Converting a Natural-Light-Driven Outward Proton Pump Rhodopsin into an Artificial Inward Proton Pump.

Microbial rhodopsins are a large family of photoreceptive membrane proteins with diverse light-regulated functions. While the most ubiquitous microbial rhodopsins are light-driven outward proton (H + ) pumps, new subfamilies of microbial rhodopsins transporting H + inwardly, i.e., light-driven inward H + pumps, have been discovered recently. Although structural and spectroscopic studies provide insights into their ion transport mechanisms, the minimum key element(s) that determine the direction of H + transport have not yet been clarified. Here, we conducted the first functional conversion study by substituting key amino acids in a natural outward H + -pumping rhodopsin ( Psp R) with those in inward H + -pumping rhodopsins. Consequently, an artificial inward H + pump was constructed by mutating only three residues of Psp R. This result indicates that these residues govern the key processes that discriminate between outward and inward H + pumps. Spectroscopic studies revealed the presence of an inward H + -accepting residue in the H + transport pathway and direct H + uptake from the extracellular solvent. This finding of the simple element for determining H + transport would provide a new basis for understanding the concept of ion transport not only by microbial rhodopsins but also by other ion-pumping proteins.