Identification of casein kinase I substrates by in vitro expression cloning screening

Structure, regulation, and (patho-)physiological functions of the stress-induced protein kinase CK1 delta (CSNK1D)

Members of the highly conserved pleiotropic CK1 family of serine/threonine-specific kinases are tightly regulated in the cell and play crucial regulatory roles in multiple cellular processes from protozoa to human. Since their dysregulation as well as mutations within their coding regions contribute to the development of various different pathologies, including cancer and neurodegenerative diseases, they have become interesting new drug targets within the last decade. However, to develop optimized CK1 isoform-specific therapeutics in personalized therapy concepts, a detailed knowledge of the regulation and functions of the different CK1 isoforms, their various splice variants and orthologs is mandatory. In this review we will focus on the stress-induced CK1 isoform delta (CK1δ), thereby addressing its regulation, physiological functions, the consequences of its deregulation for the development and progression of diseases, and its potential as therapeutic drug target.

Protein phosphorylation and its role in the regulation of Annexin A2 function

BACKGROUND

Annexin A2 (AnxA2) is a multifunctional protein involved in endocytosis, exocytosis, membrane domain organisation, actin remodelling, signal transduction, protein assembly, transcription and mRNA transport, as well as DNA replication and repair.

SCOPE OF REVIEW

The current knowledge of the role of phosphorylation in the functional regulation of AnxA2 is reviewed. To provide a more comprehensive treatment of this topic, we also address in depth the phosphorylation process in general and discuss its possible conformational effects. Furthermore, we discuss the apparent limitations of the methods used to investigate phosphoproteins, as exemplified by the study of AnxA2.

MAJOR CONCLUSIONS

AnxA2 is subjected to complex regulation by post-translational modifications affecting its cellular functions, with Ser11, Ser25 and Tyr23 representing important phosphorylation sites. Thus, Ser phosphorylation of AnxA2 is involved in the recruitment and docking of secretory granules, the regulation of its association with S100A10, and sequestration of perinuclear, translationally inactive mRNP complexes. By contrast, Tyr phosphorylation of AnxA2 regulates its role in actin dynamics and increases its association with endosomal compartments. Modification of its three main phosphorylation sites is not sufficient to discriminate between its numerous functions. Thus, fine-tuning of AnxA2 function is mediated by the joint action of several post-translational modifications.

GENERAL SIGNIFICANCE

AnxA2 participates in malignant cell transformation, and its overexpression and/or phosphorylation is associated with cancer progression and metastasis. Thus, tight regulation of AnxA2 function is an integral aspect of cellular homeostasis. The presence of AnxA2 in cancer cell-derived exosomes, as well as the potential regulation of exosomal AnxA2 by phosphorylation or other PTMs, are topics of great interest.

How do glycolytic enzymes favour cancer cell proliferation by nonmetabolic functions?

Cancer cells enhance their glycolysis, producing lactate, even in the presence of oxygen. Glycolysis is a series of ten metabolic reactions catalysed by enzymes whose expression is most often increased in tumour cells. HKII and phosphoglucose isomerase (PGI) have mainly an antiapoptotic effect; PGI and glyceraldehyde-3-phosphate dehydrogenase activate survival pathways (Akt and so on); phosphofructokinase 1 and triose phosphate isomerase participate in cell cycle activation; aldolase promotes epithelial mesenchymal transition; PKM2 enhances various nuclear effects such as transcription, stabilisation and so on. This review outlines the multiple non-glycolytic roles of glycolytic enzymes, which are essential for promoting cancer cells' survival, proliferation, chemoresistance and dissemination.

The CK1 Family: Contribution to Cellular Stress Response and Its Role in Carcinogenesis

Members of the highly conserved and ubiquitously expressed pleiotropic CK1 family play major regulatory roles in many cellular processes including DNA-processing and repair, proliferation, cytoskeleton dynamics, vesicular trafficking, apoptosis, and cell differentiation. As a consequence of cellular stress conditions, interaction of CK1 with the mitotic spindle is manifold increased pointing to regulatory functions at the mitotic checkpoint. Furthermore, CK1 is able to alter the activity of key proteins in signal transduction and signal integration molecules. In line with this notion, CK1 is tightly connected to the regulation and degradation of β-catenin, p53, and MDM2. Considering the importance of CK1 for accurate cell division and regulation of tumor suppressor functions, it is not surprising that mutations and alterations in the expression and/or activity of CK1 isoforms are often detected in various tumor entities including cancer of the kidney, choriocarcinomas, breast carcinomas, oral cancer, adenocarcinomas of the pancreas, and ovarian cancer. Therefore, scientific effort has enormously increased (i) to understand the regulation of CK1 and its involvement in tumorigenesis- and tumor progression-related signal transduction pathways and (ii) to develop CK1-specific inhibitors for the use in personalized therapy concepts. In this review, we summarize the current knowledge regarding CK1 regulation, function, and interaction with cellular proteins playing central roles in cellular stress-responses and carcinogenesis.

Nuclear localization of aldolase A correlates with cell proliferation

Muscle fructose 1,6-bisphosphate aldolase (ALDA) is a glycolytic enzyme which may localize both in nuclei and cytoplasm of cells, however its role in the nuclei is unclear. Here, we demonstrate the links between subcellular localization of ALDA and the cell cycle progression as well as the availability of energetic substrates. Results of our studies indicate that nuclear localization of ALDA correlates with the proliferative activity of the cells and with the expression of Ki-67, a marker of proliferation, both in the KLN-205 (mouse lung cancer cells) and human squamous cell lung cancer cells (hSCC). Chemically-induced block of cell cycle entry in S phase and the inhibition of transcription stimulate removal of ALDA from cells nuclei suggesting that nuclear ALDA is involved in cells proliferation. On the other hand, subcellular distribution of the enzyme also depends on the stress and pro-survival signals mediated by the Akt and the p38 pathways and, in non-proliferating cells, on the availability of glucose and lactate. The results presented here point to ALDA as a factor involved in the regulation of cells proliferation.

TERRA, hnRNP A1, and DNA-PKcs Interactions at Human Telomeres

Maintenance of telomeres, repetitive elements at eukaryotic chromosomal termini, and the end-capping structure and function they provide, are imperative for preserving genome integrity and stability. The discovery that telomeres are transcribed into telomere repeat containing RNA (TERRA) has revolutionized our view of this repetitive, rather unappreciated region of the genome. We have previously shown that the non-homologous end-joining, shelterin associated DNA dependent protein kinase catalytic subunit (DNA-PKcs) participates in mammalian telomeric end-capping, exclusively at telomeres created by leading-strand synthesis. Here, we explore potential roles of DNA-PKcs and its phosphorylation target heterogeneous nuclear ribonucleoprotein A1 (hnRNP A1) in the localization of TERRA at human telomeres. Evaluation of co-localized foci utilizing RNA-FISH and three-dimensional (3D) reconstruction strategies provided evidence that both inhibition of DNA-PKcs kinase activity and siRNA depletion of hnRNP A1 result in accumulation of TERRA at individual telomeres; depletion of hnRNP A1 also resulted in increased frequencies of fragile telomeres. These observations are consistent with previous demonstrations that decreased levels of the nonsense RNA-mediated decay factors SMG1 and UPF1 increase TERRA at telomeres and interfere with replication of leading-strand telomeres. We propose that hTR mediated stimulation of DNA-PKcs and subsequent phosphorylation of hnRNP A1 influences the cell cycle dependent distribution of TERRA at telomeres by contributing to the removal of TERRA from telomeres, an action important for progression of S-phase, and thereby facilitating efficient telomere replication and end-capping.

Regulation of ribosomal protein S6 phosphorylation by casein kinase 1 and protein phosphatase 1

Ribosomal protein S6 (rpS6) is a critical component of the 40 S ribosomal subunit that mediates translation initiation at the 5'-m(7)GpppG cap of mRNA. In response to mitogenic stimuli, rpS6 undergoes ordered C-terminal phosphorylation by p70 S6 kinases and p90 ribosomal S6 kinases on four conserved Ser residues (Ser-235, Ser-236, Ser-240, and Ser-244) whose modification potentiates rpS6 cap binding activity. A fifth site, Ser-247, is also known to be phosphorylated, but its function and regulation are not well characterized. In this study, we employed phospho-specific antibodies to show that Ser-247 is a target of the casein kinase 1 (CK1) family of protein kinases. CK1-dependent phosphorylation of Ser-247 was induced by mitogenic stimuli and required prior phosphorylation of upstream S6 kinase/ribosomal S6 kinase residues. CK1-mediated phosphorylation of Ser-247 also enhanced the phosphorylation of upstream sites, which implies that bidirectional synergy between C-terminal phospho-residues is required to sustain rpS6 phosphorylation. Consistent with this idea, CK1-dependent phosphorylation of rpS6 promotes its association with the mRNA cap-binding complex in vitro. Additionally, we show that protein phosphatase 1 (PP1) antagonizes rpS6 C terminus phosphorylation and cap binding in intact cells. These findings further our understanding of rpS6 phospho-regulation and define a direct link between CK1 and translation initiation.

Linking the kinome and phosphorylome--a comprehensive review of approaches to find kinase targets

Protein phosphorylation is associated with most cell signaling and developmental processes in eukaryotes. Despite the vast extent of the phosphoproteome within the cell, connecting specific kinases with relevant targets remains a significant experimental frontier. The challenge of linking kinases and their substrates reflects the complexity of kinase function. For example, kinases tend to exert their biological effects through supernumerary, redundant phosphorylation, often on multiple protein complex components. Although these types of phosphorylation events are biologically significant, those kinases responsible are often difficult to identify. Recent methods for global analysis of protein phosphorylation promise to substantially accelerate efforts to map the dynamic phosphorylome. Here, we review both conventional methods to identify kinase targets and more comprehensive genomic and proteomic approaches to connect the kinome and phosphorylome.

Yeast Polo-like kinase substrates are nailed with the right tools

A platform has been built combining chemical genetics and bioinformatics to screen the proteome for physiological substrates of the Polo-like kinase in budding yeast. A novel role for this kinase in regulating the mitotic spindle is revealed.

A Wnt-CKIvarepsilon-Rap1 pathway regulates gastrulation by modulating SIPA1L1, a Rap GTPase activating protein

Noncanonical Wnt signals control morphogenetic movements during vertebrate gastrulation. Casein kinase I epsilon (CKIvarepsilon) is a Wnt-regulated kinase that regulates Wnt/beta-catenin signaling and has a beta-catenin-independent role(s) in morphogenesis that is poorly understood. Here we report the identification of a CKIvarepsilon binding partner, SIPA1L1/E6TP1, a GAP (GTPase activating protein) of the Rap small GTPase family. We show that CKIvarepsilon phosphorylates SIPA1L1 to reduce its stability and thereby increase Rap1 activation. Wnt-8, which activates CKIvarepsilon, enhances the CKIvarepsilon-dependent phosphorylation and degradation of SIPA1L1. In early Xenopus or zebrafish development, inactivation of the Rap1 pathway results in abnormal gastrulation and a shortened anterior-posterior axis. Although CKIvarepsilon also transduces Wnt/beta-catenin signaling, inhibition of Rap1 does not alter beta-catenin-regulated gene expression. Our data demonstrate a role for CKIvarepsilon in noncanonical Wnt signaling and indicate that Wnt regulates morphogenesis in part through CKIvarepsilon-mediated control of Rap1 signaling.

Analysis of protein phosphorylation: methods and strategies for studying kinases and substrates

Protein phosphorylation is a highly conserved mechanism for regulating protein function, being found in all prokaryotes and eukaryotes examined. Phosphorylation can alter protein activity or subcellular localization, target proteins for degradation and effect dynamic changes in protein complexes. In many cases, different kinases may be involved in each of these processes for a single protein, allowing a large degree of combinatorial regulation at the post-translational level. Therefore, knowing which kinases are activated during a response and which proteins are substrates is integral to understanding the mechanistic regulation of a wide range of biological processes. In this paper, I will describe methods for monitoring kinase activity, investigating kinase-substrate specificity, examining phosphorylation in planta and the determination of phosphorylation sites in a protein. In addition, strategic considerations for experimental design and variables will be discussed.

Involvement of PPS3 phosphorylated by elicitor-responsive mitogen-activated protein kinases in the regulation of plant cell death

Mitogen-activated protein kinase (MAPK) cascades play pivotal roles in plant innate immunity. Overexpression of StMEK1(DD), a constitutively active MAPK kinase that activates salicylic acid-induced protein kinase (SIPK) and wound-induced protein kinase (WIPK), provokes hypersensitive response-like cell death in Nicotiana benthamiana. Here we purified a 51-kD MAPK, which was activated in potato (Solanum tuberosum) tubers treated with hyphal wall elicitor of a plant pathogen, and isolated the cDNA designated StMPK1. The deduced amino acid sequence of the StMPK1 showed strong similarity to stress-responsive MAPKs, such as tobacco (Nicotiana tabacum) SIPK and Arabidopsis (Arabidopsis thaliana) AtMPK6. To investigate the downstream signaling of StMPK1, we identified several proteins phosphorylated by StMPK1 (PPSs) using an in vitro expression cloning method. To dissect the biological function of PPSs in the plant defense, we employed virus-induced gene silencing (VIGS) in N. benthamiana. VIGS of NbPPS3 significantly delayed cell death induced by the transient expression of StMEK1(DD) and treatment with hyphal wall elicitor. Furthermore, the mobility shift of NbPPS3 on SDS-polyacrylamide gel was induced by transient expression of StMEK1(DD). The mobility shift of NbPPS3 induced by StMEK1(DD) was not compromised by VIGS of WIPK or SIPK alone, but drastically reduced by the silencing of both WIPK and SIPK. This work strongly supports the idea that PPS3 is a physiological substrate of StMPK1 and is involved in cell death activated by a MAPK cascade.

Control of mammalian circadian rhythm by CKIepsilon-regulated proteasome-mediated PER2 degradation

The mammalian circadian regulatory proteins PER1 and PER2 undergo a daily cycle of accumulation followed by phosphorylation and degradation. Although phosphorylation-regulated proteolysis of these inhibitors is postulated to be essential for the function of the clock, inhibition of this process has not yet been shown to alter mammalian circadian rhythm. We have developed a cell-based model of PER2 degradation. Murine PER2 (mPER2) hyperphosphorylation induced by the cell-permeable protein phosphatase inhibitor calyculin A is rapidly followed by ubiquitination and degradation by the 26S proteasome. Proteasome-mediated degradation is critically important in the circadian clock, as proteasome inhibitors cause a significant lengthening of the circadian period in Rat-1 cells. CKIepsilon (casein kinase Iepsilon) has been postulated to prime PER2 for degradation. Supporting this idea, CKIepsilon inhibition also causes a significant lengthening of circadian period in synchronized Rat-1 cells. CKIepsilon inhibition also slows the degradation of PER2 in cells. CKIepsilon-mediated phosphorylation of PER2 recruits the ubiquitin ligase adapter protein beta-TrCP to a specific site, and dominant negative beta-TrCP blocks phosphorylation-dependent degradation of mPER2. These results provide a biochemical mechanism and functional relevance for the observed phosphorylation-degradation cycle of mammalian PER2. Cell culture-based biochemical assays combined with measurement of cell-based rhythm complement genetic studies to elucidate basic mechanisms controlling the mammalian clock.

Drosophila Genome-Scale Screen for PAN GU Kinase Substrates Identifies Mat89Bb as a Cell Cycle Regulator

Although traditional organism-based mutational analysis is powerful in identifying genes involved in specific biological processes, limitations include incomplete coverage and time required for gene identification. Biochemical screens using cell transfection or yeast two-hybrid methods are rapid, but they are limited by cDNA library quality. The recent establishment of "uni-gene sets" has made it feasible to biochemically screen an organism's entire genome. Radiolabeled protein pools prepared from the Drosophila Gene Collection were used in a Drosophila in vitro expression cloning ("DIVEC") screen for substrates of PAN GU kinase, which is crucial for S-M embryonic cell cycles. Ablation of one identified substrate, Mat89Bb, by RNAi produces a polyploid phenotype similar to that of pan gu mutants. Xenopus embryos injected with Mat89Bb morpholinos arrest with polyploid nuclei, and Mat89Bb RNAi in HeLa cells gives rise to multinucleated cells. Thus, Mat89Bb plays an evolutionarily conserved role as a crucial regulator of both cell cycle and development.

Regulation of heterogenous nuclear ribonucleoprotein A1 transport by phosphorylation in cells stressed by osmotic shock

Heterogeneous nuclear ribonucleoprotein (hnRNP) A1 is an alternative splicing factor that is mainly nuclear, although it shuttles rapidly between nuclear and cytoplasmic compartments. Cells stressed by osmotic shock (OSM) activate the mitogen-activated protein kinase kinase(3/6)-p38 signaling pathway, which in turn results in accumulation of hnRNP A1 in the cytoplasm. This effect modulates alternative splicing regulation in vivo and correlates with increased hnRNP A1 phosphorylation. We have characterized the molecular mechanism involved in the cytoplasmic accumulation of hnRNP A1 in NIH 3T3 cells subjected to OSM. This treatment results in serine-specific phosphorylation within a C-terminal peptide, dubbed the "F-peptide," which is adjacent to the M9 motif that mediates bidirectional transport of hnRNP A1. Analysis of mutants in which the F-peptide serines were replaced by aspartic acids or alanines showed that F-peptide phosphorylation is required for the subcellular redistribution of hnRNP A1 in cells subjected to OSM. Furthermore, F-peptide phosphorylation modulates the interaction of hnRNP A1 with transportin Trn1. Our findings suggest that the phosphorylation of F-peptide by cell-signaling pathways regulates the rate of hnRNP A1 nuclear import.

The casein kinase 1 family: participation in multiple cellular processes in eukaryotes

Phosphorylation of serine, threonine and tyrosine residues by cellular protein kinases plays an important role in the regulation of various cellular processes. The serine/threonine specific casein kinase 1 and 2 protein kinase families--(CK1 and CK2)--were among the first protein kinases that had been described. In recent years our knowledge of the regulation and function of mammalian CK1 kinase family members has rapidly increased. Extracellular stimuli, the subcellular localization of CK1 isoforms, their interaction with various cellular structures and proteins, as well as autophosphorylation and proteolytic cleavage of their C-terminal regulatory domains influence CK1 kinase activity. Mammalian CK1 isoforms phosphorylate many different substrates among them key regulatory proteins involved in the control of cell differentiation, proliferation, chromosome segregation and circadian rhythms. Deregulation and/or the incidence of mutations in the coding sequence of CK1 isoforms have been linked to neurodegenerative diseases and cancer. This review will summarize our current knowledge about the function and regulation of mammalian CK1 isoforms.

Modulation of protein translation by Nck-1

In mammals, Nck represented by two genes, is a 47-kDa SH2/SH3 domain-containing protein lacking intrinsic enzymatic function. Here, we reported that the first and the third SH3 domains of Nck-1 interact with the C-terminal region of the beta subunit of the eukaryotic initiation factor 2 (eIF2 beta). Binding of eIF2 beta was specific to the SH3 domains of Nck-1, and in vivo, the interaction Nck/eIF2 beta was demonstrated by reciprocal coimmunoprecipitations. In addition, Nck was detected in a molecular complex with eIF2 beta in an enriched ribosomal fraction, whereas no other SH2/SH3 domain-containing adapters were found. Cell fractionation studies demonstrated that the presence of Nck in purified ribosomal fractions was enhanced after insulin stimulation, suggesting that growth factors dynamically regulate translocation of Nck to ribosomes. In HEK293 cells, we observed that transient overexpression of Nck-1 significantly enhanced Cap-dependent and -independent protein translation. This effect of Nck-1 required the integrity of its first and third SH3 domains originally found to interact with eIF2 beta. Finally, in vitro, Nck-1 also increased protein translation, revealing a direct role for Nck-1 in this process. Our study demonstrates that in addition to mediate receptor tyrosine kinase signaling, Nck-1 modulates protein translation potentially through its direct interaction with an intrinsic component of the protein translation machinery.

Annexins: from structure to function

Annexins are Ca2+ and phospholipid binding proteins forming an evolutionary conserved multigene family with members of the family being expressed throughout animal and plant kingdoms. Structurally, annexins are characterized by a highly alpha-helical and tightly packed protein core domain considered to represent a Ca2+-regulated membrane binding module. Many of the annexin cores have been crystallized, and their molecular structures reveal interesting features that include the architecture of the annexin-type Ca2+ binding sites and a central hydrophilic pore proposed to function as a Ca2+ channel. In addition to the conserved core, all annexins contain a second principal domain. This domain, which NH2-terminally precedes the core, is unique for a given member of the family and most likely specifies individual annexin properties in vivo. Cellular and animal knock-out models as well as dominant-negative mutants have recently been established for a number of annexins, and the effects of such manipulations are strikingly different for different members of the family. At least for some annexins, it appears that they participate in the regulation of membrane organization and membrane traffic and the regulation of ion (Ca2+) currents across membranes or Ca2+ concentrations within cells. Although annexins lack signal sequences for secretion, some members of the family have also been identified extracellularly where they can act as receptors for serum proteases on the endothelium as well as inhibitors of neutrophil migration and blood coagulation. Finally, deregulations in annexin expression and activity have been correlated with human diseases, e.g., in acute promyelocytic leukemia and the antiphospholipid antibody syndrome, and the term annexinopathies has been coined.

Casein kinase I phosphorylates and destabilizes the beta-catenin degradation complex

Wnt signaling plays a key role in cell proliferation and development. Recently, casein kinase I (CKI) and protein phosphatase 2A (PP2A) have emerged as positive and negative regulators of the Wnt pathway, respectively. However, it is not clear how these two enzymes with opposing functions regulate Wnt signaling. Here we show that both CKI delta and CKI epsilon interacted directly with Dvl-1, and that CKI phosphorylated multiple components of the Wnt-regulated beta-catenin degradation complex in vitro, including Dvl-1, adenomatous polyposis coli (APC), axin, and beta-catenin. Comparison of peptide maps from in vivo and in vitro phosphorylated beta-catenin and axin suggests that CKI phosphorylates these proteins in vivo as well. CKI abrogated beta-catenin degradation in Xenopus egg extracts. Notably, CKI decreased, whereas inhibition of CKI increased, the association of PP2A with the beta-catenin degradation complex in vitro. Additionally, inhibition of CKI in vivo stabilized the beta-catenin degradation complex, suggesting that CKI actively destabilizes the complex in vivo. The ability of CKI to induce secondary body axes in Xenopus embryos was reduced by the B56 regulatory subunit of PP2A, and kinase-dead CKI epsilon acted synergistically with B56 in inhibiting Wnt signaling. The data suggest that CKI phosphorylates and destabilizes the beta-catenin degradation complex, likely through the dissociation of PP2A, providing a mechanism by which CKI stabilizes beta-catenin and propagates the Wnt signal.

Conventional and nonconventional roles of the nucleolus

As the most prominent of subnuclear structures, the nucleolus has a well-established role in ribosomal subunit assembly. Additional nucleolar functions, not related to ribosome biogenesis, have been discovered within the last decade. Built around multiple copies of the genes for preribosomal RNA (rDNA), nucleolar structure is largely dependent on the process of ribosome assembly. The nucleolus is disassembled during mitosis at which time preribosomal RNA transcription and processing are suppressed; it is reassembled at the end of mitosis in part from components preserved from the previous cell cycle. Expression of preribosomal RNA (pre-rRNA) is regulated by the silencing of individual rDNA genes via alterations in chromatin structure or by controlling RNA polymerase I initiation complex formation. Preribosomal RNA processing and posttranscriptional modifications are guided by a multitude of small nucleolar RNAs. Nearly completed ribosomal subunits are exported to the cytoplasm by an established nuclear export system with the aid of specialized adapter molecules. Some preribosomal and nucleolar components are transiently localized in Cajal bodies, presumably for modification or assembly. The nonconventional functions of nucleolus include roles in viral infections, nuclear export, sequestration of regulatory molecules, modification of small RNAs, RNP assembly, and control of aging, although some of these functions are not well established. Additional progress in defining the mechanisms of each step in ribosome biogenesis as well as clarification of the precise role of the nucleolus in nonconventional activities is expected in the next decade.

Nuclear entry of the circadian regulator mPER1 is controlled by mammalian casein kinase I epsilon

The molecular oscillator that keeps circadian time is generated by a negative feedback loop. Nuclear entry of circadian regulatory proteins that inhibit transcription from E-box-containing promoters appears to be a critical component of this loop in both Drosophila and mammals. The Drosophila double-time gene product, a casein kinase I epsilon (CKIepsilon) homolog, has been reported to interact with dPER and regulate circadian cycle length. We find that mammalian CKIepsilon binds to and phosphorylates the murine circadian regulator mPER1. Unlike both dPER and mPER2, mPER1 expressed alone in HEK 293 cells is predominantly a nuclear protein. Two distinct mechanisms appear to retard mPER1 nuclear entry. First, coexpression of mPER2 leads to mPER1-mPER2 heterodimer formation and cytoplasmic colocalization. Second, coexpression of CKIepsilon leads to masking of the mPER1 nuclear localization signal and phosphorylation-dependent cytoplasmic retention of both proteins. CKIepsilon may regulate mammalian circadian rhythm by controlling the rate at which mPER1 enters the nucleus.