Cryptochrome

Cryptochrome 3 from Arabidopsis thaliana, showing the ligands FAD (lower front) and MTHF (upper back)

Cryptochromes (from the Greek κρυπτό χρώμα, hidden colour) are a class of blue light photoreceptors of plants and animals. They form a family of flavoproteins that regulate germination, elongation, photoperiodism, and other responses in higher plants. Cryptochromes are involved in the circadian rhythm of plants and animals, and in the sensing of magnetic fields in a number of species.

Blue light also mediates phototropism, but this response is now known to have its own set of photoreceptors, the phototropins. Cryptochromes are not kinases, unlike phytochromes and phototropins. Their flavine chromofore is reduced by light and transported into the nucleus, where it affects the turgor pressure, which causes subsequent stem elongation in the plant.

Evolutionary history

Cryptochromes are evolutionarily very old and highly conserved molecules. They are derived from and closely related to photolyase, a bacterial enzyme that is activated by light and participates in DNA damage repair. In eukaryotes the cryptochromes have lost their original enzymatic activity.

The genes coding for two cryptochromes, CRY1 and CRY2, are found in many species - including in humans on chromosomes 12 and 11.

Light capture mechanism

Cryptochromes possess two chromophores: pterin (in the form of 5,10-methenyl-6,7,8-tri-hydrofolic acid (MHF)) and flavin (in the form of flavin adenine dinucleotide (FAD)). Both may absorb a photon; in the plant Arabidopsis thaliana the pterin appears to absorb at a wave length of 380nm and flavin at 450nm. Energy captured by pterin is transferred to flavin.^[1] FAD is then reduced to FADH, which probably mediates the phosphorylation of a certain domain in cryptochrome. This may trigger a signal transduction chain possibly affecting gene regulation in the cell nucleus.

Function in circadian rhythm

Studies in animals and plants suggest that cryptochromes play a pivotal role in the generation and maintenance of circadian rhythms.^[2] In corals they are part of the mechanism that triggers coordinated spawning for a few nights after a full moon in the spring.^[3]

In mammals, cryptochromes (mCRY1 and mCRY2) act as transcriptional repressors within the circadian clockwork.^[4] In Drosophila, cryptochrome (dCRY) acts as a blue-light photoreceptor, directly modulating light input into the circadian clock.^[5] Some insects, including the monarch butterfly, have both a mammal-like and a Drosophila-like version of the protein, providing evidence for an ancestral clock mechanism involving both light sensing and transcriptional repression roles for cryptochrome.^[6][7]

Function in magnetoception

Cryptochromes in the photoreceptor neurons of the eyes of birds are involved in magnetic orientation during migration.^[8] Cryptochromes are also essential for the light-dependent ability of the fruit fly Drosophila melanogaster to sense magnetic fields.^[9] Furthermore, magnetic fields affect the cryptochromes in the plant Arabidopsis thaliana: growth behavior is affected by magnetic fields in the presence of blue (but not red) light.^[10]

According to one model^[11], cryptochrome when exposed to blue light forms a pair of two radicals (molecules with a single unpaired electron) where the spins of the two unpaired electrons are correlated. The occurence of such light-generated radical pairs and the correlation of the radical pair state have been confirmed recently in a cryptochrome of Xenopus laevis^[12]. The surrounding magnetic field affects the kind of this correlation (parallel or anti-parallel), and this in turn affects the length of time cryptochrome stays in its activated state. Activation of cryptochrome may affect the light-sensitivity of retinal neurons, with the overall result that the animal can "see" the magnetic field.^[13]

References

External links

• MeSH cryptochrome
• Cryptochrome circadian clock in Monarch Butterflies, by Steven M. Reppert, Department of Neurobiology, University of Massachusetts
• Cryptochrome and Magnetic Sensing, Theoretical and Computational Biophysics Group at the University of Illinois at Urbana-Champaign
• 2IJG at the Protein Data Bank; 3-D structure of Arabidopsis cryptochrome 3, obtained by X-ray crystallography.