An Inward Proton Transport Using Anabaena Sensory Rhodopsin

ATP is synthesized by an enzyme that utilizes proton motive force and thus nature creates various proton pumps. The best understood proton pump is bacteriorhodopsin (BR), an outward-directed light-driven proton pump in Halobacterium salinarum. Many archaeal and eubacterial rhodopsins are now known to show similar proton transport activity. Proton pumps must have a specific mechanism to exclude transport in the reverse direction to maintain a proton gradient, and in the case of BR, a highly hydrophobic cytoplasmic domain may constitute such machinery. Although an inward proton pump has neither been created naturally nor artificially, we recently reported that an inward-directed proton transport can be engineered from a bacterial rhodopsin by a single amino acid replacement. Anabaena sensory rhodopsin (ASR) is a photochromic sensor in freshwater cyanobacteria, possessing little proton transport activity. When we replace Asp217 at the cytoplasmic domain (distance ~15 angstrom from the retinal chromophore) to Glu, ASR is converted into an inward proton transport, driven by absorption of a single photon. FTIR spectra clearly show an increased proton affinity for Glu217, which presumably controls the unusual directionality opposite to normal proton pumps.