Feedback control of mitosis in budding yeast

Chromosome segregation and cancer: cutting through the mystery

Mitosis is the most dramatic--and potentially dangerous--event in the cell cycle, as sister chromatids are irreversibly segregated to daughter cells. Defects in the checkpoints that normally maintain the fidelity of this process can lead to chromosomal instability (CIN) and cancer. However, CIN--a driving force of tumorigenesis--could be the cancer cell's ultimate vulnerability. An important goal is to identify novel anticancer compounds that directly target the mitotic errors at the heart of CIN.

[When chromosomal dynamics control cell division]

In most tumor cells a chromosomal instability leads to an abnormal chromosome number (aneuploidy). The mitotic checkpoint is essential for ensuring accurate chromosome segregation by allowing mitotic delay in response to a spindle defect. This checkpoint delays the onset of anaphase until all the chromosomes are correctly aligned on the mitotic spindle. When unattached kinetochores are present, the metaphase/anaphase transition is not allowed and the time available for chromosome-microtubule capture increases. Genes required for this delay were first identified in Saccharomyces cerevisiae (the MAD, BUB and MPS1 genes) and subsequently, homologs have been identified in higher eucaryotes showing that the spindle checkpoint pathway is highly conserved. The checkpoint functions by preventing an ubiquitin ligase called the anaphase-promoting complex/cyclosome (APC) from ubiquitinylating proteins whose destruction is required for anaphase onset.

Checkpoint inhibition of the APC/C in HeLa cells is mediated by a complex of BUBR1, BUB3, CDC20, and MAD2

The mitotic checkpoint prevents cells with unaligned chromosomes from prematurely exiting mitosis by inhibiting the anaphase-promoting complex/cyclosome (APC/C) from targeting key proteins for ubiquitin-mediated proteolysis. We have examined the mechanism by which the checkpoint inhibits the APC/C by purifying an APC/C inhibitory factor from HeLa cells. We call this factor the mitotic checkpoint complex (MCC) as it consists of hBUBR1, hBUB3, CDC20, and MAD2 checkpoint proteins in near equal stoichiometry. MCC inhibitory activity is 3,000-fold greater than that of recombinant MAD2, which has also been shown to inhibit APC/C in vitro. Surprisingly, MCC is not generated from kinetochores, as it is also present and active in interphase cells. However, only APC/C isolated from mitotic cells was sensitive to inhibition by MCC. We found that the majority of the APC/C in mitotic lysates is associated with the MCC, and this likely contributes to the lag in ubiquitin ligase activity. Importantly, chromosomes can suppress the reactivation of APC/C. Chromosomes did not affect the inhibitory activity of MCC or the stimulatory activity of CDC20. We propose that the preformed interphase pool of MCC allows for rapid inhibition of APC/C when cells enter mitosis. Unattached kinetochores then target the APC/C for sustained inhibition by the MCC.

The ZW10 and Rough Deal checkpoint proteins function together in a large, evolutionarily conserved complex targeted to the kinetochore

The zeste-white 10 (zw10) and rough deal (rod) genes of Drosophila both encode kinetochore components, and mutations in either gene greatly increase the missegregation of sister chromatids during mitosis. Here, we present genetic, cytological and biochemical evidence for a close, evolutionarily conserved relationship between the ROD and ZW10 proteins. We show that the phenotypes caused by disruption of either gene’s function are similar in Drosophila and in C. elegans. No additive effects are observed in zw10; rod double null mutants. In flies, the two proteins always colocalize and, moreover, require each other for their recruitment to the mitotic apparatus. The human ROD and ZW10 homologs also colocalize on HeLa cell kinetochores or kinetochore microtubules throughout most but not all of mitosis. Finally, we show that in both Drosophila and human cells, ROD and ZW10 are in fact physically associated, and in Drosophila these proteins are together constituents of a large (700-900 kDa), soluble macromolecular complex.

The art and design of genetic screens: yeast

Understanding the biology of complex systems is facilitated by comparing them with simpler organisms. Budding and fission yeasts provide ideal model systems for eukaryotic cell biology. Although they differ from one another in terms of a range of features, these yeasts share powerful genetic and genomic tools. Classical yeast genetics remains an essential element in discovering and characterizing the genes that make up a eukaryotic cell.

Mitotic checkpoint protein hsMAD2 as a marker predicting liver metastasis of human gastric cancers

hsMAD2, the human homologue of mitotic arrest deficient 2 (MAD2), is a key component of the mitotic checkpoint system. Recently, mutations and decreased expression of mitotic checkpoint genes including hsMAD2 have been reported in cancer cell lines with defective mitotic checkpoint. However, the genetic alterations in the genomic hsMAD2 gene have not been determined in gastric cancers. Moreover, the biological implications of the overexpressed hsMAD2 in primary cancers are unknown. In this study, we analyzed 32 primary gastric cancers with polymerase chain reaction (PCR) amplification of all exons, including flanking intronic sequences, of the genomic hsMAD2 gene followed by direct DNA sequencing. We also measured the hsMAD2 protein levels in cancer and normal tissues by semi-quantitative immunoblotting. No mutations were found in the coding sequences, although three single nucleotide polymorphisms (SNPs) were identified in the noncoding sequences in 13 of 32 patients. These SNPs were not associated with either hsMAD2 expression or disease progression. The semi-quantitative western blot analysis showed hsMAD2 was significantly overexpressed in gastric cancer tissues compared with corresponding normal tissues (P < 0.001). The calculated ratio of the hsMAD2 protein in cancer tissue (C) to that in corresponding normal tissue (N) (C / N ratio) was significantly higher in patients with well differentiated adenocarcinoma (P = 0.0274) or with synchronous liver metastasis (P = 0.0025). A C / N ratio greater than 3 was observed more frequently in patients with synchronous liver metastasis. Therefore, C / N ratio > 3 may be clinically important as a predictive indicator for metachronous liver metastasis of gastric cancers.

Evolutionary conservation between budding yeast and human kinetochores

Accurate chromosome segregation during mitosis requires the correct assembly of kinetochores--complexes of centromeric DNA and proteins that link chromosomes to spindle microtubules. Studies on the kinetochore of the budding yeast Saccharomyces cerevisiae have revealed functionally novel components of the kinetochore and its regulatory complexes, some of which are highly conserved in humans.

Mad2-Independent inhibition of APCCdc20 by the mitotic checkpoint protein BubR1

The mitotic checkpoint blocks the activation of the anaphase-promoting complex (APC) until all sister chromatids have achieved bipolar attachment to the spindle. A checkpoint complex containing BubR1 and Bub3 has been purified from mitotic human cells. Upon checkpoint activation, the BubR1-Bub3 complex interacts with Cdc20. In the absence of Mad2, BubR1 inhibits the activity of APC by blocking the binding of Cdc20 to APC. Surprisingly, the kinase activity of BubR1 is not required for the inhibition of APCCdc20. BubR1 also prevents the activation of APCCdc20 in Xenopus egg extracts, and restores the mitotic arrest in Cdc20-overexpressing cells treated with nocodazole. Because BubR1 also interacts with the mitotic motor CENP-E, the ability of BubR1 to inhibit APC may be regulated by kinetochore tension or occupancy.

Identification of an overlapping binding domain on Cdc20 for Mad2 and anaphase-promoting complex: model for spindle checkpoint regulation

Activation of the anaphase-promoting complex (APC) is required for anaphase initiation and for exit from mitosis in mammalian cells. Cdc20, which specifically recognizes APC substrates involved in the metaphase-to-anaphase transition, plays a pivotal role in APC activation through direct interaction with the APC. The activation of the APC by Cdc20 is prevented by the interaction of Cdc20 with Mad2 when the spindle checkpoint is activated. Using deletion mutagenesis and peptide mapping, we have identified the sequences in Cdc20 that target it to Mad2 and the APC, respectively. These sequences are distinct but overlapping, providing a possible structural explanation for the internal modulation of the APC-Cdc20 complex by Mad2. In the course of these studies, a truncation mutant of Cdc20 (1-153) that constitutively binds Mad2 but fails to bind the APC was identified. Overexpression of this mutant induces the formation of multinucleated cells and increases their susceptibility to undergoing apoptosis when treated with microtubule-inhibiting drugs. Our experiments demonstrate that disruption of the Mad2-Cdc20 interaction perturbs the mitotic checkpoint, leading to premature activation of the APC, sensitizing the cells to the cytotoxic effects of microtubule-inhibiting drugs.

Conditional mutations in gamma-tubulin reveal its involvement in chromosome segregation and cytokinesis

gamma-Tubulin is a conserved essential protein required for assembly and function of the mitotic spindle in humans and yeast. For example, human gamma-tubulin can replace the gamma-tubulin gene in Schizosaccharomyces pombe. To understand the structural/functional domains of gamma-tubulin, we performed a systematic alanine-scanning mutagenesis of human gamma-tubulin (TUBG1) and studied phenotypes of each mutant allele in S. pombe. Our screen, both in the presence and absence of the endogenous S. pombe gamma-tubulin, resulted in 11 lethal mutations and 12 cold-sensitive mutations. Based on structural mapping onto a homology model of human gamma-tubulin generated by free energy minimization, all deleterious mutations are found in residues predicted to be located on the surface, some in positions to interact with alpha- and/or beta-tubulins in the microtubule lattice. As expected, one class of tubg1 mutations has either an abnormal assembly or loss of the mitotic spindle. Surprisingly, a subset of mutants with abnormal spindles does not arrest in M phase but proceeds through anaphase followed by abnormal cytokinesis. These studies reveal that in addition to its previously appreciated role in spindle microtubule nucleation, gamma-tubulin is involved in the coordination of postmetaphase events, anaphase, and cytokinesis.

Molecular analyses of the mitotic checkpoint components hsMAD2, hBUB1 and hBUB3 in human cancer

During the metaphase-anaphase transition, the spindle checkpoint prevents segregation of chromosomes if the spindle assembly is perturbed. Critical components of this checkpoint are the MAD and BUB families of proteins, which prevent the proteolysis of Pds1 and B cyclins, producing mitotic arrest. In the present study, we first intended to resolve the role of the hsMAD2 gene in human cancer by determining the potential presence of hsMAD2 mutations in 44 primary bladder tumors, 42 soft-tissue sarcomas and 10 hepatocellular carcinomas. The entire coding region of the hsMAD2 gene was analyzed using PCR-SSCP and sequencing. One of the bladder tumor samples showed a point mutation consisting of a transition, ATC-->GTC (Ile-->Val) in codon 190 of hsMAD2. However, no differences were found in the mitotic arrest between cells transfected with mutant and wild-type MAD2 cDNA. We also identified mobility shifts in hsMAD2 in both normal and tumor DNA in 3 bladder tumors, 3 soft-tissue sarcomas and 1 hepatocellular carcinoma, consistent with a polymorphism at codon 143, CCA-->CCG (Pro-->Pro). Another polymorphism was identified in a hepatocellular carcinoma case at codon 22, GAG-->GAA (Glu-->Glu). In addition, a subgroup of 67 primary tumors was analyzed by Southern blot hybridization. No deletion or visible re-arrangements were detected by comparing tumor and normal DNA band signals. Two other important components of the spindle mitotic checkpoint, hBUB1 and hBUB3, were also screened for mutations: hBUB1 in 43 bladder tumors and 9 bladder cell lines and hBUB3 only in the cell lines. Two polymorphisms were found in hBUB1 at positions 144, CAG-->CAA (Gln-->Gln) in 1 primary tumor and 1 bladder cell line, and 913 (ATC-->ATT, Ile-->Ile) in 1 primary tumor. We did not find sequence alterations in hBUB3. These results suggest that mutations of the hsMAD2, hBUB1 and hBUB3 genes are very rare in bladder tumors and that hsMAD2 alterations are also infrequent in soft-tissue sarcomas and hepatocellular carcinomas.

Evidence that replication fork components catalyze establishment of cohesion between sister chromatids

Accurate chromosome segregation requires that replicated sister chromatids are held together until anaphase, when their "cohesion" is dissolved, and they are pulled to opposite spindle poles by microtubules. Establishment of new cohesion between sister chromatids in the next cell cycle is coincident with replication fork passage. Emerging evidence suggests that this temporal coupling is not just a coincident timing of independent events, but rather that the establishment of cohesion is likely to involve the active participation of replication-related activities. These include PCNA, a processivity clamp for some DNA polymerases, Trf4/Pol final sigma (formerly Trf4/Pol kappa), a novel and essential DNA polymerase, and a modified Replication Factor C clamp--loader complex. Here we describe recent advances in how cohesion establishment is linked to replication, highlight important unanswered questions in this new field, and describe a "polymerase switch" model for how cohesion establishment is coupled to replication fork progression. Building the bridges between newly synthesized sister chromatids appears to be a fundamental but previously unrecognized function of the eukaryotic replication machinery.

Inhibition of Cdh1-APC by the MAD2-related protein MAD2L2: a novel mechanism for regulating Cdh1

Exit from mitosis requires the degradation of regulatory proteins including the mitotic cyclins and securin through ubiquitination by the anaphase promoting complex (APC) bound to Cdc20 or Cdh1. Cdc20-APC is regulated through inhibition by the spindle assembly checkpoint protein MAD2. Knowledge of Cdh1-APC regulation is limited to the phosphorylation-dependent dissociation of Cdh1 from APC. We report a novel means of regulating Cdh1 by the MAD2-related gene, MAD2L2. MAD2L2 specifically binds and inhibits Cdh1-APC, paralleling the effect of MAD2 on Cdc20. We suggest that MAD2L2 and MAD2 inhibit the release of substrates from APC and propose a mechanism of inhibition.

The mouse Mps1p-like kinase regulates centrosome duplication

The yeast Mps1p protein kinase acts in centrosome duplication and the spindle assembly checkpoint. We demonstrate here that a mouse Mps1p ortholog (esk, which we designate mMps1p) regulates centrosome duplication. Endogenous mMps1p and overexpressed GFP-mMps1p localize to centrosomes and kinetochores in mouse cells. Overexpression of GFP-mMps1p causes reduplication of centrosomes during S phase arrest. In contrast, a kinase-deficient mutant blocks centrosome duplication altogether. Control of centrosome duplication by mMps1p requires a known regulator of the process, Cdk2. Inhibition of Cdk2 prevents centrosome reduplication and destabilizes mMps1p, causing its subsequent loss from centrosomes, suggesting that Cdk2 promotes mMps1p's centrosome duplication function by regulating its stability during S phase. Thus, mMps1p, an in vitro Cdk2 substrate, regulates centrosome duplication jointly with Cdk2.

Mps1 is a kinetochore-associated kinase essential for the vertebrate mitotic checkpoint

The mitotic checkpoint acts to inhibit entry into anaphase until all chromosomes have successfully attached to spindle microtubules. Unattached kinetochores are believed to release an activated form of Mad2 that inhibits APC/C-dependent ubiquitination and subsequent proteolysis of components needed for anaphase onset. Using Xenopus egg extracts, a vertebrate homolog of yeast Mps1p is shown here to be a kinetochore-associated kinase, whose activity is necessary to establish and maintain the checkpoint. Since high levels of Mad2 overcome checkpoint loss in Mps1-depleted extracts, Mps1 acts upstream of Mad2-mediated inhibition of APC/C. Mps1 is essential for the checkpoint because it is required for recruitment and retention of active CENP-E at kinetochores, which in turn is necessary for kinetochore association of Mad1 and Mad2.

Fission yeast ch-TOG/XMAP215 homologue Alp14 connects mitotic spindles with the kinetochore and is a component of the Mad2-dependent spindle checkpoint

The TOG/XMAP215-related proteins play a role in microtubule dynamics at its plus end. Fission yeast Alp14, a newly identified TOG/XMAP215 family protein, is essential for proper chromosome segregation in concert with a second homologue Dis1. We show that the alp14 mutant fails to progress towards normal bipolar spindle formation. Intriguingly, Alp14 itself is a component of the Mad2-dependent spindle checkpoint cascade, as upon addition of microtubule-destabilizing drugs the alp14 mutant is incapable of maintaining high H1 kinase activity, which results in securin destruction and premature chromosome separation. Live imaging of Alp14-green fluorescent protein shows that during mitosis, Alp14 is associated with the peripheral region of the kinetochores as well as with the spindle poles. This is supported by ChIP (chromatin immunoprecipitation) and overlapping localization with the kinetochore marker Mis6. An intact spindle is required for Alp14 localization to the kinetochore periphery, but not to the poles. These results indicate that the TOG/XMAP215 family may play a central role as a bridge between the kinetochores and the plus end of pole to chromosome microtubules.

Metaphase arrest with centromere separation in polo mutants of Drosophila

The Drosophila gene polo encodes a conserved protein kinase known to be required to organize spindle poles and for cytokinesis. Here we report two strongly hypomorphic mutations of polo that arrest cells of the larval brain at a point in metaphase when the majority of sister kinetochores have separated by between 20–50% of the total spindle length in intact cells. In contrast, analysis of sister chromatid separation in squashed preparations of cells indicates that some 83% of sisters remain attached. This suggests the separation seen in intact cells requires the tension produced by a functional spindle. The point of arrest corresponds to the spindle integrity checkpoint; Bub1 protein and the 3F3/2 epitope are present on the separated kinetochores and the arrest is suppressed by a bub1 mutation. The mutant mitotic spindles are anastral and have assembled upon centrosomes that are associated with Centrosomin and the abnormal spindle protein (Asp), but neither with γ-tubulin nor CP190. We discuss roles for Polo kinase in recruiting centrosomal proteins and in regulating progression through the metaphase–anaphase checkpoint.

Spindle checkpoint protein Bub1 is required for kinetochore localization of Mad1, Mad2, Bub3, and CENP-E, independently of its kinase activity

The spindle checkpoint inhibits the metaphase to anaphase transition until all the chromosomes are properly attached to the mitotic spindle. We have isolated a Xenopus homologue of the spindle checkpoint component Bub1, and investigated its role in the spindle checkpoint in Xenopus egg extracts. Antibodies raised against Bub1 recognize a 150-kD phosphoprotein at both interphase and mitosis, but the molecular mass is reduced to 140 upon dephosphorylation in vitro. Bub1 is essential for the establishment and maintenance of the checkpoint and is localized to kinetochores, similar to the spindle checkpoint complex Mad1-Mad2. However, Bub1 differs from Mad1-Mad2 in that Bub1 remains on kinetochores that have attached to microtubules; the protein eventually dissociates from the kinetochore during anaphase. Immunodepletion of Bub1 abolishes the spindle checkpoint and the kinetochore binding of the checkpoint proteins Mad1, Mad2, Bub3, and CENP-E. Interestingly, reintroducing either wild-type or kinase-deficient Bub1 protein restores the checkpoint and the kinetochore localization of these proteins. Our studies demonstrate that Bub1 plays a central role in triggering the spindle checkpoint signal from the kinetochore, and that its kinase activity is not necessary for the spindle checkpoint in Xenopus egg extracts.

The mitotic checkpoint protein hBUB3 and the mRNA export factor hRAE1 interact with GLE2p-binding sequence (GLEBS)-containing proteins

The mRNA export factor RAE1 (also called GLE2) and the mitotic checkpoint protein BUB3 share extensive sequence homology in yeast as well as higher eukaryotes, although the biological relevance of their similarity is unclear. Previous work in HeLa cells has shown that human (h)RAE1 binds the nuclear pore complex protein hNUP98 via a short NUP98 motif called GLEBS (for GLE2p-binding sequence). Here we report that the two known binding partners of hBUB3, the mitotic checkpoint proteins hBUB1 and hBUBR1, both carry a region with remarkable similarity to the GLEBS motif of hNUP98. We show that the GLEBS-like motifs of mouse (m)BUB1 and mBUBR1 are sufficient for mBUB3 binding. mBUB3 lacks affinity for the hNUP98 GLEBS, demonstrating its binding specificity for GLEBS motifs of mitotic checkpoint proteins. Interestingly, mRAE1 does not exclusively bind to the GLEBS motif of hNUP98 and can cross-interact with the mBUB1 GLEBS. We show that full-length RAE1 and BUB1 proteins interact in mammalian cells and accumulate both at the kinetochores of prometaphase chromosomes. Our findings demonstrate that GLEBS motifs reside in mammalian nucleoporins and mitotic checkpoint proteins and apparently serve as specific binding sites for either BUB3, RAE1, or both.

Identification of RFC(Ctf18p, Ctf8p, Dcc1p): an alternative RFC complex required for sister chromatid cohesion in S. cerevisiae

We have identified and characterized an alternative RFC complex RFC(Ctf18p, Ctf8p, Dcc1p) that is required for sister chromatid cohesion and faithful chromosome transmission. Ctf18p, Ctf8p, and Dcc1p interact physically in a complex with Rfc2p, Rfc3p, Rfc4p, and Rfc5p but not with Rfc1p or Rad24p. Deletion of CTF18, CTF8, or DCC1 singly or in combination (ctf18Deltactf8Deltadcc1Delta) leads to sensitivity to microtubule depolymerizing drugs and a severe sister chromatid cohesion defect. Furthermore, temperature-sensitive mutations in RFC4 result in precocious sister chromatid separation. Our results highlight a novel function of the RFC proteins and support a model in which sister chromatid cohesion is established at the replication fork via a polymerase switching mechanism and a replication-coupled remodeling of chromatin.

Stu2 promotes mitotic spindle elongation in anaphase

During anaphase, mitotic spindles elongate up to five times their metaphase length. This process, known as anaphase B, is essential for correct segregation of chromosomes. Here, we examine the control of spindle length during anaphase in the budding yeast Saccharomyces cerevisiae. We show that microtubule stabilization during anaphase requires the microtubule-associated protein Stu2. We further show that the activity of Stu2 is opposed by the activity of the kinesin-related protein Kip3. Reexamination of the kinesin homology tree suggests that KIP3 is the S. cerevisiae orthologue of the microtubule-destabilizing subfamily of kinesins (Kin I). We conclude that a balance of activity between evolutionally conserved microtubule-stabilizing and microtubule-destabilizing factors is essential for correct spindle elongation during anaphase B.

Mammalian mad2 and bub1/bubR1 recognize distinct spindle-attachment and kinetochore-tension checkpoints

Metaphase checkpoint controls sense abnormalities of chromosome alignment during mitosis and prevent progression to anaphase until proper alignment has been attained. A number of proteins, including mad2, bub1, and bubR1, have been implicated in the metaphase checkpoint control in mammalian cells. Metaphase checkpoints have been shown, in various systems, to read loss of either spindle tension or microtubule attachment at the kinetochore. Characteristically, HeLa cells arrest in metaphase in response to low levels of microtubule inhibitors that leave an intact spindle and a metaphase plate. Here we show that the arrest induced by nanomolar vinblastine correlates with loss of tension at the kinetochore, and that in response the checkpoint proteins bub1 and bubR1 are recruited to the kinetochore but mad2 is not. mad2 remains competent to respond and is recruited at higher drug doses that disrupt spindle association with the kinetochores. Further, although mad2 forms a complex with cdc20, it does not associate with bub1 or bubR1. We conclude that mammalian bub1/bubR1 and mad2 operate as elements of distinct pathways sensing tension and attachment, respectively.

Vanadocenes as potent anti-proliferative agents disrupting mitotic spindle formation in cancer cells

We present experimental data which establish the organometallic compounds vanadocene dichloride (VDC) and vanadocene acetylacetonate (VDacac) as potent anti-proliferative agents. We first examined the effects of VDC and VDacac on the rapid embryonic cell division and development of Zebrafish. Both compounds were capable of causing cell division block at the 8-16 cell stage of embryonic development followed by total cell fusion and developmental arrest. We next examined the effect of VDC and VDacac on proliferation of human breast cancer and glioblastoma cell lines using MTT assays. VDC inhibited the proliferation of the breast cancer cell line BT-20 as well as the glioblastoma cell line U373 in a concentration-dependent fashion with IC50 values of 11.0, 14.9 and 18.6 microM, respectively. VDacac inhibited cellular proliferation with IC50 values of 9.1, 26.9 and 35.5 microM, respectively. Whereas in vehicle-treated control cancer cells mitotic spindles were organized as a bipolar microtubule array and the DNA was organized on a metaphase plate, vanadocene-treated cancer cells had aberrant monopolar mitotic structures where microtubules were detected only on one side of the chromosomes and the chromosomes were arranged in a circular pattern. In contrast to control cells which showed a single focus of gamma-tubulin at each pole of the bipolar mitotic spindle, VDC- or VDacac-treated cells had two foci of gamma-tubulin on the same side of the chromosomes resulting in a broad centrosome at one pole. All monopolar spindles examined had two foci of gamma-tubulin labeling consistent with a mechanism in which the centrosomes duplicate but do not separate properly to form a bipolar spindle. These results provide unprecedented evidence that organometallic compounds can block cell division in human cancer cells by disrupting bipolar spindle formation. In accordance with these results vanadocene treatment caused an arrest at the G2/M phase of the cell cycle. This unique mechanism of anti-mitotic function warrants further development of vanadocene complexes as anti-cancer drugs.

Mitotic checkpoint proteins HsMAD1 and HsMAD2 are associated with nuclear pore complexes in interphase

Mad1 was first identified in budding yeast as an essential component of the checkpoint system that monitors spindle assembly in mitosis and prevents premature anaphase onset. Using antibodies to the human homologue of Mad1 (HsMAD1), we have begun to characterize this protein in mammalian cells. HsMad1 is found localized at kinetochores in mitosis. The labeling is brightest in prometaphase and is absent from kinetochores at metaphase and anaphase. In cells where most chromosomes have reached the metaphase plate, those aligned at the plate show no labeling while remaining, unaligned chromosomes are still brightly labeled. We find HsMad1 associated with HsMad2. Association with p55CDC, a protein previously shown to bind HsMad2, was not detected. Surprisingly, unlike any other known mitotic checkpoint proteins, HsMad1 and HsMAD2 were found localized at nuclear pores throughout interphase. This was confirmed by co-labeling with an antibody to known nuclear pore complex proteins and by their co-purification with enriched nuclear envelope fractions. HsMad1 was identified serendipitously by its binding to a viral protein, HTLV-1 Tax, which affects transcription of viral and human proteins. The localization of HsMad1 to nuclear pore complexes suggests an alternate, non-mitotic role for the Mad1/Tax interaction in the viral transformation of cells.

Spindle assembly in animal cells

Chromosome segregation during mitosis and meiosis is driven by a complex superstructure called the spindle. Microtubules are the primary structural component of spindles, and spindle assembly and function are intimately linked to the intrinsic dynamics of microtubules. This review summarizes spindle structure and highlights recent findings regarding the mechanisms and molecules involved in organizing microtubules into spindles. In addition, mechanisms for chromosome movement and segregation are discussed.

Bub1 is activated by the protein kinase p90(Rsk) during Xenopus oocyte maturation

BACKGROUND

The kinetochore attachment (spindle assembly) checkpoint arrests cells in metaphase to prevent exit from mitosis until all the chromosomes are aligned properly at the metaphase plate. The checkpoint operates by preventing activation of the anaphase-promoting complex (APC), which triggers anaphase by degrading mitotic cyclins and other proteins. This checkpoint is active during normal mitosis and upon experimental disruption of the mitotic spindle. In yeast, the serine/threonine protein kinase Bub1 and the WD-repeat protein Bub3 are elements of a signal transduction cascade that regulates the kinetochore attachment checkpoint. In mammalian cells, activated MAPK is present on kinetochores during mitosis and activity is upregulated by the spindle assembly checkpoint. In vertebrate unfertilized eggs, a special form of meiotic metaphase arrest by cytostatic factor (CSF) is mediated by MAPK activation of the protein kinase p90(Rsk), which leads to inhibition of the APC. However, it is not known whether CSF-dependent metaphase arrest caused by p90(Rsk) involves components of the spindle assembly checkpoint.

RESULTS

xBub1 is present in resting oocytes and its protein level increases slightly during oocyte maturation and early embryogenesis. In Xenopus oocytes, Bub1 is localized to kinetochores during both meiosis I and meiosis II, and the electrophoretic mobility of Bub1 upon SDS-PAGE decreases during meiosis I, reflecting phosphorylation and activation of the enzyme. The activation of Bub1 can be induced in interphase egg extracts by selective stimulation of the MAPK pathway by c-Mos, a MAPKKK. In oocytes treated with the MEK1 inhibitor U0126, the MAPK pathway does not become activated, and Bub1 remains in its low-activity, unshifted form. Injection of a constitutively active target of MAPK, the protein kinase p90(Rsk), restores the activation of Bub1 in the presence of U0126. Moreover, purified p90(Rsk) phosphorylates Bub1 in vitro and increases its protein kinase activity.

CONCLUSIONS

Bub1, an upstream component of the kinetochore attachment checkpoint, is activated during meiosis in Xenopus in a MAPK-dependent manner. Moreover, a single substrate of MAPK, p90(Rsk), is sufficient to activate Bub1 in vitro and in vivo. These results indicate that in vertebrate eggs, kinetochore attachment/spindle assembly checkpoint proteins, including Bub1, are downstream of p90(Rsk) and may be effectors of APC inhibition and CSF-dependent metaphase arrest by p90(Rsk).

Molecular analysis of the mitotic checkpoint genes BUB1, BUBR1 and BUB3 in human lung cancers

Our previous studies showed that mitotic checkpoint impairment is present in about 40% of human lung cancer cell lines but that mutations in the MAD mitotic checkpoint genes are infrequent. In the present study, we examined 44 lung cancer cases for the potential involvement of the other gene family involved in the mitotic checkpoint, i.e. BUB. We found that the BUB gene family members including BUB1, BUBR1 and BUB3 are not frequent targets for mitotic checkpoint defects in lung cancers, if present at all. Further studies are thus warranted to elucidate the molecular basis for the acquisition of mitotic checkpoint defects in order to better understand the molecular pathogenesis of lung cancers.

Mitotic spindle integrity and kinetochore function linked by the Duo1p/Dam1p complex

Duo1p and Dam1p were previously identified as spindle proteins in the budding yeast, Saccharomyces cerevisiae. Here, analyses of a diverse collection of duo1 and dam1 alleles were used to develop a deeper understanding of the functions and interactions of Duo1p and Dam1p. Based on the similarity of mutant phenotypes, genetic interactions between duo1 and dam1 alleles, interdependent localization to the mitotic spindle, and Duo1p/Dam1p coimmunoprecipitation from yeast protein extracts, these analyses indicated that Duo1p and Dam1p perform a shared function in vivo as components of a protein complex. Duo1p and Dam1p are not required to assemble bipolar spindles, but they are required to maintain metaphase and anaphase spindle integrity. Immunofluorescence and electron microscopy of duo1 and dam1 mutant spindles revealed a diverse variety of spindle defects. Our results also indicate a second, previously unidentified, role for the Duo1p/Dam1p complex. duo1 and dam1 mutants show high rates of chromosome missegregation, premature anaphase events while arrested in metaphase, and genetic interactions with a subset of kinetochore components consistent with a role in kinetochore function. In addition, Duo1p and Dam1p localize to kinetochores in chromosome spreads, suggesting that this complex may serve as a link between the kinetochore and the mitotic spindle.

Mitosis: a history of division

Mitosis has been studied since the early 1880s, to the extent that we now have a detailed, but still incomplete, description of spindle dynamics and mechanics, a sense of potential mechanochemical and regulatory mechanisms at a molecular level, and a long list of mitotic proteins. Here we present a personal view of how far we have come, and where we need to go to fully understand the mechanisms involved in mitosis.

Human Zw10 and ROD are mitotic checkpoint proteins that bind to kinetochores

Here we show that human Zeste White 10 (Zw10) and Rough deal (Rod) are new components of the mitotic checkpoint, as cells lacking these proteins at kinetochores fail to arrest in mitosis when exposed to microtubule inhibitors. Checkpoint failure and premature mitotic exit may explain why cells defective for hZw10 and hRod divide with lagging chromosomes. As Zw10 and Rod are not conserved in yeast, our data, combined with an accompanying study of Drosophila Zw10 and Rod, indicate that metazoans may require an elaborate spindle checkpoint to monitor complex kinetochore functions.

A chemical genomics approach toward understanding the global functions of the target of rapamycin protein (TOR)

The target of rapamycin protein (TOR) is a highly conserved ataxia telangiectasia-related protein kinase essential for cell growth. Emerging evidence indicates that TOR signaling is highly complex and is involved in a variety of cellular processes. To understand its general functions, we took a chemical genomics approach to explore the genetic interaction between TOR and other yeast genes on a genomic scale. In this study, the rapamycin sensitivity of individual deletion mutants generated by the Saccharomyces Genome Deletion Project was systematically measured. Our results provide a global view of the rapamycin-sensitive functions of TOR. In contrast to conventional genetic analysis, this approach offers a simple and thorough analysis of genetic interaction on a genomic scale and measures genetic interaction at different possible levels. It can be used to study the functions of other drug targets and to identify novel protein components of a conserved core biological process such as DNA damage checkpoint/repair that is interfered with by a cell-permeable chemical compound.

The fission yeast gamma-tubulin complex is required in G(1) phase and is a component of the spindle assembly checkpoint

Microtubule polymerization is initiated from the microtubule organizing centre (MTOC), which contains the gamma-tubulin complex. We have identified fission yeast Alp4 and Alp6, which are homologues of the gamma-tubulin-interacting proteins Sc.Spc97/Hs.Gcp2 and Sc. Spc98/Hs.Gcp3, respectively. The size of the fission yeast gamma-tubulin complex is large (>2000 kDa), comparable to that in metazoans. Both Alp4 and Alp6 localize to the spindle pole body (SPB) and also to the equatorial MTOC. Temperature-sensitive (ts) alp4 and alp6 mutants show two types of microtubular defects. First, monopolar mitotic spindles form. Secondly, abnormally long cytoplasmic microtubules appear that do not stop at the cell tips and are still associated with the SPB. Alp4 function is required in G(1) phase and ts mutants become lethal before S-phase. alp4 and alp6 mutants are hypersensitive to the microtubule- destabilizing drug thiabendazole (TBZ) and show a lethal 'cut' phenotype in its presence. Furthermore, alp4mad2 double mutants show an exaggerated multiple septation phenotype in TBZ. These results indicate that Alp4 and Alp6 may play a crucial role in the spindle pole-mediated checkpoint pathway.

Loss of a protein phosphatase 2A regulatory subunit (Cdc55p) elicits improper regulation of Swe1p degradation

CDC55 encodes a Saccharomyces cerevisiae protein phosphatase 2A (PP2A) regulatory subunit. cdc55-null cells growing at low temperature exhibit a failure of cytokinesis and produce abnormally elongated buds, but cdc55-null cells producing the cyclin-dependent kinase Cdc28-Y19F, which is unable to be inhibited by Y19 phosphorylation, show a loss of the abnormal morphology. Furthermore, cdc55-null cells exhibit a hyperphosphorylation of Y19. For these reasons, we have examined in wild-type and cdc55-null cells the levels and activities of the kinase (Swe1p) and phosphatase (Mih1p) that normally regulate the extent of Cdc28 Y19 phosphorylation. We find that Mih1p levels are comparable in the two strains, and an estimate of the in vivo and in vitro phosphatase activity of this enzyme in the two cell types indicates no marked differences. By contrast, while Swe1p levels are similar in unsynchronized and S-phase-arrested wild-type and cdc55-null cells, Swe1 kinase is found at elevated levels in mitosis-arrested cdc55-null cells. This excess Swe1p in cdc55-null cells is the result of ectopic stabilization of this protein during G(2) and M, thereby accounting for the accumulation of Swe1p in mitosis-arrested cells. We also present evidence indicating that, in cdc55-null cells, misregulated PP2A phosphatase activity is the cause of both the ectopic stabilization of Swe1p and the production of the morphologically abnormal phenotype.

Carboxyl methylation of the phosphoprotein phosphatase 2A catalytic subunit promotes its functional association with regulatory subunits in vivo

The phosphoprotein phosphatase 2A (PP2A) catalytic subunit contains a methyl ester on its C-terminus, which in mammalian cells is added by a specific carboxyl methyltransferase and removed by a specific carboxyl methylesterase. We have identified genes in yeast that show significant homology to human carboxyl methyltransferase and methylesterase. Extracts of wild-type yeast cells contain carboxyl methyltransferase activity, while extracts of strains deleted for one of the methyltransferase genes, PPM1, lack all activity. Mutation of PPM1 partially disrupts the PP2A holoenzyme in vivo and ppm1 mutations exhibit synthetic lethality with mutations in genes encoding the B or B' regulatory subunit. Inactivation of PPM1 or overexpression of PPE1, the yeast gene homologous to bovine methylesterase, yields phenotypes similar to those observed after inactivation of either regulatory subunit. These phenotypes can be reversed by overexpression of the B regulatory subunit. These results demonstrate that Ppm1 is the sole PP2A methyltransferase in yeast and that its activity is required for the integrity of the PP2A holoenzyme.

Identification of genes potentially involved in LMO2-induced leukemogenesis

The most common translocations in childhood T cell acute lymphoblastic leukemias involve the LMO2 locus on chromosome 11p13 and cause ectopic expression of the LMO2 gene in thymocytes. Transgenic mice with enforced expression of LMO2 in their thymocytes develop T cell leukemias thus demonstrating the role of LMO2 in leukemogenesis. The physiologic and leukemogenic functions of LMO2 are mediated through its transcriptional regulatory activities, but the identity of the target genes is completely unknown. In this report, we have used cDNA representational difference analysis (cDNA-RDA) to identify genes that are over-expressed and are likely to play a role in the LMO2 induced leukemias. cDNA-RDA was performed using very small amounts of mRNA pool (from 1 microg of total RNA) to reverse transcribe the cDNAs from leukemic cells or normal thymocytes. The cDNA-RDA led to the isolation of nine distinct clones that were specifically overexpressed in the leukemic cells. Sequence analysis revealed that five of the nine clones had identity or homology to known genes that are known to play a role in the pathogenesis of leukemias or other cancers. Three clones had no significant homology to any known genes and thus represent novel candidate genes. Our study demonstrates that cDNA-RDA using very small amounts of total RNA is a highly efficient method to identify novel genes that may play a role in leukemogenesis.

Saccharomyces cerevisiae BUB2 prevents mitotic exit in response to both spindle and kinetochore damage

The spindle assembly checkpoint-mediated mitotic arrest depends on proteins that signal the presence of one or more unattached kinetochores and prevents the onset of anaphase in the presence of kinetochore or spindle damage. In the presence of either damage, bub2 cells initiate a preanaphase delay but do not maintain it. Inappropriate sister chromatid separation in nocodazole-treated bub2 cells is prevented when mitotic exit is blocked using a conditional tem1(c) mutant, indicating that the preanaphase failure in bub2 cells is a consequence of events downstream of TEM1 in the mitotic exit pathway. Using a conditional bub2(tsd) mutant, we demonstrate that the continuous presence of Bub2 protein is required for maintaining spindle damage-induced arrest. BUB2 is not required to maintain a DNA damage checkpoint arrest, revealing a specificity for spindle assembly checkpoint function. In a yeast two-hybrid assay and in vitro, Bub2 protein interacts with the septin protein Cdc3, which is essential for cytokinesis. These data support the view that the spindle assembly checkpoint encompasses regulation of distinct mitotic steps, including a MAD2-directed block to anaphase initiation and a BUB2-directed block to TEM1-dependent exit.

Mutation analysis of mitotic checkpoint genes (hBUB1 and hBUBR1) and microsatellite instability in adult T-cell leukemia/lymphoma

Adult T-cell leukemia/lymphoma (ATLL) is a neoplasm of T-lymphocytes, and human T-cell lymphotropic virus type-I (HTLV-I) is etiologically considered as the causative virus of ATLL. The karyotypes of ATLL are very complex in both number and structure, although no specific karyotype abnormalities have been identified. HTLV-I is thought to integrate its provirus into random sites in host chromosomal DNA and induces chromosomal instability. The BUB gene is a component of the mitotic checkpoint in budding yeast. Recently, human homologues of the BUB were identified and mutant alleles of hBUB1 and hBUBR1 were detected in two colorectal tumor cell lines, which showed microsatellite instability (MIN). In vitro, BUB proteins form a complex of monomers. These proteins interact with the human MAD1 gene product, a target of the HTLV-1 tax oncogene. We examined the role of checkpoint gene in the chromosomal abnormalities of ATLL by investigating mutations of hBUB1 and hBUBR1, and MIN of replication errors of BAX, insulin-like growth factor, and transforming growth factor beta type II. We analyzed ten cases with ATLL and eight B-cell lymphomas (five diffuse large cell lymphomas, three follicular lymphomas). Complex chromosomal abnormalities were detected in ATLL, while B-cell lymphomas showed only simple or minimal chromosomal abnormalities. Significant mutations/deletion of hBUB1 or hBUBR1 were detected in four of ten cases with ATLL, including two heterozygous point mutations, one homozygous point mutation, and one with a 47 bp deletion. In contrast, only one of eight B-cell lymphomas showed nonsense mutation of hBUBR1. None of the ATLL and B-cell lymphomas showed MIN. In the multistage process of leukemogenesis of ATLL, our findings indicate that mutations of mitotic checkpoint genes may play an important role in the induction of complex chromosomal abnormalities.

Morphological study of metaphase-specific apoptosis in MRL mouse testis

Apoptosis of male germ cells is a complex phenomenon in many animal species. Understanding its mechanisms could be useful in the diagnosis and therapy of male infertility. To examine the differences of distribution of apoptosis among mouse strains, the terminal transferase-mediated nick end labelling (TUNEL) method was employed. In the testes of MRL mice, many TUNEL-positive cells were identified at the metaphases of meiotic spermatocytes. Morphometrical analysis revealed that metaphase-specific apoptosis occurred at the region between secondary spermatocytes and step 1 spermatids in stage XII seminiferous tubules. In the investigation of the developing first-wave of seminiferous tubules, there were some metaphases showing apoptotic morphology prior to becoming secondary spermatocytes. Details of the apoptotic structure revealed by electron microscopy showed that cellular arrest occurred after the beginning of the M phase of the cell cycle. These results suggested that metaphase-specific apoptosis in the testis of the MRL mouse strain took place at least at the first meiotic division, perhaps showing the spindle assembly checkpoint of the cell cycle.

p55CDC/hCDC20 is associated with BUBR1 and may be a downstream target of the spindle checkpoint kinase

Eukaryotic cells have evolved a mechanism that delays the progression of mitosis until condensed chromosomes are properly positioned on the mitotic spindle. We have been studying genes that regulated the spindle checkpoint in human cells. Enforced expression of human BUBR1, but not a BUBR1 mutant allele, enhances accumulation of mitotic cells. Yeast two-hybrid system and GST-pulldown analyses show that p55CDC/hCdc20, a protein known to link spindle checkpoint components such as MAD2 to anaphase promoting complex (APC), interacts with BUBR1. In addition, p55CDC is capable of pulling down BUBR1 in sf-9 cells infected with both p55CDC and His6-BUBR1 recombinant baculoviruses but not in the cells infected with p55CDC baculoviruses or with the baculoviral vector alone. Moreover, immunoprecipitation followed by Western blot analyses confirmed that native p55CDC is associated with BUBR1 in HeLa cells. Spindle checkpoint activation by nocodazole treatment enhances the association between p55CDC and His6-BUBR1. In nocodazole-arrested mitotic cells, both CDC16 and hyperphosphorylated CDC27, two APC components, preferentially associate with His6-BUBR1 resins, but not the control resins. Furthermore, BUBR1 phosphorylates p55CDC in vitro, and the phosphorylation of p55CDC by BUBR1 appears to be correlated with spindle checkpoint activation. Together, our studies strongly suggest that BUBR1 may target APC via p55CDC.

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

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

CENP-E as an essential component of the mitotic checkpoint in vitro

Accurate chromatid separation is monitored by a checkpoint mechanism that delays anaphase onset until all centromeres are correctly attached to the mitotic spindle. Using Xenopus egg extracts, the kinetochore-associated microtubule motor protein CENP-E is now found to be required for establishing and maintaining this checkpoint. When CENP-E function is disrupted by immunodepletion or antibody addition, extracts fail to arrest in response to spindle damage. Mitotic arrest can be restored by addition of high levels of soluble MAD2, demonstrating that the absence of CENP-E eliminates kinetochore-dependent signaling but not the downstream steps in checkpoint signal transduction. Because it directly binds both to spindle microtubules and to the kinetochore-associated checkpoint kinase BUBR1, CENP-E is a central component in the vertebrate checkpoint that modulates signaling activity in a microtubule-dependent manner.

Bub3 gene disruption in mice reveals essential mitotic spindle checkpoint function during early embryogenesis

Bub3 is a conserved component of the mitotic spindle assembly complex. The protein is essential for early development in Bub3 gene-disrupted mice, evident from their failure to survive beyond day 6.5-7.5 postcoitus (pc). Bub3 null embryos appear normal up to day 3.5 pc but accumulate mitotic errors from days 4.5-6.5 pc in the form of micronuclei, chromatin bridging, lagging chromosomes, and irregular nuclear morphology. Null embryos treated with a spindle-depolymerising agent fail to arrest in metaphase and show an increase in mitotic disarray. The results confirm Bub3 as a component of the essential spindle checkpoint pathway that operates during early embryogenesis.

Probing spindle assembly mechanisms with monastrol, a small molecule inhibitor of the mitotic kinesin, Eg5

Monastrol, a cell-permeable small molecule inhibitor of the mitotic kinesin, Eg5, arrests cells in mitosis with monoastral spindles. Here, we use monastrol to probe mitotic mechanisms. We find that monastrol does not inhibit progression through S and G2 phases of the cell cycle or centrosome duplication. The mitotic arrest due to monastrol is also rapidly reversible. Chromosomes in monastrol-treated cells frequently have both sister kinetochores attached to microtubules extending to the center of the monoaster (syntelic orientation). Mitotic arrest-deficient protein 2 (Mad2) localizes to a subset of kinetochores, suggesting the activation of the spindle assembly checkpoint in these cells. Mad2 localizes to some kinetochores that have attached microtubules in monastrol-treated cells, indicating that kinetochore microtubule attachment alone may not satisfy the spindle assembly checkpoint. Monastrol also inhibits bipolar spindle formation in Xenopus egg extracts. However, it does not prevent the targeting of Eg5 to the monoastral spindles that form. Imaging bipolar spindles disassembling in the presence of monastrol allowed direct observations of outward directed forces in the spindle, orthogonal to the pole-to-pole axis. Monastrol is thus a useful tool to study mitotic processes, detection and correction of chromosome malorientation, and contributions of Eg5 to spindle assembly and maintenance.

CHL1 is a nuclear protein with an essential ATP binding site that exhibits a size-dependent effect on chromosome segregation

Saccharomyces cerevisiae chl1 mutants have a significant increase in the rate of chromosome missegregation. CHL1 encodes a 99 kDa predicted protein with an ATP binding site consensus, a putative helix-turn-helix DNA binding motif, and homology to helicases. Using site-directed mutagenesis, I show that mutations that are predicted to abolish ATP binding in CHL1 inactivate its function in chromosome segregation. Furthermore, overexpression of these mutations interferes with chromosome transmission of a 125 kb chromosome fragment in a wild-type strain. Polyclonal antibodies against CHL1 show that CHL1 is predominantly in the nuclear fraction of S. CEREVISIAE: CHL1 function is more critical for the segregation of small chromosomes. In chl1Delta1/chl1Delta1 mutants, artificial circular or linear chromosomes <150 kb in size exhibit near random segregation (0.12 per cell division), whereas all chromosomes tested >225 kb were lost at rates (5 x 10(-)(3) per cell division) comparable to that observed for endogenous chromosome III. These results reveal an important role for ATPases/DNA helicases in chromosome segregation. Such enzymes may alter DNA topology to allow loading of proteins involved in maintaining sister chromatid cohesion.

CENP-E forms a link between attachment of spindle microtubules to kinetochores and the mitotic checkpoint

Here we show that suppression of synthesis of the microtubule motor CENP-E (centromere-associated protein E), a component of the kinetochore corona fibres of mammalian centromeres, yields chromosomes that are chronically mono-orientated, with spindles that are flattened along the plane of the substrate. Despite apparently normal microtubule numbers and the continued presence at kinetochores of other microtubule motors, spindle poles fragment in the absence of CENP-E, which implicates this protein in delivery of components from kinetochores to poles. CENP-E represents a link between attachment of spindle microtubules and the mitotic checkpoint signalling cascade, as depletion of this motor leads to profound checkpoint activation, whereas immunoprecipitation reveals a nearly stoichiometric association of CENP-E with the checkpoint kinase BubR1 during mitosis.

The interaction between Ran and NTF2 is required for cell cycle progression

The small GTPase Ran is required for the trafficking of macromolecules into and out of the nucleus. Ran also has been implicated in cell cycle control, specifically in mitotic spindle assembly. In interphase cells, Ran is predominately nuclear and thought to be GTP bound, but it is also present in the cytoplasm, probably in the GDP-bound state. Nuclear transport factor 2 (NTF2) has been shown to import RanGDP into the nucleus. Here, we examine the in vivo role of NTF2 in Ran import and the effect that disruption of Ran imported into the nucleus has on the cell cycle. A temperature-sensitive (ts) mutant of Saccharomyces cerevisiae NTF2 that does not bind to Ran is unable to import Ran into the nucleus at the nonpermissive temperature. Moreover, when Ran is inefficiently imported into the nucleus, cells arrest in G(2) in a MAD2 checkpoint-dependent manner. These findings demonstrate that NTF2 is required to transport Ran into the nucleus in vivo. Furthermore, we present data that suggest that depletion of nuclear Ran triggers a spindle-assembly checkpoint-dependent cell cycle arrest.

Histone-histone interactions and centromere function

Cse4p is a structural component of the core centromere of Saccharomyces cerevisiae and is a member of the conserved CENP-A family of specialized histone H3 variants. The histone H4 allele hhf1-20 confers defects in core centromere chromatin structure and mitotic chromosome transmission. We have proposed that Cse4p and histone H4 interact through their respective histone fold domains to assemble a nucleosome-like structure at centromeric DNA. To test this model, we targeted random mutations to the Cse4p histone fold domain and isolated three temperature-sensitive cse4 alleles in an unbiased genetic screen. Two of the cse4 alleles contain mutations at the Cse4p-H4 interface. One of these requires two widely separated mutations demonstrating long-range cooperative interactions in the structure. The third cse4 allele is mutated at its helix 2-helix 3 interface, a region required for homotypic H3 fold dimerization. Overexpression of wild-type Cse4p and histone H4 confer reciprocal allele-specific suppression of cse4 and hhf1 mutations, providing strong evidence for Cse4p-H4 protein interaction. Overexpression of histone H3 is dosage lethal in cse4 mutants, suggesting that histone H3 competes with Cse4p for histone H4 binding. However, the relative resistance of the Cse4p-H4 pathway to H3 interference argues that centromere chromatin assembly must be highly regulated.