timeless: Biological Overview | Evolutionary Homologs | Regulation | Protein Interactions and Post-transcriptional Regulation | Developmental Biology | Effects of Mutation | References

Gene name - timeless

Synonyms -

Cytological map position - 23F5-6

Function - transcription factor regulator of photoperiod

Keywords - neural - photoperiod response

Symbol - tim

FlyBase ID:FBgn0014396

Genetic map position -

Classification - novel protein

Cellular location - nuclear and cytoplasmic

NCBI links: Precomputed BLAST | Entrez Gene

Recent literature

Montelli, S., Mazzotta, G., Vanin, S., Caccin, L., Corra, S., De Pitta, C., Boothroyd, C., Green, E. W., Kyriacou, C. P. and Costa, R. (2015). period and timeless mRNA splicing profiles under natural conditions in Drosophila melanogaster. J Biol Rhythms 30: 217-227. PubMed ID: 25994101.
Analysis of Drosophila circadian behavior under natural conditions has revealed a number of novel and unexpected features. This study focused on the oscillations of per and tim mRNAs and their posttranscriptional regulation and observe significant differences in molecular cycling under laboratory and natural conditions. In particular, robust per mRNA cycling from fly heads is limited to the summers, whereas tim RNA cycling is observed throughout the year. When both transcripts do cycle, their phases are similar, except for the very warmest summer months. The natural splicing profiles of per and tim transcripts were also studied and a clear relationship was observed between temperature and splicing. In natural conditions, the relationship between accumulation of the perspliced variant, low temperature, and the onset of the evening component of locomotor activity, first described in laboratory conditions, was also confirmed. Intriguingly, in the case of tim splicing, the opposite relationship was observed, with timspliced expression increasing at higher temperatures. A first characterization of the 4 different TIM protein isoforms (resulting from the combination of the natural N-terminus length polymorphism and the C-terminus alternative splicing) using the 2-hybrid assay showed that the TIMunspliced isoforms have a stronger affinity for CRY, but not for PER, suggesting that the tim 3' splicing could have physiological significance, possibly in temperature entrainment and/or adaptation to seasonal environments.

Tataroglu, O., Zhao, X., Busza, A., Ling, J., O'Neill, J. S. and Emery, P. (2015). Calcium and SOL protease mediate temperature resetting of circadian clocks. Cell 163: 1214-1224. PubMed ID: 26590423
Circadian clocks integrate light and temperature input to remain synchronized with the day/night cycle. Although light input to the clock is well studied, the molecular mechanisms by which circadian clocks respond to temperature remain poorly understood. This study found that temperature phase shifts Drosophila circadian clocks through degradation of the pacemaker protein Tim. This degradation is mechanistically distinct from photic Cry-dependent Tim degradation. Thermal Tim degradation is triggered by cytosolic calcium increase and Calmodulin binding to Tim and is mediated by the atypical calpain protease Sol. This thermal input pathway and Cry-dependent light input thus converge on Tim, providing a molecular mechanism for the integration of circadian light and temperature inputs. Mammals use body temperature cycles to keep peripheral clocks synchronized with their brain pacemaker. Interestingly, downregulating the mammalian Sol homolog SOLH blocks thermal mPER2 (see Drosophila Per) degradation and phase shifts. Thus, it is proposed that circadian thermosensation in insects and mammals share common principles.

Kidd, P. B., Young, M. W. and Siggia, E. D. (2015). Temperature compensation and temperature sensation in the circadian clock. Proc Natl Acad Sci U S A 112: E6284-6292. PubMed ID: 26578788
All known circadian clocks have an endogenous period that is remarkably insensitive to temperature, a property known as temperature compensation, while at the same time being readily entrained by a diurnal temperature oscillation. Although temperature compensation and entrainment are defining features of circadian clocks, their mechanisms remain poorly understood. Most models presume that multiple steps in the circadian cycle are temperature-dependent, thus facilitating temperature entrainment, but then insist that the effect of changes around the cycle sums to zero to enforce temperature compensation. An alternative theory proposes that the circadian oscillator evolved from an adaptive temperature sensor: a gene circuit that responds only to temperature changes. This theory implies that temperature changes should linearly rescale the amplitudes of clock component oscillations but leave phase relationships and shapes unchanged. This study shows using timeless luciferase reporter measurements and Western blots against Timeless protein that this prediction is satisfied by the Drosophila circadian clock. Evidence is reviewed for pathways that couple temperature to the circadian clock; previously unidentified evidence is shown for coupling between the Drosophila clock and the heat-shock pathway.

Fischer, R., Helfrich-Forster, C. and Peschel, N. (2016). GSK-3 β does not stabilize Cryptochrome in the circadian clock of Drosophila. PLoS One 11: e0146571. PubMed ID: 26741981
Cryptochrome (CRY) is the primary photoreceptor of Drosophila's circadian clock. It resets the circadian clock by promoting light-induced degradation of the clock protein Timeless (TIM) in the proteasome. Under constant light, the clock stops because TIM is absent, and the flies become arrhythmic. In addition to TIM degradation, light also induces CRY degradation. This depends on the interaction of CRY with several proteins such as the E3 ubiquitin ligases Jetlag (JET) and Ramshackle (BRWD3). However, CRY can seemingly also be stabilized by interaction with the kinase Shaggy (SGG), the GSK-3 beta fly orthologue. Consequently, flies with SGG overexpression in certain dorsal clock neurons are reported to remain rhythmic under constant light. This study investigated the interaction between CRY, Ramshackle and SGG and started to perform protein interaction studies in S2 cells. Surprisingly, it was not possible to replicate the results that SGG overexpression does stabilize CRY, neither in S2 cells nor in the relevant clock neurons. SGG rather does the contrary. Furthermore, flies with SGG overexpression in the dorsal clock neurons became arrhythmic as did wild-type flies. Nevertheless, the published interaction of SGG with TIM was reproducible, since flies with SGG overexpression in the lateral clock neurons shortened their free-running period. It is concluded that SGG does not directly interact with CRY but rather with TIM. Furthermore, it was demonstrated that an unspecific antibody explains the observed stabilization effects on CRY.

Chen, X. and Rosbash, M. (2016). mir-276a strengthens Drosophila circadian rhythms by regulating timeless expression. Proc Natl Acad Sci U S A [Epub ahead of print]. PubMed ID: 27162360
Circadian rhythms in metazoan eukaryotes are controlled by an endogenous molecular clock. It functions in many locations, including subsets of brain neurons (clock neurons) within the central nervous system. Although the molecular clock relies on transcription/translation feedback loops, posttranscriptional regulation also plays an important role. This study shows that the abundant Drosophila melanogaster microRNA mir-276a regulates molecular and behavioral rhythms by inhibiting expression of the important clock gene timeless (tim). Misregulation of mir-276a in clock neurons alters tim expression and increases arrhythmicity under standard constant darkness (DD) conditions. mir-276a expression itself appears to be light-regulated because its levels oscillate under 24-h light-dark (LD) cycles but not in DD. mir-276a is regulated by the transcription activator Chorion factor 2 in flies and in tissue-culture cells. Evidence from flies mutated using the clustered, regularly interspaced, short palindromic repeats (CRISPR) tool shows that mir-276a inhibits tim expression: Deleting the mir-276a-binding site in the tim 3' UTR causes elevated levels of TIM and approximately 50% arrhythmicity. It is suggested that this pathway contributes to the more robust rhythms observed under light/dark LD conditions than under DD conditions.

Top, D., Harms, E., Syed, S., Adams, E. L. and Saez, L. (2016). GSK-3 and CK2 kinases converge on Timeless to regulate the master clock. Cell Rep [Epub ahead of print]. PubMed ID: 27346344
The molecular clock relies on a delayed negative feedback loop of transcriptional regulation to generate oscillating gene expression. Although the principal components of the clock are present in all circadian neurons, different neuronal clusters have varying effects on rhythmic behavior, suggesting that the clocks they house are differently regulated. Combining biochemical and genetic techniques in Drosophila, this study identified a phosphorylation program native to the master pacemaker neurons that regulates the timing of nuclear accumulation of the Period/Timeless repressor complex. GSK-3/SGG binds and phosphorylates Period-bound Timeless, triggering a CK2-mediated phosphorylation cascade. Mutations that block the hierarchical phosphorylation of Timeless in vitro also delay nuclear accumulation in both tissue culture and in vivo and predictably change rhythmic behavior. This two-kinase phosphorylation cascade is anatomically restricted to the eight master pacemaker neurons, distinguishing the regulatory mechanism of the molecular clock within these neurons from the other clocks that cooperate to govern behavioral rhythmicity.
Tapanainen, R., Parker, D. J. and Kankare, M. (2018). Photosensitive alternative splicing of the circadian clock gene timeless is population specific in a cold-adapted Fly, Drosophila montana. G3 (Bethesda). PubMed ID: 29472309
To function properly, organisms must adjust their physiology, behavior and metabolism in response to a suite of varying environmental conditions. One of the central regulators of these changes is organisms' internal circadian clock, and recent evidence has suggested that the clock genes are also important in the regulation of seasonal adjustments. In particular, thermosensitive splicing of the core clock gene timeless in a cosmopolitan fly, Drosophila melanogaster, has implicated this gene to be involved in thermal adaptation. To further investigate this link, the splicing of timeless was examined in a northern malt fly species, Drosophila montana, which can withstand much colder climatic conditions than its southern relative. Northern and southern populations from two different continents (North America and Europe) were examined to find out whether and how the splicing of this gene varies in response to different temperatures and day lengths. Interestingly, it was found that the expression of timeless splice variants was sensitive to differences in light conditions, and while the flies of all study populations showed a change in the usage of splice variants in constant light compared to LD 22:2, the direction of the shift varied between populations. Overall, these findings suggest that the splicing of timeless in northern Drosophila montana flies is photosensitive, rather than thermosensitive and highlights the value of studying multiple species and populations in order to gain perspective on the generality of gene function changes in different kinds of environmental conditions.
Szabo, A., Papin, C., Cornu, D., Chelot, E., Lipinszki, Z., Udvardy, A., Redeker, V., Mayor, U. and Rouyer, F. (2018). Ubiquitylation dynamics of the clock cell proteome and TIMELESS during a circadian cycle. Cell Rep 23(8): 2273-2282. Pubmed ID: 29791839
Circadian clocks have evolved as time-measuring molecular devices to help organisms adapt their physiology to daily changes in light and temperature. Transcriptional oscillations account for a large fraction of rhythmic protein abundance. However, cycling of various posttranslational modifications, such as ubiquitylation, also contributes to shape the rhythmic protein landscape. An in vivo ubiquitin labeling assay was used to investigate the circadian ubiquitylated proteome of Drosophila melanogaster. Cyclic ubiquitylation affects Megator (Mtor), a chromatin-associated nucleoporin that, in turn, feeds back to regulate the core molecular oscillator. Furthermore, the ubiquitin ligase subunits Cullin-3 (Cul-3) and Supernumerary limbs Slbm) cooperate for ubiquitylating the Timeless protein. These findings stress the importance of ubiquitylation pathways in the Drosophila circadian clock and reveal a key component of this system.
Shakhmantsir, I., Nayak, S., Grant, G. R. and Sehgal, A. (2018). Spliceosome factors target timeless (tim) mRNA to control clock protein accumulation and circadian behavior in Drosophila. Elife 7. PubMed ID: 30516472
Transcription-translation feedback loops that comprise eukaryotic circadian clocks rely upon temporal delays that separate the phase of active transcription of clock genes, such as Drosophila period (per) and timeless (tim), from negative feedback by the two proteins. However, understanding of the mechanisms involved is incomplete. Through an RNA interference screen, this study found that pre-mRNA processing 4 (PRP4) kinase, a component of the U4/U5.U6 triple small nuclear ribonucleoprotein (tri-snRNP) spliceosome, and other tri-snRNP components regulate cycling of the molecular clock as well as rest:activity rhythms. Unbiased RNA-Sequencing uncovered an alternatively spliced intron in tim whose increased retention upon prp4 downregulation leads to decreased TIM levels. The splicing of tim is rhythmic with a phase that parallels delayed accumulation of the protein in a 24 hr cycle. It is proposed that alternative splicing constitutes an important clock mechanism for delaying the daily accumulation of clock proteins, and thereby negative feedback by them.
Grima, B., Papin, C., Martin, B., Chelot, E., Ponien, P., Jacquet, E. and Rouyer, F. (2019). Period-controlled deadenylation of the timeless transcript in the Drosophila circadian clock. Proc Natl Acad Sci U S A 116(12): 5721-5726. PubMed ID: 30833404
The Drosophila circadian oscillator relies on a negative transcriptional feedback loop, in which the Period (Per) and Timeless (Tim) proteins repress the expression of their own gene by inhibiting the activity of the Clock (Clk) and Cycle (Cyc) transcription factors. A series of posttranslational modifications contribute to the oscillations of the Per and Tim proteins but few posttranscriptional mechanisms have been described that affect mRNA stability. This study reports that down-regulation of the POP2 deadenylase, a key component of the CCR4-NOT deadenylation complex, alters behavioral rhythms. Down-regulating POP2 specifically increases Tim protein and tim mRNA but not tim pre-mRNA, supporting a posttranscriptional role. Indeed, reduced POP2 levels induce a lengthening of tim mRNA poly(A) tail. Surprisingly, such effects are lost in per (0) mutants, supporting a Per-dependent inhibition of tim mRNA deadenylation by POP2. This study reports a deadenylation mechanism that controls the oscillations of a core clock gene transcript.


The fly has a circadian cycle of activity; its activities respond to and are influenced by light (see reviews by Hardin, The Circadian Timekeeping System of Drosophila and Vallone, Start the clock! Circadian rhythms and development). How does a biological system regulate such photoperiod responses? The circadian clock is entrained (reset) by changes in the light regime. In the laboratory, the light cycle is 12 hours on, 12 off. At the beginning of the photoperiod day (lights on), TIM and PER (Period) RNA levels begin to rise. The regulation of transcription and nuclear localization in this two protein system is at the core of the photoperiod response. Regulation of PER nuclear entry is critical. Without Timeless, PER cannot enter the nucleus. Nuclear entry of PER takes place only during photoperiod night (lights off), and only when PER and TIM are combined as a protein heterodimer. When lights are on, both PER and TIM proteins are initially degraded; then the cycle begins anew. PER nuclear entry is also regulated by phosphorylation.

PER is unstable in timeless mutants, but TIM protein is stable in per mutants. Moreover, light induces loss of TIM without requiring PER, and therefore TIM light dependent cycling is independent of PER. This result suggests that TIM is responsible for coupling intracellular circadian cycles to light stimulation. If a light pulse is given in the middle of the night, TIM diminution is immediately followed by reaccumulation. This resets the molecular cycle of TIM to an earlier time point, and results in a phase delay in locomotor activity. The magnitude of the molecular phase shift corresponds with that of the behavioral shift (Myers, 1996).

A model for entrainment of the circadian pacemaker shows TIM protein along with PER declining at dawn, due to the light sensitive response of TIM. The drop in PER and TIM levels kindles the initiation of per and tim transcription during the day, and by dusk, PER and TIM mRNA has reached high enough levels to begin to yield significant TIM and PER protein levels. TIM nuclear localization depends on PER. In the middle of the night, nuclear PER and TIM reach sufficiently high concentration to begin suppressing PER and TIM mRNA accumulation, slowing accumulation of the messenger RNA. At daybreak, TIM light sensitivity results in the destruction of both TIM and its dimerization partner PER (Myers, 1996).

Two papers appearing in 1998, one by Yang et al. and another by Suri, et al., suggest that photic information is transduced in a cell autonomous fashion to the endogenous clock directly through the timeless gene product. This signal transduction takes place directly in the brain and does not involve light reception by the eyes.

In Drosophila, as in mammals, the eyes contain an autonomous oscillator that is thought to mediate an eye-specific function. However, the eyes appear to be generally dispensable for the generation of behavioral rhythms, as well as for their entrainment to light-dark cycles; thus, the existence of extraretinal photoreceptors is generally speculated. Nonetheless, there are data indicating defects in behavioral rhythms in at least one mutant that lacks eyes. The degree to which the visual transduction pathway is involved in circadian photoreception remains a major unanswered question. In Drosophila, visual transduction involves the sequential activation of a rhodopsin molecule, then signal transduction by a heterotrimeric G protein. This leads to the the involvement of phospholipase C, which activates the opening of ion channels resulting in a light-activated conductance composed mostly of calcium ions. norpA flies, containing a mutation in the eye-specific phospholipase C that results in vision blindness, display circadian rhythms that are 1 hr shorter than wild-type but otherwise entrain normally to light-dark cycles (Yang, 1998 and references).

In Drosophila, recent experiments suggest that photic information is transduced to the clock through the timeless gene product, Tim. Genetic and spectral evidence is provided supporting the relevance of Tim light responses to clock resetting. Since flies that lack eyes can, in general, entrain to light-dark cycles, one would expect that the lateral neurons in these mutants can still perceive light. However, an acute effect of light was never assayed in eyeless flies, leaving open the possibility that they might be impaired in this more sensitive test of a circadian light response. To determine whether external photoreceptor cells are required for nonparametric entrainment (entrainment to discrete pulses), the Tim response was assayed in the lateral neurons of sine oculisD (soD) flies, which lack eyes. As lateral neurons account for a very small part of the total Tim signal in the adult fly head, immunofluorescence assays, rather than Western blots, were used to assay soD flies. Flies were exposed to a 1 min light pulse of 2800 lux at ZT21 (nine hours after the beginning of the dark period) and were collected 90 min later. Lateral neurons were localized by costaining sections with antibodies to pigment-dispersing hormone (PDH). Light eliminates Tim expression in the lateral neurons of soD mutants, suggesting that acute circadian responses to light are maintained in eyeless flies (Yang, 1998).

To address the question of whether the circadian rhythm transduction mechanisms are similar to those used in the visual system, the Tim response to light was measured in visual transduction mutants. The trp gene, encoding a Ca2+ channel, is required for a light-activated conductance in the visual transduction cascade. Together with the product of the trpl gene, it mediates calcium conductance in photoreceptor cells. In the transient receptor potentialcm (trpcm) mutant, the TIM response to light is normal in photoreceptor cells but is reduced in lateral neurons: it is eliminated in some lateral neurons but persists in others.Double mutants that lack both trp and trpl gene products were obtained and the Tim response was assayed in photoreceptor cells and lateral neurons. In the double mutants, the Tim response to a 1 min pulse of 2800 lux is reduced in both cell types. A 10 min light pulse elicits an increased response in the double mutant, but it is still weaker than that seen in wild-type flies: staining in photoreceptor cells and lateral neurons is still visible in the mutants after the light treatment. The Tim response is also unaffected in ninaE flies that lack the rhodopsin protein (rh1). These results support the hypothesis that circadian entrainment does not rely on the visual system and likely involves a dedicated pathway for photoreception. (Yang, 1998)

The similar features of Per and Tim cycling are likely related to the fact that both proteins are present in a heterodimeric complex. Stimuli that phase shift the clock might cause rapid changes in one or more clock components, such as Per and Tim. Light is one such stimulus and affects several features of circadian oscillations: phase, period, and amplitude. Tim appears more closely connected to light than Per, because a decrease in Tim levels appears to be the first detectable response of a molecular clock component to acute light exposure. Tim levels respond to illumination even in arrhythmic per01 flies, i.e., in the absence of Per, and it has been proposed that a signal transduction pathway from a light receptor to Tim is an important part of the light-mediated entrainment pathway in Drosophila. In many organisms, the circadian light receptor is unknown. Dose-response curves and action spectra for light effects on circadian rhythms have been determined in plants and animals. During certain times of day, there is little or no response to light, whereas at other times there are pronounced delays or advances (dead zone, delay zone, and advance zone, respectively). Studies of the action spectrum for the eclosion delay zone phase shift of Drosophila pseudoobscura have shown that the action spectra for advance and delay phase shifts are very similar. The most effective wavelengths are between 420 and 480 nm, with a sharp decline above 550 nm; the eclosion clock is virtually insensitive to wavelength above 570 nm. This similarity suggests that both phase advances and phase delays are mediated by the same type of photoreceptor. The spectrum is significantly different from that of the major route of visual photoreception in Drosophila, which involves a rhodopsin species in all of the outer photoreceptors, suggesting that a different type of photopigment may be involved in the circadian gating of eclosion (Suri, 1998 and references).

During the past 25 years, D. melanogaster has generally replaced D. pseudoobscura as the most common experimental fly species for circadian rhythm research. Locomotor activity rhythms have also replaced adult eclosion as the most common Drosophila rhythm assay. This is due in part to the fact that the periods and responses to environmental stimuli of individual flies can be measured in the locomotor activity assays. But no action spectra have been reported for locomotor activity rhythms in D. melanogaster. Moreover, no action spectra data exist for the recently described light-mediated decrease in Tim levels. Since Tim is proposed to be the key clock molecule mediating light reception, the Tim disappearance spectrum should parallel the behavioral action spectrum. This is shown to be the case and the the results support the notion that the visual phototransduction pathway is dispensable for Tim degradation (Suri, 1998).

Per's expression pattern in the fly head has been extensively studied. It is present in photoreceptor cells, putative glia in various ganglia, and neurons in lateral as well as dorsal regions of the central brain. Expression in lateral neurons is probably necessary and may be sufficient for locomotor activity rhythms. Per undergoes robust cycling in all of these tissues as assayed by histochemistry. Although less well studied, the expression pattern of Tim is similar, as expected from the biochemical relationship between the two proteins. Multiple expression sites raise a problem for comparing the behavioral and biochemical action spectra: the former probably reflects changes in Tim levels in brain lateral neurons, whereas the latter largely reflects changes in Tim levels in eye photoreceptor cells (Suri, 1998).

Decrease in Tim levels in the eye is shown to be independent of brain pacemaker neurons. A comparison of the Western blot Tim signal from eyeless heads with that from wild-type flies indicates that in head extracts, about two-thirds of Tim comes from the eye (i.e., the photoreceptor cells). The similar action spectra for behavior and Tim level changes might therefore reflect a shared photoreceptor tissue, which detects light and relays the appropriate circadian information to the relevant tissues. Alternatively, these measurements might reflect the fact that the eye and pacemaker neurons in the CNS have independent circadian photoreceptors but share the same photopigment. The extreme version of this second possibility is that the Tim response to light is entirely cell autonomous and reflects a ubiquitous photoreceptor. To begin to address this question, the Tim degradation pattern was examined in disco flies. In this mutant strain, the lateral neurons are largely eliminated, ganglia normally located between the brain and the eyes are disrupted or eliminated, and free-running locomotor activity behavior is largely arrhythmic. This implies that signals that normally emanate from the pacemaker neurons are largely eliminated and that the same or perhaps a slightly greater fraction of Tim in disco extracts is still derived from eyes. To optimize sensitivity, Tim levels were examined as a function of intensity with 450 nm light. It was found that in both disco and wild-type genotypes, the Tim level decrease is a function of light intensity. There was little or no effect of disco, indicating that the pacemaker neurons have little or no effect on light-mediated decrease in Tim levels in the eye. This is consistent with the possibility that the Tim light response in photoreceptors may be cell autonomous; i.e., that the eye may contain all of the components necessary for a light-mediated decrease in TIM levels (Suri, 1998).

Genetic evidence is provided that light-mediated decrease in Tim levels is relevant to the effects of light on behavior. A missense mutant tim, TIM-SL, exhibits greater sensitivity to light in both Tim protein disappearance and locomotor activity phase shifting assays. Analysis of the dose response of Tim disappearance in a variety of mutant genotypes suggests cell-autonomous light responses that are largely independent of the canonical visual transduction pathway (Suri, 1998).

The simplest interpretation of these results is that all cycling tissues contain cell-autonomous oscillators with the same, as yet unidentified, photoreceptor. The data also suggest that the same oscillator tissues contain signal transduction components that impact rapidly on Tim. This interpretation is supported by a report of robust circadian cycling and light-mediated phase shifting in the Malpighian tubules of decapitated flies, where no cross-talk with the brain or eye is possible. It is also consistent with a more recent report demonstrating in vitro light sensitivity of a number of oscillating adult Drosophila tissues. The data indicate that Drosophila has a specialized circadian photoreceptor, and the action spectra suggest that flavin- and/or pterin-based molecules, such as cryptochromes, may be involved. Every oscillator tissue, perhaps every cycling cell, may contain the same machinery for light entrainment as well as for free-running rhythms. The machinery may include novel components for photoreception, signal transduction, protein processing, and gene regulation. This raises the exciting possibility that other components of the light-input pathway may be encoded by novel clock genes. The light effects on Tim levels should aid in the identification of the photoreceptor as well as the unknown signal transduction pathways that link it to the Per-Tim clock cycles (Suri, 1998).

Heat regulates the Drosophila biological clock, and the role of Period and Timeless in this process will be dealt with here. Circadian (approximately 24-h) rhythms are governed by endogenous biochemical oscillators (clocks) that in a wide variety of organisms can be phase shifted (i.e., delayed or advanced) by brief exposure to light and changes in temperature. However, it is not known how changes in temperature reset circadian timekeeping mechanisms. Cyclical change in locomotor activity is one circadian rhythm that is sensitive to temperature shift. For example, flies raised at 25 degreesC were placed at 37 degreesC for 30 minutes at 15 hours (T15) after the last dark to light transition (three hours after the beginning of the dark period). Their behavior was monitored for 7 to 10 days in constant darkness. Heat treatment delays peak locomotion by 2.3 hours from its normal peak (12.7 hours after the dark to light transition). To begin to address the biochemical basis of the behavioral change, the effects of short-duration heat pulses were measured on the protein and mRNA products from the Drosophila circadian clock genes period (per) and timeless (tim) (Sidote, 1998).

Heat treatment at T15 (15 hours after the last dark to light transition, that is 3 hours into the dark period) elicits the rapid disappearance of both Per and Tim proteins. Clear reductions are first observed between 3 to 5 min following the start of the 37 degree incubation, and essentially undetectable levels are reached after 10 to 15 min of heat treatment. Similar heat-induced decreases in the levels of both proteins were observed at all times in a daily cycle. No changes in the levels of either per or tim transcripts are detected during the first 20 min of the heat pulses, strongly suggesting that a posttranscriptional mechanism is solely responsible for mediating the heat-induced decreases in the levels of Per and Tim. The magnitude of the phase shift in the locomotor activity rhythm is also proportional to the temperature of the pulse, consistent with a causal relationship between the heat-induced degradation of Per, Tim, or both and phase resetting. It is thought that the majority of the heat-induced disappearance of Per and Tim is due to protein degradation. It is noteworthy that brief pulses at 37 degreesC elicit a full-blown heat shock response in D. melanogaster, raising the likelyhood that this pathway participates in mediating the enhanced degradation of Per and Tim. For example, thermally denatured proteins are prime targets for proteolysis by the ubiquitin-proteasome system (Sidote, 1998).

Per protein level is sensitive to heat but not light, indicating that individual clock components can markedly differ in sensitivity to environmental stimuli. A similar resetting mechanism involving delays in the per-tim transcriptional-translational feedback loop likely underlies the observation that when heat and light signals are administered in the early night, they both evoke phase delays in behavioral rhythms. Heat induces the degradation of Per and Tim independently, since the heat induced degradation of each protein takes place in flies mutant for the other protein. The results indicate that Per and Tim can be independently regulated by heat and that this degradation does not require a functional clock (Sidote, 1998).

The light-induced degradation of Tim in the late night is accompanied by stable phase advances in the temporal regulation of the Per and Tim biochemical rhythms (Per and Tim protein and phosphorylation levels). A 37 degree heat pulse at T15 evokes stable phase delays of several hours in the biochemical oscillations of Per and Tim, consistent with the magnitude and direction of the phase shift in locomotor activity rhythms produced by identical temperature treatments. The heat-induced delays in the temporal regulation of Per and Tim abundance and phosphorylation are stable for at least 2 days after the environmental perturbation (Sidote, 1998)

The initial heat-induced degradation of Per and Tim in the late night, unlike treatment in the early night, is followed by a transient and rapid increase in the speed of the Per-Tim temporal program. The net effect of these heat-induced changes results in an oscillatory mechanism with a steady-state phase similar to that of the unperturbed control situation; at the same time, there is little effect on locomotion. These findings can account for the lack of apparent steady-state shifts in Drosophila behavioral rhythms by heat pulses applied in the late night (Sidote, 1998).

An intriguing observation is that Per is sensitive to heat but not light, whereas Tim is sensitive to both stimuli. Thus, individual clock components can markedly differ in sensitivity to the two most important environmental entraining cues. Although not well studied, it is highly likely that under natural conditions a wide variety of organisms manifest circadian rhythms that are influenced by multiple temporal cues. In the case of Drosophila, it appears that the photic and heat signal transduction pathways converge at the level of regulating the stability of one or more key clock proteins. The observation that Per and Tim interact to form a functional complex that is involved in an autoregulatory circuit that is central to the timekeeping mechanism might ensure that the effects of light and temperature on individual clock proteins are combined into a coherent temporal cue resulting in daily rhythms that are optimally adapted to the precise local conditions. It will be of interest to determine whether other circadian timekeeping devices are assembled with components that differ in sensitivity to different environmental entraining cues (Sidote, 1998).

Sequential nuclear accumulation of the clock proteins Period and Timeless in the pacemaker neurons

Antisera against the circadian clock proteins Period (Per) and Timeless (Tim) were used to construct a detailed time course of Per and Tim expression and subcellular localization in a subset of the ventrolateral neurons (vLNs) in the Drosophila accessory medulla (AMe). These neurons, which express pigment-dispersing factor, play a central role in the control of behavioral rhythms. The data reveal several unexpected features of the circadian clock in Drosophila: (1) Tim but not Per is restricted to the cytoplasm of vLNs throughout most of the early night; (2) the timing of Tim and Per nuclear accumulation is substantially different; (3) the two subsets of vLNs, the large and small vLNs, have a similar timing of Per nuclear accumulation but differ by 3-4 hr in the phase of Tim nuclear accumulation. These aspects of Per and Tim expression were not predicted by the current mechanistic model of the circadian clock in Drosophila and are inconsistent with the hypothesis that Per and Tim function as obligate heterodimers. The differing profiles of Tim and Per nuclear accumulation suggest that Per and Tim have distinct functions in the nuclei of vLNs (Shafer, 2002).

One concern of an immunocytochemical study is that differences in staining intensity for two epitopes may reflect a disparity in antibody affinity rather than differences in epitope abundance. Attempts were made to allay this concern by using two to three different Per and Tim antibodies. Similar intensities and patterns of staining are seen for all of the antibodies used in this study, suggesting that differential antibody affinity is not responsible for the differences in staining observed for these two proteins. The best estimate for the relative abundance of Per and Tim has come from the analysis of whole-head extracts; the Tim/Per ratio is ~2:1 at ZT 16 and 1:1 by ZT 23. A variation of this magnitude in the vLNs would be consistent with the relative intensities of staining observed in this study. Given this transient disparity between Tim and Per abundance, the persistence of strong cytoplasmic Tim signals (at times when Per is predominantly nuclear) must not be considered as evidence that these proteins enter the nucleus sequentially. Rather, it is the differing profiles of Tim and Per immunoreactivity in the nuclei of the vLNs on which this conclusion is based (Shafer, 2002).

Several lines of evidence had suggested that Per and Tim are transported into nuclei as a dimeric complex. Biochemical assays in fly-head extracts show that Per and Tim are present as heterodimers throughout much of the night, but the data did not exclude the fact that a fraction of Per and Tim are monomeric or complexed with other proteins. Importantly, it is also uncertain what fraction of the Per/Tim heterodimers are present in vivo and what fraction form in vitro during extract preparation and analysis. Furthermore, these extracts reflect thousands of cells of many different types, including all of the photoreceptors from the compound eye. These contribute the majority of the Per and Tim to the homogenate, effectively swamping out the contribution of the vLNs. There is therefore no reason to expect that the biochemistry of the vLNs will be identical to what has been observed in whole-head extracts. In flies, little or no nuclear Per accumulation is seen without Tim, but this observation is likely attributable to Per instability and consequent low Per levels in mutants that lack Tim. The coexpression of Per and Tim has been shown to be required for the nuclear transport of both proteins in cultured S2 cells. Although these cells have not been shown to express Per and Tim rhythmically, this codependence fits well with the notion that Per and Tim enter the nucleus as a heterodimeric complex. Based on these considerations, it is expected that a time course study would reveal the simultaneous nuclear accumulation of Per and Tim in the vLNs. Instead, the data show that Per appears in nuclei at least 3 hr before Tim, indicating that most, and perhaps all, nuclear Per is not complexed with Tim at these times (Shafer, 2002).

In the large vLNs, Per is already nuclear as well as cytoplasmic when it is first detectable at ZT 16. In the large vLNs, nuclear Per is first detected at times that correlate well with the onset of repression in head extracts. This begins at approximately ZT 15-17 by biochemical criteria. The discrepancy between the timing of Per nuclear accumulation in the small cells and the onset of transcriptional repression in whole-head extracts might reflect differences between the cells and tissues that contribute to the biochemical data (Shafer, 2002).

In contrast to Per, Tim is not nuclear until after ZT 18. This difference with Per is especially prominent in the small cells, where Tim remains cytoplasmic until ZT 22. In both cell types, Tim is degraded in the early morning and is therefore nuclear for only a short time. In other words, Tim becomes nuclear well after transcriptional repression is thought to begin and is degraded well before the next cycle of transcription begins anew (Shafer, 2002).

A central feature of the molecular model of the Drosophila clock is that Per and Tim function as obligate heterodimers in their nuclear transport as well as in the repression of Clock/Cycle-based transcription. However, the different times of nuclear accumulation suggest that Per and Tim are transported independently to nuclei. The lack of high-resolution data for the Doubletime kinase makes it difficult to assess whether the Doubletime nuclear accumulation pattern correlates better with that of Per or of Tim. Importantly, these results do not preclude a role for Tim in the nuclear entry of Per. For example Tim could act catalytically in the cytoplasm to potentiate some required Per phosphorylation event. Moreover, Per and Tim could still enter nuclei as Per/Tim heterodimers, but this would require the subsequent export of Tim to the cytoplasm to explain the apparent absence of nuclear Tim during the middle of the night (Shafer, 2002).

In biochemical assays, Per and Tim are both capable of blocking per and tim transcription in the absence of the other, suggesting that repression does not require the Per/Tim heterodimer. The data indicate that little or no Tim is present in the nuclei of vLNs during times associated with repression in whole-head extracts, suggesting that Per might influence transcription independently of Tim. It has been suggested that Per is still capable of repressing transcription after a mutant form of Tim (TimUL) is degraded by light exposure. It has been concluded that Per can repress transcription in the absence of Tim and it has been suggested that Tim-independent repression by Per normally occurs after dawn. However, there was no reason to challenge the notion that the Per/Tim heterodimer is the agent of nuclear entry and the initiator of repression. The current data indicate that the Tim-independent repression by Per also applies to the onset of repression in the middle of the night. Tim may therefore play no direct role in the feedback repression of Per and Tim transcription (Shafer, 2002).

The persistence of Tim oscillations in the small vLNs but the absence of oscillations in the large vLNs under DD conditions has been noted. This observation has been confirmed, but these data extend this result in several important ways. Per was assayed as well as Tim and these two proteins were followed throughout the first DD cycle. This provides a finer temporal resolution and leads to a surprising conclusion: that the molecular oscillation in the large vLNs appears to arrest during the first subjective day. Based on the patterns of Per and Tim, these cells stop in a state that corresponds to ZT 8-10 under LD conditions (Shafer, 2002).

The individual features of Per and Tim distribution in the large and small vLNs can be added to a growing list of differences between these two classes of PDF-expressing neurons. Cell-specific differences in Per and Tim nuclear accumulation underscore the importance of studying the brain clock in situ and may presage an even more profound heterogeneity among other clock-containing neurons within the brain. This view also reflects the fact that the PDF-positive vLNs represent a minority of clock gene-expressing cells in the central brain. One might therefore expect that the circadian system will vary to an even greater degree among other clock neurons. Such diversity might offer clues as to how different groups of clock neurons interact to create the temporal complexity of behavioral rhythms (Shafer, 2002).

A key temporal delay in the circadian cycle of Drosophila is mediated by a nuclear localization signal in the timeless protein

Regulated nuclear entry of the Period (Per) and Timeless (Tim) proteins, two components of the Drosophila circadian clock, is essential for the generation and maintenance of circadian behavior. Per and Tim shift from the cytoplasm to the nucleus daily, and the length of time that Per and Tim reside in the cytoplasm is an important determinant of the period length of the circadian rhythm. This study identified a Tim nuclear localization signal (NLS) that is required for appropriately timed nuclear accumulation of both Per and Tim. Transgenic flies with a mutated Tim NLS produced circadian rhythms with a period of ~30 hr. In pacemaker cells of the brain, Per and Tim proteins rise to abnormally high levels in the cytoplasm of timδNLS mutants, but show substantially reduced nuclear accumulation. In cultured S2 cells, the mutant TIMδNLS protein significantly delays nuclear accumulation of both Tim and wild-type Per proteins. These studies confirm that Tim is required for the nuclear localization of Per and point to a key role for the Tim NLS in the regulated nuclear accumulation of both proteins (Saez, 2011).

This study identified a functional nuclear localization signal in the Tim protein. A role was demonstrated for this NLS in specifying the timing of nuclear accumulation for both Tim and its partner protein, Per. Modification of the Tim NLS impaired the nuclear entry of Per and Tim in vivo and produced an aberrant, but specific, delay in the onset of nuclear accumulation in living cultured cells. The cytoplasmic residence of Per and TimδNLS exceeds that of wild-type Per and Tim proteins by ~4 hr in S2 cells. A similar delay in vivo might be expected to extend period length to ~28 hr, which is close to the change in behavioral rhythmicity seen in tim0; timδNLS mutant flies. Although levels of nuclear accumulation were too low to be assessed directly in neural pacemaker cells (sLNV's) of tim0; timδNLS flies, the rhythmic behavior of these flies makes it likely that periodic Per/Tim δNLS nuclear accumulation also occurs in vivo. Alternatively, the long-period rhythms of tim0; timδNLS flies could be the result of Per and Tim activity in clock cells other than the sLNV’s (Saez, 2011).

The altered profiles of Per and TimδNLS nuclear accumulation do not appear to be due to a loss of physical association between the two proteins. As shown by inmmunoprecipitation and FRET analyses, Per and TimδNLS are capable of binding to each other and do so in cultured cells with kinetics that are similar to wild-type Per and Tim. Surprisingly, mutating the identified Tim NLS only delays the time of nuclear translocation in S2 cells. These results suggest that a sequence that poorly resembles a nuclear localization signal can promote nuclear entry in the absence of the known Tim NLS (Saez, 2011).

The contemporaneous movement of Tim and Per (or of TimδNLS and Per) to the nucleus in S2 cells suggests that the Tim NLS has an important role in determining the time of nuclear entry, but not the co-dependent nuclear accumulation of Per and Tim. That is, in the absence of the identified Tim NLS, although nuclear transfer is substantially delayed, the onset of nuclear accumulation is shifted for both the TimδNLSprotein and for wild-type Per expressed in the same cell. That wild-type Per does not accumulate in the nucleus with kinetics that are independent of TimδNLS indicates that other features of the Tim (and Per) proteins coordinate their timed subcellular movements. Prior studies have repeatedly shown that the presence of Tim significantly enhances the nuclear accumulation of Per. This tight coupling is most likely to reflect the physical association of Per and Tim as they accumulate in the cytoplasm and immediately prior to the time of their nuclear translocation. The NLS of Tim may participate in a mechanism that facilitates nuclear entry of both proteins. One possibility is that a specific importin recognizes the Tim NLS in cytoplasmic Per/Tim complexes and promotes an association of these complexes with the nuclear pore. As earlier studies have indicated that Per and Tim dissociate at the time of nuclear transfer, this model would require recognition of the Tim NLS prior to separation of the proteins, but could account for the similar timing of Per and Tim nuclear accumulation that has been observed in S2 cells. Possibly a Per NLS or alternate segment of the Tim protein can promote a delayed association with the nuclear pore in timδNLS mutants (Saez, 2011).

In summary, these studies have confirmed a role for a specific Tim protein sequence in the regulated nuclear accumulation of Tim and Per. Mutation of the Tim NLS suppressed nuclear accumulation of Per and Tim in neural pacemaker cells of Drosophila and lengthened the period of molecular and behavioral rhythms in these flies to ~30 hr. The same timδNLS mutation also affected nuclear accumulation of Per and Tim when expressed in cultured Drosophila cells and caused delays in the onset of Per/Tim nuclear accumulation that are well correlated with the long-period circadian rhythms of timδNLS flies (Saez, 2011).


Amino Acids - 1389

Structural Domains

TIM protein has a central acidic region followed by a basic region. The basic region has been shown to interact with the PAS domain of PER (Meyers, 1995).


The complete sequence of a tim gene is reported from Drosophila virilis. Between D. melanogaster and D. virilis, TIM is better conserved than is the PER protein (76 vs. 54% overall amino acid identity); putative functional domains, such as the PER interaction domains and the nuclear localization signal, are highly conserved. The acidic domain and the cytoplasmic localization domain, however, are within the least conserved regions. In addition, the initiating methionine in the D. virilis tim gene lies downstream of the proposed translation start for the original D. melanogaster tim cDNA and corresponds to the one used by D. simulans and D. yakuba. Among the most conserved parts of TIM is a region of unknown function near the N terminus. Deletion of a 32 amino acid segment within this region affects rescue of rhythms in arrhythmic tim01 flies. Flies carrying a full-length tim transgene display rhythms with ~24-hr periods, indicating that a fully functional clock can be restored in tim01 flies through expression of a tim transgene. Deletion of the conserved segment results in very long activity rhythms with periods ranging from 30.5 to 48 hr (Ousleya, 1998).

The clock genes period and timeless of Drosophila encode cardinal components of the fly's endogenous clock. Their mRNAs and proteins oscillate in abundance with a lag of about 4-6 hours between their respective peaks in numerous cells throughout the Drosophila body and brain. Per and Tim proteins participate in a negative autoregulatory feedback loop, providing a relentless circadian molecular oscillation. A mammalian tim homolog appears to play a minor role in the mammalian clock. In addition, and in contrast to Drosophila, in which a null mutation is viable, a knockout of murine tim (mmTim) is lethal, revealing an essential requirement for mmTim in development. These discrepancies between the tim genes of the two taxa prompted an investigation of the possible existence of paralogs of the tim gene. Southern hybridization of Musca domestica genomic DNA with a D. melanogaster tim probe indicated the possible presence of two tim related sequences. Furthermore, partial sequence data from a Bombyx mori tim-like cDNA clone revealed an unexpectedly higher similarity with mammalian tim compared to Drosophila tim. A database search of the recently completed Drosophila genome was launched, and a similar sequence in the fly was identified using the human TIM (hsTIM) protein for comparison. Subsequent RT-PCR from an Oregon-R strain, followed by DNA sequencing, confirmed the existence of a corresponding 4.3 kb transcript, which encodes a putative protein of 1384 residues. The gene, which has been called timeless2 (tim2; GenBank accession number AF279586), spans 75 kb of genomic DNA and is located on chromosome 3R (87E1-87E3), adjacent to the single-minded (sim) gene. The two proteins show 12% identity and 22% similarity. The corresponding hsTIM and dmTIM2 comparison has 29% identity and 44% similarity. The amino terminus is clearly more conserved than the carboxyl terminus and includes some of the regions of Tim that are proposed to interact with Per proteins. A phylogeny does not indicate when the duplication of tim arose. The absence of more than a single copy of a tim2-like sequence in the Caenorhabditis elegans databases, suggests that tim2 may be the ancestral gene from which tim1 originated after the split between the arthropod and nematode lineages. On the basis of the phylogeny, however, it cannot be ruled out that duplication occurred at an earlier stage, and that the tim1 gene has simply been lost in the worm. If the duplication occurred before the divergence of vertebrates and arthropods, the existence of tim1 genes might be expected in mammals. If it occurred afterwards, then mammals would be predicted to have only tim2 type genes. In mammals, the duplicated mPer and mCry (cryptochrome) genes, have largely taken on the roles that tim1 plays in the fly clock. This in itself could suggest that tim1 genes may not be present in mammals, thereby indirectly favouring the scenario in which the duplication of tim is a relatively recent event, largely confined to the arthropods. Consequently, tim1 may have been freed from its original function and evolved a more central role in the insect clockworks. This could explain the long branch lengths for the insect tim1 genes in the phylogeny, which might reflect a rapid evolutionary rate. Further comparative analysis should clarify the evolutionary history of the tim gene family (Benna, 2000).

Mouse (mTim) and human (hTIM) orthologs of the Drosophila timeless (dtim) gene have been cloned and characterized. The mammalian Tim genes are widely expressed in a variety of tissues; however, unlike Drosophila, mTim mRNA levels do not oscillate in the suprachiasmatic nucleus (SCN) or retina. Importantly, hTIM interacts with the Drosophila PERIOD (dPER) protein as well as the mouse PER1 and PER2 proteins in vitro. In Drosophila (S2) cells, hTIM and dPER interact and translocate into the nucleus. Finally, hTIM and mPER1 specifically inhibit CLOCK-BMAL1-induced transactivation of the mPer1 promoter. Taken together, these results demonstrate that mTim and hTIM are mammalian orthologs of timeless and provide a framework for a basic circadian autoregulatory loop in mammals (Sangoram, 1998).

The mouse cDNA of a mammalian homolog of the Drosophila timeless gene has been isolated. The mTim protein shows five homologous regions with Drosophila TIM. The first conserved region (C1) encompasses the amino-terminal regions of both the mouse and fly proteins. Between C1 and C2 of dTIM, there is a stretch of 223 residues not found in mTIM. mTIM appears to lack the 5' half of the first PER interactive domain defined in dTIM. The second PER interactive domain (IAD-2) of dTIM is present in C2-C4 of mTIM. In mTIM, however, this domain is interrupted by two long stretches of amino acids not present in dTIM. Between C4 and C5 of dTIM, there is a stretch of 175 amino acids not found in mTIM. C5 represents a small area in the carboxyl end of mTIM that is highly conserved among dTIM and mTIM and also in silkmoth TIM. Within the nonconserved region between C2 and C3 of mTIM, there is a stretch of 10 basic amino acids and a stretch of 11 acidic amino acids. An acidic region resides in the nonconserved region of dTIM between C1 and C2. No motifs of structural significance are detected in the mTIM protein. mTim is weakly expressed in the suprachiasmatic nuclei (SCN) but exhibits robust expression in the hypophyseal pars tuberalis (PT). mTim RNA levels do not oscillate in the SCN nor are they acutely altered by light exposure during subjective night. mTim RNA is expressed at low levels in several peripheral tissues, including eyes, and is heavily expressed in spleen and testis. Yeast two-hybrid assays reveal an array of interactions between the various mPER proteins but no mPER-mTIM interactions. The data suggest that PER-PER interactions have replaced the function of PER-TIM dimers in the molecular workings of the mammalian circadian clock. Since mTim is expressed in the SCN and eyes, it is still possible that mTIM has a clock-relevant function but that its function is distinct from that described for dTIM. It is also conceivable that an mTIM homolog other that the one characterized here might exist that interacts with the mPER proteins (Zylka, 1998).

The timeless (tim) gene is essential for circadian clock function in Drosophila. A putative mouse homolog, mTimeless (mTim), has been difficult to place in the circadian clock of mammals. Although mTim is expressed in the SCN, neither its RNA nor protein levels oscillate there under constant conditions. Moreover, mTIM is a nuclear protein, and SCN levels are not altered by light pulses that shift circadian behavior. Mammalian TIM does not interact with the mPER proteins in yeast or in the SCN, but it is reported to interact with mPER1 in COS-7 cells and with Drosophila Per in vitro. Immunofluorescence studies of over-expressed proteins in cell culture fail to show changes in cellular location of mPER proteins when coexpressed with mTIM. mTIM interacts with mCRY1 and mCRY2, both in vivo and in vitro. Here it is shown that mTim is essential for embryonic development, but does not have substantiated circadian function. At embryonic day 7.5, presumptive homozygous embryos lack cellular organization, with necrotic cell debris filling the amniotic cavity, and resorption by maternal tissues has already begun. Developmental abnormalities are observed in embryos as early as embryonic day 5.5, indicating that mTim is essential for development around implantation. The mechanism behind the essential role of mTIM for mouse development is currently not known. At embryonic day 7.5, in situ hybridization shows that mTim RNA is expressed throughout the embryo, particularly in the embryonic germ cell layers and in the ectoplacental cone (Gotter, 2000).

Faithful transmission of the genome requires that a protein complex called cohesin establishes and maintains the regulated linkage between replicated chromosomes before their segregation. This study reports the unforeseen participation of C. elegans TIM-1, a paralog of the Drosophila clock protein Timeless, in the regulation of chromosome cohesion. Biochemical experiments defined the C. elegans cohesin complex and revealed its physical association with TIM-1. Functional relevance of the interaction was demonstrated by aberrant mitotic chromosome behavior, embryonic lethality and defective meiotic chromosome cohesion caused by the disruption of either TIM-1 or cohesin. TIM-1 depletion prevents the assembly of non-SMC (structural maintenance of chromosome) cohesin subunits onto meiotic chromosomes; however, and unexpectedly, a partial cohesin complex composed of SMC components still loads. Further disruption of cohesin activity in meiosis by the simultaneous depletion of TIM-1 and an SMC subunit decreases homologous chromosome pairing before synapsis, revealing a new role for cohesin in metazoans. On the basis of comparisons between Timeless homologs in worms, flies and mice, it is proposed that chromosome cohesion, rather than circadian clock regulation, is the ancient and conserved function for Timeless-like proteins (Chan, 2003).

Given the vital role of C. elegans tim-1 in chromosome cohesion and the essential function of mouse mTim1 in embryo viability, it is suggested that the evolution of the involvement of timeless in the Drosophila circadian rhythm results from a duplication and subsequent specialization of the fly timeout gene, which more closely resembles the single paralog present in C. elegans, mice and humans. Comparison of predicted protein structures reinforces this view. Drosophila Timeout, C. elegans TIM-1 and mouse mTim1 all have contiguous HEAT/Armadillo (Arm) repeats that reside within a similar region of each protein. In contrast, the HEAT/Arm repeats of Drosophila Timeless are interrupted by one of two Period-interaction domains, indicating that this change could enhance the association between Period and Timeless. Given the unique structure of Drosophila Timeless and the conclusions of the present study, it is proposed that chromosome cohesion, rather than circadian rhythm regulation, is the ancient and conserved function for timeless paralogs in metazoans (Chan, 2003).

The Timeless protein is essential for circadian rhythm in Drosophila. The Timeless orthologue in mice is essential for viability and appears to be required for the maintenance of a robust circadian rhythm as well. The human Timeless protein interacts with both the circadian clock protein cryptochrome 2 and with the cell cycle checkpoint proteins Chk1 and the ATR-ATRIP complex and plays an important role in the DNA damage checkpoint response. Down-regulation of Timeless in human cells seriously compromises replication and intra-S checkpoints, indicating an intimate connection between the circadian cycle and the DNA damage checkpoints that is in part mediated by the Timeless protein (Unsal-Kacmaz, 2005).

An unexpected role for the clock protein timeless in developmental apoptosis

Programmed cell death is critical not only in adult tissue homeostasis but for embryogenesis as well. One of the earliest steps in development, formation of the proamniotic cavity, involves coordinated apoptosis of embryonic cells. Recent work has demonstrated that c-Src protein-tyrosine kinase activity triggers differentiation of mouse embryonic stem (mES) cells to primitive ectoderm-like cells. In this report, Timeless (Tim), the mammalian ortholog of a Drosophila circadian rhythm protein, was identified as a binding partner and substrate for c-Src, and its role in the differentiation of mES cells was probed. To determine whether Tim is involved in ES cell differentiation, Tim protein levels were stably suppressed using shRNA. Tim-defective ES cell lines were then tested for embryoid body (EB) formation, which models early mammalian development. Remarkably, confocal microscopy revealed that EBs formed from the Tim-knockdown ES cells failed to cavitate. Cells retained within the centers of the failed cavities strongly expressed the pluripotency marker Oct4, suggesting that further development is arrested without Tim. Immunoblots revealed reduced basal Caspase activity in the Tim-defective EBs compared to wild-type controls. Furthermore, EBs formed from Tim-knockdown cells demonstrated resistance to staurosporine-induced apoptosis, consistent with a link between Tim and programmed cell death during cavitation. These data demonstrate a novel function for the clock protein Tim during a key stage of early development. Specifically, EBs formed from ES cells lacking Tim showed reduced caspase activity and failed to cavitate. As a consequence, further development was halted, and the cells present in the failed cavity remained pluripotent. These findings reveal a new function for Tim in the coordination of ES cell differentiation, and raise the intriguing possibility that circadian rhythms and early development may be intimately linked (O'Reilly, 2011).

CRTC potentiates light-independent timeless transcription to sustain circadian rhythms in Drosophila

Light is one of the strongest environmental time cues for entraining endogenous circadian rhythms. Emerging evidence indicates that CREB-regulated transcription co-activator 1 (CRTC1) is a key player in this pathway, stimulating light-induced Period1 (Per1) transcription in mammalian clocks. This study demonstrates a light-independent role of Drosophila CRTC in sustaining circadian behaviors. Genomic deletion of the crtc locus causes long but poor locomotor rhythms in constant darkness. Overexpression or RNA interference-mediated depletion of CRTC in circadian pacemaker neurons similarly impairs the free-running behavioral rhythms, implying that Drosophila clocks are sensitive to the dosage of CRTC. The crtc null mutation delays the overall phase of circadian gene expression yet it remarkably dampens light-independent oscillations of TIMELESS (TIM) proteins in the clock neurons. In fact, CRTC overexpression enhances CLOCK/CYCLE (CLK/CYC)-activated transcription from tim but not per promoter in clock-less S2 cells whereas CRTC depletion suppresses it. Consistently, TIM overexpression partially but significantly rescues the behavioral rhythms in crtc mutants. Taken together, these data suggest that CRTC is a novel co-activator for the CLK/CYC-activated tim transcription to coordinate molecular rhythms with circadian behaviors over a 24-hour time-scale. The study proposes that CRTC-dependent clock mechanisms have co-evolved with selective clock genes among different species (Kim, 2016).

CREB-dependent transcription has long been implicated in different aspects of circadian gene expression. In mammalian clocks, light exposure triggers intracellular signaling pathways that activate CREB-dependent Per1 transcription, thereby adjusting the circadian phase of master circadian pacemaker neurons in the suprachiasmatic nucleus (SCN). The phase-resetting process involves the specific CREB coactivator CRTC1 and its negative regulator SIK1, constituting a negative feedback in the photic entrainment via a CREB pathway (Sakamoto, 2013; Jagannath, 2013). This report demonstrates a novel role of Drosophila CRTC that serves to coordinate circadian gene expression with 24-hour locomotor rhythms even in the absence of light. CRTC may regulate several clock-relevant genes, including those clock output genes that might be involved in the rhythmic arborizations and PDF cycling of the circadian pacemaker neurons. However, tim transcription was identified as one of the primary targets of Drosophila CRTC to sustain circadian rhythms in the free-running conditions, thus defining its light-independent clock function (Kim, 2016).

CREB could employ another transcriptional coactivator CBP (CREB-binding protein) to activate CRE-dependent transcription. In fact, CBP is a rather general coactivator recruited to gene promoters by other DNA-binding transcription factors. Previous studies have shown that Drosophila CBP associates with CLK, titrating its transcriptional activity. Mammalian CBP and the closely related coactivator p300 also form a complex with CLOCK-BMAL1, a homolog of the Drosophila CLK-CYC heterodimer, to stimulate their transcriptional activity. One possible explanation for CRTC-activated tim transcription is that Drosophila CRTC may analogously target the CLK-CYC heterodimer to stimulate CLK-CYC-tivate CRE-dependent transcription. Under these circumstances, a circadian role of light-sensitive TIM might have degenerated, while-induced clock genes. Moreover, CRTC associates with the bZIP domain in CREB protein, whereas CBP/p300 binds CREB through the phosphorylated KID domain, indicating that they might not necessarily target the same transcription factors apart from CREB. Finally, a protein complex of CLK and CRTC could not be detected in Drosophila S2 cells. Thus, it is likely that CRTC and CBP/p300 play unique roles in circadian transcription through their interactions with different DNA-binding transcription factors (Kim, 2016).

If CRTC augments CLK-CYC-dependent tim transcription indirectly, then why do crtc effects require CLK? A recent study suggested that mammalian CLOCK-BMAL1 may regulate the rhythmic access of other DNA-binding transcription factors to their target promoters in the context of chromatin, acting as a pioneer-like transcription factor. Given the structural and functional homology between Drosophila CLK-CYC and mammalian CLOCK-BMAL1, the presence of CLK-CYC in the tim promoter might allow the recruitment of additional transcription factors (e.g., CREB) and their co-activators including CRTC for maximal tim transcription. The transcriptional context of tim promoter might thus define its sensitivity to crtc effects among other clock promoters. In addition, the differential assembly of transcription factors on the tim promoter could explain tissue-specific effects of crtc on TIM oscillations (i.e., circadian pacemaker neurons versus peripheral clock tissues). Interestingly, chromatin immunoprecipitation with V5-tagged CLK protein revealed that CLK-CYC heterodimers associate with both tim and Sik2 gene promoters in fly heads. In LD cycles, however, their rhythmic binding to the Sik2 promoter is phase-delayed by ~4.5 hours compared with that to the tim promoter. These modes of transcriptional regulation may gate crtc effects on tim transcription in a clock-dependent manner, particularly in the increasing phase of tim transcription (Kim, 2016).

Transcription from CREB-responsive reporter genes shows daily oscillations, both in Drosophila and mammals, implicating this transcriptional strategy in the evolution of molecular clocks. In fact, cAMP signaling and CRE-dependent transcription constitute the integral components of core molecular clocks, serving to regulate daily rhythmic transcription of circadian clock genes. For instance, reciprocal regulation of dCREB2 and per at the transcription level has been reported to sustain free-running circadian rhythms in Drosophila. During fasting in mammals, a transcriptional program for hepatic gluconeogenesis is induced by CREB phosphorylation and CRTC2 dephosphorylation. Fasting-activated CREB-CRTC2 then stimulates Bmal1 expression61, whereas CLOCK-BMAL1-induced CRY rhythmically gates CREB activity in this process by modulating G protein-coupled receptor activity and inhibiting cAMP-induced CREB phosphorylation62. This molecular feedback circuit thus mutually links mammalian clocks and energy metabolism in terms of CREB-dependent transcription (Kim, 2016).

On the basis of these observations, a model is proposed for the evolution of CRTC-dependent clocks to explain the distinctive circadian roles of CRTC homologs (see A model for the evolution of CRTC-dependent clocks). CRTC is a transcriptional effector that integrates various cellular signals (Altarejos, 2011). It was reasoned that ancestral clocks may have employed CREB-CRTC-mediated transcription to sense extracellular time cues cell-autonomously and integrate this timing information directly into the earliest transcription-translation feedback loop (TTFL). This strategy would have generated simple but efficient molecular clocks to tune free-running molecular rhythms in direct response to environmental zeitgebers, such as light and the availability of nutrients. A circadian role of CRTC then has differentially evolved along with a selective set of clock targets. In poikilothermic Drosophila, light is accessible directly to circadian pacemaker neurons in the adult fly brain. Therefore, TIM degradation by the blue-light photoreceptor CRY plays a major role in the light entrainment of Drosophila clocks, although the photic induction of CLK/CYC-dependent tim transcription has been reported specifically at lower temperatures. Accordingly, Drosophila CRTC retained a constitutive co-activator function from the ancestral TTFL to support CLK/CYC-activated tim transcription and sustain free-running circadian behaviors. In homeothermic mammals, light input to the SCN is indirectly mediated by neurotransmitter release from presynaptic termini of the retinohypothalamic tract (RHT). Intracellular signaling relays in the SCN converge on the dephosphorylation and nuclear translocation of CRTC1 to activate CRE-dependent transcription. Under these circumstances, a circadian role of light-sensitive TIM might have degenerated, while per took over a role in the light-entrainment pathway by retaining CREB-CRTC1-dependent transcriptional regulation from the primitive TTFL. Consequently, mammalian clocks have lost a homolog of the Drosophila-like cry gene family, but instead evolved CRY homologs of the vertebrate-like cry gene family with transcriptional repressor activities in CLOCK-BMAL1-dependent transcription (Kim, 2016).

Regulation of metabolism and stress responses by neuronal CREB-CRTC-SIK pathways has been well documented in Drosophila. Given the demonstration of a circadian role of CRTC in the pacemaker neurons, it is possible that CRTC might sense metabolic cues in the context of circadian neural circuits to entrain molecular clocks cell-autonomously. Alternatively, but not exclusively, CRTC could participate in the regulation of clock-relevant metabolism as clock outputs from pacemaker neurons. These hypotheses remain to be validated in future studies (Kim, 2016).

timeless: Biological Overview | Regulation | Protein Interactions and Post-transcriptional Regulation | Developmental Biology | Effects of Mutation | References

date revised: 30 July 2006

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