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Biochem J. 2016 Nov 1;473(21):3819-3836. Epub 2016 Sep 13.

Quaternary structures of opsin in live cells revealed by FRET spectrometry.

Author information

1
Department of Physics, University of Wisconsin Milwaukee, Milwaukee, WI, USA.
2
Department of Ophthalmology and Visual Sciences, Case Western Reserve University, Cleveland, OH, USA.
3
Department of Biological Sciences, University of Wisconsin Milwaukee, Milwaukee, WI, USA.
4
Laboratory of Biomodeling, Faculty of Chemistry and Biological and Chemical Research Centre, University of Warsaw, Warsaw, Poland.
5
Department of Physics, University of Wisconsin Milwaukee, Milwaukee, WI, USA Department of Biological Sciences, University of Wisconsin Milwaukee, Milwaukee, WI, USA.

Abstract

Rhodopsin is a prototypical G-protein-coupled receptor (GPCR) that initiates phototransduction in the retina. The receptor consists of the apoprotein opsin covalently linked to the inverse agonist 11-cis retinal. Rhodopsin and opsin have been shown to form oligomers within the outer segment disc membranes of rod photoreceptor cells. However, the physiological relevance of the observed oligomers has been questioned since observations were made on samples prepared from the retina at low temperatures. To investigate the oligomeric status of opsin in live cells at body temperatures, we utilized a novel approach called Förster resonance energy transfer spectrometry, which previously has allowed the determination of the stoichiometry and geometry (i.e. quaternary structure) of various GPCRs. In the current study, we have extended the method to additionally determine whether or not a mixture of oligomeric forms of opsin exists and in what proportion. The application of this improved method revealed that opsin expressed in live Chinese hamster ovary (CHO) cells at 37°C exists as oligomers of various sizes. At lower concentrations, opsin existed in an equilibrium of dimers and tetramers. The tetramers were in the shape of a near-rhombus. At higher concentrations of the receptor, higher-order oligomers began to form. Thus, a mixture of different oligomeric forms of opsin is present in the membrane of live CHO cells and oligomerization occurs in a concentration-dependent manner. The general principles underlying the concentration-dependent oligomerization of opsin may be universal and apply to other GPCRs as well.

KEYWORDS:

G-protein-coupled receptors; fluorescence resonance energy transfer; membrane proteins; oligomerization; quaternary structure

PMID:
27623775
PMCID:
PMC5085863
DOI:
10.1042/BCJ20160422
[Indexed for MEDLINE]
Free PMC Article

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