Much remains to be elucidated about the assembly and activity of these core clock proteins, but insight into the molecular basis of circadian period determination is growing thanks to the integration of genetic studies from model organisms and humans with biochemical, structural and cell-based studies. High-resolution structures have now been determined for most of the globular domains of core clock proteins and some of their complexes, with a growing appreciation for the important role that flexible linkers and intrinsically disordered regions play in tuning clock protein function and clock timing. We will focus here on recent advances in our understanding of the mechanisms of period control by some of the negative elements of the core feedback loop , highlighting the nanoscale structural and dynamic properties of clock proteins that influence their functional roles as repressors within the core TTFL. For a review of mutations in human CRY1 and CRY2 that influence circadian timing, please refer to.Although transcriptional regulation by CLOCK:BMAL1 and downstream transcription factors is essential for generation of robust circadian rhythms, it is becoming increasingly clear that post-transcriptional and post-translational modifications of core clock components play an important role in both the generation of circadian rhythms and determination of its intrinsic period. While many studies have identified roles for post-translational modification of CRY and PER proteins in clock timing, we will focus here on an in-depth analysis of the regulation of PER2 by CK1δ/ε,vertical growing systems as it relies on an elaborate integration of post-translational modifications that ultimately determine the relative abundance of PER2 needed to maintain a ~24-hour period. PER1 and PER2 serve as interaction hubs for both CK1δ/ε and cryptochromes with dedicated binding sites that maintain stable complexes with these core clock proteins throughout most of the repressive phase of the clock each day , a feature that is notably absent in PER3.
The flexibility of PER proteins likely contributes to their role as labile scaffolds for transcriptional regulators of the clock; PER1–3 are all predominantly intrinsically disordered, conferring a susceptibility to regulation by post translational modifications and promiscuity for interaction partners that is common among other intrinsically disordered proteins. Two tandem PER-ARNT-SIM domains present in the N-terminus of all three PER isoforms allow them to form homodimers and heterodimers mediated by the β-sheet surface of the PAS-B domain . Deletion of the core PAS-B motif in the Per2 Brdm1 mutant leads to a loss of circadian rhythms, demonstrating that protein-protein interactions facilitated by this region are essential for clock function. Notably, mutation of just a single residue, W419E, at the dimer interface in the mouse PER2 PAS-B domain potently disrupts formation of homodimers and was recently shown to reduce phosphorylation by CK1δ. Moreover, the PER2 earlydoors mouse possesses a point mutation in the interdomain linker connecting the PAS-A and B domains that reduces PER2 stability to shorten the circadian period. Collectively, these findings demonstrate that dimerization via the PER PAS domains is critical for protein stability and clock timing, although more work is needed to understand the exact role that PAS domains play in orchestrating clock protein complexes and PER turnover.The turnover of PER2 is primarily mediated by CK1δ/ε-dependent phosphodegrons to intimately link kinase activity with PER2 stability. Dimerization of the PER2 PAS-AB domains positions the phosphodegron site of each monomer to protrude from the same face of the PAS-AB domain homodimer. This phosphodegron largely conforms to the canonical β-TrCP recognition motif, DSGϕXS, where ϕ is a hydrophobic residue and the two conserved serines become phosphorylated to make the substrate competent for β-TrCP recognition.
CK1δ/ε phosphorylation of S478 in mouse PER2, the first of the two serines in the motif, is required for interaction with the E3 ubiquitin ligases β-TrCP1/2, leading to ubiquitination of PER2 and its proteasomal degradation. However, this PAS-B phosphodegron is unique to PER2; PER1 utilizes a different CK1δ/ε-dependent phosphodegron N-terminal to the tandem PAS domains. This N-terminal phosphodegron is also conserved in PER2 and may play an auxiliary role in its turnover, as clock timing was only modestly impacted in the PER2 S478A transgenic mouse. An interaction with the E3 ubiquitin ligase MDM2 also influences PER2 stability independently of CK1δ/ε activity, opening the door for a complex integration of signals to mediate PER2 degradation. Although other kinases such as CK1α, CK2, SIK3 and Cdk5 phosphorylate PER2, CK1δ/ε are the only kinases that stably associate with PER2 throughout the night, moving from the cytoplasm into the nucleus with the other core clock proteins. CK1δ and the related isoform CK1ε bind to the Casein Kinase-Binding Domain in PER2 via two conserved motifs that flank a serine-rich region. Notably, mutation of the first residue in a series of five consecutive serines that are phosphorylated in human PER2 markedly decreases its stability and shortens circadian period by ~4 hours to manifest as Familial Advanced Sleep Phase Syndrome. Because CK1δ/ε-dependent phosphorylation of PER2 in this region links circadian timekeeping to this human sleep disorder, the serine-rich cluster in the CKBD has been named the FASP region. Recent studies have begun to elucidate the molecular basis for CK1δ/ε activity in the FASP region of PER2 to understand how it exerts such powerful control over circadian period. Phosphorylation of the first serine in this cluster by CK1δ leads to the obligately sequential phosphorylation of downstream serines.Therefore, the human S662G FASPS allele eliminates the ability of CK1δ to prime its activity downstream, disrupting all phosphorylation in the FASP region.
There is strong evidence that FASP phosphorylation plays a critical role in stabilizing PER2 protein, as the S662G mutation in human PER2 leads to premature turnover of the protein and a dramatically shorter circadian period of ~20 hours in a transgenic mouse model, while use of a phosphomimetic mutation in human PER2 that presumably leads to constitutive priming of sequential FASP phosphorylation confers a long period of ~25 hours in vivo. Although it is not yet known how FASP phosphorylation contributes to regulation of PER2 stability, mutation of the priming serine that blocks phosphorylation of FASP downstream serines increases CK1δ/ε activity at the phosphodegron site S478 to suggest that the phospho-FASP region could antagonize CK1δ/ε activity at the phosphodegron site S478. The opposing effects of FASP and phosphodegron phosphorylation likely involves cellular phosphatases like PP1 that contribute to CK1δ/ε-dependent regulation of circadian period through PER2, although there could also be a direct mechanistic link between FASP phosphorylation and regulation of CK1δ/ε activity. In fact, the functional linkage of phosphorylation at the FASP region and phosphodegron by CK1δ/ε has been described as a phospho switch that introduces a phase-specific delay to PER2 degradation necessary for proper circadian timekeeping. Interestingly, while introduction of the analogous priming site mutation in mouse PER1 destabilized PER1 and led to a shorter circadian period, it also caused an advance in feeding rhythms not seen in the PER2 mutant, suggesting that further study of the regulation of PER turnover could help uncouple distinct functions of PER1 and PER2 in control of circadian period and clock outputs. PER proteins are also subjected to a number of other post-translational modifications aside from phosphorylation. PER2 is O-GlcNacylated within the FASP region,outdoor vertical plant stands modifying the priming serine along with two sites downstream. Both O-GlcNAc transferase and O-GlcNAcase , the enzymes responsible for adding or hydrolyzing O-GlcNAc, respectively, are expressed or activated in a circadian manner, and factors that increase O-GlcNacylation also lead to a concomitant decrease in FASP phosphorylation that reduces PER2 protein levels.
These results support a model for competition between OGlcNacylation and phosphorylation at this key regulatory region, suggesting a mechanism by which glucose metabolism could modulate the circadian clock by antagonizing phosphorylation of PER2 in the stabilizing FASP region to represent a direct link between the circadian clock and metabolism as a “nutrition switch” . Acetylation also plays an important yet enigmatic role in PER2 regulation of the clock, first observed through manipulation of SIRT1, the nicotinamide adenine dinucleotide – dependent deacetylase. Although PER2 becomes acetylated as it accumulates throughout the repressive phase of the circadian clock, the identity of the acetyltransferase that modify PER2 is currently not known, nor is it known where PER2 is acetylated throughout the protein. Nonetheless, loss of SIRT1 results in elevated levels of acetylated PER2 in mouse liver to attenuate the robustness of circadian rhythms, while overexpression of SIRT1 facilitates PER2 degradation. Because acetylation and ubiquitination both target lysine residues, it is possible that these modifications compete for the same residues to directly control PER2 stability. However, there is some evidence that regulation of PER2 by acetylation could be more complicated, as acetylation at K680 on mouse PER2, located downstream of the serine cluster in the FASP region, is hyperacetylated following inhibition of SIRT1 and leads to a decrease in FASP phosphorylation, suggesting that an interplay between acetylation and phosphorylation of the FASP region could also control CK1δ/ε activity on PER2 to regulate the balance of the phosphoswitch. The timing of eukaryotic circadian rhythms from green algae to humans is heavily influenced by CK1δ/ε and its related orthologs. Like other Ser/Thr kinases, CK1δ/ε has a typical two-lobed structure , but little is known about the molecular mechanisms by which activity of the CK1 family is regulated. The activation loop is one key feature that distinguishes the CK1 family from other Ser/Thr kinases. Unlike many other kinases, the kinase domain of CK1 family members is not regulated by activation loop phosphorylation; therefore, they are considered to be constitutively active. The CK1 family acts as phosphate-directed kinases that preferentially recognize a D/E/pSxxS consensus motif, where a phosphorylated serine or a similar negative charge within the substrate templates activity at a serine located 3 or 4 residues downstream. Interestingly, at least two functionally important CK1δ/ε-dependent phosphorylation sites on PER2, the phosphodegron and the FASP priming site, do not conform to this consensus motif and likely serve as slow, rate-limiting steps for PER2 regulation. Therefore, understanding the molecular basis for kinase activity and substrate selectivity by CK1δ/ε has the potential to yield important insights into circadian timekeeping. In particular, a better understanding of the molecular mechanisms underpinning CK1δ/ε-dependent phosphorylation of PER will provide a framework for treating circadian disorders by targeting CK1δ/ε to modulate the clock. The kinase domain of CK1δ/ε contains several highly conserved anion binding sites located around the C-terminal lobe, including two that flank either side of the substrate binding cleft. We recently showed that these anion binding sites regulate the overall kinase activity of CK1δ/ε, as well as influence the substrate specificity of the kinase at both consensus and non-consensus sites. The significance of these highly conserved anion binding sites was initially suggested by the discovery of the first period-altering allele in mammals, the CK1ε tau allele that causes a dramatically shortened circadian period of ~20 hours. The R178C substitution in the tau kinase was predicted to disrupt an anion-binding pocket near the substrate binding region to decrease CK1δ/ε activity. While the tau mutant kinase did exhibit reduced activity on some generic kinase substrates as well as the FASP region of PER2, it led to a paradoxical gain of function at the PER2 phosphodegron that decreased stability of the protein. Crystal structures of the tau kinase domain recently revealed that disruption of the anion binding pocket at S1 in the mutant is linked to an allosteric structural switch in the activation loop that encodes a preference for the PER2 PAS-B phosphodegron site S478. Allostery is a common regulatory feature of protein kinases that allows for a switch-like, ultrasensitive regulation of their biological activity. The activation loop and flanking regions distinguish CK1 from all other Ser/Thr kinases, containing residues involved in the coordination of anions at three conserved sites, S1-S3. Therefore, these sites likely play a role in the CK1 family-specific regulation of kinase activity, perhaps through binding of anionic, phosphorylated residues. Interestingly, the entire substrate binding cleft that allosterically links anion binding to substrate selectivity is 95% identical from humans to green algae, suggesting that the mechanisms discovered in mammalian CK1δ/ε may also regulate kinase activity and circadian period across other eukaryotic clocks.