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Rhodopsin Channel Activity Can Be Evaluated by Measuring the Photocurrent Voltage Dependence in Planar Bilayer Lipid Membranes

Tatyana I. Rokitskaya1,a*, Nina Maliar2, Kirill V. Kovalev2,3, Oleksandr Volkov4,5, Valentin I. Gordeliy2,3,4,5, and Yuri N. Antonenko1

1Belozersky Institute of Physico-Chemical Biology, Lomonosov Moscow State University, 119991 Moscow, Russia

2Moscow Institute of Physics and Technology, 141701 Dolgoprudny, Moscow Region, Russia

3Institut de Biologie Structurale (IBS), Université Grenoble Alpes, CEA, CNRS, 38044 Grenoble, France

4Institute of Biological Information Processing (IBI-7: Structural Biochemistry), Forschungszentrum Jülich GmbH, 52425 Juelich, Germany

5JuStruct: Jülich Center for Structural Biology, Forschungszentrum Jülich GmbH, 52425 Juelich, Germany

* To whom correspondence should be addressed.

Received December 4, 2020; Revised January 22, 2021; Accepted January 22, 2021
The studies of the functional properties of retinal-containing proteins often include experiments in model membrane systems, e.g., measurements of electric current through planar bilayer lipid membranes (BLMs) with proteoliposomes adsorbed on one of the membrane surfaces. However, the possibilities of this method have not been fully explored yet. We demonstrated that the voltage dependence of stationary photocurrents for two light-sensitive proteins, bacteriorhodopsin (bR) and channelrhodopsin 2 (ChR2), in the presence of protonophore had very different characteristics. In the case of the bR (proton pump), the photocurrent through the BLM did not change direction when the polarity of the applied voltage was switched. In the case of the photosensitive channel protein ChR2, the photocurrent increased with the increase in voltage and the current polarity changed with the change in the voltage polarity. The protonophore 4,5,6,7-tetrachloro-2-trifluoromethyl benzimidazole (TTFB) was more efficient in the maximizing stationary photocurrents. In the presence of carbonyl cyanide-m-chlorophenylhydrazone (CCCP), the amplitude of the measured photocurrents for bR significantly decreased, while in the case of ChR2, the photocurrents virtually disappeared. The difference between the effects of TTFB and CCCP was apparently due to the fact that, in contrast to TTFB, CCCP transfers protons across the liposome membranes with a higher rate than through the decane-containing BLM used as a surface for the proteoliposome adsorption.
KEY WORDS: retinal-containing proteins, channelrhodopsin, proton pump, proteoliposomes, bilayer lipid membrane, protonophore

DOI: 10.1134/S0006297921040039