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Cryptochromes (from the Greek κρυπτός χρώμα, "hidden colour") are a class of flavoproteins that are sensitive to blue light. They are found in plants and animals. Cryptochromes are involved in the circadian rhythms of plants and animals, and in the sensing of magnetic fields in a number of species. The name cryptochrome was proposed as a portmanteau combining the cryptic nature of the photoreceptor, and the cryptogamic organisms on which many blue-light studies were carried out.
The two genes Cry1 and Cry2 code for the two cryptochrome proteins CRY1 and CRY2. In insects and plants, CRY1 regulates the circadian clock in a light-dependent fashion, whereas, in mammals, CRY1 and CRY2 act as light-independent inhibitors of CLOCK-BMAL1 components of the circadian clock. In plants, blue-light photoreception can be used to cue developmental signals.
Although Charles Darwin first documented plant responses to blue light in the 1800s, it was not until the 1980s that research began to identify the pigment responsible. In 1980, researchers discovered that the HY4 gene of the plant Arabidopsis thaliana was necessary for the plant's blue light sensitivity, and, when the gene was sequenced in 1993, it showed high sequence homology with photolyase, a DNA repair protein activated by blue light. By 1995, it became clear that the products of the HY4 gene and its two human homologs did not exhibit photolyase activity and were instead a new class of blue light photoreceptor hypothesized to be circadian photopigments. In 1996 and 1998, Cry homologs were identified in Drosophila and mice, respectively.
Evolutionary history and structure
Cryptochromes (CRY1, CRY2) are evolutionarily old and highly conserved proteins that belong to the flavoproteins superfamily that exists in all kingdoms of life. All members of this superfamily have the characteristics of an N-terminal photolyase homology (PHR) domain. The PHR domain can bind to the flavin adenine dinucleotide (FAD) cofactor and a light-harvesting chromophore. Cryptochromes are derived from and closely related to photolyases, which are bacterial enzymes that are activated by light and involved in the repair of UV-induced DNA damage. In eukaryotes, cryptochromes no longer retain this original enzymatic activity.
The structure of cryptochrome involves a fold very similar to that of photolyase, with a single molecule of FAD noncovalently bound to the protein. These proteins have variable lengths and surfaces on the C-terminal end, due to the changes in genome and appearance that result from the lack of DNA repair enzymes. The Ramachandran plot shows that the secondary structure of the CRY1 protein is primarily a right-handed alpha helix with little to no steric overlap. The structure of CRY1 is almost entirely made up of alpha helices, with several loops and few beta sheets. The molecule is arranged as an orthogonal bundle.