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 µA 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 trans-membrane 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 trans-membrane 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 eucaryotic, H+ pumping rhodopsin and of microbial-type rhodopsins in general.