The spindle checkpoint: two transitions, two pathways

Rad9-mediated checkpoint activation is responsible for elevated expansions of GAA repeats in CST-deficient yeast

Large-scale expansion of (GAA)n repeats in the first intron of the FXN gene is responsible for the severe neurodegenerative disease, Friedreich's ataxia in humans. We have previously conducted an unbiased genetic screen for GAA repeat instability in a yeast experimental system. The majority of genes that came from this screen encoded the components of DNA replication machinery, strongly implying that replication irregularities are at the heart of GAA repeat expansions. This screen, however, also produced two unexpected hits: members of the CST complex, CDC13 and TEN1 genes, which are required for telomere maintenance. To understand how the CST complex could affect intra-chromosomal GAA repeats, we studied the well-characterized temperature-sensitive cdc13-1 mutation and its effects on GAA repeat instability in yeast. We found that in-line with the screen results, this mutation leads to ∼10-fold increase in the rate of large-scale expansions of the (GAA)100 repeat at semi-permissive temperature. Unexpectedly, the hyper-expansion phenotype of the cdc13-1 mutant largely depends on activation of the G2/M checkpoint, as deletions of individual genes RAD9, MEC1, RAD53, and EXO1 belonging to this pathway rescued the increased GAA expansions. Furthermore, the hyper-expansion phenotype of the cdc13-1 mutant depended on the subunit of DNA polymerase δ, Pol32. We hypothesize, therefore, that increased repeat expansions in the cdc13-1 mutant happen during post-replicative repair of nicks or small gaps within repetitive tracts during the G2 phase of the cell cycle upon activation of the G2/M checkpoint.

The Mitotic Exit Network integrates temporal and spatial signals by distributing regulation across multiple components

GTPase signal transduction pathways control cellular decision making by integrating multiple cellular events into a single signal. The Mitotic Exit Network (MEN), a Ras-like GTPase signaling pathway, integrates spatial and temporal cues to ensure that cytokinesis only occurs after the genome has partitioned between mother and daughter cells during anaphase. Here we show that signal integration does not occur at a single step of the pathway. Rather, sequential components of the pathway are controlled in series by different signals. The spatial signal, nuclear position, regulates the MEN GTPase Tem1. The temporal signal, commencement of anaphase, is mediated by mitotic cyclin-dependent kinase (CDK) phosphorylation of the GTPase's downstream kinases. We propose that integrating multiple signals through sequential steps in the GTPase pathway represents a generalizable principle in GTPase signaling and explains why intracellular signal transmission is a multi-step process. Serial signal integration rather than signal amplification makes multi-step signal transduction necessary.

DNA replication after mutagenic treatment in Hordeum vulgare

The temporal and spatial properties of DNA replication in plants related to DNA damage and mutagenesis is poorly understood. Experiments were carried out to explore the relationships between DNA replication, chromatin structure and DNA damage in nuclei from barley root tips. We quantitavely analysed the topological organisation of replication foci using pulse EdU labelling during the S phase and its relationship with the DNA damage induced by mutagenic treatment with maleic hydrazide (MH), nitroso-N-methyl-urea (MNU) and gamma ray. Treatment with mutagens did not change the characteristic S-phase patterns in the nuclei; however, the frequencies of the S-phase-labelled cells after treatment differed from those observed in the control cells. The analyses of DNA replication in barley nuclei were extended to the micronuclei induced by mutagens. Replication in the chromatin of the micronuclei was rare. The results of simultanous TUNEL reaction to identify cells with DNA strand breaks and the labelling of the S-phase cells with EdU revealed the possibility of DNA replication occurring in damaged nuclei. For the first time, the intensity of EdU fluorescence to study the rate of DNA replication was analysed.

Positive Feedback Keeps Duration of Mitosis Temporally Insulated from Upstream Cell-Cycle Events

Cell division is characterized by a sequence of events by which a cell gives rise to two daughter cells. Quantitative measurements of cell-cycle dynamics in single cells showed that despite variability in G1-, S-, and G2 phases, duration of mitosis is short and remarkably constant. Surprisingly, there is no correlation between cell-cycle length and mitotic duration, suggesting that mitosis is temporally insulated from variability in earlier cell-cycle phases. By combining live cell imaging and computational modeling, we showed that positive feedback is the molecular mechanism underlying the temporal insulation of mitosis. Perturbing positive feedback gave rise to a sluggish, variable entry and progression through mitosis and uncoupled duration of mitosis from variability in cell cycle length. We show that positive feedback is important to keep mitosis short, constant, and temporally insulated and anticipate it might be a commonly used regulatory strategy to create modularity in other biological systems.

CK1 is required for a mitotic checkpoint that delays cytokinesis

Failure to accurately partition genetic material during cell division causes aneuploidy and drives tumorigenesis. Cell-cycle checkpoints safeguard cells from such catastrophes by impeding cell-cycle progression when mistakes arise. FHA-RING E3 ligases, including human RNF8 and CHFR and fission yeast Dma1, relay checkpoint signals by binding phosphorylated proteins via their FHA domains and promoting ubiquitination of downstream targets. Upon mitotic checkpoint activation, S. pombe Dma1 concentrates at spindle pole bodies (SPBs) in an FHA-dependent manner and ubiquitinates Sid4, a scaffold of Polo kinase, to suspend cytokinesis. However, the kinase or kinases that phosphoprime Sid4 for Dma1-mediated ubiquitination are unknown. Here, we report that the highly conserved protein kinase CK1 transmits the signal necessary to stall cytokinesis by phosphopriming Sid4 for Dma1-mediated ubiquitination. Like Dma1, CK1 accumulates at SPBs during a mitotic arrest and associates stably with SPB components, including Sid4. Our results establish CK1 as an integral component of a mitotic, ubiquitin-mediated checkpoint pathway.

The TP53 fertility network

The TP53 gene, first described in 1979, was identified as a tumor suppressor gene in 1989, when it became clear that its product, the p53 nuclear phosphoprotein, was frequently inactivated in many different forms of cancers. Nicknamed "guardian of the genome", TP53 occupies a central node in stress response networks. The p53 protein has a key role as transcription factor in limiting oncogenesis through several growth suppressive functions, such as initiating apoptosis, senescence, or cell cycle arrest. The p53 protein is directly inactivated in about 50% of all tumors as a result of somatic gene mutations or deletions, and over 80% of tumors demonstrate dysfunctional p53 signaling. Beyond the undeniable importance of p53 as a tumor suppressor, an increasing number of new functions for p53 have been reported, including its ability to regulate energy metabolism, to control autophagy, and to participate in various aspects of differentiation and development. Recently, studies on genetic variations in TP53 among different populations have led to the notion that the p53 protein might play an important role in regulating fertility. This review summarizes current knowledge on the basic functions of different genes of the TP53 family and TP53 pathway with respect to fertility. We also provide original analyses based on genomic and genotype databases, providing further insights into the possible roles of the TP53 pathway in human reproduction.

Mitotic Kinases and p53 Signaling

Mitosis is tightly regulated and any errors in this process often lead to aneuploidy, genomic instability, and tumorigenesis. Deregulation of mitotic kinases is significantly associated with improper cell division and aneuploidy. Because of their importance during mitosis and the relevance to cancer, mitotic kinase signaling has been extensively studied over the past few decades and, as a result, several mitotic kinase inhibitors have been developed. Despite promising preclinical results, targeting mitotic kinases for cancer therapy faces numerous challenges, including safety and patient selection issues. Therefore, there is an urgent need to better understand the molecular mechanisms underlying mitotic kinase signaling and its interactive network. Increasing evidence suggests that tumor suppressor p53 functions at the center of the mitotic kinase signaling network. In response to mitotic spindle damage, multiple mitotic kinases phosphorylate p53 to either activate or deactivate p53-mediated signaling. p53 can also regulate the expression and function of mitotic kinases, suggesting the existence of a network of mutual regulation, which can be positive or negative, between mitotic kinases and p53 signaling. Therefore, deciphering this regulatory network will provide knowledge to overcome current limitations of targeting mitotic kinases and further improve the results of targeted therapy.

Computational modeling of signaling pathways mediating cell cycle checkpoint control and apoptotic responses to ionizing radiation-induced DNA damage

The shape of dose response of ionizing radiation (IR) induced cancer at low dose region, either linear non-threshold or J-shaped, has been a debate for a long time. This dose response relationship can be influenced by built-in capabilities of cells that minimize the fixation of IR-mediated DNA damage as pro-carcinogenic mutations. Key capabilities include sensing of damage, activation of cell cycle checkpoint arrests that provide time needed for repair of the damage as well as apoptosis. Here we describe computational modeling of the signaling pathways that link sensing of DNA damage and checkpoint arrest activation/apoptosis to investigate how these molecular-level interactions influence the dose response relationship for IR induced cancer. The model provides qualitatively accurate descriptions of the IR-mediated activation of cell cycle checkpoints and the apoptotic pathway, and of time-course activities and dose response of relevant regulatory proteins (e.g. p53 and p21). Linking to a two-stage clonal growth cancer model, the model described here successfully captured a monotonically increasing to a J-shaped dose response curve and identified one potential mechanism leading to the J-shape: the cell cycle checkpoint arrest time saturates with the increase of the dose.

Dependence of Chs2 ER export on dephosphorylation by cytoplasmic Cdc14 ensures that septum formation follows mitosis

Sequestration of Cdc14 from the cytoplasm ensures Chs2 ER retention after MEN activation. The interdependence of chromosome segregation, MEN activation, decrease in mitotic CDK activity, and Cdc14 dispersal provides an effective mechanism for cells to order late mitotic events.

Cytokinesis, which leads to the physical separation of two dividing cells, is normally restrained until after nuclear division. In Saccharomyces cerevisiae, chitin synthase 2 (Chs2), which lays down the primary septum at the mother–daughter neck, also ensures proper actomyosin ring constriction during cytokinesis. During the metaphase-to-anaphase transition, phosphorylation of Chs2 by the mitotic cyclin-dependent kinase (Cdk1) retains Chs2 at the endoplasmic reticulum (ER), thereby preventing its translocation to the neck. Upon Cdk1 inactivation at the end of mitosis, Chs2 is exported from the ER and targeted to the neck. The mechanism for triggering Chs2 ER export thus far is unknown. We show here that Chs2 ER export requires the direct reversal of the inhibitory Cdk1 phosphorylation sites by Cdc14 phosphatase, the ultimate effector of the mitotic exit network (MEN). We further show that only Cdc14 liberated by the MEN after completion of chromosome segregation, and not Cdc14 released in early anaphase by the Cdc fourteen early anaphase release pathway, triggers Chs2 ER exit. Presumably, the reduced Cdk1 activity in late mitosis further favors dephosphorylation of Chs2 by Cdc14. Thus, by requiring declining Cdk1 activity and Cdc14 nuclear release for Chs2 ER export, cells ensure that septum formation is contingent upon chromosome separation and exit from mitosis.

Premature chromosome condensation induced by caffeine, 2-aminopurine, staurosporine and sodium metavanadate in S-phase arrested HeLa cells is associated with a decrease in Chk1 phosphorylation, formation of phospho-H2AX and minor cytoskeletal rearrangements

Here, we demonstrate that in HeLa cells, Ser317 of Chk1 undergoes phosphorylation in response to replication stress induced by hydroxyurea. We also demonstrate the existence of constitutive (interphase and mitotic) Chk1 kinase phosphorylation, the translocation of its phosphorylated form from the nucleus to cytoplasm in prometaphase as well as strong labeling of apoptotic nuclei with α-Chk1(S317) antibodies. Additionally, we show that caffeine, 2-aminopurine, staurosporine and sodium metavanadate can induce premature chromosome condensation (PCC) by the abrogation of the S-M checkpoint. Staurosporine appeared to be the most effective PCC inductor, and as in the case of the remaining inductors, the addition of hydroxyurea each time brought about an increase in the number of cells showing PCC symptoms (synergic effect). The forced premature mitosis was accompanied by an increasing index of double-strand breaks marked by the phosphorylation of histone H2AX on Ser139. Moreover, we found that the chemicals used brought about minor actin and tubulin network rearrangements that occurred following either replication stress or drug-induced cell cycle delay. At the same time, it was found that the extent of the cytoskeleton rearrangement did not hinder PCC in all its subperiods, i.e., from PCC-type prophase to PCC-type telophase.

Analysis of changes in protein level and subcellular localization during cell cycle progression using the budding yeast Saccharomyces cerevisiae

Methods are described here to monitor changes in protein level and subcellular localization during the cell cycle progression in the budding yeast Saccharomyces cerevisiae. Cell synchronization is achieved by an α-factor-mediated block-and-release protocol. Cells are collected at different time points for the first two cell cycles upon release. Cellular DNA contents are analyzed by flow cytometry. Trichloroacetic acid protein precipitates are prepared for monitoring levels of cell cycle regulated proteins by Western blotting. The dynamic changes in protein subcellular localization patterns are examined by indirect immunofluorescence microscopy.

Dma1 ubiquitinates the SIN scaffold, Sid4, to impede the mitotic localization of Plo1 kinase

Proper cell division requires strict coordination between mitotic exit and cytokinesis. In the event of a mitotic error, cytokinesis must be inhibited to ensure equal partitioning of genetic material. In the fission yeast, Schizosaccharomyces pombe, the checkpoint protein and E3 ubiquitin ligase, Dma1, delays cytokinesis by inhibiting the septation initiation network (SIN) when chromosomes are not attached to the mitotic spindle. To elucidate the mechanism by which Dma1 inhibits the SIN, we screened all SIN components as potential Dma1 substrates and found that the SIN scaffold protein, Sid4, is ubiquitinated in vivo in a Dma1-dependent manner. To investigate the role of Sid4 ubiquitination in checkpoint function, a ubiquitination deficient sid4 allele was generated and our data indicate that Sid4 ubiquitination by Dma1 is required to prevent cytokinesis during a mitotic checkpoint arrest. Furthermore, Sid4 ubiquitination delays recruitment of the Polo-like kinase and SIN activator, Plo1, to spindle pole bodies (SPBs), while at the same time prolonging residence of the SIN inhibitor, Byr4, providing a mechanistic link between Dma1 activity and cytokinesis inhibition.

The role of p53 gene family in reproduction

The p53 family of genes (p53, p63, and p73) is conserved over evolutionary time scales. Although the functions of p53 gene and its protein as a tumor suppressor have been firmly established, the earliest functions for the p53 ancestral genes in worms and flies are to ensure germ-line genomic integrity and the fidelity of the developmental process. In vertebrates, the p53 family of genes retains those functions in germ-line genomic integrity but have added important functions in regulation of reproduction. Loss of the p53, p63, or p73 genes in female mice leads to a significant decrease of fertility. The p53 gene product regulates maternal reproduction at the implantation stage of the embryo. p63 and p73 play important roles in monitoring the genomic quality of oocytes. The p53 pathway appears to play a similar role in human fertility. In humans, certain alleles containing a functional single-nucleotide polymorphism (SNP) in the p53 pathway are under positive evolutionary selection. Selected alleles of these SNPs in the p53 pathway are associated with decreased fertility. This important function of the p53 pathway in reproduction provides a plausible explanation for the evolution of p53 as a tumor suppressor gene and the positive selection of some alleles in the p53 gene and its pathway. These observations provide a good possible example of antagonistic pleiotrophy for fertility, tumor suppression, and longevity.

Heat shock protein inhibitors, 17-DMAG and KNK437, enhance arsenic trioxide-induced mitotic apoptosis

Arsenic trioxide (ATO) has recently emerged as a promising therapeutic agent in leukemia because of its ability to induce apoptosis. However, there is no sufficient evidence to support its therapeutic use for other types of cancers. In this study, we investigated if, and how, 17-dimethylaminoethylamino-17-demethoxy-geldanamycin (17-DMAG), an antagonist of heat shock protein 90 (HSP90), and KNK437, a HSP synthesis inhibitor, potentiated the cytotoxic effect of ATO. Our results showed that cotreatment with ATO and either 17-DMAG or KNK437 significantly increased ATO-induced cell death and apoptosis. siRNA-mediated attenuation of the expression of the inducible isoform of HSP70 (HSP70i) or HSP90alpha/beta also enhanced ATO-induced apoptosis. In addition, cotreatment with ATO and 17-DMAG or KNK437 significantly increased ATO-induced mitotic arrest and ATO-induced BUBR1 phosphorylation and PDS1 accumulation. Cotreatment also significantly increased the percentage of mitotic cells with abnormal mitotic spindles and promoted metaphase arrest as compared to ATO treatment alone. These results indicated that 17-DMAG or KNK437 may enhance ATO cytotoxicity by potentiating mitotic arrest and mitotic apoptosis possibly through increased activation of the spindle checkpoint.

TAp73 regulates the spindle assembly checkpoint by modulating BubR1 activity

The role of various p73 isoforms in tumorigenesis has been controversial. However, as we have recently shown, the generation of TAp73-deficient (TAp73(-/-)) mice reveals that TAp73 isoforms exert tumor-suppressive functions, indicating an emerging role for Trp-73 in the maintenance of genomic stability. Unlike mice lacking all p73 isoforms, TAp73(-/-) mice show a high incidence of spontaneous tumors. Moreover, TAp73(-/-) mice are infertile and produce oocytes exhibiting spindle abnormalities. These data suggest a link between TAp73 activities and the common molecular machinery underlying meiosis and mitosis. Previous studies have indicated that the spindle assembly checkpoint (SAC) complex, whose activation leads to mitotic arrest, also regulates meiosis. In this study, we demonstrate in murine and human cells that TAp73 is able to interact directly with several partners of the SAC complex (Bub1, Bub3, and BubR1). We also show that TAp73 is involved in SAC protein localization and activities. Moreover, we show that decreased TAp73 expression correlates with increases of SAC protein expression in patients with lung cancer. Our results establish TAp73 as a regulator of SAC responses and indicate that TAp73 loss can lead to mitotic arrest defects. Our data suggest that SAC impairment in the absence of functional TAp73 could explain the genomic instability and increased aneuploidy observed in TAp73-deficient cells.

Pds1p is required for meiotic recombination and prophase I progression in Saccharomyces cerevisiae

Sister-chromatid separation at the metaphase-anaphase transition is regulated by a proteolytic cascade. Destruction of the securin Pds1p liberates the Esp1p separase, which ultimately targets the mitotic cohesin Mcd1p/Scc1p for destruction. Pds1p stabilization by the spindle or DNA damage checkpoints prevents sister-chromatid separation while mutants lacking PDS1 (pds1Delta) are temperature sensitive for growth due to elevated chromosome loss. This report examined the role of the budding yeast Pds1p in meiotic progression using genetic, cytological, and biochemical assays. Similar to its mitotic function, Pds1p destruction is required for metaphase I-anaphase I transition. However, even at the permissive temperature for growth, pds1Delta mutants arrest with prophase I spindle and nuclear characteristics. This arrest was partially suppressed by preventing recombination initiation or by inactivating a subset of recombination checkpoint components. Further studies revealed that Pds1p is required for recombination in both double-strand-break formation and synaptonemal complex assembly. Although deleting PDS1 did not affect the degradation of the meiotic cohesin Rec8p, Mcd1p was precociously destroyed as cells entered the meiotic program. This role is meiosis specific as Mcd1p destruction is not altered in vegetative pds1Delta cultures. These results define a previously undescribed role for Pds1p in cohesin maintenance, recombination, and meiotic progression.

Exploring the mechanisms of action of the novel microtubule inhibitor vinflunine

Microtubules have been identified as a suitable target for anticancer therapy, primarily based on their biological importance in coordinating chromosomal segregation at mitosis. Two main classes of microtubule-targeted agents, the taxanes and vinca alkaloids, suppress the dynamic behavior of spindle microtubules, inducing mitotic arrest and subsequent apoptotic cell death. Clinical activity of taxanes and first-generation vinca alkaloids in the treatment of solid tumors and hematologic malignancies, respectively, has prompted further research for novel analogs with improved clinical efficacy and safety. Such efforts have led to the development of vinflunine, a bifluorinated vinca alkaloid endowed with unique antitumor properties. Highlighted in this review are the key features of vinflunine that lead to effective suppression of microtubule dynamics and induction of cell death in cancer cells.

Histone H3 K56 hyperacetylation perturbs replisomes and causes DNA damage

Deacetylation of histone H3 K56, regulated by the sirtuins Hst3p and Hst4p, is critical for maintenance of genomic stability. However, the physiological consequences of a lack of H3 K56 deacetylation are poorly understood. Here we show that cells lacking Hst3p and Hst4p, in which H3 K56 is constitutively hyperacetylated, exhibit hallmarks of spontaneous DNA damage, such as activation of the checkpoint kinase Rad53p and upregulation of DNA-damage inducible genes. Consistently, hst3 hst4 cells display synthetic lethality interactions with mutations that cripple genes involved in DNA replication and DNA double-strand break (DSB) repair. In most cases, synthetic lethality depends upon hyperacetylation of H3 K56 because it can be suppressed by mutation of K56 to arginine, which mimics the nonacetylated state. We also show that hst3 hst4 phenotypes can be suppressed by overexpression of the PCNA clamp loader large subunit, Rfc1p, and by inactivation of the alternative clamp loaders CTF18, RAD24, and ELG1. Loss of CTF4, encoding a replisome component involved in sister chromatid cohesion, also suppresses hst3 hst4 phenotypes. Genetic analysis suggests that CTF4 is a part of the K56 acetylation pathway that converges on and modulates replisome function. This pathway represents an important mechanism for maintenance of genomic stability and depends upon proper regulation of H3 K56 acetylation by Hst3p and Hst4p. Our data also suggest the existence of a precarious balance between Rfc1p and the other RFC complexes and that the nonreplicative forms of RFC are strongly deleterious to cells that have genomewide and constitutive H3 K56 hyperacetylation.

The AAA-ATPase FIGL-1 controls mitotic progression, and its levels are regulated by the CUL-3MEL-26 E3 ligase in the C. elegans germ line

Members of the AAA-ATPase (ATPases associated with diverse cellular activities) family use the energy from ATP hydrolysis to disrupt protein complexes involved in many cellular processes. Here, we report that FIGL-1 (Fidgetin-like 1), the single Caenorhabditis elegans homolog of mammalian fidgetin and fidgetin-like 1 AAA-ATPases, controls progression through mitosis in the germ line and the early embryo. Loss of figl-1 function leads to the accumulation of mitotic nuclei in the proliferative zone of the germ line, resulting in sterility owing to depletion of germ cells. Like the AAA-ATPase MEI-1 (also known as katanin), FIGL-1 interacts with microtubules and with MEL-26, a specificity factor of CUL-3-based E3 ligases involved in targeting proteins for ubiquitin-dependent degradation by the 26S proteasome. In the germ line, FIGL-1 is enriched in nuclei of mitotic cells, but it disappears at the transition into meiosis. Conversely, MEL-26 expression is low in nuclei of the mitotic zone and induced during meiosis. FIGL-1 accumulates in the germ line and spreads to the meiotic zone after inactivation of mel-26 or cul-3 in vivo. We conclude that degradation of FIGL-1 by the CUL-3MEL-26 E3 ligase spatially restricts FIGL-1 function to mitotic cells, where it is required for correct progression through mitosis.

Farnesyl diphosphate analogues with omega-bioorthogonal azide and alkyne functional groups for protein farnesyl transferase-catalyzed ligation reactions

Eleven farnesyl diphosphate analogues, which contained omega-azide or alkyne substituents suitable for bioorthogonal Staudinger and Huisgen [3 + 2] cycloaddition coupling reactions, were synthesized. The analogues were evaluated as substrates for the alkylation of peptide cosubstrates by yeast protein farnesyl transferase. Five of the diphosphates were good alternative substrates for farnesyl diphosphate (FPP). Steady-state kinetic constants were measured for the active compounds, and the products were characterized by HPLC and LC-MS. Two of the analogues gave steady-state kinetic parameters (kcat and Km) very similar to those of the natural substrate.

Detailed map of oxidative post-translational modifications of human p21ras using Fourier transform mass spectrometry

P21ras, the translation product of the most commonly mutated oncogene, is a small guanine nucleotide exchange protein. Oxidant-induced post-translational modifications of p21ras including S-nitrosation and S-glutathiolation have been demonstrated to modulate its activity. Structural characterization of this protein is critical to further understanding of the biological functions of p21ras. In this study, high-resolution and high mass accuracy Fourier transform mass spectrometry was utilized to map, in detail, the post-translational modifications of p21ras (H-ras) exposed to oxidants by combining bottom-up and top-down techniques. For peroxynitrite-treated p21ras, five oxidized methionines, five nitrated tyrosines, and at least two oxidized cysteines (including C118) were identified by "bottom-up" analysis, and the major oxidative modification of C118, Cys118-SO3H, was confirmed by several tandem mass spectrometry experiments. Additionally, "top-down" analysis was conducted on p21ras S-glutathiolated by oxidized glutathione and identified C118 as the major site of glutathiolation among the four surface cysteines. The present study provides a paradigm for an effective and efficient method not only for mapping post-translational modifications of proteins but also for predicting the relative selectivity and specificity of oxidative post-translational modifications, especially using top-down analysis.

Pds1/Esp1-dependent and -independent sister chromatid separation in mutants defective for protein phosphatase 2A

Spindle disruption or DNA damage prevents sister chromatid separation through the activation of checkpoint pathways that inhibit anaphase entry by stabilizing the anaphase inhibitor Pds1. Mutation of CDC55, which encodes a B regulatory subunit of protein phosphatase 2A (PP2A), results in precocious sister chromatid separation when spindle is disrupted. Here we report that decreased Pds1 levels in Deltacdc55 mutants contribute to sister chromatid separation in the presence of nocodazole, a microtubule-depolymerizing drug. However, in the presence of DNA damage, Deltacdc55 mutant cells separate sister chromatids without noticeable decrease of Pds1 or cohesin Mcd1/Scc1 levels. Further analysis demonstrates that Deltacdc55 mutants lose cohesion along the entire chromosomes when the spindle is disrupted. In contrast, separation of sister chromatids is limited to the centromeric regions in Deltacdc55 cells after DNA damage. Moreover, mutation of TPD3, which encodes the A regulatory subunit of PP2A, also results in sister chromatid separation in DNA- or spindle-damage-arrested cells. These data suggest that PP2A regulates sister chromatid cohesion in Pds1-dependent and -independent manners.

Chromosome segregation in fission yeast with mutations in the tubulin folding cofactor D

Faithful chromosome segregation requires the combined activities of the microtubule-based mitotic spindle and the multiple proteins that form mitotic kinetochores. Here, we show that the fission yeast mitotic mutant, tsm1-512, is an allele of the tubulin folding chaperone, cofactor D. Chromosome segregation in this and in an additional cofactor D mutant depends on growth conditions that are monitored specifically by the mitotic checkpoint proteins Mad1, 2, 3 and Bub3. The temperature-sensitive mutants we have used disrupt the function of cofactor D to different extents, but both strains form a mitotic spindle in which the poles separate in anaphase. However, chromosome segregation is often unequal, apparently due to a defect in kinetochore-microtubule interactions. Mutations in cofactor D render cells particularly sensitive to the expression levels of a CENP-B-like protein, Abp1p, which works as an allele-specific, high-copy suppressor of cofactor D. This and other genetic interactions between cofactor D mutants and specific kinetochore and spindle components suggest their critical role in establishing the normal kinetochore-microtubule interface.

Mouse Homologue of Skin-specific Retroviral-like Aspartic Protease Involved in Wrinkle Formation

Retroviral proteases are encoded in the retroviral genome and are responsible for maturation and assembly of infectious virus particles. A number of retroviral protease sequences with retroviral elements are integrated in every eukaryotic genome as endogenous retroviruses. Recently, retroviral-like aspartic proteases that were not embedded within endogenous retroviral elements were identified throughout the eukaryotic and prokaryotic genomes. However, the physiological role of this novel protease family, especially in mammals, is not known. During the high throughput in situ hybridization screening of mouse epidermis, as a granular layer-expressing clone, we identified a mouse homologue of SASPase (Skin ASpartic Protease), a recently identified retroviral-like aspartic protease. We detected and purified the endogenous 32-kDa (mSASP32) and 15-kDa (mSASP15) forms of mSASP from mouse stratum corneum extracts and determined their amino acid sequences. Next, we bacterially produced recombinant mSASP15 via autoprocessing of GST-mSASP32. Purified recombinant mSASP15 cleaved a quenched fluorogenic peptide substrate, designed from the autoprocessing site for mSASP32 maximally at pH 5.77, which is close to the pH of the epidermal surface. Finally, we generated mSASP-deficient mice that at 5 weeks of age showed fine wrinkles that ran parallel on the lateral trunk without apparent epidermal differentiation defects. These results indicate that the retroviral-like aspartic protease, SASPase, is involved in prevention of fine wrinkle formation via activation in a weakly acidic stratum corneum environment. This study provides the first evidence that retroviral-like aspartic protease is functionally important in mammalian tissue organization.

A mitotic topoisomerase II checkpoint in budding yeast is required for genome stability but acts independently of Pds1/securin

Topoisomerase II (Topo II) performs topological modifications on double-stranded DNA molecules that are essential for chromosome condensation, resolution, and segregation. In mammals, G2 and metaphase cell cycle delays induced by Topo II poisons have been proposed to be the result of checkpoint activation in response to the catenation state of DNA. However, the apparent lack of such controls in model organisms has excluded genetic proof that Topo II checkpoints exist and are separable from the conventional DNA damage checkpoint controls. But here, we define a Topo II-dependent G2/M checkpoint in a genetically amenable eukaryote, budding yeast, and demonstrate that this checkpoint enhances cell survival. Conversely, a lack of the checkpoint results in aneuploidy. Neither DNA damage-responsive pathways nor Pds1/securin are needed for this checkpoint. Unusually, spindle assembly checkpoint components are required for the Topo II checkpoint, but checkpoint activation is not the result of failed chromosome biorientation or a lack of spindle tension. Thus, compromised Topo II function activates a yeast checkpoint system that operates by a novel mechanism.

Regulation of the cell cycle by protein phosphatase 2A in Saccharomyces cerevisiae

Protein phosphatase 2A (PP2A) has long been implicated in cell cycle regulation in many different organisms. In the yeast Saccharomyces cerevisiae, PP2A controls cell cycle progression mainly through modulation of cyclin-dependent kinase (CDK) at the G(2)/M transition. However, CDK does not appear to be a direct target of PP2A. PP2A affects CDK activity through its roles in checkpoint controls. Inactivation of PP2A downregulates CDK by activating the morphogenesis checkpoint and, consequently, delays mitotic entry. Defects in PP2A also compromise the spindle checkpoint and predispose the cell to an error-prone mitotic exit. In addition, PP2A is involved in controlling the G(1)/S transition and cytokinesis. These findings suggest that PP2A functions in many stages of the cell cycle and its effect on cell cycle progression is pleiotropic.

Non-SELEX selection of aptamers

Aptamers are typically selected from libraries of random DNA (or RNA) sequences by SELEX, which involves multiple rounds of alternating steps of partitioning and PCR amplification. Here we report, for the first time, non-SELEX selection of aptamers-a process that involves repetitive steps of partitioning with no amplification between them. A highly efficient affinity method, non-equilibrium capillary electrophoresis of equilibrium mixtures (NECEEM), was used for partitioning. We found that three steps of NECEEM-based partitioning in the non-SELEX approach were sufficient to improve the affinity of a DNA library to a target protein by more than 4 orders of magnitude. The resulting affinity was higher than that of the enriched library obtained in three rounds of NECEEM-based SELEX. Remarkably, NECEEM-based non-SELEX selection took only 1 h in contrast to several days or several weeks required for a typical SELEX procedure by conventional partitioning methods. In addition, NECEEM-based non-SELEX allowed us to accurately measure the abundance of aptamers in the library. Not only does this work introduce an extremely fast and economical method for aptamer selection, but it also suggests that aptamers may be much more abundant than they are thought to be. Finally, this work opens the opportunity for selection of drug candidates from libraries of small molecules, which cannot be PCR-amplified and thus are not approachable by SELEX.

Chemoprevention of squamous cell carcinoma of the oral cavity

Squamous cell carcinoma of the oral cavity has long been seen as an attractive candidate for chemoprevention strategies. Because of the poor out-comes associated with the disease, the presence of identifiable premalignant lesions, and the failure of local preventive therapies, such as surgery, many investigators have hoped to find an effective chemopreventive compound. Initial enthusiasm surrounding high-dose retinoids gave way to concerns regarding toxicity and short duration of response. Although many of the other agents discussed above have shown promise, as yet none have been proven safe and effective in large-scale randomized trials. Much has been learned,however, about the molecular process of oral carcinogenesis from studies of these agents. Ongoing and future studies of chemopreventive agents in oral cancer hopefully will be able to exploit our expanding knowledge of these molecular pathways.

Lipid Modifications of a Ras Peptide Exhibit Altered Packing and Mobility versus Host Membrane as Detected by 2H Solid-State NMR

The human N-ras protein binds to cellular membranes by insertion of two covalently bound posttranslational lipid modifications, which is crucial for its function in signal transduction and cell proliferation. Mutations in ras may lead to unregulated cell growth and eventually cancer, making it an important therapeutic target. Here we have investigated the molecular details of the membrane binding mechanism. A heptapeptide derived from the C-terminus of the human N-ras protein was synthesized including two hexadecyl modifications. Solid-state 2H NMR was used to determine the packing and molecular dynamics of the ras lipid chains as well as the phospholipid matrix. Separately labeling the chains of the peptide and the phospholipids with 2H enabled us to obtain atomically resolved parameters relevant to their structural dynamics. While the presence of ras only marginally affected the packing of DMPC membranes, dramatically lower order parameters (S(CD)) were observed for the ras acyl chains indicating modified packing properties. Essentially identical projected lengths of the 16:0 ras chains and the 14:0 DMPC chains were found, implying that the polypeptide backbone is located at the lipid-water interface. Dynamical properties of both the ras and phospholipid chains were determined from spin-lattice 2H relaxation (R1Z) measurements. Plots of R1Z rates versus the corresponding squared segmental order parameters revealed striking differences. We propose the ras peptide is confined to microdomains containing DMPC chains which are in exchange with the bulk bilayer on the 2H NMR time scale (approximately 10(-5) s). Compared to the host DMPC matrix, the ras lipid modifications are extremely flexible and undergo relatively large amplitude motions. It is hypothesized that this flexibility is a requirement for the optimal anchoring of lipid-modified proteins to cellular membranes.

EVI5 is a novel centrosomal protein that binds to alpha- and gamma-tubulin

The human EVI5 protein carries a TBC domain indicative of Rab GTPase activating protein (GAP) activity, and an extensive coiled-coil motif in the C-terminal region. EVI5 is ubiquitously expressed in adult, fetal, and cancer tissues and exists as two mRNA species resulting from differential use of polyadenylation signals. Western blot analysis suggests that different molecular weight protein species are probably generated by posttranslational modification. FPLC analysis demonstrates that EVI5 protein can form dimers and confocal microscopy indicates that EVI5, in addition to a diffuse localization in the nucleus, also preferentially localizes to the pericentriolar material in interphase cells. Immunoprecipitation and GST pull-down experiments demonstrate that EVI5 exists in complexes with both alpha- and gamma-tubulin. Both interactions are localized to the N-terminal part of the EVI5 protein. Thus, EVI5 is a novel centrosomal protein with a complex expression pattern and subcellular localization, possibly involved in centrosome stability and dynamics.

Novel indication for cancer therapy: Chk1 inhibition sensitizes tumor cells to antimitotics

Paclitaxel (Taxol) is the most-prescribed anti-mitotic agent for a variety of advanced metastatic cancers. It induces mitotic arrest leading to apoptosis through microtubule stabilization. Chk1 is the major cell-cycle checkpoint kinase mediating S- and G2-arrests in response to various DNA-damages. Chk1 inhibitor is anticipated and has been demonstrated to potentiate the cytotoxicity of DNA-damaging agents through abrogation of cell-cycle checkpoints. Paclitaxel does not, however, induce Chk1 activation, and Chk1 has not been shown to function in mitotic checkpoint. Thus, Chk1 inhibitor is not expected to enhance the toxicity of paclitaxel. Here we show that downregulation of Chk1 sensitizes tumor cells to the toxicity of paclitaxel in cell proliferation assay. Fluorescence microscopy showed that Chk1 knockdown augments mitotic catastrophe and apoptosis in paclitaxel-treated cancer cells. Further, we elucidated the mechanism of this sensitization. Chk1 inhibition facilitates paclitaxel-induced M-phase entry by activation of Cdc2 kinase and accumulation of cyclin B1, the required cofactor for Cdc2 kinase activity. Moreover, Chk1 downregulation inhibits M phase exit through induction of the anaphase inhibitor, securin/PDS1. Collectively, Chk1 elimination sustains a more effective mitotic arrest as demonstrated by the more efficient accumulation of M-phase marker phospho-histone H3. We show that Chk1 elimination attenuates the paclitaxel-induced activation of the anti-apoptotic p42/p44 (ERK1/2) MAP kinase pathway, additionally contributing to the sensitization. Our results suggest that in addition to its well-established role as an enforcer of S and G2-checkpoints in response to genotoxic stress, Chk1 also plays a protective role in mitotic checkpoint to lessen mitotic catastrophe and thereby limits cell-death. Therefore Chk1 downregulation can not only potentiate DNA-damaging agents, but also enhance the toxicity of anti-microtubule agents, which significantly broadens its therapeutic applications.

Oral cancer prevention and the evolution of molecular-targeted drug development

The multifaceted rationale for molecular-targeted prevention of oral cancer is strong. Oral cancer is a major global threat to public health, causing great morbidity and mortality rates that have not improved in decades. Oral cancer development is a tobacco-related multistep and multifocal process involving field carcinogenesis and intraepithelial clonal spread. Biomarkers of genomic instability, such as aneuploidy and allelic imbalance, can accurately measure the cancer risk of oral premalignant lesions, or intraepithelial neoplasia (IEN). Retinoid-oral IEN studies (eg, of retinoic acid receptor-beta, p53, genetic instability, loss of heterozygosity, and cyclin D1) have advanced the overall understanding of the biology of intraepithelial carcinogenesis and of preventive agent molecular mechanisms and targets-important advances for monitoring preventive interventions and assessing cancer risk and pharmacogenomics. Clinical management of oral IEN varies from watchful waiting to complete resection, although complete resection does not prevent oral cancer in high-risk patients. New approaches, such as interventions with molecular-targeted agents and agent combinations in molecularly defined high-risk oral IEN patients, are urgently needed to reduce the devastating worldwide consequences of oral cancer.

Abrogation of the Chk1-Pds1 checkpoint leads to tolerance of persistent single-strand breaks in Saccharomyces cerevisiae

In budding yeast, Apn1, Apn2, Tpp1, and Rad1/Rad10 are important enzymes in the removal of spontaneous DNA lesions. apn1 apn2 rad1 yeast are inviable due to accumulation of abasic sites and strand breaks with 3' blocking lesions. We found that tpp1 apn1 rad1 yeast exhibited slow growth but frequently gave rise to spontaneous slow growth suppressors that segregated as single-gene mutations. Using a candidate gene approach, we identified several tpp1 apn1 rad1 suppressors. Deleting uracil glycosylase suppressed both tpp1 apn1 rad1 and apn1 apn2 rad1 growth defects by reducing the abasic site burden. Mutants affecting the Chk1-Pds1 metaphase-anaphase checkpoint only suppressed tpp1 apn1 rad1 slow growth. In contrast, most S-phase checkpoint mutants were synthetically lethal in a tpp1 apn1 rad1 background. Epistasis analyses showed an additive effect between chk1 and ung1, indicating different mechanisms of suppression. Loss of Chk1 partially restored cell-growth parameters in tpp1 apn1 rad1 yeast, but at the same time exacerbated chromosome instability. We propose a model in which recombinational repair during S phase coupled with failure of the metaphase-anaphase checkpoint allows for tolerance of persistent single-strand breaks at the expense of genome stability.

Characterization of a Saccharomyces cerevisiae thermosensitive lytic mutant leads to the identification of a new allele of the NUD1 gene

To improve our understanding of the factors involved in the osmotic stability of yeast cells, a search for novel conditional Saccharomyces cerevisiae cell lysis mutants was performed. Ten temperature-sensitive (ts) mutant strains of S. cerevisiae were isolated that lyse at the restrictive temperature on hypotonic, but not on osmotically supported medium. The ten mutants fell into four complementation groups: ts1 to ts4. To clone the wild-type gene corresponding to the ts4 mutation, a strategy aimed at complementing the thermosensitive phenotype-using low-copy and high-copy DNA libraries--was followed, but only two extragenic suppressors were identified. Another approach, in which classic genetic methods were combined with the use of yeast artificial chromosomes and traditional cloning procedures, allowed the identification of the NUD1 gene--which codes for a component of the spindle-pole body-as the wild-type gene corresponding to the ts4 mutation. Cloning and sequencing of the defective allele from the chromosome of the mutant cells resulted in the identification of a point mutation that produces a single amino acid change in the protein: a Gly-to-Glu change at position 585 (the nud1-G585E allele). Further analysis revealed that cells carrying this allele show a thermosensitive growth defect. At the restrictive temperature, the cells arrest with large buds, elongated spindles, and duplicated nuclei. In addition, with longer incubation times they are unable to maintain cellular integrity and lyse. Our results have allowed the identification of the first single amino acid mutation in NUD1, and suggest a link between cell cycle progression and cellular integrity.

The yeast rRNA biosynthesis factor Ebp2p is also required for efficient nuclear division

Molecular genetic analysis of the yeast Ebp2 protein has revealed that it is an essential, nucleolar protein that functions in the rRNA biosynthesis pathway. Temperature-sensitive ebp2-1 mutants are defective in the processing of the 27 SA precursor rRNA, and the point substitutions that disrupt this activity cluster towards the central, more highly conserved region of the Ebp2 protein. We report here that other ebp2 mutants exhibit deficiencies associated with defects in chromosome segregation. Yeast cells bearing a 50 amino acid C-terminal truncation allele (ebp2 delta C50) display a slow-growth phenotype and exhibit an increased percentage of cells with the nucleus positioned at the bud neck. The ebp2-1 and ebp2 delta C50 alleles genetically complement each other, and ebp2 delta C50 mutants exhibit nuclear division defects that are distinct from the rRNA biosynthesis-related phenotypes of ebp2-1 mutants. Cytological and FACS analysis of the ebp2 delta C50 deletion mutants indicate that the chromosome segregation related activities of the Ebp2 protein are monitored by Mad2p, a mitotic checkpoint protein. The finding that yeast Ebp2p functions in nuclear division is consistent with the growing body of evidence that supports the role that human EBP2 plays in chromosome segregation.

HTLV-I Tax directly binds the Cdc20-associated anaphase-promoting complex and activates it ahead of schedule

Expression of the human T lymphotropic virus type I (HTLV-I) transactivator/oncoprotein, Tax, leads to faulty mitosis as reflected by chromosome aneuploidy, cytokinesis failure, and formation of micro- and multinucleated cells. Here we show that HTLV-I-transformed T cells progress through S/G(2)/M phases of the cell cycle with a delay. This delay is correlated with a decrease in the levels of cyclin A, cyclin B1, and securin. In tax-expressing cells, the Cdc20-associated anaphase promoting complex (APC(Cdc20)), an E3 ubiquitin ligase that controls metaphase to anaphase transition, becomes active before cellular entry into mitosis as evidenced by premature cyclin B1 polyubiquitination and degradation during S/G(2). Consistent with the notion that Tax activates APC(Cdc20) directly, Tax is found to coimmunoprecipitate with Cdc20 and Cdc27/APC3. The APC(Cdc20) activity prematurely activated by Tax remains sensitive to spindle checkpoint inhibition. Unscheduled activation of APC(Cdc20) by Tax provides an explanation for the mitotic abnormalities in HTLV-I-infected cells and is likely to play an important role in the development of adult T cell leukemia.

A spindle checkpoint functions during mitosis in the early Caenorhabditis elegans embryo

During mitosis, chromosome segregation is regulated by a spindle checkpoint mechanism. This checkpoint delays anaphase until all kinetochores are captured by microtubules from both spindle poles, chromosomes congress to the metaphase plate, and the tension between kinetochores and their attached microtubules is properly sensed. Although the spindle checkpoint can be activated in many different cell types, the role of this regulatory mechanism in rapidly dividing embryonic animal cells has remained controversial. Here, using time-lapse imaging of live embryonic cells, we show that chemical or mutational disruption of the mitotic spindle in early Caenorhabditis elegans embryos delays progression through mitosis. By reducing the function of conserved checkpoint genes in mutant embryos with defective mitotic spindles, we show that these delays require the spindle checkpoint. In the absence of a functional checkpoint, more severe defects in chromosome segregation are observed in mutants with abnormal mitotic spindles. We also show that the conserved kinesin CeMCAK, the CENP-F-related proteins HCP-1 and HCP-2, and the core kinetochore protein CeCENP-C all are required for this checkpoint. Our analysis indicates that spindle checkpoint mechanisms are functional in the rapidly dividing cells of an early animal embryo and that this checkpoint can prevent chromosome segregation defects during mitosis.

Meiosis: cell-cycle controls shuffle and deal

Meiosis is the type of cell division that gives rise to eggs and sperm. Errors in the execution of this process can result in the generation of aneuploid gametes, which are associated with birth defects and infertility in humans. Here, we review recent findings on how cell-cycle controls ensure the coordination of meiotic events, with a particular focus on the segregation of chromosomes.

What Criteria for the Definition of Oocyte Quality?

Although the spermatozoon provides an essential contribution to the generation of a new individual, the developmental fate of the embryo is principally dictated by the oocyte. Oocyte competencies are acquired throughout oogenesis, via the interaction with somatic cells. The ability to reinitiate the meiotic process and undergo preimplantation development is progressively determined during the antral phase. It is known that these changes involve the nuclear and cytoplasmic compartments, respectively, but the underlying cellular and molecular mechanisms are still poorly understood. Analysis of various aspects of oocyte morphology (cytoplasm, zona pellucida, and polar body) via conventional phase-contrast microscopy has generated contrasting evidence on the possibility of establishing reliable criteria for the prediction of developmental potential. The introduction of a newly developed microscopy technique based on the detection of polarized light generated by birefringent cell structures has offered the possibility of visualizing noninvasively the meiotic spindle, whose presence is critical for fertilization and later developmental stages. However, further studies are needed to standardize and interpret the information accessible through such a technique. Although unable to preserve cell viability and therefore provide a method by which to select oocytes with superior developmental competence, invasive techniques can make a fundamental contribution to defining objective criteria of oocyte quality. In particular, immunofluorescence analysis, which is able to identify critical anomalies of the meiotic spindle and cytoskeleton organization that can account for oocyte quality, is an important method for assessing the efficiency of in vitro maturation systems.

Mitotic checkpoint function in the formation of gross chromosomal rearrangements in Saccharomyces cerevisiae

The accumulation of gross chromosomal rearrangements (GCRs) is characteristic of cancer cells. Multiple pathways that prevent GCRs, including S-phase cell cycle checkpoints, homologous recombination, telomere maintenance, suppression of de novo telomere addition, chromatin assembly, and mismatch repair, have been identified in Saccharomyces cerevisiae. However, pathways that promote the formation of GCRs are not as well understood. Of these, the de novo telomere addition pathway and nonhomologous end-joining are the best characterized. Here, we demonstrate that defects in the mitotic checkpoint and the mitotic exit network can suppress GCRs in strains containing defects that increase the GCR rate. These data suggest that functional mitotic checkpoints can play a role in the formation of genome rearrangements.

Novel management of oral cancer: a paradigm of predictive oncology

The rationale for molecular-targeted prevention of oral cancer is strong. Oral cancer is a major global threat to public health with 300,000 new cases diagnosed worldwide on an annual basis. Notably, the great morbidity and mortality rates of this devastating disease have not improved in decades. Oral cancer development is a tobacco-related multistep and multifocal process involving field carcinogenesis and intraepithelial clonal spread. Biomarkers of genomic instability, such as aneuploidy and allelic imbalance, can accurately measure the cancer risk of oral premalignant lesions or intraepithelial neoplasia (IEN). Retinoid-oral IEN studies (e.g., retinoid acid receptor-beta, p53, genetic instability, loss of heterozygosity, and cyclin D1) have advanced the overall understanding of the biology of intraepithelial carcinogenesis and preventive agent molecular mechanisms and targets, important advances for monitoring preventive interventions, assessing cancer risk, and pharmacogenomics. Clinical management of oral IEN varies from watchful waiting to complete resection, although complete resection does not prevent oral cancer in high-risk patients. New approaches, such as interventions with molecular-targeted agents and agent combinations in molecularly defined high-risk oral IEN patients, are urgently needed to reduce the devastating worldwide consequences of oral cancer.

The Ras/protein kinase A pathway acts in parallel with the Mob2/Cbk1 pathway to effect cell cycle progression and proper bud site selection

In Saccharomyces cerevisiae, Ras proteins connect nutrient availability to cell growth through regulation of protein kinase A (PKA) activity. Ras proteins also have PKA-independent functions in mitosis and actin repolarization. We have found that mutations in MOB2 or CBK1 confer a slow-growth phenotype in a ras2Delta background. The slow-growth phenotype of mob2Delta ras2Delta cells results from a G1 delay that is accompanied by an increase in size, suggesting a G1/S role for Ras not previously described. In addition, mob2Delta strains have imprecise bud site selection, a defect exacerbated by deletion of RAS2. Mob2 and Cbk1 act to properly localize Ace2, a transcription factor that directs daughter cell-specific transcription of several genes. The growth and budding phenotypes of the double-deletion strains are Ace2 independent but are suppressed by overexpression of the PKA catalytic subunit, Tpk1. From these observations, we conclude that the PKA pathway and Mob2/Cbk1 act in parallel to determine bud site selection and promote cell cycle progression.

Dynactin is involved in a checkpoint to monitor cell wall synthesis in Saccharomyces cerevisiae

Checkpoint controls ensure the completion of cell cycle events with high fidelity in the correct order. Here we show the existence of a novel checkpoint that ensures coupling of cell wall synthesis and mitosis. In response to a defect in cell wall synthesis, S. cerevisiae cells arrest the cell-cycle before spindle pole body separation. This arrest results from the regulation of the M-phase cyclin Clb2p at the transcriptional level through the transcription factor Fkh2p. Components of the dynactin complex are required to achieve the G2 arrest whilst keeping cells highly viable. Thus, the dynactin complex has a function in a checkpoint that monitors cell wall synthesis.

Capillary high-performance liquid chromatography/mass spectrometric analysis of proteins from affinity-purified plasma membrane

Proteomics analysis of plasma membranes is a potentially powerful strategy for the discovery of proteins involved in membrane remodeling under diverse cellular environments and identification of disease-specific membrane markers. A key factor for successful analysis is the preparation of plasma membrane fractions with low contamination from subcellular organelles. Here we report the characterization of plasma membrane prepared by an affinity-purification method, which involves biotinylation of cell-surface proteins and subsequent affinity enrichment with strepavidin beads. Western blotting analysis showed this method was able to achieve a 1600-fold relative enrichment of plasma membrane versus mitochondria and a 400-fold relative enrichment versus endoplasmic reticulum, two major contaminants in plasma membrane fractions prepared by conventional ultracentrifugation methods. Capillary-HPLC/MS analysis of 30 microg of affinity-purified plasma membrane proteins led to the identification of 918 unique proteins, which include 16.4% integral plasma membrane proteins and 45.5% cytosol proteins (including 8.6% membrane-associated proteins). Notable among the identified membrane proteins include 30 members of ras superfamily, receptors (e.g., EGF receptor, integrins), and signaling molecules. The low number of endoplasmic reticulum and mitochondria proteins (approximately 3.3% of the total) suggests the plasma membrane preparation has minimum contamination from these organelles. Given the importance of integral membrane proteins for drug design and membrane-associated proteins in the regulation cellular behaviors, the described approach will help expedite the characterization of plasma membrane subproteomes, identify signaling molecules, and discover therapeutic membrane-protein targets in diseases.

The role of Cdh1p in maintaining genomic stability in budding yeast

Cdh1p, a substrate specificity factor for the cell cycle-regulated ubiquitin ligase, the anaphase-promoting complex/cyclosome (APC/C), promotes exit from mitosis by directing the degradation of a number of proteins, including the mitotic cyclins. Here we present evidence that Cdh1p activity at the M/G(1) transition is important not only for mitotic exit but also for high-fidelity chromosome segregation in the subsequent cell cycle. CDH1 showed genetic interactions with MAD2 and PDS1, genes encoding components of the mitotic spindle assembly checkpoint that acts at metaphase to prevent premature chromosome segregation. Unlike cdh1delta and mad2delta single mutants, the mad2delta cdh1delta double mutant grew slowly and exhibited high rates of chromosome and plasmid loss. Simultaneous deletion of PDS1 and CDH1 caused extensive chromosome missegregation and cell death. Our data suggest that at least part of the chromosome loss can be attributed to kinetochore/spindle problems. Our data further suggest that Cdh1p and Sic1p, a Cdc28p/Clb inhibitor, have overlapping as well as nonoverlapping roles in ensuring proper chromosome segregation. The severe growth defects of both mad2delta cdh1delta and pds1delta cdh1dDelta strains were rescued by overexpressing Swe1p, a G(2)/M inhibitor of the cyclin-dependent kinase, Cdc28p/Clb. We propose that the failure to degrade cyclins at the end of mitosis leaves cdh1delta mutant strains with abnormal Cdc28p/Clb activity that interferes with proper chromosome segregation.

DAP-like kinase, a member of the death-associated protein kinase family, associates with centrosomes, centromers, and the contractile ring during mitosis

DAP-like kinase (Dlk) is a nuclear serine/threonine-specific kinase which has been implicated in apoptosis. However, induction of apoptosis by Dlk requires its relocation to the cytoplasm, particularly association with the actin cytoskeleton, which is achieved through interaction with pro-apoptotic protein Par-4. On the other hand, nuclear Dlk does not induce apoptosis and has rather been implicated in transcription. To further explore the biological functions of Dlk, we established a cell clone of MCF-7 cells stably expressing a GFP-Dlk fusion protein at low level. Ectopic expression of GFP-Dlk did not affect the growth properties of the cells. During interphase, GFP-Dlk showed a diffuse nuclear distribution with punctate staining in a subpopulation of cells. During mitosis, however, Dlk was associated with centrosomes, centromeres, and the contractile ring, but not with the mitotic spindle. Association with centrosomes, as confirmed by colocalization with gamma-tubulin and pericentrin persisted throughout mitosis but was also seen in interphase cells. Interestingly, GFP-Dlk and gamma-tubulin could be co-immunoprecipitated indicating that they are present in the same protein complex. Association of Dlk with centromeres, as verified by confocal fluorescence microscopy with centromere-specific antibodies was more restricted and discernable from prophase to early anaphase. Centromere association of Dlk coincides with H3 phosphorylation at Thr11 that is specifically phosphorylated by Dlk in vitro (U. Preuss, G. Landsberg, K. H. Scheidtmann, Nucleic Acids Res. 31, 878-885, 2003). During cytokinesis, Dlk was enriched in the contractile acto-myosin ring and colocalized with Ser19-phosphorylated myosin light chain, which is an in vitro substrate of Dlk. Strikingly, a C-terminal truncation mutant of Dlk generated multi-nucleated cells. Together, these data suggest that Dlk participates in regulation and, perhaps, coordination of mitotis and cytokinesis.

Cdc14 and condensin control the dissolution of cohesin-independent chromosome linkages at repeated DNA

Chromosome segregation is triggered by the cleavage of cohesins by separase. Here we show that in budding yeast separation of the ribosomal DNA (rDNA) and telomeres also requires Cdc14, a protein phosphatase known for its role in mitotic exit. Cdc14 shares this role with the FEAR network, which activates Cdc14 during early anaphase, but not the mitotic exit network, which promotes Cdc14 activity during late anaphase. We further show that CDC14 is necessary and sufficient to promote condensin enrichment at the rDNA locus and to trigger rDNA segregation in a condensin-dependent manner. We propose that Cdc14 released by the FEAR network mediates the partitioning of rDNA by facilitating the localization of condensin thereto. This dual role of the FEAR network in initiating mitotic exit and promoting chromosome segregation ensures that exit from mitosis is coupled to the completion of chromosome segregation.