FOCUS_Optogenetics

Custom-Tailored Molecules

Chlamydomonas reinhardtii, a single-celled green alga, can’t see much at all with its eye composed solely of photosensitive molecules. Yet there is more to algal rhodopsin than one would expect. In recent years, it has triggered a revolution in neurobiology. from the Max Planck Institute of Biophysics in Frankfurt helped make it famous. He is now researching these molecules and developing new variants for basic research and medical applications. n

TEXT CATARINA PIETSCHMANN

ll Chlamydomonas requires when light straightens out the , are transduced faster into an electrical in order to see is an accu- which takes on a bent shape in the dark. signal in an algal cell than they are in mulation of proteins, known In humans and other mammals, this the human eye. as an eyespot. Under a mi- activates the rhodopsin and, via a mul- The researchers named this partic- croscope, the eyespot ap- tistage process, blocks positive ions from ular protein “.” A pears as a yellow dot in an otherwise flooding into the cell. They soon realized that this protein green algal cell. It allows Chlamydomo- harbors great potential for science. In nas to see what it needs to see – light, ALL-IN-ONE PHOTORECEPTOR a comprehensive patent specification dark, and a few shades in between – so AND published following their discovery, that the cell can swim closer to or fur- they even included a detailed list of ther away from the water surface, de- In 2002, Bamberg and , to- possible applications in the fields of pending on the light conditions. gether with from neurobiology and biomedicine. “In The eyespot is composed of around Humboldt University in Berlin, discov- retrospect, that was almost a rather 200 different proteins, including pho- ered the mechanism of algal rhodop- presumptuous thing to do, but almost tosensitive rhodopsin molecules. Simi- sins. The researchers transferred the all of it has since come true. Today, lar can also be found in the rhodopsin gene to egg cells of a clawed there are hardly any applications for human eye, or more specifically, in the frog and observed that the proteins that aren’t includ- retinal photoreceptors, where they combine the photoreceptor and the ed in our patent,” says Ernst Bamberg. transduce the incident light into an ion channel into one single protein. To name just one example: a license electrical signal that is then transmit- The rhodopsin in algae thus has a dif- extract for treating eye diseases has al- ted to the brain for further processing. ferent function than the rhodopsins in ready been granted to a large pharma- Rhodopsins are made up of two mammals: the itself forms an ceutical company. components: the protein opsin and the ion channel, which can be opened by It all sounds very simple, and with carotenoid retinal, a photosensitive mol- light so that the ions can then pass the methods of modern molecular bi-

ecule. In the eye, the act of seeing starts through. As a result, the light stimuli ology, it is just that: When the gene for Graphic: Angewandte Chemie 2013-125/37/Thomas Sattig, Christian Rickert, Ernst Bamberg, Heinz Jürgen Steinhoff, Baman

26 MaxPlanckResearch 4 | 14 A channelrhodopsin-2 molecule before and after exposure to light: The protein’s amino acid chain is rolled up into a spiral measuring seven times the diameter of the cell membrane. When exposed to incident light, helix 2 (turquoise) twists out (green), opening the ion channel for ca