A WD repeat protein controls the cell cycle and differentiation by negatively regulating Cdc2/B-type cyclin complexes

Cip1, a CDK regulator, determines heterothallic mating or homothallic selfing in a protist

Mating type (sex) plays a crucial role in regulating sexual reproduction in most extant eukaryotes. One of the functions of mating types is ensuring self-incompatibility to some extent, thereby promoting genetic diversity. However, heterothallic mating is not always the best mating strategy. For example, in low-density populations or specific environments, such as parasitic ones, species may need to increase the ratio of potential mating partners. Consequently, many species allow homothallic selfing (i.e., self-fertility or intraclonal mating). Throughout the extensive evolutionary history of species, changes in environmental conditions have influenced mating strategies back and forth. However, the mechanisms through which mating-type recognition regulates sexual reproduction and the dynamics of mating strategy throughout evolution remain poorly understood. In this study, we show that the Cip1 protein is responsible for coupling sexual reproduction initiation to mating-type recognition in the protozoal eukaryote Tetrahymena thermophila. Deletion of the Cip1 protein leads to the loss of the selfing-avoidance function of mating-type recognition, resulting in selfing without mating-type recognition. Further experiments revealed that Cip1 is a regulatory subunit of the Cdk19-Cyc9 complex, which controls the initiation of sexual reproduction. These results reveal a mechanism that regulates the choice between mating and selfing. This mechanism also contributes to the debate about the ancestral state of sexual reproduction.

Computational modeling of chromosome re-replication in mutant strains of fission yeast

Typically cells replicate their genome only once per division cycle, but under some circumstances, both natural and unnatural, cells synthesize an overabundance of DNA, either in a disorganized manner (“overreplication”) or by a systematic doubling of chromosome number (“endoreplication”). These variations on the theme of DNA replication and division have been studied in strains of fission yeast, Schizosaccharomyces pombe, carrying mutations that interfere with the function of mitotic cyclin-dependent kinase (Cdk1:Cdc13) without impeding the roles of DNA-replication loading factor (Cdc18) and S-phase cyclin-dependent kinase (Cdk1:Cig2). Some of these mutations support endoreplication, and some overreplication. In this paper, we propose a dynamical model of the interactions among the proteins governing DNA replication and cell division in fission yeast. By computational simulations of the mathematical model, we account for the observed phenotypes of these re-replicating mutants, and by theoretical analysis of the dynamical system, we provide insight into the molecular distinctions between overreplicating and endoreplicating cells. In the case of induced overproduction of regulatory proteins, our model predicts that cells first switch from normal mitotic cell cycles to growth-controlled endoreplication, and ultimately to disorganized overreplication, parallel to the slow increase of protein to very high levels.

Requirement of PP2A-B56Par1 for the Stabilization of the CDK Inhibitor Rum1 and Activation of APC/CSte9 during Pre-Start G1 in S. pombe

Exit from the cell cycle during the establishment of quiescence and upon cell differentiation requires the sustained inactivation of CDK complexes. Fission yeast cells deprived of nitrogen halt cell cycle progression in pre-Start G1, before becoming quiescent or undergoing sexual differentiation. The CDK inhibitor Rum1 and the APC/C activator Ste9 are fundamental for this arrest, but both are down-regulated by CDK complexes. Here, we show that PP2A-B56^Par1 is instrumental for Rum1 stabilization and Ste9 activation. In the absence of PP2A-B56^Par1, cells fail to accumulate Rum1, and this results in persistent CDK activity, Ste9 inactivation, retention of the mitotic cyclin Cdc13, and impaired withdrawal from the cell cycle during nitrogen starvation. Importantly, mutation of a putative B56 interacting motif in Rum1 recapitulates these defects. These results underscore the relevance of CDK-counteracting phosphatases in cell differentiation, establishment of the quiescent state, and escape from it in cancer cells.

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PP2A-B56^Par1 is required for cell-cycle arrest and mating upon nitrogen deprivation

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Loss of Par1 impairs degradation of Cdc13 under nitrogen starvation

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Absence of Par1 impedes proper dephosphorylation of Ste9 and accumulation of Rum1

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Mutation of a Rum1 putative PP2A-B56 SLiM depicts similar defects as the loss Par1

Protein Phosphatases in G1 Regulation

Eukaryotic cells make the decision to proliferate, to differentiate or to cease dividing during G1, before passage through the restriction point or Start. Keeping cyclin-dependent kinase (CDK) activity low during this period restricts commitment to a new cell cycle and is essential to provide the adequate timeframe for the sensing of environmental signals. Here, we review the role of protein phosphatases in the modulation of CDK activity and as the counteracting force for CDK-dependent substrate phosphorylation, in budding and fission yeast. Moreover, we discuss recent findings that place protein phosphatases in the interface between nutritional signalling pathways and the cell cycle machinery.

Nutritional cell cycle reprogramming reveals that inhibition of Cdk1 is required for proper MBF-dependent transcription

In nature, cells and in particular unicellular microorganisms are exposed to a variety of nutritional environments. Fission yeast cells cultured in nitrogen-rich media grow fast, divide with a large size and show a short G1 and a long G2. However, when cultured in nitrogen-poor media, they exhibit reduced growth rate and cell size and a long G1 and a short G2. In this study, we compared the phenotypes of cells lacking the highly conserved cyclin-dependent kinase (Cdk) inhibitor Rum1 and the anaphase-promoting complex/cyclosome (APC/C) activator Ste9 in nitrogen-rich and nitrogen-poor media. Rum1 and Ste9 are dispensable for cell division in nitrogen-rich medium. However, in nitrogen-poor medium they are essential for generating a proper wave of MluI cell-cycle box binding factor (MBF)-dependent transcription at the end of G1, which is crucial for promoting a successful S phase. Mutants lacking Rum1 and Ste9 showed premature entry into S phase and a reduced wave of MBF-dependent transcription, leading to replication stress, DNA damage and G2 cell cycle arrest. This work demonstrates how reprogramming the cell cycle by changing the nutritional environment may reveal new roles for cell cycle regulators.

Genomic evidence for adaptation by gene duplication

Gene duplication is widely believed to facilitate adaptation, but unambiguous evidence for this hypothesis has been found in only a small number of cases. Although gene duplication may increase the fitness of the involved organisms by doubling gene dosage or neofunctionalization, it may also result in a simple division of ancestral functions into daughter genes, which need not promote adaptation. Hence, the general validity of the adaptation by gene duplication hypothesis remains uncertain. Indeed, a genome-scale experiment found similar fitness effects of deleting pairs of duplicate genes and deleting individual singleton genes from the yeast genome, leading to the conclusion that duplication rarely results in adaptation. Here we contend that the above comparison is unfair because of a known duplication bias among genes with different fitness contributions. To rectify this problem, we compare homologous genes from the budding yeast Saccharomyces cerevisiae and the fission yeast Schizosaccharomyces pombe. We discover that simultaneously deleting a duplicate gene pair in S. cerevisiae reduces fitness significantly more than deleting their singleton counterpart in S. pombe, revealing post-duplication adaptation. The duplicates-singleton difference in fitness effect is not attributable to a potential increase in gene dose after duplication, suggesting that the adaptation is owing to neofunctionalization, which we find to be explicable by acquisitions of binary protein-protein interactions rather than gene expression changes. These results provide genomic evidence for the role of gene duplication in organismal adaptation and are important for understanding the genetic mechanisms of evolutionary innovation.

Complementary and dose-dependent action of AtCCS52A isoforms in endoreduplication and plant size control

· The dimension of organs depends on the number and the size of their component cells. Formation of polyploid cells by endoreduplication cycles is predominantly associated with increases in the cell size and implicated in organ growth. In plants, the CCS52A proteins play a major role in the switch from mitotic to endoreduplication cycles controlling thus the number of mitotic cells and the endoreduplication events in the differentiating cells. · Arabidopsis has two CCS52A isoforms; AtCCS52A1 and AtCCS52A2. Here we focused on their roles in endoreduplication and cell size control during plant development. We demonstrate their complementary and dose-dependent actions that are dependent on their expression patterns. Moreover, the impact of CCS52A overexpression on organ size in transgenic plants was dependent on the expression level; while enhanced expression of the CCS52A genes positively correlated with the ploidy levels, organ sizes were negatively affected by strong overexpression whereas milder overexpression resulted in a significant increase in the organ sizes. · Taken together, these finding support both complementary and dose-dependent actions for the Arabidopsis CCS52A isoforms in plant development and demonstrate that elevated ectopic CCS52A expression positively correlates with organ size, opening a route to higher biomass production.

Degradation of MONOCULM 1 by APC/C(TAD1) regulates rice tillering

A rice tiller is a specialized grain-bearing branch that contributes greatly to grain yield. The MONOCULM 1 (MOC1) gene is the first identified key regulator controlling rice tiller number; however, the underlying mechanism remains to be elucidated. Here we report a novel rice gene, Tillering and Dwarf 1 (TAD1), which encodes a co-activator of the anaphase-promoting complex (APC/C), a multi-subunit E3 ligase. Although the elucidation of co-activators and individual subunits of plant APC/C involved in regulating plant development have emerged recently, the understanding of whether and how this large cell-cycle machinery controls plant development is still very limited. Our study demonstrates that TAD1 interacts with MOC1, forms a complex with OsAPC10 and functions as a co-activator of APC/C to target MOC1 for degradation in a cell-cycle-dependent manner. Our findings uncovered a new mechanism underlying shoot branching and shed light on the understanding of how the cell-cycle machinery regulates plant architecture.

Potential role of the rice OsCCS52A gene in endoreduplication

In eukaryotes, the cell cycle consists of four distinct phases: G1, S, G2 and M. In certain condition, the cells skip M-phase and undergo endoreduplication. Endoreduplication, occurring during a modified cell cycle, duplicates the entire genome without being followed by M-phase. A cycle of endoreduplication is common in most of the differentiated cells of plant vegetative tissues and it occurs extensively in cereal endosperm cells. Endoreduplication occurs when CDK/Cyclin complex low or inactive caused by ubiquitin-mediated degradation by APC and their activators. In this study, rice cell cycle switch 52 A (OsCCS52A), an APC activator, is functionally characterized using the reverse genetic approach. In rice, OsCCS52A is highly expressed in seedlings, flowers, immature panicles and 15 DAP kernels. Localization studies revealed that OsCCS52A is a nuclear protein. OsCCS52A interacts with OsCdc16 in yeast. In addition, overexpression of OsCCS52A inhibits mitotic cell division and induces endoreduplication and cell elongation in fission yeast. The homozygous mutant exhibits dwarfism and smaller seeds. Further analysis demonstrated that endoreduplication cycles in the endosperm of mutant seeds were disturbed, evidenced by reduced nuclear and cell sizes. Taken together, these results suggest that OsCCS52A is involved in maintaining normal seed size formation by mediating the exit from mitotic cell division to enter the endoreduplication cycles in rice endosperm.

Control of Arabidopsis root development

The Arabidopsis root has been the subject of intense research over the past decades. This research has led to significantly improved understanding of the molecular mechanisms underlying root development. Key insights into the specification of individual cell types, cell patterning, growth and differentiation, branching of the primary root, and responses of the root to the environment have been achieved. Transcription factors and plant hormones play key regulatory roles. Recently, mechanisms involving protein movement and the oscillation of gene expression have also been uncovered. Root gene regulatory networks controlling root development have been reconstructed from genome-wide profiling experiments, revealing novel molecular connections and models. Future refinement of these models will lead to a more complete description of the complex molecular interactions that give rise to a simple growing root.

Functional cDNA expression cloning: pushing it to the limit

The 1970s and the following decade are the era of the birth and early development of recombinant DNA technologies, which have entirely revolutionized the modern life science by providing tools that enable us to know the structures of genes and genomes and to dissect their components and understand their functions at the molecular and submolecular levels. One major objective of the life sciences is to achieve molecular and chemical understandings of the functions of genes and their encoded proteins, which are responsible for the manifestation of all biological phenomena in organisms. In the early 1980s, I developed, together with Paul Berg, a new technique that enables the cloning of full-length complementary DNAs (cDNAs) on the basis of their functional expression in a given cell of interest. I review the development, application and future implications in the life sciences of this gene-cloning technique.

Non-mitotic functions of the Anaphase-Promoting Complex

The Anaphase-Promoting Complex or Cyclosome (APC/C) is an E3 ubiquitin ligase whose activation requires the binding of a cofactor, either Cdc20 or Cdh1. While APC/C-Cdc20 is a major player during mitotic exit, APC/C-Cdh1 plays a central role in maintaining quiescence and controlling the onset of DNA replication. In addition, APC/C-Cdh1 is essential for endoreduplication, a process in which several rounds of DNA synthesis occur without mitosis. Recent data suggest that the APC/C is also involved in differentiation and metabolism, and plays important roles in postmitotic cells such as neurons. Thus, the APC/C is not only critical for anaphase onset but also regulates many other cellular processes during G1/S or in quiescent cells.

The fission yeast rDNA-binding protein Reb1 regulates G1 phase under nutritional stress

Yeast Reb1 and its mammalian ortholog TTF1 are conserved Myb-type DNA-binding proteins that bind to specific sites near the 3'-end of rRNA genes (rDNA). Here, they participate in the termination of transcription driven by RNA polymerase I and block DNA replication forks approaching in the opposite direction. We found that Schizosaccharomyces pombe Reb1 also upregulates transcription of the ste9(+) gene that is required for nitrogen-starvation-induced growth arrest with a G1 DNA content and sexual differentiation. Ste9 activates the anaphase-promoting complex or cyclosome ('APC/C') in G1, targeting B-cyclin for proteasomal degradation in response to nutritional stress. Reb1 binds in vivo and in vitro to a specific DNA sequence at the promoter of ste9(+), similar to the sequence recognized in the rDNA, and this binding is required for ste9(+) transcriptional activation and G1 arrest. This suggests that Reb1 acts as a link between rDNA metabolism and cell cycle control in response to nutritional stress. In agreement with this new role for Reb1 in the regulation of the G1-S transition, reb1Δ and wee1(ts) mutations are synthetically lethal owing to the inability of these cells to lengthen G1 before entering S phase. Similarly, reb1Δ cdc10(ts) cells are unable to arrest in G1 and die at the semi-permissive temperature.

APC/C-CCS52A complexes control meristem maintenance in the Arabidopsis root

Plant organs originate from meristems where stem cells are maintained to produce continuously daughter cells that are the source of different cell types. The cell cycle switch gene CCS52A, a substrate specific activator of the anaphase promoting complex/cyclosome (APC/C), controls the mitotic arrest and the transition of mitotic cycles to endoreduplication (ER) cycles as part of cell differentiation. Arabidopsis, unlike other organisms, contains 2 CCS52A isoforms. Here, we show that both of them are active and regulate meristem maintenance in the root tip, although through different mechanisms. The CCS52A1 activity in the elongation zone of the root stimulates ER and mitotic exit, and contributes to the border delineation between dividing and expanding cells. In contrast, CCS52A2 acts directly in the distal region of the root meristem to control identity of the quiescent center (QC) cells and stem cell maintenance. Cell proliferation assays in roots suggest that this control involves CCS52A2 mediated repression of mitotic activity in the QC cells. The data indicate that the CCS52A genes favor a low mitotic state in different cell types of the root tip that is required for meristem maintenance, and reveal a previously undescribed mechanism for APC/C mediated control in plant development.

FZR2/CCS52A1 expression is a determinant of endoreduplication and cell expansion in Arabidopsis

Endoreduplication, a modified cell cycle that allows cells to increase ploidy without subsequent cell division, is a key component of plant growth and development. In this work, we show that some, but not all, of the endoreduplication of Arabidopsis (Arabidopsis thaliana) is mediated by the expression of a WD40 gene, FIZZY-RELATED2 (FZR2). Loss-of-function alleles show reduced endoreduplication and reduced expansion in trichomes and other leaf cells. Misexpression of FZR2 is sufficient to drive ectopic or extra endoreduplication in leaves, roots, and flowers, leading to alteration of cell sizes and, sometimes, organ size and shape. Our data, which suggest that reduced cell size can be compensated by increased cell proliferation to allow normal leaf morphology, are discussed with respect to the so-called compensation mechanism of plant development.

Cdk5 phosphorylates Cdh1 and modulates cyclin B1 stability in excitotoxicity

Anaphase-promoting complex/cyclosome (APC/C), an E3 ubiquitin ligase that destabilizes cell cycle proteins, is activated by Cdh1 in post-mitotic neurons, where it regulates axonal growth, synaptic plasticity and survival. The APC/C-Cdh1 substrate, cyclin B1, has been found to accumulate in degenerating brain areas in Alzheimer's disease and stroke. This highlights the importance of elucidating cyclin B1 regulation by APC/C-Cdh1 in neurons under stress conditions relevant to neurological disease. Here, we report that stimulation of N-methyl-D-aspartate receptors (NMDARs) that occurs in neurodegenerative diseases promoted the accumulation of cyclin B1 in the nuclei of cortical neurons; this led the neurons to undergo apoptotic death. Moreover, we found that the Ser-40, Thr-121 and Ser-163 triple phosphorylation of Cdh1 by the cyclin-dependent kinase-5 (Cdk5)-p25 complex was necessary and sufficient for cyclin B1 stabilization and apoptotic death after NMDAR stimulation. These results reveal Cdh1 as a novel Cdk5 substrate that mediates cyclin B1 neuronal accumulation in excitotoxicity.

Diminishing HDACs by drugs or mutations promotes normal or abnormal sister chromatid separation by affecting APC/C and adherin

Histone acetyltransferases (HATs) and histone deacetylases (HDACs) play important roles in cell regulation, including cell cycle progression, although their precise role in mitotic progression remains elusive. To address this issue, the effects of HDAC inhibition were examined upon a variety of mitotic mutants of the fission yeast Schizosaccharomyces pombe, which contains three HDACs that are sensitive to trichostatin A (TSA) and are similar to human HDACs. Here it is shown that HDACs are implicated in sister chromatid cohesion and separation. A mutant of the cohesin loader Mis4 (adherin) was hypersensitive to TSA and synthetically lethal with HDAC deletion mutations. TSA treatment of mis4 mutant cells decreased chromatin-bound cohesins in the chromosome arm region. By contrast, HDAC inhibitors and clr6 HDAC mutations rescued temperature sensitive (ts) phenotypes of the mutants of the ubiquitin ligase complex anaphase-promoting complex/cyclosome (APC/C), which display metaphase arrest. This suppression coincided with facilitated complex formation of APC/C. Moreover, our mass spectrometry analysis showed that an APC/C subunit, Cut23/APC8, is acetylated. HATs and HDACs might directly target adherin and APC/C to ensure proper chromosome segregation, and anti-tumour effects of HDAC inhibitors could be attributed to this deregulation.

Spindle checkpoint activation at meiosis I advances anaphase II onset via meiosis-specific APC/C regulation

During mitosis, the spindle assembly checkpoint (SAC) inhibits the Cdc20-activated anaphase-promoting complex/cyclosome (APC/C(Cdc20)), which promotes protein degradation, and delays anaphase onset to ensure accurate chromosome segregation. However, the SAC function in meiotic anaphase regulation is poorly understood. Here, we examined the SAC function in fission yeast meiosis. As in mitosis, a SAC factor, Mad2, delayed anaphase onset via Slp1 (fission yeast Cdc20) when chromosomes attach to the spindle improperly. However, when the SAC delayed anaphase I, the interval between meiosis I and II shortened. Furthermore, anaphase onset was advanced and the SAC effect was reduced at meiosis II. The advancement of anaphase onset depended on a meiosis-specific, Cdc20-related factor, Fzr1/Mfr1, which contributed to anaphase cyclin decline and anaphase onset and was inefficiently inhibited by the SAC. Our findings show that impacts of SAC activation are not confined to a single division at meiosis due to meiosis-specific APC/C regulation, which has probably been evolved for execution of two meiotic divisions.

Genomic stability and tumour suppression by the APC/C cofactor Cdh1

The anaphase promoting complex or cyclosome (APC/C) is a ubiquitin protein ligase that, together with Cdc20 or Cdh1, targets cell-cycle proteins for degradation. APC/C-Cdh1 specifically promotes protein degradation in late mitosis and G1. Mutant embryos lacking Cdh1 die at E9.5-E10.5 due to defects in the endoreduplication of trophoblast cells and placental malfunction. This lethality is prevented when Cdh1 is expressed in the placenta. Cdh1-deficient cells proliferate inefficiently and accumulate numeric and structural chromosomal aberrations, indicating that Cdh1 contributes to the maintenance of genomic stability. Cdh1 heterozygous animals show increased susceptibility to spontaneous tumours, suggesting that Cdh1 functions as a haploinsufficient tumour suppressor. These heterozygous mice also show several defects in behaviour associated with increased proliferation of stem cells in the nervous system. These results indicate that Cdh1 is required for preventing unscheduled proliferation of specific progenitor cells and protecting mammalian cells from genomic instability.

Nuc2p, a subunit of the anaphase-promoting complex, inhibits septation initiation network following cytokinesis in fission yeast

In most cell types, mitosis and cytokinesis are tightly coupled such that cytokinesis occurs only once per cell cycle. The fission yeast Schizosaccharomyces pombe divides using an actomyosin-based contractile ring and is an attractive model for the study of the links between mitosis and cytokinesis. In fission yeast, the anaphase-promoting complex/cyclosome (APC/C) and the septation initiation network (SIN), a spindle pole body (SPB)–associated GTPase-driven signaling cascade, function sequentially to ensure proper coordination of mitosis and cytokinesis. Here, we find a novel interplay between the tetratricopeptide repeat (TPR) domain–containing subunit of the APC/C, Nuc2p, and the SIN, that appears to not involve other subunits of the APC/C. Overproduction of Nuc2p led to an increase in the presence of multinucleated cells, which correlated with a defect in actomyosin ring maintenance and localization of the SIN component protein kinases Cdc7p and Sid1p to the SPBs, indicative of defective SIN signaling. Conversely, loss of Nuc2p function led to increased SIN signaling, characterized by the persistent localization of Cdc7p and Sid1p on SPBs and assembly of multiple actomyosin rings and division septa. Nuc2p appears to function independently of the checkpoint with FHA and ring finger (CHFR)–related protein Dma1p, a known inhibitor of the SIN in fission yeast. Genetic and biochemical analyses established that Nuc2p might influence the nucleotide state of Spg1p GTPase, a key regulator of the SIN. We propose that Nuc2p, by inhibiting the SIN after cell division, prevents further deleterious cytokinetic events, thereby contributing to genome stability.

Cytokinesis is the process by which a mother cell is physically partitioned into two daughter cells. Cytokinesis is well coordinated with segregation of the genetic material to ensure that the genome is not damaged by the cell division apparatus. How untimely cytokinesis is prevented is not fully understood, and is a topic of current interest. Studies of the mechanisms of segregation of the genetic material and cytokinesis have benefited extensively from the use of the fission yeast Schizosaccharomyces pombe. In this study, we make the discovery that fission yeast Nuc2p, a protein previously known to form part of a multi-protein machine that regulates genome segregation, has a second function in regulating cytokinesis. Nuc2p appears to dampen the septation initiation network, which is an important signaling pathway that is essential for cytokinesis. Thus, Nuc2p prevents the occurrence of cytokinetic events prior to segregation of the genetic material and thereby contributes to genome stability. Since the multi-component machinery that Nuc2p forms part of, as well as Nuc2p itself, has relatives in essentially all eukaryotic cells, a similar mechanism might operate in other cells as well.

Genome re-duplication and irregular segregation occur during the cell cycle of Entamoeba histolytica

Heterogeneity of genome content is commonly observed in axenic cultures of Entamoeba histolytica. Cells with multiple nuclei and nuclei with heterogenous genome contents suggest that regulatory mechanisms that ensure alternation of DNA synthesis and mitosis are absent in this organism. Therefore, several endo-reduplicative cycles may occur without mitosis. The data also shows that unlike other endo-reduplicating organisms, E.histolytica does not undergo a precise number of endo-reduplicative cycles. We propose that irregular endo-reduplication and genome partitioning lead to heterogeneity in the genome content of E.histolytica trophozoites in their proliferative phase. The goal of future studies should be aimed at understanding the mechanisms that are involved in (a) accumulation of multiple genome contents in a single nucleus; (b) genome segregation in nuclei that contain multiple genome contents and (c) maintenance of genome fidelity in E. histolytica.

Cdc2p controls the forkhead transcription factor Fkh2p by phosphorylation during sexual differentiation in fission yeast

In most eukaryotes, cyclin-dependent kinases (Cdks) play a central role in control of cell-cycle progression. Cdks are inactivated from the end of mitosis to the start of the next cell cycle as well as during sexual differentiation. The forkhead-type transcription factor Fkh2p is required for the periodic expression of many genes and for efficient mating in the fission yeast Schizosaccharomyces pombe. However, the mechanism responsible for coordination of cell-cycle progression with sexual differentiation is still unknown. We now show that Fkh2p is phosphorylated by Cdc2p (Cdk1) and that phosphorylation of Fkh2p on T314 or S462 by this Cdk blocks mating in S. pombe by preventing the induction of ste11+ transcription, which is required for the onset of sexual development. We propose that functional interaction between Cdks and forkhead transcription factors may link the mitotic cell cycle and sexual differentiation.

Molecular mechanisms underlying the mitosis-meiosis decision

Most eukaryotic cells possess genetic potential to perform meiosis, but the vast majority of them never initiate it. The entry to meiosis is strictly regulated by developmental and environmental conditions, which vary significantly from species to species. Molecular mechanisms underlying the mitosis-meiosis decision are unclear in most organisms, except for a few model systems including fission yeast Schizosaccharomyces pombe. Nutrient limitation is a cue to the entry into meiosis in this microbe. Signals from nutrients converge on the activity of Mei2 protein, which plays pivotal roles in both induction and progression of meiosis. Here we outline the current knowledge of how a set of environmental stimuli eventually activates Mei2, and discuss how Mei2 governs the meiotic program molecularly, especially focusing on a recent finding that Mei2 antagonizes selective elimination of meiotic messenger RNAs.

L2dtl Is Essential for Cell Survival and Nuclear Division in Early Mouse Embryonic Development

l(2)dtl (lethal (2) denticleless), is an embryonic lethal homozygous mutation initially identified in Drosophila melanogaster that produces embryos that lack ventral denticle belts. In addition to nucleotide sequence, bioinformatic analysis has revealed a conservation of critical functional motifs among the human L2DTL, mouse L2dtl, and Drosophila l(2)dtl proteins. The function of the L2DTL protein in the development of mammalian embryos was studied using targeted disruption of the L2dtl gene in mice. The knock-out resulted in early embryonic lethality. L2dtl-/- embryos were deformed and terminated development at the 4-8-cell stage. Microinjection of a small interfering RNA (siRNA) vector (siRNA-L2dtl) into the two-cell stage nuclei of wild-type mouse embryos led to cell cycle progression failure, termination of cell division, and, eventually, embryonic death during the preimplantation stage. Morphological studies of the embryos 54 h after injection showed fragmentation of mitotic chromosomes and chromosomal lagging, hallmarks of mitotic catastrophe. The siRNA-L2dtl-treated embryos eventually lysed and failed to develop into blastocysts after 72 h of in vitro culturing. However, the embryos developed normally after they were microinjected into one nucleus of the two-celled embryos. The siRNA studies in HeLa cells showed that L2dtl protein depletion results in multinucleation and down-regulation of phosphatidylinositol 3-kinase, proliferating cell nuclear antigen, and PTTG1/securin, which might partially explain the mitotic catastrophe observed in L2dtl-depleted mouse embryos. Based on these findings, we conclude that L2dtl gene expression is essential for very early mouse embryonic development.

Fission yeast Tor2 promotes cell growth and represses cell differentiation

The fission yeast Schizosaccharomyces pombe is an excellent model system in which to study the coordination of cell growth and cell differentiation. In the presence of nutrients, fission yeast cells grow and divide; in the absence of nutrients, they stop growing and undergo cell differentiation. The molecular mechanisms underlying this response are not fully understood. Here, we demonstrate that Tor2, a fission yeast member of the TOR protein kinase family, is central to controlling the switch between cell growth and cell differentiation in response to nutrient availability. Tor2 controls cell growth and ribosome biogenesis by regulating ribosomal protein gene expression. We have found that Tor2 has an additional function in repressing sexual differentiation. Tor2 overexpression strongly represses mating, meiosis and sporulation efficiency, whereas Tor2 inactivation has the opposite effect, leading to cell differentiation, regardless of the nutritional conditions. This newly revealed function of Tor2 appears to operate by interfering with the functions of the transcription factor Ste11 and the meiosis-promoting RNA-binding protein Mei2. Thus, our data reveal a unique regulatory function of the Tor pathway – ensuring that growth and cell differentiation become mutually exclusive and that the choice between them depends on environmental conditions.

Pathocycles: Ustilago maydis as a model to study the relationships between cell cycle and virulence in pathogenic fungi

Activation of virulence in pathogenic fungi often involves differentiation processes that need the reset of the cell cycle and induction of a new morphogenetic program. Therefore, the fungal capability to modify its cell cycle constitutes an important determinant in carrying out a successful infection. The dimorphic fungus Ustilago maydis is the causative agent of corn smut disease and has lately become a highly attractive model in addressing fundamental questions about development in pathogenic fungi. The different morphological and genetic changes of U. maydis cells during the pathogenic process advocate an accurate control of the cell cycle in these transitions. This is why this model pathogen deserves attention as a powerful tool in analyzing the relationships between cell cycle, morphogenesis, and pathogenicity. The aim of this review is to summarize recent advances in the unveiling of cell cycle regulation in U. maydis. We also discuss the connection between cell cycle and virulence and how cell cycle control is an important downstream target in the fungus-plant interaction.

Cdh1/Hct1-APC is essential for the survival of postmitotic neurons

Cell division at the end of mitosis and G1 is controlled by Cdh1/Hct1, an activator of the E3-ubiquitin ligase anaphase-promoting complex (APC) that promotes the ubiquitylation and degradation of mitotic cyclins and other substrates. Cdh1-APC is active in postmitotic neurons, where it regulates axonal growth and patterning in the developing brain. However, it remains unknown whether Cdh1-APC is involved in preventing cell-cycle progression in terminally differentiated neurons. To address this issue, we used the small hairpin RNA strategy to deplete Cdh1 in postmitotic neurons. We observed that Cdh1 silencing rapidly triggered apoptotic neuronal death. To investigate the underlying mechanism, we focused on cyclin B1, a major Cdh1-APC substrate. Our results demonstrate that Cdh1 is required to prevent the accumulation of cyclin B1 in terminally differentiated neurons. Moreover, by keeping cyclin B1 low, Cdh1 prevented these neurons from entering an aberrant S phase that led to apoptotic cell death. These results provide an explanation for the mechanism of cyclin B1 reactivation that occurs in the brain of patients suffering from neurodegenerative diseases, such as Alzheimer's disease.

The WD-repeat protein GRWD1: potential roles in myeloid differentiation and ribosome biogenesis

A cDNA fragment originally identified in U-937 cells as a vitamin D(3)-regulated gene is here designated the glutamate-rich WD-repeat (GRWD1) gene. WD-repeat proteins are a class of functionally divergent molecules that cooperate with other proteins to regulate cellular processes. GRWD1 encodes a 446-amino-acid protein containing a glutamate-rich region followed by four WD repeats. The yeast homologue of GRWD1, Rrb1, has been shown to be an essential protein involved in ribosome biogenesis. Northern analysis of GRWD1 message levels in the myeloid cell line HL-60 undergoing differentiation induced by vitamin D(3) or retinoic acid demonstrate downregulation coincident with slowing of cellular proliferation. A siRNA designed to downregulate GRWD1 similarly results in a decrease in cellular proliferation within 293 cells. Metabolic labeling of cells expressing the siRNA to GRWD1 shows a decrease in global protein synthesis. Finally, nuclear fractionation studies show cosedimentation of GRWD1 with preribosomal complexes, as well as the WD-repeat-containing protein Bop1, which has previously been implicated in ribosome biogenesis. These studies suggest that within mammalian cells GRWD1 plays a role in ribosome biogenesis and during myeloid differentiation its levels are regulated.

A member of the Fizzy-related family of APC activators is regulated by cAMP and is required at different stages of plant infection by Ustilago maydis

Here, we identified a new member of the Fizzy-related family of APC activators, Cru1, which is required for virulence in the corn smut fungus Ustilago maydis. We show that Cru1 promotes the degradation of B-type cyclins in U. maydis. Cells deficient in the Cru1 protein show defects in cell size, adaptation to nutritional conditions and cell separation. We propose that the phenotypes observed are a consequence of the inability of cru1 Delta cells to keep under control the levels of mitotic cyclins during G1. The levels of cru1 mRNA are controlled by nutritional conditions and cAMP levels, implicating the cAMP/protein kinase A pathway in the transmission of environmental conditions to the cell cycle. Cells deficient in Cru1 function are severely impaired in their ability to infect corn plants. This low rate of plant infection is caused by several defects. First, a low level of expression of the pheromone-encoding gene, mfa1, resulted in a low frequency of dikaryotic infective filament formation. Second, proliferation of fungal cells inside the plant is also affected, resulting in the inability to induce tumors in plants. Finally, the formation and germination of teliospores is also impaired. Our results support the hypothesis that virulence and cell cycle are connected in U. maydis. We propose that along the infection process, Cru1 is required to keep the appropriate G1 length necessary for the adaptation of fungal cells to host environment through the different stages of the plant infection.

Analysis of the Role of Phosphorylation in Fission Yeast Cdc13p/CyclinB Function

The Cdk1p-cyclin B complex drives entry into mitosis in all eukaryotes. Cdc13p is the single essential cyclin in Schizosaccharomyces pombe and a member of the cyclin B family. Cdc13p abundance rises during G(2)-phase and falls as cells progress through mitosis and G(1). Cdc13p degradation, mediated by the anaphase-promoting complex, is an important mechanism of Cdk1p inhibition and mitotic exit. Cdk1p-cyclin B1 complexes shuttle between the nucleus and cytoplasm, and preventing nuclear accumulation of Cdk1p-cyclin B1 in mammalian cells appears to be one mechanism of preventing entry into mitosis during a DNA damage-induced checkpoint delay. In vertebrates, phosphorylation plays a key role in regulating the intracellular distribution of cyclins. Previous mass spectrometric analysis identified sites of Cdc13p phosphorylation. Here, we have confirmed that these sites are the sole in vivo Cdc13p phosphorylation sites and have studied the role that phosphorylation plays in Cdc13p localization and function. Our data indicate that Cdc13p accumulates in the nucleolus in response to G(2) checkpoint delays, rather than in the cytoplasm, and that phosphorylation plays no role in Cdc13p localization or function.

Cdk inhibitor ste9p/srw1p is involved in response to protein synthesis inhibition in fission yeast

It remains unknown whether the cell cycle system responds properly to protein synthesis inhibition. In this paper I report finding in Schizosaccharomyces pombe that partially deleted elongation factor 3 genes rescue various mitotic catastrophe mutants depending on deltaste9 in a dominant-negative manner. In response to protein synthesis inhibitors, deltaste9 and some other mutants delay halting the cell cycle at G2-M and the combined cdc2-M26 deltaste9 mutant greatly loses viability. It is suggested that cell cycle be positively controlled in an ste9-dependent manner before essential factors for viability and other important functions are exhausted when protein synthesis is inhibited.

Two ubiquitin-conjugating enzymes, UbcP1/Ubc4 and UbcP4/Ubc11, have distinct functions for ubiquitination of mitotic cyclin

Cell cycle events are regulated by sequential activation and inactivation of Cdk kinases. Mitotic exit is accomplished by the inactivation of mitotic Cdk kinase, which is mainly achieved by degradation of cyclins. The ubiquitin-proteasome system is involved in this process, requiring APC/C (anaphase-promoting complex/cyclosome) as a ubiquitin ligase. In Xenopus and clam oocytes, the ubiquitin-conjugating enzymes that function with APC/C have been identified as two proteins, UBC4 and UBCx/E2-C. Previously we reported that the fission yeast ubiquitin-conjugating enzyme UbcP4/Ubc11, a homologue of UBCx/E2-C, is required for mitotic transition. Here we show that the other fission yeast ubiquitin-conjugating enzyme, UbcP1/Ubc4, which is homologous to UBC4, is also required for mitotic transition in the same manner as UbcP4/Ubc11. Both ubiquitin-conjugating enzymes are essential for cell division and directly required for the degradation of mitotic cyclin Cdc13. They function nonredundantly in the ubiquitination of CDC13 because a defect in ubcP1/ubc4+ cannot be suppressed by high expression of UbcP4/Ubc11 and a defect in ubcP4/ubc11+ cannot be suppressed by high expression of UbcP1/Ubc4. In vivo analysis of the ubiquitinated state of Cdc13 shows that the ubiquitin chains on Cdc13 were short in ubcP1/ubc4 mutant cells while ubiquitinated Cdc13 was totally reduced in ubcP4/ubc11 mutant cells. Taken together, these results indicate that the two ubiquitin-conjugating enzymes play distinct and essential roles in the degradation of mitotic cyclin Cdc13, with the UbcP4/Ubc11-pathway initiating ubiquitination of Cdc13 and the UbcP1/Ubc4-pathway elongating the short ubiquitin chains on Cdc13.

The dynamics of cell cycle regulation

Major events of the cell cycle--DNA synthesis, mitosis and cell division-are regulated by a complex network of protein interactions that control the activities of cyclin-dependent kinases. The network can be modeled by a set of nonlinear differential equations and its behavior predicted by numerical simulation. Computer simulations are necessary for detailed quantitative comparisons between theory and experiment, but they give little insight into the qualitative dynamics of the control system and how molecular interactions determine the fundamental physiological properties of cell replication. To that end, bifurcation diagrams are a useful analytical tool, providing new views of the dynamical organization of the cell cycle, the role of checkpoints in assuring the integrity of the genome, and the abnormal regulation of cell cycle events in mutants. These claims are demonstrated by an analysis of cell cycle regulation in fission yeast.

A novel chk1-dependent G1/M checkpoint in fission yeast

Fission yeast cells with a temperature-sensitive Orp1 protein, a component of the origin recognition complex, cannot perform DNA replication at the restrictive temperature. Seventy percent of orp1-4 cells arrest with a 1C DNA content, whereas 30% proceed to mitosis ('cut'). The arrest depends upon the checkpoint Rad proteins and, surprisingly, the Chk1 protein, which is thought to act only from late S phase. The arrested cells maintain a 1C DNA content, as judged by flow cytometry, and the early origin ars3001 has not been initiated, as judged by 2D gel analysis. We show that in G1-arrested orp1-4 cells, Wee1 phosphorylates and inactivates Cdc2. Activation of Chk1 occurs earlier than Cdc2 phosphorylation, indicating a novel role for Chk1, namely to induce and/or maintain Cdc2 phosphorylation upon checkpoint activation in G1. We also show that commitment to cutting occurs already in early G1 phase.

Checking cell size in yeast

To remain viable, cells have to coordinate cell growth with cell division. In yeast, this occurs at two control points: the boundaries between G1 and S phases, also known as Start, and between G2 and M phases. Theoretically, coordination can be achieved by independent regulation of growth and division, or by participation of surveillance mechanisms in which cell size feeds back into cell-cycle control. This article discusses recent advances in the identification of sizing mechanisms in budding and in fission yeast, and how these mechanisms integrate with environmental stimuli. A comparison of the G1-S and G2-M size-control modules in the two species reveals a degree of conservation higher than previously thought. This reinforces the notion that internal sizing could be a conserved feature of cell-cycle control throughout eukaryotes.

Essential role of MCM proteins in premeiotic DNA replication

A critical event in eukaryotic DNA replication involves association of minichromosome maintenance (MCM2-7) proteins with origins, to form prereplicative complexes (pre-RCs) that are competent for initiation. The ability of mutants defective in MCM2-7 function to complete meiosis had suggested that pre-RC components could be irrelevant to premeiotic S phase. We show here that MCM2-7 proteins bind to chromatin in fission yeast cells preparing for meiosis and during premeiotic S phase in a manner suggesting they in fact are required for DNA replication in the meiotic cycle. This is confirmed by analysis of a degron mcm4 mutant, which cannot carry out premeiotic DNA replication. Later in meiosis, Mcm4 chromatin association is blocked between meiotic nuclear divisions, presumably accounting for the absence of a second round of DNA replication. Together, these results emphasize similarity between replication mechanisms in mitotic and meiotic cell cycles.

Rca1 inhibits APC-Cdh1(Fzr) and is required to prevent cyclin degradation in G2

We demonstrate that Rca1 is an essential inhibitor of the anaphase-promoting complex/cyclosome (APC) in Drosophila. APC activity is restricted to mitotic stages and G1 by its activators Cdc20-Fizzy (Cdc20(Fzy)) and Cdh1-Fizzy-related (Cdh1(Fzr)), respectively. In rca1 mutants, cyclins are degraded prematurely in G2 by APC-Cdh1(Fzr)-dependent proteolysis, and cells fail to execute mitosis. Overexpression of Cdh1(Fzr) mimics the rca1 phenotype, and coexpression of Rca1 blocks this Cdh1(Fzr) function. We show that Rca1 and Cdh1(Fzr) are in a complex that also includes the APC component Cdc27. Previous studies have shown that phosphorylation of Cdh1 prevents its interaction with the APC. Our data reveal a different mode of APC regulation by Rca1 at the G2 stage, when low Cdk activity is unable to inhibit Cdh1(Fzr) interaction.

Expression and stability of Arabidopsis CDC6 are associated with endoreplication

Studies on the CDC6 protein, which is crucial to the control of DNA replication in yeast and animal cells, are lacking in plants. We have isolated an Arabidopsis cDNA encoding the AtCDC6 protein and studied its possible connection to the occurrence of developmentally regulated endoreplication cycles. The AtCDC6 gene is expressed maximally in early S-phase, and its promoter contains an E2F consensus site that mediates the binding of a plant E2F/DP complex. Transgenic plants carrying an AtCDC6 promoter-beta-glucuronidase fusion revealed that it is active in proliferating cells and, interestingly, in endoreplicating cells. In particular, the extra endoreplication cycle that occurs in dark-grown hypocotyl cells is associated with upregulation of the AtCDC6 gene. This was corroborated using ctr1 Arabidopsis mutants altered in their endoreplication pattern. The ectopic expression of AtCDC6 in transgenic plants induced endoreplication and produced a change in the somatic ploidy level. AtCDC6 was degraded in a ubiquitin- and proteosome-dependent manner by extracts from proliferating cells, but it was degraded poorly by extracts from dark-grown hypocotyl endoreplicating cells. Our results indicate that endoreplication is associated with expression of the AtCDC6 gene and, most likely, the stability of its product; it also apparently requires activation of the retinoblastoma/E2F/DP pathway. These conclusions may apply to endoreplicating cells in other tissues of the plant and to endoreplicating cells in other eukaryotes.

Expression and stability of Arabidopsis CDC6 are associated with endoreplication

Studies on the CDC6 protein, which is crucial to the control of DNA replication in yeast and animal cells, are lacking in plants. We have isolated an Arabidopsis cDNA encoding the AtCDC6 protein and studied its possible connection to the occurrence of developmentally regulated endoreplication cycles. The AtCDC6 gene is expressed maximally in early S-phase, and its promoter contains an E2F consensus site that mediates the binding of a plant E2F/DP complex. Transgenic plants carrying an AtCDC6 promoter-beta-glucuronidase fusion revealed that it is active in proliferating cells and, interestingly, in endoreplicating cells. In particular, the extra endoreplication cycle that occurs in dark-grown hypocotyl cells is associated with upregulation of the AtCDC6 gene. This was corroborated using ctr1 Arabidopsis mutants altered in their endoreplication pattern. The ectopic expression of AtCDC6 in transgenic plants induced endoreplication and produced a change in the somatic ploidy level. AtCDC6 was degraded in a ubiquitin- and proteosome-dependent manner by extracts from proliferating cells, but it was degraded poorly by extracts from dark-grown hypocotyl endoreplicating cells. Our results indicate that endoreplication is associated with expression of the AtCDC6 gene and, most likely, the stability of its product; it also apparently requires activation of the retinoblastoma/E2F/DP pathway. These conclusions may apply to endoreplicating cells in other tissues of the plant and to endoreplicating cells in other eukaryotes.

Activation of Cdh1-dependent APC is required for G1 cell cycle arrest and DNA damage-induced G2 checkpoint in vertebrate cells

Anaphase-promoting complex (APC) is activated by two regulatory proteins, Cdc20 and Cdh1. In yeast and Drosophila, Cdh1-dependent APC (Cdh1-APC) activity targets mitotic cyclins from the end of mitosis to the G1 phase. To investigate the function of Cdh1 in vertebrate cells, we generated clones of chicken DT40 cells disrupted in their Cdh1 loci. Cdh1 was dispensable for viability and cell cycle progression. However, similarly to yeast and Drosophila, loss of Cdh1 induced unscheduled accumulation of mitotic cyclins in G1, resulting in abrogation of G1 arrest caused by treatment with rapamycin, an inducer of p27(Kip1). Further more, we found that Cdh1(-/-) cells fail to maintain DNA damage-induced G2 arrest and that Cdh1-APC is activated by X-irradiation-induced DNA damage. Thus, activation of Cdh1-APC plays a crucial role in both cdk inhibitor-dependent G1 arrest and DNA damage-induced G2 arrest.

Flp1, a fission yeast orthologue of theS. cerevisiae CDC14gene, is not required for cyclin degradation or rum1p stabilisation at the end of mitosis

In Saccharomyces cerevisiae, the phosphoprotein phosphatase Cdc14p plays a central role in exit from mitosis, by promoting B-type cyclin degradation and allowing accumulation of the cyclin-dependent kinase inhibitor Sic1p. Cdc14p is sequestered in the nucleolus during interphase, from where it is released at the end of mitosis, dependent upon mitotic exit network function. The CDC14 gene is essential and loss-of-function mutants arrest at the end of mitosis. We have identified a fission yeast orthologue of CDC14 through database searches. A Schizosaccharomyces pombe flp1 (cdc fourteen-like-phosphatase) null mutant is viable, divides at a reduced size and shows defects in septation. flp1p is not the essential effector of the S. pombe septation initiation network, but may potentiate signalling of the onset of septation. In contrast to S. cerevisiae Cdc14p, flp1p is not required for the accumulation or destruction of the B-type cyclin cdc13p, the cyclin-dependent kinase inhibitor rum1p, or for dephosphorylation of the APC/C specificity factor ste9p in G1. Like its budding yeast counterpart, flp1p is restricted to the nucleolus until mitosis, when it is dispersed through the nucleus. In contrast to S. cerevisiae Cdc14p, flp1p is also present on the mitotic spindle and contractile ring. The potential roles of flp1p in cell cycle control are discussed.

Fission yeast Clp1p phosphatase regulates G2/M transition and coordination of cytokinesis with cell cycle progression

BACKGROUND

In Saccharomyces cerevisiae the mitotic-exit network (MEN) functions in anaphase to promote the release of the Cdc14p phosphatase from the nucleolus. This release causes mitotic exit via inactivation of the cyclin-dependent kinase (Cdk). Cdc14p-like proteins are highly conserved; however, it is unclear if these proteins regulate mitotic exit as in S. cerevisiae. In Schizosaccharomyces pombe a signaling pathway homologous to the MEN and termed the septation initiation network (SIN) is required not for mitotic exit, but for initiation of cytokinesis and for a cytokinesis checkpoint that inhibits further cell cycle progression until cytokinesis is complete.

RESULTS

We have identified the S. pombe Cdc14p homolog, Clp1p, and show that it is not required for mitotic exit but rather functions together with the SIN in coordinating cytokinesis with the nuclear-division cycle. As cells enter mitosis, Clp1p relocalizes from the nucleolus to the spindle and site of cell division. Clp1p exit from the nucleolus does not depend on the SIN, but the SIN is required for keeping Clp1p out of the nucleolus until completion of cytokinesis. Clp1p, in turn, may promote the activation of the SIN by antagonizing Cdk activity until cytokinesis is complete and thus ensuring that cytokinesis is completed prior to the initiation of the next cell cycle. In addition to its roles in anaphase, Clp1p regulates the G2/M transition since cells deleted for clp1 enter mitosis precociously and cells overexpressing Clp1p delay mitotic entry. Unlike Cdc14p, Clp1p appears to antagonize Cdk activity by preventing dephosphorylation of Cdc2p on tyrosine.

CONCLUSIONS

S. pombe Clp1p affects cell cycle progression in a markedly different manner than its S. cerevisiae homolog, Cdc14p. This finding raises the possibility that related phosphatases in animal cells will prove to have important roles in coordinating the onset of cytokinesis with the events of mitosis.

Fission yeast mfr1 activates APC and coordinates meiotic nuclear division with sporulation

Meiosis is the developmental program by which sexually reproducing diploid organisms generate haploid gametes. In yeast, meiosis is followed by spore morphogenesis. These two events are normally coordinated in such a way that spore formation is dependent upon completion of the meiotic nuclear divisions. Here we describe a meiosis-specific protein, mfr1, that is involved in this coordination. mfr1 is an activator of the anaphase-promoting complex (APC), which is necessary for the rapid degradation of the cdc13 cyclin at the end of meiosis II, prior to the formation of spores. An mfr1 null mutant completes meiosis II but remains with high levels of cdc13 and cdc2 kinase activity and has considerably delayed spore formation. By analogy with the mitotic cell cycle, where proteolysis and inactivation of cdc2 kinase are necessary to trigger mitotic exit and cytokinesis, we propose that at the end of meiosis rapid and timely proteolysis of cyclins is required to switch on the differentiation program that eventually leads to the formation of haploid gametes.

A conserved cyclin-binding domain determines functional interplay between anaphase-promoting complex-Cdh1 and cyclin A-Cdk2 during cell cycle progression

Periodic activity of the anaphase-promoting complex (APC) ubiquitin ligase determines progression through multiple cell cycle transitions by targeting cell cycle regulators for destruction. At the G(1)/S transition, phosphorylation-dependent dissociation of the Cdh1-activating subunit inhibits the APC, allowing stabilization of proteins required for subsequent cell cycle progression. Cyclin-dependent kinases (CDKs) that initiate and maintain Cdh1 phosphorylation have been identified. However, the issue of which cyclin-CDK complexes are involved has been a matter of debate, and the mechanism of how cyclin-CDKs interact with APC subunits remains unresolved. Here we substantiate the evidence that mammalian cyclin A-Cdk2 prevents unscheduled APC reactivation during S phase by demonstrating its periodic interaction with Cdh1 at the level of endogenous proteins. Moreover, we identified a conserved cyclin-binding motif within the Cdh1 WD-40 domain and show that its disruption abolished the Cdh1-cyclin A-Cdk2 interaction, eliminated Cdh1-associated histone H1 kinase activity, and impaired Cdh1 phosphorylation by cyclin A-Cdk2 in vitro and in vivo. Overexpression of cyclin binding-deficient Cdh1 stabilized the APC-Cdh1 interaction and induced prolonged cell cycle arrest at the G(1)/S transition. Conversely, cyclin binding-deficient Cdh1 lost its capability to support APC-dependent proteolysis of cyclin A but not that of other APC substrates such as cyclin B and securin Pds1. Collectively, these data provide a mechanistic explanation for the mutual functional interplay between cyclin A-Cdk2 and APC-Cdh1 and the first evidence that Cdh1 may activate the APC by binding specific substrates.

Mathematical model of the cell division cycle of fission yeast

Much is known about the genes and proteins controlling the cell cycle of fission yeast. Can these molecular components be spun together into a consistent mechanism that accounts for the observed behavior of growth and division in fission yeast cells? To answer this question, we propose a mechanism for the control system, convert it into a set of 14 differential and algebraic equations, study these equations by numerical simulation and bifurcation theory, and compare our results to the physiology of wild-type and mutant cells. In wild-type cells, progress through the cell cycle (G1-->S-->G2-->M) is related to cyclic progression around a hysteresis loop, driven by cell growth and chromosome alignment on the metaphase plate. However, the control system operates much differently in double-mutant cells, wee1(-) cdc25Delta, which are defective in progress through the latter half of the cell cycle (G2 and M phases). These cells exhibit "quantized" cycles (interdivision times clustering around 90, 160, and 230 min). We show that these quantized cycles are associated with a supercritical Hopf bifurcation in the mechanism, when the wee1 and cdc25 genes are disabled. (c) 2001 American Institute of Physics.

Cell cycle regulation in Schizosaccharomyces pombe

Cdc2, a cyclin-dependent kinase, controls cell cycle progression in fission yeast. New details of Cdc2 regulation and function have been uncovered in recent studies. These studies involve cyclins that associate with Cdc2 in G1-phase and the proteins that regulate inhibitory phosphorylation of Cdc2 during S-phase and G2-phase. Recent investigations have also provided a better understanding of proteins that regulate DNA replication and that are directly or indirectly controlled by Cdc2.

High dosage expression of a zinc finger protein, Grt1, suppresses a mutant of fission yeast slp1(+), a homolog of CDC20/p55CDC/Fizzy

Selective proteolysis at and after the onset of anaphase is a key cell cycle event required for sister chromatid separation as well as for exit from mitosis. It requires ubiquitination of substrates by Anaphase Promoting Complex(APC)/Cyclosome. Slp1, a WD-repeat protein, is a putative activator for APC in fission yeast. With another WD- repeat protein, Ste9/Srw1, it is thought to promote the proteolysis in a substrate-specific manner. We report here characterization of a temperature-sensitive (ts) slp1 mutant and its high-dosage suppressor, grt1(+). In cells arrested in metaphase, wild-type Slp1 was preferentially found in a complex with hyperphosphorylated Cut9 (subunit of APC), whereas the ts Slp1 protein, lacking the last 113 amino acids, failed to interact with Cut9. The temperature sensitivity was suppressed by high dosage expression of a zinc finger protein, Grt1. The ts slp1 mutant was unable to maintain the normal level of Grt1 protein. The reduction in the Grt1 level may be a primary defect since high dosage expression of grt1(+) rescues the slp1 mutant. The grt1-suppression had an additive effect to ste9 and wee1-50, both of which partially suppress the ts slp1 mutant. Therefore, grt1(+) would define an independent pathway that facilitates the function of Slp1.

A role for the START gene-specific transcription factor complex in the inactivation of cyclin B and Cut2 destruction

Hyperactivation of Cdc2 in fission yeast causes cells to undergo a lethal premature mitosis called mitotic catastrophe. This phenotype is observed in cdc2-3w wee1-50 cells at high temperature. Eleven of 17 mutants that suppress this phenotype define a single complementation group, mcs1. The mcs1-77 mutant also suppresses lethal inactivation of the Wee1 and Mik1 tyrosine kinases and thus delays mitosis independently of Cdc2 tyrosine phosphorylation. We have cloned mcs1 by isolating suppressors of the cell cycle arrest phenotype of mcs1-77 cdc25-22 cells and found that it encodes Res2, a component of the START gene-specific transcription factor complex MBF (also known as DSC-1). The mcs1-77 mutant bears a single point mutation in the DNA-binding domain of Res2 that causes glycine 68 to be replaced by a serine residue. Importantly, two substrates of the anaphase-promoting complex (APC), the major B-type cyclin, Cdc13, and the anaphase inhibitor, Cut2, are unstable in G2-phase mcs1-77 cells. Consistent with this, we observe abnormal sister chromatid separation in mcs1-77 cdc25-22 cells at the restrictive temperature. Mutation of either Cdc10 or Res1 also deregulates MBF-dependent transcription and causes a G2 delay. We find that this cell cycle delay is abolished in the absence of the APC regulator Ste9/Srw1 and that the periodic expression of Ste9/Srw1 is controlled by the MBF complex. These data suggest that in fission yeast the MBF complex plays a key role in the inactivation of cyclin B and Cut2 destruction by controlling the periodic production of APC regulators.