Feedback control of mitosis in budding yeast

Release of mouse eggs from metaphase arrest by protein synthesis inhibition in the absence of a calcium signal or microtubule assembly

Mouse egg activation, which includes release from meiotic metaphase II arrest, results from fertilization-induced increase in intracellular calcium concentration ([Ca2+]i). However, during egg activation caused by exposure to the protein synthesis inhibitor, cycloheximide, [Ca2+]i did not change. Although eggs fertilized in the presence of microtubule inhibitors remain arrested at metaphase, eggs treated for 32 hr with cycloheximide and the microtubule inhibitor, colcemid, formed nuclei. In untreated eggs aged in culture for 24 hr, the microtubule spindles became deformed. These eggs formed nuclei after exposure to cycloheximide, but not the calcium ionophore A23187. Our results indicate that eggs in which protein synthesis is inhibited are released from metaphase without an increase in [Ca2+]i, and despite disruption of the spindle.

The Drosophila cell cycle gene fizzy is required for normal degradation of cyclins A and B during mitosis and has homology to the CDC20 gene of Saccharomyces cerevisiae

The Drosophila cell cycle gene fizzy (fzy) is required for normal execution of the metaphase-anaphase transition. We have cloned fzy, and confirmed this by P-element mediated germline transformation rescue. Sequence analysis predicts that fzy encodes a protein of 526 amino acids, the carboxy half of which has significant homology to the Saccharomyces cerevisiae cell cycle gene CDC20. A monoclonal antibody against fzy detects a single protein of the expected size, 59 kD, in embryonic extracts. In early embryos fzy is expressed in all proliferating tissues; in late embryos fzy expression declines in a tissue-specific manner correlated with cessation of cell division. During interphase fzy protein is present in the cytoplasm; while in mitosis fzy becomes ubiquitously distributed throughout the cell except for the area occupied by the chromosomes. The metaphase arrest phenotype caused by fzy mutations is associated with failure to degrade both mitotic cyclins A and B, and an enrichment of spindle microtubules at the expense of astral microtubules. Our data suggest that fzy function is required for normal cell cycle-regulated proteolysis that is necessary for successful progress through mitosis.

The role of Saccharomyces cerevisiae Cdc40p in DNA replication and mitotic spindle formation and/or maintenance

Successful progression through the cell cycle requires the coupling of mitotic spindle formation to DNA replication. In this report we present evidence suggesting that, in Saccharomyces cerevisiae, the CDC40 gene product is required to regulate both DNA replication and mitotic spindle formation. The deduced amino acid sequence of CDC40 (455 amino acids) contains four copies of a beta-transducin-like repeat. Cdc40p is essential only at elevated temperatures, as a complete deletion or a truncated protein (deletion of the C-terminal 217 amino acids in the cdc40-1 allele) results in normal vegetative growth at 23 degrees C, and cell cycle arrest at 36 degrees C. In the mitotic cell cycle Cdc40p is apparently required for at least two steps: (1) for entry into S phase (neither DNA synthesis, nor mitotic spindle formation occurs at 36 degrees C and (2) for completion of S-phase (cdc40::LEU2 cells cannot complete the cell cycle when returned to the permissive temperature in the presence of hydroxyurea). The role of Cdc40p as a regulatory protein linking DNA synthesis, spindle assembly/maintenance, and maturation promoting factor (MPF) activity is discussed.

A p53-dependent mouse spindle checkpoint

Cell cycle checkpoints enhance genetic fidelity by causing arrest at specific stages of the cell cycle when previous events have not been completed. The tumor suppressor p53 has been implicated in a G1 checkpoint. To investigate whether p53 also participates in a mitotic checkpoint, cultured fibroblasts from p53-deficient mouse embryos were exposed to spindle inhibitors. The fibroblasts underwent multiple rounds of DNA synthesis without completing chromosome segregation, thus forming tetraploid and octaploid cells. Deficiency of p53 was also associated with the development of tetraploidy in vivo. These results suggest that murine p53 is a component of a spindle checkpoint that ensures the maintenance of diploidy.

Oocyte activation and passage through the metaphase/anaphase transition of the meiotic cell cycle is blocked in clams by inhibitors of HMG-CoA reductase activity

Cell cycle progression for postembryonic cells requires the activity of 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMG-R), the enzyme which catalyzes the production of the isoprenoid precursor, mevalonate. In this study, we examine the requirements of HMG-R activity for cell cycle progression during the meiotic and early mitotic divisions using oocytes and dividing embryos from the surf clam, Spisula solidissima. Using two different inhibitors of HMG-R, we find that the activity of this enzyme appears to be required at three distinct points of the cell cycle during meiosis. Depending on the stage at which these inhibitors are added to synchronous clam cultures, a reversible cell cycle block is triggered at the time of activation or at metaphase of either meiosis I or II, whereas there is not block to the mitotic cell cycle. Inhibition of HMG-R activity in activated oocytes does not affect the transient activation of p42MAPK but results in a block at metaphase of meiosis I that is accompanied by the stabilization of cyclins A and B and p34cdc2 kinase activity. Our results suggest that metabolites from the mevalonate biosynthetic pathway can act to influence the process of activation, as well as the events later in the cell cycle that lead to cyclin proteolysis and the exit from M phase during clam meiosis.

Suppression of a conditional mutation in alpha-tubulin by overexpression of two checkpoint genes

To identify proteins that regulate microtubule assembly in Saccharomyces cerevisiae, we screened for multicopy suppressors of a conditional mutation in alpha-tubulin. Cells expressing the recessive allele tub1-729 as their sole alpha-tubulin gene grow normally at permissive temperature. However, at 15 degrees C the cells lose viability and arrest primarily with large buds and quantitatively diminished microtubule structures. Transformation of mutant cells with genomic libraries repeatedly identified three different suppressors: the two wild-type alpha-tubulin genes, TUB1 and TUB3; and BUB3. BUB3 is a checkpoint gene that permits entry into mitosis depending upon the assembly state of microtubules. Excess BUB3 rescues both the loss of viability and microtubule defects but not the benomyl supersensitivity associated with tub1-729. The suppression is specific for the mutation ALA422VAL in TUB1, and does not affect several other mutations in TUB1 that produce the ‘no microtubule’ phenotype. Overexpression of BUB1, which interacts genetically with BUB3 and which is involved in the same checkpoint pathway, also rescues the cold sensitivity of tub1-729, but another checkpoint gene, MAD2, does not. Overexpression of BUB3 in wild-type cells has no detectable growth or microtubule defect, but disruption of the BUB3 gene produces slow growth and benomyl supersensitivity. Our results suggest that BUB1 and BUB3 overexpression modulate an event required for mitotic spindle function which is rate limiting for tub1-729 cells at the restrictive temperature.

Metaphase arrest in newly matured or microtubule-depleted mouse eggs after calcium stimulation

In mouse eggs arrested at meiotic metaphase II, the increase in intracellular calcium that results from fertilisation induces nuclear formation in both newly ovulated and older eggs. In contrast, the calcium increase that results from exposure to the calcium ionophore A23187 induces nuclear formation in older, but not young, newly ovulated eggs. When treated with the microtubule inhibitor colcemid, and fertilised, young eggs remained at metaphase, but many older eggs formed nuclei, although older eggs treated with colcemid and A23187 remained at metaphase. However, young A23187-treated eggs, young colcemid-treated fertilised eggs, and older colcemid- and A23187-treated eggs, formed nuclei when treated, in addition, with the protein synthesis inhibitor cycloheximide, or the protein kinase inhibitor 6-dimethylaminopurine (6-DMAP). The possibility is discussed that metaphase in newly matured eggs and microtubule-depleted eggs may be maintained by similar mechanisms involving short-lived phosphorylated proteins.

The Schizosaccharomyces pombe hus5 gene encodes a ubiquitin conjugating enzyme required for normal mitosis

Normal eukaryotic cells do not enter mitosis unless DNA is fully replicated and repaired. Controls called ‘checkpoints’, mediate cell cycle arrest in response to unreplicated or damaged DNA. Two independent Schizosaccharomyces pombe mutant screens, both of which aimed to isolate new elements involved in checkpoint controls, have identified alleles of the hus5+ gene that are abnormally sensitive to both inhibitors of DNA synthesis and to ionizing radiation. We have cloned and sequenced the hus5+ gene. It is a novel member of the E2 family of ubiquitin conjugating enzymes (UBCs). To understand the role of hus5+ in cell cycle control we have characterized the phenotypes of the hus5 mutants and the hus5 gene disruption. We find that, whilst the mutants are sensitive to inhibitors of DNA synthesis and to irradiation, this is not due to an inability to undergo mitotic arrest. Thus, the hus5+ gene product is not directly involved in checkpoint control. However, in common with a large class of previously characterized checkpoint genes, it is required for efficient recovery from DNA damage or S-phase arrest and manifests a rapid death phenotype in combination with a temperature sensitive S phase and late S/G2 phase cdc mutants. In addition, hus5 deletion mutants are severely impaired in growth and exhibit high levels of abortive mitoses, suggesting a role for hus5+ in chromosome segregation. We conclude that this novel UBC enzyme plays multiple roles and is virtually essential for cell proliferation.

The genetics of cell cycle checkpoints

Checkpoints help in the prevention of genetic damage by giving cells time to repair damaged structures before proceeding in the cell cycle. Genetic analyses in budding and fission yeast have identified a large number of cell cycle checkpoint genes. Several of these encode proteins related to components of other signal transduction pathways, including protein kinases, lipid kinases, and 14-3-3 proteins. In fission yeast, checkpoints play an important role in keeping cells from entering mitosis before they pass Start.

beta-Tubulin mutation suppresses microtubule dynamics in vitro and slows mitosis in vivo

Microtubule (MT) dynamics vary both spatially and temporally within cells and are thought to be important for proper MT cellular function. Because MT dynamics appear to be closely tied to the guanosine triphosphatase (GTPase) activity of beta-tubulin subunits, we examined the importance of MT dynamics in the budding yeast S. cerevisiae by introducing a T107K point mutation into a region of the single beta-tubulin gene, TUB2, known to affect the assembly-dependent GTPase activity of MTs in vitro. Analysis of MT dynamic behavior by video-enhanced differential interference contrast microscopy, revealed that T107K subunits slowed both the growth rates and catastrophic disassembly rates of individual MTs in vitro. In haploid cells tub2-T107K is lethal; but in tub2-T107K/tub2-590 heterozygotes the mutation is viable, dominant, and slows cell-cycle progression through mitosis, without causing wholesale disruption of cellular MTs. The correlation between the slower growing and shortening rates of MTs in vitro, and the slower mitosis in vivo suggests that MT dynamics are important in budding yeast and may regulate the rate of nuclear movement and segregation. The slower mitosis in mutant cells did not result in premature cytokinesis and cell death, further suggesting that cell-cycle control mechanisms "sense" the mitotic slowdown, possibly by monitoring MT dynamics directly.

The formation and functioning of yeast mitotic spindles

The mitotic spindle contains the machinery responsible for sister chromatid segregation. It is composed of a complex and dynamic array of microtubules, which are nucleated from the spindle poles. Studies of yeast spindle functions by molecular genetic analysis and by in vitro functional analysis have identified proteins that are mitosis-specific and present at very low concentrations in the cell, and have revealed the molecular bases of several processes required for the formation and functioning of the mitotic spindle. Here I review the current knowledge of the processes that are common to most eukaryotes: microtubule nucleation at the spindle poles, bipolar spindle assembly, maintenance of the spindle structure, chromosome attachment to the spindle and chromosome separation on the spindle.

The Saccharomyces cerevisiae checkpoint gene BUB1 encodes a novel protein kinase

Normal cell multiplication requires that the events of mitosis occur in a carefully ordered fashion. Cells employ checkpoints to prevent cycle progression until some prerequisite step has been completed. To explore the mechanisms of checkpoint enforcement, we previously screened for mutants of Saccharomyces cerevisiae which are unable to recover from a transient treatment with a benzimidazole-related microtubule inhibitor because they fail to inhibit subsequent cell cycle steps. Two of the identified genes, BUB2 and BUB3, have been cloned and described (M. A. Hoyt, L. Totis, and B. T. Roberts, Cell 66:507-517, 1991). Here we present the characterization of the BUB1 gene and its product. Genetic evidence was obtained suggesting that Bub1 and Bub3 are mutually dependent for function, and immunoprecipitation experiments demonstrated a physical association between the two. Sequence analysis of BUB1 revealed a domain with similarity to protein kinases. In vitro experiments confirmed that Bub1 possesses kinase activity; Bub1 was able to autophosphorylate and to catalyze phosphorylation of Bub3. In addition, overproduced Bub1 was found to localize to the cell nucleus.

Cell cycle control and cancer

Multiple genetic changes occur during the evolution of normal cells into cancer cells. This evolution is facilitated in cancer cells by loss of fidelity in the processes that replicate, repair, and segregate the genome. Recent advances in our understanding of the cell cycle reveal how fidelity is normally achieved by the coordinated activity of cyclin-dependent kinases, checkpoint controls, and repair pathways and how this fidelity can be abrogated by specific genetic changes. These insights suggest molecular mechanisms for cellular transformation and may help to identify potential targets for improved cancer therapies.

Bypassing anaphase by fission yeast cut9 mutation: requirement of cut9+ to initiate anaphase

A novel anaphase block phenotype was found in fission yeast temperature-sensitive cut9 mutants. Cells enter mitosis with chromosome condensation and short spindle formation, then block anaphase, but continue to progress into postanaphase events such as degradation of the spindle, reformation of the postanaphase cytoplasmic microtubule arrays, septation, and cytokinesis. The cut9 mutants are defective in the onset of anaphase and possibly in the restraint of postanaphase events until the completion of anaphase. The cut9+ gene encodes a 78-kD protein containing the 10 34-amino acid repeats, tetratricopeptide repeats (TPR), and similar to budding yeast Cdc16. It is essential for viability, and the mutation sites reside in the TPR. The three genes, namely, nuc2+, scn1+, and scn2+, genetically interact with cut9+. The nuc2+ and cut9+ genes share an essential function to initiate anaphase. The cold-sensitive scn1 and scn2 mutations, defective in late anaphase, can suppress the ts phenotype of cut9.

Anaphase onset in vertebrate somatic cells is controlled by a checkpoint that monitors sister kinetochore attachment to the spindle

To test the popular but unproven assumption that the metaphase-anaphase transition in vertebrate somatic cells is subject to a checkpoint that monitors chromosome (i.e., kinetochore) attachment to the spindle, we filmed mitosis in 126 PtK1 cells. We found that the time from nuclear envelope breakdown to anaphase onset is linearly related (r2 = 0.85) to the duration the cell has unattached kinetochores, and that even a single unattached kinetochore delays anaphase onset. We also found that anaphase is initiated at a relatively constant 23-min average interval after the last kinetochore attaches, regardless of how long the cell possessed unattached kinetochores. From these results we conclude that vertebrate somatic cells possess a metaphase-anaphase checkpoint control that monitors sister kinetochore attachment to the spindle. We also found that some cells treated with 0.3-0.75 nM Taxol, after the last kinetochore attached to the spindle, entered anaphase and completed normal poleward chromosome motion (anaphase A) up to 3 h after the treatment--well beyond the 9-48-min range exhibited by untreated cells. The fact that spindle bipolarity and the metaphase alignment of kinetochores are maintained in these cells, and that the chromosomes move poleward during anaphase, suggests that the checkpoint monitors more than just the attachment of microtubules at sister kinetochores or the metaphase alignment of chromosomes. Our data are most consistent with the hypothesis that the checkpoint monitors an increase in tension between kinetochores and their associated microtubules as biorientation occurs.

The Saccharomyces cerevisiae checkpoint gene BUB1 encodes a novel protein kinase

Normal cell multiplication requires that the events of mitosis occur in a carefully ordered fashion. Cells employ checkpoints to prevent cycle progression until some prerequisite step has been completed. To explore the mechanisms of checkpoint enforcement, we previously screened for mutants of Saccharomyces cerevisiae which are unable to recover from a transient treatment with a benzimidazole-related microtubule inhibitor because they fail to inhibit subsequent cell cycle steps. Two of the identified genes, BUB2 and BUB3, have been cloned and described (M. A. Hoyt, L. Totis, and B. T. Roberts, Cell 66:507-517, 1991). Here we present the characterization of the BUB1 gene and its product. Genetic evidence was obtained suggesting that Bub1 and Bub3 are mutually dependent for function, and immunoprecipitation experiments demonstrated a physical association between the two. Sequence analysis of BUB1 revealed a domain with similarity to protein kinases. In vitro experiments confirmed that Bub1 possesses kinase activity; Bub1 was able to autophosphorylate and to catalyze phosphorylation of Bub3. In addition, overproduced Bub1 was found to localize to the cell nucleus.

Cell cycle checkpoints

Checkpoints help ensure that cell cycle events occur in the correct order. Studies on mammalian cells identified inhibitors of complexes of cyclins and cyclin-dependent kinases as components of cell cycle checkpoints and provide the first glimpse of the molecular pathways that prevent cells with damaged DNA from replicating their DNA. In embryos, the extent to which checkpoints arrest the cell cycle reflects the relative strength of inhibitory checkpoints and the machinery driving the cell cycle forward.

Signal transduction in the budding yeast Saccharomyces cerevisiae

As a key part of the mechanism which controls growth and division, cells are able to respond to a variety of intracellular and extracellular stimuli. Significant progress has been made in the understanding of the biochemical mechanisms underlying mating-factor signal transduction in Saccharomyces cerevisiae. Some of these mechanisms may be relevant to the regulation of other signal transduction pathways.

A MAP kinase-dependent spindle assembly checkpoint in Xenopus egg extracts

Like early Xenopus embryos, extracts made from Xenopus eggs lack the cell cycle checkpoint that keeps anaphase from occurring before spindle assembly is complete. At very high densities of sperm nuclei, however, microtubule depolymerization arrests the extracts in mitosis. The arrested extracts have high levels of maturation-promoting factor activity, fail to degrade cyclin B, and contain activated ERK2/mitogen-activated protein (MAP) kinase. The addition of the purified MAP kinase-specific phosphatase MKP-1 demonstrates that MAP kinase activity is required for both the establishment and maintenance of the mitotic arrest induced by spindle depolymerization. Increased calcium concentrations, which release unfertilized frog eggs from their natural arrest in metaphase of meiosis II, have no effect on the mitotic arrest.

Meiotic cycle checkpoints in mammalian oocytes

Recent Spectacular achievements have enabled the identification of key molecules responsible for mitotic cell cycle progression through the stages of G1, the gap before DNA replication; S, the phase of DNA synthesis; G2, the gap before chromosome segregation; and M, mitosis itself. The last stage has been most intensively studied, where MPE, maturation promotion factor, has been found responsible for the nuclear events associated with chromosomal segregation and the prodcution of two identical daughter cells (see Murray & Hunt, 1993).

Microtubule dependency of p34cdc2 inactivation and mitotic exit in mammalian cells

The protein kinase inhibitor 2-aminopurine induces checkpoint override and mitotic exit in BHK cells which have been arrested in mitosis by inhibitors of microtubule function (Andreassen, P. R., and R. L. Margolis. 1991. J. Cell Sci. 100:299-310). Mitotic exit is monitored by loss of MPM-2 antigen, by the reformation of nuclei, and by the extinction of p34cdc2-dependent H1 kinase activity. 2-AP-induced inactivation of p34cdc2 and mitotic exit depend on the assembly state of microtubules. During mitotic arrest generated by the microtubule assembly inhibitor nocodazole, the rate of mitotic exit induced by 2-AP decreases proportionally with increasing nocodazole concentrations. At nocodazole concentrations of 0.12 microgram/ml or greater, 2-AP induces no apparent exit through 75 min of treatment. In contrast, 2-AP brings about a rapid exit (t1/2 = 20 min) from mitotic arrest by taxol, a drug which causes inappropriate overassembly of microtubules. In control mitotic cells, p34cdc2 localizes to kinetochores, centrosomes, and spindle microtubules. We find that efficient exit from mitosis occurs under conditions where p34cdc2 remains associated with centrosomal microtubules, suggesting it must be present on these microtubules in order to be inactivated. Mitotic slippage, the natural reentry of cells into G1 during prolonged mitotic block, is also microtubule dependent. At high nocodazole concentrations slippage is prevented and mitotic arrest approaches 100%. We conclude that essential components of the machinery for exit from mitosis are present on the mitotic spindle, and that normal mitotic exit thereby may be regulated by the microtubule assembly state.

The SAD1/RAD53 protein kinase controls multiple checkpoints and DNA damage-induced transcription in yeast

Inhibition of DNA synthesis prevents mitotic entry through the action of the S-phase checkpoint. We have isolated S-phase arrest-defective (sad) mutants that show lethality in the presence of the DNA synthesis inhibitor hydroxyurea (HU). Several of these mutants show phenotypes consistent with inappropriate mitotic entry in the presence of unreplicated DNA, indicating a defect in the S-phase checkpoint. sad1 mutants are additionally defective for the G1 and G2 DNA damage checkpoints, and for DNA damage-induced transcription of RNR2 and RNR3. The transcriptional response to DNA damage requires activation of the Dun1 protein kinase. Activation of Dun1 in response to replication blocks or DNA damage is blocked in sad1 mutants. The HU sensitivity of sad1 mutants is suppressed by mutations in CKS1, a subunit of the p34CDC28 kinase, further establishing a link between cell cycle progression and lethality. sad1 mutants are allelic to rad53, a radiation-sensitive mutant. SAD1 encodes an essential protein kinase. The observation that SAD1 controls three distinct checkpoints suggests a common mechanism for cell cycle arrest at these points. Together, these observations implicate protein phosphorylation in the cellular response to DNA damage and replication blocks.

Mitosis

Within the last decade, the study of mitosis has evolved into a multidisciplinary science in which findings from fields as diverse as chromosome biology and cytoskeletal architecture have converged to present a more cohesive understanding of the complex events that occur when cells divide. The largest strides have been made in the identification and characterization of regulatory enzymes (kinases and phosphatases) that modulate mitotic activity, as well as a number of the proteins and structural components (spindle, chromosomes, nuclear envelope) which carry out the mitotic instructions. One emerging theme appears to be that molecular signalling, which can involve modification of components (such as phosphorylation) or even their specific destruction, monitors the state of the mitotic cell at all stages. One of the major challenges for the future will be the identification of additional targets of the signalling machinery, as well as new regulatory components and their targets on the chromosomes, on the spindle, and at the cleavage furrow.

Symmetric cell division in pseudohyphae of the yeast Saccharomyces cerevisiae

Laboratory strains of Saccharomyces cerevisiae are dimorphic; in response to nitrogen starvation they switch from a yeast form (YF) to a filamentous pseudohyphal (PH) form. Time-lapse video microscopy of dividing cells reveals that YF and PH cells differ in their cell cycles and budding polarity. The YF cell cycle is controlled at the G1/S transition by the cell-size checkpoint Start. YF cells divide asymmetrically, producing small daughters from full-sized mothers. As a result, mothers and daughters bud asynchronously. Mothers bud immediately but daughters grow in G1 until they achieve a critical cell size. By contrast, PH cells divide symmetrically, restricting mitosis until the bud grows to the size of the mother. Thus, mother and daughter bud synchronously in the next cycle, without a G1 delay before Start. YF and PH cells also exhibit distinct bud-site selection patterns. YF cells are bipolar, producing their second and subsequent buds at either pole. PH cells are unipolar, producing their second and subsequent buds only from the end opposite the junction with their mother. We propose that in PH cells a G2 cell-size checkpoint delays mitosis until bud size reaches that of the mother cell. We conclude that yeast and PH forms are distinct cell types each with a unique cell cycle, budding pattern, and cell shape.

The Drosophila maternal-effect mutation grapes causes a metaphase arrest at nuclear cycle 13

grapes (grp) is a second chromosome (36A-B) maternal-effect lethal mutation in Drosophila melanogaster. We demonstrate that the syncytial nuclear divisions of grp-derived embryos are normal through metaphase of nuclear cycle 12. However, as the embryos progress into telophase of cycle 12, the microtubule structures rapidly deteriorate and midbodies never form. Immediately following the failure of midbody formation, sister telophase products collide and form large tetraploid nuclei. These observations suggest that the function of the midbody in the syncytial embryo is to maintain separation of sister nuclei during telophase of the cortical divisions. After an abbreviated nuclear cycle 13 interphase, these polyploid nuclei progress through prophase and arrest in metaphase. The spindles associated with the arrested nuclei are stable for hours even though the microtubules are rapidly turning over. The nuclear cycle 13 anaphase separation of sister chromatids never occurs and the chromosomes, still encompassed by spindles, assume a telophase conformation. Eventually neighboring arrested spindles begin to associate and form large clusters of spindles and nuclei. To determine whether this arrest was the result of a disruption in normal developmental events that occur at this time, both grp-derived and wild-type embryos were exposed to X-irradiation. Syncytial wild-type embryos exhibit a high division error rate, but not a nuclear-cycle arrest after exposure to low doses of X-irradiation. In contrast, grp-derived embryos exhibit a metaphase arrest in response to equivalent doses of X-irradiation. This arrest can be induced even in the early syncytial divisions prior to nuclear migration. These results suggest that the nuclear cycle 13 metaphase arrest of unexposed grp-derived embryos is independent of the division-cycle transitions that also occur at this stage. Instead, it may be the result of a previously unidentified feedback mechanism.

Type 1 protein phosphatase acts in opposition to IpL1 protein kinase in regulating yeast chromosome segregation

The IPL1 gene is required for high-fidelity chromosome segregation in the budding yeast Saccharomyces cerevisiae. Conditional ipl1ts mutants missegregate chromosomes severely at 37 degrees C. Here, we report that IPL1 encodes an essential putative protein kinase whose function is required during the later part of each cell cycle. At 26 degrees C, the permissive growth temperature, ipl1 mutant cells are defective in the recovery from a transient G2/M-phase arrest caused by the antimicrotubule drug nocodazole. In an effort to identify additional gene products that participate with the Ipl1 protein kinase in regulating chromosome segregation in yeast, a truncated version of the previously identified DIS2S1/GLC7 gene was isolated as a dosage-dependent suppressor of ipl1ts mutations. DIS2S1/GLC7 is predicted to encode a catalytic subunit (PP1C) of type 1 protein phosphatase. Overexpression of the full-length DIS2S1/GLC7 gene results in chromosome missegregation in wild-type cells and exacerbates the mutant phenotype in ipl1 cells. In addition, the glc7-1 mutation can partially suppress the ipl1-1 mutation. These results suggest that type 1 protein phosphatase acts in opposition to the Ipl1 protein kinase in vivo to ensure the high fidelity of chromosome segregation.

Inactivation of a Cdk2 inhibitor during interleukin 2-induced proliferation of human T lymphocytes

Peripheral blood T lymphocytes require two sequential mitogenic signals to reenter the cell cycle from their natural, quiescent state. One signal is provided by stimulation of the T-cell antigen receptor, and this induces the synthesis of both cyclins and cyclin-dependent kinases (CDKs) that are necessary for progression through G1. Antigen receptor stimulation alone, however, is insufficient to promote activation of G1 cyclin-Cdk2 complexes. This is because quiescent lymphocytes contain an inhibitor of Cdk2 that binds directly to this kinase and prevents its activation by cyclins. The second mitogenic signal, which can be provided by the cytokine interleukin 2, leads to inactivation of this inhibitor, thereby allowing Cdk2 activation and progression into S phase. Enrichment of the Cdk2 inhibitor from G1 lymphocytes by cyclin-CDK affinity chromatography indicates that it may be p27Kip1. These observations show how sequentially acting mitogenic signals can combine to promote activation of cell cycle proteins and thereby cause cell proliferation to start. CDK inhibitors have been shown previously to be induced by signals that negatively regulate cell proliferation. Our new observations show that similar proteins are down-regulated by positively acting signals, such as interleukin 2. This finding suggests that both positive and negative growth signals converge on common targets which are regulators of G1 cyclin-CDK complexes. Inactivation of G1 cyclin-CDK inhibitors by mitogenic growth factors may be one biochemical pathway underlying cell cycle commitment at the restriction point in G1.

Type 1 protein phosphatase acts in opposition to IpL1 protein kinase in regulating yeast chromosome segregation

The IPL1 gene is required for high-fidelity chromosome segregation in the budding yeast Saccharomyces cerevisiae. Conditional ipl1ts mutants missegregate chromosomes severely at 37 degrees C. Here, we report that IPL1 encodes an essential putative protein kinase whose function is required during the later part of each cell cycle. At 26 degrees C, the permissive growth temperature, ipl1 mutant cells are defective in the recovery from a transient G2/M-phase arrest caused by the antimicrotubule drug nocodazole. In an effort to identify additional gene products that participate with the Ipl1 protein kinase in regulating chromosome segregation in yeast, a truncated version of the previously identified DIS2S1/GLC7 gene was isolated as a dosage-dependent suppressor of ipl1ts mutations. DIS2S1/GLC7 is predicted to encode a catalytic subunit (PP1C) of type 1 protein phosphatase. Overexpression of the full-length DIS2S1/GLC7 gene results in chromosome missegregation in wild-type cells and exacerbates the mutant phenotype in ipl1 cells. In addition, the glc7-1 mutation can partially suppress the ipl1-1 mutation. These results suggest that type 1 protein phosphatase acts in opposition to the Ipl1 protein kinase in vivo to ensure the high fidelity of chromosome segregation.

Inactivation of a Cdk2 inhibitor during interleukin 2-induced proliferation of human T lymphocytes

Peripheral blood T lymphocytes require two sequential mitogenic signals to reenter the cell cycle from their natural, quiescent state. One signal is provided by stimulation of the T-cell antigen receptor, and this induces the synthesis of both cyclins and cyclin-dependent kinases (CDKs) that are necessary for progression through G1. Antigen receptor stimulation alone, however, is insufficient to promote activation of G1 cyclin-Cdk2 complexes. This is because quiescent lymphocytes contain an inhibitor of Cdk2 that binds directly to this kinase and prevents its activation by cyclins. The second mitogenic signal, which can be provided by the cytokine interleukin 2, leads to inactivation of this inhibitor, thereby allowing Cdk2 activation and progression into S phase. Enrichment of the Cdk2 inhibitor from G1 lymphocytes by cyclin-CDK affinity chromatography indicates that it may be p27Kip1. These observations show how sequentially acting mitogenic signals can combine to promote activation of cell cycle proteins and thereby cause cell proliferation to start. CDK inhibitors have been shown previously to be induced by signals that negatively regulate cell proliferation. Our new observations show that similar proteins are down-regulated by positively acting signals, such as interleukin 2. This finding suggests that both positive and negative growth signals converge on common targets which are regulators of G1 cyclin-CDK complexes. Inactivation of G1 cyclin-CDK inhibitors by mitogenic growth factors may be one biochemical pathway underlying cell cycle commitment at the restriction point in G1.

Feedback control of the metaphase-anaphase transition in sea urchin zygotes: role of maloriented chromosomes

To help ensure the fidelity of chromosome transmission during mitosis, sea urchin zygotes have feedback control mechanisms for the metaphase-anaphase transition that monitor the assembly of spindle microtubules and the complete absence of proper chromosome attachment to the spindle. The way in which these feedback controls work has not been known. In this study we directly test the proposal that these controls operate by maloriented chromosomes producing a diffusible inhibitor of the metaphase-anaphase transition. We show that zygotes having 50% of their chromosomes (approximately 20) unattached or monoriented initiate anaphase at the same time as the controls, a time that is well within the maximum period these zygotes will spend in mitosis. In vivo observations of the unattached maternal chromosomes indicate that they are functionally within the sphere of influence of the molecular events that cause chromosome disjunction in the spindle. Although the unattached chromosomes disjoin (anaphase onset without chromosome movement) several minutes after spindle anaphase onset, their disjunction is correlated with the time of spindle anaphase onset, not the time their nucleus breaks down. This suggests that the molecular events that trigger chromosome disjunction originate in the central spindle and propagate outward. Our results show that the mechanisms for the feedback control of the metaphase-anaphase transition in sea urchin zygotes do not involve a diffusible inhibitor produced by maloriented chromosomes. Even though the feedback controls for the metaphase-anaphase transition may detect the complete absence of properly attached chromosomes, they are insensitive to unattached or mono-oriented chromosomes as long as some chromosomes are properly attached to the spindle.

Ca2+ triggers premature inactivation of the cdc2 protein kinase in permeabilized sea urchin embryos

Exit from mitosis requires inactivation of the cyclin B-p34cdc2 protein kinase complex. Since increased cytosolic Ca2+ has been implicated as a potential trigger of mitotic progression, we directly tested the possibility that Ca2+ triggers the pathway responsible for inactivating the cdc2 kinase, using sea urchin embryos permeabilized at various stages of the cell cycle. In cells permeabilized during late interphase and prophase, micromolar Ca2+ induced premature inactivation of the cdc2 kinase without affecting the absolute amount of p34cdc2 protein. Inactivation was selective for the cdc2 kinase, as elevated Ca2+ had no effect on cAMP-dependent protein kinase activity. Premature cdc2 kinase inactivation did not require cyclin B destruction, but did coincide with the dissociation of cyclin B-p34cdc2 complexes. In cells permeabilized during prometaphase and metaphase, cdc2 kinase inactivation was Ca(2+)-independent, presumably because at these later times the inactivating pathway had been enabled prior to permeabilization. This work provides evidence that Ca2+ is the physiological trigger enabling cdc2 kinase inactivation during mitosis.

Cell cycle analysis and chromosomal localization of human Plk1, a putative homologue of the mitotic kinases Drosophila polo and Saccharomyces cerevisiae Cdc5

polo and CDC5 are two genes required for passage through mitosis in Drosophila melanogaster and Saccharomyces cerevisiae, respectively. Both genes encode structurally related protein kinases that have been implicated in regulating the function of the mitotic spindle. Here, we report the characterization of a human protein kinase that displays extensive sequence similarity to Drosophila polo and S. cerevisiae Cdc5; we refer to this kinase as Plk1 (for polo-like kinase 1). The largest open reading frame of the Plk1 cDNA encodes a protein of 68,254 daltons, and a protein of this size is detected by immunoblotting of HeLa cell extracts with monoclonal antibodies raised against the C-terminal part of Plk1 expressed in Escherichia coli. Northern blot analysis of RNA isolated from human cells and mouse tissues shows that a single Plk1 mRNA of 2.3 kb is highly expressed in tissues with a high mitotic index, consistent with a possible function of Plk1 in cell proliferation. The Plk1 gene maps to position p12 on chromosome 16, a locus for which no associations with neoplastic malignancies are known. The Plk1 protein levels and its distribution change during the cell cycle, in a manner consistent with a role of Plk1 in mitosis. Thus, like Drosophila polo and S. cerevisiae Cdc5, human Plk1 is likely to function in cell cycle progression.

An inhibitor of yeast cyclin-dependent protein kinase plays an important role in ensuring the genomic integrity of daughter cells

The gene encoding a 40-kDa protein, previously studied as a substrate and inhibitor of the yeast cyclin-dependent protein kinase, Cdc28, has been cloned. The DNA sequence reveals that p40 is a highly charged protein of 32,187 Da with no significant homology to other proteins. Overexpression of the gene encoding p40, SIC1, produces cells with an elongated but morphology similar to that of cells with depleted levels of the CLB gene products, suggesting that p40 acts as an inhibitor of Cdc28-Clb complexes in vivo. A SIC1 deletion is viable and has highly increased frequencies of broken and lost chromosomes. The deletion strain segregates out many dead cells that are primarily arrested at the G2 checkpoint in an asymmetric fashion. Only daughters and young mothers display the lethal defect, while experienced mothers appear to grow normally. These results suggest that negative regulation of Cdc28 protein kinase activity by p40 is important for faithful segregation of chromosomes to daughter cells.

Identification of yeast component A: reconstitution of the geranylgeranyltransferase that modifies Ypt1p and Sec4p

Members of a large family of small GTP-binding proteins, termed Rabs in mammalian cells or Ypt and Sec4 in yeast, regulate vesicular traffic in all eukaryotic cells. These proteins are able to bind to membranes because they are modified by the type II geranylgeranyltransferase (GGTase-II), a multisubunit complex. Component A, encoded by the choroideremia gene in humans, is an escort protein that brings Rabs to component B, the catalytic alpha/beta heterodimer. Mutations in the catalytic subunits of the yeast GGTase-II (Bet2p/Mad2p) disrupt the membrane attachment of Ypt1p and Sec4p and this in turn blocks membrane traffic. In mammalian cells, deletions in choroideremia lead only to retinal degeneration, even though GGTase-II activity is defective. The yeast MRS6 gene encodes a protein that is approximately 30% identical to the choroideremia gene product. Here we show that the addition of recombinant Mrs6p to bacterially expressed Bet2p (beta subunit) and Mad2p (alpha subunit) reconstitutes GGTase-II activity in vitro, demonstrating that Mrs6p is yeast component A. Like Bet2p and Mad2p, Mrs6p is required for the membrane attachment of Ypt1p and Sec4p in vivo. In contrast to what has been observed before for the loss of function of the choroideremia gene, the depletion of Mrs6p from yeast cells blocks vesicular transport. Thus, these findings suggest that there is one essential escort protein in yeast, while more than one may exist in mammalian cells.

CENP-C is required for maintaining proper kinetochore size and for a timely transition to anaphase

The human autoantigen CENP-C has been demonstrated by immunoelectron microscopy to be a component of the inner kinetochore plate. Here we have used antibodies raised against various portions of CENP-C to probe its function in mitosis. We show that nuclear microinjection of anti-CENP-C antibodies during interphase causes a transient arrest at the following metaphase. Injection of the same antibodies after the initiation of prophase, however, does not disrupt mitosis. Correspondingly, indirect immunofluorescence using affinity-purified human anti-CENP-C antibodies reveals that levels of CENP-C staining are reduced at centromeres in cells that were injected during interphase, but appear unaffected in cells which were injected during mitosis. Thus, we suggest that the injected antibodies cause metaphase arrest by reducing the amount of CENP-C at centromeres. Examination of kinetochores in metaphase-arrested cells by electron microscopy reveals that the number of trilaminar structures is reduced. More surprisingly, the few remaining kinetochores in these cells retain a normal trilaminar morphology but are significantly reduced in diameter. In cells arrested for extended periods, these small kinetochores become disrupted and apparently no longer bind microtubules. These observations are consistent with an involvement of CENP-C in kinetochore assembly, and suggest that CENP-C plays a critical role in both establishing and/or maintaining proper kinetochore size and stabilizing microtubule attachments. These findings also support the idea that proper assembly of kinetochores may be monitored by the cell cycle checkpoint preceding the transition to anaphase.

An inhibitor of yeast cyclin-dependent protein kinase plays an important role in ensuring the genomic integrity of daughter cells

The gene encoding a 40-kDa protein, previously studied as a substrate and inhibitor of the yeast cyclin-dependent protein kinase, Cdc28, has been cloned. The DNA sequence reveals that p40 is a highly charged protein of 32,187 Da with no significant homology to other proteins. Overexpression of the gene encoding p40, SIC1, produces cells with an elongated but morphology similar to that of cells with depleted levels of the CLB gene products, suggesting that p40 acts as an inhibitor of Cdc28-Clb complexes in vivo. A SIC1 deletion is viable and has highly increased frequencies of broken and lost chromosomes. The deletion strain segregates out many dead cells that are primarily arrested at the G2 checkpoint in an asymmetric fashion. Only daughters and young mothers display the lethal defect, while experienced mothers appear to grow normally. These results suggest that negative regulation of Cdc28 protein kinase activity by p40 is important for faithful segregation of chromosomes to daughter cells.

The JNM1 gene in the yeast Saccharomyces cerevisiae is required for nuclear migration and spindle orientation during the mitotic cell cycle

JNM1, a novel gene on chromosome XIII in the yeast Saccharomyces cerevisiae, is required for proper nuclear migration. jnm1 null mutants have a temperature-dependent defect in nuclear migration and an accompanying alteration in astral microtubules. At 30 degrees C, a significant proportion of the mitotic spindles is not properly located at the neck between the mother cell and the bud. This defect is more severe at low temperature. At 11 degrees C, 60% of the cells accumulate with large buds, most of which have two DAPI staining regions in the mother cell. Although mitosis is delayed and nuclear migration is defective in jnm1 mutant, we rarely observe more than two nuclei in a cell, nor do we frequently observe anuclear cells. No loss of viability is observed at 11 degrees C and cells continue to grow exponentially with increased doubling time. At low temperature the large budded cells of jnm1 mutants exhibit extremely long astral microtubules that often wind around the periphery of the cell. jnm1 mutants are not defective in chromosome segregation during mitosis, as assayed by the rate of chromosome loss, or nuclear migration during conjugation, as assayed by the rate of mating and cytoduction. The phenotype of a jnm1 mutant is strikingly similar to that for mutants in the dynein heavy chain gene (Eshel, D., L. A. Urrestarazu, S. Vissers, J.-C. Jauniaux, J. C. van Vliet-Reedijk, R. J. Plants, and I. R. Gibbons. 1993. Proc. Natl. Acad. Sci. USA. 90:11172-11176; Li, Y. Y., E. Yeh, T. Hays, and K. Bloom. 1993. Proc. Natl. Acad. Sci. USA. 90:10096-10100). The JNM1 gene product is predicted to encode a 44-kD protein containing three coiled coil domains. A JNM1:lacZ gene fusion is able to complement the cold sensitivity and microtubule phenotype of a jnm1 deletion strain. This hybrid protein localizes to a single spot in the cell, most often near the spindle pole body in unbudded cells and in the bud in large budded cells. Together these results point to a specific role for Jnm1p in spindle migration, possibly as a subunit or accessory protein for yeast dynein.

Determinants of Drosophila zw10 protein localization and function

We have examined several issues concerning how the Drosophila l(1)zw10 gene product functions to ensure proper chromosome segregation. (a) We have found that in zw10 mutant embryos and larval neuroblasts, absence of the zw10 protein has no obvious effect on either the congression of chromosomes to the metaphase plate or the morphology of the metaphase spindle, although many aberrations are observed subsequently in anaphase. This suggests that activity of the zw10 protein becomes essential at anaphase onset, a time at which the zw10 protein is redistributed to the kinetochore region of the chromosomes. (b) The zw10 protein appears to bind to kinetochores in mitotically arrested cells, eventually accumulating to high levels within the chromosome mass. Our results imply that zw10 may act as part of a novel feedback pathway that normally renders sister chromatid separation dependent upon spindle integrity. (c) The localization of zw10 protein is altered by two mitotic mutations, rough deal and abnormal anaphase resolution, that specifically disrupt anaphase. These findings indicate that the zw10 protein functions as part of a multicomponent mechanism ensuring proper chromosome segregation at the beginning of anaphase.

A chromosome breakage assay to monitor mitotic forces in budding yeast

During the eukaryotic cell cycle, genetic material must be accurately duplicated and faithfully segregated to each daughter cell. Segregation of chromosomes is dependent on the centromere, a region of the chromosome which interacts with mitotic spindle microtubules during cell division. Centromere function in the budding yeast, Saccharomyces cerevisiae, can be regulated by placing an inducible promotor adjacent to centromere DNA. This conditional centromere can be integrated into chromosome III to generate a conditionally functional dicentric chromosome. Activation of the dicentric chromosome results in a transient mitotic delay followed by the generation of monocentric derivatives. The propagation of viable cells containing these monocentric derivative chromosomes is dependent upon the DNA repair gene RAD52, indicating that double-strand DNA breaks are structural intermediates in the dicentric repair pathway. We have used these conditionally dicentric chromosomes to monitor the exertion of mitotic forces during cell division. Analysis of synchronized cells reveal that lethality in dicentric, rad52 mutant cells occurs during G2/M phase and is concomitant with the transient mitotic delay. the delay is largely dependent upon the cell cycle checkpoint gene RAD9, which is involved in monitoring DNA damage. These data demonstrate that DNA lesions resulting from dicentric activation are responsible for signalling the mitotic delay. Since the delay precedes the decline of p34cdc28 kinase activity, mitotic forces sufficient to result in dicentric chromosome breakage are generated prior to spindle elongation and anaphase onset in yeast.

GDI1 encodes a GDP dissociation inhibitor that plays an essential role in the yeast secretory pathway

GTP binding proteins of the Sec4/Ypt/rab family regulate distinct vesicular traffic events in eukaryotic cells. We have cloned GDI1, an essential homolog of bovine rab GDI (GDP dissociation inhibitor) from the yeast Saccharomyces cerevisiae. Analogous to the bovine protein, purified Gdi1p slows the dissociation of GDP from Sec4p and releases the GDP-bound form from yeast membranes. Depletion of Gdi1p in vivo leads to loss of the soluble pool of Sec4p and inhibition of protein transport at multiple stages of the secretory pathway. Complementation analysis indicates that GDI1 is allelic to sec19-1. These results establish that Gdi1p plays an essential function in membrane traffic and are consistent with a role for Gdi1p in the recycling of proteins of the Sec4/Ypt/rab family from their target membranes back to their vesicular pools.

Saccharomyces cerevisiae cells lacking the homologous pairing protein p175SEP1 arrest at pachytene during meiotic prophase

Saccharomyces cerevisiae cells containing null mutations in the SEP1 gene, which encodes the homologous pairing and strand exchange protein p175SEP1, enter pachytene with a delay. They arrest uniformly at this stage of meiotic prophase, probably revealing a checkpoint in the transition from pachytene to meiosis I. At the arrest point, the cells remain largely viable and are cytologically characterized by the duplicated but unseparated spindle pole bodies of equal size and by the persistence of the synaptonemal complex, a cytological marker for pachytene. In addition, fluorescence in situ hybridization revealed that in arrested mutant cells maximal chromatin condensation and normal homolog pairing is achieved, typical for pachytene in wild type. A hallmark of meiosis is the high level of homologous recombination, which was analyzed both genetically and physically. Formation and processing of the double-strand break intermediate in meiotic recombination is achieved prior to arrest. Physical intragenic (conversion) and intergenic (crossover) products are formed just prior to, or directly at, the arrest point. Structural deficits in synaptonemal complex morphology, failure to separate spindle pole bodies, and/or defects in prophase DNA metabolism might be responsible for triggering the observed arrest. The pachytene arrest in sep1 cells is likely to be regulatory, but is clearly different from the RAD9 checkpoint in meiotic prophase, which occurs prior to the pachytene stage.

Mitotic checkpoint genes in budding yeast and the dependence of mitosis on DNA replication and repair

In eukaryotes a cell-cycle control termed a checkpoint causes arrest in the S or G2 phases when chromosomes are incompletely replicated or damaged. Previously, we showed in budding yeast that RAD9 and RAD17 are checkpoint genes required for arrest in the G2 phase after DNA damage. Here, we describe a genetic strategy that identified four additional checkpoint genes that act in two pathways. Both classes of genes are required for arrest in the G2 phase after DNA damage, and one class of genes is also required for arrest in S phase when DNA replication is incomplete. The G2-specific genes include MEC3 (for mitosis entry checkpoint), RAD9, RAD17, and RAD24. The genes common to both S phase and G2 phase pathways are MEC1 and MEC2. The MEC2 gene proves to be identical to the RAD53 gene. Checkpoint mutants were identified by their interactions with a temperature-sensitive allele of the cell division cycle gene CDC13; cdc13 mutants arrested in G2 and survived at the restrictive temperature, whereas all cdc13 checkpoint double mutants failed to arrest in G2 and died rapidly at the restrictive temperature. The cell-cycle roles of the RAD and MEC genes were examined by combination of rad and mec mutant alleles with 10 cdc mutant alleles that arrest in different stages of the cell cycle at the restrictive temperature and by the response of rad and mec mutant alleles to DNA damaging agents and to hydroxyurea, a drug that inhibits DNA replication. We conclude that the checkpoint in budding yeast consists of overlapping S-phase and G2-phase pathways that respond to incomplete DNA replication and/or DNA damage and cause arret of cells before mitosis.

CDC44: a putative nucleotide-binding protein required for cell cycle progression that has homology to subunits of replication factor C

To investigate the means by which a cell regulates the progression of the mitotic cell cycle, we characterized cdc44, a mutation that causes Saccharomyces cerevisiae cells to arrest before mitosis. CDC44 encodes a 96-kDa basic protein with significant homology to a human protein that binds DNA (PO-GA) and to three subunits of human replication factor C (also called activator 1). The hypothesis that Cdc44p is involved in DNA metabolism is supported by the observations that (i) levels of mitotic recombination suggest elevated rates of DNA damage in cdc44 mutants and (ii) the cell cycle arrest observed in cdc44 mutants is alleviated by the DNA damage checkpoint mutations rad9, mec1, and mec2. The predicted amino acid sequence of Cdc44p contains GTPase consensus sites, and mutations in these regions cause a conditional cell cycle arrest. Taken together, these observations suggest that the essential CDC44 gene may encode the large subunit of yeast replication factor C.

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.

Protein prenylation in eukaryotic microorganisms: genetics, biology and biochemistry

Modification of proteins at C-terminal cysteine residue(s) by the isoprenoids farnesyl (C15) and geranylgeranyl (C20) is essential for the biological function of a number of eukaryotic proteins including fungal mating factors and the small, GTP-binding proteins of the Ras superfamily. Three distinct enzymes, conserved between yeast and mammals, have been identified that prenylate proteins: farnesyl protein transferase, geranylgeranyl protein transferase type I and geranylgeranyl protein transferase type II. Each prenyl protein transferase has its own protein substrate specificity. Much has been learned about the biology, genetics and biochemistry of protein prenylation and prenyl protein transferases through studies of eukaryotic microorganisms, particularly Saccharomyces cerevisiae. The functional importance of protein prenylation was first demonstrated with fungal mating factors. The initial genetic analysis of prenyl protein transferases was in S. cerevisiae with the isolation and subsequent characterization of mutations in the RAM1, RAM2, CDC43 and BET2 genes, each of which encodes a prenyl protein transferase subunit. We review here these and other studies on protein prenylation in eukaryotic microbes and how they relate to and have contributed to our knowledge about protein prenylation in all eukaryotic cells.

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.