Characterization of a Cyanobacterial Chloride-Pumping Rhodopsin and Its Conversion into a Proton Pump [Bioenergetics]

November 17th, 2015 by Hasemi, T., Kikukawa, T., Kamo, N., Demura, M.

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.