At the center of the mammalian circadian clock is a negative transcription/translation-based feedback loop: The BMAL1:CLOCK/NPAS2 heterodimer transactivates CRY and PER genes by binding E-box elements in their promoters; the CRY and PER proteins then inhibit transactivation by BMAL1:CLOCK/NPAS2. BMAL1:CLOCK/NPAS2 activates transcription of CRY, PER, and several other genes in the morning. Levels of PER and CRY proteins rise during the day and inhibit expression of CRY, PER, and other BMAL1:CLOCK/NPAS2-activated genes in the afternoon and evening. During the night CRY and PER proteins are targeted for degradation by phosphorylation and polyubiquitination, allowing the cycle to commence again in the morning. Transcription of the BMAL1 (ARNTL) gene is controlled by ROR-alpha and REV-ERBA, both of which are targets of BMAL1:CLOCK/NPAS2 in mice and both of which compete for the same element (RORE) in the BMAL1 promoter. ROR-alpha activates transcription of BMAL1; REV-ERBA represses transcription of BMAL1. This mutual control forms a secondary, reinforcing loop of the circadian clock. REV-ERBA shows strong circadian rhythmicity and confers circadian expression on BMAL1. BMAL1 can form heterodimers with either CLOCK or NPAS2, which act redundantly but show different tissue specificity. The BMAL1:CLOCK and BMAL1:NPAS2 heterodimers activate a set of genes that possess E-box elements (consensus CACGTG) in their promoters. This confers circadian expression on the genes. The PER genes (PER1, PER2, PER3) and CRY genes (CRY1, CRY2) are among those activated by BMAL1:CLOCK and BMAL1:NPAS2. PER and CRY mRNA accumulates during the morning and the proteins accumulate during the afternoon. PER and CRY proteins form complexes in the cytosol and these are bound by either CSNK1D or CSNK1E kinases which phosphorylate PER and CRY. The phosphorylated PER:CRY:kinase complex is translocated into the nucleus due to the nuclear localization signal of PER and CRY. Within the nucleus the PER:CRY complexes bind BMAL1:CLOCK and BMAL1:NPAS2, inhibiting their transactivation activity and their phosphorylation. This reduces expression of the target genes of BMAL1:CLOCK and BMAL1:NPAS2 during the afternoon and evening. PER:CRY complexes also traffic out of the nucleus into the cytosol due to the nuclear export signal of PER. During the night PER:CRY complexes are polyubiquitinated and degraded, allowing the cycle to begin again. Phosphorylated PER is bound by Beta-TrCP1, a cytosolic F-box type component of some SCF E3 ubiquitin ligases. CRY is bound by FBXL3, a nucleoplasmic F-box type component of some SCF E3 ubiquitin ligases. Phosphorylation of CRY1 by Adenosine monophosphate-activated kinase (AMPK) enhances degradation of CRY1. PER and CRY are subsequently polyubiquitinated and proteolyzed by the 26S proteasome. The circadian clock is cell-autonomous and some, but not all cells of the body exhibit circadian rhythms in metabolism, cell division, and gene transcription. The suprachiasmatic nucleus (SCN) in the hypothalamus is the major clock in the body and receives its major input from light (via retinal neurons) and a minor input from nutrient intake. The SCN and other brain tissues determine waking and feeding cycles and influence the clocks in other tissues by hormone secretion and nervous stimulation. Independently of the SCN, other tissues such as liver receive inputs from signals from the brain and from nutrients.
van der Spek PJ, Kobayashi K, Bootsma D, Takao M, Eker AP, Yasui A.; ''Cloning, tissue expression, and mapping of a human photolyase homolog with similarity to plant blue-light receptors.''; PubMedEurope PMCScholia
Xu Y, Toh KL, Jones CR, Shin JY, Fu YH, Ptácek LJ.; ''Modeling of a human circadian mutation yields insights into clock regulation by PER2.''; PubMedEurope PMCScholia
Motzkus D, Loumi S, Cadenas C, Vinson C, Forssmann WG, Maronde E.; ''Activation of human period-1 by PKA or CLOCK/BMAL1 is conferred by separate signal transduction pathways.''; PubMedEurope PMCScholia
Tei H, Okamura H, Shigeyoshi Y, Fukuhara C, Ozawa R, Hirose M, Sakaki Y.; ''Circadian oscillation of a mammalian homologue of the Drosophila period gene.''; PubMedEurope PMCScholia
Ueda HR, Hayashi S, Chen W, Sano M, Machida M, Shigeyoshi Y, Iino M, Hashimoto S.; ''System-level identification of transcriptional circuits underlying mammalian circadian clocks.''; PubMedEurope PMCScholia
Ikeda M, Nomura M.; ''cDNA cloning and tissue-specific expression of a novel basic helix-loop-helix/PAS protein (BMAL1) and identification of alternatively spliced variants with alternative translation initiation site usage.''; PubMedEurope PMCScholia
Kobayashi K, Kanno S, Smit B, van der Horst GT, Takao M, Yasui A.; ''Characterization of photolyase/blue-light receptor homologs in mouse and human cells.''; PubMedEurope PMCScholia
Matsumura R, Matsubara C, Node K, Takumi T, Akashi M.; ''Nuclear receptor-mediated cell-autonomous oscillatory expression of the circadian transcription factor, neuronal PAS domain protein 2 (NPAS2).''; PubMedEurope PMCScholia
Yin L, Wu N, Lazar MA.; ''Nuclear receptor Rev-erbalpha: a heme receptor that coordinates circadian rhythm and metabolism.''; PubMedEurope PMCScholia
Wu N, Yin L, Hanniman EA, Joshi S, Lazar MA.; ''Negative feedback maintenance of heme homeostasis by its receptor, Rev-erbalpha.''; PubMedEurope PMCScholia
Ko CH, Takahashi JS.; ''Molecular components of the mammalian circadian clock.''; PubMedEurope PMCScholia
Miyajima N, Horiuchi R, Shibuya Y, Fukushige S, Matsubara K, Toyoshima K, Yamamoto T.; ''Two erbA homologs encoding proteins with different T3 binding capacities are transcribed from opposite DNA strands of the same genetic locus.''; PubMedEurope PMCScholia
Tamaru T, Hirayama J, Isojima Y, Nagai K, Norioka S, Takamatsu K, Sassone-Corsi P.; ''CK2alpha phosphorylates BMAL1 to regulate the mammalian clock.''; PubMedEurope PMCScholia
Adelmant G, Bègue A, Stéhelin D, Laudet V.; ''A functional Rev-erb alpha responsive element located in the human Rev-erb alpha promoter mediates a repressing activity.''; PubMedEurope PMCScholia
Poliandri AH, Gamsby JJ, Christian M, Spinella MJ, Loros JJ, Dunlap JC, Parker MG.; ''Modulation of clock gene expression by the transcriptional coregulator receptor interacting protein 140 (RIP140).''; PubMedEurope PMCScholia
Puigserver P, Wu Z, Park CW, Graves R, Wright M, Spiegelman BM.; ''A cold-inducible coactivator of nuclear receptors linked to adaptive thermogenesis.''; PubMedEurope PMCScholia
Xu Y, Padiath QS, Shapiro RE, Jones CR, Wu SC, Saigoh N, Saigoh K, Ptácek LJ, Fu YH.; ''Functional consequences of a CKIdelta mutation causing familial advanced sleep phase syndrome.''; PubMedEurope PMCScholia
Dardente H, Fortier EE, Martineau V, Cermakian N.; ''Cryptochromes impair phosphorylation of transcriptional activators in the clock: a general mechanism for circadian repression.''; PubMedEurope PMCScholia
Raghuram S, Stayrook KR, Huang P, Rogers PM, Nosie AK, McClure DB, Burris LL, Khorasanizadeh S, Burris TP, Rastinejad F.; ''Identification of heme as the ligand for the orphan nuclear receptors REV-ERBalpha and REV-ERBbeta.''; PubMedEurope PMCScholia
Nakamura K, Inoue I, Takahashi S, Komoda T, Katayama S.; ''Cryptochrome and Period Proteins Are Regulated by the CLOCK/BMAL1 Gene: Crosstalk between the PPARs/RXRalpha-Regulated and CLOCK/BMAL1-Regulated Systems.''; PubMedEurope PMCScholia
Thompson CL, Bowes Rickman C, Shaw SJ, Ebright JN, Kelly U, Sancar A, Rickman DW.; ''Expression of the blue-light receptor cryptochrome in the human retina.''; PubMedEurope PMCScholia
Crumbley C, Wang Y, Kojetin DJ, Burris TP.; ''Characterization of the core mammalian clock component, NPAS2, as a REV-ERBalpha/RORalpha target gene.''; PubMedEurope PMCScholia
Reick M, Garcia JA, Dudley C, McKnight SL.; ''NPAS2: an analog of clock operative in the mammalian forebrain.''; PubMedEurope PMCScholia
Sato TK, Yamada RG, Ukai H, Baggs JE, Miraglia LJ, Kobayashi TJ, Welsh DK, Kay SA, Ueda HR, Hogenesch JB.; ''Feedback repression is required for mammalian circadian clock function.''; PubMedEurope PMCScholia
Crumbley C, Burris TP.; ''Direct regulation of CLOCK expression by REV-ERB.''; PubMedEurope PMCScholia
Muñoz E, Baler R.; ''The circadian E-box: when perfect is not good enough.''; PubMedEurope PMCScholia
Toh KL, Jones CR, He Y, Eide EJ, Hinz WA, Virshup DM, Ptácek LJ, Fu YH.; ''An hPer2 phosphorylation site mutation in familial advanced sleep phase syndrome.''; PubMedEurope PMCScholia
Wu Z, Huang X, Feng Y, Handschin C, Feng Y, Gullicksen PS, Bare O, Labow M, Spiegelman B, Stevenson SC.; ''Transducer of regulated CREB-binding proteins (TORCs) induce PGC-1alpha transcription and mitochondrial biogenesis in muscle cells.''; PubMedEurope PMCScholia
Pilegaard H, Saltin B, Neufer PD.; ''Exercise induces transient transcriptional activation of the PGC-1alpha gene in human skeletal muscle.''; PubMedEurope PMCScholia
Shearman LP, Zylka MJ, Weaver DR, Kolakowski LF, Reppert SM.; ''Two period homologs: circadian expression and photic regulation in the suprachiasmatic nuclei.''; PubMedEurope PMCScholia
Takahashi JS, Hong HK, Ko CH, McDearmon EL.; ''The genetics of mammalian circadian order and disorder: implications for physiology and disease.''; PubMedEurope PMCScholia
Juge-Aubry C, Pernin A, Favez T, Burger AG, Wahli W, Meier CA, Desvergne B.; ''DNA binding properties of peroxisome proliferator-activated receptor subtypes on various natural peroxisome proliferator response elements. Importance of the 5'-flanking region.''; PubMedEurope PMCScholia
Akashi M, Tsuchiya Y, Yoshino T, Nishida E.; ''Control of intracellular dynamics of mammalian period proteins by casein kinase I epsilon (CKIepsilon) and CKIdelta in cultured cells.''; PubMedEurope PMCScholia
Eide EJ, Vielhaber EL, Hinz WA, Virshup DM.; ''The circadian regulatory proteins BMAL1 and cryptochromes are substrates of casein kinase Iepsilon.''; PubMedEurope PMCScholia
Giguère V, Tini M, Flock G, Ong E, Evans RM, Otulakowski G.; ''Isoform-specific amino-terminal domains dictate DNA-binding properties of ROR alpha, a novel family of orphan hormone nuclear receptors.''; PubMedEurope PMCScholia
Hastings MH, Maywood ES, O'Neill JS.; ''Cellular circadian pacemaking and the role of cytosolic rhythms.''; PubMedEurope PMCScholia
Lee C, Etchegaray JP, Cagampang FR, Loudon AS, Reppert SM.; ''Posttranslational mechanisms regulate the mammalian circadian clock.''; PubMedEurope PMCScholia
Larrouy D, Vidal H, Andreelli F, Laville M, Langin D.; ''Cloning and mRNA tissue distribution of human PPARgamma coactivator-1.''; PubMedEurope PMCScholia
Latres E, Chiaur DS, Pagano M.; ''The human F box protein beta-Trcp associates with the Cul1/Skp1 complex and regulates the stability of beta-catenin.''; PubMedEurope PMCScholia
Rutter J, Reick M, Wu LC, McKnight SL.; ''Regulation of clock and NPAS2 DNA binding by the redox state of NAD cofactors.''; PubMedEurope PMCScholia
Raspè E, Mautino G, Duval C, Fontaine C, Duez H, Barbier O, Monte D, Fruchart J, Fruchart JC, Staels B.; ''Transcriptional regulation of human Rev-erbalpha gene expression by the orphan nuclear receptor retinoic acid-related orphan receptor alpha.''; PubMedEurope PMCScholia
Yin L, Lazar MA.; ''The orphan nuclear receptor Rev-erbalpha recruits the N-CoR/histone deacetylase 3 corepressor to regulate the circadian Bmal1 gene.''; PubMedEurope PMCScholia
Steeves TD, King DP, Zhao Y, Sangoram AM, Du F, Bowcock AM, Moore RY, Takahashi JS.; ''Molecular cloning and characterization of the human CLOCK gene: expression in the suprachiasmatic nuclei.''; PubMedEurope PMCScholia
Yin L, Wu N, Curtin JC, Qatanani M, Szwergold NR, Reid RA, Waitt GM, Parks DJ, Pearce KH, Wisely GB, Lazar MA.; ''Rev-erbalpha, a heme sensor that coordinates metabolic and circadian pathways.''; PubMedEurope PMCScholia
Hogenesch JB, Gu YZ, Jain S, Bradfield CA.; ''The basic-helix-loop-helix-PAS orphan MOP3 forms transcriptionally active complexes with circadian and hypoxia factors.''; PubMedEurope PMCScholia
Busino L, Bassermann F, Maiolica A, Lee C, Nolan PM, Godinho SI, Draetta GF, Pagano M.; ''SCFFbxl3 controls the oscillation of the circadian clock by directing the degradation of cryptochrome proteins.''; PubMedEurope PMCScholia
Camacho F, Cilio M, Guo Y, Virshup DM, Patel K, Khorkova O, Styren S, Morse B, Yao Z, Keesler GA.; ''Human casein kinase Idelta phosphorylation of human circadian clock proteins period 1 and 2.''; PubMedEurope PMCScholia
Keesler GA, Camacho F, Guo Y, Virshup D, Mondadori C, Yao Z.; ''Phosphorylation and destabilization of human period I clock protein by human casein kinase I epsilon.''; PubMedEurope PMCScholia
Gekakis N, Staknis D, Nguyen HB, Davis FC, Wilsbacher LD, King DP, Takahashi JS, Weitz CJ.; ''Role of the CLOCK protein in the mammalian circadian mechanism.''; PubMedEurope PMCScholia
Miyazaki K, Nagase T, Mesaki M, Narukawa J, Ohara O, Ishida N.; ''Phosphorylation of clock protein PER1 regulates its circadian degradation in normal human fibroblasts.''; PubMedEurope PMCScholia
Phelan CA, Gampe RT, Lambert MH, Parks DJ, Montana V, Bynum J, Broderick TM, Hu X, Williams SP, Nolte RT, Lazar MA.; ''Structure of Rev-erbalpha bound to N-CoR reveals a unique mechanism of nuclear receptor-co-repressor interaction.''; PubMedEurope PMCScholia
Zhou YD, Barnard M, Tian H, Li X, Ring HZ, Francke U, Shelton J, Richardson J, Russell DW, McKnight SL.; ''Molecular characterization of two mammalian bHLH-PAS domain proteins selectively expressed in the central nervous system.''; PubMedEurope PMCScholia
Hogenesch JB, Chan WK, Jackiw VH, Brown RC, Gu YZ, Pray-Grant M, Perdew GH, Bradfield CA.; ''Characterization of a subset of the basic-helix-loop-helix-PAS superfamily that interacts with components of the dioxin signaling pathway.''; PubMedEurope PMCScholia
Shirogane T, Jin J, Ang XL, Harper JW.; ''SCFbeta-TRCP controls clock-dependent transcription via casein kinase 1-dependent degradation of the mammalian period-1 (Per1) protein.''; PubMedEurope PMCScholia
Knutti D, Kaul A, Kralli A.; ''A tissue-specific coactivator of steroid receptors, identified in a functional genetic screen.''; PubMedEurope PMCScholia
Isojima Y, Nakajima M, Ukai H, Fujishima H, Yamada RG, Masumoto KH, Kiuchi R, Ishida M, Ukai-Tadenuma M, Minami Y, Kito R, Nakao K, Kishimoto W, Yoo SH, Shimomura K, Takao T, Takano A, Kojima T, Nagai K, Sakaki Y, Takahashi JS, Ueda HR.; ''CKIepsilon/delta-dependent phosphorylation is a temperature-insensitive, period-determining process in the mammalian circadian clock.''; PubMedEurope PMCScholia
Griffin EA, Staknis D, Weitz CJ.; ''Light-independent role of CRY1 and CRY2 in the mammalian circadian clock.''; PubMedEurope PMCScholia
BMAL1 (ARNTL) contains both a nuclear localization signal and a nuclear export signal. The shuttling of BMAL1 between the nucleus and cytoplasm is important for transactivation by BMAL1:CLOCK/NPAS2 and degradation of BMAL1:CLOCK/NPAS2. BMAL1 initially forms a heterodimer with CLOCK or NPAS2 in the cytosol. The heterodimer is then phosphorylated and translocated into the nucleus.
As inferred from mouse, PER proteins can form homodimers and CRY proteins can form heterodimers with PER proteins. CRY and PER proteins may therefore form trimers (PER:PER:CRY).
MED1 is a component of each of the various Mediator complexes, that function as transcription co-activators. The MED1-containing compolexes include the DRIP, ARC, TRIP and CRSP compllexes.
The NAMPT (NamPRT) gene is transcribed to yield mRNA and the mRNA is translated to yield protein. As inferred from mouse, the BMAL1:CLOCK heterodimer enhances transcription of NAMPT.
The NPAS2 gene is transcribed to yield mRNA and the mRNA is translated to yield protein. Transcription of NPAS2 is enhanced by the RORA:Coactivator complex and repressed by the REV-ERBA:Corepressor complex.
The PPARGC1A (PGC-1alpha) gene is transcribed to yield mRNA and the mRNA is translated to yield protein. PPARGC1A protein is located in the nucleus where it coactivates transcription.
NR1D1 (REV-ERBA) binds heme. The REV-ERBA:heme complex is then able to recruit the corepressors NCoR and HDAC3. Corepressors do not bind REV-ERBA in the absence of heme.
The AVP gene is transcribed to yield mRNA and the mRNA is translated to yield protein. As inferred from mouse, BMAL1:CLOCK heterodimers bind an E-box enhancer in the promoter of the AVP gene and activate transcription of AVP.
The CRYPTOCHROME-2 (CRY2) gene is transcribed to yield mRNA and the mRNA is translated to yield protein. As inferred from mouse, the CRY2 protein shows circadian rhythm in the suprachiasmatic nucleus (SCN) and in peripheral tissues. The mRNA shows circadian rhythm in muscle but not in the SCN. Expression is dependent on CLOCK.
In mouse, BMAL1, CLOCK, and NPAS2 are phosphorylated by unknown kinases. The phosphorylation is dependent on the heterodimerization of BMAL1 with CLOCK or NPAS2. Phosphorylated BMAL1:CLOCK/NPAS2 is a much stronger transactivator of gene expression than is unphosphorylated BMAL1:CLOCK/NPAS2.
The ternary complex containing phosphorylated CRY and PER proteins with a kinase (CSNK1D or CSNK1E) is translocated to the nucleus. Phosphorylation controls transfer to the nucleus and retention in the nucleus.
CRY (CRY1 and CRY2) and PER (PER1, PER2, PER3) proteins form complex in the cytoplasm where they are phosphorylated by CSNK1D and CSNK1E kinases. CRY:PER complexes appear to form stable complexes with a kinase. Because of the nuclear localization signals of PER and CRY, the complexes are translocated to the nucleus where they bind BMAL1:CLOCK/NPAS2 heterodimers and inhibit the transactivation activity of BMAL1:CLOCK/NPAS2. CRY and PER proteins are themselves transcriptionally activated by BMAL1:CLOCK/NPAS2 thus they participate in a negative loop inhibiting their own synthesis and the synthesis of other targets of BMAL1:CLOCK/NPAS2. Experiments with two-hybrid interactions and in vitro associations show that CRY1, CRY2, and PER2 bind BMAL1 at two different sites on BMAL1. PER2 but not CRY1 or CRY2 binds CLOCK. Different combinations of PER and CRY proteins in PER:CRY complexes have different inhibitory activities.
The PPARA gene is transcribed to yield mRNA and the mRNA is translated to yield protein. As inferred from mouse, BMAL1:CLOCK heterodimers bind the scond intron of the PPARA gene and activate transcription of PPARA.
Beta-TrCP1 is an F-box type component of a particular SKP/CUL/F-Box (SCF) E3 ubiquitin ligase. Beta-TrCP1 interacts specifically with phosphorylated PER proteins and directs their polyubiquitination.
The FACTOR VII (F7) gene is transcribed to yield mRNA and the mRNA is translated to yield protein. In mouse the F7 gene shows circadian expression due to activation by the Bmal1:Clock heterodimer.
FBXL3 is an F-box type component of a particular SKP/CUL/F-Box E3 ubiquitin ligase. FBXL3 interacts specifically with CRY1 and CRY2 in the cytosol to direct the polyubiquitination of CRY1 and CRY2. It is unknown if FBXL3 requires phosphorylation or other modification of CRY proteins in order to bind and ubiquitinate them. Phosphorylation of CRY by Adenosine monophosphate-dependent kinase increases degradation of CRY, apparently by increasing association of CRY with FBXL3 Polyubiquitination of CRY proteins directs them to the 26S proteasome for degradation.
The PAI-1 gene is transcribed to yield mRNA and the mRNA is translated to yield protein. The PAI-1 gene shows circadian expression due to direct transcriptional activation by the BMAL1:CLOCK heterodimer and the BMAL2(CLIF, ARNTL2):CLOCK heterodimer.
In the cytosol the kinases CSNK1D (casein kinase I delta) and CSNK1E (casein kinase I epsilon) phosphorylate PER1, PER2, CRY1, and CRY2 at multiple sites. Evidence indicates that PER:CRY complexes form a stable ternary complex with either CSNK1E or CSNK1D. Both kinases are able to bind and phosphorylate PER proteins. CSNK1E has been shown to phosphorylate CRY proteins only when they are complexed with PER proteins. PER proteins contain a nuclear localization sequence and a nuclear export sequence allowing their movement into and out of the nucleus. Phosphorylation is required for transit of PER:CRY:kinase complexes into the nucleus and for interaction of PER proteins with the ubiquitin-mediated degradation process in the cytoplasm. A mutation at Serine662 of PER2 is responsible for familial advanced phase sleep syndrome, however the particular kinase responsible for phosphorylating Serine662 is unknown.
The DEC1 (BHLHE40, BHLHB2) gene is transcribed to yield mRNA and the mRNA is translated to yield protein. The DEC1 gene contains E-box elements in its promoter which bind the BMAL:CLOCK heterodimer and confer circadian rhythm on its expression.
Polyubiquitination of PER proteins is directed by the Beta-TrCP1 component of SCF E3 ubiquitin ligase. The polyubiquitinated PER proteins are recognized and degraded by the 26S proteasome. Degradation of PER proteins occurs during the night and is necessary to allow new transcription of BMAL1:CLOCK/NPAS2 targets in the morning during the circadian cycle.
The DEC2 (BHLHE41, BHLHB3) gene is transcribed to yield mRNA and the mRNA is translated to yield protein. As inferred from mouse, the BMAL1:CLOCK heterodimer binds E-box elements in the DEC2 promoter and activates transcription of DEC2.
The NR1D1 (REV-ERBA) gene is transcribed to yield mRNA and the mRNA is translated to yield protein. In mouse the Rev-erba gene shows circadian expression due to transactivation by the BMAL1:CLOCK heterodimer. REV-ERBA binds the promoter of its own gene and represses its own expression (Adelmont et al. 1996).
The BMAL1 (ARNTL) gene is transcribed to yield mRNA and the mRNA is translated to yield protein. The ROR-alpha transcription factor binds the RORE element of the BMAL1 (ARNTL) promoter and activates transcription of the BMAL1 gene. The REV-ERBA transcription factor binds the same RORE element and represses transcription of the BMAL1 gene.
CRY1 and CRY2 bind the unphosphorylated BMAL1:CLOCK heterodimer (and by homology the BMAL1:NPAS2 heterodimer) and prolong its half-life. The unphosphorylated BMAL1:CLOCK heterodimer only weakly activates transcription and is therefore believed to competitively reduce transcription by phosphorylated BMAL1:CLOCK heterodimer. The complex of unphosphorylated BMAL1:CLOCK with CRY may contain additional components and may traffic into the nucleus.
The PERIOD-1 (PER1) gene is transcribed to yield mRNA and the mRNA is translated to yield protein. The promoter of the PER1 gene contains E-boxes which are bound by the BMAL1:CLOCK heterodimer (and probably also the BMAL1:NPAS2 heterodimer). The BMAL1:CLOCK heterodimer activates transcription of PER1.
The DBP gene is transcribed to yield mRNA and the mRNA is translated to yield protein. As inferred from mouse, BMAL1:CLOCK heterodimers bind E-boxes in the DBP promoter and activate transcription of DBP.
The PERIOD-2 (PER2) gene is transcribed to yield mRNA and the mRNA is translated to yield protein. The promoter of the PER2 gene contains an E-box which binds the BMAL1:CLOCK heterodimer (and probably also the BMAL1:NPAS2 heterodimer). The BMAL1:CLOCK heterodimer activates transcription of PER2.
The CRYPTOCHROME-1 (CRY1) gene is transcribed to yield mRNA and the mRNA is translated to yield protein. CRY1 mRNA and protein show circadian expression. The promoter of the CRY1 gene contains an E-box which is bound by the BMAL1:CLOCK heterodimer (and probably also the BMAL1:NPAS2 heterodimer), which activates transcription of CRY1.
Polyubiquitination of CRY proteins is directed by the FBXL3 component of SCF E3 ubiquitin ligase. The polyubiquitinated CRY proteins are recognized and degraded by the 26S proteasome. Degradation of CRY proteins occurs during the night and is necessary to allow new transcription of BMAL1:CLOCK/NPAS2 targets in the morning during the circadian cycle.
The NOCTURNIN gene is transcribed to yield mRNA and the mRNA is translated to yield protein. The NOCTURNIN gene shows circadian expression because the BMAL1:CLOCK heterodimer binds an E-box element in the NOCTURNIN promoter and activates transcription.
BMAL1 (ARNTL), CLOCK, and NPAS2 are basic helix-loop-helix transcription factors. In humans BMAL1 has been demonstrated to form a heterodimer with CLOCK. In mouse, BMAL1 can form a heterodimer with either CLOCK or NPAS2. By analogy with other basic helix-loop-helix proteins the basic domain binds DNA, in this case the E-box motif, and the helix-loop-helix domains interact to form the heterodimer. BMAL1 and CLOCK/NPAS2 are codependently phosphorylated by unknown kinases after dimerization. The phosphorylation enhances transactivation activity and is inhibited by PER:CRY complexes. Both CLOCK and NPAS2 are expressed in the suprachiasmatic nucleus of the hypothalamus and act redundantly there. The tissue distributions of CLOCK and NPAS2 do not entirely overlap, however. For example, NPAS2 but not CLOCK is found in forebrain.
Transcription of the BMAL1 (ARNTL) gene is controlled by ROR-alpha and REV-ERBA, both of which are targets of BMAL1:CLOCK/NPAS2 in mice and both of which compete for the same element (RORE) in the BMAL1 promoter. ROR-alpha activates transcription of BMAL1; REV-ERBA represses transcription of BMAL1. This mutual control forms a secondary, reinforcing loop of the circadian clock. REV-ERBA shows strong circadian rhythmicity and confers circadian expression on BMAL1.
BMAL1 can form heterodimers with either CLOCK or NPAS2, which act redundantly but show different tissue specificity. The BMAL1:CLOCK and BMAL1:NPAS2 heterodimers activate a set of genes that possess E-box elements (consensus CACGTG) in their promoters. This confers circadian expression on the genes. The PER genes (PER1, PER2, PER3) and CRY genes (CRY1, CRY2) are among those activated by BMAL1:CLOCK and BMAL1:NPAS2. PER and CRY mRNA accumulates during the morning and the proteins accumulate during the afternoon. PER and CRY proteins form complexes in the cytosol and these are bound by either CSNK1D or CSNK1E kinases which phosphorylate PER and CRY. The phosphorylated PER:CRY:kinase complex is translocated into the nucleus due to the nuclear localization signal of PER and CRY. Within the nucleus the PER:CRY complexes bind BMAL1:CLOCK and BMAL1:NPAS2, inhibiting their transactivation activity and their phosphorylation. This reduces expression of the target genes of BMAL1:CLOCK and BMAL1:NPAS2 during the afternoon and evening.
PER:CRY complexes also traffic out of the nucleus into the cytosol due to the nuclear export signal of PER. During the night PER:CRY complexes are polyubiquitinated and degraded, allowing the cycle to begin again. Phosphorylated PER is bound by Beta-TrCP1, a cytosolic F-box type component of some SCF E3 ubiquitin ligases. CRY is bound by FBXL3, a nucleoplasmic F-box type component of some SCF E3 ubiquitin ligases. Phosphorylation of CRY1 by Adenosine monophosphate-activated kinase (AMPK) enhances degradation of CRY1. PER and CRY are subsequently polyubiquitinated and proteolyzed by the 26S proteasome.
The circadian clock is cell-autonomous and some, but not all cells of the body exhibit circadian rhythms in metabolism, cell division, and gene transcription. The suprachiasmatic nucleus (SCN) in the hypothalamus is the major clock in the body and receives its major input from light (via retinal neurons) and a minor input from nutrient intake. The SCN and other brain tissues determine waking and feeding cycles and influence the clocks in other tissues by hormone secretion and nervous stimulation. Independently of the SCN, other tissues such as liver receive inputs from signals from the brain and from nutrients.
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CLOCK/NPAS2 CRY
PERCLOCK/NPAS2
CRYPER
Kinasep-CLOCK/NPAS2
DNAp-PER
Kinasep-PER
KinaseAnnotated Interactions
CRY and PER proteins are themselves transcriptionally activated by BMAL1:CLOCK/NPAS2 thus they participate in a negative loop inhibiting their own synthesis and the synthesis of other targets of BMAL1:CLOCK/NPAS2.
Experiments with two-hybrid interactions and in vitro associations show that CRY1, CRY2, and PER2 bind BMAL1 at two different sites on BMAL1. PER2 but not CRY1 or CRY2 binds CLOCK. Different combinations of PER and CRY proteins in PER:CRY complexes have different inhibitory activities.
PER proteins contain a nuclear localization sequence and a nuclear export sequence allowing their movement into and out of the nucleus. Phosphorylation is required for transit of PER:CRY:kinase complexes into the nucleus and for interaction of PER proteins with the ubiquitin-mediated degradation process in the cytoplasm.
A mutation at Serine662 of PER2 is responsible for familial advanced phase sleep syndrome, however the particular kinase responsible for phosphorylating Serine662 is unknown.
p-CLOCK/NPAS2
DNAp-CLOCK/NPAS2
DNAp-CLOCK/NPAS2
DNAp-CLOCK/NPAS2
DNAp-CLOCK/NPAS2
DNAp-CLOCK/NPAS2
DNAp-CLOCK/NPAS2
DNAp-CLOCK/NPAS2
DNAp-CLOCK/NPAS2
DNAp-CLOCK/NPAS2
DNAp-CLOCK/NPAS2
DNAp-CLOCK/NPAS2
DNAp-CLOCK/NPAS2
DNAp-CLOCK/NPAS2
DNAp-PER
Kinase