H+ -pumping rhodopsin from the marine alga Acetabularia.

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.