A proliferation of cyclins

Morphological Changes Induced by TKS4 Deficiency Can Be Reversed by EZH2 Inhibition in Colorectal Carcinoma Cells

BACKGROUND

The scaffold protein tyrosine kinase substrate 4 (TKS4) undergoes tyrosine phosphorylation by the epidermal growth factor receptor (EGFR) pathway via Src kinase. The TKS4 deficiency in humans is responsible for the manifestation of a genetic disorder known as Frank-Ter Haar syndrome (FTHS). Based on our earlier investigation, the absence of TKS4 triggers migration, invasion, and epithelial-mesenchymal transition (EMT)-like phenomena while concurrently suppressing cell proliferation in HCT116 colorectal carcinoma cells. This indicates that TKS4 may play a unique role in the progression of cancer. In this study, we demonstrated that the enhancer of zeste homolog 2 (EZH2) and the histone methyltransferase of polycomb repressive complex 2 (PRC2) are involved in the migration, invasion, and EMT-like changes in TKS4-deficient cells (KO). EZH2 is responsible for the maintenance of the trimethylated lysine 27 on histone H3 (H3K27me3).

METHODS

We performed transcriptome sequencing, chromatin immunoprecipitation, protein and RNA quantitative studies, cell mobility, invasion, and proliferation studies combined with/without the EZH2 activity inhibitor 3-deazanoplanocine (DZNep).

RESULTS

We detected an elevation of global H3K27me3 levels in the TKS4 KO cells, which could be reduced with treatment with DZNep, an EZH2 inhibitor. Inhibition of EZH2 activity reversed the phenotypic effects of the knockout of TKS4, reducing the migration speed and wound healing capacity of the cells as well as decreasing the invasion capacity, while the decrease in cell proliferation became stronger. In addition, inhibition of EZH2 activity also reversed most epithelial and mesenchymal markers. We investigated the wider impact of TKS4 deletion on the gene expression profile of colorectal cancer cells using transcriptome sequencing of wild-type and TKS4 knockout cells, particularly before and after treatment with DZNep. Additionally, we observed changes in the expression of several protein-coding genes and long non-coding RNAs that showed a recovery in expression levels following EZH2 inhibition.

CONCLUSIONS

Our results indicate that the removal of TKS4 causes a notable disruption in the gene expression pattern, leading to the disruption of several signal transduction pathways. Inhibiting the activity of EZH2 can restore most of these transcriptomics and phenotypic effects in colorectal carcinoma cells.

Cell Cycle-Dependent Expression Dynamics of G1/S Specific Cyclin, Cellulose Synthase and Cellulase in the Dinoflagellate Prorocentrum donghaiense

Dinoflagellates undergo a typical eukaryotic cell cycle consisting of G1, S, G2, and M phases and some of the typical cell cycle related genes have been computationally identified. However, very few of these genes have been experimentally linked to the cell cycle phases. Besides, although thecate dinoflagellates are known to possess theca composed of cellulose, information on cellulose synthesis and degradation associated with the cell cycle is also limited. In this study, we isolated G1/S cyclin, cellulose synthase and cellulase encoding genes in dinoflagellate Prorocentrum donghaiense. Further, using reverse transcription quantitative PCR (RT-qPCR), we characterized the expression profiles of the three genes throughout the cell cycle. All three showed clear expression dynamics throughout the cell cycle, with fold changes of 26, 2.4 and 9.3 for G1/S cyclin, cellulose synthase and cellulase gene, respectively. The transcript abundance of G1/S cyclin increased in late G1 phase and dropped in early S phase, indicating that this protein is involved in the G1/S transition. Throughout the cell cycle, the average transcript level of cellulose synthase was 4.5-fold higher than that of cellulase. Cellulose synthase and cellulase gene expressions showed peak transcript abundances at middle G1 phase and G2M phase, respectively, indicating the respective roles of these enzymes in the growth of newly divided cells and in cytokinesis. Our results suggest that G1/S cyclin, cellulase, and cellulose synthase genes associated with G1/S transition, G2M, and G1 phases of the cell cycle and are candidates of biomarkers for assessing growth status of P. donghaiense.

3,6-Epidioxy-1,10-bisaboladiene inhibits G1 -specific transcription through Swi4/Swi6 and Mbp1/Swi6 via the Hog1 stress pathway in yeast

3,6-Epidioxy-1,10-bisaboladiene (EDBD), a bisabolane sesquiterpene endoperoxide compound, was previously isolated from Cacalia delphiniifolia and C. hastata in northern Japan. EDBD has cytotoxic effects and induces apoptosis via phosphorylation of p38 mitogen-activated protein kinase in human promyelocytic leukemia HL60 cells. However, the mechanism of action of EDBD has not yet been fully elucidated. In this study, we examined the molecular mechanisms of EDBD in the budding yeast Saccharomyces cerevisiae. EDBD arrested the growth of S. cerevisiae cells by suppressing progression from the G1 phase to the S phase and from the G2 phase to the M phase. Moreover, biochemical and genetic analyses revealed that EDBD activated environmental stress-response pathways involving Hog1 and affected Cln3/G1 cyclin activity, thereby inhibiting the expression of SCB-binding factor and MCB-binding factor target genes. Our results provided important insights into the functions of EDBD in tumor cells and may facilitate the development of EDBD-based antitumor therapies.

STRUCTURED DIGITAL ABSTRACT

•Swi4 physically interacts with Swi6 by anti tag coimmunoprecipitation (View interaction).

Eupolyphaga sinensis Walker inhibits human chronic myeloid leukemia cell K562 growth by inducing G2-M phase cell cycle arrest and targeting EGFR signaling pathway and in S180 tumor-bearing mice

Eupolyphaga sinensis Walker is not only used as a food to enhance immunity, but is used as a traditional Chinese medicine and is known as the "preferred drug to regulate blood flow". Previous studies have reported its potential biological activities including anticoagulation, antithrombotic, liver protective effect and antitumor effects. Our results indicated that E. sinensis Walker 70% ethanol extract exhibited anti-tumor effects on S180 (murine sarcoma cell line) cells implanted mice. It effectively inhibited K562 (human chronic myeloid leukemia cell line) cells proliferation and induced G(2)-M phase arrest accompanying through up-regulation of cyclin B1, cdc2 and down-regulation of cyclin D1, cyclin E1, cdc25c and p53. In addition, it inhibited EGF secretion and EGFR kinase activity. Western blotting analysis indicated that it also inhibited the phosphorylation EGFR and activation of its downstream signaling molecules AKT and ERK. These results suggested that the antitumor mechanism of E. sinensis Walker involved altering the cell cycle and inhibiting EGFR phosphorylation in the EGFR signaling pathway.

Among B-type cyclins only CLB5 and CLB6 promote premeiotic S phase in Saccharomyces cerevisiae

The Saccharomyces cerevisiae cyclin Clb5 is required for premeiotic S phase, meiotic recombination, and successful progression through meiosis. Clb5 is not essential for mitotic proliferation because Clb1-Clb4 can support DNA replication in clb5 clb6 mutants. Clb1, Clb3, and Clb4 accumulate in clb5 clb6 cells during meiotic differentiation yet fail to promote premeiotic DNA replication. When expressed under the regulation of the CLB5 promoter, Clb1 and Clb3 accumulate and are active in the early stages of meiotic differentiation but cannot induce premeiotic DNA replication, suggesting that they do not target Cdk1 to the necessary substrates. The Clb5 hydrophobic patch (HP) residues are important for Clb5 function but this motif alone does not provide the specificity required for Clb5 to induce premeiotic S phase. Domain exchange experiments demonstrated that the amino terminus of Clb5 when fused to Clb3 confers upon Clb3 the ability to induce premeiotic S phase. Chimeric cyclins containing smaller regions of the Clb5 amino terminus displayed reduced ability to activate premeiotic DNA replication despite being more abundant and having greater associated histone H1 kinase activity than endogenous Clb5. These observations suggest that Clb5 has a unique ability to trigger premeiotic S phase and that the amino-terminal region of Clb5 contributes to its specificity and regulates the functions performed by the cyclin-Cdk complex.

The BAR domain proteins: molding membranes in fission, fusion, and phagy

The Bin1/amphiphysin/Rvs167 (BAR) domain proteins are a ubiquitous protein family. Genes encoding members of this family have not yet been found in the genomes of prokaryotes, but within eukaryotes, BAR domain proteins are found universally from unicellular eukaryotes such as yeast through to plants, insects, and vertebrates. BAR domain proteins share an N-terminal BAR domain with a high propensity to adopt alpha-helical structure and engage in coiled-coil interactions with other proteins. BAR domain proteins are implicated in processes as fundamental and diverse as fission of synaptic vesicles, cell polarity, endocytosis, regulation of the actin cytoskeleton, transcriptional repression, cell-cell fusion, signal transduction, apoptosis, secretory vesicle fusion, excitation-contraction coupling, learning and memory, tissue differentiation, ion flux across membranes, and tumor suppression. What has been lacking is a molecular understanding of the role of the BAR domain protein in each process. The three-dimensional structure of the BAR domain has now been determined and valuable insight has been gained in understanding the interactions of BAR domains with membranes. The cellular roles of BAR domain proteins, characterized over the past decade in cells as distinct as yeasts, neurons, and myocytes, can now be understood in terms of a fundamental molecular function of all BAR domain proteins: to sense membrane curvature, to bind GTPases, and to mold a diversity of cellular membranes.

Genes involved in the initiation of DNA replication in yeast

Replication and segregation of the information contained in genomic DNA are strictly regulated processes that eukaryotic cells alternate to divide successfully. Experimental work on yeast has suggested that this alternation is achieved through oscillations in the activity of a serine/threonine kinase complex, CDK, which ensures the timely activation of DNA synthesis. At the same time, this CDK-mediated activation sets up the basis of the mechanism that ensures ploidy maintenance in eukaryotes. DNA synthesis is initiated at discrete sites of the genome called origins of replication on which a prereplicative complex (pre-RC) of different protein subunits is formed during the G1 phase of the cell division cycle. Only after pre-RCs are formed is the genome competent to be replicated. Several lines of evidence suggest that CDK activity prevents the assembly of pre-RCs ensuring single rounds of genome replication during each cell division cycle. This review offers a descriptive discussion of the main molecular events that a unicellular eukaryote such as the budding yeast Saccharomyces cerevisiae undergoes to initiate DNA replication.

Molecular cloning and characterisation of p15(CDK-BP), a novel CDK-binding protein

The suc1/Cks proteins are well-conserved regulatory components of cyclin-dependent kinases 1 and 2 (CDK1/2). These small molecular mass proteins form a stable complex with CDK1/2 and are essential for normal regulation of CDKs during the cell division cycle and for degradation of p27(kip1). Despite the high degree of homology between the nine known CDKs, only CDK1, CDK2 and, to a lesser extent, CDK3 are able to bind to the suc1/Cks proteins. No additional suc1/Cks-related proteins interacting with other CDKs have been reported. We have purified, from starfish oocytes, a 15 kDa protein, p15(CDK-BP), which cross-reacts with anti-Cks antibodies (L. Azzi, L. Meijer, A.C. Ostvold, J. Lew, J.H. Wang, J. Biol. Chem. 269 (1994)). Following microsequencing of internal peptides and generation of corresponding oligonucleotides we cloned two cDNAs encoding two closely related proteins, p15A and p15B. The predicted protein sequences display distant but distinct homology with the Suc1/Cks proteins, including the genuine starfish Cks homologue protein, p9(CksMg). P15 transcripts are essentially expressed in oocytes. Recombinant p15B or native p15(CDK-BP) bind a 34 kDa protein cross-reacting with anti-PSTAIRE antibodies, a feature characteristic of CDK-related proteins. In addition p15B interacts tightly with CDK4, CDK6, CDK8 and the yeast CDC28-related kinase Pho85, but not with CDK1, CDK2 or CDK7. P15 does not appear to alter the catalytic activity of the bound kinases.

Expression of Cyclin E and the Cyclin-Dependent Kinase Inhibitor p27 in Malignant Lymphomas—Prognostic Implications

Cyclin E and the cyclin-dependent kinase inhibitor p27 are two important regulators of the G1-S transition modulating the activity of cyclin-dependent kinases. Aberrations in the cell cycle control are often observed in tumors and might even be mandatory in tumor development. To investigate the importance of cell-cycle defects in malignant lymphomas we have characterized the expression of cyclin E and p27 in 105 newly diagnosed lymphomas using immunohistochemistry. A significant, inverse correlation between p27 and cyclin E expression was observed (rs = −.24, P = .02) and both proteins correlated with the S-phase fraction (rs = −.35, P < .001 andrs = .45, P < .001, respectively). The inverse relationship between p27 expression and proliferation was abrogated in some lymphomas, suggesting that p27 downregulation can represent a genuine aberration. Survival analysis was performed in 105 patients with a median observation time of 86 months. Low p27 and high cyclin E expression were significantly associated with a poor prognosis (P = .0001 and .03, respectively). In a multivariate Cox analysis, p27 expression, stage, serum lactate dehydrogenase level, grade, and age were independent prognostic factors, in contrast to S-phase fraction and cyclin E expression. This is the first report showing that p27 expression in malignant lymphomas has independent prognostic significance, which necessitates future studies regarding its more precise biological role in lymphoid tumorogenesis.

© 1998 by The American Society of Hematology.

Expression of Cyclin E and the Cyclin-Dependent Kinase Inhibitor p27 in Malignant Lymphomas—Prognostic Implications

Cyclin E and the cyclin-dependent kinase inhibitor p27 are two important regulators of the G1-S transition modulating the activity of cyclin-dependent kinases. Aberrations in the cell cycle control are often observed in tumors and might even be mandatory in tumor development. To investigate the importance of cell-cycle defects in malignant lymphomas we have characterized the expression of cyclin E and p27 in 105 newly diagnosed lymphomas using immunohistochemistry. A significant, inverse correlation between p27 and cyclin E expression was observed (rs = −.24, P = .02) and both proteins correlated with the S-phase fraction (rs = −.35, P &lt; .001 andrs = .45, P &lt; .001, respectively). The inverse relationship between p27 expression and proliferation was abrogated in some lymphomas, suggesting that p27 downregulation can represent a genuine aberration. Survival analysis was performed in 105 patients with a median observation time of 86 months. Low p27 and high cyclin E expression were significantly associated with a poor prognosis (P = .0001 and .03, respectively). In a multivariate Cox analysis, p27 expression, stage, serum lactate dehydrogenase level, grade, and age were independent prognostic factors, in contrast to S-phase fraction and cyclin E expression. This is the first report showing that p27 expression in malignant lymphomas has independent prognostic significance, which necessitates future studies regarding its more precise biological role in lymphoid tumorogenesis.

© 1998 by The American Society of Hematology.

Inhibitors of calcineurin block expression of cyclins A and E induced by fibroblast growth factor in Swiss 3T3 fibroblasts

In Swiss 3T3 fibroblasts, growth factor-stimulated progression from G1 to S phase involves activation of the Ca2+/calmodulin-dependent serine/threonine-specific protein phosphatase 2B (calcineurin). Here we report that both cobalt and the calcium chelator EGTA, inhibitors of calcium uptake, as well as cyclosporin A and FK-506, specific inhibitors of calcineurin function, abolished fibroblast growth factor (FGF)-induced expression of cyclins A and E, but not cyclin D1. At 0.1 microM concentration cyclosporin A completely blocked FGF-induced expression of cyclins E and A and it inhibited FGF-stimulated DNA synthesis by 40%; full inhibition of DNA synthesis required 10 microM cyclosporin A. PD 98059, an inhibitor of mitogen-activated protein (MAP) kinase kinase, and hemicholinium-3, an inhibitor of FGF-induced MAP kinase activity, did not inhibit the stimulatory effect of FGF on the expression of cyclin E. On the other hand, the inhibitory effect of 0.1 microM cyclosporin A on FGF-stimulated DNA synthesis was additive with that of hemicholinium-3, suggesting that the two inhibitors acted by different mechanisms. The inhibitors of calcineurin and calcium uptake also completely blocked the stimulatory effects of lysophosphatidic acid on the expression of cyclins E and A, but not cyclin D1. The results suggest that FGF- or lysophosphatidic acid-induced transcription of cyclin A and cyclin E genes is mediated by calcineurin involving a MAP kinase-independent mechanism and that increased expression of cyclins A and E is required for the maximal stimulatory effects of these mitogens on DNA synthesis.

An essential domain within Cdc34p is required for binding to a complex containing Cdc4p and Cdc53p in Saccharomyces cerevisiae

The CDC34 gene of the yeast Saccharomyces cerevisiae encodes a ubiquitin-conjugating protein that transfers ubiquitin onto substrates to signal rapid degradation via the proteasome. Cdc34p has been implicated in signaling the destruction of a variety of substrates including the cyclin-dependent kinase inhibitor, Sic1p, which must be degraded for cells to enter S-phase. Mutants lacking CDC34 activity fail to degrade Sic1p and fail to enter S-phase, a phenotype that is also shared with cells lacking CDC4 and CDC53 activity. Here we demonstrate that Cdc4p, Cdc34p, and Cdc53p interact in vivo. We have mapped a Cdc4p/Cdc53p-binding region on Cdc34p; this region is essential for S-phase entry and thus the association of these three proteins is required for Sic1p degradation. All three proteins migrate in gel filtration to sizes that greatly exceed their actual size suggesting that they form stable associations with other proteins and we observe Cdc4p, Cdc34p, and Cdc53p fractionating into overlapping families of high molecular weight complexes. Finally, we demonstrate that Cdc4p, Cdc34p, and Cdc53p are stable throughout the cell cycle and that Cdc34p permanently resides as part of a complex throughout the cell cycle. This suggests that all Cdc34p substrates are ubiquitinated by a similar high molecular weight complex.

G2-and early-M-specific expression of the NTCYC1 cyclin gene in Nicotiana tabacum cells

We have previously reported the isolation of a cDNA encoding a mitotic cyclin, NTCYC1, from a tobacco cell suspension library. Here we describe the expression patterns of NTCYC1 and of Ntsuc1, a suc 1 plant homologue, in synchronized tobacco cell suspensions. Furthermore, the expression pattern of this cyclin is compared to that of Ntcdc2-1, a Nicotiana tabacum homologue of cdc2. While no NTCYC1 transcript was detected in cells synchronized in the G1 and S phases, NTCYC1 expression was observed in late G2 and early M phases, disappearing in the G1' of a new cell cycle. On the other hand, Ntsuc1 and Ntcdc2-1 exhibited a constitutive expression during the cell cycle. A functional analysis performed by microinjecting NTCYC1 mRNA into immature Xenopus oocytes, indicates that NTCYC1 could participate in the control of the G2/M transition in plant cells. Subsequently NTCYC1 expression was used to assess the status of mesophyll cells in expanded leaves of N. tabacum. Depending on leaf position along the shoot axis, a large population of mesophyll cells appeared with a 4C DNA content, suggesting a G2 arrest. It was found that leaves with such a population also contained high levels of NTCYC1 transcripts. With respect to these results concerning a naturally occurring G2-arrested cell population, the regulation of NTCYC1 expression in planta is discussed.

Cyclin A expression is under negative transcriptional control during the cell cycle

Transcription of the gene coding for cyclin A, a protein required for S-phase transit, is cell cycle regulated and is restricted to proliferating cells. To further explore transcriptional regulation linked to cell division cycle control, a genomic clone containing 5' flanking sequences of the murine cyclin A gene was isolated. When it was fused to a luciferase reporter gene, it was shown to function as a proliferation-regulated promoter in NIH 3T3 cells. Transcription of the mouse cyclin A gene is negatively regulated by arrest of cell proliferation. A mutation of a GC-rich sequence conserved between mice and humans is sufficient to relieve transcriptional repression, resulting in a promoter with constitutively high activity. In agreement with this result, in vivo footprinting reveals a protection of the cell cycle-responsive element in G0/early G1 cells which is not observed at later stages of the cell cycle. Moreover, the footprint is present in dimethyl sulfoxide-induced differentiating and not in proliferating Friend erythroleukemia cells. Conversely, two other sites, which in vitro bind ATF-1 and NF-Y, respectively, are constitutively occupied throughout cell cycle progression.

Xe-p9, a Xenopus Suc1/Cks homolog, has multiple essential roles in cell cycle control

The small Suc1/Cks protein is a ubiquitous subunit of Cdk/cyclin complexes, but its precise function has remained unclear. We have isolated a Xenopus homolog, Xe-p9, of the Suc1/Cks protein by virtue of its ability to rescue a fission yeast mutant that enters mitosis prematurely. To assess its functional role in cell cycle control, we have both overexpressed p9 in Xenopus egg extracts and immunodepleted the protein from these extracts. We found that addition of recombinant His6-p9 to egg extracts results in a pronounced delay of mitosis that can be attributed to an inhibition of the tyrosine dephosphorylation of the inactive Cdc2/cyclin B complex. In immunodepletion studies, we observed that the consequences of removing p9 from egg extracts depend on the stage of the cell cycle. Specifically, in the case of interphase extracts, the removal of p9 abolishes the entry into mitosis as a result of a failure in the activation of the Cdc2/cyclin B complex by tyrosine dephosphorylation. Furthermore, mitotic extracts lacking p9 fail to exit mitosis because of a defect in the destruction of cyclin B. Collectively, these results indicate that p9 has multiple essential roles in the cell cycle by governing the interaction of the Cdc2/cyclin B complex with both positive and negative regulators.

Cell cycle regulation by the retinoblastoma family of growth inhibitory proteins

The retinoblastoma family of growth-inhibitory proteins act by binding and inhibiting several proteins with growth-stimulatory activity, the most prominent of which is the cellular transcription factor E2F. In higher organisms, progression through the cell division cycle is accompanied by the cyclical activation of a number of protein kinases, the cyclin-dependent kinases. Phosphorylation of retinoblastoma family proteins by these cyclin-dependent kinases leads to release of the associated growth-stimulatory proteins which in turn mediate progression through the cell division cycle.

Pds1p is required for faithful execution of anaphase in the yeast, Saccharomyces cerevisiae

To identify mutations that cause defects in mitosis, a collection of mutants in Saccharomyces cerevisiae was screened by a rapid visual assay for abnormal chromosome segregation. From this screen we identified one mutation, pds1-1 that was independently identified in an alternative screen for mutants that exhibit inviability after transient exposure to nocodazole and precocious disassociation of sister chromatids (Guacci, V., A. Yamamoto, A. Strunnikov, J. Kingsbury, E. Hogan, P. Meluh, and D. Koshland. 1993. CSH Symp. Quant. Biol. 58:677-685; Yamamoto, T.J., G. Li, B. Schaar, I. Szilak, and D.W. Cleveland. 1992. Nature (Lond.). 359:536-539). At 23 degrees C pds1-1 mutants exhibit frequent cell death and a 300-fold increase in chromosome loss compared to wild type. At 37 degrees C pds1-1 cells fail to elongate their spindles during anaphase. This spindle defect of pds1 mutants results from a temperature-sensitive step that occurs around the G1/S boundary about the time of spindle assembly. In the absence of spindle elongation pds1 mutants undergo cytokinesis, leading to the missegregation of both chromosomes and spindle pole bodies. After abnormal cell division pds1-1 mutants also initiate new rounds of DNA replication, spindle pole body duplication, and bud formation. Thus, in the pds1-1 mutant at 37 degrees C, cell cycle progression is uncoupled from the completion of anaphase. A pds1 deletion allele has similar phenotypes to the original allele. Taken together these results suggest that Pds1 protein plays an important role in chromosome segregation at 23 degrees C and an essential role for this process at 37 degrees C. The PDS1 gene encodes a novel 42-kD nuclear protein that has both basic and acidic domains. The level of PDS1 mRNA varies with the cell cycle with maximal accumulation around the G1/S boundary. The stability of Pds1 protein also appears to change during the cell cycle as overproduced Pds1p is stable in S and M but degraded in early G1. Therefore, expression of Pds1p is regulated apparently both transcriptionally and postranslationally during the cell cycle. The phenotypes of pds1 mutants and expression pattern of Pds1p are discussed in the context of other spindle-defective mutants and the knowledge that Pds1 protein is an inhibitor of anaphase (Yamamoto, T.J., G. Li, B. Schaar, I. Szilak, and D.W. Cleveland. 1992. Nature (Lond.). 359:536-539).

Cig2, a B-type cyclin, promotes the onset of S in Schizosaccharomyces pombe

Cdc2, a catalytic subunit of cyclin-dependent kinases, is required for both the G1-to-S and G2-to-M transitions in the fission yeast Schizosaccharomyces pombe. Cdc13, a B-type cyclin, is required for the M-phase induction function of Cd2. Two additional B-type cyclins, Cig1 and Cig2, have been identified in S. pombe, but none of the B-type cyclins are individually required for the onset of S. We report that Cdc13 is important for DNA replication in a strain lacking Cig2. Unlike deltacdc13 cells, double-mutant deltacdc13 deltacig2 cells are defective in undergoing multiple rounds of DNA replication. The conclusion that Cig2 promotes S is further supported by the finding that Cig2 protein and Cig2-associated kinase activity appear soon after the completion of M and peak during S, as well as the observation that S is delayed in deltacig2 cells as they recover from a G1 arrest induced by nitrogen starvation. These studies indicate that Cig2 is the primary S-phase-promoting cyclin in S. pombe but that Cdc13 can effectively substitute for Cig2 in deltacig2 cells. These observations also suggest that the gradual increase in the activity of Cdc2-Cdc13 kinase can be sufficient for the correct temporal ordering of S and M phases in deltacig2 cells.

Pds1p, an inhibitor of anaphase in budding yeast, plays a critical role in the APC and checkpoint pathway(s)

We report the isolation and characterization of pds1 mutants in Saccharomyces cerevisiae. The initial pds1-1 allele was identified by its inviability after transient exposure to microtubule inhibitors and its precocious dissociation of sister chromatids in the presence of these microtubule inhibitors. These findings suggest that pds1 mutants might be defective in anaphase arrest that normally is imposed by a spindle-damage checkpoint. To further examine a role for Pds1p in anaphase arrest, we compared the cell cycle arrest of pds1 mutants and PDS1 cells after: (a) the inactivation of Cdc16p or Cdc23p, two proteins that are required for the degradation of mitotic cyclins and are putative components of the yeast anaphase promoting complex (APC); (b) the inactivation of Cdc20p, another protein implicated in the degradation of mitotic cyclins; and (c) the inactivation of Cdc13 protein or gamma irradiation, two circumstances that induce a DNA-damage checkpoint. Under all these conditions, anaphase is inhibited in PDS1 cells but not in pds1 mutants. From these results we suggest that Pds1 protein is an anaphase inhibitor in PDS1 cells but not in pds1 mutants. From these results we suggest that Pds1 protein is an anaphase inhibitor that plays a critical role in the control of anaphase by both APC and checkpoints. We also show that pds1 mutants exit mitosis and initiate new rounds of cell division after gamma irradiation and Cdc13p inactivation but no after nocodazole-treatment or inactivation of Cdc16p, Cdc20p or Cdc23p function. Therefore, in the DNA-damage checkpoint, Pds1p is required for the inhibition of cytokinesis and DNA replication as well as anaphase. The role of Pds1 protein in anaphase inhibition and general cell cycle regulation is discussed.

Helical fold prediction for the cyclin box

The smooth progression of the eukaryotic cell cycle relies on the periodic activation of members of a family of cell cycle kinases by regulatory proteins called cyclins. Outside of the cell cycle, cyclin homologs play important roles in regulating the assembly of transcription complexes; distant structural relatives of the conserved cyclin core or "box" can also function as general transcription factors (like TFIIB) or survive embedded in the chain of the tumor suppressor, retinoblastoma protein. The present work attempts the prediction of the canonical secondary, supersecondary, and tertiary fold of the minimal cyclin box domain using a combination of techniques that make use of the evolutionary information captured in a multiple alignment of homolog sequences. A tandem set of closely packed, helical modules are predicted to form the cyclin box domain.

Negative regulation of G1 and G2 by S-phase cyclins of Saccharomyces cerevisiae

Cell cycle progression in the budding yeast Saccharomyces cerevisiae is controlled by the Cdc28 protein kinase, which is sequentially activated by different sets of cyclins. Previous genetic analysis has revealed that two B-type cyclins, Clb5 and Clb6, have a positive role in DNA replication. In the present study, we show, in addition, that these cyclins negatively regulate G1- and G2-specific functions. The consequences of this negative regulation were most apparent in clb6 mutants, which had a shorter pre-Start G1 phase as well as a shorter G2 phase than congenic wild-type cells. As a consequence, clb6 mutants grew and proliferated more rapidly than wild-type cells. It was more difficult to assess the role of Clb5 in G1 and G2 by genetic analysis because of the extreme prolongation of S phase in clb5 mutants. Nevertheless, both Clb5 and Clb6 were shown to be responsible for down-regulation of the protein kinase activities associated with Cln2, a G1 cyclin, and Clb2, a mitotic cyclin, in vivo. These observations are consistent with the observed cell cycle phase accelerations associated with the clb6 mutant and are suggestive of similar functions for Clb5. Genetic evidence suggested that the inhibition of mitotic cyclin-dependent kinase activities was dependent on and possibly mediated through the CDC6 gene product. Thus, Clb5 and Clb6 may stabilize S phase by promoting DNA replication while inhibiting other cell cycle activities.

Cyclin A-associated kinase activity is rate limiting for entrance into S phase and is negatively regulated in G1 by p27Kip1

We have created fibroblast cell lines that express cyclin A under the control of a tetracycline-repressible promoter. When stimulated to reenter the cell cycle after serum withdrawal, these cells were advanced prematurely into S phase by induction of cyclin A. In an asynchronous population, induction of cyclin A caused a decrease in the percentage of cells in G1. These results demonstrate that expression of cyclin A is rate limiting for the G1-to-S transition and suggest that cyclin A can function as a G1 cyclin. Although the level of exogenous cyclin A was constant throughout the cell cycle, its associated kinase activity increased as cells approached S phase. Low kinase activity in early G1 was found to correlate with the presence of p27Kip1 in cyclin A-associated complexes, while high kinase activity in late G1 was correlated with its absence. These results suggest that a function of p27Kip1 in G1 is to prevent premature activation of cyclin A-associated kinase. Cyclin A expression in early G1 led to phosphorylation of the product of the retinoblastoma susceptibility gene (pRb). Thus, cyclin A expression can be rate limiting for pRb phosphorylation, implicating pRb as a physiological substrate of the cyclin A-dependent kinase. Taken together, these results demonstrate that deregulated expression of cyclin A can perturb the normal regulation of the G1-to-S transition.

Different roles for cyclins D1 and E in regulation of the G1-to-S transition

Ectopic expression of cyclins D1 and E was previously shown to accelerate the G1/S-phase transition, indicating that both classes of G1 cyclin control an event(s) that is rate limiting for entry into S phase. In order to determine whether cyclins D1 and E control the same or two different rate-limiting events, we have created cell lines that express both cyclins in an inducible manner. We show here that ectopic expression of both cyclins E and D1 in the same cell has an additive effect on shortening of the G1 interval relative to expression of any single cyclin. In order to further explore the molecular basis for G1 cyclin action, we used cell lines capable of expressing cyclin D1, E, or both prematurely and measured the effect of cyclin expression in early G1 on phosphorylation of the retinoblastoma susceptibility gene product (pRb). We show here that while premature expression of either cyclin alone advances the G1/S-phase transition to the same extent, premature expression of cyclin D1 leads to immediate appearance of hyperphosphorylated pRb, while premature expression of cyclin E does not. Ectopic expression of both cyclins E and D1 in the same cell has an additive effect on shortening of the G1 interval, while the effect on pRb phosphorylation is similar to the effect of cyclin D1 alone. These results suggest that cyclins E and D1 control two different events, both rate limiting for the G1/S-phase transition, and that pRb phosphorylation might be the rate-limiting event controlled by cyclin D1.

A method to predict functional residues in proteins

The biological activity of a protein typically depends on the presence of a small number of functional residues. Identifying these residues from the amino acid sequences alone would be useful. Classically, strictly conserved residues are predicted to be functional but often conservation patterns are more complicated. Here, we present a novel method that exploits such patterns for the prediction of functional residues. The method uses a simple but powerful representation of entire proteins, as well as sequence residues as vectors in a generalised 'sequence space'. Projection of these vectors onto a lower-dimensional space reveals groups of residues specific for particular subfamilies that are predicted to be directly involved in protein function. Based on the method we present testable predictions for sets of functional residues in SH2 domains and in the conserved box of cyclins.

Expression and activity of p40MO15, the catalytic subunit of cdk-activating kinase, during Xenopus oogenesis and embryogenesis

Threonine 161 phosphorylation of p34cdc2 and its equivalent threonine 160 in p33cdk2 by cdk-activating kinase (CAK) is essential for the activation of these cyclin-dependent kinases. We have studied the expression and associated kinase activity of p40MO15, the catalytic subunit of CAK, during Xenopus oogenesis, meiotic maturation, and early development to understand in more detail how cdk kinases are regulated during these events. We find that p40MO15 is a stable protein with a half-life > 16 h that is accumulated during oogenesis. p40MO15 protein and its associated CAK activity are localized predominantly to the germinal vesicle; however, a small but significant proportion is found in the cytoplasm. The amount of p40MO15 detected in stage VI oocytes remains unchanged through meiotic maturation, fertilization, and early embryogenesis. Significantly, p40MO15 was found to be constitutively active during oogenesis, meiotic maturation, and the rapid mitotic cycles of early development. This suggests that regulation of p34cdc2 and p33cdk2 activity during cell cycle progression does not involve changes in the level or activity of p40MO15/CAK.

P40SDB25, a putative CDK inhibitor, has a role in the M/G1 transition in Saccharomyces cerevisiae

The Saccharomyces cerevisiae protein kinase Dbf2 carries out an essential function in late mitosis, and its kinase activity is cell-cycle regulated around anaphase/telophase. We have isolated SDB25, a high copy suppressor of temperature-sensitive dbf2 mutants, and genetic analysis suggests that the two proteins may function in parallel pathways in late mitosis. SDB25 encodes p40, a previously characterized substrate and potent inhibitor of Cdc28 kinase activity. Sdb25 is a phosphoprotein, and Sdb25 immunoprecipitates have a histone H1 kinase activity that is CDC28-dependent. Remarkably, Sdb25 transcript levels, protein levels, and associated kinase activity are precisely cell-cycle regulated, sharing a common peak in late mitosis. Moreover, Sdb25 protein levels and associated kinase activity are sharply up-regulated at the peak of Dbf2 kinase activity in cells released from a dbf2 ts block. The Sdb25 protein then disappears around Start in the next cell cycle. This indicates that SDB25 function is confined to M/G1, and morphological analysis of sdb25 delta cells supports this conclusion. Our data suggest that Sdb25 functions in a pathway in late mitosis leading to the down-regulation of Cdc28 kinase activity as cells traverse the M/G1 boundary.

A gene from the hypotrichous ciliate Stylonychia lemnae coding for a protein with homology to cyclin B

A nucleotide (nt) sequence of a DNA molecule from Stylonychia lemnae with an open reading frame encoding a protein showing homology to cyclin B has been determined. The DNA molecule is 3791-nt long and the deduced 444-amino-acid (aa) sequence shares about 30% identity with the sequences of two yeast cyclin-B homologs over a length of about 210 aa.

p53-dependent inhibition of cyclin-dependent kinase activities in human fibroblasts during radiation-induced G1 arrest

gamma-Irradiation of human diploid fibroblasts in the G1 interval caused arrest of the cell cycle prior to S phase. This cell cycle block was correlated with a lack of activation of both cyclin E-Cyclin-dependent kinase 2 (Cdk2) and cyclin A-Cdk2 kinases and depended on wild-type p53. Although the accumulation of cyclin A was strongly inhibited in gamma-irradiated cells, cyclin E accumulated and bound Cdk2 at normal levels but remained in an inactive state. We found that both whole-cell lysates and inactive cyclin E-Cdk2 complexes prepared from irradiated cells contained an activity capable of inactivating cyclin E-Cdk2 complexes. The protein responsible for this activity was shown to be p21CIP1/WAF1, recently described as a p53-inducible Cdk inhibitor. Our data suggest a model in which ionizing radiation confers G1 arrest via the p53-mediated induction of a Cdk inhibitor protein.

Acceleration of the G1/S phase transition by expression of cyclins D1 and E with an inducible system

Conditional overexpression of human cyclins B1, D1, and E was accomplished by using a synthetic cDNA expression system based on the Escherichia coli tetracycline repressor. After induction of these cyclins in asynchronous Rat-1 fibroblasts, a decrease in the length of the G1 interval was observed for cyclins D1 and E, consistent with an acceleration of the G1/S phase transition. We observed, in addition, a compensatory lengthening of S phase and G2 so that the mean cell cycle length in populations constitutively expressing these cyclins was unchanged relative to those of their uninduced counterparts. We found that expression of cyclin B1 had no effect on cell cycle dynamics, despite elevated levels of cyclin B-associated histone H1 kinase activity. Induction of cyclins D1 and E also accelerated entry into S phase for synchronized cultures emerging from quiescence. However, whereas cyclin E exerted a greater effect than cyclin D1 in asynchronous cycling cells, cyclin D1 conferred a greater effect upon stimulation from quiescence, suggesting a specific role for cyclin D1 in the G0-to-G1 transition. Overexpression of cyclins did not prevent cells from entering into quiescence upon serum starvation, although a slight delay in attainment of quiescence was observed for cells expressing either cyclin D1 or cyclin E. These results suggest that cyclins D1 and E are rate-limiting activators of the G1-to-S phase transition and that cyclin D1 might play a specialized role in facilitating emergence from quiescence.

Evidence for a protein domain superfamily shared by the cyclins, TFIIB and RB/p107

Cyclins, TFIIB and RB play major roles in cell cycle and/or gene regulation. Earlier work has suggested common ancestry for the TFIIB repeats and RB pocket B which share 20% sequence identity. We now report that database searches with profiles based on a multiple alignment of cyclin core regions (the 'cyclin box') detect the TFIIB repeats with equivalent scores to divergent cyclins. Several features of the sequences support the notion of common ancestry: e.g. cyclins A/B, C and D share approximately 20-30% identity but each have approximately 15-20% identity with vertebrate TFIIB, showing that conserved cyclin features underlie the match. These results suggest the presence of a domain superfamily, which we term the TR domain, in nuclear regulatory proteins belonging to the TFIIB, cyclin and RB families, that has been duplicated many times during eukaryotic evolution. The TR domain appears to function in protein-protein interactions.

Acceleration of the G1/S phase transition by expression of cyclins D1 and E with an inducible system

Conditional overexpression of human cyclins B1, D1, and E was accomplished by using a synthetic cDNA expression system based on the Escherichia coli tetracycline repressor. After induction of these cyclins in asynchronous Rat-1 fibroblasts, a decrease in the length of the G1 interval was observed for cyclins D1 and E, consistent with an acceleration of the G1/S phase transition. We observed, in addition, a compensatory lengthening of S phase and G2 so that the mean cell cycle length in populations constitutively expressing these cyclins was unchanged relative to those of their uninduced counterparts. We found that expression of cyclin B1 had no effect on cell cycle dynamics, despite elevated levels of cyclin B-associated histone H1 kinase activity. Induction of cyclins D1 and E also accelerated entry into S phase for synchronized cultures emerging from quiescence. However, whereas cyclin E exerted a greater effect than cyclin D1 in asynchronous cycling cells, cyclin D1 conferred a greater effect upon stimulation from quiescence, suggesting a specific role for cyclin D1 in the G0-to-G1 transition. Overexpression of cyclins did not prevent cells from entering into quiescence upon serum starvation, although a slight delay in attainment of quiescence was observed for cells expressing either cyclin D1 or cyclin E. These results suggest that cyclins D1 and E are rate-limiting activators of the G1-to-S phase transition and that cyclin D1 might play a specialized role in facilitating emergence from quiescence.

Reversal of terminal differentiation and control of DNA replication: cyclin A and Cdk2 specifically localize at subnuclear sites of DNA replication

DNA replication in mammalian cells occurs in discrete nuclear foci. Here we show that terminally differentiated myotubes can be induced to reenter S phase and show the same pattern of replication foci as cycling cells. We used this cellular system to analyze the interaction of cell cycle proteins with these foci in vivo. Cyclin A and cdk2, but not cyclin B1 and cdc2, were specifically localized at nuclear replication foci, just like the replication protein proliferating cell nuclear antigen. A potential target of cyclin A and cdk2 is the 34 kd subunit of replication protein A (RPA34). In contrast with the 70 kd subunit, which localizes to the foci, RPA34 was not detected at these replication sites, which may reflect a transient interaction. The specific localization of cyclin A and cdk2 at nuclear replication foci provides a direct link between cell cycle regulation and DNA replication.

Mechanisms that help the yeast cell cycle clock tick: G2 cyclins transcriptionally activate G2 cyclins and repress G1 cyclins

In budding yeast, G1 cyclins such as CLN1 and CLN2 are expressed in G1 and S phases, while mitotic cyclins such as CLB1 and CLB2 are expressed in G2 and M phases. We find that the CLBs play a central role in the transition from CLNs to CLBs: the CLBs stimulate their own expression while repressing that of CLNs. This negative regulation of CLNs may occur via the transcription factor SWI4, because CLBs are necessary for G2 repression of SCB-regulated genes like CLN1 and CLN2 but not for repression of MCB-regulated genes like DNA polymerase and CLB5. Furthermore, SW14 associates with CLB2 protein and is a substrate for the CLB2-associated CDC28 kinase in vitro.

A three-dimensional model of the Cdc2 protein kinase: localization of cyclin- and Suc1-binding regions and phosphorylation sites

The Cdc2 protein kinase requires cyclin binding for activity and also binds to a small protein, Suc1. Charged-to-alanine scanning mutagenesis of Cdc2 was used previously to localize cyclin A- and B- and Suc1-binding sites (B. Ducommun, P. Brambilla, and G. Draetta, Mol. Cell. Biol. 11:6177-6184, 1991). Those sites were mapped by building a Cdc2 model based on the crystallographic coordinates of the catalytic subunit of cyclic AMP-dependent protein kinase (cAPK) (D. R. Knighton, J. Zheng, L. F. Ten Eyck, V. A. Ashford, N.-H. Xuong, S. S. Taylor, and J. M. Sowadski, Science 253:407-414, 1991). On the basis of this model, additional mutations were made and tested for cyclin A and Suc1 binding and for kinase activity. Mutations that interfere with cyclin A binding are localized primarily on the small lobe near its interface with the cleft and include an acidic patch on the B helix and R-50 in the highly conserved PSTAIRE sequence. Two residues in the large lobe, R-151 and T-161, influence cyclin binding, and both are at the surface of the cleft near its interface with the PSTAIRE motif. Cyclin-dependent phosphorylation of T-161 in Cdc2 is essential for activation, and the model provides insights into the importance of this site. T-161 is equivalent to T-197, a stable phosphorylation site in cAPK. On the basis of the model, cyclin binding very likely alters the surface surrounding T-161 to allow for T-161 phosphorylation. The two major ligands to T-197 in cAPK are conserved as R-127 and R-151 in Cdc2. The equivalent of the third ligand, H-87, is T-47 in the PSTAIRE sequence motif. Once phosphorylated, T-161 is predicted to play a major structural role in Cdc2, comparable to that of T-197 in cAPK, by assembling the active conformation required for peptide recognition. The inhibitory phosphorylation at Y-15 also comes close to the cleft interface and on the basis of this model would disrupt the cleft interface and the adjacent peptide recognition site rather than prevent ATP binding. In contrast to cyclin A, both lobes influence Suc1 binding; however, the Suc1-binding sites are far from the active site. Several mutants map to the surface in cAPK, which is masked in part by the N-terminal 40 residues that lie outside the conserved catalytic core. The other Suc1-binding site maps to the large lobe near a 25-residue insert and includes R-215.

A three-dimensional model of the Cdc2 protein kinase: localization of cyclin- and Suc1-binding regions and phosphorylation sites

The Cdc2 protein kinase requires cyclin binding for activity and also binds to a small protein, Suc1. Charged-to-alanine scanning mutagenesis of Cdc2 was used previously to localize cyclin A- and B- and Suc1-binding sites (B. Ducommun, P. Brambilla, and G. Draetta, Mol. Cell. Biol. 11:6177-6184, 1991). Those sites were mapped by building a Cdc2 model based on the crystallographic coordinates of the catalytic subunit of cyclic AMP-dependent protein kinase (cAPK) (D. R. Knighton, J. Zheng, L. F. Ten Eyck, V. A. Ashford, N.-H. Xuong, S. S. Taylor, and J. M. Sowadski, Science 253:407-414, 1991). On the basis of this model, additional mutations were made and tested for cyclin A and Suc1 binding and for kinase activity. Mutations that interfere with cyclin A binding are localized primarily on the small lobe near its interface with the cleft and include an acidic patch on the B helix and R-50 in the highly conserved PSTAIRE sequence. Two residues in the large lobe, R-151 and T-161, influence cyclin binding, and both are at the surface of the cleft near its interface with the PSTAIRE motif. Cyclin-dependent phosphorylation of T-161 in Cdc2 is essential for activation, and the model provides insights into the importance of this site. T-161 is equivalent to T-197, a stable phosphorylation site in cAPK. On the basis of the model, cyclin binding very likely alters the surface surrounding T-161 to allow for T-161 phosphorylation. The two major ligands to T-197 in cAPK are conserved as R-127 and R-151 in Cdc2. The equivalent of the third ligand, H-87, is T-47 in the PSTAIRE sequence motif. Once phosphorylated, T-161 is predicted to play a major structural role in Cdc2, comparable to that of T-197 in cAPK, by assembling the active conformation required for peptide recognition. The inhibitory phosphorylation at Y-15 also comes close to the cleft interface and on the basis of this model would disrupt the cleft interface and the adjacent peptide recognition site rather than prevent ATP binding. In contrast to cyclin A, both lobes influence Suc1 binding; however, the Suc1-binding sites are far from the active site. Several mutants map to the surface in cAPK, which is masked in part by the N-terminal 40 residues that lie outside the conserved catalytic core. The other Suc1-binding site maps to the large lobe near a 25-residue insert and includes R-215.

The Cdk-associated protein Cks1 functions both in G1 and G2 in Saccharomyces cerevisiae

The CKS1 gene of Saccharomyces cerevisiae encodes a small essential protein shown to interact genetically and physically with the Cdc28 protein kinase. To investigate the specific functions of the CKS1 gene product, conditional temperature-sensitive mutant alleles were generated. The mutations were found to impair the ability of cells to undergo both the G1/S-phase and G2/M-phase transitions of the cell cycle, as well as the ability to bud. Mutants were not defective, however, in their ability to activate Cdc28 kinase as assayed in vitro on the substrate histone H1. It is likely, therefore, that Cks1 mediates a more specialized function of the Cdc28 kinase such as its ability to form specific multimeric complexes or to localize properly in cellular compartments.

Differential function and expression of Saccharomyces cerevisiae B-type cyclins in mitosis and meiosis

We have studied the patterns of expression of four B-type cyclins (Clbs), Clb1, Clb2, Clb3, and Clb4, and their ability to activate p34cdc28 during the mitotic and meiotic cell cycles of Saccharomyces cerevisiae. During the mitotic cell cycle, Clb3 and Clb4 were expressed and induced a kinase activity in association with p34cdc28 from early S phase up to mitosis. On the other hand, Clb1 and Clb2 were expressed and activated p34cdc28 later in the mitotic cell cycle, starting in late S phase and continuing up to mitosis. The pattern of expression of Clb3 and Clb4 suggests a possible role in the regulation of DNA replication as well as mitosis. Clb1 and Clb2, whose pattern of expression is similar to that of other known Clbs, are likely to have a role predominantly in the regulation of M phase. During the meiotic cell cycle, Clb1, Clb3, and Clb4 were expressed and induced a p34cdc28-associated kinase activity just before the first meiotic division. The fact that Clb3 and Clb4 were not synthesized earlier, in S phase, suggests that these cyclins, which probably have a role in S phase during the mitotic cell cycle, are not implicated in premeiotic S phase. Clb2, the primary mitotic cyclin in S. cerevisiae, was not detectable during meiosis. Sporulation experiments on strains deleted for one, two, or three Clbs indicate, in agreement with the biochemical data, that Clb1 is the primary cyclin for the regulation of meiosis, while Clb2 is not involved at all.

Control of the yeast cell cycle by the Cdc28 protein kinase

It is becoming increasingly apparent that the diverse functions of Cdc28 during the yeast cell cycle are performed by forms of the kinase that are distinguished by their cyclin subunits. Entry into the cell cycle at START involves the Cln cyclins. S phase needs Clb5 or Clb6 B-type cyclins. Bipolar mitotic spindle formation involves Clb1-4 B-type cyclins. Much of the order and timing of the cell cycle events may involve the progressive activation of Cdc28 kinase activities associated with different cyclins, whose periodicity during the cycle is determined by both transcriptional and post-transcriptional controls.

Differential function and expression of Saccharomyces cerevisiae B-type cyclins in mitosis and meiosis

We have studied the patterns of expression of four B-type cyclins (Clbs), Clb1, Clb2, Clb3, and Clb4, and their ability to activate p34cdc28 during the mitotic and meiotic cell cycles of Saccharomyces cerevisiae. During the mitotic cell cycle, Clb3 and Clb4 were expressed and induced a kinase activity in association with p34cdc28 from early S phase up to mitosis. On the other hand, Clb1 and Clb2 were expressed and activated p34cdc28 later in the mitotic cell cycle, starting in late S phase and continuing up to mitosis. The pattern of expression of Clb3 and Clb4 suggests a possible role in the regulation of DNA replication as well as mitosis. Clb1 and Clb2, whose pattern of expression is similar to that of other known Clbs, are likely to have a role predominantly in the regulation of M phase. During the meiotic cell cycle, Clb1, Clb3, and Clb4 were expressed and induced a p34cdc28-associated kinase activity just before the first meiotic division. The fact that Clb3 and Clb4 were not synthesized earlier, in S phase, suggests that these cyclins, which probably have a role in S phase during the mitotic cell cycle, are not implicated in premeiotic S phase. Clb2, the primary mitotic cyclin in S. cerevisiae, was not detectable during meiosis. Sporulation experiments on strains deleted for one, two, or three Clbs indicate, in agreement with the biochemical data, that Clb1 is the primary cyclin for the regulation of meiosis, while Clb2 is not involved at all.

Morphogenesis in the yeast cell cycle: regulation by Cdc28 and cyclins

Analysis of cell cycle regulation in the budding yeast Saccharomyces cerevisiae has shown that a central regulatory protein kinase, Cdc28, undergoes changes in activity through the cell cycle by associating with distinct groups of cyclins that accumulate at different times. The various cyclin/Cdc28 complexes control different aspects of cell cycle progression, including the commitment step known as START and mitosis. We found that altering the activity of Cdc28 had profound effects on morphogenesis during the yeast cell cycle. Our results suggest that activation of Cdc28 by G1 cyclins (Cln1, Cln2, or Cln3) in unbudded G1 cells triggers polarization of the cortical actin cytoskeleton to a specialized pre-bud site at one end of the cell, while activation of Cdc28 by mitotic cyclins (Clb1 or Clb2) in budded G2 cells causes depolarization of the cortical actin cytoskeleton and secretory apparatus. Inactivation of Cdc28 following cyclin destruction in mitosis triggers redistribution of cortical actin structures to the neck region for cytokinesis. In the case of pre-bud site assembly following START, we found that the actin rearrangement could be triggered by Cln/Cdc28 activation in the absence of de novo protein synthesis, suggesting that the kinase may directly phosphorylate substrates (such as actin-binding proteins) that regulate actin distribution in cells.

Neuronal cdc2-like kinase: a cdc2-related protein kinase with predominantly neuronal expression

Recent studies have shown that there exists a family of protein kinases structurally and functionally related to the yeast cell cycle regulatory kinase cdc2 [Meyerson, M., Faha, B., Su, L.-K., Harlow, E. & Tsai, L.-H. (1991) Cold Spring Harbor Symp. Quant. Biol. 56, 177-186 and Meyerson, M., Enders, G. H., Wu, C.-L., Su, L.-K., Gorka, C., Nelson, C., Harlow, E. & Tsai, L.-H. (1992) EMBO J. 11, 2909-2917]. Two members of cdc2 family, p34cdc2 (also named cdk1) and cdk2, have been identified in mammalian cells. cdk1 kinase regulates the progression from G2 to M phase, and cdk2 kinase has been proposed to regulate the progression from G1 to S phase. In this work, we have cloned and structurally characterized a third member of the cdc2 kinase family with 58% amino acid sequence identity to mouse cdk1 and 61% identity to human cdk2. We call this kinase neuronal cdc2-like kinase (nclk) because, in contrast to either cdk1 or cdk2, nclk is expressed at high levels in terminally differentiated neurons no longer in the cell cycle. Previous studies have shown [Hisanaga, S., Kusubata, M., Okumura, E. & Kishimoto, T. (1991) J. Biol. Chem. 266, 21798-21803 and Guan, R. J., Hall, F. L. & Cohlberg, J. A. (1992) J. Neurochem. 58, 1365-1371] that cdk1 kinase, but not other structurally defined protein kinases, could phosphorylate the repeated Lys-Ser-Pro (KSP) motifs found in mammalian high and middle molecular mass neurofilament subunits in vitro, but the precise molecular nature of the endogenous neuronal KSP kinase has remained undefined. The structural similarity of nclk to cdk1 kinase and its high level of expression in terminally differentiated neurons suggest that nclk may play a role in the phosphorylation of the neurofilament KSP repeats in vivo, a function distinct from cell cycle regulation.

Fission yeast cdc21+ belongs to a family of proteins involved in an early step of chromosome replication

The cdc21+ gene of Schizosaccharomyces pombe was originally identified in a screen for cdc mutants affecting S phase and nuclear division. Here we show that the cdc21+ gene product belongs to a family of proteins implicated in DNA replication. These include the Saccharomyces cerevisiae MCM2 and MCM3 proteins, which are needed for the efficient function of certain replication origins, and S.cerevisiae CDC46, which is required for the initiation of chromosome replication. The cdc21 mutant is defective in the mitotic maintenance of some plasmids, like mcm2 and mcm3. The mutant arrests with a single nucleus containing two genome equivalents of DNA, and maintains a cytoplasmic microtubular configuration. Activation of most, but not all, replication origins in the mutant may result in failure to replicate a small proportion of the genome, and this could explain the arrest phenotypes. Using the polymerase chain reaction technique, we have identified new cdc21(+)-related genes in S.cerevisiae, S.pombe and Xenopus laevis. Our results suggest that individual members of the cdc21(+)-related family are highly conserved in evolution.

Cyclin-B homologs in Saccharomyces cerevisiae function in S phase and in G2

We have cloned four cyclin-B homologs from Saccharomyces cerevisiae, CLB1-CLB4, using the polymerase chain reaction and low stringency hybridization approaches. These genes form two classes based on sequence relatedness: CLB1 and CLB2 show highest homology to the Schizosaccharomyces pombe cyclin-B homolog cdc13 involved in the initiation of mitosis, whereas CLB3 and CLB4 are more highly related to the S. pombe cyclin-B homolog cig1, which appears to play a role in G1 or S phase. CLB1 and CLB2 mRNA levels peak late in the cell cycle, whereas CLB3 and CLB4 are expressed earlier in the cell cycle but peak later than the G1-specific cyclin, CLN1. Analysis of null mutations suggested that the CLB genes exhibit some degree of redundancy, but clb1,2 and clb2,3 cells were inviable. Using clb1,2,3,4 cells rescued by conditional overproduction of CLB1, we showed that the CLB genes perform an essential role at the G2/M-phase transition, and also a role in S phase. CLB genes also appear to share a role in the assembly and maintenance of the mitotic spindle. Taken together, these analyses suggest that CLB1 and CLB2 are crucial for mitotic induction, whereas CLB3 and CLB4 might participate additionally in DNA replication and spindle assembly.