Getting down to the phosphorylated ‘nuts and bolts’ of spindle checkpoint signalling

Copper(II) Complexes with Isomeric Morpholine-Substituted 2-Formylpyridine Thiosemicarbazone Hybrids as Potential Anticancer Drugs Inhibiting Both Ribonucleotide Reductase and Tubulin Polymerization: The Morpholine Position Matters

The development of copper(II) thiosemicarbazone complexes as potential anticancer agents, possessing dual functionality as inhibitors of R2 ribonucleotide reductase (RNR) and tubulin polymerization by binding at the colchicine site, presents a promising avenue for enhancing therapeutic effectiveness. Herein, we describe the syntheses and physicochemical characterization of four isomeric proligands H2L3-H2L6, with the methylmorpholine substituent at pertinent positions of the pyridine ring, along with their corresponding Cu(II) complexes 3-6. Evidently, the position of the morpholine moiety and the copper(II) complex formation have marked effects on the in vitro antiproliferative activity in human uterine sarcoma MES-SA cells and the multidrug-resistant derivative MES-SA/Dx5 cells. Activity correlated strongly with quenching of the tyrosyl radical (Y•) of mouse R2 RNR protein, inhibition of RNR activity in the cancer cells, and inhibition of tubulin polymerization. Insights into the mechanism of antiproliferative activity, supported by experimental results and molecular modeling calculations, are presented.

Impact of UV radiation on Mxene-mediated tubulin dissociation and mitochondrial apoptosis in breast cancer cells

Breast cancer is a global health concern that requires personalized therapies to prevent relapses, as conventional treatments may develop resistance over time. Photothermal therapy using spectral radiation or intense light emission is a broad-spectrum treatment that induces hyperthermia-mediated cancer cell death. MXene, a two-dimensional material, has been reported to have potential biological applications in photothermal therapy for cancer treatment. In this study, we investigated the apoptotic activity of MXene and UV-irradiated MXene in MCF-7 breast cancer cells by treating them with varying concentrations of MXene. The cytotoxicity of MXene and UV was evaluated by analyzing cellular morphology, nuclei condensation, caspase activation, and apoptotic cell death. We also assessed the effect of the combined treatment on the expression and cellular distribution of Tubulin, a key component of microtubules required for cell division. At low concentrations of MXene (up to 100 µg/ml), the level of cytotoxicity in MCF-7 cells was low. However, the combined treatment of MXene and UV resulted in a synergistic increase in cytotoxicity, causing rounded cellular morphology, condensed nuclei, caspase activation, and apoptotic cell death. Furthermore, the treatment reduced Tubulin protein expression and cellular distribution, indicating a potent inducer of cell death with potential application for cancer treatment. The study demonstrates that the combined treatment of MXene and UVB irradiation is a promising strategy for inducing apoptotic cell death in breast cancer cells, suggesting its potential as a therapeutic intervention for breast cancer.

Fission yeast Wee1 is required for stable kinetochore-microtubule attachment

Wee1 is a cell cycle regulator that phosphorylates Cdk1/Cdc2 and inhibits G2/M transition. Loss of Wee1 in fission yeast results in an early onset of mitosis. Interestingly, we found that cells lacking Wee1 require the functional spindle checkpoint for their viability. Genetic analysis indicated that the requirement is not attributable to the early onset of mitosis. Live-cell imaging revealed that some kinetochores are not attached or bioriented in the wee1 mutant. Furthermore, Mad2, a component of the spindle checkpoint known to recognize unattached kinetochores, accumulates in the vicinity of the spindle, representing activation of the spindle checkpoint in the mutant. It appears that the wee1 mutant cannot maintain stable kinetochore-microtubule attachment, and relies on the delay imposed by the spindle checkpoint for establishing biorientation of kinetochores. This study revealed a role of Wee1 in ensuring accurate segregation of chromosomes during mitosis, and thus provided a basis for a new principle of cancer treatment with Wee1 inhibitors.

Identification of Potential Antitubulin Agents with Anticancer Assets from a Series of Imidazo[1,2-a]quinoxaline Derivatives: In Silico and In Vitro Approaches

Computer-aided drug design is a powerful and promising tool for drug design and development, with a reduced cost and time. In the current study, we rationally selected a library of 34 fused imidazo[1,2-a]quinoxaline derivatives and performed virtual screening, molecular docking, and molecular mechanics for a lead identification against tubulin as an anticancer molecule. The computational analysis and pharmacophoric features were represented as 1A2; this was a potential lead against tubulin, with a maximized affinity and binding score at the colchicine-binding site of tubulin. The efficiency of this lead molecule was further identified using an in vitro assay on a tubulin enzyme and the anticancer potential was established using an MTT assay. Compound 1A2 (IC₅₀ = 4.33-6.11 µM against MCF-7, MDA-MB-231, HCT-116, and A549 cell lines) displayed encouraging results similar to the standard drug colchicine in these in vitro studies, which further confirmed the effectiveness of CADD in new drug developments. Thus, we successfully applied the utility of in silico techniques to identify the best plausible leads from the fused azaheterocycles.

Design, Synthesis and Biological Evaluation of Imidazole-Substituted/Fused Aryl Derivatives Targeting Tubulin Polymerization as Anticancer Agents

The development of new pharmacologically active molecules targeting tubulin polymerization has recently attracted great interest in research groups. In efforts to develop new potent anticancer compounds, imidazole-tethered/fused pharmacologically active aryl derivatives possessing different substitution patterns targeting tubulin polymerization have been rationally designed and synthesized. The target molecules (P1-5 and KG1-5) were synthesized by multistep syntheses involving the reaction of intermediate 2-aminophenyl-tethered imidazoles with appropriate reactants in the presence of p-TsOH under different conditions. The synthesized compounds displayed moderate to good cytotoxicity, comparable to that of colchicine, against four cancer cell lines (MCF-7, MD-MBA-231, A549, and HCT-116). Compounds P2 and P5, with an imidazoloquinoxaline moiety, emerged as potential leads with cytotoxicity profiles against these cell lines similar to colchicine. Compounds P2 and P5 arrested cell division at the G2/M phase and prevented cancerous cell growth through induced apoptosis. These results favored the hypothesis that the compounds might act by binding to the colchicine binding site, which was further confirmed with the help of a tubulin polymerization inhibition assay. The results encourage the further exploration of imidazoloquinoxalines as promising leads that deserve advanced clinical investigation.

CENP-F-dependent DRP1 function regulates APC/C activity during oocyte meiosis I

Chromosome segregation is initiated by cohesin degradation, which is driven by anaphase-promoting complex/cyclosome (APC/C). Chromosome cohesin is removed by activated separase, with the degradation of securin and cyclinB1. Dynamin-related protein 1 (DRP1), a component of the mitochondrial fission machinery, is related to cyclin dynamics in mitosis progression. Here, we show that DRP1 is recruited to the kinetochore by centromeric Centromere protein F (CENP-F) after nuclear envelope breakdown in mouse oocytes. Loss of DRP1 during prometaphase leads to premature cohesin degradation and chromosome segregation. Importantly, acute DRP1 depletion activates separase by initiating cyclinB1 and securin degradation during the metaphase-to-anaphase transition. Finally, we demonstrate that DRP1 is bound to APC2 to restrain the E3 ligase activity of APC/C. In conclusion, DRP1 is a CENP-F-dependent atypical spindle assembly checkpoint (SAC) protein that modulates metaphase-to-anaphase transition by controlling APC/C activity during meiosis I in oocytes.

Structure-activity relationships and antiproliferative effects of 1,2,3,4-4H-quinoxaline derivatives as tubulin polymerization inhibitors

Colchicine binding site inhibitors (CBSIs) hold great potential for the treatment of various tumors and they can overcome multidrug resistance which the existing tubulin inhibitors such as paclitaxel and vinorelbine are faced with. Herein, we report the design, synthesis and biological evaluation of a series of tetrahydro-quinoxaline derivatives as colchicine binding site inhibitors. All the synthesized compounds were evaluated for their in vitro antiproliferative activities against HT-29 and Hela cancer cell lines, and most of the target compounds demonstrated moderate to strong activities towards two tumor cell lines. In addition, the structure-activity relationships of these derivatives were also discussed. Among them, compounds 11a and 11b showed the most potent activities. Moreover, compound 11a inhibited the tubulin polymerization in both cell-free and cellular assays. Further profiling of compound 11a revealed that it arrested cell cycle in G2/M and induced cell apoptosis in a dose-dependent manner. Furthermore, molecular docking study proved that compound 11a acted on the colchicine binding site. Therefore, 11a is a promising candidate for the discovery of colchicine binding site inhibitors.

Structure-Activity Relationship Study of Novel 6-Aryl-2-benzoyl-pyridines as Tubulin Polymerization Inhibitors with Potent Antiproliferative Properties

We recently reported the crystal structure of tubulin in complex with a colchicine binding site inhibitor (CBSI), ABI-231, having 2-aryl-4-benzoyl-imidazole (ABI). Based on this and additional crystal structures, here we report the structure-activity relationship study of a novel series of pyridine analogues of ABI-231, with compound 4v being the most potent one (average IC₅₀ ∼ 1.8 nM) against a panel of cancer cell lines. We determined the crystal structures of another potent CBSI ABI-274 and 4v in complex with tubulin and confirmed their direct binding at the colchicine site. 4v inhibited tubulin polymerization, strongly suppressed A375 melanoma tumor growth, induced tumor necrosis, disrupted tumor angiogenesis, and led to tumor cell apoptosis in vivo. Collectively, these studies suggest that 4v represents a promising new generation of tubulin inhibitors.

The anaphase-promoting complex: A key mitotic regulator associated with somatic mutations occurring in cancer

The anaphase-promoting complex/cyclosome (APC/C) is an E3 ubiquitin ligase that helps control chromosome separation and exit from mitosis in many different kinds of organisms, including yeast, flies, worms, and humans. This review represents a new perspective on the connection between APC/C subunit mutations and cancer. The complex nature of APC/C and limited mutation analysis of its subunits has made it difficult to determine the relationship of each subunit to cancer. In this work, cancer genomic data were examined to identify APC/C subunits with a greater than 5% alteration frequency in 11 representative cancers using the cBioPortal database. Using the Genetic Determinants of Cancer Patient Survival database, APC/C subunits were also studied and found to be significantly associated with poor patient prognosis in several cases. In comparing these two kinds of cancer genomics data to published large-scale genomic analyses looking for cancer driver genes, ANAPC1 and ANAPC3/CDC27 stood out as being represented in all three types of analyses. Seven other subunits were found to be associated both with >5% alteration frequency in certain cancers and being associated with an effect on cancer patient prognosis. The aim of this review is to provide new approaches for investigators conducting in vivo studies of APC/C subunits and cancer progression. In turn, a better understanding of these APC/C subunits and their role in different cancers will help scientists design drugs that are more precisely targeted to certain cancers, using APC/C mutation status as a biomarker.

The Anaphase Promoting Complex/Cyclosome (APC/C): A Versatile E3 Ubiquitin Ligase

In the present chapter we discuss the essential roles of the human E3 ubiquitin ligase Anaphase Promoting Complex/Cyclosome (APC/C) in mitosis as well as the emerging evidence of important APC/C roles in cellular processes beyond cell division control such as regulation of genomic integrity and cell differentiation of the nervous system. We consider the potential incipient role of APC/C dysregulation in the pathophysiology of the neurological disorder Alzheimer's disease (AD). We also discuss how certain Deoxyribonucleic Acid (DNA) and Ribonucleic Acid (RNA) viruses take control of the host's cell division regulatory system through harnessing APC/C ubiquitin ligase activity and hypothesise the plausible molecular mechanisms underpinning virus manipulation of the APC/C. We also examine how defects in the function of this multisubunit protein assembly drive abnormal cell proliferation and lastly argue the potential of APC/C as a promising therapeutic target for the development of innovative therapies for the treatment of chronic malignancies such as cancer.

Centromeric signaling proteins boost G1 cyclin degradation and modulate cell size in budding yeast

Cell size scales with ploidy in a great range of eukaryotes, but the underlying mechanisms remain unknown. Using various orthogonal single-cell approaches, we show that cell size increases linearly with centromere (CEN) copy number in budding yeast. This effect is due to a G1 delay mediated by increased degradation of Cln3, the most upstream G1 cyclin acting at Start, and specific centromeric signaling proteins, namely Mad3 and Bub3. Mad3 binds both Cln3 and Cdc4, the adaptor component of the Skp1/Cul1/F-box (SCF) complex that targets Cln3 for degradation, these interactions being essential for the CEN-dosage dependent effects on cell size. Our results reveal a pathway that modulates cell size as a function of CEN number, and we speculate that, in cooperation with other CEN-independent mechanisms, it could assist the cell to attain efficient mass/ploidy ratios.

An Emerging Role for Tubulin Isotypes in Modulating Cancer Biology and Chemotherapy Resistance

Tubulin proteins, as components of the microtubule cytoskeleton perform critical cellular functions throughout all phases of the cell cycle. Altered tubulin isotype composition of microtubules is emerging as a feature of aggressive and treatment refractory cancers. Emerging evidence highlighting a role for tubulin isotypes in differentially influencing microtubule behaviour and broader functional networks within cells is illuminating a complex role for tubulin isotypes regulating cancer biology and chemotherapy resistance. This review focuses on the role of different tubulin isotypes in microtubule dynamics as well as in oncogenic changes that provide a survival or proliferative advantage to cancer cells within the tumour microenvironment and during metastatic processes. Consideration of the role of tubulin isotypes beyond their structural function will be essential to improving the current clinical use of tubulin-targeted chemotherapy agents and informing the development of more effective cancer therapies.

Loss of the Methyl-CpG-Binding Protein ZBTB4 Alters Mitotic Checkpoint, Increases Aneuploidy, and Promotes Tumorigenesis

Chromosome segregation during mitosis is monitored by the mitotic checkpoint and is dependent upon DNA methylation. ZBTB4 is a mammalian epigenetic regulator with high affinity for methylated CpGs that localizes at pericentromeric heterochromatin and is frequently downregulated in cancer. Here, we report that decreased ZBTB4 expression correlates with high genome instability across many frequent human cancers. In human cell lines, ZBTB4 depletion was sufficient to increase the prevalence of micronuclei and binucleated cells in parallel with aberrant mitotic checkpoint gene expression, a weakened mitotic checkpoint, and an increased frequency of lagging chromosomes during mitosis. To extend these findings, we generated Zbtb4-deficient mice. Zbtb4^-/- mice were smaller than their wild-type littermates. Primary cells isolated from Zbtb4^-/- mice exhibited diminished mitotic checkpoint activity, increased mitotic defects, aneuploid cells marked by a specific transcriptional signature, and increased genomic instability. Zbtb4^-/- mice were also more susceptible to 7,12-dimethylbenz(a)anthracene/12-O-tetradecanoylphorbol-13-acetate (DMBA/TPA)-induced skin carcinogenesis. Our results establish the epigenetic regulator ZBTB4 as an essential component in maintaining genomic stability in mammals. Cancer Res; 77(1); 62-73. ©2016 AACR.

The mitotic checkpoint complex (MCC): looking back and forth after 15 years

The mitotic checkpoint is a specialized signal transduction pathway that contributes to the fidelity of chromosome segregation. The signaling of the checkpoint originates from defective kinetochore-microtubule interactions and leads to formation of the mitotic checkpoint complex (MCC), a highly potent inhibitor of the Anaphase Promoting Complex/Cyclosome (APC/C)—the E3 ubiquitin ligase essential for anaphase onset. Many important questions concerning the MCC and its interaction with APC/C have been intensively investigated and debated in the past 15 years, such as the exact composition of the MCC, how it is assembled during a cell cycle, how it inhibits APC/C, and how the MCC is disassembled to allow APC/C activation. These efforts have culminated in recently reported structure models for human MCC:APC/C supra-complexes at near-atomic resolution that shed light on multiple aspects of the mitotic checkpoint mechanisms. However, confusing statements regarding the MCC are still scattered in the literature, making it difficult for students and scientists alike to obtain a clear picture of MCC composition, structure, function and dynamics. This review will comb through some of the most popular concepts or misconceptions about the MCC, discuss our current understandings, present a synthesized model on regulation of CDC20 ubiquitination, and suggest a few future endeavors and cautions for next phase of MCC research.

Mps1Mph1 Kinase Phosphorylates Mad3 to Inhibit Cdc20Slp1-APC/C and Maintain Spindle Checkpoint Arrests

The spindle checkpoint is a mitotic surveillance system which ensures equal segregation of sister chromatids. It delays anaphase onset by inhibiting the action of the E3 ubiquitin ligase known as the anaphase promoting complex or cyclosome (APC/C). Mad3/BubR1 is a key component of the mitotic checkpoint complex (MCC) which binds and inhibits the APC/C early in mitosis. Mps1^Mph1 kinase is critical for checkpoint signalling and MCC-APC/C inhibition, yet few substrates have been identified. Here we identify Mad3 as a substrate of fission yeast Mps1^Mph1 kinase. We map and mutate phosphorylation sites in Mad3, producing mutants that are targeted to kinetochores and assembled into MCC, yet display reduced APC/C binding and are unable to maintain checkpoint arrests. We show biochemically that Mad3 phospho-mimics are potent APC/C inhibitors in vitro, demonstrating that Mad3p modification can directly influence Cdc20^Slp1-APC/C activity. This genetic dissection of APC/C inhibition demonstrates that Mps1^Mph1 kinase-dependent modifications of Mad3 and Mad2 act in a concerted manner to maintain spindle checkpoint arrests.

When cells divide they need to ensure that a complete copy of their genetic material is transmitted to both daughter cells. Cells have evolved many controls to ensure that every division is carried out with very high fidelity. The spindle checkpoint is one such control, which acts as a surveillance system during mitosis. Defects in this checkpoint control lead to unequal segregation of DNA/chromosomes, termed aneuploidy, which is responsible for human birth defects and is very common in tumour cells.

The molecular components of the spindle checkpoint, identified initially through yeast genetics, include several protein kinases. Surprisingly few of their substrates have been identified. Here we identify the checkpoint protein Mad3 as an important substrate of the Mps1^Mph1 kinase. We show that Mps1^Mph1-dependent modification of Mad3 and Mad2 acts to delay cell division in situations where the genetic material would not be equally inherited by daughter cells. This delay enables the cell to correct any problems within the division machinery and thus avoid aneuploidy.

Spindle Checkpoint Factors Bub1 and Bub2 Promote DNA Double-Strand Break Repair by Nonhomologous End Joining

The nonhomologous end-joining (NHEJ) pathway is essential for the preservation of genome integrity, as it efficiently repairs DNA double-strand breaks (DSBs). Previous biochemical and genetic investigations have indicated that, despite the importance of this pathway, the entire complement of genes regulating NHEJ remains unknown. To address this, we employed a plasmid-based NHEJ DNA repair screen in budding yeast (Saccharomyces cerevisiae) using 369 putative nonessential DNA repair-related components as queries. Among the newly identified genes associated with NHEJ deficiency upon disruption are two spindle assembly checkpoint kinases, Bub1 and Bub2. Both observation of resulting phenotypes and chromatin immunoprecipitation demonstrated that Bub1 and -2, either alone or in combination with cell cycle regulators, are recruited near the DSB, where phosphorylated Rad53 or H2A accumulates. Large-scale proteomic analysis of Bub kinases phosphorylated in response to DNA damage identified previously unknown kinase substrates on Tel1 S/T-Q sites. Moreover, Bub1 NHEJ function appears to be conserved in mammalian cells. 53BP1, which influences DSB repair by NHEJ, colocalizes with human BUB1 and is recruited to the break sites. Thus, while Bub is not a core component of NHEJ machinery, our data support its dual role in mitotic exit and promotion of NHEJ repair in yeast and mammals.

The composition, functions, and regulation of the budding yeast kinetochore

The propagation of all organisms depends on the accurate and orderly segregation of chromosomes in mitosis and meiosis. Budding yeast has long served as an outstanding model organism to identify the components and underlying mechanisms that regulate chromosome segregation. This review focuses on the kinetochore, the macromolecular protein complex that assembles on centromeric chromatin and maintains persistent load-bearing attachments to the dynamic tips of spindle microtubules. The kinetochore also serves as a regulatory hub for the spindle checkpoint, ensuring that cell cycle progression is coupled to the achievement of proper microtubule-kinetochore attachments. Progress in understanding the composition and overall architecture of the kinetochore, as well as its properties in making and regulating microtubule attachments and the spindle checkpoint, is discussed.

Mitosis as an anti-cancer drug target

Suppression of cell proliferation by targeting mitosis is one potential cancer intervention. A number of existing chemotherapy drugs disrupt mitosis by targeting microtubule dynamics. While efficacious, these drugs have limitations, i.e. neuropathy, unpredictability and development of resistance. In order to overcome these issues, a great deal of effort has been spent exploring novel mitotic targets including Polo-like kinase 1, Aurora kinases, Mps1, Cenp-E and KSP/Eg5. Here we summarize the latest developments in the discovery and clinical evaluation of new mitotic drug targets.

Analysis of protein phosphatase-1 and aurora protein kinase suppressors reveals new aspects of regulatory protein function in Saccharomyces cerevisiae

Protein phosphatase-1 (PP1) controls many processes in eukaryotic cells. Modulation of mitosis by reversing phosphorylation of proteins phosphorylated by aurora protein kinase is a critical function for PP1. Overexpression of the sole PP1, Glc7, in budding yeast, Saccharomyces cerevisiae, is lethal. This work shows that lethality requires the function of Glc7 regulatory proteins Sds22, Reg2, and phosphorylated Glc8. This finding shows that Glc7 overexpression induced cell death requires a specific subset of the many Glc7-interacting proteins and therefore is likely caused by promiscuous dephosphorylation of a variety of substrates. Additionally, suppression can occur by reducing Glc7 protein levels by high-copy Fpr3 without use of its proline isomerase domain. This divulges a novel function of Fpr3. Most suppressors of GLC7 overexpression also suppress aurora protein kinase, ipl1, temperature-sensitive mutations. However, high-copy mutant SDS22 genes show reciprocal suppression of GLC7 overexpression induced cell death or ipl1 temperature sensitivity. Sds22 binds to many proteins besides Glc7. The N-terminal 25 residues of Sds22 are sufficient to bind, directly or indirectly, to seven proteins studied here including the spindle assembly checkpoint protein, Bub3. These data demonstrate that Sds22 organizes several proteins in addition to Glc7 to perform functions that counteract Ipl1 activity or lead to hyper Glc7 induced cell death. These data also emphasize that Sds22 targets Glc7 to nuclear locations distinct from Ipl1 substrates.

Construction of protein phosphorylation networks by data mining, text mining and ontology integration: analysis of the spindle checkpoint

Knowledge representation of the role of phosphorylation is essential for the meaningful understanding of many biological processes. However, such a representation is challenging because proteins can exist in numerous phosphorylated forms with each one having its own characteristic protein–protein interactions (PPIs), functions and subcellular localization. In this article, we evaluate the current state of phosphorylation event curation and then present a bioinformatics framework for the annotation and representation of phosphorylated proteins and construction of phosphorylation networks that addresses some of the gaps in current curation efforts. The integrated approach involves (i) text mining guided by RLIMS-P, a tool that identifies phosphorylation-related information in scientific literature; (ii) data mining from curated PPI databases; (iii) protein form and complex representation using the Protein Ontology (PRO); (iv) functional annotation using the Gene Ontology (GO); and (v) network visualization and analysis with Cytoscape. We use this framework to study the spindle checkpoint, the process that monitors the assembly of the mitotic spindle and blocks cell cycle progression at metaphase until all chromosomes have made bipolar spindle attachments. The phosphorylation networks we construct, centered on the human checkpoint kinase BUB1B (BubR1) and its yeast counterpart MAD3, offer a unique view of the spindle checkpoint that emphasizes biologically relevant phosphorylated forms, phosphorylation-state–specific PPIs and kinase–substrate relationships. Our approach for constructing protein phosphorylation networks can be applied to any biological process that is affected by phosphorylation.

Database URL:http://www.yeastgenome.org/

Use of the protein ontology for multi-faceted analysis of biological processes: a case study of the spindle checkpoint

As a member of the Open Biomedical Ontologies (OBO) foundry, the Protein Ontology (PRO) provides an ontological representation of protein forms and complexes and their relationships. Annotations in PRO can be assigned to individual protein forms and complexes, each distinguishable down to the level of post-translational modification, thereby allowing for a more precise depiction of protein function than is possible with annotations to the gene as a whole. Moreover, PRO is fully interoperable with other OBO ontologies and integrates knowledge from other protein-centric resources such as UniProt and Reactome. Here we demonstrate the value of the PRO framework in the investigation of the spindle checkpoint, a highly conserved biological process that relies extensively on protein modification and protein complex formation. The spindle checkpoint maintains genomic integrity by monitoring the attachment of chromosomes to spindle microtubules and delaying cell cycle progression until the spindle is fully assembled. Using PRO in conjunction with other bioinformatics tools, we explored the cross-species conservation of spindle checkpoint proteins, including phosphorylated forms and complexes; studied the impact of phosphorylation on spindle checkpoint function; and examined the interactions of spindle checkpoint proteins with the kinetochore, the site of checkpoint activation. Our approach can be generalized to any biological process of interest.

BRK1, a Bub1-related kinase, is essential for generating proper tension between homologous kinetochores at metaphase I of rice meiosis

Bub1 (for budding uninhibited by benzimidazole 1), one of the main spindle checkpoint kinases, acts as a kinetochore scaffold for assembling other checkpoint proteins. Here, we identify a plant Bub1-related kinase 1 (BRK1) in rice (Oryza sativa). The brk1 mutants are sterile due to the precocious separation of sister chromatids at the onset of anaphase I. The centromeric recruitment of SHUGOSHIN1 and phosphorylation of histone H2A at Thr-134 (H2A-pT134) depend on BRK1. Although the homologs can faithfully separate from each other at the end of meiosis I, the uncorrected merotelic attachment of paired sister kinetochores at the early stage of metaphase I in brk1 reduces the tension across homologous kinetochores, causes the metaphase I spindle to be aberrantly shaped, and subsequently affects the synchronicity of homolog separation at the onset of anaphase I. In addition, the phosphorylation of inner centromeric histone H3 at Ser-10 (H3-pS10) during diakinesis depends on BRK1. Therefore, we speculate that BRK1 may be required for normal localization of Aurora kinase before the onset of metaphase I, which is responsible for correcting the merotelic attachment.

Phosphoregulation of Spc105 by Mps1 and PP1 regulates Bub1 localization to kinetochores

Kinetochores are the macromolecular complexes that interact with microtubules to mediate chromosome segregation. Accurate segregation requires that kinetochores make bioriented attachments to microtubules from opposite poles. Attachments between kinetochores and microtubules are monitored by the spindle checkpoint, a surveillance system that prevents anaphase until every pair of chromosomes makes proper bioriented attachments. Checkpoint activity is correlated with the recruitment of checkpoint proteins to the kinetochore. Mps1 is a conserved protein kinase that regulates segregation and the spindle checkpoint, but few of the targets that mediate its functions have been identified. Here, we show that Mps1 is the major kinase activity that copurifies with budding yeast kinetochore particles and identify the conserved Spc105/KNL-1/blinkin kinetochore protein as a substrate. Phosphorylation of conserved MELT motifs within Spc105 recruits the Bub1 protein to kinetochores, and this is reversed by protein phosphatase I (PP1). Spc105 mutants lacking Mps1 phosphorylation sites are defective in the spindle checkpoint and exhibit growth defects. Together, these data identify Spc105 as a key target of the Mps1 kinase and show that the opposing activities of Mps1 and PP1 regulate the kinetochore localization of the Bub1 protein.

Kinase activity of fission yeast Mph1 is required for Mad2 and Mad3 to stably bind the anaphase promoting complex

Defects in chromosome segregation result in aneuploidy, which can lead to disease or cell death [1, 2]. The spindle checkpoint delays anaphase onset until all chromosomes are attached to spindle microtubules in a bipolar fashion [3, 4]. Mad2 is a key checkpoint component that undergoes conformational activation, catalyzed by a Mad1-Mad2 template enriched at unattached kinetochores [5]. Mad2 and Mad3 (BubR1) then bind and inhibit Cdc20 to form the mitotic checkpoint complex (MCC), which binds and inhibits the anaphase promoting complex (APC/C). Checkpoint kinases (Aurora, Bub1, and Mps1) are critical for checkpoint signaling, yet they have poorly defined roles and few substrates have been identified [6–8]. Here we demonstrate that a kinase-dead allele of the fission yeast MPS1 homolog (Mph1) is checkpoint defective and that levels of APC/C-associated Mad2 and Mad3 are dramatically reduced in this mutant. Thus, MCC binding to fission yeast APC/C is dependent on Mph1 kinase activity. We map and mutate several phosphorylation sites in Mad2, producing mutants that display reduced Cdc20-APC/C binding and an inability to maintain checkpoint arrest. We conclude that Mph1 kinase regulates the association of Mad2 with its binding partners and thereby mitotic arrest.

The architecture of the BubR1 tetratricopeptide tandem repeat defines a protein motif underlying mitotic checkpoint-kinetochore communication

The accurate and timely transmission of the genetic material to progeny during successive rounds of cell division is sine qua non for the maintenance of genome stability. Eukaryotic cells have evolved a surveillance mechanism, the mitotic spindle assembly checkpoint (SAC), to prevent premature advance to anaphase before every chromosome is properly attached to microtubules of the mitotic spindle. The architecture of the KNL1-BubR1 complex reveals important features of the molecular recognition between SAC components and the kinetochore. The interaction is important for a functional SAC as substitution of BubR1 residues engaged in KNL1 binding impaired the SAC and BubR1 recruitment into checkpoint complexes in stable cell lines. Here we discuss the implications of the disorder-to-order transition of KNL1 upon BubR1 binding for SAC signaling and propose a mechanistic model of how BUBs binding may affect the recognition of KNL1 by its other interacting partners.

The flavonoid eupatorin inactivates the mitotic checkpoint leading to polyploidy and apoptosis

The spindle assembly checkpoint (SAC) is a conserved mechanism that ensures the fidelity of chromosome distribution in mitosis by preventing anaphase onset until the correct bipolar microtubule-kinetochore attachments are formed. Errors in SAC function may contribute to tumorigenesis by inducing numerical chromosome anomalies (aneuploidy). On the other hand, total disruption of SAC can lead to massive genomic imbalance followed by cell death, a phenomena that has therapeutic potency. We performed a cell-based high-throughput screen with a compound library of 2000 bioactives for novel SAC inhibitors and discovered a plant-derived phenolic compound eupatorin (3',5-dihydroxy-4',6,7-trimethoxyflavone) as an anti-mitotic flavonoid. The premature override of the microtubule drug-imposed mitotic arrest by eupatorin is dependent on microtubule-kinetochore attachments but not interkinetochore tension. Aurora B kinase activity, which is essential for maintenance of normal SAC signaling, is diminished by eupatorin in cells and in vitro providing a mechanistic explanation for the observed forced mitotic exit. Eupatorin likely has additional targets since eupatorin treatment of pre-mitotic cells causes spindle anomalies triggering a transient M phase delay followed by impaired cytokinesis and polyploidy. Finally, eupatorin potently induces apoptosis in multiple cancer cell lines and suppresses cancer cell proliferation in organotypic 3D cell culture model.

Monitoring the fidelity of mitotic chromosome segregation by the spindle assembly checkpoint

Accurate chromosome segregation relies on activity of the spindle assembly checkpoint, a surveillance mechanism that prevents premature anaphase onset until all chromosomes are properly attached to the mitotic spindle apparatus and aligned at the metaphase plate. Defects in this mechanism contribute to chromosome instability and aneuploidy, a hallmark of malignant cells. Here, we review the molecular mechanisms of activation and silencing of the spindle assembly checkpoint and its relationship to tumourigenesis.

Ubiquitin ligase Ufd2 is required for efficient degradation of Mps1 kinase

Ufd2 is a U-box-containing ubiquitylation enzyme that promotes ubiquitin chain assembly on substrates. The physiological function of Ufd2 remains poorly understood. Here, we show that ubiquitylation and degradation of the cell cycle kinase Mps1, a known target of the anaphase-promoting complex E3, require Ufd2 enzyme. Yeast cells lacking UFD2 exhibit altered chromosome stability and several spindle-related phenotypes, expanding the biological function of Ufd2. We demonstrate that Ufd2-mediated Mps1 degradation is conserved in humans. Our results underscore the significance of Ufd2 in proteolysis and further suggest that Ufd2-like enzymes regulate far more substrates than previously envisioned.

emb-1 encodes the APC16 subunit of the Caenorhabditis elegans anaphase-promoting complex

In the nematode Caenorhabditis elegans, temperature-sensitive mutants of emb-1 arrest as one-cell embryos in metaphase of meiosis I in a manner that is indistinguishable from embryos that have been depleted of known subunits of the anaphase-promoting complex or cyclosome (APC/C). Here we show that the emb-1 phenotype is enhanced in double mutant combinations with known APC/C subunits and suppressed in double mutant combinations with known APC/C suppressors. In addition to its meiotic function, emb-1 is required for mitotic proliferation of the germline. These studies reveal that emb-1 encodes K10D2.4, a homolog of the small, recently discovered APC/C subunit, APC16.

Quantitative mass spectrometry analysis reveals similar substrate consensus motif for human Mps1 kinase and Plk1

BACKGROUND

Members of the Mps1 kinase family play an essential and evolutionarily conserved role in the spindle assembly checkpoint (SAC), a surveillance mechanism that ensures accurate chromosome segregation during mitosis. Human Mps1 (hMps1) is highly phosphorylated during mitosis and many phosphorylation sites have been identified. However, the upstream kinases responsible for these phosphorylations are not presently known.

METHODOLOGY/PRINCIPAL FINDINGS

Here, we identify 29 in vivo phosphorylation sites in hMps1. While in vivo analyses indicate that Aurora B and hMps1 activity are required for mitotic hyper-phosphorylation of hMps1, in vitro kinase assays show that Cdk1, MAPK, Plk1 and hMps1 itself can directly phosphorylate hMps1. Although Aurora B poorly phosphorylates hMps1 in vitro, it positively regulates the localization of Mps1 to kinetochores in vivo. Most importantly, quantitative mass spectrometry analysis demonstrates that at least 12 sites within hMps1 can be attributed to autophosphorylation. Remarkably, these hMps1 autophosphorylation sites closely resemble the consensus motif of Plk1, demonstrating that these two mitotic kinases share a similar substrate consensus.

CONCLUSIONS/SIGNIFICANCE

hMps1 kinase is regulated by Aurora B kinase and its autophosphorylation. Analysis on hMps1 autophosphorylation sites demonstrates that hMps1 has a substrate preference similar to Plk1 kinase.

Orthologues of the anaphase-promoting complex/cyclosome coactivators Cdc20p and Cdh1p are important for mitotic progression and morphogenesis in Candida albicans

The conserved anaphase-promoting complex/cyclosome (APC/C) system mediates protein degradation during mitotic progression. Conserved coactivators Cdc20p and Cdh1p regulate the APC/C during early to late mitosis and G(1) phase. Candida albicans is an important fungal pathogen of humans, and it forms highly polarized cells when mitosis is blocked through depletion of the polo-like kinase Cdc5p or other treatments. However, the mechanisms governing mitotic progression and associated polarized growth in the pathogen are poorly understood. In order to gain insights into these processes, we characterized C. albicans orthologues of Cdc20p and Cdh1p. Cdc20p-depleted cells were blocked in early or late mitosis with elevated levels of Cdc5p and the mitotic cyclin Clb2p, suggesting that Cdc20p is essential and has some conserved functions during mitosis. However, the yeast cells formed highly polarized buds in contrast to the large doublets of S. cerevisiae cdc20 mutants, implying a distinct role in morphogenesis. In comparison, cdh1Δ/cdh1Δ cells were viable but showed enrichment of Clb2p and Cdc5p, suggesting that Cdh1p may influence mitotic exit. The cdh1Δ/cdh1Δ phenotype was pleiotropic, consisting of normal or enlarged yeast, pseudohyphae, and some elongated buds, whereas S. cerevisiae cdh1Δ yeast cells were reduced in size. Thus, C. albicans Cdh1p may have some distinct functions. Finally, absence of Cdh1p or Cdc20p had a minor or no effect on hyphal development, respectively. Overall, the results suggest that Cdc20p and Cdh1p may be APC/C activators that are important for mitosis but also morphogenesis in C. albicans. Their novel features imply additional variations in function and underscore rewiring in the emerging mitotic regulatory networks of the pathogen.

Antitumor agents 283. Further elaboration of desmosdumotin C analogs as potent antitumor agents: activation of spindle assembly checkpoint as possible mode of action

In our ongoing study of the desmosdumotin C (1) series, twelve new analogues, 21-32, mainly with structural modifications in ring-A, were prepared and evaluated for in vitro antiproliferative activity against several human tumor cell lines. Among them, the 4'-iodo-3,3,5-tripropyl-4-methoxy analogue (31) showed significant antiproliferative activity against multiple human tumor cell lines with ED(50) values of 1.1-2.8 μM. Elongation of the C-3 and C-5 carbon chains reduced activity relative to propyl substituted analogues; however, activity was still better than that of natural compound 1. Among analogues with various ether groups on C-4, compounds with methyl (2) and propyl (26) ethers inhibited cell growth of multiple tumor cells lines, while 28 with an isobutyl ether showed selective antiproliferative activity against lung cancer A549 cells (ED(50) 1.7 μM). The gene expression profiles showed that 3 may modulate the spindle assembly checkpoint (SAC) and chromosome separation, and thus, arrest cells at the G2/M-phase.

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.

BUB1 and BUBR1: multifaceted kinases of the cell cycle

The multidomain protein kinases BUB1 and BUBR1 (Mad3 in yeast, worms and plants) are central components of the mitotic checkpoint for spindle assembly (SAC). This evolutionarily conserved and essential self-monitoring system of the eukaryotic cell cycle ensures the high fidelity of chromosome segregation by delaying the onset of anaphase until all chromosomes are properly bi-oriented on the mitotic spindle. Despite their amino acid sequence conservation and similar domain organization, BUB1 and BUBR1 perform different functions in the SAC. Recent structural information provides crucial molecular insights into the regulation and recognition of BUB1 and BUBR1, and a solid foundation to dissect the roles of these proteins in the control of chromosome segregation in normal and oncogenic cells.

Defining the molecular basis of BubR1 kinetochore interactions and APC/C-CDC20 inhibition

BubR1 is essential for the mitotic checkpoint that prevents aneuploidy in cellular progeny by triggering anaphase delay in response to kinetochores incorrectly/not attached to the mitotic spindle. Here, we define the molecular architecture of the functionally significant N-terminal region of human BubR1 and present the 1.8 A crystal structure of its tetratricopeptide repeat (TPR) domain. The structure reveals divergence from the classical TPR fold and is highly similar to the TPR domain of budding yeast Bub1. Shared distinctive features include a disordered loop insertion, a 3(10)-helix, a tight turn involving glycine positive Phi angles, and noncanonical packing of and between the TPR motifs. We also define the molecular determinants of the interaction between BubR1 and kinetochore protein Blinkin. We identify a shallow groove on the concave surface of the BubR1 TPR domain that forms multiple discrete and potentially cooperative interactions with Blinkin. Finally, we present evidence for a direct interaction between BubR1 and Bub1 mediated by regions C-terminal to their TPR domains. This interaction provides a mechanism for Bub1-dependent kinetochore recruitment of BubR1. We thus present novel molecular insights into the structure of BubR1 and its interactions at the kinetochore-microtubule interface. Our studies pave the way for future structure-directed engineering aimed at dissecting the roles of kinetochore-bound and other pools of BubR1 in vivo.