The Casein kinase 1 family () of
protein kinase
A protein kinase is a kinase which selectively modifies other proteins by covalently adding phosphates to them (phosphorylation) as opposed to kinases which modify lipids, carbohydrates, or other molecules. Phosphorylation usually results in a fu ...
s are
serine
Serine (symbol Ser or S) is an α-amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated − form under biological conditions), a carboxyl group (which is in the deprotonated − form un ...
/
threonine
Threonine (symbol Thr or T) is an amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated −NH form under biological conditions), a carboxyl group (which is in the deprotonated −COO ...
-selective enzymes that function as regulators of
signal transduction
Signal transduction is the process by which a chemical or physical signal is transmitted through a cell as a series of molecular events, most commonly protein phosphorylation catalyzed by protein kinases, which ultimately results in a cellula ...
pathways in most eukaryotic cell types. CK1 isoforms are involved in Wnt signaling, circadian rhythms, nucleo-cytoplasmic shuttling of transcription factors, DNA repair, and DNA transcription.
Discovery
By the early 1950s it was known from metabolic labeling studies using radioactive
phosphate
In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthophosphoric acid .
The phosphate or orthophosphate ion is derived from phospho ...
that phosphate groups attached to
phosphoprotein
A phosphoprotein is a protein that is posttranslationally modified by the attachment of either a single phosphate group, or a complex molecule such as 5'-phospho-DNA, through a phosphate group. The target amino acid is most often serine, threonin ...
s inside cells can sometimes undergo rapid exchange of new phosphate for old. In order to perform experiments that would allow isolation and characterization of the
enzyme
Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products. A ...
s involved in attaching and removing phosphate from proteins, there was a need for convenient
substrates for
protein kinase
A protein kinase is a kinase which selectively modifies other proteins by covalently adding phosphates to them (phosphorylation) as opposed to kinases which modify lipids, carbohydrates, or other molecules. Phosphorylation usually results in a fu ...
s and
protein phosphatases
A protein phosphatase is a phosphatase enzyme that removes a phosphate group from the phosphorylated amino acid residue of its substrate protein. Protein phosphorylation is one of the most common forms of reversible protein posttranslational modif ...
.
Casein
Casein ( , from Latin ''caseus'' "cheese") is a family of related phosphoproteins (CSN1S1, αS1, aS2, CSN2, β, K-casein, κ) that are commonly found in mammalian milk, comprising about 80% of the proteins in cow's milk and between 20% and 60% of ...
has been used as a substrate since the earliest days of research on protein
phosphorylation
In chemistry, phosphorylation is the attachment of a phosphate group to a molecule or an ion. This process and its inverse, dephosphorylation, are common in biology and could be driven by natural selection. Text was copied from this source, wh ...
. By the late 1960s,
cyclic AMP-dependent protein kinase had been purified, and most attention was centered on kinases and phosphatases that could regulate the activity of important enzymes. Casein kinase activity associated with the
endoplasmic reticulum
The endoplasmic reticulum (ER) is, in essence, the transportation system of the eukaryotic cell, and has many other important functions such as protein folding. It is a type of organelle made up of two subunits – rough endoplasmic reticulum ( ...
of mammary glands was first characterized in 1974, and its activity was shown to not depend on
cyclic AMP
Cyclic adenosine monophosphate (cAMP, cyclic AMP, or 3',5'-cyclic adenosine monophosphate) is a second messenger important in many biological processes. cAMP is a derivative of adenosine triphosphate (ATP) and used for intracellular signal transd ...
.
CK1 family
The CK1 family of monomeric serine–threonine protein kinases is found in eukaryotic organisms from
yeast
Yeasts are eukaryotic, single-celled microorganisms classified as members of the fungus kingdom. The first yeast originated hundreds of millions of years ago, and at least 1,500 species are currently recognized. They are estimated to constitut ...
to
humans
Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, culture, ...
. Mammals have seven family members (sometimes referred to as
isoforms
A protein isoform, or "protein variant", is a member of a set of highly similar proteins that originate from a single gene or gene family and are the result of genetic differences. While many perform the same or similar biological roles, some isof ...
, but encoded by distinct genes): alpha, beta 1, gamma 1, gamma 2, gamma 3, delta, and epsilon. Isoforms range from 22 to 55
kDa
The dalton or unified atomic mass unit (symbols: Da or u) is a non-SI unit of mass widely used in physics and chemistry. It is defined as of the mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state and at re ...
and have been identified in the membranes, nucleus, and cytoplasm of eukaryotes and additionally in the mitotic spindle in mammalian cells.
The family members have the highest homology in their kinase domains (53%–98% identical) and differ from most other protein kinases by the presence of the sequence S-I-N instead of A-P-E in kinase domain VIII. The family members appear to have similar substrate specificity ''in vitro'',
and substrate selection is thought to be regulated in vivo via subcellular localization and docking sites in specific substrates. One consensus phosphorylation site is S/Tp-X-X-S/T, where S/Tp refers to a phospho-serine or phospho-threonine, X refers to any amino acid, and the underlined residues refer to the target site. Thus, this CKI consensus site requires priming by another kinase. CKI also phosphorylates a related unprimed site, which optimally contains a cluster of acidic amino acids N-terminal to the target S/T including an acidic residue at n − 3 and a hydrophobic region C-terminal to the target S/T.
[ A single acidic residue in the n − 3 position is not sufficient for CKI phosphorylation. In contrast, in several important targets, NF-AT and beta-catenin, CKI does not require n − 3 priming but, instead, phosphorylates the first serine in the sequence S-L-S, which is followed by a cluster of acidic residues, albeit less efficiently than the optimal sites.
]
Roles
Casein kinase activity was found to be present in most cell types and to be associated with multiple enzymes. The type 1 casein kinase family of related gene products are now given designations such as "casein kinase 1 alpha" and "casein kinase 1 epsilon".
Wnt signaling pathway
Casein kinase 1 epsilon has been suggested to play a role in phosphorylation of Disheveled in the Wnt signaling pathway
The Wnt signaling pathways are a group of signal transduction pathways which begin with proteins that pass signals into a cell through cell surface receptors. The name Wnt is a portmanteau created from the names Wingless and Int-1. Wnt signaling p ...
. Casein kinase 1 alpha (CK1α) binds to and phosphorylates β‑catenin
In plants the phosphorylation of protein Jade-1 is regulated by casein kinase 1. In humans there are three casein kinase 1 gamma enzymes.
''Xenopus
''Xenopus'' () (Gk., ξενος, ''xenos''=strange, πους, ''pous''=foot, commonly known as the clawed frog) is a genus of highly aquatic frogs native to sub-Saharan Africa. Twenty species are currently described within it. The two best-known ...
'' casein kinase 1 gamma (CK1gamma) is associated with the cell membrane and binds to LRP. CK1gamma was found to be needed for Wnt signaling through LRP, and is both necessary and sufficient to transduce LRP6 signaling in vertebrates
Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, ...
and ''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 species ...
'' cells. Wnt binding to LRP causes a rapid increase in phosphorylation of the cytoplasmic domain of LRP by CK1gamma. Phosphorylation of LRP6 by CK1gamma promotes binding of axin to LRP and activation of the Wnt signaling pathway.
Circadian rhythm
CK1ε and CK1δ are essential in the genetic transcription-translation (and post-translation) feedback loops that generate circadian rhythm
A circadian rhythm (), or circadian cycle, is a natural, internal process that regulates the sleep–wake cycle and repeats roughly every 24 hours. It can refer to any process that originates within an organism (i.e., Endogeny (biology), endogeno ...
in mammals.
The previously-characterized CK1ε isoform was first implicated as a clock gene when its ''Drosophila'' homolog, double-time (Doubletime (gene)
Doubletime (dbt) also known as discs overgrown (dco) is a gene that encodes the double-time protein (DBT) in ''Drosophila melanogaster''. The double-time protein is a kinase that phosphorylates PER protein that regulates the molecularly-driven, ...
), was discovered in 1998. Double-time is 86% identical to human CK1ε. Kloss ''et al'' and Price ''et al'' showed that mutations in double-time altered circadian rhythm. They found two DBT mutants that had abnormal free-running periods and one that was pupal-lethal but resulted in accumulations of hypophosphorylated PER
Per is a Latin preposition which means "through" or "for each", as in per capita.
Per or PER may also refer to:
Places
* IOC country code for Peru
* Pér, a village in Hungary
* Chapman code for Perthshire, historic county in Scotland
Math ...
protein. Since then, double-time's protein product DBT has been well characterized for its role in phosphorylating PER, the protein product of clock gene period
Period may refer to:
Common uses
* Era, a length or span of time
* Full stop (or period), a punctuation mark
Arts, entertainment, and media
* Period (music), a concept in musical composition
* Periodic sentence (or rhetorical period), a concept ...
in Drosophila, and its mammalian homologs appear to play a similar role.
In 2021, scientists reported the development of a light-responsive days-lasting modulator of circadian rhythms of tissues via Ck1 inhibition. Such modulators may be useful for chronobiology
Chronobiology is a field of biology that examines timing processes, including periodic (cyclic) phenomena in living organisms, such as their adaptation to solar- and lunar-related rhythms. These cycles are known as biological rhythms. Chronobi ...
research and repair of organs that are "out of sync".
Interactions
DBT has been shown to physically interact with PER in vitro and in vivo, and to create a stable complex with PER throughout the circadian cycle. PER that has been phosphorylated by DBT is recognized by the Slimb protein. Slimb is a component of the Skp1/Cullin/F-box protein (SCF) ubiquitin ligase complex, which marks proteins for proteosomal degradation in a phosphorylation-dependent manner. Enhanced PER degradation in the cytoplasm is predicted to delay nuclear translocation of both PER and TIM, and to thus affect the period of circadian rhythms.
The mutation dbtS, associated with a proline
Proline (symbol Pro or P) is an organic acid classed as a proteinogenic amino acid (used in the biosynthesis of proteins), although it does not contain the amino group but is rather a secondary amine. The secondary amine nitrogen is in the prot ...
to serine substitution at residue 47 47S shortens period length by about 6 h. dbtL contains an amino acid substitution of isoleucine
Isoleucine (symbol Ile or I) is an α-amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated −NH form under biological conditions), an α-carboxylic acid group (which is in the deprot ...
for methionine
Methionine (symbol Met or M) () is an essential amino acid in humans. As the precursor of other amino acids such as cysteine and taurine, versatile compounds such as SAM-e, and the important antioxidant glutathione, methionine plays a critical ro ...
at residue 80 (M80I) and lengthens period to 29 h. A third mutation, dbtAR, is associated with a change from histidine
Histidine (symbol His or H) is an essential amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated –NH3+ form under biological conditions), a carboxylic acid group (which is in the de ...
126 to tyrosine
-Tyrosine or tyrosine (symbol Tyr or Y) or 4-hydroxyphenylalanine is one of the 20 standard amino acids that are used by cells to synthesize proteins. It is a non-essential amino acid with a polar side group. The word "tyrosine" is from the Gr ...
and causes arrhythmia. PER protein in this mutant is hypophosphorylated. Each of these mutations maps to the kinase domain of DBT gene. The short- and long-period alleles of DBT enhance or attenuate, respectively, PER degradation in the nucleus, further demonstrating the importance of timely PER degradation as a critical determinant in establishing 24-h rhythmicity. In addition to influencing protein degradation, DBT affects the timing of nuclear accumulation of PER. The short-period mutant dbtS delays PER nuclear accumulation, which is independent of PER protein stability, and arrhythmic alleles of dbt cause nuclear accumulation of PER in clock-containing cells of larval and adult ''Drosophila''.
Both mammalian CK1δ and CK1ε contain closely related 123-amino-acid carboxy-terminal domains that can auto-regulate kinase activity. CK1δ and CK1ε are 53% identical. These domains are not related to the carboxy-terminal domain of double-time, suggesting a split in the evolution of the mammalian and fly homologs.
A similar function for casein kinase 2
Casein kinase 2 ()(CK2/CSNK2) is a serine/threonine-selective protein kinase that has been implicated in cell cycle control, DNA repair, regulation of the circadian rhythm, and other cellular processes. De-regulation of CK2 has been linked to tumor ...
has been reported in ''Arabidopsis thaliana
''Arabidopsis thaliana'', the thale cress, mouse-ear cress or arabidopsis, is a small flowering plant native to Eurasia and Africa. ''A. thaliana'' is considered a weed; it is found along the shoulders of roads and in disturbed land.
A winter a ...
'', ''Drosophila'', and ''Neurospora
''Neurospora'' is a genus of Ascomycete fungi. The genus name, meaning "nerve spore" refers to the characteristic striations on the spores that resemble axons.
The best known species in this genus is ''Neurospora crassa'', a common model organi ...
''.
Positive and negative feedback
In the negative feedback loops, CK1ε periodically binds to and phosphorylates the PER proteins (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 memb ...
, PER2
PER2 is a protein in mammals encoded by the ''PER2'' gene. ''PER2'' is noted for its major role in circadian rhythms.
Discovery
The ''per ''gene'' ''was first discovered using forward genetics in '' Drosophilla melanogaster'' in 1971. Mammalia ...
, and PER3
The PER3 gene encodes the period circadian protein homolog 3 protein in humans. PER3 is a paralog to the PER1 and PER2 genes. It is a circadian gene associate with delayed sleep phase syndrome in humans.
History
The Per3 gene was independently ...
), which form heterodimers with each other and interact with CRY1
Cryptochromes (from the Greek κρυπτός χρώμα, "hidden colour") are a class of flavoproteins found in plants and animals that are sensitive to blue light. They are involved in the circadian rhythms and the sensing of magnetic fields i ...
and CRY2
Cryptochromes (from the Greek κρυπτός χρώμα, "hidden colour") are a class of flavoproteins found in plants and animals that are sensitive to blue light. They are involved in the circadian rhythms and the sensing of magnetic fields i ...
. The effects of phosphorylation are two-fold. It has been shown in Drosophila that phosphorylation of the PER proteins increase their ubiquitination, which leads to degradation. Phosphorylation of the PER proteins also leaves them unable to enter the nucleus, where they suppress transcription of clock genes. The blocking of nuclear translocation occurs via phosphorylation of PER at the nuclear localization signal A nuclear localization signal ''or'' sequence (NLS) is an amino acid sequence that 'tags' a protein for import into the cell nucleus by nuclear transport. Typically, this signal consists of one or more short sequences of positively charged lysines o ...
, which masks the signal and prevents nuclear entry. However, this CK1ε-mediated constraint to the cytoplasm can be overcome when the PER protein complex is bound to CRY. CK1ε has been shown to phosphorylate CRY when both CK1ε and CRY are complexed with PER in vitro, but the functional significance of this remains undetermined.
CK1ε may also have a role in positive feedback
Positive feedback (exacerbating feedback, self-reinforcing feedback) is a process that occurs in a feedback loop which exacerbates the effects of a small disturbance. That is, the effects of a perturbation on a system include an increase in the ...
; the transcription factor 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 ...
is a CK1ε substrate in vitro, and increased CK1ε activity has been shown to positively regulate transcription of genes under the influence of BMAL1-dependent circadian gene promoters. This has not yet been studied 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 ...
.
Significance in disease
CK1δ and CK1ε have been shown to be relevant in human disease. Recent findings indicate that pharmaceutical inhibition of CK1 may be a promising therapeutic for aberrant circadian rhythm. Mutations and variants of the CK1ε phosphorylation site of PER2 are associated with cases of Familial Advanced Sleep Phase Syndrome
Advanced Sleep Phase Disorder (ASPD), also known as the advanced sleep-phase type (ASPT) of circadian rhythm sleep disorder, is a condition that is characterized by a recurrent pattern of early evening (e.g. 7-9 PM) sleepiness and very early morn ...
(FASPS). Similarly, length variations in the CK1ε phosphorylation site of PER3 have been found to correlate with 'morningness' and 'eveningness'; longer alleles are associated with early risers while shorter alleles are associated with late risers. Additionally, 75% of patients with Delayed sleep phase syndrome
Delayed sleep phase disorder (DSPD), more often known as delayed sleep phase syndrome and also as delayed sleep–wake phase disorder, is a delaying of a person's circadian rhythm (biological clock) compared to those of societal norms. The diso ...
are homozygous for the shorter allele.
Mutations in CK1 have been shown to alter circadian behavior in other mammals, as well. In 1988, the golden hamster ''tau'' mutant, which has a freerunning period of 22hrs, was the first mammalian circadian mutant discovered. Twelve years later in 2000, the ''tau'' mutation was mapped to CK1ε. Since its discovery, the ''tau'' mutant has proven to be a valuable research tool in circadian biology. CK1ɛtau, a T178C substitution, is a gain-of function mutation that causes an increase in degradation of PER, but not CRY. This creates a disruption in the PER-regulated feedback loop and consequently an acceleration of molecular oscillations. Homozygous
Zygosity (the noun, zygote, is from the Greek "yoked," from "yoke") () is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism.
Mo ...
mutants (CK1ε(''tau/tau'')) show a significant decrease in period, both in vivo (behaviorally) and in vitro (measured by firing rates of the suprachiasmatic nucleus
The suprachiasmatic nucleus or nuclei (SCN) is a tiny region of the brain in the hypothalamus, situated directly above the optic chiasm. It is responsible for controlling circadian rhythms. The neuronal and hormonal activities it generates regula ...
). Recent research has also identified a link between mutations in the CK1δ gene and familial migraine and advanced sleep phase, a finding that was replicated in mice migraine models.
Roles of isoforms
CK1δ and CK1ε were thought to be generally redundant in circadian cycle length and protein stability. Recent research, however, has shown that CK1δ deficiency lengthens circadian period while CK1ε deficiency does not. Also, CK1α has recently been suggested to play a role redundant to CK1δ in phosphorylating PER1 although this is not consistent with other data
Nucleo-cytoplasmic regulation of transcription factors
CKIα or CKIδ is essential in modulating the nuclear export of eukaryotic translation initiation factor 6 (eIF6
Eukaryotic translation initiation factor 6 (EIF6), also known as Integrin beta 4 binding protein (ITGB4BP), is a human gene.
Hemidesmosomes are structures which link the basal lamina to the intermediate filament cytoskeleton. An important functio ...
), a protein with essential nuclear and cytoplasmic roles in biogenesis
Spontaneous generation is a superseded scientific theory that held that living creatures could arise from nonliving matter and that such processes were commonplace and regular. It was hypothesized that certain forms, such as fleas, could arise ...
of the 60S subunit of the eukaryotic ribosome
Ribosomes ( ) are macromolecular machines, found within all cells, that perform biological protein synthesis (mRNA translation). Ribosomes link amino acids together in the order specified by the codons of messenger RNA (mRNA) molecules to ...
. Phosphorylation of Ser-174 and Ser-175 by CKI promotes nuclear export of eIF6 while dephosphorylation by calcineurin
Calcineurin (CaN) is a calcium and calmodulin dependent serine/threonine protein phosphatase (also known as protein phosphatase 3, and calcium-dependent serine-threonine phosphatase). It activates the T cells of the immune system and can be bloc ...
promotes nuclear accumulation of eIF6. It is unclear whether the same mechanism is responsible for eIF6 cycling in yeast and if other kinases also play roles in these processes.
CKI homologs are also implicated in cytoplasmic shuttling of nuclear factor of activated T-cells (NFAT
Nuclear factor of activated T-cells (NFAT) is a family of transcription factors shown to be important in immune response. One or more members of the NFAT family is expressed in most cells of the immune system. NFAT is also involved in the developme ...
) through observation that the transcription factor Crz1p is phosphorylated by a CKI homolog in yeast.
Interphase, mitosis and DNA repair
CKIδ activity is implicated in mitosis
In cell biology, mitosis () is a part of the cell cycle in which replicated chromosomes are separated into two new nuclei. Cell division by mitosis gives rise to genetically identical cells in which the total number of chromosomes is mainta ...
and in response to DNA damage. During interphase
Interphase is the portion of the cell cycle that is not accompanied by visible changes under the microscope, and includes the G1, S and G2 phases. During interphase, the cell grows (G1), replicates its DNA (S) and prepares for mitosis (G2). A c ...
, CKIδ associates with the Golgi Apparatus
The Golgi apparatus (), also known as the Golgi complex, Golgi body, or simply the Golgi, is an organelle found in most eukaryotic cells. Part of the endomembrane system in the cytoplasm, it packages proteins into membrane-bound vesicles ins ...
and appears to regulate the budding of clathrin
Clathrin is a protein that plays a major role in the formation of coated vesicles. Clathrin was first isolated and named by Barbara Pearse in 1976. It forms a triskelion shape composed of three clathrin heavy chains and three light chains. Whe ...
coated vesicles
Vesicle may refer to:
; In cellular biology or chemistry
* Vesicle (biology and chemistry), a supramolecular assembly of lipid molecules, like a cell membrane
* Synaptic vesicle
; In human embryology
* Vesicle (embryology), bulge-like features o ...
from the TGN; it also appears to associate with tubulin
Tubulin in molecular biology can refer either to the tubulin protein superfamily of globular proteins, or one of the member proteins of that superfamily. α- and β-tubulins polymerize into microtubules, a major component of the eukaryotic cytoske ...
. While undamaged mitotic cells shows no CKIδ association with tubulin
Tubulin in molecular biology can refer either to the tubulin protein superfamily of globular proteins, or one of the member proteins of that superfamily. α- and β-tubulins polymerize into microtubules, a major component of the eukaryotic cytoske ...
, the kinase was recruited during mitosis in cells with DNA damage, indicative of a role for CKIδ in arranging the microtubule
Microtubules are polymers of tubulin that form part of the cytoskeleton and provide structure and shape to eukaryotic cells. Microtubules can be as long as 50 micrometres, as wide as 23 to 27 nm and have an inner diameter between 11 an ...
network during mitosis. The mechanisms for these biochemical interactions remain unknown.
See also
*Casein kinase 2
Casein kinase 2 ()(CK2/CSNK2) is a serine/threonine-selective protein kinase that has been implicated in cell cycle control, DNA repair, regulation of the circadian rhythm, and other cellular processes. De-regulation of CK2 has been linked to tumor ...
— a distinct protein kinase family
References
{{DEFAULTSORT:Casein Kinase 1
Signal transduction
Protein kinases