Cryptochrome
Encyclopedia
Cryptochromes are a class of blue
light
-sensitive flavoproteins found in plants and animal
s. Cryptochromes are involved in the circadian rhythm
s of plants and animals, and in the sensing of magnetic fields
in a number of species. The name Cryptochrome was proposed as a pun combining the cryptic nature of the photoreceptor, and the cryptogamic organisms on which many blue light studies were carried out.
The two gene
s Cry1 and Cry2 code for the two cryptochrome protein
s 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.
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 photopigment
s. In 1996 and 1998, Cry homologs were identified in Drosophila
and mice
, respectively.
(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.
, or directional growth towards a light source, in response to blue light. This response is now known to have its own set of photoreceptors, the phototropin
s.
Unlike phytochrome
s and phototropins, cryptochromes are not kinases. Their flavin
chromophore is reduced by light and transported into the cell
nucleus
, where it affects the turgor pressure
and causes subsequent stem elongation. Specifically, Cry2 is responsible for blue-light mediated cotyledon
and leaf expansion. Cry2 overexpression in transgenic 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 was shown to accelerates it during long and short days, which suggests that Arabidopsis CRY2 may play a role in accelerating flowering time during continuous light.
and Arabidopsis thaliana is still poorly understood. Cryptochromes are known to possess two chromophores: pterin
(in the form of 5,10-methenyltetrahydrofolic acid (MHF)) and flavin (in the form of FAD). Both may absorb a photon
, 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 in the cell nucleus.
Recent research has indicated that 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 the FAD is in an oxidized form , while others support a model in which the flavin cofactor exists in anion radical
form, FAD•-. Recently, researchers have observed that oxidized FAD is readily reduced to FAD•- by light. Furthermore, mutations that blocked photoreduction had no effect on light-induced degradation of CRY, while mutations that altered the stability of FAD•- destroyed CRY photoreceptor function.These observations provide support for a ground state of FAD•-. Researchers have also recently proposed a model in which FAD•- is excited to its doublet or quartet state by absorption of a photon, which then leads to a conformational change in the CRY protein.
repressor
s 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
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 cryb, 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
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 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-lateral
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, but is not required for the operation of the clock because the rhodopsin pathway is sufficient in the absence of the CRY pathway. 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
-based mechanism that is dependent on potassium channel conductance. This CRY mediated light response has been shown to increase action potential
firing within seconds of a light response in opsin
-knockout Drosophila.
Cryptochrome, like many genes involved in circadian rhythm, shows circadian cycling in mRNA 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.
, CLOCK, and BMAL1. In this loop, CLOCK and BMAL1 proteins are transcriptional activators, 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, particularly melanopsin cells which 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
binding elements (RREs) in the gene’s first intron. Transfection
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.
. Cryptochromes are also essential for the light-dependent ability of Drosophila to sense magnetic fields. Furthermore, magnetic fields affect cryptochromes in Arabidopsis thaliana: growth behavior is affected by magnetic fields in the presence of blue ( but not red) light.
According to one model, cryptochrome forms a pair of two radical
s with correlated spin
s when exposed to blue light. The occurrence of such light-generated radical pairs and the correlation of the radical pair state have been confirmed recently in a cryptochrome of Xenopus laevis. However, recent evidence from Arabidopsis thaliana cryptochrome also suggests that radical pairs can be generated by the light-independent dark reoxidation of Flavin protein by molecular oxygen through the formation of a spin-correlated FADH-superoxide radical pairs. Magnetoception is hypothesized to function through the surrounding magnetic fields effect on the correlation (parallel or anti-parallel) of these radicals, which affects the duration that cryptochrome remains activated. Activation of cryptochrome may affect the light-sensitivity of retina
l neurons, with the overall result that the animal can "see" the magnetic field.
Blue
Blue is a colour, the perception of which is evoked by light having a spectrum dominated by energy with a wavelength of roughly 440–490 nm. It is considered one of the additive primary colours. On the HSV Colour Wheel, the complement of blue is yellow; that is, a colour corresponding to an equal...
light
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
-sensitive flavoproteins found in plants and animal
Animal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...
s. Cryptochromes are involved in the circadian rhythm
Circadian rhythm
A circadian rhythm, popularly referred to as body clock, is an endogenously driven , roughly 24-hour cycle in biochemical, physiological, or behavioural processes. Circadian rhythms have been widely observed in plants, animals, fungi and cyanobacteria...
s of plants and animals, and in the sensing of magnetic fields
Magnetoception
Magnetoception is the ability to detect a magnetic field to perceive direction, altitude or location. This sense plays a role in the navigational abilities of several animal species and has been postulated as a method for animals to develop regional maps.Magnetoception is most commonly observed in...
in a number of species. The name Cryptochrome was proposed as a pun combining the cryptic nature of the photoreceptor, and the cryptogamic organisms on which many blue light studies were carried out.
The two gene
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...
s Cry1 and Cry2 code for the two cryptochrome protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
s 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
CLOCK
Clock is a gene encoding a basic-helix-loop-helix-PAS transcription factor that affects both the persistence and period of circadian rhythms...
-BMAL1
ARNTL
Aryl hydrocarbon receptor nuclear translocator-like, also known as ARNTL, Bmal1, or Mop3, is a gene which is associated with susceptibility to hypertension and type 2 diabetes.-Function:...
components of the circadian clock. In plants, blue light photoreception can be used to cue developmental signals.
Discovery
Although Charles DarwinCharles Darwin
Charles Robert Darwin FRS was an English naturalist. He established that all species of life have descended over time from common ancestry, and proposed the scientific theory that this branching pattern of evolution resulted from a process that he called natural selection.He published his theory...
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
Arabidopsis thaliana
Arabidopsis thaliana is a small flowering plant native to Europe, Asia, and northwestern Africa. A spring annual with a relatively short life cycle, arabidopsis is popular as a model organism in plant biology and genetics...
was necessary for the plant's blue light sensitivity, and when the gene was sequenced in 1993, it showed high sequence homology with photolyase
Photolyase
Photolyases are DNA repair enzymes that repair damage caused by exposure to ultraviolet light. This enzyme mechanism requires visible light, preferentially from the violet/blue end of the spectrum, and is known as photoreactivation....
, 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
Homology (biology)
Homology forms the basis of organization for comparative biology. In 1843, Richard Owen defined homology as "the same organ in different animals under every variety of form and function". Organs as different as a bat's wing, a seal's flipper, a cat's paw and a human hand have a common underlying...
did not exhibit photolyase activity and were instead a new class of blue light photoreceptor hypothesized to be circadian photopigment
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 photoreception...
s. In 1996 and 1998, Cry homologs were identified in Drosophila
Drosophila
Drosophila is a genus of small flies, belonging to the family Drosophilidae, whose members are often called "fruit flies" or more appropriately pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many species to linger around overripe or rotting fruit...
and mice
Mouse
A mouse is a small mammal belonging to the order of rodents. The best known mouse species is the common house mouse . It is also a popular pet. In some places, certain kinds of field mice are also common. This rodent is eaten by large birds such as hawks and eagles...
, 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 homologyDNA photolyase
DNA photolyase is an evolutionary conserved protein domain. This domain binds a light harvesting cofactor.Deoxyribodipyrimidine photolyase is a DNA repair enzyme. It binds to UV-damaged DNA containing pyrimidine dimers and, upon absorbing a near-UV photon , breaks the cyclobutane ring joining the...
(PHR) domain. The PHR domain can bind to the flavin adenine dinucleotide (FAD) cofactor
Cofactor
Cofactor may refer to any of the following:* Cofactor , the signed minor of a matrix* Minor , an alternative name for the determinant of a smaller matrix than that which it describes...
and a light-harvesting chromophore
Chromophore
A chromophore is the part of a molecule responsible for its color. The color arises when a molecule absorbs certain wavelengths of visible light and transmits or reflects others. The chromophore is a region in the molecule where the energy difference between two different molecular orbitals falls...
. 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
Ramachandran plot
-Introduction and early history:A Ramachandran plot , originally developed in 1963 by G. N. Ramachandran C. Ramakrishnan and V...
shows that the secondary structure
Secondary structure
In biochemistry and structural biology, secondary structure is the general three-dimensional form of local segments of biopolymers such as proteins and nucleic acids...
of the CRY1 protein is primarily a right-handed alpha helix
Alpha helix
A common motif in the secondary structure of proteins, the alpha helix is a right-handed coiled or spiral conformation, in which every backbone N-H group donates a hydrogen bond to the backbone C=O group of the amino acid four residues earlier...
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.
Phototropism
In plants, cryptochromes mediate phototropismPhototropism
Phototropism is directional growth in which the direction of growth is determined by the direction of the light source. In other words, it is the growth and response to a light stimulus. Phototropism is most often observed in plants, but can also occur in other organisms such as fungi...
, or directional growth towards a light source, in response to blue light. This response is now known to have its own set of photoreceptors, the phototropin
Phototropin
Phototropins are photoreceptor proteins that mediate phototropism responses in higher plants. Along with cryptochromes and phytochromes they allow plants to respond and alter their growth in response to the light environment...
s.
Unlike phytochrome
Phytochrome
Phytochrome is a photoreceptor, a pigment that plants use to detect light. It is sensitive to light in the red and far-red region of the visible spectrum. Many flowering plants use it to regulate the time of flowering based on the length of day and night and to set circadian rhythms...
s and phototropins, cryptochromes are not kinases. Their flavin
Flavin
Flavin is the common name for a group of organic compounds based on pteridine, formed by the tricyclic heteronuclear organic ring isoalloxazine. The biochemical source is the vitamin riboflavin...
chromophore is reduced by light and transported into the cell
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
nucleus
Cell nucleus
In cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...
, where it affects the turgor pressure
Turgor pressure
Turgor Pressure or turgidity is the main pressure of the cell contents against the cell wall in plant cells and bacteria cells, determined by the water content of the vacuole, resulting from osmotic pressure, i.e...
and causes subsequent stem elongation. Specifically, Cry2 is responsible for blue-light mediated cotyledon
Cotyledon
A cotyledon , is a significant part of the embryo within the seed of a plant. Upon germination, the cotyledon may become the embryonic first leaves of a seedling. The number of cotyledons present is one characteristic used by botanists to classify the flowering plants...
and leaf expansion. Cry2 overexpression in transgenic 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 was shown to accelerates it during long and short days, which suggests that Arabidopsis CRY2 may play a role in accelerating flowering time during continuous light.
Light capture
Despite much research on the topic, cryptochrome photoreception and phototransduction in DrosophilaDrosophila
Drosophila is a genus of small flies, belonging to the family Drosophilidae, whose members are often called "fruit flies" or more appropriately pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many species to linger around overripe or rotting fruit...
and Arabidopsis thaliana is still poorly understood. Cryptochromes are known to possess two chromophores: pterin
Pterin
Pterin is a heterocyclic compound composed of a pteridine ring system, with a keto group and an amino group on positions 4 and 2 respectively...
(in the form of 5,10-methenyltetrahydrofolic acid (MHF)) and flavin (in the form of FAD). Both may absorb a photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
, 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
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
to FADH, which probably mediates the phosphorylation
Phosphorylation
Phosphorylation is the addition of a phosphate group to a protein or other organic molecule. Phosphorylation activates or deactivates many protein enzymes....
of a certain domain in cryptochrome. This could then trigger a signal transduction
Signal transduction
Signal transduction occurs when an extracellular signaling molecule activates a cell surface receptor. In turn, this receptor alters intracellular molecules creating a response...
chain, possibly affecting gene regulation in the cell nucleus.
Recent research has indicated that 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 the FAD is in an oxidized form , while others support a model in which the flavin cofactor exists in anion radical
Radical (chemistry)
Radicals are atoms, molecules, or ions with unpaired electrons on an open shell configuration. Free radicals may have positive, negative, or zero charge...
form, FAD•-. Recently, researchers have observed that oxidized FAD is readily reduced to FAD•- by light. Furthermore, mutations that blocked photoreduction had no effect on light-induced degradation of CRY, while mutations that altered the stability of FAD•- destroyed CRY photoreceptor function.These observations provide support for a ground state of FAD•-. Researchers have also recently proposed a model in which FAD•- is excited to its doublet or quartet state by absorption of a photon, which then leads to a conformational change in the CRY protein.
Circadian rhythm
Studies in animals and plants suggest that cryptochromes play a pivotal role in the generation and maintenance of circadian rhythms. 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 transcriptionTranscription (genetics)
Transcription is the process of creating a complementary RNA copy of a sequence of DNA. Both RNA and DNA are nucleic acids, which use base pairs of nucleotides as a complementary language that can be converted back and forth from DNA to RNA by the action of the correct enzymes...
repressor
Repressor
In molecular genetics, a repressor is a DNA-binding protein that regulates the expression of one or more genes by binding to the operator and blocking the attachment of RNA polymerase to the promoter, thus preventing transcription of the genes. This blocking of expression is called...
s within the circadian clockwork. Some insects, including the monarch butterfly
Monarch butterfly
The Monarch butterfly is a milkweed butterfly , in the family Nymphalidae. It is perhaps the best known of all North American butterflies. Since the 19th century, it has been found in New Zealand, and in Australia since 1871 where it is called the Wanderer...
, 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
Mutant
In biology and especially genetics, a mutant is an individual, organism, or new genetic character, arising or resulting from an instance of mutation, which is a base-pair sequence change within the DNA of a gene or chromosome of an organism resulting in the creation of a new character or trait not...
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 cryb, 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
PER1
Period circadian protein homolog 1 is a protein that in humans is encoded by the PER1 gene....
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 TIMTimeless (gene)
Timeless is a gene in Drosophila which encodes TIM, an essential protein that regulates circadian rhythms. Timeless mRNA and protein oscillate rhythmically with time as part of a transcription-translation [negative feedback] loop involving the period gene and its protein.-Discovery:Timeless was...
, in a light-dependent manner. Once bound by dCRY, dTIM is committed to degradation by the ubiquitin-proteasome
Proteasome
Proteasomes are very large protein complexes inside all eukaryotes and archaea, and in some bacteria. In eukaryotes, they are located in the nucleus and the cytoplasm. The main function of the proteasome is to degrade unneeded or damaged proteins by proteolysis, a chemical reaction that breaks...
system.
Although light pulses do not entrain, full photoperiod LD cycles can still drive cycling in the ventral-lateral
Lateral
Lateral may refer to:*Lateral , an anatomical direction*Lateral canal, a canal built beside another stream*Lateral consonant, an ℓ-like consonant in which air flows along the sides of the tongue...
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
Rhodopsin
Rhodopsin, also known as visual purple, is a biological pigment of the retina that is responsible for both the formation of the photoreceptor cells and the first events in the perception of light. Rhodopsins belong to the G-protein coupled receptor family and are extremely sensitive to light,...
pathway. Therefore, CRY is involved in light perception and is an input to the circadian clock, but is not required for the operation of the clock because the rhodopsin pathway is sufficient in the absence of the CRY pathway. 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
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
-based mechanism that is dependent on potassium channel conductance. This CRY mediated light response has been shown to increase action potential
Action potential
In physiology, an action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a consistent trajectory. Action potentials occur in several types of animal cells, called excitable cells, which include neurons, muscle cells, and...
firing within seconds of a light response in opsin
Opsin
Opsins are a group of light-sensitive 35–55 kDa membrane-bound G protein-coupled receptors of the retinylidene protein family found in photoreceptor cells of the retina. Five classical groups of opsins are involved in vision, mediating the conversion of a photon of light into an electrochemical...
-knockout Drosophila.
Cryptochrome, like many genes involved in circadian rhythm, shows circadian cycling in mRNA 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)Period (gene)
Period is a gene located on the X chromosome of Drosophila melanogaster. Oscillations in levels of both per transcript and its corresponding protein PER have a period of approximately 24 hours and together play a central role in the molecular mechanism of the Drosophila biological clock driving...
, CLOCK, and BMAL1. In this loop, CLOCK and BMAL1 proteins are transcriptional activators, 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
Suprachiasmatic nucleus
The suprachiasmatic nucleus or nuclei, abbreviated SCN, is a tiny region on the brain's midline, situated directly above the optic chiasm. It is responsible for controlling circadian rhythms...
, 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
C-Fos
In the field of molecular biology and Genetics, c-Fos is a protein encoded by the FOS gene.-Structure and function:c-Fos is a cellular proto-oncogene belonging to the immediate early gene family of transcription factors. c-Fos has a leucine-zipper DNA binding domain, and a transactivation domain at...
transcription, a mediator of light sensitivity, significantly drops. In recent years, data have supported melanopsin
Melanopsin
Melanopsin is a photopigment found in specialized photosensitive ganglion cells of the retina that are involved in the regulation of circadian rhythms, pupillary light reflex, and other non-visual responses to light. In structure, melanopsin is an opsin, a retinylidene protein variety of...
as the main circadian photoreceptor, particularly melanopsin cells which mediate entrainment and communication between the eye
Eye
Eyes are organs that detect light and convert it into electro-chemical impulses in neurons. The simplest photoreceptors in conscious vision connect light to movement...
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
E-box
An E-box is a DNA sequence which usually lies upstream of a gene in a promoter region. It is a transcription factor binding site where the specific sequence of DNA, CANNTG, is recognized by proteins that can bind to it to help initiate its transcription. Once transcription factors bind to...
and D-box elements in the promoter and RevErbA
Rev-ErbA
The Rev-ErbA proteins are members of the nuclear receptor family of intracellular transcription factors. There are two forms of the receptor, alpha and beta, each encoded by a separate gene ....
/ROR
RAR-related orphan receptor
The RAR-related orphan receptors are members of the nuclear receptor family of intracellular transcription factors. There are three forms of ROR, ROR-α, -β, and -γ and each is encoded by a separate gene...
binding elements (RREs) in the gene’s first intron. Transfection
Transfection
Transfection is the process of deliberately introducing nucleic acids into cells. The term is used notably for non-viral methods in eukaryotic cells...
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.
Magnetoception
Cryptochromes in the photoreceptor neurons of birds' eyes are involved in magnetic orientation during migrationBird migration
Bird migration is the regular seasonal journey undertaken by many species of birds. Bird movements include those made in response to changes in food availability, habitat or weather. Sometimes, journeys are not termed "true migration" because they are irregular or in only one direction...
. Cryptochromes are also essential for the light-dependent ability of Drosophila to sense magnetic fields. Furthermore, magnetic fields affect cryptochromes in Arabidopsis thaliana: growth behavior is affected by magnetic fields in the presence of blue ( but not red) light.
According to one model, cryptochrome forms a pair of two radical
Radical (chemistry)
Radicals are atoms, molecules, or ions with unpaired electrons on an open shell configuration. Free radicals may have positive, negative, or zero charge...
s with correlated spin
Spin (physics)
In quantum mechanics and particle physics, spin is a fundamental characteristic property of elementary particles, composite particles , and atomic nuclei.It is worth noting that the intrinsic property of subatomic particles called spin and discussed in this article, is related in some small ways,...
s when exposed to blue light. The occurrence of such light-generated radical pairs and the correlation of the radical pair state have been confirmed recently in a cryptochrome of Xenopus laevis. However, recent evidence from Arabidopsis thaliana cryptochrome also suggests that radical pairs can be generated by the light-independent dark reoxidation of Flavin protein by molecular oxygen through the formation of a spin-correlated FADH-superoxide radical pairs. Magnetoception is hypothesized to function through the surrounding magnetic fields effect on the correlation (parallel or anti-parallel) of these radicals, which affects the duration that cryptochrome remains activated. Activation of cryptochrome may affect the light-sensitivity of retina
Retina
The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...
l neurons, with the overall result that the animal can "see" the magnetic field.
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 BankProtein Data BankThe Protein Data Bank is a repository for the 3-D structural data of large biological molecules, such as proteins and nucleic acids....
; 3-D structure of Arabidopsis cryptochrome 3, obtained by X-ray crystallography. - Animated model of Murine circadian pathway, including role of Cry