Molecular cloning and sequence analysis of two novel fission yeast casein kinase-1 isoforms

Djck1α Is Required for Proper Regeneration and Maintenance of the Medial Tissues in Planarians

CK1α (Casein kinase 1α) is a member of the casein kinase 1(CK1) family that is involved in diverse cellular processes, but its functions remain unclear in stem cell development. Freshwater planarians are capable of whole-body regeneration, making it a classic model for the study of regeneration, tissue homeostasis, and polarity in vivo. To investigate the roles of CK1α in regeneration and homeostasis progress, we characterize a homolog of CK1α from planarian Dugesia japonica. We find that Djck1α, which shows an enriched expression pattern in the nascent tissues, is widely expressed especially in the medial regions of planarians. Knockdown of CK1α by RNAi presents a thicker body due to dorsal hyperplasia, along with defects in the medial tissues including nerve proliferation, missing epidermis, intestine disturbance, and hyper-proliferation during the progression of regeneration and homeostasis. Moreover, we find that the ck1α RNAi animals exhibit expansion of the midline marker slit. The eye deficiency induced by slit RNAi can be rescued by ck1α and slit double RNAi. These results suggest that ck1α is required for the medial tissue regeneration and maintenance in planarian Dugesia japonica by regulating the expression of slit, which helps to further investigate the regulation of planarian mediolateral axis.

Yeast casein kinase 2 governs morphology, biofilm formation, cell wall integrity, and host cell damage of Candida albicans

The regulatory networks governing morphogenesis of a pleomorphic fungus, Candida albicans are extremely complex and remain to be completely elucidated. This study investigated the function of C. albicans yeast casein kinase 2 (CaYck2p). The yck2Δ/yck2Δ strain displayed constitutive pseudohyphae in both yeast and hyphal growth conditions, and formed enhanced biofilm under non-biofilm inducing condition. This finding was further supported by gene expression analysis of the yck2Δ/yck2Δ strain which showed significant upregulation of UME6, a key transcriptional regulator of hyphal transition and biofilm formation, and cell wall protein genes ALS3, HWP1, and SUN41, all of which are associated with morphogenesis and biofilm architecture. The yck2Δ/yck2Δ strain was hypersensitive to cell wall damaging agents and had increased compensatory chitin deposition in the cell wall accompanied by an upregulation of the expression of the chitin synthase genes, CHS2, CHS3, and CHS8. Absence of CaYck2p also affected fungal-host interaction; the yck2Δ/yck2Δ strain had significantly reduced ability to damage host cells. However, the yck2Δ/yck2Δ strain had wild-type susceptibility to cyclosporine and FK506, suggesting that CaYck2p functions independently from the Ca+/calcineurin pathway. Thus, in C. albicans, Yck2p is a multifunctional kinase that governs morphogenesis, biofilm formation, cell wall integrity, and host cell interactions.

HIV-1 Protease in the Fission Yeast Schizosaccharomyces pombe

BACKGROUND

HIV-1 protease (PR) is an essential viral enzyme. Its primary function is to proteolyze the viral Gag-Pol polyprotein for production of viral enzymes and structural proteins and for maturation of infectious viral particles. Increasing evidence suggests that PR cleaves host cellular proteins. However, the nature of PR-host cellular protein interactions is elusive. This study aimed to develop a fission yeast (Schizosaccharomyces pombe) model system and to examine the possible interaction of HIV-1 PR with cellular proteins and its potential impact on cell proliferation and viability.

RESULTS

A fission yeast strain RE294 was created that carried a single integrated copy of the PR gene in its chromosome. The PR gene was expressed using an inducible nmt1 promoter so that PR-specific effects could be measured. HIV-1 PR from this system cleaved the same indigenous viral p6/MA protein substrate as it does in natural HIV-1 infections. HIV-1 PR expression in fission yeast cells prevented cell proliferation and induced cellular oxidative stress and changes in mitochondrial morphology that led to cell death. Both these PR activities can be prevented by a PR-specific enzymatic inhibitor, indinavir, suggesting that PR-mediated proteolytic activities and cytotoxic effects resulted from enzymatic activities of HIV-1 PR. Through genome-wide screening, a serine/threonine kinase, Hhp2, was identified that suppresses HIV-1 PR-induced protease cleavage and cell death in fission yeast and in mammalian cells, where it prevented PR-induced apoptosis and cleavage of caspase-3 and caspase-8.

CONCLUSIONS

This is the first report to show that HIV-1 protease is functional as an enzyme in fission yeast, and that it behaves in a similar manner as it does in HIV-1 infection. HIV-1 PR-induced cell death in fission yeast could potentially be used as an endpoint for mechanistic studies, and this system could be used for developing a high-throughput system for drug screenings.

Casein Kinase 1 and Phosphorylation of Cohesin Subunit Rec11 (SA3) Promote Meiotic Recombination through Linear Element Formation

Proper meiotic chromosome segregation, essential for sexual reproduction, requires timely formation and removal of sister chromatid cohesion and crossing-over between homologs. Early in meiosis cohesins hold sisters together and also promote formation of DNA double-strand breaks, obligate precursors to crossovers. Later, cohesin cleavage allows chromosome segregation. We show that in fission yeast redundant casein kinase 1 homologs, Hhp1 and Hhp2, previously shown to regulate segregation via phosphorylation of the Rec8 cohesin subunit, are also required for high-level meiotic DNA breakage and recombination. Unexpectedly, these kinases also mediate phosphorylation of a different meiosis-specific cohesin subunit Rec11. This phosphorylation in turn leads to loading of linear element proteins Rec10 and Rec27, related to synaptonemal complex proteins of other species, and thereby promotes DNA breakage and recombination. Our results provide novel insights into the regulation of chromosomal features required for crossing-over and successful reproduction. The mammalian functional homolog of Rec11 (STAG3) is also phosphorylated during meiosis and appears to be required for fertility, indicating wide conservation of the meiotic events reported here.

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.

Casein kinase 1 regulates sterol regulatory element-binding protein (SREBP) to control sterol homeostasis

Sterol homeostasis is tightly controlled by the sterol regulatory element-binding protein (SREBP) transcription factor that is highly conserved from fungi to mammals. In fission yeast, SREBP functions in an oxygen-sensing pathway to promote adaptation to decreased oxygen supply that limits oxygen-dependent sterol synthesis. Low oxygen stimulates proteolytic cleavage of the SREBP homolog Sre1, generating the active transcription factor Sre1N that drives expression of sterol biosynthetic enzymes. In addition, low oxygen increases the stability and DNA binding activity of Sre1N. To identify additional signals controlling Sre1 activity, we conducted a genetic overexpression screen. Here, we describe our isolation and characterization of the casein kinase 1 family member Hhp2 as a novel regulator of Sre1N. Deletion of Hhp2 increases Sre1N protein stability and ergosterol levels in the presence of oxygen. Hhp2-dependent Sre1N degradation by the proteasome requires Hhp2 kinase activity, and Hhp2 binds and phosphorylates Sre1N at specific residues. Our results describe a role for casein kinase 1 as a direct regulator of sterol homeostasis. Given the role of mammalian Hhp2 homologs, casein kinase 1δ and 1ε, in regulation of the circadian clock, these findings may provide a mechanism for coordinating circadian rhythm and lipid metabolism.

Shugoshin-PP2A counteracts casein-kinase-1-dependent cleavage of Rec8 by separase

During meiosis, the cohesin complexes that maintain sister chromatid cohesion are lost in a stepwise manner. At meiosis I the cohesin subunit Rec8 is cleaved only along the chromosome arms; until meiosis II it is protected at centromeres by the action of shugoshin (Sgo1)-protein phosphatase 2A (PP2A). Although this regulation hypothetically involves phosphorylation that is antagonized by Sgo1-PP2A, the kinase and substrate that are responsible are as yet unknown. Using a genetic screen for 'anti-shugoshin', we identify Hhp2, an orthologue of casein kinase 1delta/epsilon (CK1), as a factor required for Rec8 cleavage in fission yeast. We show that CK1, rather than a Polo-like kinase that is widely believed to do so, acts as the cohesin kinase to promote this cleavage during meiosis. Crucially, forced localization of excess Hhp2 at the pericentromeric region abrogates the ability of Sgo1-PP2A to protect centromeric Rec8. Thus, our studies prove the key notion that the balance between Rec8 phosphorylation and its dephosphorylation by Sgo1-PP2A regulates the step-wise loss of chromosomal cohesion in meiosis.

Construction of conditional analog-sensitive kinase alleles in the fission yeast Schizosaccharomyces pombe

Reversible protein phosphorylation is a major regulatory mechanism in a cell. A chemical-genetic strategy to conditionally inactivate protein kinases has been developed recently. Mutating a single residue in the ATP-binding pocket confers sensitivity to small-molecule inhibitors. The inhibitor can only bind to the mutant kinase and not to any other wild-type kinase, allowing specific inactivation of the modified kinase. Here, we describe a protocol to construct conditional analog-sensitive kinase alleles in the fission yeast Schizosaccharomyces pombe. This protocol can be completed in about 3 weeks and should be applicable to other organisms as well.

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.

Protein kinase CK1 from Trypanosoma cruzi

A protein kinase activity, which uses casein as a substrate, has been purified to homogeneity from the epimastigote stage of Trypanosoma cruzi, by sequential chromatography on Q sepharose, heparin sepharose, phenyl sepharose, and alpha-casein agarose. An apparent molecular weight of 36,000 was estimated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Gel filtration chromatography and sedimentation analyses demonstrated that the purified native enzyme is a monomer with a sedimentation coefficient of 2.9 S. The hydrodynamic parameters indicated that the shape of the protein is globular with a frictional ratio f/f(o) = 1.36 and a Stokes radius of 27.7 A. When two selective peptide substrates for protein kinases CK1 and CK2 were used (RRKDLHDDEEDEAM. SITA and RRRADDSDDDDD, respectively), the purified kinase was shown to predominantly phosphorylate the CK1-specific peptide. Additionally, the enzyme was inhibited by N-(2-amino-ethyl)-5-chloroisoquinoline-8-sulfonamide, a specific inactivator of CK1s from mammals. Based on these results, we concluded that the purified kinase corresponds to a parasite CK1.

Casein kinase I: spatial organization and positioning of a multifunctional protein kinase family

The casein kinase I family of serine/threonine protein kinases is highly conserved from yeast to humans. Until only recently, both the function and regulation of these enzymes remained poorly uncharacterised in that they appeared to be constitutively active and were capable of phosphorylating an untold number of other proteins. While relatively little was known regarding the exact function of the higher eukaryotic isoforms, the casein kinase I (CKI) isoforms from yeast have been genetically linked to vesicular trafficking, DNA repair, cell cycle progression and cytokinesis. All five S. cerevisiae isoforms are known to be associated with discrete cellular compartments and this localization has been shown to be absolutely essential for their respective functions. New evidence now suggests that the CKI isoforms in more complex systems also exhibit non-homogeneous subcellular distributions that may prove vital to defining the function and regulation of these enzymes. In particular, CKIalpha, the most-characterized vertebrate isoform, is associated with cytosolic vesicles, the mitotic spindle and structures within the nucleus. Functions associated with these localizations coincide with those previously reported in yeast, suggesting a conservation of function. Other reports have indicated that each of the remaining CKI isoforms have the capacity to make associations with components of several signal transduction pathways, thereby channeling CKI function toward specific regulatory events. This review will examine what is now known about the higher eukaryotic CKI family members from the perspective localization as a means of gaining a better understanding of the function and regulation of these kinases.

Identification of a novel casein kinase-1 homolog in fission yeast Schizosaccharomyces pombe

Fission yeast cells lacking either the ste9+- or rum1+ function cannot enter the cell differentiation pathway upon nutritional starvation. Sterility in both mutants is suppressed by the srs1-S41 mutation. A gene encoding a novel casein kinase-1 (CK1) isoform, cki3+, was isolated as a high-copy-number suppressor gene of the srs1 mutation. Cki3 protein is structurally more related to the Cki/Yck subfamily proteins than those of the Hhp/Hrr25 subfamily. A mutant cki3 gene in which a highly conserved lysine residue in the kinase subdomain II was substituted to arginine lost the ability to recover the growth defect in the srs1 mutant, indicating that catalytic activity was necessary for suppression. Gene disruption revealed that cki3+ was dispensable for cell viability, and cells lacking functional cki3+ exhibited no characteristic phenotype. Thus, S. pombe has three highly related CK1 isoforms (Cki1, Cki2 and Cki3), but none of them has an essential function.

The effect of heparin on the activity of Trichosporon cutaneum casein kinase I

Casein kinase I from Trichosporon cutaneum ribosome-free extracts was purified. Its molecular mass was calculated for 33 kDa. It was shown that casein, phosvitin and Trichosporon cutaneum ribosomal protein of 15 kDa were preferable substrates for the enzyme. It was found that heparin can stimulate or inhibit CKI activity depending on the substrate used. Stimulation of casein and inhibition of phosvitin phosphorylation was observed. In addition it was shown that ribosomal proteins of 19 kDa and 38 kDa were phosphorylated by CKI only in the presence of heparin.

Identification, cloning, and mutational analysis of the casein kinase 1 cDNA of the malaria parasite, Plasmodium falciparum. Stage-specific expression of the gene

The cDNA for casein kinase 1 (CK1) of Plasmodium falciparum was cloned, sequenced, and expressed in bacteria. The single major open reading frame of the 1.2-kilobase pair cDNA coded for a 324-amino acid polypeptide of approximately 37 kDa, the predicted sequence of which showed strong identity with known CK1 isoforms. The purified recombinant enzyme exhibited properties characteristic of CK1, such as inhibition by CK1-7, the ability to phosphorylate a highly specific peptide substrate, and a strong preference for ATP over GTP. A casein kinase activity, partially purified from soluble extracts of P. falciparum by affinity chromatography through CK1-7 columns displayed identical properties. The activity showed a stage-specific expression in the parasite, in the order trophozoite > ring >> schizont. Northern analysis indicated the existence of two major CK1 mRNAs, 2.4 and 3.2 kilobase pairs long, the levels of which were in the order ring > schizont > trophozoite. Mutagenesis of recombinant CK1 defined important amino acid residues and their potential role in the conformation of the enzyme. The malarial CK1 appeared to be the one of the smallest and perhaps the most primitive CK1 enzymes known, containing little sequence information beyond the minimal catalytic domain.

Proteins in unexpected locations

Members of all classes of proteins--cytoskeletal components, secreted growth factors, glycolytic enzymes, kinases, transcription factors, chaperones, transmembrane proteins, and extracellular matrix proteins--have been identified in cellular compartments other than their conventional sites of action. Some of these proteins are expressed as distinct compartment-specific isoforms, have novel mechanisms for intercompartmental translocation, have distinct endogenous biological actions within each compartment, and are regulated in a compartment-specific manner as a function of physiologic state. The possibility that many, if not most, proteins have distinct roles in more than one cellular compartment has implications for the evolution of cell organization and may be important for understanding pathological conditions such as Alzheimer's disease and cancer.

Role of COOH-terminal phosphorylation in the regulation of casein kinase I delta

Casein kinase I delta is a member of the casein kinase I (CKI) family, a group of second messenger independent protein kinases. We present evidence that the COOH-terminal domain of CKI delta has regulatory properties. CKI delta expressed in Escherichia coli was activated by heparin, as found previously, and by treatment with the catalytic subunit of type-1 protein phosphatase (CS1). Concomitant with activation by CS1, there was a reduction in the apparent molecular weight of CKI delta from 55,000 to 49,000 as judged by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate. Truncation of CKI delta by removal of the COOH-terminal 110 amino acids eliminated the ability of CS1 to activate or to increase electrophoretic mobility. Casein kinase I alpha, a 37-kDa isoform that lacks an extended COOH-terminal domain, was not activated by CS1 or the presence of heparin. However, a chimeric enzyme consisting of CKI alpha fused to the COOH-terminal domain of CKI delta was activated by both heparin and CS1. Analysis of the effects of CS1 on a series of CKI delta COOH-terminal truncation mutants identified an inhibitory region between His317 and Pro342, which contained six potential phosphorylation sites. From analysis of the specific activites of these truncation mutants, removal of the same region resulted in enzyme with a specific activity nearly 10-fold greater than wild-type. Thus, CKI delta activity can be regulated by phosphorylation of its COOH terminus, which may serve to create an autoinhibitory domain. This mechanism of regulation could have important consequences in vivo.

Isolation and characterization of human casein kinase I epsilon (CKI), a novel member of the CKI gene family

The casein kinase I (CKI) gene family is a rapidly enlarging group whose members have been implicated in the control of cytoplasmic and nuclear processes, including DNA replication and repair. We report here the cloning and characterization of a novel isoform of CKI from a human placental cDNA library. The cDNA for this isoform, hCKI epsilon, predicts a basic polypeptide of 416 amino acids and a molecular mass of 47.3 kDa. It encodes a core kinase domain of 285 amino acids and a carboxyl-terminal tail of 123 amino acids. The kinase domain is 53-98% identical to the kinase domains of other CKI family members and is most closely related to the delta isoform. Localization of the hCKI epsilon gene to chromosome 22q12-13 and the hCKI delta gene to chromosome 17q25 confirms that these are distinct genes in the CKI family. Northern blot analysis shows that hCKI epsilon is expressed in multiple human cell lines. Recombinant hCKI epsilon is an active enzyme that phosphorylates known CKI substrates including a CKI-specific peptide substrate and is inhibited by CKI-7, a CKI-specific inhibitor. A budding yeast isoform of CKI, HRR25, has been implicated in DNA repair responses. Expression of hCKI epsilon but not hCKI alpha rescued the slow-growth phenotype of a Saccharomyces cerevisiae strain with a deletion of HRR25. Human CKI epsilon is a novel CKI isoform with properties that overlap those of previously described CKI isoforms.

Casein kinase I gamma subfamily. Molecular cloning, expression, and characterization of three mammalian isoforms and complementation of defects in the Saccharomyces cerevisiae YCK genes

Casein kinase I, one of the first protein kinases identified biochemically, is known to exist in multiple isoforms in mammals. Using a partial cDNA fragment corresponding to an isoform termed CK1 gamma, three full-length rat testis cDNAs were cloned that defined three separate members of this subfamily. The isoforms, designated CK1 gamma 1, CK1 gamma 2, and CK1 gamma 3, have predicted molecular masses of 43,000, 45,500, and 49,700. CK1 gamma 3 may also exist in an alternatively spliced form. The proteins are more than 90% identical to each other within the protein kinase domain but only 51-59% identical to other casein kinase I isoforms within this region. Messages for CK1 gamma 1 (2 kilobases (kb)), CK1 gamma 2 (1.5 and 2.4 kb), and CK1 gamma 3 (2.8 kb) were detected by Northern hybridization of testis RNA. Message for CK1 gamma 3 was also observed in brain, heart, kidney, lung, liver, and muscle whereas CK1 gamma 1 and CK1 gamma 2 messages were restricted to testis. All three CK1 gamma isoforms were expressed as active enzymes in Escherichia coli and partially purified. The enzymes phosphorylated typical in vitro casein kinase I substrates such as casein, phosvitin, and a synthetic peptide, D4. Phosphorylation of the D4 peptide was activated by heparin whereas phosphorylation of the protein substrates was inhibited. The known casein kinase I inhibitor CK1-7 also inhibited the CK1 gamma s although less effectively than the CK1 alpha or CK1 delta isoforms. All three CK1 gamma s underwent autophosphorylation when incubated with ATP and Mg2+. The YCK1 and YCK2 genes in Saccharomyces cerevisiae encode casein kinase I homologs, defects in which lead to aberrant morphology and growth arrest. Expression of mammalian CK1 gamma 1 or CK1 gamma 3 restored growth and normal morphology to a yeast mutant carrying a disruption of YCK1 and a temperature-sensitive allele of YCK2, suggesting overlap of function between the yeast Yck proteins and these CK1 isoforms.