Algae belong to the most abundant lower eukaryotes on this planet. They are exceptionally rich in sensory photoreceptors which control developmental processes and orientation of the algae in their light environment.

During the recent years we have identified several sensory photoreceptors that are quite different from those that are known from prokaryotes, green plants or animals.

In channelrhodopsins the light switch is retinal that is covalentlylinked to the protein via a Schiff base. Light is absorbed by the retinal chromophore and isomerizes the retinal from all-trans into 13-cis. This isomerization initiates a cascade if conformational changes of the protein and opening of the ion channel ca. 200 ms after a light flash. Recently, in cooperation with the work group of Dr. Oliver Ernst VChR could be expressed in COS cells(cultivated green monkey cells) and purified by affinity chromatography. Usingtime resolved absorption spectroscopy (Fig.4) allowed the identification of defined photoproducts that are arranged in a cyclic reaction scheme in Fig.5. P510 is considered as the conducting state since it rises with a similar rate as the photocurrent rises after a laser flash.

In the last three years Channelrhodopsins were expressed in neuronal cells, tissues and living animals in order to trigger action potentials simply by light. The new technology based on Channelrhodopsin is named “Optogenetics”. For more information see:

Acetabularia Rhodopsin, a new proton pumping rhodopsin from green algae

Acetabularia acetabulum is a green algea typically found in subtropical see waters. Acetabularia acetabulum is a giant single-cell organism that grows as large as ~10 cm tall, having three anatomic parts: a bottom rhizoid resembling a set of roots, a long stalk with whorl of hairs, and an umbrella that fuse into a cap. Its large cell volume allows performing intercellular voltage clamp experiments that demonstrate hyperpolarization of the plasmamenbrane by light illumination, therefore having postulated the presence of a rhodopsin in a plant system. We cloned a cDNA that encodes a rhodopsin-related protein from this organism. The protein named AR (Acetabularia Rhodopsin) has 279 amino acids with high homology with bacteriorhodopsin (BR).

AR was heterogously expressed in Xenopus laevis oocytes and its function was studied by electrophysiological approach, so called Two-Electrodes-Voltage-Clamp (TEVC). We found out that AR mediates light-induced proton pump, in which the current is always outward directed. AR is the first ion-pumping rhodopsin identified in a plant system. AR was induced its maximum proton conductance by 520 nm wavelength light. The photocurrent comprises of 3 components, the initial peak current, a, followed by steady-state level, b, and slow decay current after light-off, c. All components are dependent on transmembrane potential and on the intensity of applied light . Blue-light causes a shunt of the photocycle.

Fig. 1 A, Images of Acetabularia acetabulum in different stages of growth. B, AR is supposed to have 7 transmembrane helices and bind a all-trans retinal to form a chlomophore, same as bacteriorhodopsin. It absorbs light and transport ptorons.

Fig. 2 Observations of photocurrents from AR. A, Illumination of green light mediates the current flow to outward direction in AR expressed cell (a), while red light does not (b). No photo-induced current is seen without AR (c). B, Photocurrent is independent from the cations and anion species, but dependedt on pH (proton), indicating AR as a proton pump.

Fig. 3 The photocurrent comprises of 3 components, the initial peak current, a, followed by steady-state level, b, and slow decay current after light-off, c. All of their sizes are dependent on membrane-voltage (A) and light intensity (B). Additional blue light reduces the photocurrent due to accellerated photochemical conversion back to the dark state (C).

Fig. 4 Tentative reaction scheme of AR photocycle.

Although AR is functionally characterised in oocyte in detail, the physiological role in the intact cell remains still open. It is enigma why light-driven proton pump exists in an algae. Bacteriorhodopsin, one of the most studied light-driven proton pump, creates proton gradient, which is utilized in bacteria as a main energy source for various cellular activities such as ATP production and flagellar movement. On the other hand, the plant has a photo synthesis system as an energy source. Thus, it is unlikely that AR contributes for energy producing. Further investigation is required to understand the physiological relevance of AR.

For reference see: Tsunoda SP, Ewers D, Gazzarrini S, Moroni A, Gradmann D, Hegemann P. (2006) H⁺-pumping rhodopsin from the marine alga Acetabularia. Biophys J. 91,1471-9.