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Most organisms use photoreceptors to sense and respond to light. Examples of photoreceptors include bacteriorhodopsins and bacteriophytochromes in some bacteria, phytochromes in plants, and rhodopsins in the photoreceptor cells of the vertebral retina. The light-sensitive property of these receptors is because of the bound chromophores, such as bilin in the phytochromes and retinal in the rhodopsins.
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Enzymes like flippase, floppase, and scramblase transfer phospholipids from one layer to another in the membrane, thereby affecting membrane asymmetry.
Flippase
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A Fluorescence-based Assay of Phospholipid Scramblase Activity
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Phospholipid scrambling by rhodopsin.

Oliver P Ernst1, Anant K Menon2

  • 1Department of Biochemistry, University of Toronto, Toronto, ON, Canada M5S 1A8 and Department of Molecular Genetics, University of Toronto, Toronto, ON, Canada M5S 1A8. oliver.ernst@utoronto.ca.

Photochemical & Photobiological Sciences : Official Journal of the European Photochemistry Association and the European Society for Photobiology
|July 17, 2015
PubMed
Summary

Rhodopsin, known for vision, also acts as a phospholipid scramblase, rapidly moving lipids across membranes without ATP. This newly found function may be crucial for retinal health.

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Area of Science:

  • Biochemistry
  • Molecular Biology
  • Ophthalmology

Background:

  • Rhodopsin is a well-known visual pigment and G protein-coupled receptor essential for dim-light vision.
  • Phospholipid scramblases are enzymes that facilitate the movement of lipids across cell membranes, a process critical for membrane homeostasis.
  • The transporter ABCA4 plays a vital role in the retina, preventing the buildup of toxic retinoid compounds.

Purpose of the Study:

  • To investigate the newly discovered phospholipid scramblase activity of rhodopsin.
  • To explore the potential mechanism behind rhodopsin's scramblase function.
  • To hypothesize the necessity of rhodopsin's scramblase activity for ABCA4 transporter function in the retina.

Main Methods:

  • Reconstitution of purified rhodopsin into large unilamellar vesicles (LUVs).
  • Measurement of phospholipid transbilayer translocation rates facilitated by rhodopsin.
  • Analysis of rhodopsin's role in the context of ABCA4 transporter function and retinoid metabolism.

Main Results:

  • Rhodopsin demonstrates significant ATP-independent phospholipid scramblase activity, accelerating lipid translocation over 1000-fold.
  • The translocation rate exceeds 10,000 phospholipids per rhodopsin per second.
  • This activity is independent of ATP, distinguishing it from other known scramblases.

Conclusions:

  • Rhodopsin possesses a potent, ATP-independent phospholipid scramblase function.
  • This scramblase activity is hypothesized to be essential for the proper functioning of the ABCA4 transporter.
  • Understanding rhodopsin's dual role may offer insights into retinal diseases linked to retinoid accumulation.