The observation of sensor-effector coupling in the full-length structure of a protein responsive to red light could aid in the development of red-light-regulated optogenetic tools for targeted cell stimulation. Light-regulated enzymes have potential for optogenetic applications. However, current knowledge of the molecular mechanisms underlying the modularity of sensor-effector couples has inhibited the design of efficient, novel sensor-effector combinations. A schematic representation of the illumination of the sensor domain of a photoreceptor and the molecular propagation of the light signal to the effector (in red on the right-hand edge of the image). Courtesy of TU Graz/IBC. A team of scientists from Graz University of Technology (TU Graz) and the Medical University of Graz have observed the molecular details of a red-light photoreceptor engaged in producing a central bacterial messenger molecule, and the structure of a full-length light receptor together with its enzymatic effector. The team determined the structural details of a red-light-regulated full-length system and described the detailed mechanisms of signal transmission over long distances at a molecular level. “By using a combination of x-ray structural analysis and hydrogen-deuterium exchange, by which the structural dynamics and conformational changes can be analyzed, we managed to better understand the functional characteristics of this helical coupling element,” said TU Graz researcher Andreas Winkler. “We were able to show that illuminating the sensor with red light resulted in a rotation-like change in the coiled coil linker region, which in turn effects the enzymatic activity of the neighboring effector.” The coupling of sensory modules with enzymatic effectors allows direct allosteric regulation of cellular signaling molecules in response to diverse stimuli. The findings of the Graz team could lead to a better understanding of the modularity of naturally occurring protein domains. The researchers believe that further investigation of the molecular principles of sensor-effector coupling and dynamically driven allostery could open a path to the design of novel red-light-regulated optogenetic tools. They point out that diverse combinations of different sensor modules are found in nature, such as red-light sensors, blue-light sensors and pH sensors — sometimes with identical and sometimes with different effectors. Based on this observation, the researchers surmise that there are molecular similarities in signal transduction and, therefore, that rational and completely arbitrary combinations of sensors and effectors that do not occur in nature are conceivable. “We are currently limited to naturally occurring systems to a great extent in the use of directly regulated enzymatic functionalities,” said Winkler. “The long-term aim is to generate new light-regulated systems which can overcome the limitations of nature and which would be of great interest for different applications in optogenetics.” The research was published in Science Advances (doi: 10.1126/sciadv.1602498).