Microbial rhodopsins – from structure to function and ecological adaptation

Rhodopsin proteins are found in all three domains of life, and the two best-known types to date are animal and microbial rhodopsins. Animal rhodopsins are found only in animals, while microbial rhodopsins are found in microorganisms in all domains, and mainly in protists among eukaryotes. All known rhodopsins have a similar structure, consisting of seven transmembrane α-helices and a retinal ligand. Animal and microbial types do not show sequence similarity, suggesting their convergent evolution. Animal rhodopsins are responsible for vision and control of the biological clock, while microbial rhodopsins are responsible for cell phototaxis and also act as hydrogen or ion pumps. These processes may be involved in converting energy from photons into energy used by the cell. A lot more is known about rhodopsins in marine microorganisms than in freshwater ones. The differences between rhodopsins appearing in these two ecosystems can be significant because they are characterized by different environmental conditions, which lead to different optical properties, consequently affecting the sequences and structure of rhodopsins.