minimizing spontaneous quenching.

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