Functional expression of the eukaryotic proton pump rhodopsin OmR2 in Escherichia coli and its photochemical characterization

Microbial rhodopsins are photoswitchable seven-transmembrane proteins that are widely distributed in three domains of life, archaea, bacteria and eukarya. Rhodopsins allow the transport of protons outwardly across the membrane and are indispensable for light-energy conversion in microorganisms. Archaeal and bacterial proton pump rhodopsins have been characterized using an Escherichia coli expression system because that enables the rapid production of large amounts of recombinant proteins, whereas no success has been reported for eukaryotic rhodopsins. Here, we report a phylogenetically distinct eukaryotic rhodopsin from the dinoflagellate Oxyrrhis marina (O. marina rhodopsin-2, OmR2) that can be expressed in E. coli cells. E. coli cells harboring the OmR2 gene showed an outward proton-pumping activity, indicating its functional expression. Spectroscopic characterization of the purified OmR2 protein revealed several features as follows: (1) an absorption maximum at 533 nm with all-trans retinal chromophore, (2) the possession of the deprotonated counterion (pK_a = 3.0) of the protonated Schiff base and (3) a rapid photocycle through several distinct photointermediates. Those features are similar to those of known eukaryotic proton pump rhodopsins. Our successful characterization of OmR2 expressed in E. coli cells could build a basis for understanding and utilizing eukaryotic rhodopsins.