Completion of DNA replication is monitored by a feedback system that controls the initiation of mitosis in vitro: studies in Xenopus

Membrane localization of the kinase which phosphorylates p34cdc2 on threonine 14

The key regulator of entry into mitosis is the serine/threonine kinase p34cdc2. This kinase is regulated both by association with cyclins and by phosphorylation at several sites. Phosphorylation at Tyr 15 and Thr 14 are believed to inhibit the kinase activity of cdc2. In Schizosaccharomyces pombe, the wee1 (and possibly mik1) protein kinase catalyzes phosphorylation of Tyr 15. It is not clear whether these or other, as yet unidentified, protein kinases phosphorylate Thr 14. In this report we show, using extracts of Xenopus eggs, that the Thr 14-directed kinase is tightly membrane associated. Specifically, we have shown that a purified membrane fraction, in the absence of cytoplasm, can promote phosphorylation of cdc2 on both Thr 14 and Tyr 15. In contrast, the cytoplasm can phosphorylate cdc2 only on Tyr 15, suggesting the existence of at least two distinctly localized subpopulations of cdc2 Tyr 15-directed kinases. The membrane-associated Tyr 15 and Thr 14 kinase activities behaved similarly during salt or detergent extraction and were similarly regulated during the cell cycle and by the checkpoint machinery that delays mitosis while DNA is being replicated. This suggests the possibility that a dual-specificity membrane-associated protein kinase may catalyze phosphorylation of both Tyr 15 and Thr 14.

The B subunit of the DNA polymerase alpha-primase complex in Saccharomyces cerevisiae executes an essential function at the initial stage of DNA replication

The four-subunit DNA polymerase alpha-primase complex is unique in its ability to synthesize DNA chains de novo, and some in vitro data suggest its involvement in initiation and elongation of chromosomal DNA replication, although direct in vivo evidence for a role in the initiation reaction is still lacking. The function of the B subunit of the complex is unknown, but the Saccharomyces cerevisiae POL12 gene, which encodes this protein, is essential for cell viability. We have produced different pol12 alleles by in vitro mutagenesis of the cloned gene. The in vivo analysis of our 18 pol12 alleles indicates that the conserved carboxy-terminal two-thirds of the protein contains regions that are essential for cell viability, while the more divergent NH2-terminal portion is partially dispensable. The characterization of the temperature-sensitive pol12-T9 mutant allele demonstrates that the B subunit is required for in vivo DNA synthesis and correct progression through S phase. Moreover, reciprocal shift experiments indicate that the POL12 gene product plays an essential role at the early stage of chromosomal DNA replication, before the hydroxyurea-sensitive step. A model for the role of the B subunit in initiation of DNA replication at an origin is presented.

The B subunit of the DNA polymerase alpha-primase complex in Saccharomyces cerevisiae executes an essential function at the initial stage of DNA replication

The four-subunit DNA polymerase alpha-primase complex is unique in its ability to synthesize DNA chains de novo, and some in vitro data suggest its involvement in initiation and elongation of chromosomal DNA replication, although direct in vivo evidence for a role in the initiation reaction is still lacking. The function of the B subunit of the complex is unknown, but the Saccharomyces cerevisiae POL12 gene, which encodes this protein, is essential for cell viability. We have produced different pol12 alleles by in vitro mutagenesis of the cloned gene. The in vivo analysis of our 18 pol12 alleles indicates that the conserved carboxy-terminal two-thirds of the protein contains regions that are essential for cell viability, while the more divergent NH2-terminal portion is partially dispensable. The characterization of the temperature-sensitive pol12-T9 mutant allele demonstrates that the B subunit is required for in vivo DNA synthesis and correct progression through S phase. Moreover, reciprocal shift experiments indicate that the POL12 gene product plays an essential role at the early stage of chromosomal DNA replication, before the hydroxyurea-sensitive step. A model for the role of the B subunit in initiation of DNA replication at an origin is presented.

A model for regulation of the cell cycle incorporating cyclin A, cyclin B and their complexes

A mathematical model for the cell cycle is proposed that incorporates the known biochemical reactions involving both cyclin A and cyclin B, the interactions of these cyclins with cdc2 and cdk2, and the controlling effects of cdc25 and weel. The model also postulates the existence of an as yet unknown phosphatase involved in the formation of maturation promoting factor. The model produces solutions that agree qualitatively with a wide variety of experimentally observed cell-cycle behaviour. Conditions under which the model could explain the initial rapid divisions of embryonic cells and the transition to the slower somatic cell cycle are also discussed.

Theoretical dynamics of the cyclin B-MPF system: a possible role for p13suc1

In dividing cells, entry into mitosis is caused by maturation promoting factor (MPF), which is formed autocatalytically by activation of a complex of p34cdc2 and cyclin B. This biochemical system may oscillate, causing repeated mitosis. It is shown mathematically that the oscillatory tendency would be enhanced by a cofactor which binds to MPF and inhibits its autocatalytic action. A candidate for such a cofactor is the suc1 gene product p13, which binds to p34cdc2/cyclin B complex and inhibits MPF-induced MPF activation. At a steady rate of cyclin biosynthesis, with small amounts converted to MPF, p13suc1 would have to be titrated by MPF before autocatalysis could begin. This would have three possibly important effects: (1) it would determine the 'threshold' cyclin accumulation (and hence the corresponding time-delay) for MPF activation; (2) it would cause the accumulation of a backlog of MPF precursor (tyrosine-phosphorylated p34cdc2/cyclin B) sufficient to produce a substantial MPF pulse when MPF autocatalysis begins; (3) it would give the autocatalysis a high reaction order, which tends to destabilize the steady state, promote autonomous oscillations, and enhance the triggering property (excitability) of the system. The MPF pulse generated by this system may be essential for the proper triggering of the events of M phase, including the cyclin degradation which inactivates MPF at the end of M phase. This model offers explanations for several puzzling effects of p13suc1, including the fact that p13suc1, though an inhibitor of MPF activation, is nevertheless necessary for mitosis.

p27Kip1, a cyclin-Cdk inhibitor, links transforming growth factor-beta and contact inhibition to cell cycle arrest

Cell-cell contact and TGF-beta can arrest the cell cycle in G1. Mv1Lu mink epithelial cells arrested by either mechanism are incapable of assembling active complexes containing the G1 cyclin, cyclin E, and its catalytic subunit, Cdk2. These growth inhibitory signals block Cdk2 activation by raising the threshold level of cyclin E necessary to activate Cdk2. In arrested cells the threshold is set higher than physiological cyclin E levels and is determined by an inhibitor that binds to cyclin E-Cdk2 complexes. A 27-kD protein that binds to and prevents the activation of cyclin E-Cdk2 complexes can be purified from arrested cells but not from proliferating cells, using cyclin E-Cdk2 affinity chromatography. p27 is present in proliferating cells, but it is sequestered and unavailable to interact with cyclin E-Cdk2 complexes. Cyclin D2-Cdk4 complexes bind competitively to and down-regulate the activity of p27 and may thereby act in a pathway that reverses Cdk2 inhibition and enables G1 progression.

Abnormal cytoskeletal and chromosome distribution in po, ms4 and ms6; mutant alleles of polymitotic that disrupt the cell cycle progression from meiosis to mitosis in maize

The maize cell cycle regulation mutant polymitotic (po) progresses through abnormal cell cycles, characterized by premature cell divisions without chromosome duplication of the daughter cells produced by meiosis during microsporogenesis and macrosporogenesis. There are three recessive alleles of the Po gene; po, ms4, and ms6. A new method of permeabilizing cells based on freeze-fracture technology was used to study the distribution of microtubules in wild-type and mutant microspores. Here we show that an abnormal distribution of microtubules is correlated with changes in chromosome morphology in a cell cycle-dependent manner in po, ms4 and ms6 mutant alleles. After meiosis II, the cell cycle is complete and becomes progressively less synchronous in po homozygotes compared with wild-type cells. During microsporogenesis, the distribution of microtubules is abnormal, and chromosome morphology is altered in both po, ms4 and ms6 mutants. However, more chromosome fragments or micronuclei associated with minispindles are present in ms6 than po and ms4. After microspores are released from the tetrads, disruptions in structure and organization of chromosomes and microtubules continues in subsequent abnormal cell cycles. However, these cell cycles are incomplete since phragmoplasts are not formed. During these incomplete cell cycles, abnormal spindles and microtubule arrays are induced and extra microtubule arrays are associated with irregularly distributed chromosome fragments. States corresponding to interphase, prophase, metaphase and anaphase can be recognized in the mutant microspores. Abnormal cell cycles also occur after female meiosis during ms4 macrospore development. Since only the cell that normally undergoes embryo sac development (the chazal-most cell) undergoes supernumerary divisions this suggests that the po phenotype can be characterized as premature haploid divisions rather than repetition of meiosis II.

Numerical analysis of a comprehensive model of M-phase control in Xenopus oocyte extracts and intact embryos

To contribute to a deeper understanding of M-phase control in eukaryotic cells, we have constructed a model based on the biochemistry of M-phase promoting factor (MPF) in Xenopus oocyte extracts, where there is evidence for two positive feedback loops (MPF stimulates its own production by activating Cdc25 and inhibiting Wee1) and a negative feedback loop (MPF stimulates its own destruction by indirectly activating the ubiquitin pathway that degrades its cyclin subunit). To uncover the full dynamical possibilities of the control system, we translate the regulatory network into a set of differential equations and study these equations by graphical techniques and computer simulation. The positive feedback loops in the model account for thresholds and time lags in cyclin-induced and MPF-induced activation of MPF, and the model can be fitted quantitatively to these experimental observations. The negative feedback loop is consistent with observed time lags in MPF-induced cyclin degradation. Furthermore, our model indicates that there are two possible mechanisms for autonomous oscillations. One is driven by the positive feedback loops, resulting in phosphorylation and abrupt dephosphorylation of the Cdc2 subunit at an inhibitory tyrosine residue. These oscillations are typical of oocyte extracts. The other type is driven by the negative feedback loop, involving rapid cyclin turnover and negligible phosphorylation of the tyrosine residue of Cdc2. The early mitotic cycles of intact embryos exhibit such characteristics. In addition, by assuming that unreplicated DNA interferes with M-phase initiation by activating the phosphatases that oppose MPF in the positive feedback loops, we can simulate the effect of addition of sperm nuclei to oocyte extracts, and the lengthening of cycle times at the mid-blastula transition of intact embryos.

Cell cycle-regulated nuclear localization of MCM2 and MCM3, which are required for the initiation of DNA synthesis at chromosomal replication origins in yeast

MCM2 and MCM3 are two genetically interacting and structurally related proteins essential for growth in Saccharomyces cerevisiae. Mutants defective in these proteins affect the stability of minichromosomes in general, but the severity of the defect is dependent on the autonomously replicating sequence (ARS) that drives the replication of that plasmid. In this paper we show by two-dimensional gel electrophoresis that the initiation of DNA synthesis at chromosomal replication origins is also reduced in frequency in these mutants. We show further that the nuclear and subnuclear localizations of the MCM2 and MCM3 proteins are temporally regulated with respect to the cell cycle. These proteins enter the nucleus at the end of mitosis, persist there throughout G1 phase, and disappear from it at the beginning of S phase. Once inside the nucleus, a fraction of the MCM2 and MCM3 proteins becomes tightly associated with DNA. The association of MCM2 and MCM3 with chromatin presumably leads to the initiation of DNA synthesis, and their subsequent disappearance from the nucleus presumably prevents reinitiation of DNA synthesis at replication origins. This temporally and spatially restricted localization of MCM2 and MCM3 in the nucleus may serve to ensure that DNA replication occurs once and only once per cell cycle.

Distinct roles of cdk2 and cdc2 in RP-A phosphorylation during the cell cycle

RP-A is a single-stranded DNA-binding protein, which has been shown to be required for DNA replication using an SV40 model system. The protein has also been shown to be phosphorylated at the G1-S phase transition. Using Xenopus cell-free extracts we have investigated the role of RP-A in nuclear replication and characterized the kinases and conditions that lead to phosphorylation of RP-A during the cell cycle. By immunodepleting RP-A from Xenopus extracts we have shown that RP-A is essential for replication of chromosomal DNA. Our results show that, during S phase, only that RP-A which is associated with nuclei is phosphorylated. Furthermore our results indicate that during S phase RP-A is only phosphorylated when associated with single-stranded DNA. By immunodepleting cdk2 kinase we show that cdk2 kinase is required for the observed phosphorylation of RP-A in nuclei during S phase. However, using purified cdk2 kinase and RP-A we are unable to detect a direct phosphorylation of RP-A by cdk2 kinase. This observation suggests that phosphorylation of DNA-bound RP-A at S phase is carried out by a kinase distinct from cdk2. Consistent with this we find that when single-stranded DNA is added to S phase extracts depleted of cdk2 kinase, RP-A is phosphorylated. Together these results suggest that cdk2 kinase participates in the activation of DNA replication at a stage prior to the binding of RP-A to the initiation complex. In addition to RP-A phosphorylation in S phase, we have also found that at the onset of mitosis RP-A is quantitatively phosphorylated and that phosphorylation is directly mediated by cdc2 kinase. However, at this time during the cell cycle, cdc2-dependent phosphorylation of RP-A is independent of DNA binding. These observations further demonstrate the distinctions between cdk2 and cdc2 kinases.

Petunia p34cdc2 protein kinase activity in G2/M cells obtained with a reversible cell cycle inhibitor, mimosine

Protoplasts isolated from petunia leaf mesophyll are non-cycling cells mostly with 2C content. Cells regenerating from protoplast culture enter mitosis after 48 h. This experimental model is used to relate p34cdc2 kinase activity to cell cycle phase. Our results show that the histone H1 phosphorylation, and hence p34cdc2 kinase activity, peaks with G2+early M cell cycle phase. However, a trace kinase activity was already present when most cells were entering S phase. To obtain a maximum of cells in G1+S phases, the protoplast culture was treated with the rare amino acid, mimosine. Mimosine blocked plant cells derived from protoplast culture both at G1 and in early and mid S phase. Despite the increased G1+S level, p34cdc2 kinase activity did not increase. This suggests that the trace activity appearing when the majority of cells are entering S does not correspond to any putative p34cdc2 activation at G1/S transition but to the activation of the minor 4C population initially present in the leaf: the hypothesis remains that p34cdc2 kinase activity is solely related to G2+M phase in petunia.

Human wee1 maintains mitotic timing by protecting the nucleus from cytoplasmically activated Cdc2 kinase

The wee1 tyrosine kinase and cdc25 tyrosine phosphatase of fission yeast play antagonistic roles in the induction of mitosis through cdc2 regulation. We show here that the human wee1-like tyrosine kinase is a nuclear protein that ensures the completion of DNA replication prior to mitosis in cells expressing otherwise catastrophic levels of cdc2 activators. Paradoxically, wee1-rescued cells display very high levels of mitotic cdc2 kinase activity. We account for this anomaly by our observation that the cdc2 activator, cdc25C, is a cytoplasmic protein that, like cyclin B1, enters the nucleus at the G2/M transition. Thus, cdc2 is likely to be activated in the cytoplasm and requires nuclear localization to initiate both cytoplasmic and nuclear mitotic transformations. The human wee1 kinase appears to coordinate the transition between DNA replication and mitosis by protecting the nucleus from this cytoplasmically activated cdc2 kinase.

Full activation of p34CDC28 histone H1 kinase activity is unable to promote entry into mitosis in checkpoint-arrested cells of the yeast Saccharomyces cerevisiae

In most cells, mitosis is dependent upon completion of DNA replication. The feedback mechanisms that prevent entry into mitosis by cells with damaged or incompletely replicated DNA have been termed checkpoint controls. Studies with the fission yeast Schizosaccharomyces pombe and Xenopus egg extracts have shown that checkpoint controls prevent activation of the master regulatory protein kinase, p34cdc2, that normally triggers entry into mitosis. This is achieved through inhibitory phosphorylation of the Tyr-15 residue of p34cdc2. However, studies with the budding yeast Saccharomyces cerevisiae have shown that phosphorylation of this residue is not essential for checkpoint controls to prevent mitosis. We have investigated the basis for checkpoint controls in this organism and show that these controls can prevent entry into mitosis even in cells which have fully activated the cyclin B (Clb)-associated forms of the budding yeast homolog of p34cdc2, p34CDC28, as assayed by histone H1 kinase activity. However, the active complexes in checkpoint-arrested cells are smaller than those in cycling cells, suggesting that assembly of mitosis-inducing complexes requires additional steps following histone H1 kinase activation.

Full activation of p34CDC28 histone H1 kinase activity is unable to promote entry into mitosis in checkpoint-arrested cells of the yeast Saccharomyces cerevisiae

In most cells, mitosis is dependent upon completion of DNA replication. The feedback mechanisms that prevent entry into mitosis by cells with damaged or incompletely replicated DNA have been termed checkpoint controls. Studies with the fission yeast Schizosaccharomyces pombe and Xenopus egg extracts have shown that checkpoint controls prevent activation of the master regulatory protein kinase, p34cdc2, that normally triggers entry into mitosis. This is achieved through inhibitory phosphorylation of the Tyr-15 residue of p34cdc2. However, studies with the budding yeast Saccharomyces cerevisiae have shown that phosphorylation of this residue is not essential for checkpoint controls to prevent mitosis. We have investigated the basis for checkpoint controls in this organism and show that these controls can prevent entry into mitosis even in cells which have fully activated the cyclin B (Clb)-associated forms of the budding yeast homolog of p34cdc2, p34CDC28, as assayed by histone H1 kinase activity. However, the active complexes in checkpoint-arrested cells are smaller than those in cycling cells, suggesting that assembly of mitosis-inducing complexes requires additional steps following histone H1 kinase activation.

Dephosphorylation of cdc25-C by a type-2A protein phosphatase: specific regulation during the cell cycle in Xenopus egg extracts

We have examined the roles of type-1 (PP-1) and type-2A (PP-2A) protein-serine/threonine phosphatases in the mechanism of activation of p34cdc2/cyclin B protein kinase in Xenopus egg extracts. p34cdc2/cyclin B is prematurely activated in the extracts by inhibition of PP-2A by okadaic acid but not by specific inhibition of PP-1 by inhibitor-2. Activation of the kinase can be blocked by addition of the purified catalytic subunit of PP-2A at a twofold excess over the activity in the extract. The catalytic subunit of PP-1 can also block kinase activation, but very high levels of activity are required. Activation of p34cdc2/cyclin B protein kinase requires dephosphorylation of p34cdc2 on Tyr15. This reaction is catalysed by cdc25-C phosphatase that is itself activated by phosphorylation. We show that, in interphase extracts, inhibition of PP-2A by okadaic acid completely blocks cdc25-C dephosphorylation, whereas inhibition of PP-1 by specific inhibitors has no effect. This indicates that a type-2A protein phosphatase negatively regulates p34cdc2/cyclin B protein kinase activation primarily by maintaining cdc25-C phosphatase in a dephosphorylated, low activity state. In extracts containing active p34cdc2/cyclin B protein kinase, dephosphorylation of cdc25-C is inhibited, whereas the activity of PP-2A (and PP-1) towards other substrates is unaffected. We propose that this specific inhibition of cdc25-C dephosphorylation is part of a positive feedback loop that also involves direct phosphorylation and activation of cdc25-C by p34cdc2/cyclin B. Dephosphorylation of cdc25-C is also inhibited when cyclin A-dependent protein kinase is active, and this may explain the potentiation of p34cdc2/cyclin B protein kinase activation by cyclin A. In extracts supplemented with nuclei, the block on p34cdc2/cyclin B activation by unreplicated DNA is abolished when PP-2A is inhibited or when stably phosphorylated cdc25-C is added, but not when PP-1 is specifically inhibited. This suggests that unreplicated DNA inhibits p34cdc2/cyclin B activation by maintaining cdc25-C in a low activity, dephosphorylated state, probably by keeping the activity of a type-2A protein phosphatase towards cdc25-C at a high level.

Spinach thioredoxin m inhibits DNA synthesis in fertilized Xenopus eggs

A role for thioredoxin in metazoan DNA synthesis has been assessed by injecting rapidly dividing Xenopus eggs with purified heterologous thioredoxins, which might act as inhibitors if they were to replace resident thioredoxins in some but not all reaction steps. Of 10 tested proteins, spinach chloroplast thioredoxin m is the most potent inhibitor. Eggs cleave and produce cells lacking nuclei. DNA synthesis is severely reduced. Development arrests before gastrulation. In egg extracts, thioredoxin m inhibits incorporation of radioactive dCTP into DNA of sperm nuclei and M13 phage. Inhibition exceeds 90% when thioredoxin m and M13 DNA are preincubated together. The data support the interpretation that thioredoxins normally participate in initiation of metazoan DNA synthesis.

Characterization of the unusually rapid cell cycles during rat gastrulation

The onset of gastrulation in rodents is associated with the start of differentiation within the embryo proper and a dramatic increase in the rate of growth and proliferation. We have determined the duration of the cell cycle for mesodermal and ectodermal cells of rat embryos during gastrulation (days 8.5 to 9.5 of gestation) using a stathmokinetic analysis. These embryonic cells are the most rapidly dividing mammalian cells yet described. Most cells of the ectoderm and mesoderm had a cell cycle time of 7 to 7.5 hours, but the cells of the primitive streak divided every 3 to 3.5 hours. Total cell cycle time was reduced by shortening S and G2, as well as G1, in contrast to cells later in development, when cell cycle duration is modulated largely by varying the length of G1. In the ectoderm and mesoderm, G1 was 1.5 to 2 hours, S was 3.5 to 4 hours, and G2 was 30 to 40 minutes. G1, S and G2 were shortened even further in the cells of the primitive streak: G1 was less than 30 minutes, S was 2 to 2.75 hours, and G2 was less than 20 minutes. Thus, progress of cells through all phases of the cell cycle is extensively modified during rodent embryogenesis. Specifically, the increased growth rate during gastrulation is associated with radical changes in cell cycle structure and duration. Further, the commitment of cells to become mesoderm and endoderm by entering the primitive streak is associated with expression of a very short cell cycle during transit of the primitive streak, such that developmental decisions determining germ layer fate are reflected in differences in cell cycle regulation.

Ran/TC4: a small nuclear GTP-binding protein that regulates DNA synthesis

Ran/TC4, first identified as a well-conserved gene distantly related to H-RAS, encodes a protein which has recently been shown in yeast and mammalian systems to interact with RCC1, a protein whose function is required for the normal coupling of the completion of DNA synthesis and the initiation of mitosis. Here, we present data indicating that the nuclear localization of Ran/TC4 requires the presence of RCC1. Transient expression of a Ran/TC4 protein with mutations expected to perturb GTP hydrolysis disrupts host cell DNA synthesis. These results suggest that Ran/TC4 and RCC1 are components of a GTPase switch that monitors the progress of DNA synthesis and couples the completion of DNA synthesis to the onset of mitosis.

The efficiency and timing of plasmid DNA replication in Xenopus eggs: correlations to the extent of prior chromatin assembly

Injection of the circular plasmid FV1 (derived from type I bovine papilloma virus) into Xenopus eggs before the start of the first cell cycle dramatically increases the efficiency of plasmid replication once eggs are chemically activated. We call this the preloading effect and report kinetic and quantitative characterization of this phenomenon here. The timing and the amount of FV1 synthesis were measured by both BrdUTP density labelling and an optimized method of selective enzymatic digestion of replicated and unreplicated molecules using the three methyladenosine-sensitive isoschizomers, DpnI, MboI and Sau3a. DpnI in 100 mM NaCl proved particularly useful for distinguishing and quantitating unreplicated, once-replicated, and repeatedly replicated molecules accumulated over several cell cycles. Our results reveal that both the amount of DNA replicated and the timing of synthesis during the first S-phase correlate with the length of the preloading period. Longer preloading leads to larger amounts of DNA being replicated sooner. In fact, up to 30–50% of 1 ng injected plasmid can replicate in a semiconservative cell cycle-dependent manner during the first S-phase. But such high levels of synthesis during the first cell cycle appear to limit the egg's ability to rereplicate this material in subsequent cell cycles. The preloading effect does not depend on synthesis of either viral or egg proteins, but does appear to correlate with the extent of plasmid assembly into chromatin before the start of the cell cycle. We postulate that each plasmid molecule must achieve a critical degree of chromatin assembly before it can proceed along the replication pathway. These observations illuminate some of the difficulties inherent in building a vector for gene insertion into Xenopus embryos, but also suggest an experimental strategy toward this aim.

The ts41 mutation in Chinese hamster cells leads to successive S phases in the absence of intervening G2, M, and G1

The ts41 mutation of Chinese hamster cells was first isolated and characterized by Hirschberg and Marcus (1982) who showed that at nonpermissive temperature, cells accumulate up to 16C equivalents of DNA. Here we show that the mutation is recessive and at nonpermissive temperature, cells replicate their genome normally, but instead of going on into G2, M, and G1, they pass directly into a second S phase. Entry into a second S phase does not require serum nor is it inhibited by G2 checkpoints or mitotic inhibitors. Temperature-shift experiments suggest that the ts41 gene product participates in two functions in the cell cycle: entry into mitosis and inhibition of entry into S phase. The ts41 mutation seems to define a class of cell cycle mutant that couples the sequential events of DNA replication and mitosis.

Fission yeast genes involved in coupling mitosis to completion of DNA replication

We have isolated fission yeast mutants that enter mitosis when DNA replication is blocked with hydroxyurea. The mutants define eight linkage groups, three of which consist of alleles of the rad1, rad3, and rad17 genes. Recently, these fission yeast genes have been shown to be required for radiation-induced cell cycle arrest, as is the budding yeast RAD9 gene. The other five genes are called hus (hydroxyurea sensitive) 1-5. We propose that these genes participate in an intracellular signal transduction pathway that monitors the completion of DNA replication and transmits information to the mitotic control protein cdc2. Mutations that bypass the requirement for cdc25 (an activator of the mitotic regulator cdc2) also uncouple mitosis from DNA replication. However, mitosis is blocked by inhibitors of DNA replication in strains in which the cdc25 gene has been deleted, indicating that although cdc25 influences the coupling of mitosis to the completion of DNA replication, it is not essential for this control.

Creative blocks: cell-cycle checkpoints and feedback controls

Before division, cells must ensure that they finish DNA replication, DNA repair and chromosome segregation. They do so by using feedback controls which can detect the failure to complete replication, repair or spindle assembly to arrest the progress of the cell cycle at one of three checkpoints. Failures in feedback controls can contribute to the generation of cancer.

The synthesis of p58cyclin A and the phosphorylation of p34cdc2 are inhibited in human lymphoid cells arrested in G1 by alpha-interferon

Daudi cells arrest in G1 in the presence of alpha-interferon. Such cells have little p58cyclin A, probably due to inhibition of p58cyclin A synthesis. The phosphorylation-associated migration shift of p34cdc2 is not seen in alpha-interferon-arrested cells. Cells arrested in late G1 by aphidicolin have abundant p58cyclin A and phosphorylated p34cdc2. Cell sorting showed that p58cyclin A increases in proliferating cells in late G1 and coincides with phosphorylation of p34cdc2.

Checkpoint controls that couple mitosis to completion of DNA replication

Cells treated with inhibitors of DNA synthesis do not normally enter mitosis. Incompletely replicated DNA apparently activates a regulatory mechanism that prevents activation of the mitotic inducer M-phase kinase by controlling the dephosphorylation of a critical tyrosine residue in the active site of the kinase. The control system may also target a second mitotic inducer, possibly the NIMA protein kinase. Unreplicated DNA may be detected and signalled by a complex of RCC1, a DNA-binding protein, and Ran, a Ras-related protein. This article reviews these recent developments and discusses the possibility that the control system also operates in the normal cell cycle, to ensure that mitosis strictly follows S phase.

Control of the cell cycle

Cell division is arguably the most fundamental developmental process for single-celled and multicellular organisms alike. The pathway from one cell division to the next is known as the cell cycle. A conserved biochemical regulatory network controls progress along this pathway in plants, animals, and yeasts. This review is intended to serve as a primer on the current state of the eukaryotic cell cycle regulatory model, an introduction to the special roles of cell division and its control in plant development, and a review of recent progress in applying the universal mitotic control paradigm to higher plant systems.

Cyclin A potentiates maturation-promoting factor activation in the early Xenopus embryo via inhibition of the tyrosine kinase that phosphorylates cdc2

We have produced human cyclin A in Escherichia coli and investigated how it generates H1 kistone kinase activity when added to cyclin-free extracts prepared from parthenogenetically activated Xenopus eggs. Cyclin A was found to form a major complex with cdc2, and to bind cdk2/Eg1 only poorly. No lag phase was detected between the time when cyclin A was added and the time when H1 histone kinase activity was produced in frog extracts, even in the presence of 2 mM vanadate, which blocks cdc25 activity. Essentially identical results were obtained using extracts prepared from starfish oocytes. We conclude that formation of an active cyclin A-cdc2 kinase during early development escapes an inhibitory mechanism that delays formation of an active cyclin B-cdc2 kinase. This inhibitory mechanism involves phosphorylation of cdc2 on tyrosine 15. Okadaic acid (OA) activated cyclin B-cdc2 kinase and strongly reduced tyrosine phosphorylation of cyclin B-associated cdc2, even in the presence of vanadate. 6-dimethylamino-purine, a reported inhibitor of serine-threonine kinases, suppressed OA-dependent activation of cyclin B-cdc2 complexes. This indicates that the kinase(s) which phosphorylate(s) cdc2 on inhibitory sites can be inactivated by a phosphorylation event, itself antagonized by an OA-sensitive, most likely type 2A phosphatase. We also found that cyclin B- or cyclin A-cdc2 kinases can induce or accelerate conversion of the cyclin B-cdc2 complex from an inactive into an active kinase. Cyclin B-associated cdc2 does not undergo detectable phosphorylation on tyrosine in egg extracts containing active cyclin A-cdc2 kinase, even in the presence of vanadate. We propose that the active cyclin A-cdc2 kinase generated without a lag phase from neo-synthesized cyclin A and cdc2 may cause a rapid switch in the equilibrium of cyclin B-cdc2 complexes to the tyrosine-dephosphorylated and active form of cdc2 during early development, owing to strong inhibition of the cdc2-specific tyrosine kinase(s). This may explain why early cell cycles are so rapid in many species.

RCC1, a regulator of mitosis, is essential for DNA replication

Temperature-sensitive mutants in the RCC1 gene of BHK cells fail to maintain a correct temporal order of the cell cycle and will prematurely condense their chromosomes and enter mitosis at the restrictive temperature without having completed S phase. We have used Xenopus egg extracts to investigate the role that RCC1 plays in interphase nuclear functions and how this role might contribute to the known phenotype of temperature-sensitive RCC1 mutants. By immunodepleting RCC1 protein from egg extracts, we find that it is required for neither chromatin decondensation nor nuclear formation but that it is absolutely required for the replication of added sperm chromatin DNA. Our results further suggest that RCC1 does not participate enzymatically in replication but may be part of a structural complex which is required for the formation or maintenance of the replication machinery. By disrupting the replication complex, the loss of RCC1 might lead directly to disruption of the regulatory system which prevents the initiation of mitosis before the completion of DNA replication.

In vitro cell cycle arrest induced by using artificial DNA templates

In cell extracts of Xenopus eggs which oscillate between S and M phases of the cell cycle, the onset of mitosis is blocked by the presence of incompletely replicated DNA. In this report, we show that several artificial DNA templates (M13 single-stranded DNA and double-stranded plasmid DNA) can trigger this feedback pathway, which inhibits mitosis. Single-stranded M13 DNA is much more effective than double-stranded plasmid DNA at inhibiting the onset of mitosis. Furthermore, we have shown that low levels of M13 single-stranded DNA and high levels of double-stranded plasmid DNA can elevate the tyrosine kinase activity responsible for phosphorylating p34cdc2, thereby inactivating maturation-promoting factor and inhibiting entry into mitosis. This constitutes a simplified system with which to study the signal transduction pathway from the DNA template to the tyrosine kinase responsible for inhibiting p34cdc2 activity.

Periodic changes in phosphorylation of the Xenopus cdc25 phosphatase regulate its activity

The cdc25 tyrosine phosphatase is known to activate cdc2 kinase in the G2/M transition by dephosphorylation of tyrosine 15. To determine how entry into M-phase in eukaryotic cells is controlled, we have investigated the regulation of the cdc25 protein in Xenopus eggs and oocytes. Two closely related Xenopus cdc25 genes have been cloned and sequenced and specific antibodies generated. The cdc25 phosphatase activity oscillates in both meiotic and mitotic cell cycles, being low in interphase and high in M-phase. Increased activity of cdc25 at M-phase is accompanied by increased phosphorylation that retards electrophoretic mobility in gels from 76 to 92 kDa. Treatment of cdc25 with either phosphatase 1 or phosphatase 2A removes phosphate from cdc25, reverses the mobility shift, and decreases its ability to activate cdc2 kinase. Furthermore, the addition of okadaic acid to egg extracts arrested in S-phase by aphidicolin causes phosphorylation and activation of the cdc25 protein before cyclin B/cdc2 kinase activation. These results demonstrate that the activity of the cdc25 phosphatase at the G2/M transition is directly regulated through changes in its phosphorylation state.

RCC1, a regulator of mitosis, is essential for DNA replication

Temperature-sensitive mutants in the RCC1 gene of BHK cells fail to maintain a correct temporal order of the cell cycle and will prematurely condense their chromosomes and enter mitosis at the restrictive temperature without having completed S phase. We have used Xenopus egg extracts to investigate the role that RCC1 plays in interphase nuclear functions and how this role might contribute to the known phenotype of temperature-sensitive RCC1 mutants. By immunodepleting RCC1 protein from egg extracts, we find that it is required for neither chromatin decondensation nor nuclear formation but that it is absolutely required for the replication of added sperm chromatin DNA. Our results further suggest that RCC1 does not participate enzymatically in replication but may be part of a structural complex which is required for the formation or maintenance of the replication machinery. By disrupting the replication complex, the loss of RCC1 might lead directly to disruption of the regulatory system which prevents the initiation of mitosis before the completion of DNA replication.

Regulation of the cdc25 protein during the cell cycle in Xenopus extracts

The cdc25 protein is a highly specific tyrosine phosphatase that triggers mitosis by dephosphorylating the cdc2 protein kinase. Using Xenopus extracts, we have found that the cdc25 protein is active at a low level throughout interphase. Near the onset of mitosis, the cdc25 protein undergoes a marked elevation in phosphatase activity that coincides with an extensive phosphorylation of the protein in its N-terminal region. In vitro dephosphorylation of this hyperphosphorylated form of cdc25 reduces its phosphatase activity back to the interphase level. Moreover, treatment of interphase Xenopus extracts with okadaic acid, a phosphatase inhibitor that accelerates the entry into mitosis, elicits both the premature hyperphosphorylation of cdc25 and the stimulation of its cdc2-specific tyrosine phosphatase activity. These experiments demonstrate the existence of a cdc25 regulatory system consisting of both a stimulatory kinase that phosphorylates a putative regulatory domain of the cdc25 protein and an inhibitory serine/threonine phosphatase that counteracts this kinase activity.

In vitro cell cycle arrest induced by using artificial DNA templates

In cell extracts of Xenopus eggs which oscillate between S and M phases of the cell cycle, the onset of mitosis is blocked by the presence of incompletely replicated DNA. In this report, we show that several artificial DNA templates (M13 single-stranded DNA and double-stranded plasmid DNA) can trigger this feedback pathway, which inhibits mitosis. Single-stranded M13 DNA is much more effective than double-stranded plasmid DNA at inhibiting the onset of mitosis. Furthermore, we have shown that low levels of M13 single-stranded DNA and high levels of double-stranded plasmid DNA can elevate the tyrosine kinase activity responsible for phosphorylating p34cdc2, thereby inactivating maturation-promoting factor and inhibiting entry into mitosis. This constitutes a simplified system with which to study the signal transduction pathway from the DNA template to the tyrosine kinase responsible for inhibiting p34cdc2 activity.

Multiple roles for protein phosphatase 1 in regulating the Xenopus early embryonic cell cycle

Using cytostatic factor metaphase II-arrested extracts as a model system, we show that protein phosphatase 1 is regulated during early embryonic cell cycles in Xenopus. Phosphatase 1 activity peaks during interphase and decreases shortly before the onset of mitosis. A second peak of activity appears in mitosis at about the same time that cdc2 becomes active. If extracts are inhibited in S-phase with aphidicolin, then phosphatase 1 activity remains high. The activity of phosphatase 1 appears to determine the timing of exit from S-phase and entry into M-phase; inhibition of phosphatase 1 by the specific inhibitor, inhibitor 2 (Inh-2), causes premature entry into mitosis, whereas exogenously added phosphatase 1 lengthens the interphase period. Analysis of DNA synthesis in extracts treated with Inh-2, but lacking the A- and B-type cyclins, shows that phosphatase 1 is also required for the process of DNA replication. These data indicate that phosphatase 1 is a component of the signaling pathway that ensures that M-phase is not initiated until DNA synthesis is complete.

A dominant negative allele of p34cdc2 shows altered phosphoamino acid content and sequesters p56cdc13 cyclin

The cdc2 gene product, a 34-kDa phosphoprotein with serine/threonine protein kinase activity, has been implicated as the key component in the regulation of the eucaryotic cell cycle. Activation of the cdc2 protein kinase is regulated by its phosphorylation state and by interaction with other proteins. We have mutagenized the fission yeast cdc2 gene to obtain conditionally dominant negative alleles. One of these mutants, named DL2, is characterized in this report. Overexpression of the mutant protein in a wild-type cdc2 background is lethal and leads to arrest in the G2 phase of the cell cycle. The mutant phenotype is the result of a single amino acid change in the GDSEID motif of the protein, a region of identity in all cdc2 homologs, and results in a nonfunctional protein that shows an altered content of phosphothreonine. Multicopy suppressors of the dominant negative phenotype have been isolated, and one of these has been shown to encode the cdc13 cyclin B gene product.

A dominant negative allele of p34cdc2 shows altered phosphoamino acid content and sequesters p56cdc13 cyclin

The cdc2 gene product, a 34-kDa phosphoprotein with serine/threonine protein kinase activity, has been implicated as the key component in the regulation of the eucaryotic cell cycle. Activation of the cdc2 protein kinase is regulated by its phosphorylation state and by interaction with other proteins. We have mutagenized the fission yeast cdc2 gene to obtain conditionally dominant negative alleles. One of these mutants, named DL2, is characterized in this report. Overexpression of the mutant protein in a wild-type cdc2 background is lethal and leads to arrest in the G2 phase of the cell cycle. The mutant phenotype is the result of a single amino acid change in the GDSEID motif of the protein, a region of identity in all cdc2 homologs, and results in a nonfunctional protein that shows an altered content of phosphothreonine. Multicopy suppressors of the dominant negative phenotype have been isolated, and one of these has been shown to encode the cdc13 cyclin B gene product.

2-Aminopurine overrides multiple cell cycle checkpoints in BHK cells

BHK cells blocked at any of several points in the cell cycle override their drug-induced arrest and proceed in the cycle when exposed concurrently to the protein kinase inhibitor 2-aminopurine (2-AP). For cells arrested at various points in interphase, 2-AP-induced cell cycle progression is made evident by arrival of the drug-treated cell population in mitosis. Cells that have escaped from mimosine G1 arrest, from hydroxyurea or aphidicolin S-phase arrest, or from VM-26-induced G2 arrest subsequently have all the hallmarks of mitosis--such as a mitotic microtubule array, nuclear envelope breakdown, and chromatin condensation. In a synchronous population, the time course of arrival in mitosis and its duration in 2-AP-treated cells that have escaped drug-induced cell cycle blocks is indistinguishable from control cells. Cells arrested in mitosis by nocodazole or taxol quickly escape mitotic arrest and enter interphase when exposed to 2-AP. 2-AP by itself does not influence the timing of cell cycle progression. We conclude that 2-AP acts to override checkpoints in every phase of the cell cycle, perhaps by inhibiting a protein kinase responsible for control of multiple cell cycle checkpoints.

Treatment of myeloid leukemic cells with the phosphatase inhibitor okadaic acid induces cell cycle arrest at either G1/S or G2/M depending on dose

The phosphatase inhibitor okadaic acid was found to induce cell cycle arrest of human myeloid leukemic cell lines HL-60 and U937 in a concentration- and time-dependent manner. Exposure to low concentrations of okadaic acid (2-8nM) for 24-48 hr caused greater than 70% of cells to arrest at G2/M, with up to 40% of the cells arrested in early mitosis. Cell viability decreased rapidly after 48 hr of treatment, and morphological and DNA structure analysis indicated that this was primarily due to the induction of apoptosis. The cells arrested in mitosis by 8 nM okadaic acid could be highly enriched by density gradient centrifugation and underwent apoptosis when further cultured either with or without okadaic acid, indicating that the effects of okadaic acid were irreversible. In contrast to the effects of low concentrations of okadaic acid, high concentrations (500 nM), inhibited proliferation in less than 3 hr. Remarkably, the majority of cells also entered a mitosis-like state characterized by dissolution of the nuclear membrane and condensation and partial separation of chromosomes. However, these cells had a diploid content of DNA, indicating that the cell cycle arrest occurred at G1/S with premature chromosome condensation (PCC), rather than at G2/M. If cells were first blocked at G1/S with hydroxyurea and then treated with okadaic acid, greater than 90% developed PCC in less than 3 hr without replicating their DNA. Caffeine was not able to induce PCC in these cells, either with or without prior inhibition of DNA synthesis.(ABSTRACT TRUNCATED AT 250 WORDS)

Coupling of mitosis to the completion of S phase in Xenopus occurs via modulation of the tyrosine kinase that phosphorylates p34cdc2

In cell-free extracts derived from Xenopus eggs which oscillate between S phase and mitosis, incompletely replicated DNA blocks the activation of p34cdc2-cyclin by maintaining p34cdc2 in a tyrosine-phosphorylated form. We used a recombinant cyclin fusion protein to generate a substrate to measure the ability of the tyrosine kinase(s) to phosphorylate and inactivate p34cdc2 in the absence of tyrosine phosphatase activity. p34cdc2 tyrosine phosphorylation is highly regulated during the cell cycle, being elevated in S phase and attenuated in mitosis. The elevation in p34cdc2 tyrosine phosphorylation rate occurs in response to the presence of incompletely replicated DNA. Moreover, okadaic acid and caffeine, which uncouple the dependence of mitosis on the completion of S phase, increase unphosphorylated p34cdc2 by attenuating tyrosine kinase function. These data indicate that the control system, which monitors the state of DNA replication, modulates the function of the tyrosine kinase by a phosphorylation/dephosphorylation mechanism, ensuring that mitosis occurs only when S phase is complete.

The requirements for protein synthesis and degradation, and the control of destruction of cyclins A and B in the meiotic and mitotic cell cycles of the clam embryo

Fertilization of clam oocytes initiates a series of cell divisions, of which the first three--meiosis I, meiosis II, and the first mitotic division--are highly synchronous. After fertilization, protein synthesis is required for the successful completion of every division except meiosis I. When protein synthesis is inhibited, entry into meiosis I and the maintenance of M phase for the normal duration of meiosis occur normally, but the chromosomes fail to interact correctly with the spindle in meiosis II metaphase. By contrast, inhibition of protein synthesis immediately after completion of meiosis or mitosis stops cells entering the next mitosis. We describe the behavior of cyclins A and B in relation to these "points of no return." The cyclins are synthesized continuously and are rapidly destroyed shortly before the metaphase-anaphase transition of the mitotic cell cycles, with cyclin A being degraded in advance of cyclin B. Cyclin destruction normally occurs during a 5-min window in mitosis, but in the monopolar mitosis that occurs after parthenogenetic activation of clam oocytes, or when colchicine is added to fertilized eggs about to enter first mitosis, the destruction of cyclin B is strongly delayed, whereas proteolysis of cyclin A is maintained in an activated state for the duration of metaphase arrest. Under either of these abnormal conditions, inhibition of protein synthesis causes a premature return to interphase that correlates with the time when cyclin B disappears.

Uncoupled cell cycle without mitosis induced by a protein kinase inhibitor, K-252a

The staurosporine analogues, K-252a and RK-286C, were found to cause DNA re-replication in rat diploid fibroblasts (3Y1) without an intervening mitosis, producing tetraploid cells. Analysis of cells synchronized in early S phase in the presence of K-252a revealed that initiation of the second S phase required a lag period of 8 h after completion of the previous S phase. Reinitiation of DNA synthesis was inhibited by cycloheximide, actinomycin D, and serum deprivation, but not by Colcemid, suggesting that a functional G1 phase dependent on de novo synthesis of protein and RNA is essential for entry into the next S phase. In a src-transformed 3Y1 cell line, as well as other cell lines, giant cells containing polyploid nuclei with DNA contents of 16C to 32C were produced by continuous treatment with K-252a, indicating that the agent induced several rounds of the incomplete cell cycle without mitosis. Although the effective concentration of K-252a did not cause significant inhibition of affinity-purified p34cdc2 protein kinase activity in vitro, in vivo the full activation of p34cdc2 kinase during the G2/M was blocked by K-252a. On the other hand, the cyclic fluctuation of partially activated p34cdc2 kinase activity peaking in S phase still continued. These results suggest that a putative protein kinase(s) sensitive to K-252a plays an important role in the mechanism for preventing over-replication after completion of previous DNA synthesis. They also suggest that a periodic activation of p34cdc2 is required for S phases in the cell cycle without mitosis.

Role for cyclin A in the dependence of mitosis on completion of DNA replication

THE cyclins were first identified by their cell-cycle-dependent synthesis and destruction and have a key role in the control of mitosis in Xenopus embryonic cell cycles. All higher eukaryotes have at least two types of cyclins, the A- and B-type, which can be distinguished by sequence motifs and the timing of their destruction in the cell cycle. The degradation of both cyclins is required for exit from mitosis, but the activation and destruction of cyclin A occur earlier in the cell cycle than with the B-type cyclins. This suggests that cyclin A has a distinct role in cell-cycle progression. We have used an antisense oligodeoxy-nucleotide directed against cyclin A to investigate this role. Ablation of cyclin A messenger RNA in cytostatic factor/metaphase-arrested extracts of Xenopus eggs, followed by in vitro progression into interphase, resulted in the premature appearance of cyclin B-cdc2-associated H1 kinase activity and premature entry into mitosis. Although cyclin A-ablated extracts could initiate DNA synthesis during interphase, S phase was not completed before entry into mitosis. The effects of cyclin A ablation were reversed by the addition of cyclin A mRNA or cyclin A protein to the extracts.

Controlling cell cycle progress in the fission yeast Schizosaccharomyces pombe

The suitability of fission yeast as a model for understanding the eukaryotic cell cycle has been validated in five years of exciting developments. We review recent advances in understanding the nature of the controls that regulate progression through the cell cycle and the coordination of DNA replication and mitosis.