title: Proton circulation during the photocycle of sensory rhodopsin II.
abstract: Sensory rhodopsin II (SRII) in Halobacterium salinarum membranes is a phototaxis receptor that signals through its bound transducer HtrII for avoidance of blue-green light. In the present study we investigated the proton movements during the photocycle of SRII in the HtrII-free and HtrII-complexed form. We monitored sustained light-induced pH changes with a pH electrode, and laser flash-induced pH changes with the pH indicator pyranine using sealed membrane vesicles and open sheets containing the free or the complexed receptor. The results demonstrated that SRII takes up a proton in M-to-O conversion and releases it during O-decay. The uptake and release are from and to the extracellular side, and therefore SRII does not transport the proton across the membrane. The pH dependence of the SRII photocycle indicated the presence of a protonatable group (pK(a) approximately 7.5) in the extracellular proton-conducting path, which plays a role in proton uptake by the Schiff base in the M-to-O conversion. The extracellular proton circulation produced by SRII was not blocked by HtrII complexation, unlike the cytoplasmic proton conduction in SRI that was found in the same series of measurements to be blocked by its transducer, HtrI. The implications of this finding for current models of SRI and SRII signaling are discussed.
authors.affiliation: Department of Microbiology & Molecular Genetics, University of Texas Medical School, Houston, Texas 77030 USA.
authors.forename: J
authors.initials: J
authors.lastname: Sasaki
authors.forename: J L
authors.initials: JL
authors.lastname: Spudich
mesh_terms: D000042:Absorption; D019843:Archaeal Proteins; D001190:Arylsulfonates / Q000378:metabolism; D001426:Bacterial Proteins / Q000378:metabolism; D001436:Bacteriorhodopsins / Q000378:metabolism*; D002338:Carotenoids; D002462:Cell Membrane / Q000378:metabolism; D005456:Fluorescent Dyes / Q000378:metabolism; D019617:Halobacterium salinarum / Q000166:cytology / Q000378:metabolism*; D025602:Halorhodopsins; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D007834:Lasers; D008027:Light; D055537:Light Signal Transduction; D008565:Membrane Proteins / Q000378:metabolism; D008954:Models, Biological; D010775:Photic Stimulation; D010782:Photolysis; D011522:Protons; D012545:Schiff Bases; D025603:Sensory Rhodopsins
chemical_list: D019843:Archaeal Proteins; D001190:Arylsulfonates; D001426:Bacterial Proteins; D005456:Fluorescent Dyes; D025602:Halorhodopsins; C100812:HtrII protein, Halobacterium salinarium; D008565:Membrane Proteins; D011522:Protons; C061497:SRI protein, Halobacterium; D012545:Schiff Bases; D025603:Sensory Rhodopsins; C082504:htrI protein, Halobacterium salinarium; C061508:sensory rhodopsin II protein, archaeal; D002338:Carotenoids; D001436:Bacteriorhodopsins; C005047:pyranine