Cryptochromes (from the

Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...

κρυπτός χρώμα, "hidden colour") are a class of

flavoproteins Flavoproteins are proteins that contain a nucleic acid derivative of riboflavin. Flavoproteins are involved in a wide array of biological processes, including removal of radicals contributing to oxidative stress, photosynthesis, and DNA repai ...

found in

plants Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae exclude ...

and

animal Animals are multicellular, eukaryotic organisms in the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic material, Cellular respiration#Aerobic respiration, breathe oxygen, are Motilit ...

s that are sensitive to

blue light Blue Light or Blue light may refer to: Science and technology * Portion of the visible spectrum related to the blue color ** Blue laser ** Blue LED * Cherenkov radiation, the physical phenomenon responsible for the characteristic blue glow in nucl ...

. They are involved in the circadian rhythms and the sensing of magnetic fields in a number of species. The name ''cryptochrome'' was proposed as a ''

portmanteau A portmanteau word, or portmanteau (, ) is a blend of wordschromatic'' nature of the photoreceptor, and the ''

cryptogam A cryptogam (scientific name Cryptogamae) is a plant (in the wide sense of the word) or a plant-like organism that reproduces by spores, without flowers or seeds. The name ''Cryptogamae'' () means "hidden reproduction", referring to the fact ...

ic'' organisms on which many blue-light studies were carried out. The two

gene In biology, the word gene (from , ; "... Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a b ...

s ''Cry1'' and ''Cry2'' code the two cryptochrome

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...

s CRY1 and CRY2. In

insects Insects (from Latin ') are pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body (head, thorax and abdomen), three pairs of j ...

and plants, CRY1 regulates the circadian clock in a light-dependent fashion, whereas in

mammals Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur o ...

, CRY1 and CRY2 act as light-independent inhibitors of

CLOCK A clock or a timepiece is a device used to measure and indicate time. The clock is one of the oldest human inventions, meeting the need to measure intervals of time shorter than the natural units such as the day, the lunar month and t ...

BMAL1 Aryl hydrocarbon receptor nuclear translocator-like protein 1 (ARNTL) or brain and muscle ARNT-Like 1 (BMAL1) is a protein that in humans is encoded by the gene on chromosome 11, region p15.3. It's also known as ''BMAL1'', ''MOP3'', and, less com ...

components of the circadian clock. In plants, blue-light photoreception can be used to cue developmental signals. Besides chlorophylls, cryptochromes are the only proteins known to form photoinduced radical-pairs ''

in vivo Studies that are ''in vivo'' (Latin for "within the living"; often not italicized in English) are those in which the effects of various biological entities are tested on whole, living organisms or cells, usually animals, including humans, and ...

''. These appear to enable some animals to detect magnetic fields. Cryptochromes have been the focus of several current efforts in

optogenetics Optogenetics is a biological technique to control the activity of neurons or other cell types with light. This is achieved by expression of light-sensitive ion channels, pumps or enzymes specifically in the target cells. On the level of individ ...

. Employing

transfection Transfection is the process of deliberately introducing naked or purified nucleic acids into eukaryotic cells. It may also refer to other methods and cell types, although other terms are often preferred: " transformation" is typically used to des ...

, initial studies on yeast have capitalized on the potential of Cry2

heterodimerization In biochemistry, a protein dimer is a macromolecular complex formed by two protein monomers, or single proteins, which are usually non-covalently bound. Many macromolecules, such as proteins or nucleic acids, form dimers. The word ''dimer'' has ...

to control cellular processes, including gene expression, by light.

Discovery

Although

Charles Darwin Charles Robert Darwin ( ; 12 February 1809 – 19 April 1882) was an English naturalist, geologist, and biologist, widely known for his contributions to evolutionary biology. His proposition that all species of life have descended ...

first documented plant responses to blue light in the 1880s, 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 Photolyases () are DNA repair enzymes that repair damage caused by exposure to ultraviolet light. These enzymes require visible light (from the violet/blue end of the spectrum) both for their own activation and for the actual DNA repair. The DN ...

, 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 A couple of homologous chromosomes, or homologs, are a set of one maternal and one paternal chromosome that pair up with each other inside a cell during fertilization. Homologs have the same genes in the same loci where they provide points alon ...

did not exhibit photolyase activity and were instead a new class of

photoreceptor hypothesized to be circadian

photopigment Photopigments are unstable pigments that undergo a chemical change when they absorb light. The term is generally applied to the non-protein chromophore moiety of photosensitive chromoproteins, such as the pigments involved in photosynthesis and pho ...

s. In 1996 and 1998, ''Cry'' homologs were identified in ''

Drosophila ''Drosophila'' () is a genus of flies, belonging to the family Drosophilidae, whose members are often called "small fruit flies" or (less frequently) pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many speci ...

'' 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

s, 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 In biochemistry, a Ramachandran plot (also known as a Rama plot, a Ramachandran diagram or a �,ψplot), originally developed in 1963 by G. N. Ramachandran, C. Ramakrishnan, and V. Sasisekharan, is a way to visualize energetically allowed regions ...

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 beta sheet, (β-sheet) (also β-pleated sheet) is a common motif of the regular protein secondary structure. Beta sheets consist of beta strands (β-strands) connected laterally by at least two or three backbone hydrogen bonds, forming a gen ...

. The molecule is arranged as an orthogonal bundle.

Function

Phototropism

In plants, cryptochromes mediate phototropism, or directional growth toward a light source, in response to blue light. This response is now known to have its own set of photoreceptors, the

phototropin Phototropins are photoreceptor proteins (more specifically, flavoproteins) that mediate phototropism responses in higher plants. Phototropins can be found throughout the leaves of a plant. Along with cryptochromes and phytochromes they allow plants ...

s. Unlike

phytochrome Phytochromes are a class of photoreceptor in plants, bacteria and fungi used to detect light. They are sensitive to light in the red and far-red region of the visible spectrum and can be classed as either Type I, which are activated by far-re ...

s and phototropins, cryptochromes are not

kinases In biochemistry, a kinase () is an enzyme that catalyzes the transfer of phosphate groups from high-energy, phosphate-donating molecules to specific substrates. This process is known as phosphorylation, where the high-energy ATP molecule don ...

. Their flavin chromophore is reduced by light and transported into the cell nucleus, where it affects the

turgor pressure Turgor pressure is the force within the cell that pushes the plasma membrane against the cell wall. It is also called ''hydrostatic pressure'', and is defined as the pressure in a fluid measured at a certain point within itself when at equilibriu ...

and causes subsequent stem elongation. To be specific, ''Cry2'' is responsible for blue-light-mediated

cotyledon A cotyledon (; ; ; , gen. (), ) is a significant part of the embryo within the seed of a plant, and is defined as "the embryonic leaf in seed-bearing plants, one or more of which are the first to appear from a germinating seed." The num ...

and leaf expansion. ''Cry2'' overexpression in

transgenic A transgene is a gene that has been transferred naturally, or by any of a number of genetic engineering techniques, from one organism to another. The introduction of a transgene, in a process known as transgenesis, has the potential to change the ...

plants increases blue-light-stimulated cotyledon expansion, which results in many broad leaves and no flowers rather than a few primary leaves with a flower. A double loss-of-function mutation in Arabidopsis thaliana Early Flowering 3 (elf3) and Cry2 genes delays flowering under continuous light and was shown to accelerate it during long and short days, which suggests that Arabidopsis CRY2 may play a role in accelerating flowering time during continuous light.

Photomorphogenesis

Cryptochromes receptors cause plants to respond to blue light via photomorphogenesis. They help control seed and seedling development, as well as the switch from the vegetative to the flowering stage of development. In Arabidopsis, it has been shown that cryptochromes controls plant growth during sub-optimal blue-light conditions.

Light capture

Despite much research on the topic, cryptochrome

photoreception A photoreceptor cell is a specialized type of neuroepithelial cell found in the retina that is capable of visual phototransduction. The great biological importance of photoreceptors is that they convert light (visible electromagnetic radiatio ...

and phototransduction in ''

'' and ''Arabidopsis thaliana'' is still poorly understood. Cryptochromes are known to possess two chromophores:

pterin Pterin is a heterocyclic compound composed of a pteridine ring system, with a "keto group" (a lactam) and an amino group on positions 4 and 2 respectively. It is structurally related to the parent bicyclic heterocycle called pteridine. Pterins, ...

(in the form of

5,10-methenyltetrahydrofolic acid 5,10-Methenyltetrahydrofolate (5,10-CH=THF) is a form of tetrahydrofolate that is an intermediate in metabolism. 5,10-CH=THF is a coenzyme that accepts and donates methenyl (CH=) groups. It is produced from 5,10-methylenetetrahydrofolate by eith ...

(MTHF)) and flavin (in the form of FAD). Both may absorb a

photon A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they a ...

, and in ''Arabidopsis'', pterin appears to absorb at a wavelength of 380 nm and flavin at 450 nm. Past studies have supported a model by which energy captured by pterin is transferred to flavin. Under this model of phototransduction, FAD would then be reduced to FADH, which probably mediates the phosphorylation of a certain domain in cryptochrome. This could then trigger a signal transduction chain, possibly affecting

gene regulation Regulation of gene expression, or gene regulation, includes a wide range of mechanisms that are used by cells to increase or decrease the production of specific gene products (protein or RNA). Sophisticated programs of gene expression are wi ...

in the cell nucleus. A new hypothesis proposes that in plant cryptochromes, the transduction of the light signal into a chemical signal that might be sensed by partner molecules could be triggered by a photo-induced negative charge within the protein - on the FAD cofactor or on the neighbouring aspartic acid. This negative charge would electrostatically repel the protein-bound ATP molecule and thereby also the protein C-terminal domain, which covers the ATP binding pocket prior to photon absorption. The resulting change in protein conformation could lead to phosphorylation of previously inaccessible phosphorylation sites on the C-terminus and the given phosphorylated segment could then liberate the transcription factor HY5 by competing for the same binding site at the negative regulator of photomorphogenesis

COP1 ''For the membrane coated vesicle used in transport, see here.'' Fagol Caspase recruitment domain-containing protein 16 is an enzyme that in humans is encoded by the ''CARD16'' gene In biology, the word gene (from , ; "...Wilhelm Johannse ...

. A different mechanism may function in ''Drosophila''. The true ground state of the flavin cofactor in ''Drosophila'' CRY is still debated, with some models indicating that the FAD is in an oxidized form, while others support a model in which the flavin cofactor exists in

anion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...

radical form, •. Recently, researchers have observed that oxidized FAD is readily reduced to • by light. Furthermore, mutations that blocked photoreduction had no effect on light-induced degradation of CRY, while mutations that altered the stability of • destroyed CRY photoreceptor function. These observations provide support for a ground state of •. Researchers have also recently proposed a model in which is excited to its doublet or quartet state by absorption of a photon, which then leads to a conformational change in the CRY protein. Also the ring eyes of the demosponge larva of ''Amphimedon queenslandica'' express a blue-light-sensitive cryptochrome (Aq-Cry2), which might mediate phototaxis. In contrast, the eyes of most animals use photo-sensitive

opsin Animal opsins are G-protein-coupled receptors and a group of proteins made light-sensitive via a chromophore, typically retinal. When bound to retinal, opsins become Retinylidene proteins, but are usually still called opsins regardless. Most ...

s expressed in photoreceptor cells, which communicate information about light from the environment to the nervous system. However, ''A. queenslandica'' lacks a nervous system, like other

sponge Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the diploblasts. They are multicellular organisms that have bodies full of pores and channels allowing water to circulate throug ...

s. And it does not have an

in its fully sequenced

genome In the fields of molecular biology and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding g ...

either, despite having many other

G-protein-coupled receptors G protein-coupled receptors (GPCRs), also known as seven-(pass)-transmembrane domain receptors, 7TM receptors, heptahelical receptors, serpentine receptors, and G protein-linked receptors (GPLR), form a large group of evolutionarily-related p ...

(GPCRs). Therefore, the sponge's unique eyes must have evolved a different mechanism to detect light and mediate phototaxis, possibly with cryptochromes or other proteins.

Iris function

The iris of chicken embryos senses short-wavelength light via a cryptochrome, rather than opsins.

Circadian rhythm

Studies in animals and plants suggest that cryptochromes play a pivotal role in the generation and maintenance of circadian rhythms. Similarly, cryptochromes play an important role in the entrainment of circadian rhythms in plants. In ''Drosophila'', cryptochrome (dCRY) acts as a blue-light photoreceptor that directly modulates light input into the circadian clock, while in mammals, cryptochromes (CRY1 and CRY2) act as

transcription Transcription refers to the process of converting sounds (voice, music etc.) into letters or musical notes, or producing a copy of something in another medium, including: Genetics * Transcription (biology), the copying of DNA into RNA, the fir ...

repressors within the circadian clockwork. Some insects, including the monarch butterfly, have both a mammal-like and a ''Drosophila''-like version of cryptochrome, providing evidence for an ancestral clock mechanism involving both light-sensing and transcriptional-repression roles for cryptochrome. ''Cry''

mutant In biology, and especially in genetics, a mutant is an organism or a new genetic character arising or resulting from an instance of mutation, which is generally an alteration of the DNA sequence of the genome or chromosome of an organism. It ...

s have altered circadian rhythms, showing that ''Cry'' affects the circadian pacemaker. ''Drosophila'' with mutated ''Cry'' exhibit little to no mRNA cycling. A point mutation in ''cry^b,'' which is required for flavin association in CRY protein, results in no PER or TIM protein cycling in either DD or LD. In addition, mice lacking ''Cry1'' or ''Cry2'' genes exhibit differentially altered free running periods, but are still capable of photoentrainment. However, mice that lack both ''Cry1'' and ''Cry2'' are arrhythmic in both LD and DD and always have high

Per1 The PER1 gene encodes the period circadian protein homolog 1 protein in humans. Function The PER1 protein is important to the maintenance of circadian rhythms in cells, and may also play a role in the development of cancer. This gene is a mem ...

mRNA levels. These results suggest that cryptochromes play a photoreceptive role, as well as acting as negative regulators of Per gene expression in mice.

In ''Drosophila''

In ''Drosophila'', cryptochrome functions as a blue light photoreceptor. Exposure to blue light induces a conformation similar to that of the always-active CRY mutant with a C-terminal deletion (CRYΔ). The half-life of this conformation is 15 minutes in the dark and facilitates the binding of CRY to other clock gene products, PER and TIM, in a light-dependent manner. Once bound by dCRY, dTIM is committed to degradation by the ubiquitin- proteasome system. Although light pulses do not entrain, full photoperiod LD cycles can still drive cycling in the

ventral Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...

lateral Lateral is a geometric term of location which may refer to: Healthcare *Lateral (anatomy), an anatomical direction * Lateral cricoarytenoid muscle * Lateral release (surgery), a surgical procedure on the side of a kneecap Phonetics *Lateral co ...

neurons in the ''Drosophila'' brain. These data along with other results suggest that CRY is the cell-autonomous photoreceptor for body clocks in ''Drosophila'' and may play a role in nonparametric entrainment (entrainment by short discrete light pulses). However, the lateral neurons receive light information through both the blue light CRY pathway and the rhodopsin pathway. Therefore, CRY is involved in light perception and is an input to the circadian clock, however it is not the only input for light information, as a sustained rhythm has been shown in the absence of the CRY pathway, in which it is believed that the rhodopsin pathway is providing some light input. Recently, it has also been shown that there is a CRY-mediated light response that is independent of the classical circadian CRY-TIM interaction. This mechanism is believed to require a flavin

redox Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a ...

-based mechanism that is dependent on potassium channel conductance. This CRY-mediated light response has been shown to increase

action potential An action potential occurs when the membrane potential of a specific cell location rapidly rises and falls. This depolarization then causes adjacent locations to similarly depolarize. Action potentials occur in several types of animal cells, ...

firing within seconds of a light response in

-knockout ''Drosophila''. Cryptochrome, like many genes involved in circadian rhythm, shows circadian cycling in

mRNA In molecular biology, messenger ribonucleic acid (mRNA) is a single-stranded molecule of RNA that corresponds to the genetic sequence of a gene, and is read by a ribosome in the process of synthesizing a protein. mRNA is created during the ...

and protein levels. In ''Drosophila'', ''Cry'' mRNA concentrations cycle under a light-dark cycle (LD), with high levels in light and low levels in the dark. This cycling persists in constant darkness (DD), but with decreased amplitude. The transcription of the ''Cry'' gene also cycles with a similar trend. CRY protein levels, however, cycle in a different manner than ''Cry'' transcription and mRNA levels. In LD, CRY protein has low levels in light and high levels in dark, and, in DD, CRY levels increase continuously throughout the subjective day and night. Thus, CRY expression is regulated by the clock at the transcriptional level and by light at the translational and posttranslational level. Overexpression of ''Cry'' also affects circadian light responses. In ''Drosophila'', ''Cry'' overexpression increases flies' sensitivity to low-intensity light. This light regulation of CRY protein levels suggests that CRY has a circadian role upstream of other clock genes and components.

In mammals

Cryptochrome is one of the four groups of mammalian clock genes/proteins that generate a transcription-translation negative-feedback loop (TTFL), along with Period (PER), CLOCK, and BMAL1. In this loop, CLOCK and BMAL1 proteins are

transcriptional activators A transcriptional activator is a protein (transcription factor) that increases transcription of a gene or set of genes. Activators are considered to have ''positive'' control over gene expression, as they function to promote gene transcription and ...

, which together bind to the promoters of the ''Cry'' and ''Per'' genes and activate their transcription. The CRY and PER proteins then bind to each other, enter the nucleus, and inhibit CLOCK-BMAL1-activated transcription. In mice, ''Cry1'' expression displays circadian rhythms in the suprachiasmatic nucleus, a brain region involved in the generation of circadian rhythms, with mRNA levels peaking during the light phase and reaching a minimum in the dark. These daily oscillations in expression are maintained in constant darkness. While CRY has been well established as a TIM homolog in mammals, the role of CRY as a photoreceptor in mammals has been controversial. Early papers indicated that CRY has both light-independent and -dependent functions. A study in 2000 indicated that mice without rhodopsin but with cryptochrome still respond to light; however, in mice without either rhodopsin or cryptochrome, '' c-Fos'' transcription, a mediator of light sensitivity, significantly drops. In recent years, data have supported melanopsin as the main circadian photoreceptor, in particular melanopsin cells that mediate entrainment and communication between the eye and the suprachiasmatic nucleus (SCN). One of the main difficulties in confirming or denying CRY as a mammalian photoreceptor is that when the gene is knocked out the animal goes arrhythmic, so it is hard to measure its capacity as purely a photoreceptor. However, some recent studies indicate that human CRY may mediate light response in peripheral tissues. Normal mammalian circadian rhythm relies critically on delayed expression of ''Cry1'' following activation of the ''Cry1'' promoter. Whereas rhythms in ''Per2'' promoter activation and ''Per2'' mRNA levels have almost the same phase, ''Cry1'' mRNA production is delayed by approximately four hours relative to ''Cry1'' promoter activation. This delay is independent of CRY1 or CRY2 levels and is mediated by a combination of E/E'-box and D-box elements in the promoter and RevErbA/

ROR Ror is a caste found primarily in Haryana and Uttar Pradesh. In the parts of Baiswara in Uttar Pradesh that are inhabited by Ror people, Rors along with groups such as the Chauhans and Tomars are associated with Delhi and its outskirts. Occu ...

binding elements (RREs) in the gene's first intron.

Transfection Transfection is the process of deliberately introducing naked or purified nucleic acids into eukaryotic cells. It may also refer to other methods and cell types, although other terms are often preferred: " transformation" is typically used to des ...

of arrhythmic ''Cry1''^−/− ''Cry2''^−/− double-knockout cells with only the ''Cry1'' promoter (causing constitutive ''Cry1'' expression) is not sufficient to rescue rhythmicity. Transfection of these cells with both the promoter and the first intron is required for restoration of circadian rhythms in these cells.

Magnetoreception

Magnetoreception is a sense which allows an organism to detect a magnetic field to perceive direction, altitude or location. Experimental data suggests that cryptochromes in the photoreceptor neurons of birds' eyes are involved in magnetic orientation during

migration Migration, migratory, or migrate may refer to: Human migration * Human migration, physical movement by humans from one region to another ** International migration, when peoples cross state boundaries and stay in the host state for some minimum le ...

. Cryptochromes are also thought to be essential for the light-dependent ability of ''

'' to sense

magnetic fields A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...

. Magnetic fields were once reported to affect cryptochromes also in ''Arabidopsis thaliana'' plants: growth behavior seemed to be affected by magnetic fields in the presence of blue (but not red) light. Nevertheless, these results have later turned out to be irreproducible under strictly controlled conditions in another laboratory, suggesting that plant cryptochromes do not respond to magnetic fields. Quantum Magnetoreception in Birds

Cryptochrome forms a pair of radicals with correlated spins when exposed to blue light. Radical pairs can also be generated by the light-independent dark reoxidation of the flavin cofactor by molecular oxygen through the formation of a spin-correlated FADH-superoxide radical pairs. Magnetoreception is hypothesized to function through the surrounding magnetic field's effect on the correlation (parallel or anti-parallel) of these radicals, which affects the lifetime of the activated form of cryptochrome. Activation of cryptochrome may affect the light-sensitivity of

retina The retina (from la, rete "net") is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs. The optics of the eye create a focused two-dimensional image of the visual world on the retina, which then ...

l neurons, with the overall result that the animal can sense the magnetic field. Animal cryptochromes and closely related animal (6-4) photolyases contain a longer chain of electron-transferring tryptophans than other proteins of the cryptochrome-photolyase superfamily (a tryptophan tetrad instead of a triad). The longer chain leads to a better separation and over 1000× longer lifetimes of the photoinduced flavin-tryptophan radical pairs than in proteins with a triad of tryptophans. The absence of spin-selective recombination of these radical pairs on the nanosecond to microsecond timescales seems to be incompatible with the suggestion that magnetoreception by cryptochromes is based on the forward light reaction.

References

External links

*
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 The Protein Data Bank (PDB) is a database for the three-dimensional structural data of large biological molecules, such as proteins and nucleic acids. The data, typically obtained by X-ray crystallography, NMR spectroscopy, or, increasingly, ...

; 3-D structure of ''Arabidopsis'' cryptochrome 3, obtained by X-ray crystallography.
Animated model of murine circadian pathway, including role of Cry
* {{Flavoproteins Physiology Biological pigments Sensory receptors Plant cognition