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Melanopsin forms a functional short-wavelength photopigment
Lucy A Newman1, Marquis T Walker, R Lane Brown
1Department of Biological Sciences, University of Maryland, Baltimore County, Baltimore, Maryland 21250, USA.
Biochemistry
|November 5, 2003
Summary
Melanopsin, a key photopigment in the mammalian circadian system, was studied. While it activates a G-protein, its blue light absorption differs from the required spectrum for circadian entrainment.
Area of Science:
- Neuroscience
- Photobiology
- Chronobiology
Background:
- Melanopsin is a candidate photopigment for the mammalian circadian system.
- It's expressed in retinal ganglion cells projecting to the suprachiasmatic nucleus (SCN), the circadian pacemaker.
- Light synchronization of circadian rhythms doesn't require rod or cone photoreceptors.
Purpose of the Study:
- To characterize melanopsin's properties as a photopigment.
- To determine if melanopsin's spectral characteristics match those required for circadian entrainment.
Main Methods:
- Heterologous expression of melanopsin in COS cells.
- Measurement of melanopsin's light absorption spectrum after reconstitution with 11-cis-retinal.
- Assay of melanopsin's ability to activate the G-protein transducin in a light-dependent manner.
Main Results:
- Melanopsin maximally absorbed light at 424 nm after reconstitution with 11-cis-retinal.
- Melanopsin demonstrated light-dependent activation of transducin.
- The measured absorbance spectrum (peak ~424 nm) is inconsistent with the action spectrum for circadian entrainment (peak ~484 nm).
Conclusions:
- Melanopsin forms a functional photopigment capable of G-protein activation.
- Melanopsin's spectral properties do not align with the known requirements for synchronizing the mammalian circadian system to light-dark cycles.