The Institute for Solid State Physics, the University of Tokyo
Microbial rhodopsin is photoreceptive membrane protein family in unicellular microbes. In the 20th, microbial rhodopsins are considered to present in finite species such as hyperhalophilic archaea. However, recent genomic and environmental metagenomic analyses identified more than thousands of microbial rhodopsins from diverse microorganisms to use sun-light energy for various biological events.
We studied the molecular functional mechanism of various types of microbial rhodopsins with time-resolved laser spectroscopy . We also found many rhodopsin genes have highly different amino-acid sequences compared with reported ones. As the results, light-driven outward Na+
pump [2-4] and inward H+ pump  rhodopsins were newly identified, and their transport mechanism was also revealed. In 2018, a new distinct class of rhodopsin was discovered by functional metagenomics, and it was named heliorhodopsin (HeR) meaning “the rhodopsin of the sun” in Greek [6,7]. HeR has an orientation in membrane in which N- and C-termini face cytoplasmic and extracellular side. This is opposed to all classical rhodopsins. The variety of rhodopsin proteins is expected to increase, and new many insights about the essence of protein functionality will be obtained by studying them.
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 A. Pushkarev†, K. Inoue† et al., Nature 558, 595-599 (2018) †: equally contributed
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