title: Cyanobacterial light-driven proton pump, gloeobacter rhodopsin: complementarity between rhodopsin-based energy production and photosynthesis.
abstract: A homologue of type I rhodopsin was found in the unicellular Gloeobacter violaceus PCC7421, which is believed to be primitive because of the lack of thylakoids and peculiar morphology of phycobilisomes. The Gloeobacter rhodopsin (GR) gene encodes a polypeptide of 298 amino acids. This gene is localized alone in the genome unlike cyanobacterium Anabaena opsin, which is clustered together with 14 kDa transducer gene. Amino acid sequence comparison of GR with other type I rhodopsin shows several conserved residues important for retinal binding and H+ pumping. In this study, the gene was expressed in Escherichia coli and bound all-trans retinal to form a pigment (λmax  = 544 nm at pH 7). The pKa of proton acceptor (Asp121) for the Schiff base, is approximately 5.9, so GR can translocate H+ under physiological conditions (pH 7.4). In order to prove the functional activity in the cell, pumping activity was measured in the sphaeroplast membranes of E. coli and one of Gloeobacter whole cell. The efficient proton pumping and rapid photocycle of GR strongly suggests that Gloeobacter rhodopsin functions as a proton pumping in its natural environment, probably compensating the shortage of energy generated by chlorophyll-based photosynthesis without thylakoids.
authors.affiliation: Department of Life Science and Institute of Biological Interfaces, Sogang University, Seoul, Korea.
authors.forename: Ah Reum
authors.initials: AR
authors.lastname: Choi
authors.affiliation: Department of Physics, University of Guelph, Ontario, Canada.
authors.forename: Lichi
authors.initials: L
authors.lastname: Shi
authors.affiliation: Department of Physics, University of Guelph, Ontario, Canada.
authors.forename: Leonid S
authors.initials: LS
authors.lastname: Brown
authors.affiliation: Department of Life Science and Institute of Biological Interfaces, Sogang University, Seoul, Korea.
authors.forename: Kwang-Hwan
authors.initials: KH
authors.lastname: Jung
mesh_terms: D000595:Amino Acid Sequence; D001692:Biological Transport; D000458:Cyanobacteria / Q000145:classification / Q000235:genetics* / Q000378:metabolism*; D004734:Energy Metabolism; D015870:Gene Expression; D008027:Light; D008969:Molecular Sequence Data; D009154:Mutation; D010788:Photosynthesis; D010802:Phylogeny; D017494:Proton Pumps / Q000235:genetics* / Q000378:metabolism; D011522:Protons; D012243:Rhodopsin / Q000235:genetics* / Q000378:metabolism
chemical_list: D017494:Proton Pumps; D011522:Protons; D012243:Rhodopsin