Characterization of a Cyanobacterial Chloride-pumping Rhodopsin and Its Conversion into a Proton Pump

Light-driven ion-pumping rhodopsins are widely distributed in microorganisms and are now classified into the categories of outward H⁺ and Na⁺ pumps and an inward Cl⁻ pump. These different types share a common protein architecture and utilize the photoisomerization of the same chromophore, retinal, to evoke photoreactions. Despite these similarities, successful pump-to-pump conversion had been confined to only the H⁺ pump bacteriorhodopsin, which was converted to a Cl⁻ pump in 1995 by a single amino acid replacement. In this study we report the first success of the reverse conversion from a Cl⁻ pump to a H⁺ pump. A novel microbial rhodopsin (MrHR) from the cyanobacterium Mastigocladopsis repens functions as a Cl⁻ pump and belongs to a cluster that is far distant from the known Cl⁻ pumps. With a single amino acid replacement, MrHR is converted to a H⁺ pump in which dissociable residues function almost completely in the H⁺ relay reactions. MrHR most likely evolved from a H⁺ pump, but it has not yet been highly optimized into a mature Cl⁻ pump.