Biophysical characterization of microbial rhodopsins with DSE motif

Microbial rhodopsins are photoreceptive transmembrane proteins that transport ions or regulate other intracellular biological processes. Recent genomic and metagenomic analyses found many microbial rhodopsins with unique sequences distinct from known ones. Functional characterization of these new types of microbial rhodopsins is expected to expand our understanding of their physiological roles. Here, we found microbial rhodopsins having a DSE motif in the third transmembrane helix from members of the Actinobacteria. Although the expressed proteins exhibited blue-green light absorption, either no or extremely small outward H⁺ pump activity was observed. The turnover rate of the photocycle reaction of the purified proteins was extremely slow compared to typical H⁺ pumps, suggesting these rhodopsins would work as photosensors or H⁺ pumps whose activities are enhanced by an unknown regulatory system in the hosts. The discovery of this rhodopsin group with the unique motif and functionality expands our understanding of the biological role of microbial rhodopsins.