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Maeda R, Hiroshima M, Yamashita T, Wada A, Nishimura S, Sako Y, Shichida Y, Imamoto Y. Single-molecule observation of the ligand-induced population shift of rhodopsin, a G-protein-coupled receptor. Biophys J 2014; 106:915-24. [PMID: 24559994 DOI: 10.1016/j.bpj.2014.01.020] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/20/2013] [Revised: 12/26/2013] [Accepted: 01/10/2014] [Indexed: 01/20/2023] Open
Abstract
Rhodopsin is a G-protein-coupled receptor, in which retinal chromophore acts as inverse-agonist or agonist depending on its configuration and protonation state. Photostimulation of rhodopsin results in a pH-dependent equilibrium between the active state (Meta-II) and its inactive precursor (Meta-I). Here, we monitored conformational changes of rhodopsin using a fluorescent probe Alexa594 at the cytoplasmic surface, which shows fluorescence increase upon the generation of active state, by single-molecule measurements. The fluorescence intensity of a single photoactivated rhodopsin molecule alternated between two states. Interestingly, such a fluorescence alternation was also observed for ligand-free rhodopsin (opsin), but not for dark-state rhodopsin. In addition, the pH-dependences of Meta-I/Meta-II equilibrium estimated by fluorescence measurements deviated notably from estimates based on absorption spectra, indicating that both Meta-I and Meta-II are mixtures of two conformers. Our observations indicate that rhodopsin molecules intrinsically adopt both active and inactive conformations, and the ligand retinal shifts the conformational equilibrium. These findings provide dynamical insights into the activation mechanisms of G-protein-coupled receptors.
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Affiliation(s)
- Ryo Maeda
- Department of Biophysics, Graduate School of Science, Kyoto University, Kyoto, Japan
| | - Michio Hiroshima
- Cellular Informatics Laboratory, RIKEN, Wako, Japan; Laboratory for Cell Signaling Dynamics, RIKEN Quantitative Biology Center, Furuedai, Suita, Japan
| | - Takahiro Yamashita
- Department of Biophysics, Graduate School of Science, Kyoto University, Kyoto, Japan
| | - Akimori Wada
- Department of Organic Chemistry for Life Science, Kobe Pharmaceutical University, Kobe, Japan
| | - Shoko Nishimura
- Cellular and Structural Physiology Institute, Nagoya University, Nagoya, Japan
| | - Yasushi Sako
- Cellular Informatics Laboratory, RIKEN, Wako, Japan
| | - Yoshinori Shichida
- Department of Biophysics, Graduate School of Science, Kyoto University, Kyoto, Japan
| | - Yasushi Imamoto
- Department of Biophysics, Graduate School of Science, Kyoto University, Kyoto, Japan.
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Woodruff ML, Wang Z, Chung HY, Redmond TM, Fain GL, Lem J. Spontaneous activity of opsin apoprotein is a cause of Leber congenital amaurosis. Nat Genet 2003; 35:158-64. [PMID: 14517541 DOI: 10.1038/ng1246] [Citation(s) in RCA: 143] [Impact Index Per Article: 6.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/25/2003] [Accepted: 08/26/2003] [Indexed: 12/19/2022]
Abstract
Mutations in Rpe65 disrupt synthesis of the opsin chromophore ligand 11-cis-retinal and cause Leber congenital amaurosis (LCA), a severe, early-onset retinal dystrophy. To test whether light-independent signaling by unliganded opsin causes the degeneration, we used Rpe65-null mice, a model of LCA. Dark-adapted Rpe65-/- mice behaved as if light adapted, exhibiting reduced circulating current, accelerated response turn-off, and diminished intracellular calcium. A genetic block of transducin signaling completely rescued degeneration irrespective of an elevated level of retinyl ester. These studies clearly show that activation of sensory transduction by unliganded opsin, and not the accumulation of retinyl esters, causes light-independent retinal degeneration in LCA. A similar mechanism may also be responsible for degeneration induced by vitamin A deprivation.
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Affiliation(s)
- Michael L Woodruff
- Department of Physiological Science, University of California Los Angeles, Los Angeles, California 90095, USA
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Abstract
The rod cell photoreceptor apoprotein, opsin, activates the G-protein, transducin, although at a much reduced level than light-activated rhodopsin. The ability of all-trans-retinal to enhance opsin apoprotein activity was investigated using a guanyl nucleotide exchange assay on transducin. All-trans-retinal enhanced opsin activity in a concentration-dependent manner. At high concentrations of all-trans-retinal, the activity of the all-trans-retinal-opsin complex was comparable to that from an equimolar amount of metarhodopsin(II). However, in contrast to metarhodopsin(II), the active all-trans-retinalopsin complex did not require a stable Schiff base linkage between opsin and all-trans-retinal. The lack of a stable Schiff base and differences in activity at high pH imply that opsin and all-trans-retinal form a complex that is distinct from metarhodopsin(II). The ability of all-trans-retinal to stimulate the transduction cascade may be a source of post-bleach noise in photoreceptors.
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Affiliation(s)
- A Surya
- Department of Biochemistry and Molecular Biology, SUNY Health Science Center at Syracuse 13210, USA
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