title: H+ -pumping rhodopsin from the marine alga Acetabularia.
abstract: An opsin-encoding cDNA was cloned from the marine alga Acetabularia acetabulum. The cDNA was expressed in Xenopus oocytes into functional Acetabularia rhodopsin (AR) mediating H+ carried outward photocurrents of up to 1.2 microA with an action spectrum maximum at 518 nm (AR518). AR is the first ion-pumping rhodopsin found in a plant organism. Steady-state photocurrents of AR are always positive and rise sigmoidally from negative to positive transmembrane voltages. Numerous kinetic details (amplitudes and time constants), including voltage-dependent recovery of the dark state after light-off, are documented with respect to their sensitivities to light, internal and external pH, and the transmembrane voltage. The results are analyzed by enzyme kinetic formalisms using a simplified version of the known photocycle of bacteriorhodopsin (BR). Blue-light causes a shunt of the photocycle under H+ reuptake from the extracellular side. Similarities and differences of AR with BR are pointed out. This detailed electrophysiological characterization highlights voltage dependencies in catalytic membrane processes of this eukaryotic, H+ -pumping rhodopsin and of microbial-type rhodopsins in general.
authors.affiliation: Experimentelle Biophysik, Fachbereich für Biologie, Humboldt-Universität zu Berlin, 10115 Berlin, Germany.
authors.forename: Satoshi P
authors.initials: SP
authors.lastname: Tsunoda
authors.forename: David
authors.initials: D
authors.lastname: Ewers
authors.forename: Sabrina
authors.initials: S
authors.lastname: Gazzarrini
authors.forename: Anna
authors.initials: A
authors.lastname: Moroni
authors.forename: Dietrich
authors.initials: D
authors.lastname: Gradmann
authors.forename: Peter
authors.initials: P
authors.lastname: Hegemann
mesh_terms: D000076:Acetabularia / Q000502:physiology* / Q000528:radiation effects; D004307:Dose-Response Relationship, Radiation; D008027:Light; D008386:Marine Biology; D008564:Membrane Potentials / Q000502:physiology* / Q000528:radiation effects; D017494:Proton Pumps / Q000502:physiology* / Q000528:radiation effects; D011829:Radiation Dosage; D012243:Rhodopsin / Q000502:physiology* / Q000528:radiation effects; D012623:Seawater / Q000382:microbiology
chemical_list: D017494:Proton Pumps; D012243:Rhodopsin