Abstract—

The blue-light protein sensors, cryptochromes, compose the widespread class of photoreceptors that regulate processes of development in plants and circadian rhythms in animals and plants. These photoreceptors can also function as magnetoreceptors. During the past decade, cryptochromes were discovered and characterized in several photosynthesizing algae, where they may act not only as regulatory photoreceptors but also as photolyases, catalyzing the repair of ultraviolet-induced DNA lesions. Cryptochrome proteins bind flavin adenine dinucleotide (FAD) as the chromophore in the photolyase homology region (PHR) domain and contain the cryptochrome C-terminal extension (CCE), which is attached to PHR near the FAD-binding site. The chromophore is responsible for photosensory properties of cryptochromes, while CCE is essential for their signaling activities. Photosensory processes are initiated by photochemical FAD conversions involving electron/proton transfer and the formation of redox forms. These reactions lead to changes in chromophore–protein interactions. The resulting conformational transitions in protein structure provide the molecular foundation of cryptochrome signaling activity in living systems. In plants, cryptochrome protein with photoreduced FAD undergoes conformational changes, causing disengagement of the PHR domain and CCE, which is accompanied by the formation of functionally active dimers/tetramers of cryptochrome molecules. Photooligomerization is considered a key process necessary for cryptochrome signaling activity, inasmuch as oligomers ensure the formation of their complexes with a variety of proteins that are the components of photoreceptor signaling pathways. Cryptochrome–protein interactions in such complexes change the protein-signaling activities, leading to gene expression alteration and photomorphogenesis. The review discusses current knowledge on photosensory and signaling properties of cryptochromes.