Clock protein

Further extensions of the model are required to address the dynamical consequences of these additional regulatory loops and of the indirect nature of the negative feedback on gene expression. Such extended models have been proposed for Drosophila [112, 113] and mammals [113]. The model for the circadian clock mechanism in mammals is schematized in Fig. 3C. The presence of additional mRNA and protein species, as well as of multiple complexes formed between the various clock proteins, complicates the model, which is now governed by a system of 16 or 19 kinetic equations. Sustained or damped oscillations can occur in this model for parameter values corresponding to continuous darkness. As observed in the experiments on the mammalian clock. Email mRNA oscillates in opposite phase with respect to Per and Cry mRNAs [97]. The model displays the property of entrainment by the ED cycle... 

Knowledge of the detailed mechanism underlying circadian rhythms continues to be refined as new experiments reveal novel facets of the oscillatory machinery. Thus, a link has recently been established between chromatin structure and the circadian oscillatory mechanism. The CLOCK protein indeed functions as a histone acetyltransferase [116]. This enzyme activity is required for oscillations so that histone modification and the associated chromatin remodeling are implicated in the origin of circadian rhythmicity. 

Recent experimental studies have uncovered a direct link between the cell cycle and circadian rhythms. Thus, the circadian clock protein BMALl induces the expression of the gene Weel, which codes for the protein kinase that inactivates through phosphorylation the kinase cdkl that controls the G2/M transition [149]. This link allows the coupling of cell division to the circadian clock and explains how the latter may entrain the cell cycle clock in a variety of cell types. 

Yagita K, Tamanini F, Yasuda M, Hoeijmakers JHJ, van der Horst GTJ, Okamura H 2002 Nucleocytoplasmic shuttling and mCRY-dependent inhibition of ubiquitylation of the mPER2 clock protein. EMBO J 21 1301—1314... 

Sehgal Chuck Weitz said something about the SCN being sufficient to drive activity rhythms. What about the cycling clock proteins in the motor cortex ... 

Sehgal What I am getting at is whether cycling clock proteins are required in the motor cortex for activity rhythms to be generated. 

Ishida We have the same data. If we feed Clock homozygote mutant mice during the daytime, the rhythmic expression of Clock is completely entrained as indicated by Per2 and Email expression patterns in the absence of clock protein. 

Kosbash The Clock mutant still has some clock protein. But that s a pertinent result, for sure. 

Roshash In the double Cry knockouts, what are the levels of the clock proteins such as PER, relative to a wild-type oscillatory cycle ... 

Weit W e have looked at most of the known clock proteins and find that there is no overlap at all between melanopsin immunoreactivity and immunoreactivity for various circadian clock proteins. It looks hke the melanopsin-containing cells do not express any clock proteins. 

Sehgal Why can t you have the clock proteins controlled post-transcriptionally Under all these conditions, they are capable of effecting transcription of downstream genes, just not their own because their promoters don t exist. 

There is growing evidence that clock proteins are regulated dynamically in both temporal (production and degradation) and spatial (nuclear and cytoplasmic) dimensions. The phosphorylation of mPERl and mPER2 by casein kinase le (CKIe) is known as an important step for the accumulation of negatively active clock proteins (Lowrey et al 2000) as in Drosophila (Kloss et al 1998). 

Garceau NY, Liu Y, Loros JJ, Dunlap JC 1997 Alternative initiation of translation and time-specific phosphorylation yield multiple forms of the essential clock protein FREQUENCY. Cell 89 469-476... 

Liu Y, Loros J, Dunlap JC 2000 Phosphorylation of the Neurospora clock protein FREQUENCY determines its degradation rate and strongly influences the period length of the circadian clock. Proc Natl Acad Sci USA 97 234—239 Loros JJ, Dunlap JC 2001 Genetic and molecular analysis of circadian rhythms in Neurospora. Annu Rev Physiol 63 757—794... 

Yang Y, Cheng P, Zhi G, Liu Y 2001 Identification of a calcium/calmodulin-dependent protein kinase that phosphorylates the Neurospora clock protein FREQUENCY. J Biol Chem 276 41064-41072... 

FIG. 1. Expression of clock proteins in mouse SCN. (a) Immunostaining of mouse SCN sampled at beginning (CTO) and end (CT12) of circadian day reveals constitutive expression of mCLOCK, and rhythmic expression of mCRY (scale bar 500 /tm). Inset high power confocal views of mCLOCK-ir and Hoescht DNA stain confirm nuclear localization of mCLOCK-ir. 

Koshash In the simple experiments of the clock protein profiles in the wild-type, do you see PER build up cytoplasmically before you see a nuclear PER ... 

Lee C, Bae K, Edery I 1998 The Drosophila CLOCK protein undergoes daily rhythms in abundance, phosphorylation and interactions with the PER-TIM complex. Neuron 4 857-867... 

FIG. 1. Rhythmic CLOCK protein is also important like its rhythmic mRNA expression. The circadian expression of clock gene products such as PER and TIM in Drosophila is thought to be important for driving overt rhythms. Several reports showed that the constitutive expression ofper or ... 

Fig. 5). The phosphorylation and proteasome degradation of circadian clock proteins may play an important role in maintaining the circadian clock even in humans. 

FIG. 5. Role of phosphorylation and degradation of clock protein PER in human normal fibroblasts. After the phosphorylation of hPERl by casein kinase, the ubiquitin-proteosome pathway may be involved in its degradation in human cells. 

Camacho F, Cilio M, Guo Y et al 2001 Human casein kinase Idelta phosphorylation of human circadian clock proteins period 1 and 2. FEES Lett 489 159-165 Delaunay F, Thisse C, Marchand O, Laudet V, Thisse B 2000 An inherited functional circadian clock in zebrafish embryos. Science 289 297-300 Dunlap J 1998 Circadian rhythms. An end in the beginning. Science 280 1548-1549 Ebisawa T, Uchiyama M, Kajimura N et al 2001 Association of structural polymorphisms in the human period3 gene with delayed sleep phase syndrome. EMBO Rep 2 342-346 Edery I, Zwiebel LJ, Dembinska ME, Rosbash M 1994 Temporal phosphorylation of the Drosophila period protein. Proc Natl Acad Sci USA 91 2260-2264 Ishida N, Kaneko M, AUada R 1999 Biological clocks. Proc Natl Acad Sci USA 96 8819-8820... 

Unhke other clock proteins, neither DBT/CKl nor SGG/GSK3 is rhythmically expressed. Their substrates, however, are rhythmically phosphorylated throughout the circadian day. How, then, are the activities of these kinases regulated ... 

Kume K, Zylka MJ, Sriram S et al 1999 mCRYl and mCRY2 are essential components of the negative limb of the circadian clock feedback loop. Cell 98 193-205 Lee C, Bae K, Edery I 1998 The Drosophila CLOCK protein undergoes daily rhythms in abundance, phosphorylation, and interactions with the PER-TIM complex. Neuron 21 857-867... 

Naidoo N, Song W, Hunter-Ensor M, Sehgal A 1999 A role for the proteasome in the light response of the timeless clock protein. Science 285 1737-1741 Panda S, Hogenesch JB, Kay SA 2002 Circadian rhythms from flies to human. Nature 417 ... 

Shafer OT, Rosbash M, Truman JW 2002 Sequential nuclear accumulation of the clock proteins Period and Timeless in the pacemaker neurons of Drosophila melanogaster. J Neurosci 22 5946-5954... 

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