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Ca2+-dependent metarhodopsin inactivation mediated by calmodulin and NINAC myosin III

Che-Hsiung Liu1, Akiko K Satoh, Marten Postma

  • 1Department of Physiology, Development and Neuroscience, Cambridge University, Cambridge CB23DY, UK.

Neuron
|September 13, 2008
PubMed

Insights

Calcium ions accelerate the inactivation of activated rhodopsin (metarhodopsin) in fly photoreceptors. This calcium-dependent mechanism, involving calmodulin and NINAC, enhances visual signaling speed and accuracy.

Area of Science:

  • Molecular Biology
  • Neuroscience
  • Vision Science

Background:

  • Phototransduction in flies represents the fastest known G protein-coupled signaling cascade.
  • The precise mechanisms underlying this rapid signaling performance are not fully understood.

Purpose of the Study:

  • To investigate the mechanism and role of rhodopsin inactivation in Drosophila phototransduction.
  • To elucidate how activated rhodopsin (metarhodopsin, M(*)) is inactivated and the factors influencing its lifetime.

Main Methods:

  • Whole-cell recordings from Drosophila photoreceptors.
  • Measuring the lifetime of activated rhodopsin (M(*)) by assessing the suppressive effect of photoreisomerization.
  • Utilizing Ca2+-free solutions and genetic mutants (calmodulin, NINAC) to probe the Ca2+ dependence of M(*) inactivation.

Main Results:

  • Activated rhodopsin (M(*)) was rapidly inactivated (tau ~20 ms) under normal conditions.
  • M(*) inactivation was significantly slower (~10-fold) in Ca2+-free solutions, indicating a strong Ca2+ dependence.
  • This Ca2+ dependence was independent of rhodopsin phosphorylation or arrestin binding but was abolished in calmodulin (CaM) or NINAC mutants.

Conclusions:

  • Calcium influx, acting through CaM and NINAC, accelerates the binding of arrestin to M(*).
  • This Ca2+-dependent inactivation mechanism is crucial for promoting quantum efficiency, temporal resolution, and fidelity in fly visual signaling.

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