Temporal and spatial control of cyclin B1 destruction in metaphase

Dual inhibition of sister chromatid separation at metaphase

Separation of sister chromatids in anaphase is mediated by separase, an endopeptidase that cleaves the chromosomal cohesin SCC1. Separase is inhibited by securin, which is degraded at the metaphase-anaphase transition. Using Xenopus egg extracts, we demonstrate that high CDC2 activity inhibits anaphase but not securin degradation. We show that separase is kept inactive under these conditions by a mechanism independent of binding to securin. Mutation of a single phosphorylation site on separase relieves the inhibition and rescues chromatid separation in extracts with high CDC2 activity. Using quantitative mass spectrometry, we show that, in intact cells, there is complete phosphorylation of this site in metaphase and significant dephosphorylation in anaphase. We propose that separase activation at the metaphase-anaphase transition requires the removal of both securin and an inhibitory phosphate.

Sub-cellular localisation of GFP-tagged tobacco mitotic cyclins during the cell cycle and after spindle checkpoint activation

We have previously shown that the tobacco cyclin B1;1 protein accumulates during the G2 phase of the cell cycle and is subsequently destroyed during mitosis. Here, we investigated the sub-cellular localisation of two different B1-types and one A3-type cyclin during the cell cycle by using confocal imaging and differential interference contrast (DIC) microscopy. The cyclins were visualised as GFP-tagged fusion proteins in living tobacco cells. Both B1-type cyclins were found in the cytoplasm and in the nucleus during G2 but when cells entered into prophase, both cyclins became associated with condensing chromatin and remained on chromosomes until metaphase. As cells exited metaphase, the B1-type cyclins became degraded, as shown by time-lapse images. A stable variant of cyclin B1;1-GFP fusion protein, in which the destruction box had been mutated, maintained its association with the nuclear material at later phases of mitosis such as anaphase and telophase. Furthermore, we demonstrated that cyclin B1;1 protein is stabilised in metaphase-arrested cells after microtubule destabilising drug treatments. In contrast to the B1-type cyclins, the cyclin A3;1 was found exclusively in the nucleus in interphase cells and disappeared earlier than the cyclin B1 proteins during mitosis.

The cyclin B1 gene is actively transcribed during mitosis in HeLa cells

In mammalian cells, the expression level of the cyclin B1 gene plays a critical role in the progression through mitosis. Here we demonstrate that the transcriptional activity of the human cyclin B1 promoter, as well as the rate of gene transcription, is high during mitosis. Indeed, the cyclin B1 promoter maintains an open chromatin configuration at the mitotic stage. Consistent with this, we show that the cyclin B1 promoter is occupied and bound to NF-Y during mitosis in vivo. Our results provide the first example of RNA polymerase II-dependent transcription during mitosis in mammalian cells.

Recruitment of cdc2 kinase by DNA topoisomerase II is coupled to chromatin remodeling

Although initiation of chromosome condensation during early prophase is linked temporally to the appearance of the mitotic cdc2 kinase in the nucleus, it is not known what targets the kinase to the nucleus and how this is coupled to chromatin remodeling. We now report that cdc2 kinase forms stable molecular complexes with the nuclear enzyme DNA topoisomerase II, which is associated with marked stimulation of both DNA binding and catalytic activity of topoisomerase II, albeit in a phosphorylation-independent manner. The molecular interaction is required for recruitment of cdc2 kinase, as shown by incubation of purified enzymes with chicken erythrocyte nuclei, which have neither endogenous topoisomerase II nor cdc2 kinase. The physical association between the two enzymes alters the DNA/topoisomerase II interaction as shown by pulse-field electrophoresis after incubation of intact nuclei with the specific topoisomerase II inhibitor VM-26. Furthermore, the presence of both enzymes, but not either enzyme alone, is accompanied by extensive chromatin remodeling converting the interphase nuclei into precondensation chromosomes with striking resemblance to early prophase structures. Our results reveal a novel property of cyclin-dependent kinases and demonstrate that the recruitment of cdc2 kinase by topoisomerase II is coupled to chromatin remodeling.

Re-evaluating centrosome function

Over the past 100 years, the centrosome has risen in status from an enigmatic organelle, located at the focus of microtubules, to a key player in cell-cycle progression and cellular control. A growing body of evidence indicates that centrosomes might not be essential for spindle assembly, whereas recent data indicate that they might be important for initiating S phase and completing cytokinesis. Molecules that regulate centrosome duplication have been identified, and the expanding list of intriguing centrosome-anchored activities, the functions of which have yet to be determined, promises continued discovery.

Characterization of an N-terminally truncated cyclin A isoform in mammalian cells

Cyclin A is essential for regulating key transitions in the eukaryotic cell cycle including initiation of DNA replication and mitosis. This paper describes the characterization of a truncated cyclin A isoform (cyclin A(t)) in vitro in cultured mammalian cells and in mouse tissues. The presence of cyclin A(t) in specific cell types correlates with the ability of cell extracts to cleave in vitro translated cyclin A. In CHO-K1 cells, cyclin A processing to cyclin A(t) occurs at the N terminus; it does not involve the 26 S proteasome, nor could it be induced by conditional overexpression of the cyclin-dependent kinase inhibitor p27(Kip1). However, high cell densities lead to increased cyclin A(t) levels. Unlike full-length cyclin A, cyclin A(t) localizes to the cytoplasm, where it binds Cdk2. The data suggest that cyclin A processing occurs in vivo to yield an N-terminally truncated isoform by an unknown mechanism that is regulated by cell density. Differential subcellular localization may provide the first insights into the physiological role of cyclin A(t).

New insights into development from mitosis of a unicellular yeast

Studies in the fission yeast Schizosaccharomyces pombe have uncovered a new spindle checkpoint.

Microtubule-dependent changes in assembly of microtubule motor proteins and mitotic spindle checkpoint proteins at PtK1 kinetochores

The ability of kinetochores to recruit microtubules, generate force, and activate the mitotic spindle checkpoint may all depend on microtubule- and/or tension-dependent changes in kinetochore assembly. With the use of quantitative digital imaging and immunofluorescence microscopy of PtK1 tissue cells, we find that the outer domain of the kinetochore, but not the CREST-stained inner core, exhibits three microtubule-dependent assembly states, not directly dependent on tension. First, prometaphase kinetochores with few or no kinetochore microtubules have abundant punctate or oblate fluorescence morphology when stained for outer domain motor proteins CENP-E and cytoplasmic dynein and checkpoint proteins BubR1 and Mad2. Second, microtubule depolymerization induces expansion of the kinetochore outer domain into crescent and ring morphologies around the centromere. This expansion may enhance recruitment of kinetochore microtubules, and occurs with more than a 20- to 100-fold increase in dynein and relatively little change in CENP-E, BubR1, and Mad2 in comparison to prometaphase kinetochores. Crescents disappear and dynein decreases substantially upon microtubule reassembly. Third, when kinetochores acquire their full metaphase complement of kinetochore microtubules, levels of CENP-E, dynein, and BubR1 decrease by three- to sixfold in comparison to unattached prometaphase kinetochores, but remain detectable. In contrast, Mad2 decreases by 100-fold and becomes undetectable, consistent with Mad2 being a key factor for the "wait-anaphase" signal produced by unattached kinetochores. Like previously found for Mad2, the average amounts of CENP-E, dynein, or BubR1 at metaphase kinetochores did not change with the loss of tension induced by taxol stabilization of microtubules.

Emi1 is a mitotic regulator that interacts with Cdc20 and inhibits the anaphase promoting complex

We have discovered an early mitotic inhibitor, Emi1, which regulates mitosis by inhibiting the anaphase promoting complex/cyclosome (APC). Emi1 is a conserved F box protein containing a zinc binding region essential for APC inhibition. Emi1 accumulates before mitosis and is ubiquitylated and destroyed in mitosis, independent of the APC. Emi1 immunodepletion from cycling Xenopus extracts strongly delays cyclin B accumulation and mitotic entry, whereas nondestructible Emi1 stabilizes APC substrates and causes a mitotic block. Emi1 binds the APC activator Cdc20, and Cdc20 can rescue an Emi1-induced block to cyclin B destruction. Our results suggest that Emi1 regulates progression through early mitosis by preventing premature APC activation, and may help explain the well-known delay between cyclin B/Cdc2 activation and cyclin B destruction.

Requirement of cyclin B2, but not cyclin B1, for bipolar spindle formation in frog (Rana japonica) oocytes

Cyclin B, the regulatory subunit of maturation-promoting factor (MPF), comprises several subtypes that are presumed to confer different functions on MPF although no direct evidence has been provided to date. To clarify the difference in the roles of cyclins B1 and B2, we used frog (Rana japonica) oocytes in which MPF is formed only after progesterone stimulation because it is possible to produce oocytes containing either cyclin B1-MPF or cyclin B2-MPF by antisense RNA-mediated translational inhibition of each mRNA. Using this advantage, we investigated the functions of cyclins B1 and B2 and obtained the following results: (a) oocytes synthesizing cyclin B2-MPF underwent meiosis I and II with formation of a bipolar spindle at each metaphase; (b) oocytes synthesizing cyclin B1-MPF formed a monopolar spindle at metaphase I and extruded an abnormal polar body; and (c) both oocytes underwent germinal vesicle breakdown (GVBD) and chromosome condensation. Immunocytochemical observations also revealed continuous localization of cyclin B2 on the spindle during meiosis. These results provide evidence of the requirement of cyclin B2, but not cyclin B1, for organizing the bipolar spindle, though either cyclin B1 or B2 is redundant for inducing GVBD and chromosome condensation.

Checking out the G(2)/M transition

Tight regulation of cell cycle progression is essential for the maintenance of genomic integrity. The orderly progression from one cell cycle phase to the other is mediated by timed activation of distinct cyclin/cdk complexes. For example, onset of mitosis is regulated by the activation of cyclin B/cdc2 and this event is controlled by several cell cycle checkpoints. Such checkpoints ensure that chromosome segregation does not occur in the case of unreplicated or damaged DNA, or misaligned chromosomes. Recently, new insights into the targets of the DNA damage checkpoint help to unravel more of the complex mechanisms of cell cycle checkpoints. This review focuses on the factors controlling the transition from G(2) phase to mitosis. Also, the pathways contributing to the DNA damage checkpoints in these phases of the cell cycle will be discussed.

Nucleolar assembly of the rRNA processing machinery in living cells

To understand how nuclear machineries are targeted to accurate locations during nuclear assembly, we investigated the pathway of the ribosomal RNA (rRNA) processing machinery towards ribosomal genes (nucleolar organizer regions [NORs]) at exit of mitosis. To follow in living cells two permanently transfected green fluorescence protein-tagged nucleolar proteins, fibrillarin and Nop52, from metaphase to G1, 4-D time-lapse microscopy was used. In early telophase, fibrillarin is concentrated simultaneously in prenucleolar bodies (PNBs) and NORs, whereas PNB-containing Nop52 forms later. These distinct PNBs assemble at the chromosome surface. Analysis of PNB movement does not reveal the migration of PNBs towards the nucleolus, but rather a directional flow between PNBs and between PNBs and the nucleolus, ensuring progressive delivery of proteins into nucleoli. This delivery appeared organized in morphologically distinct structures visible by electron microscopy, suggesting transfer of large complexes. We propose that the temporal order of PNB assembly and disassembly controls nucleolar delivery of these proteins, and that accumulation of processing complexes in the nucleolus is driven by pre-rRNA concentration. Initial nucleolar formation around competent NORs appears to be followed by regroupment of the NORs into a single nucleolus 1 h later to complete the nucleolar assembly. This demonstrates the formation of one functional domain by cooperative interactions between different chromosome territories.

Asymmetrical segregation of chromosomes with a normal metaphase/anaphase checkpoint in polyploid megakaryocytes

During differentiation, megakaryocytes increase ploidy through a process called endomitosis, whose mechanisms remain unknown. As it corresponds to abortive mitosis at anaphase and is associated with a multipolar spindle, investigation of chromosome segregation may help to better understand this cell-cycle abnormality. To examine this variation, a new method was developed to combine primed in situ labeling to label centromeres of one chromosome category and immunostaining of tubulin. Human megakaryocytes were obtained from normal bone marrow culture. By confocal microscopy, this study demonstrates an asymmetrical distribution of chromosomes (1 or 7) either between the spindle poles at anaphase stage of endomitosis and between the different lobes of interphase megakaryocyte nuclei. The metaphase/anaphase checkpoint appears normal on the evidence that under nocodazole treatment megakaryocytes progressively accumulate in pseudo-metaphase, without spontaneous escape from this blockage. Immunostaining of p55CDC/hCDC20 with similar kinetochore localization and dynamics as during normal mitosis confirms this result. HCdh1 was also expressed in megakaryocytes, and its main target, cyclin B1, was normally degraded at anaphase, suggesting that the hCdh1-anaphase-promoting complex checkpoint was also functional. This study found the explanation for these unexpected results of an asymmetrical segregation coupled to normal checkpoints by careful analysis of multipolar endomitotic spindles: whereas each aster is connected to more than one other aster, one chromosome may segregate symmetrically between 2 spindle poles and still show asymmetrical segregation when the entire complex spindle is considered.

Meiotic maturation of the mouse oocyte requires an equilibrium between cyclin B synthesis and degradation

Among the proteins whose synthesis and/or degradation is necessary for a proper progression through meiotic maturation, cyclin B appears to be one of the most important. Here, we attempted to modulate the level of cyclin B1 and B2 synthesis during meiotic maturation of the mouse oocyte. We used cyclin B1 or B2 mRNAs with poly(A) tails of different sizes and cyclin B1 or B2 antisense RNAs. Oocytes microinjected with cyclin B1 mRNA showed two phenotypes: most were blocked in MI, while the others extruded the first polar body in advance when compared to controls. Moreover, these effects were correlated with the length of the poly(A) tail. Thus it seems that the rate of cyclin B1 translation controls the timing of the first meiotic M phase and the transition to anaphase I. Moreover, overexpression of cyclin B1 or B2 was able to bypass the dbcAMP-induced germinal vesicle block, but only the cyclin B1 mRNA-microinjected oocytes did not extrude their first polar body. Oocytes injected with the cyclin B1 antisense progressed through the first meiotic M phase but extruded the first polar body in advance and were unable to enter metaphase II. This suggested that inhibition of cyclin B1 synthesis only took place at the end of the first meiotic M phase, most likely because the cyclin B1 mRNA was protected. The injection of cyclin B2 antisense RNA had no effect. The life observation of the synthesis and degradation of a cyclin B1-GFP chimera during meiotic maturation of the mouse oocyte demonstrated that degradation can only occur during a given period of time once it has started. Taken together, our data demonstrate that the rates of cyclin B synthesis and degradation determine the timing of the major events taking place during meiotic maturation of the mouse oocyte.

Centrosomes enhance the fidelity of cytokinesis in vertebrates and are required for cell cycle progression

When centrosomes are destroyed during prophase by laser microsurgery, vertebrate somatic cells form bipolar acentrosomal mitotic spindles (Khodjakov, A., R.W. Cole, B.R. Oakley, and C.L. Rieder. 2000. Curr. Biol. 10:59-67), but the fate of these cells is unknown. Here, we show that, although these cells lack the radial arrays of astral microtubules normally associated with each spindle pole, they undergo a normal anaphase and usually produce two acentrosomal daughter cells. Relative to controls, however, these cells exhibit a significantly higher (30-50%) failure rate in cytokinesis. This failure correlates with the inability of the spindle to properly reposition itself as the cell changes shape. Also, we destroyed just one centrosome during metaphase and followed the fate of the resultant acentrosomal and centrosomal daughter cells. Within 72 h, 100% of the centrosome-containing cells had either entered DNA synthesis or divided. By contrast, during this period, none of the acentrosomal cells had entered S phase. These data reveal that the primary role of the centrosome in somatic cells is not to form the spindle but instead to ensure cytokinesis and subsequent cell cycle progression.

Cyclin A is destroyed in prometaphase and can delay chromosome alignment and anaphase

Mitosis is controlled by the specific and timely degradation of key regulatory proteins, notably the mitotic cyclins that bind and activate the cyclin-dependent kinases (Cdks). In animal cells, cyclin A is always degraded before cyclin B, but the exact timing and the mechanism underlying this are not known. Here we use live cell imaging to show that cyclin A begins to be degraded just after nuclear envelope breakdown. This degradation requires the 26S proteasome, but is not affected by the spindle checkpoint. Neither deletion of its destruction box nor disrupting Cdk binding prevents cyclin A proteolysis, but Cdk binding is necessary for degradation at the correct time. We also show that increasing the levels of cyclin A delays chromosome alignment and sister chromatid segregation. This delay depends on the proteolysis of cyclin A and is not caused by a lag in the bipolar attachment of chromosomes to the mitotic spindle, nor is it mediated via the spindle checkpoint. Thus, proteolysis that is not under the control of the spindle checkpoint is required for chromosome alignment and anaphase.

Late mitotic failure in mice lacking Sak, a polo-like kinase

Polo-like kinases in yeast, flies, and mammals regulate key events in mitosis. Such events include spindle formation at G2/M, the anaphase-promoting complex (APC) at the exit from mitosis, the cleavage structure at cytokinesis, and DNA damage checkpoints in G2/M. Polo-like kinases are distinguished by two C-terminal polo box (pb) motifs, which localize the enzymes to mitotic structures. We previously identified Sak, a novel polo-like kinase found in Drosophila and mammals. Here, we demonstrate that the Sak kinase has a functional pb domain that localizes the enzyme to the nucleolus during G2, to the centrosomes in G2/M, and to the cleavage furrow during cytokinesis. To study the role of Sak in embryo development, we generated a Sak null allele, the first polo-like kinase to be mutated in mice. Sak(-/-) embryos arrested after gastrulation at E7.5, with a marked increase in mitotic and apoptotic cells. Sak(-/-) embryos displayed cells in late anaphase or telophase that continued to express cyclin B1 and phosphorylated histone H3. Our results suggest that Sak is required for the APC-dependent destruction of cyclin B1 and for exit from mitosis in the postgastrulation embryo.

The localization of human cyclins B1 and B2 determines CDK1 substrate specificity and neither enzyme requires MEK to disassemble the Golgi apparatus

In this paper, we show that substrate specificity is primarily conferred on human mitotic cyclin-dependent kinases (CDKs) by their subcellular localization. The difference in localization of the B-type cyclin-CDKs underlies the ability of cyclin B1-CDK1 to cause chromosome condensation, reorganization of the microtubules, and disassembly of the nuclear lamina and of the Golgi apparatus, while it restricts cyclin B2-CDK1 to disassembly of the Golgi apparatus. We identify the region of cyclin B2 responsible for its localization and show that this will direct cyclin B1 to the Golgi apparatus and confer upon it the more limited properties of cyclin B2. Equally, directing cyclin B2 to the cytoplasm with the NH(2) terminus of cyclin B1 confers the broader properties of cyclin B1. Furthermore, we show that the disassembly of the Golgi apparatus initiated by either mitotic cyclin-CDK complex does not require mitogen-activated protein kinase kinase (MEK) activity.

Inactivation of M-phase promoting factor at exit from first embryonic mitosis in the rat is independent of cyclin B1 degradation

Exit from M-phase and completion of cell division requires inactivation of M-phase promoting factor (MPF), a heterodimer composed of the regulatory cyclin B1 and the catalytic p34cdc2 kinase. Inactivation of MPF is associated with cyclin B1 degradation that is brought about by the ubiquitin-proteasome pathway. Our study examined the role of the proteasome in the first mitosis of rat embryos and its participation in the regulation of cyclin B1 degradation and MPF inactivation. We show that in the early zygote the proteasome is evenly distributed in the ooplasm and the nucleus, whereas during mitosis it accumulates on the spindle apparatus. We further demonstrate that inhibition of proteasomal catalytic activity prevents 1-cell embryos from undergoing mitosis. This mitotic arrest is associated with the presence of relatively high amounts of cyclin B1, which unexpectedly does not result in elevated MPF activity. Our findings strongly imply that completion of the first embryonic division depends on proteasomal degradation and that cyclin B1 is included among its target proteins. They also provide the first evidence that MPF inactivation at this stage of development is not solely dependent upon cyclin B1 degradation and is insufficient to allow the formation of the 2-cell embryo.

Securin degradation is mediated by fzy and fzr, and is required for complete chromatid separation but not for cytokinesis

We have studied the ubiquitination and degradation patterns of the human securin/PTTG protein. We show that, in contrast to budding yeast pds1, securin degradation is catalyzed by both fzy (fizzy/cdc20) and fzr (fizzy-related/cdh1/hct1). Both fzy and fzr also induce the APC/C to ubiquitinate securin in vitro. Securin degradation is mediated by an RXXL destruction box and a KEN box, and is inhibited only when both sequences are mutated. Interestingly, the non-degradable securin mutant is also partially ubiquitinated by fzy and fzr in vitro. Expressing the non-degradable securin mutant in cells frequently resulted in incomplete chromatid separation and gave rise to daughter cells connected by a thin chromatin fiber, presumably of chromosomes that failed to split completely. Strikingly, the mutant securin did not prevent the majority of sister chromatids from separating completely, nor did it prevent mitotic cyclin degradation and cytokinesis. This phenotype, reminiscent of the fission yeast cut (cells untimely torn) phenotype, is reported here for the first time in mammals.

Chromosome movement in mitosis requires microtubule anchorage at spindle poles

Anchorage of microtubule minus ends at spindle poles has been proposed to bear the load of poleward forces exerted by kinetochore-associated motors so that chromosomes move toward the poles rather than the poles toward the chromosomes. To test this hypothesis, we monitored chromosome movement during mitosis after perturbation of nuclear mitotic apparatus protein (NuMA) and the human homologue of the KIN C motor family (HSET), two noncentrosomal proteins involved in spindle pole organization in animal cells. Perturbation of NuMA alone disrupts spindle pole organization and delays anaphase onset, but does not alter the velocity of oscillatory chromosome movement in prometaphase. Perturbation of HSET alone increases the duration of prometaphase, but does not alter the velocity of chromosome movement in prometaphase or anaphase. In contrast, simultaneous perturbation of both HSET and NuMA severely suppresses directed chromosome movement in prometaphase. Chromosomes coalesce near the center of these cells on bi-oriented spindles that lack organized poles. Immunofluorescence and electron microscopy verify microtubule attachment to sister kinetochores, but this attachment fails to generate proper tension across sister kinetochores. These results demonstrate that anchorage of microtubule minus ends at spindle poles mediated by overlapping mechanisms involving both NuMA and HSET is essential for chromosome movement during mitosis.

Mitotic checkpoints: from yeast to cancer

Separation of chromosomes during mitosis is monitored by a checkpoint that leads to cell-cycle arrest if the chromosomes are not properly attached to the mitotic spindle. Molecular mechanisms controlling this checkpoint have been identified. In addition, loss of this checkpoint has been shown to result in chromosome missegregation in higher eukaryotes and may contribute to the genomic instability observed in human cancers.

The APC is dispensable for first meiotic anaphase in Xenopus oocytes

Here we show that segregation of homologous chromosomes and that of sister chromatids are differentially regulated in Xenopus and possibly in other higher eukaryotes. Upon hormonal stimulation, Xenopus oocytes microinjected with antibodies against the anaphase-promoting complex (APC) activator Fizzy or the APC core subunit Cdc27, or with the checkpoint protein Mad2, a destruction-box peptide or methylated ubiquitin, readily progress through the first meiotic cell cycle and arrest at second meiotic metaphase. However, they fail to segregate sister chromatids and remain arrested at second meiotic metaphase when electrically stimulated or when treated with ionophore A34187, two treatments that mimic fertilization and readily induce chromatid segregation in control oocytes. Thus, APC is required for second meiotic anaphase but not for first meiotic anaphase.

The relevance of drug sequence in combination chemotherapy

The concept of combining chemotherapeutic agents to increase the cytotoxic efficacy has evolved greatly over the past several years. In the past, the rationale for combination chemotherapy centered on attacking different biochemical targets, overcoming drug resistance in heterogenous tumors, and increasing the dose-density of combination chemotherapy to take advantage of tumor growth kinetics. The overall goal was to improve clinical efficacy with acceptable clinical toxicity. It is now apparent that the sequence of drug administration can significantly enhance the therapeutic effect of chemotherapy. These sequence-dependent effects can be explained by chemotherapy-induced cell cycle perturbations, or by pharmacodynamic interactions between the agents in combination. In this review, we focus on drug combinations with taxanes and camptothecins, which we believe best illustrate the importance of the cell cycle and pharmacologic interactions in the sequential administration of chemotherapy. As our understanding of the cell cycle grows, our ability to appropriately sequence chemotherapy can have a great impact on the treatment of human cancers. Copyright 2000 Harcourt Publishers Ltd.

Rough deal and Zw10 are required for the metaphase checkpoint in Drosophila

The metaphase-anaphase transition during mitosis is carefully regulated in order to assure high-fidelity transmission of genetic information to the daughter cells. A surveillance mechanism known as the metaphase checkpoint (or spindle-assembly checkpoint) monitors the attachment of kinetochores to the spindle microtubules, and inhibits anaphase onset until all chromosomes have achieved a proper bipolar orientation on the spindle. Defects in this checkpoint lead to premature anaphase onset, and consequently to greatly increased rates of aneuploidy. Here we show that the Drosophila kinetochore components Rough deal (Rod) and Zeste-White 10 (Zw10) are required for the proper functioning of the metaphase checkpoint in flies. Drosophila cells lacking either ROD or Zw10 exhibit a phenotype that is similar to that of bub1 mutants - they do not arrest in metaphase in response to spindle damage, but instead separate sister chromatids, degrade cyclin B and exit mitosis. These are the first checkpoint components to be identified that do not have obvious homologues in budding yeast.

Cell cycle-dependent proteolysis and ectopic overexpression of cyclin B1 in tobacco BY2 cells

Activation of cyclin B/Cdc2 kinase complex triggers entry into mitosis in all eukaryotic cells. Although cyclin gene expression has been extensively studied in plants, not much is known at the level of the protein stability and function. Here, we demonstrated by using the highly synchronizable tobacco BY2 cell culture, that endogenous cyclin B1 protein undergoes cell cycle-dependent proteolysis and is stabilized when the spindle checkpoint has been activated. Furthermore, we established transgenic tobacco BY2 cell cultures expressing under the control of an inducible promoter, cyclin B1 protein as well as its non-degradable form as fusion proteins with GFP and found that the ectopic expression of these proteins did not dramatically disturb the cell cycle progression. These results indicate that, to a certain extent, cell cycle exit is possible without cyclin B1 proteolysis.

Control of mitosis by changes in the subcellular location of cyclin-B1-Cdk1 and Cdc25C

Nuclear events of mitosis are initiated when the protein kinase cyclin-B1-Cdk1 is translocated into the nucleus during prophase. Recent work has unveiled many of the mechanisms that govern the localization of cyclin-B1-Cdk1 and its regulator Cdc25C. Phosphorylation-dependent changes in the rate of nuclear import and export of these proteins help to control the onset of mitosis both in normal cells and in cells delayed before mitosis by DNA damage.

Nuclear-specific degradation of Far1 is controlled by the localization of the F-box protein Cdc4

Far1 is a bifunctional protein that is required to arrest the cell cycle and establish cell polarity during yeast mating. Here we show that SCF(Cdc4) ubiquitylates Far1 in the nucleus, which in turn targets the multi-ubiquitylated protein to 26S proteasomes most likely located at the nuclear envelope. In response to mating pheromones, a fraction of Far1 was stabilized after its export into the cytoplasm by Ste21/Msn5. Preventing nuclear export destabilized Far1, while conversely cytoplasmic Far1 was stabilized, although the protein was efficiently phosphorylated in a Cdc28-Cln-dependent manner. The core SCF subunits Cdc53, Hrt1 and Skp1 were distributed in the nucleus and the cytoplasm, whereas the F-box protein Cdc4 was exclusively nuclear. A cytoplasmic form of Cdc4 was unable to complement the growth defect of cdc4-1 cells, but it was sufficient to degrade Far1 in the cytoplasm. Our results illustrate the importance of subcellular localization of F-box proteins, and provide an example of how an extracellular signal regulates protein stability at the level of substrate localization.

Visualization of Mad2 dynamics at kinetochores, along spindle fibers, and at spindle poles in living cells

The spindle checkpoint prevents errors in chromosome segregation by inhibiting anaphase onset until all chromosomes have aligned at the spindle equator through attachment of their sister kinetochores to microtubules from opposite spindle poles. A key checkpoint component is the mitotic arrest-deficient protein 2 (Mad2), which localizes to unattached kinetochores and inhibits activation of the anaphase-promoting complex (APC) through an interaction with Cdc20. Recent studies have suggested a catalytic model for kinetochore function where unattached kinetochores provide sites for assembling and releasing Mad2-Cdc20 complexes, which sequester Cdc20 and prevent it from activating the APC. To test this model, we examined Mad2 dynamics in living PtK1 cells that were either injected with fluorescently labeled Alexa 488-XMad2 or transfected with GFP-hMAD2. Real-time, digital imaging revealed fluorescent Mad2 localized to unattached kinetochores, spindle poles, and spindle fibers depending on the stage of mitosis. FRAP measurements showed that Mad2 is a transient component of unattached kinetochores, as predicted by the catalytic model, with a t(1/2) of approximately 24-28 s. Cells entered anaphase approximately 10 min after Mad2 was no longer detectable on the kinetochores of the last chromosome to congress to the metaphase plate. Several observations indicate that Mad2 binding sites are translocated from kinetochores to spindle poles along microtubules. First, Mad2 that bound to sites on a kinetochore was dynamically stretched in both directions upon microtubule interactions, and Mad2 particles moved from kinetochores toward the poles. Second, spindle fiber and pole fluorescence disappeared upon Mad2 disappearance at the kinetochores. Third, ATP depletion resulted in microtubule-dependent depletion of Mad2 fluorescence at kinetochores and increased fluorescence at spindle poles. Finally, in normal cells, the half-life of Mad2 turnover at poles, 23 s, was similar to kinetochores. Thus, kinetochore-derived sites along spindle fibers and at spindle poles may also catalyze Mad2 inhibitory complex formation.

Cell cycle- and cell growth-regulated proteolysis of mammalian CDC6 is dependent on APC-CDH1

CDC6 is conserved during evolution and is essential and limiting for the initiation of eukaryotic DNA replication. Human CDC6 activity is regulated by periodic transcription and CDK-regulated subcellular localization. Here, we show that, in addition to being absent from nonproliferating cells, CDC6 is targeted for ubiquitin-mediated proteolysis by the anaphase promoting complex (APC)/cyclosome in G(1). A combination of point mutations in the destruction box and KEN-box motifs in CDC6 stabilizes the protein in G(1) and in quiescent cells. Furthermore, APC, in association with CDH1, ubiquitinates CDC6 in vitro, and both APC and CDH1 are required and limiting for CDC6 proteolysis in vivo. Although a stable mutant of CDC6 is biologically active, overexpression of this mutant or wild-type CDC6 is not sufficient to induce multiple rounds of DNA replication in the same cell cycle. The APC-CDH1-dependent proteolysis of CDC6 in early G(1) and in quiescent cells suggests that this process is part of a mechanism that ensures the timely licensing of replication origins during G(1).

Microtubule disassembly delays the G2-M transition in vertebrates

When cell cultures in growth are treated with drugs that cause microtubules to disassemble, the mitotic index (MI) progressively increases as the cells accumulate in a C-mitosis. For many cell types, however, including rat kangaroo kidney PtK(1) cells, the MI does not increase during the first several hours of treatment [1-3] (Figure 1). This 'lag' implies either that cells are entering mitosis but rapidly escaping the block, or that they are delayed from entering division. To differentiate between these possibilities, we fixed PtK(1) cultures 0, 90 and 270 minutes after treatment with nocodazole, colcemid, lumi-colcemid, taxol or cytochalasin D. After 90 minutes, we found that the numbers of prophase cells in cultures treated with nocodazole or colcemid were reduced by approximately 80% relative to cultures treated with lumi-colcemid, cytochalasin D or taxol. Thus, destroying microtubules delays late G(2 )cells from entering prophase and, as the MI does not increase during this time, existing prophase cells do not enter prometaphase. When mid-prophase cells were treated with nocodazole, the majority (70%) decondensed their chromosomes and returned to G(2) before re-entering and completing prophase 3-10 hours later. Thus, a pathway exists in vertebrates that delays the G(2)-M transition when microtubules are disassembled during the terminal stages of G(2). As this pathway induces mid-prophase cells to transiently decondense their chromosomes, it is likely that it downregulates the cyclin A-cyclin-dependent kinase 2 (CDK2) complex, which is required in vertebrates for the early stages of prophase [4].

On the concentrations of cyclins and cyclin-dependent kinases in extracts of cultured human cells

Cyclins and cyclin-dependent kinases (CDKs) are key regulators of the human cell cycle. Here we have directly measured the concentrations of the G(1) and G(2) cyclins and their CDK partners in highly synchronized human cervical carcinoma cells (HeLa). To determine the exact concentrations of cyclins and CDKs in the cell extracts, we developed a relatively simple method that combined the use of (35)S-labeled standards produced in rabbit reticulocyte lysates and immunoblotting with specific antibodies. Using this approach, we formally demonstrated that CDC2 and CDK2 are in excess of their cyclin partners. We found that the concentrations of cyclin A2 and cyclin B1 (at their peak levels in the G(2) phase) were about 30-fold less than that of their partner CDC2. The peak levels of cyclin A2 and cyclin E1, at the G(2) phase and G(1) phase, respectively, were only about 8-fold less than that of their partner CDK2. These ratios are in good agreement with size fractionation analysis of the relative amount of monomeric and complexed forms of CDC2 and CDK2 in the cell. All the cyclin A2 and cyclin E1 are in complexes with CDC2 and CDK2, but there are some indications that a significant portion of cyclin B1 may not be in complex with CDC2. Furthermore, we also demonstrated that the concentration of the CDK inhibitor p21(CIP1/WAF1) induced after DNA damage is sufficient to overcome the cyclin-CDK2 complexes in MCF-7 cells. These direct quantitations formally confirmed the long-held presumption that CDKs are in excess of the cyclins in the cell. Moreover, similar approaches can be used to measure the concentration of any protein in cell-free extracts.

The polo-like kinase Plx1 prevents premature inactivation of the APC(Fizzy)-dependent pathway in the early Xenopus cell cycle

Members of the polo-like family of protein kinases have been involved in the control of APC (anaphase-promoting complex) during the cell cycle, yet how they activate APC is not understood in any detail. In Xenopus oocytes, Ca2+-dependent degradation of cyclin B associated with release from arrest at second meiotic metaphase was demonstrated to require the polo-like kinase Plx1. The aim of the present study was to examine, beyond Ca2+-dependent resumption of meiosis, the possible role of Plx1 in the control of cyclin degradation during the early mitotic cell cycle. Plx1 was found to be dispensable for MPF to turn on the cyclin degradation machinery. However, it is required to prevent premature inactivation of the APC-dependent proteolytic pathway. Microcystin suppresses the requirement for Plx1 in both Ca2+-dependent exit from meiosis, associated with degradation of both cyclin B and A downstream of CaMK2 activation, and prevention of premature APC(Fizzy) inactivation in the early mitotic cell cycle. These results are consistent with the view that Plx1 antagonizes an unidentified microcystin-sensitive phosphatase that inactivates APC(Fizzy).

Cyclin-dependent kinase inhibitors: novel anticancer agents

In current models of cell cycle control, the transition between different cell cycle states is regulated at checkpoints. Transition through the cell-cycle is induced by a family of protein kinase holoenzymes, the cyclin-dependent kinases (CDKs) and their heterodimeric cyclin partner. Orderly progression through the cell-cycle involves co-ordinated activation of the CDKs, which in the presence of an associated CDK-activating kinase, phosphorylate target substrates including members of the 'pocket protein' family. This family includes the product of the retinoblastoma susceptibility gene (the pRb protein) and the related p107 and p130 proteins. Activity of these holoenzymes is regulated by post-translational modification. Phosphorylation of inhibitory sites on a conserved threonine residue within the activation segment is regulated by CDK7/cyclin H, referred to as CDK-activating kinase [1]. In addition, the cdc25 phosphatases activate the CDKs by dephosphorylating their inhibitory tyrosine and threonine phosphorylated residues [2,3]. Among the many roles for endogenous inhibitors (CDKIs), including members of the p21(CIP1/Waf1) family and the p16 family, one role is to regulate cyclin activity. Cellular neoplastic transformation is accompanied by loss of regulation of cell cycle checkpoints in conjunction with aberrant expression of CDKs and/or cyclins and the loss or mutation of the negative regulators (the CDKIs or the pocket protein pRb). One strategy to inhibit malignant cellular proliferation involves inhibiting CDK activity or enhancing function of the CDKI. Novel inhibitors of CDKs showing promise in the clinic include flavopiridol and UCN-01, which show early evidence of human tolerability in clinical trials. This review examines pertinent advances in the field of CDK inhibitors.

Identification of CIP-1-associated regulator of cyclin B (CARB), a novel p21-binding protein acting in the G2 phase of the cell cycle

The cyclin-dependent kinase inhibitor p21(cip1) regulates cell cycle progression, DNA replication, and DNA repair by binding to specific cellular proteins through distinct amino- and carboxyl-terminal protein binding motifs. We have identified a novel human gene, CARB (CIP-1-associated regulator of cyclin B), whose product interacts with the p21 carboxyl terminus. Immunocytochemical analysis demonstrates that the CARB protein is perinuclear and predominantly associated with the centrosome and mitotic spindle poles. In addition, CARB is also able to associate with cyclin B1, a key regulator of mitosis. However, cyclin B1-CARB complex formation occurs preferentially in the absence of p21. Unexpectedly, overexpression of CARB is associated with a growth-inhibitory and ultimately lethal phenotype in p21(-/-) cells but not in p21(+/+) cells. These data identify a novel mechanism that may underlie the effects of p21 in the G(2)/M phases of the cell cycle.

Chromosome missegregation and apoptosis in mice lacking the mitotic checkpoint protein Mad2

The initiation of chromosome segregation at anaphase is linked by the spindle assembly checkpoint to the completion of chromosome-microtubule attachment during metaphase. To determine the function of the mitotic checkpoint protein Mad2 during normal cell division and when mitosis goes awry, we have knocked out Mad2 in mice. We find that E5.5 embryonic cells lacking Mad2, like mad2 yeast, grow normally but are unable to arrest in response to spindle disruption. At E6.5, the cells of the epiblast begin rapid cell division and the absence of a checkpoint results in widespread chromosome missegregation and apoptosis. In contrast, the postmitotic trophoblast giant cells survive without Mad2. Thus, the spindle assembly checkpoint is required for accurate chromosome segregation in mitotic mouse cells, and for embryonic viability, even in the absence of spindle damage.

Cyclin F regulates the nuclear localization of cyclin B1 through a cyclin-cyclin interaction

The key regulator of G(2)-M transition of the cell cycle is M-phase promoting factor (MPF), a complex composed of cdc2 and a B-type cyclin. Cyclin B1 nuclear localization involves phosphorylation within a region called the cytoplasmic retention signal, which also contains a nuclear export signal. The mechanism of MPF nuclear localization remains unclear since it contains no functional nuclear localization signal (NLS). We exploited the yeast two-hybrid screen to find protein(s) potentially mediating localization of cyclin B1 and identified a novel interaction between cyclin B1 and cyclin F. We found that cdc2, cyclin B1 and cyclin F form a complex that exhibits histone H1 kinase activity. Cyclin B1 and cyclin F also colocalize through immunofluorescence studies. Additionally, deletion analysis revealed that each putative NLS of cyclin F is functional. Taken together, the data suggest that the NLS regions of cyclin F regulate cyclin B1 localization to the nucleus. The interaction between cyclin B1 and cyclin F represents the first example of direct cyclin-cyclin binding, and elucidates a novel mechanism that regulates MPF localization and function.

Cdk1 is essential for mammalian cyclosome/APC regulation

The cyclosome/APC (anaphase-promoting complex), the major component of cell-cycle-specific ubiquitin-mediated proteolysis of mitotic cyclins and of other cell cycle proteins, is essential for sister chromatid separation and for exit from mitosis. Cyclosome activity and substrate specificity are modulated by phosphorylation and by transient interactions with Fizzy/cdc20 (Fzy) and Fizzy-related/Hct1/Cdh1 (Fzr). This regulation has been studied so far in Drosophila embryos, in yeast, and in cell-free extracts in vitro. Studying cyclosome regulation in mammalian cells in vivo we found that both Fzr overexpression and Cdk1 inhibition can override the prometaphase checkpoint. We further show that Fzr activation of the cyclosome is negatively regulated by Cdk1. Finally, we show that the mammalian cdc14 phosphatase, like its budding yeast homologue, plays a role in cyclosome pathway regulation. These results suggest that Cdk1 is essential for coupling various activities of the cyclosome and in particular for preventing Fzr from short-circuiting the spindle pole checkpoint. Cdk1-cyclin B is thus an inhibitor, activator, and substrate of the cyclosome.

Two different modes of cyclin clb2 proteolysis during mitosis in Saccharomyces cerevisiae

Sister chromatid separation and mitotic exit are triggered by the anaphase-promoting complex (APC/C) which is a multi-subunit ubiquitin ligase required for proteolytic degradation of various target proteins. Cdc20 and Cdh1 are substrate-specific activators of the APC/C. It was previously proposed that Cdh1 is essential for proteolysis of the yeast mitotic cyclin Clb2. We show that Clb2 proteolysis is triggered by two different modes during mitosis. A fraction of Clb2 is degraded during anaphase in the absence of Cdh1. However, a second fraction of Clb2 remains stable during anaphase and is degraded in a Cdh1-dependent manner as cells exit from mitosis. Most of cyclin Clb3 is degraded independently of Cdh1. Our data imply that degradation of mitotic cyclins is initiated by a Cdh1-independent mechanism.

Human cyclin A is required for mitosis until mid prophase

We have used microinjection and time-lapse video microscopy to study the role of cyclin A in mitosis. We have injected purified, active cyclin A/cyclin-dependent kinase 2 (CDK2) into synchronized cells at specific points in the cell cycle and assayed its effect on cell division. We find that cyclin A/CDK2 will drive G2 phase cells into mitosis within 30 min of microinjection, up to 4 h before control cells enter mitosis. Often this premature mitosis is abnormal; the chromosomes do not completely condense and daughter cells fuse. Remarkably, microinjecting cyclin A/CDK2 into S phase cells has no effect on progress through the following G2 phase or mitosis. In complementary experiments we have microinjected the amino terminus of p21(Cip1/Waf1/Sdi1) (p21N) into cells to inhibit cyclin A/CDK2 activity. We find that p21N will prevent S phase or G2 phase cells from entering mitosis, and will cause early prophase cells to return to interphase. These results suggest that cyclin A/CDK2 is a rate-limiting component required for entry into mitosis, and for progress through mitosis until late prophase. They also suggest that cyclin A/CDK2 may be the target of the recently described prophase checkpoint.

Regulation of APC activity by phosphorylation and regulatory factors

Ubiquitin-dependent proteolysis of Cut2/Pds1 and Cyclin B is required for sister chromatid separation and exit from mitosis, respectively. Anaphase-promoting complex/cyclosome (APC) specifically ubiquitinates Cut2/Pds1 at metaphase-anaphase transition, and ubiquitinates Cyclin B in late mitosis and G1 phase. However, the exact regulatory mechanism of substrate-specific activation of mammalian APC with the right timing remains to be elucidated. We found that not only the binding of the activators Cdc20 and Cdh1 and the inhibitor Mad2 to APC, but also the phosphorylation of Cdc20 and Cdh1 by Cdc2-Cyclin B and that of APC by Polo-like kinase and cAMP-dependent protein kinase, regulate APC activity. The cooperation of the phosphorylation/dephosphorylation and the regulatory factors in regulation of APC activity may thus control the precise progression of mitosis.

Four-dimensional control of the cell cycle

The cell-division cycle has to be regulated in both time and space. In the time dimension, the cell ensures that mitosis does not begin until DNA replication is completed and any damaged DNA is repaired, and that DNA replication normally follows mitosis. This is achieved by the synthesis and destruction of specific cell-cycle regulators at the right time in the cell cycle. In the spatial dimension, the cell coordinates dramatic reorganizations of the subcellular architecture at the entrance to and exit from mitosis, largely through the actions of protein kinases and phosphatases that are often localized to specific subcellular structures. Evidence is now accumulating to suggest that the spatial organization of cell-cycle regulators is also important in the temporal control of the cell cycle. Here I will focus on how the locations of the main components of the cell-cycle machinery are regulated as part of the mechanism by which the cell controls when and how it replicates and divides.

Regulation of the APC and the exit from mitosis

The events of late mitosis, from sister-chromatid separation to cytokinesis, are governed by the anaphase-promoting complex (APC), a multisubunit assembly that triggers the ubiquitin-dependent proteloysis of key regulatory proteins. An intricate regulatory network governs APC activity and helps to ensure that late mitotic events are properly timed and coordinated.