Institut de Biologie StructuraleGrenoble / France

Channelrhodopsin reveals its dark secrets

Ion channels are integral membrane proteins that upon stimulation modulate the flow of ions across the cell or organelle membrane. Channelrhodopsins (ChRs) appeared to be unusual channels. They belong to the large family of microbial rhodopsins, seven-helical transmembrane proteins containing retinal as chromophore. Photon absorption initiates retinal isomerization resulting in a photocycle, with different spectroscopically distinguishable intermediates, thereby controlling the opening and closing of the channel. In 2003, it was demonstrated that light-induced currents by heterologously expressed ChannelRhodopsin2 (ChR2) can be used to change a host`s membrane potential. This accomplishment has made ChR2 the first long thought and further used a key instrument of neuroscience. ChR2 opened a new field - optogenetics. Optogenetics is a biological technique which involves the use of light to control cells in living tissue, typically neurons, that have been genetically modified to express special light-sensitive proteins. In 2010, optogenetics was chosen as the "Method of the Year" across all fields of science and engineering by the interdisciplinary research journal Nature Methods.

Despite the extraordinary importance of this protein, a high-resolution structure and structural mechanisms of a native ChR2 (and other ChRs) have not yet been known. A step forward was the structure of a chimera between ChannelRhodopsin1 and ChannelRhodopsin 2 (C1C2). However, recent electrophysiological and Fourier transform infrared data showed that C1C2 exhibits light-induced responses that are functionally and mechanistically different from ChR2. Given that ChR2 is the most frequently used tool in optogenetics, a high-resolution structure of ChR2 is of high importance. Deciphering the structure of the native channel would shed light on how the light-induced changes at the retinal Schiff base (RSB) are linked to the channel operation.

In this work scientist from the MEMBRANE group and their collaborators expressed ChR2 in LEXSY expression system and used in meso crystallization approach to determine the crystal structure of the wild-type ChR2 and C128T slow mutant at 2.4 and 2.7 Å, respectively. The determined structures of ChR2 and its C128T mutant present the molecular basis for the understanding of ChR functioning. They provide insights into mechanisms of channel opening and closing.

Thereby, this work shed light to the molecular mechanisms of Channelrhodopsin 2 work and opens the possibilities to make engineering of enhanced optogenetic tools more efficient.

Structural insights into ion conduction by channelrhodopsin 2. Volkov O, Kovalev K., Polovinkin V, Borshchevskiy V, Bamann C, Astashkin R, Marin E, Popov A, Balandin T, Willbold D, Büldt G, Bamberg E, Gordeliy V. Science: Vol. 358, Issue 6366, pp. 1000-1001.