Many organisms have “ion channels” (gateways that selectively allow charged particles called ions to enter cells and are an integral part of cell function) called “channelrhodopsins”, which can be turned on and off using the light. Different channelrhodopsins respond to different wavelengths in the light spectrum. These channels can be expressed in foreign organisms (even human animals) by means of genetic engineering, which in turn finds applications in optogenetics, or the application of light to modulate cellular and gene functions. So far, the shortest wavelength to which a channelrhodopsin responds has been blue.
However, recently a group of scientists from the Nagoya Institute of Technology, Japan, and Jawaharlal Nehru University, India, identified a channelrhodopsin that responds to an even shorter indigo blue wavelength. In their study published in Nature’s Communications biology, the group of researchers, led by Professor Hideki Kandori and Associate Professor Satoshi P. Tsunoda, identified a new channelrhodopsin, which they named KnChR, from a species of terrestrial algae called Klebsormidium nitens. “We chose this alga because it is known to be sensitive to light, but its photoreceptor domain has not been established”, reports Professor Kandori. Unlike other discovered channelrhodopsins, KnChR has been shown to respond to indigo blue light.
KnChR is known to be composed of a seven-celled membrane region, which forms the pore that allows entry and exit of different ions. This region is followed by a protein fragment comprising a peptidoglycan binding domain. In order to study the properties of KnChR, the researchers performed numerous genetic and electrophysiological experiments.
What was perhaps the most interesting result was that they were able to identify the role of the “cytoplasmic domain”. All of the known channelrhodopsins have a large “cytoplasmic domain”, or region which is located in the internal area of the cell. As Prof. Kandori explains, “All currently known channelrhodopsins include a large cytoplasmic domain, the function of which is elusive. We have found that the cytoplasmic domain of KnChR modulates the properties of the ion channel.”
Accordingly, the results of the experiments showed that the change in the lengths of the cytoplasmic domain caused the changes in the closure of the ion channel. In particular, the shortening of the domain resulted in an increase in the “open time” of the channel by more than ten times. In addition, the researchers also identified two amino acid residues of arginine, namely R287 and R291, in the same region, which played an important role in the properties of the light currents generated. They found that KnChR exhibited maximum sensitivity at 430nm and 460nm, making it the most “ blue ” channel in rhodopsin.
Overall, researchers believe KnChR is useful in biological systems requiring specific excitation parameters. When asked about the implications of these results, Prof Tsunoda, who is the corresponding author of the study, suggests that “KnChR would extend the optogenetic toolkit, especially for dual lumen applications when excitation at short wavelength is required. ” This means that the light-operated property of KnChR can be applied to the targeted manipulation of the biological functions of an organism, in a research setting. Some examples would include the manipulation of neuronal and myocyte activities.
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