Light-Driven Domain Mechanics of a Minimal Phytochrome Photosensory Module Studied by EPR

Light-exposed organisms developed photoreceptors to transduce light signals for environmental adaptation. Phytochromes, found in bacteria, fungi, and plants, can discriminate the ratio of red and far-red light using the isomerization of a bilin chromophore bound to a photosensory module to trigger downstream conformational changes in the protein. Here, we investigated by hydrogen/deuterium exchange mass spectrometry and electron paramagnetic resonance spectroscopy the light-driven domain mechanics of a minimal monomeric photosensory module from the group II phytochrome Cph2 from Synechocystis sp. PCC 6803. We could unambiguously trace the light-driven secondary structural rearrangement of its tongue region, and we found a translational motion of the PHY domain that is related to what was found before by X-ray studies in a group I module. Our analysis demonstrates a common light response in the photosensory modules of phytochromes, orchestrated solely by the GAF-PHY bidomain independent of further quaternary interactions or the nature of downstream effector domains. [Display omitted] •H/D exchange experiments performed on a minimal photosensory module•EPR reveals light-induced changes in side chain dynamics and interspin distances•Coarse-grained model of the domain mechanics•Common light response independent of quaternary interaction and downstream modules Assafa et al. describe the conformational changes of a minimal photosensory module of a phytochrome, a protein sensing red and far-red light in plants, bacteria, and fungi. They use spin labels and measure how interspin distances and side chain dynamics change upon light excitation, unraveling its structural response.