Biophysics and Physicobiology
Online ISSN : 2189-4779
ISSN-L : 2189-4779
Commentary and Perspective
Introduction of Session 7, “Functional diversity and evolution in microbial rhodopsins”
Takeshi Murata
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2023 Volume 20 Issue Supplemental Article ID: e201012

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Microbial rhodopsin is a member of a seven-transmembrane photoreactive protein family that uses retinal pigment (vitamin-A aldehyde) as a chromophore that is covalently linked to a conserved Lys residue. The microbial rhodopsin can absorb a wide range of wavelengths of visible light, which induces trans-cis photoisomerization of the retinal chromophore, inducing a series of structural changes in the protein moiety during photoreaction. Therefore, various biological functions such as ion transport and signal transduction can be achieved. Ion-transporting microbial rhodopsins are classified as light-driven ion pumps and light-dependent ion channels, and they transport various types of ions in different directions. To date, hundreds of ion-transporting microbial rhodopsins have been characterized and developed as central optogenetics tools and used in neuroscience research. In the session 7, five researchers presented cutting-edge results regarding ‘functional diversity and evolution in microbial rhodopsins’, as described below.

The first speaker was Prof. Oded Béjà from Technion-Israel Institute of Technology, who talked about ‘the search for new microbial rhodopsins using metagenomics’. Prof. Béjà and his group discovered many new microbial rhodopsins including heliorhodopsins [1] using functional metagenomics. In his talk, he presented the functional diversity in microbial rhodopsins and light energy transfer from the widespread hydroxylated carotenoids zeaxanthin and lutein to the retinal moiety of xanthorhodopsins and proteorhodopsins, which was detected by applying functional metagenomics combined with chromophore extraction from the environment. He also discussed the potential of metagenomics for future discoveries of new rhodopsin activities.

The next speaker was Prof. Yuki Sudo from Okayama University, who talked about ‘diversity and possibility of microbial rhodopsins’. In his talk, he introduced their recent progress in microbial rhodopsin research for optogenetics from the viewpoints of (i) functional diversity, (ii) spectroscopic and structural analyses and (iii) development of tools for optogenetics. In particular, he talked about new techniques for optical control of apoptosis, a type of cell death, using light-driven proton pump rhodopsins and development of photo-induced disruptive liposomes for intracellular substance delivery using them [2].

The third speaker was Prof. Leonid S. Brown from University of Guelph, who talked about ‘convergent evolution of microbial sensory rhodopsins’. Microbial rhodopsins display several cases of convergent evolution, where the same function is produced by unrelated or very distant protein groups. He is one of the specialists of such kind of microbial rhodopsin research [3]. In his talk, he presented a new type of sensory rhodopsins that are relative to proteobacterial proton pumps, proteorhodopsins, but also appear to interact with Htr-like transducers. The new sensory rhodopsins have unique structural motifs and many conserved residues, including those around the retinal chromophore. He also proposed the structures and interactions of the receptor and the transducer.

The fourth speaker was Prof. Keiichi Inoue from The University of Tokyo, who talked about ‘what determines the rate of reaction process of ion-transporting rhodopsins?’. In the design of optogenetics tools, it is necessary to control the rate of ion transfer and channel opening and closing. However, the key factor that determines the rate of each elementary process in their photocycle remains unclear. Prof. Inoue and his group performed kinetic studies of inward proton-pumping rhodopsins and channelrhodopsins using time-resolved spectroscopies and electrophysiology experiments with a nanosecond pulsed laser. By exchanging H2O for D2O in solvent and measuring kinetic isotope effects (KIE), they determined the processes that are rate-limited by proton transfer [4]. Interestingly, the KIE of the deprotonation and reprotonation process of the retinal Schiff base in inward proton-pumping rhodopsins are quite different from those of canonical outward proton-pumping rhodopsin, bacteriorhodopsin. He also presented the elementary processes and KIE of several channelrhodopsins.

The last speaker was Dr. Johannes Vierock from Humboldt University, who talked about ‘Molecular determinants of potassium selectivity in Channelrhodopsins’. The electric excitability of muscle, heart and brain tissue relies on the precise interplay of Na+- and K+-selective ion channels. The involved ion fluxes are controlled in optogenetic studies using light-gated channelrhodopsins (ChRs). While non-selective cation-conducting ChRs are well-established for excitation, K+-selective ChRs (KCRs) for efficient inhibition have only recently come into reach. In his talk, he presented the molecular analysis of recently discovered KCRs, WiChR from Wobblia lunata that features an unmatched 80-fold preference for K+ over Na+, stable photocurrents under continuous illumination and a prolonged open state lifetime [5]. Well expressed in neurons, WiChR allows two-photon inhibition at low irradiance and reduced tissue heating, recommending WiChR as the long-awaited efficient and versatile optogenetic inhibitor.

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