Genetic enhancers of partial PLK1 inhibition reveal hypersensitivity to kinetochore perturbations

Polo-like kinase 1 (PLK1) is a serine/threonine kinase required for mitosis and cytokinesis. As cancer cells are often hypersensitive to partial PLK1 inactivation, chemical inhibitors of PLK1 have been developed and tested in clinical trials. However, these small molecule inhibitors alone are not completely effective. PLK1 promotes numerous molecular and cellular events in the cell division cycle and it is unclear which of these events most crucially depend on PLK1 activity. We used a CRISPR-based genome-wide screening strategy to identify genes whose inactivation enhances cell proliferation defects upon partial chemical inhibition of PLK1. Genes identified encode proteins that are functionally linked to PLK1 in multiple ways, most notably factors that promote centromere and kinetochore function. Loss of the kinesin KIF18A or the outer kinetochore protein SKA1 in PLK1-compromised cells resulted in mitotic defects, activation of the spindle assembly checkpoint and nuclear reassembly defects. We also show that PLK1-dependent CENP-A loading at centromeres is extremely sensitive to partial PLK1 inhibition. Our results suggest that partial inhibition of PLK1 compromises the integrity and function of the centromere/kinetochore complex, rendering cells hypersensitive to different kinetochore perturbations. We propose that KIF18A is a promising target for combinatorial therapies with PLK1 inhibitors.

Identification of PP2A-B55 targets uncovers regulation of emerin during nuclear envelope reassembly in Drosophila

Mitotic exit requires the dephosphorylation of many proteins whose phosphorylation was needed for mitosis. Protein phosphatase 2A with its B55 regulatory subunit (PP2A-B55) promotes this transition. However, the events and substrates that it regulates are incompletely understood. We used proteomic approaches in Drosophila to identify proteins that interact with and are dephosphorylated by PP2A-B55. Among several candidates, we identified emerin (otefin in Drosophila). Emerin resides in the inner nuclear membrane and interacts with the DNA-binding protein barrier-to-autointegration factor (BAF) via a LEM domain. We found that the phosphorylation of emerin at Ser50 and Ser54 near its LEM domain negatively regulates its association with BAF, lamin and additional emerin in mitosis. We show that dephosphorylation of emerin at these sites by PP2A-B55 determines the timing of nuclear envelope reformation. Genetic experiments indicate that this regulation is required during embryonic development. Phosphoregulation of the emerin-BAF complex formation by PP2A-B55 appears as a key event of mitotic exit that is likely conserved across species.

Dephosphorylation in nuclear reassembly after mitosis

In most animal cell types, the interphase nucleus is largely disassembled during mitotic entry. The nuclear envelope breaks down and chromosomes are compacted into separated masses. Chromatin organization is also mostly lost and kinetochores assemble on centromeres. Mitotic protein kinases play several roles in inducing these transformations by phosphorylating multiple effector proteins. In many of these events, the mechanistic consequences of phosphorylation have been characterized. In comparison, how the nucleus reassembles at the end of mitosis is less well understood in mechanistic terms. In recent years, much progress has been made in deciphering how dephosphorylation of several effector proteins promotes nuclear envelope reassembly, chromosome decondensation, kinetochore disassembly and interphase chromatin organization. The precise roles of protein phosphatases in this process, in particular of the PP1 and PP2A groups, are emerging. Moreover, how these enzymes are temporally and spatially regulated to ensure that nuclear reassembly progresses in a coordinated manner has been partly uncovered. This review provides a global view of nuclear reassembly with a focus on the roles of dephosphorylation events. It also identifies important open questions and proposes hypotheses.

The spindle assembly checkpoint and the spatial activation of Polo kinase determine the duration of cell division and prevent tumor formation

The maintenance of a restricted pool of asymmetrically dividing stem cells is essential for tissue homeostasis. This process requires the control of mitotic progression that ensures the accurate chromosome segregation. In addition, this event is coupled to the asymmetric distribution of cell fate determinants in order to prevent stem cell amplification. How this coupling is regulated remains poorly described. Here, using asymmetrically dividing Drosophila neural stem cells (NSCs), we show that Polo kinase activity levels determine timely Cyclin B degradation and mitotic progression independent of the spindle assembly checkpoint (SAC). This event is mediated by the direct phosphorylation of Polo kinase by Aurora A at spindle poles and Aurora B kinases at centromeres. Furthermore, we show that Aurora A-dependent activation of Polo is the major event that promotes NSC polarization and together with the SAC prevents brain tumor growth. Altogether, our results show that an Aurora/Polo kinase module couples NSC mitotic progression and polarization for tissue homeostasis.

Spatiotemporal coordination of Greatwall-Endos-PP2A promotes mitotic progression

Mitotic entry involves inhibition of protein phosphatase 2A bound to its B55/Tws regulatory subunit (PP2A-B55/Tws), which dephosphorylates substrates of mitotic kinases. This inhibition is induced when Greatwall phosphorylates Endos, turning it into an inhibitor of PP2A-Tws. How this mechanism operates spatiotemporally in the cell is incompletely understood. We previously reported that the nuclear export of Greatwall in prophase promotes mitotic progression. Here, we examine the importance of the localized activities of PP2A-Tws and Endos for mitotic regulation. We find that Tws shuttles through the nucleus via a conserved nuclear localization signal (NLS), but expression of Tws in the cytoplasm and not in the nucleus rescues the development of tws mutants. Moreover, we show that Endos must be in the cytoplasm before nuclear envelope breakdown (NEBD) to be efficiently phosphorylated by Greatwall and to bind and inhibit PP2A-Tws. Disrupting the cytoplasmic function of Endos before NEBD results in subsequent mitotic defects. Evidence suggests that this spatiotemporal regulation is conserved in humans.

Evidence for a role of spindle matrix formation in cell cycle progression by antibody perturbation

In Drosophila it has recently been demonstrated that a spindle matrix in the form of a membrane-less macromolecular assembly embeds the microtubule-based spindle apparatus. In addition, two of its constituents, Megator and Chromator, were shown to function as spatial regulators of spindle checkpoint proteins. However, whether the spindle matrix plays a wider functional role in spatially regulating cell cycle progression factors was unknown. Here using a live imaging approach we provide evidence that a number of key cell cycle proteins such as Cyclin B, Polo, and Ran co-localize with the spindle matrix during mitosis. Furthermore, prevention of spindle matrix formation by injection of a function blocking antibody against the spindle matrix protein Chromator results in cell cycle arrest prior to nuclear envelope breakdown. In such embryos the spatial dynamics of Polo and Cyclin B enrichment at the nuclear rim and kinetochores is abrogated and Polo is not imported into the nucleus. This is in contrast to colchicine-arrested embryos where the wild-type dynamics of these proteins are maintained. Taken together these results suggest that spindle matrix formation may be a general requirement for the localization and proper dynamics of cell cycle factors promoting signaling events leading to cell cycle progression.

PP2A-B55 promotes nuclear envelope reformation after mitosis in Drosophila

As a dividing cell exits mitosis and daughter cells enter interphase, many proteins must be dephosphorylated. The protein phosphatase 2A (PP2A) with its B55 regulatory subunit plays a crucial role in this transition, but the identity of its substrates and how their dephosphorylation promotes mitotic exit are largely unknown. We conducted a maternal-effect screen in Drosophila melanogaster to identify genes that function with PP2A-B55/Tws in the cell cycle. We found that eggs that receive reduced levels of Tws and of components of the nuclear envelope (NE) often fail development, concomitant with NE defects following meiosis and in syncytial mitoses. Our mechanistic studies using Drosophila cells indicate that PP2A-Tws promotes nuclear envelope reformation (NER) during mitotic exit by dephosphorylating BAF and suggests that PP2A-Tws targets additional NE components, including Lamin and Nup107. This work establishes Drosophila as a powerful model to further dissect the molecular mechanisms of NER and suggests additional roles of PP2A-Tws in the completion of meiosis and mitosis.

A unified view of spatio-temporal control of mitotic entry: Polo kinase as the key

The Polo kinase is an essential regulator of cell division. Its ability to regulate multiple events at distinct subcellular locations and times during mitosis is remarkable. In the last few years, a much clearer mechanistic understanding of the functions and regulation of Polo in cell division has emerged. In this regard, the importance of coupling changes in activity with changes in localization is striking, both for Polo itself and for its upstream regulators. This review brings together several new pieces of the puzzle that are gradually revealing how Polo is regulated, in space and time, to enable its functions in the early stages of mitosis in animal cells. As a result, a unified view of how mitotic entry is spatio-temporally regulated is emerging.

Coupling of Polo kinase activation to nuclear localization by a bifunctional NLS is required during mitotic entry

The Polo kinase is a master regulator of mitosis and cytokinesis conserved from yeasts to humans. Polo is composed of an N-term kinase domain (KD) and a C-term polo-box domain (PBD), which regulates its subcellular localizations. The PBD and KD can interact and inhibit each other, and this reciprocal inhibition is relieved when Polo is phosphorylated at its activation loop. How Polo activation and localization are coupled during mitotic entry is unknown. Here we report that PBD binding to the KD masks a nuclear localization signal (NLS). Activating phosphorylation of the KD leads to exposure of the NLS and entry of Polo into the nucleus before nuclear envelope breakdown. Failures of this mechanism result in misregulation of the Cdk1-activating Cdc25 phosphatase and lead to mitotic and developmental defects in Drosophila. These results uncover spatiotemporal mechanisms linking master regulatory enzymes during mitotic entry.

Drosophila Polo kinase is the founding member of the Polo-Like Kinase (PLK) family and a master regulator of mitosis and cytokinesis. Here the authors uncover a molecular mechanism for the spatiotemporal regulation of Polo kinase during mitotic entry through a phosphorylation event that triggers nuclear import.

Several inhibitors of the Plk1 Polo-Box Domain turn out to be non-specific protein alkylators

For almost a decade, there has been much interest in the development of chemical inhibitors of Polo-like kinase 1 (Plk1) protein interactions. Plk1 is a master regulator of the cell division cycle that controls numerous substrates. It is a promising target for cancer drug development. Inhibitors of the kinase domain of Plk1 had some success in clinical trials. However, they are not perfectly selective. In principle, Plk1 can also be inhibited by interfering with its protein interaction domain, the Polo-Box Domain (PBD). Selective chemical inhibitors of the PBD would constitute tools to probe for PBD-dependent functions of Plk1 and could be advantageous in cancer therapy. The discovery of Poloxin and thymoquinone as PBD inhibitors indicated that small, cell-permeable chemical inhibitors could be identified. Other efforts followed, including ours, reporting additional molecules capable of blocking the PBD. It is now clear that, unfortunately, most of these compounds are non-specific protein alkylators (defined here as groups covalently added via a carbon) that have little or no potential for the development of real Plk1 PBD-specific drugs. This situation should be minded by biologists potentially interested in using these compounds to study Plk1. Further efforts are needed to develop selective, cell-permeable PBD inhibitors.

Identification of Polo-like kinase 1 interaction inhibitors using a novel cell-based assay

Polo-like kinase 1 (Plk1) plays several roles in cell division and it is a recognized cancer drug target. Plk1 levels are elevated in cancer and several types of cancer cells are hypersensitive to Plk1 inhibition. Small molecule inhibitors of the kinase domain (KD) of Plk1 have been developed. Their selectivity is limited, which likely contributes to their toxicity. Polo-like kinases are characterized by a Polo-Box Domain (PBD), which mediates interactions with phosphorylation substrates or regulators. Inhibition of the PBD could allow better selectivity or result in different effects than inhibition of the KD. In vitro screens have been used to identify PBD inhibitors with mixed results. We developed the first cell-based assay to screen for PBD inhibitors, using Bioluminescence Resonance Energy Transfer (BRET). We screened through 112 983 compounds and characterized hits in secondary biochemical and biological assays. Subsequent Structure-Activity Relationship (SAR) analysis on our most promising hit revealed that it requires an alkylating function for its activity. In addition, we show that the previously reported PBD inhibitors thymoquinone and Poloxin are also alkylating agents. Our cell-based assay is a promising tool for the identification of new PBD inhibitors with more drug-like profiles using larger and more diverse chemical libraries.

Spatial regulation of greatwall by Cdk1 and PP2A-Tws in the cell cycle

Entry into mitosis requires the phosphorylation of multiple substrates by cyclin B-Cdk1, while exit from mitosis requires their dephosphorylation, which depends largely on the phosphatase PP2A in complex with its B55 regulatory subunit (Tws in Drosophila). At mitotic entry, cyclin B-Cdk1 activates the Greatwall kinase, which phosphorylates Endosulfine proteins, thereby activating their ability to inhibit PP2A-B55 competitively. The inhibition of PP2A-B55 at mitotic entry facilitates the accumulation of phosphorylated Cdk1 substrates. The coordination of these enzymes involves major changes in their localization. In interphase, Gwl is nuclear while PP2A-B55 is cytoplasmic. We recently showed that Gwl suddenly relocalizes from the nucleus to the cytoplasm in prophase, before nuclear envelope breakdown and that this controlled localization of Gwl is required for its function. We and others have shown that phosphorylation of Gwl by cyclin B-Cdk1 at multiple sites is required for its nuclear exclusion, but the precise mechanisms remained unclear. In addition, how Gwl returns to its nuclear localization was not explored. Here we show that cyclin B-Cdk1 directly inactivates a Nuclear Localization Signal in the central region of Gwl. This phosphorylation facilitates the cytoplasmic retention of Gwl, which is exported to the cytoplasm in a Crm1-dependent manner. In addition, we show that PP2A-Tws promotes the return of Gwl to its nuclear localization during cytokinesis. Our results indicate that the cyclic changes in Gwl localization at mitotic entry and exit are directly regulated by the antagonistic cyclin B-Cdk1 and PP2A-Tws enzymes.

Understanding the Polo Kinase machine

The Polo Kinase is a central regulator of cell division required for several events of mitosis and cytokinesis. In addition to a kinase domain (KD), Polo-like kinases (Plks) comprise a Polo-Box domain (PBD), which mediates protein interactions with targets and regulators of Plks. In all organisms that contain Plks, one Plk family member fulfills several essential functions in the regulation of cell division, and here we refer to this conserved protein as Polo Kinase (Plk1 in humans). The PBD and the KD are capable of both cooperation and mutual inhibition in their functions. Crystal structures of the PBD, the KD and, recently, a PBD-KD complex have helped understanding the inner workings of the Polo Kinase. In parallel, an impressive array of molecular mechanisms has been found to mediate the regulation of the protein. Moreover, the targeting of Polo Kinase in the development of anti-cancer drugs has yielded several molecules with which to chemically modulate Polo Kinase to study its biological functions. Here we review our current understanding of the protein function and regulation of Polo Kinase as a fascinating molecular device in control of cell division.

Interdomain allosteric regulation of Polo kinase by Aurora B and Map205 is required for cytokinesis

Aurora B phosphorylation of the Polo kinase activation loop disrupts its binding to Map205 and central spindle microtubules, allowing it to be recruited to the site of cytokinesis.

Drosophila melanogaster Polo and its human orthologue Polo-like kinase 1 fulfill essential roles during cell division. Members of the Polo-like kinase (Plk) family contain an N-terminal kinase domain (KD) and a C-terminal Polo-Box domain (PBD), which mediates protein interactions. How Plks are regulated in cytokinesis is poorly understood. Here we show that phosphorylation of Polo by Aurora B is required for cytokinesis. This phosphorylation in the activation loop of the KD promotes the dissociation of Polo from the PBD-bound microtubule-associated protein Map205, which acts as an allosteric inhibitor of Polo kinase activity. This mechanism allows the release of active Polo from microtubules of the central spindle and its recruitment to the site of cytokinesis. Failure in Polo phosphorylation results in both early and late cytokinesis defects. Importantly, the antagonistic regulation of Polo by Aurora B and Map205 in cytokinesis reveals that interdomain allosteric mechanisms can play important roles in controlling the cellular functions of Plks.

Cell cycle: proteomics gives it a spin

The eukaryotic cell division cycle has been studied at the molecular level for over 30 years, most fruitfully in model organisms. In the past 5 years, developments in mass spectrometry-based proteomics have been applied to the study of protein interactions and post-translational modifications involving key cell cycle regulators such as cyclin-dependent kinases and the anaphase-promoting complex, as well as effectors such as centrosomes, the kinetochore and DNA replication forks. In addition, innovations in chemical biology, functional proteomics and bioinformatics have been employed to study the cell cycle at the proteome level. This review surveys the contributions of proteomics to cell cycle research. The near future should see the application of more quantitative proteomic approaches to probe the dynamic aspects of the molecular system that underlie the cell cycle in model organisms and in human cells.

Affinity purification of protein complexes from Drosophila embryos in cell cycle studies

The ability to identify protein interactions is key to elucidating the molecular mechanisms of cellular processes, including mitosis and cell cycle regulation. Drosophila melanogaster, as a model system, provides powerful tools to study cell division using genetics, microscopy, and RNAi. Drosophila early embryos are highly enriched in mitotic protein complexes as their nuclei undergo 13 rounds of rapid, synchronous mitotic nuclear divisions in a syncytium during the first 2 h of development. Here, we describe simple methods for the affinity purification of protein complexes from transgenic fly embryos via protein A- and green fluorescent protein-tags fused to bait proteins of interest. This in vivo proteomics approach has allowed the identification of several known and novel mitotic protein interactions using mass spectrometry, and it expands the use of the Drosophila model in modern molecular biology.

The Greatwall-PP2A axis in cell cycle control

Cell cycle progression is largely controlled by reversible protein phosphorylation mediated by cyclically activated kinases and phosphatases. It has long been known that cyclin B-Cdk1 activation triggers mitotic entry, and the enzymatic network controlling its activation and inactivation has been well characterized. Much more recently protein phosphatase 2A (PP2A) together with its B55 regulatory subunit has been recognized as the major activity dephosphorylating Cdk1 targets. Moreover, PP2A-B55 activity is high in late M phase and interphase, but low at mitotic entry. A series of discoveries in the fly and frog model systems have uncovered the molecular mechanism mediating this regulation. The Greatwall (Gwl) kinase activates endosulfines, which become specific inhibitors of PP2A-B55. Cdk1-dependent activation of Gwl at mitotic entry leads to PP2A-B55 downregulation, which synergizes with Cdk1 activation to promote the phosphorylated states of several mitotic substrates. Much less is known on the mechanisms inactivating Gwl and endosulfines at mitotic exit. Recent reports show the importance of spatiotemporal regulation of Gwl, endosulfines, and PP2A-B55 for cell cycle progression. The various systems and cell types differ in their dependence on the Gwl-PP2A axis for cell cycle progression. Moreover, this pathway also regulates gene expression in yeast, and this function could be conserved in metazoans.

Cell cycle regulation of Greatwall kinase nuclear localization facilitates mitotic progression

Greatwall kinase relocation from the nucleus to the cytoplasm is required at mitotic entry and is mediated by a phosphorylation-dependent mechanism targeting its central region.

Cell division requires the coordination of critical protein kinases and phosphatases. Greatwall (Gwl) kinase activity inactivates PP2A-B55 at mitotic entry to promote the phosphorylation of cyclin B–Cdk1 substrates, but how Gwl is regulated is poorly understood. We found that the subcellular localization of Gwl changed dramatically during the cell cycle in Drosophila. Gwl translocated from the nucleus to the cytoplasm in prophase. We identified two critical nuclear localization signals in the central, poorly characterized region of Gwl, which are required for its function. The Polo kinase associated with and phosphorylated Gwl in this region, promoting its binding to 14-3-3ε and its localization to the cytoplasm in prophase. Our results suggest that cyclin B–Cdk1 phosphorylation of Gwl is also required for its nuclear exclusion by a distinct mechanism. We show that the nucleo-cytoplasmic regulation of Gwl is essential for its functions in vivo and propose that the spatial regulation of Gwl at mitotic entry contributes to the mitotic switch.

Polo-like kinase-activating kinases: Aurora A, Aurora B and what else?

The events of cell division are regulated by a complex interplay between kinases and phosphatases. Cyclin-dependent kinases (Cdks), polo-like kinases (Plks) and Aurora kinases play central roles in this process. Polo kinase (Plk1 in humans) regulates a wide range of events in mitosis and cytokinesis. To ensure the accuracy of these processes, polo activity itself is subject to complex regulation. Phosphorylation of polo in its T loop (or activation loop) increases its kinase activity several-fold. It has been shown that Aurora A kinase, with its co-factor Bora, activates Plk1 in G(2), and that this is essential for recovery from cell cycle arrest induced by DNA damage. In a recent article published in PLoS Biology, we report that Drosophila polo is activated by Aurora B kinase at centromeres, and that this is crucial for polo function in regulating chromosome dynamics in prometaphase. Our results suggest that this regulatory pathway is conserved in humans. Here, we propose a model for the collaboration between Aurora B and polo in the regulation of kinetochore attachment to microtubules in early mitosis. Moreover, we suggest that Aurora B could also function to activate Polo/Plk1 in cytokinesis. Finally, we discuss recent findings and open questions regarding the activation of polo and polo-like kinases by different kinases in mitosis, cytokinesis and other processes.

Nessun Dorma, a novel centralspindlin partner, is required for cytokinesis in Drosophila spermatocytes

Nessun Dorma is a component of the ring canal with a polysaccharide-binding domain, which is important for cytokinesis during male meiosis.

Cytokinesis, the final step of cell division, usually ends with the abscission of the two daughter cells. In some tissues, however, daughter cells never completely separate and remain interconnected by intercellular bridges or ring canals. In this paper, we report the identification and analysis of a novel ring canal component, Nessun Dorma (Nesd), isolated as an evolutionarily conserved partner of the centralspindlin complex, a key regulator of cytokinesis. Nesd contains a pectin lyase–like domain found in proteins that bind to polysaccharides, and we present evidence that it has high affinity for β-galactosides in vitro. Moreover, nesd is an essential gene in Drosophila melanogaster, in which it is required for completion of cytokinesis during male meiosis and possibly in female germline cells. Our findings indicate that Nesd is a novel carbohydrate-binding protein that functions together with centralspindlin in late cytokinesis, thus highlighting the importance of glycosylation in this process.

Emerge: European Magnets with Enhanced Remanence for Greater Efficiency: a new and cost effective magnetic material for use in mid-priced applications

Emerge is an European project based on the collaboration between three industrial partners (Rhodia, Philips and Magnetfabrik Bonn) and three research laboratories (Centre National de la Recherche Scientifique CNRS (Laboratory Louis NEEL), Trinity College of Dublin TCD, Vienna University IATP). The aim of this project is to improve the coercivity in enhanced remanence material. This three years project used statistical set-ups to reduce the number of samples to be realised and tested. The complete description of the project is developed in this paper and the main conclusions on sample composition and process issues are also discussed.

Free centrosomes: where do they all come from?

Centrosomes act as major microtubule-organizing centers in most cell types. Their functions in interphase and mitosis are usually facilitated by their association with the nucleus. This may be particularly true in very large cells. Several papers report free centrosomes in syncytial Drosophila embryos. However, this phenotype often remains little explored. Yet, free centrosomes can occur by multiple mechanisms, including functional defects of the mitotic spindle, detachment of centrosomes from the nuclear envelope, centrosome inactivation upon DNA damage, and de novo centrosome genesis. Deciphering the cellular mechanism leading to free centrosomes upon a given perturbation such as a mutation or injection of a drug, can provide valuable clues regarding the nature of the molecular pathway affected. To this end, genetic and cytological tests, as well as time-lapse imaging are available. These studies can inform on the biology of centrosomes, cell cycle regulation and cytoskeletal dynamics. Here we briefly discuss what to make of free centrosomes in the fly embryo.

Drosophila Larp associates with poly(A)-binding protein and is required for male fertility and syncytial embryo development

As the influence of mRNA translation upon cell cycle regulation becomes clearer, we searched for genes that might specify such control in Drosophila. A maternal-effect lethal screen identified mutants in the Drosophila gene for Larp (La-related protein) which displayed maternal-effect lethality and male sterility. A role for La protein has already been implicated in mRNA translation whereas Larp has been proposed to regulate mRNA stability. Here we demonstrate that Larp exists in a physical complex with, and also interacts genetically with, the translation regulator poly(A)-binding protein (PABP). Most mutant alleles of pAbp are embryonic lethal. However hypomorphic pAbp alleles show similar meiotic defects to larp mutants. We find that larp mutant-derived syncytial embryos show a range of mitotic phenotypes, including failure of centrosomes to migrate around the nuclear envelope, detachment of centrosomes from spindle poles, the formation of multipolar spindle arrays and cytokinetic defects. We discuss why the syncytial mitotic cycles and male meiosis should have a particularly sensitive requirement for Larp proteins in regulating not only transcript stability but also potentially the translation of mRNAs.

Polo-like kinases: conservation and divergence in their functions and regulation

Polo-like kinases (Plks) are potent regulators of M phase that are conserved from yeasts to humans. Their roles in mitotic entry, spindle pole functions and cytokinesis are broadly conserved despite physical and molecular differences in these processes in disparate organisms. Plks are characterized by their Polo-box domain, which mediates protein interactions. They are additionally controlled by phosphorylation, proteolysis and transcription, depending on the biological context. Plks are now recognized to link cell division to developmental processes and to function in differentiated cells. A comparison of Plk function and regulation between organisms offers insight into the rich variations of cell division.

Isolation of protein complexes involved in mitosis and cytokinesis from Drosophila cultured cells

The identification of all the individual components that constitute the plethora of complexes in each cell type represents perhaps the most exciting challenge of postgenomic biology. This is particularly important in the study of events such as mitosis and cytokinesis, in which rapid and precise protein-protein interactions regulate both the direction and accuracy of these intricate processes. Here we describe an experimental strategy to isolate protein complexes involved in mitosis and cytokinesis in cultured Drosophila cells. This method involves the tagging of the bait protein with two IgG binding domains of Protein A and the isolation of the tagged bait along with its interacting partners by a single affinity purification step. These isolated complexes can then be analysed by several methods including mass spectrometry and Western blotting. Although this method has proven very successful in isolating mitotic and cytokinetic complexes, it can also be used to characterise protein complexes involved in many other cellular processes.

A bitter PP1 fights the sweet polo

In a recent paper in Developmental Cell, Yamashiro et al. (2008) report that the PP1 regulatory subunit MYPT1 interacts with PLK1 and antagonizes essential mitotic functions of PLK1, at least in part by promoting the dephosphorylation of PLK1 at Thr210.

Yeast Polo-like kinase substrates are nailed with the right tools

A platform has been built combining chemical genetics and bioinformatics to screen the proteome for physiological substrates of the Polo-like kinase in budding yeast. A novel role for this kinase in regulating the mitotic spindle is revealed.

Mutations in Drosophila Greatwall/Scant reveal its roles in mitosis and meiosis and interdependence with Polo kinase

Polo is a conserved kinase that coordinates many events of mitosis and meiosis, but how it is regulated remains unclear. Drosophila females having only one wild-type allele of the polo kinase gene and the dominant Scant mutation produce embryos in which one of the centrosomes detaches from the nuclear envelope in late prophase. We show that Scant creates a hyperactive form of Greatwall (Gwl) with altered specificity in vitro, another protein kinase recently implicated in mitotic entry in Drosophila and Xenopus. Excess Gwl activity in embryos causes developmental failure that can be rescued by increasing maternal Polo dosage, indicating that coordination between the two mitotic kinases is crucial for mitotic progression. Revertant alleles of Scant that restore fertility to polo-Scant heterozygous females are recessive alleles or deficiencies of gwl; they show chromatin condensation defects and anaphase bridges in larval neuroblasts. One recessive mutant allele specifically disrupts a Gwl isoform strongly expressed during vitellogenesis. Females hemizygous for this allele are sterile, and their oocytes fail to arrest in metaphase I of meiosis; both homologues and sister chromatids separate on elongated meiotic spindles with little or no segregation. This allelic series of gwl mutants highlights the multiple roles of Gwl in both mitotic and meiotic progression. Our results indicate that Gwl activity antagonizes Polo and thus identify an important regulatory interaction of the cell cycle.

Recruitment of Polo kinase to the spindle midzone during cytokinesis requires the Feo/Klp3A complex

Background

Polo-like kinases control multiple events during cell division, including mitotic entry, centrosome organization, spindle formation, chromosome segregation and cytokinesis. Their roles during cytokinesis, however, are not well understood because the requirement of these kinases during early stages of mitosis complicates the study of their functions after anaphase onset.

Methodology/Principal Findings

We used time-lapse microscopy to analyze the dynamics of Polo::GFP in Drosophila tissue culture cells during mitosis. After anaphase onset, Polo::GFP concentrated at the spindle midzone, but also diffused along the entire length of the central spindle. Using RNA interference we demonstrate that the microtubule-associated proteins Feo and Klp3A are required for Polo recruitment to the spindle midzone, but not the kinesin Pavarotti as previously thought. Moreover, we show that Feo and Klp3A form a complex and that Polo co-localizes with both proteins during cytokinesis.

Conclusion/Significance

Our results reveal that the Feo/Klp3A complex is necessary for Polo recruitment to the spindle midzone. A similar finding has also been recently reported in mammalian cells [1], suggesting that this basic mechanism has been conserved during evolution, albeit with some differences. Finally, since cleavage furrow formation and ingression are unaffected following feo RNAi, our data imply that Polo recruitment to the central spindle is not required for furrowing, but some other aspect of cytokinesis.

Molecular analysis of core kinetochore composition and assembly in Drosophila melanogaster

Background

Kinetochores are large multiprotein complexes indispensable for proper chromosome segregation. Although Drosophila is a classical model organism for studies of chromosome segregation, little is known about the organization of its kinetochores.

Methodology/Principal Findings

We employed bioinformatics, proteomics and cell biology methods to identify and analyze the interaction network of Drosophila kinetochore proteins. We have shown that three Drosophila proteins highly diverged from human and yeast Ndc80, Nuf2 and Mis12 are indeed their orthologues. Affinity purification of these proteins from cultured Drosophila cells identified a further five interacting proteins with weak similarity to subunits of the SPC105/KNL-1, MIND/MIS12 and NDC80 kinetochore complexes together with known kinetochore associated proteins such as dynein/dynactin, spindle assembly checkpoint components and heterochromatin proteins. All eight kinetochore complex proteins were present at the kinetochore during mitosis and MIND/MIS12 complex proteins were also centromeric during interphase. Their down-regulation led to dramatic defects in chromosome congression/segregation frequently accompanied by mitotic spindle elongation. The systematic depletion of each individual protein allowed us to establish dependency relationships for their recruitment onto the kinetochore. This revealed the sequential recruitment of individual members of first, the MIND/MIS12 and then, NDC80 complex.

Conclusions/Significance

The Drosophila MIND/MIS12 and NDC80 complexes and the Spc105 protein, like their counterparts from other eukaryotic species, are essential for chromosome congression and segregation, but are highly diverged in sequence. Hierarchical dependence relationships of individual proteins regulate the assembly of Drosophila kinetochore complexes in a manner similar, but not identical, to other organisms.

Multiple protein phosphatases are required for mitosis in Drosophila

BACKGROUND

Approximately one-third of the Drosophila kinome has been ascribed some cell-cycle function. However, little is known about which of its 117 protein phosphatases (PPs) or subunits have counteracting roles.

RESULTS

We investigated mitotic roles of PPs through systematic RNAi. We found that G(2)-M progression requires Puckered, the JNK MAP-kinase inhibitory phosphatase and PP2C in addition to string (Cdc25). Strong mitotic arrest and chromosome congression failure occurred after Pp1-87B downregulation. Chromosome alignment and segregation defects also occurred after knockdown of PP1-Flapwing, not previously thought to have a mitotic role. Reduction of several nonreceptor tyrosine phosphatases produced spindle and chromosome behavior defects, and for corkscrew, premature chromatid separation. RNAi of the dual-specificity phosphatase, Myotubularin, or the related Sbf "antiphosphatase" resulted in aberrant mitotic chromosome behavior. Finally, for PP2A, knockdown of the catalytic or A subunits led to bipolar monoastral spindles, knockdown of the Twins B subunit led to bridged and lagging chromosomes, and knockdown of the B' Widerborst subunit led to scattering of all mitotic chromosomes. Widerborst was associated with MEI-S332 (Shugoshin) and required for its kinetochore localization.

CONCLUSIONS

We identify cell-cycle roles for 22 of 117 Drosophila PPs. Involvement of several PPs in G(2) suggests multiple points for its regulation. Major mitotic roles are played by PP1 with tyrosine PPs and Myotubularin-related PPs having significant roles in regulating chromosome behavior. Finally, depending upon its regulatory subunits, PP2A regulates spindle bipolarity, kinetochore function, and progression into anaphase. Discovery of several novel cell-cycle PPs identifies a need for further studies of protein dephosphorylation.

Cyclin and cyclin-dependent kinase substrate requirements for preventing rereplication reveal the need for concomitant activation and inhibition

DNA replication initiation in S. cerevisiae is promoted by B-type cyclin-dependent kinase (Cdk) activity. In addition, once-per-cell-cycle replication is enforced by cyclin-Cdk-dependent phosphorylation of the prereplicative complex (pre-RC) components Mcm2-7, Cdc6, and Orc1-6. Several of these controls must be simultaneously blocked by mutation to obtain rereplication. We looked for but did not obtain strong evidence for cyclin specificity in the use of different mechanisms to control rereplication: both the S-phase cyclin Clb5 and the mitotic cyclins Clb1-4 were inferred to be capable of imposing ORC-based and MCM-based controls. We found evidence that the S-phase cyclin Clb6 could promote initiation of replication without blocking reinitiation, and this activity was highly toxic when the ability of other cyclins to block reinitiation was prevented by mutation. The failure of Clb6 to regulate reinitiation was due to rapid Clb6 proteolysis, since this toxic activity of Clb6 was lost when Clb6 was stabilized by mutation. Clb6-dependent toxicity is also relieved when early accumulation of mitotic cyclins is allowed to impose rereplication controls. Cell-cycle timing of rereplication control is crucial: sufficient rereplication block activity must be available as soon as firing begins. DNA rereplication induces DNA damage, and when rereplication controls are compromised, the DNA damage checkpoint factors Mre11 and Rad17 provide additional mechanisms that maintain viability and also prevent further rereplication, and this probably contributes to genome stability.

Analysis of protein phosphorylation by hypothesis-driven multiple-stage mass spectrometry

We describe a strategy, which we term hypothesis-driven multiple-stage mass spectrometry (HMS-MS), for the sensitive detection and identification of phosphopeptides derived from enzymatic digests of phosphoproteins. In this strategy, we postulate that any or all of the potential sites of phosphorylation in a given protein may be phosphorylated. Using this assumption, we calculate the m/z values of all the corresponding singly charged phosphopeptide ions that could, in theory, be produced by the enzyme employed for proteolysis. We test ions at these m/z values for the presence of phosphoserine or phosphothreonine residues using tandem mass spectrometry (MS(2)) in a vacuum MALDI ion trap mass spectrometer, where the neutral loss of the elements of H(3)PO(4) (98 Da) provides a sensitive assay for the presence of phosphopeptides. Subsequent MS(3) analysis of the (M + H - 98)(+) peaks allows us to confirm or reject the hypotheses that the putative phosphopeptides are present in the sample. HMS-MS was successfully applied to the detection and identification of phosphopeptides from substrates of the Saccharomyces cerevisiae cyclin-dependent kinase (Cdk) Cdc28, phosphorylated in vitro (Ipl1) and in vivo (Orc6), basing hypothesis formation on the minimal Cdk consensus phosphorylation motif Ser/Thr-Pro. The method was also used to find in vitro phosphopeptides from a domain of the Drosophila melanogaster protein PERIOD, hypothesizing possible phosphorylations of all Ser/Thr residues without assuming a consensus motif. Our results demonstrate that HMS-MS is a sensitive, highly specific tool for systematically surveying proteins for Ser/Thr phosphorylation, and represents a significant step toward our goal of comprehensive phosphorylation mapping.

Disruption of mechanisms that prevent rereplication triggers a DNA damage response

Eukaryotes replicate DNA once and only once per cell cycle due to multiple, partially overlapping mechanisms efficiently preventing reinitiation. The consequences of reinitiation are unknown. Here we show that the induction of rereplication by mutations in components of the prereplicative complex (origin recognition complex [ORC], Cdc6, and minichromosome maintenance proteins) causes a cell cycle arrest with activated Rad53, a large-budded morphology, and an undivided nucleus. Combining a mutation disrupting the Clb5-Orc6 interaction (ORC6-rxl) and a mutation stabilizing Cdc6 (CDC6(Delta)NT) causes a cell cycle delay with a similar phenotype, although this background is only partially compromised for rereplication control and does not exhibit overreplication detectable by fluorescence-activated cell sorting. We conducted a systematic screen that identified genetic requirements for the viability of these cells. ORC6-rxl CDC6(Delta)NT cells depend heavily on genes required for the DNA damage response and for double-strand-break repair by homologous recombination. Our results implicate an Mre11-Mec1-dependent pathway in limiting the extent of rereplication.

Two-Faced Cyclins with Eyes on the Targets

We recently reported that the 'hydrophobic patch' (HP) of the Saccharomyces cerevisiae S-phase cyclin Clb5 facilitates its interaction with Orc6 (via its Cy or RXL motif), providing a mechanism that helps prevent rereplication from individual origins. This is the first finding of a biological function for an interaction between a cyclin and a cyclin-binding motif (Cy or RXL motif) in a target protein in Saccharomyces cerevisiae. It is also the first such example involving a B-type cyclin in any organism. Yet, some of our observations as well as work from other groups suggest that HP-RXL interactions are functionally important for cyclin-Cdk signaling to other targets. The evolutionary conservation of the HP motif suggests that it allows cyclins to carry out important and specialized functions.

Targeted Proteomic Study of the Cyclin-Cdk Module

The cell division cycle of the yeast S. cerevisiae is driven by one Cdk (cyclin-dependent kinase), which becomes active when bound to one of nine cyclin subunits. Elucidation of Cdk substrates and other Cdk-associated proteins is essential for a full understanding of the cell cycle. Here, we report the results of a targeted proteomics study using affinity purification coupled to mass spectrometry. Our study identified numerous proteins in association with particular cyclin-Cdk complexes. These included phosphorylation substrates, ubiquitination-degradation proteins, adaptors, and inhibitors. Some associations were previously known, and for others, we confirmed their specificity and biological relevance. Using a hypothesis-driven mass spectrometric approach, we also mapped in vivo phosphorylation at Cdk consensus motif-containing peptides within several cyclin-associated candidate Cdk substrates. Our results demonstrate that this approach can be used to detect a host of transient and dynamic protein associations within a biological module.

Interaction of the S-phase cyclin Clb5 with an "RXL" docking sequence in the initiator protein Orc6 provides an origin-localized replication control switch

Cyclin-dependent kinases are critical regulators of eukaryotic DNA replication. We show that the S-phase cyclin Clb5 binds stably and directly to the origin recognition complex (ORC). This interaction is mediated by an "RXL" target sequence, or "Cy" motif, in the Orc6 subunit that is recognized by the "hydrophobic patch" region on Clb5. The Clb5-Orc6 interaction requires replication initiation, and is maintained throughout the remainder of S phase and into M phase. Eliminating the Clb5-Orc6 interaction has no effect on initiation of replication but instead sensitizes cells to lethal overreplication. We propose that Clb5 binding to ORC provides an origin-localized replication control switch that specifically prevents reinitiation at replicated origins.

Genetic and biochemical evaluation of the importance of Cdc6 in regulating mitotic exit

We evaluated the hypothesis that the N-terminal region of the replication control protein Cdc6 acts as an inhibitor of cyclin-dependent kinase (Cdk) activity, promoting mitotic exit. Cdc6 accumulation is restricted to the period from mid-cell cycle until the succeeding G1, due to proteolytic control that requires the Cdc6 N-terminal region. During late mitosis, Cdc6 is present at levels comparable with Sic1 and binds specifically to the mitotic cyclin Clb2. Moderate overexpression of Cdc6 promotes viability of CLB2Deltadb strains, which otherwise arrest at mitotic exit, and rescue is dependent on the N-terminal putative Cdk-inhibitory domain. These observations support the potential for Cdc6 to inhibit Clb2-Cdk, thus promoting mitotic exit. Consistent with this idea, we observed a cytokinesis defect in cdh1Delta sic1Delta cdc6Delta2-49 triple mutants. However, we were able to construct viable strains, in three different backgrounds, containing neither SIC1 nor the Cdc6 Cdk-inhibitory domain, in contradiction to previous work. We conclude, therefore, that although both Cdc6 and Sic1 have the potential to facilitate mitotic exit by inhibiting Clb2-Cdk, mitotic exit nevertheless does not require any identified stoichiometric inhibitor of Cdk activity.

Testing a mathematical model of the yeast cell cycle

We derived novel, testable predictions from a mathematical model of the budding yeast cell cycle. A key qualitative prediction of bistability was confirmed in a strain simultaneously lacking cdc14 and G1 cyclins. The model correctly predicted quantitative dependence of cell size on gene dosage of the G1 cyclin CLN3, but it incorrectly predicted strong genetic interactions between G1 cyclins and the anaphase-promoting complex specificity factor Cdh1. To provide constraints on model generation, we determined accurate concentrations for the abundance of all nine cyclins as well as the inhibitor Sic1 and the catalytic subunit Cdc28. For many of these we determined abundance throughout the cell cycle by centrifugal elutriation, in the presence or absence of Cdh1. In addition, perturbations to the Clb-kinase oscillator were introduced, and the effects on cyclin and Sic1 levels were compared between model and experiment. Reasonable agreement was obtained in many of these experiments, but significant experimental discrepancies from the model predictions were also observed. Thus, the model is a strong but incomplete attempt at a realistic representation of cell cycle control. Constraints of the sort developed here will be important in development of a truly predictive model.

The function of interdomain interactions in controlling nucleotide exchange rates in transducin

The intramolecular contacts in heterotrimeric G proteins that determine the rates of basal and receptor-stimulated nucleotide exchange are not fully understood. The alpha subunit of heterotrimeric G proteins consists of two domains: a Ras-like domain with structural homology to the monomeric G protein Ras and a helical domain comprised of six alpha-helices. The bound nucleotide lies in a deep cleft between the two domains. Exchange of the bound nucleotide may involve opening of this cleft. Thus interactions between the domains may affect the rate of nucleotide exchange in G proteins. We have tested this hypothesis in the alpha subunit of the rod cell G protein transducin (Galpha(t)). Site-directed mutations were prepared in a series of residues located at the interdomain interface. The proteins were expressed in vitro in a reticulocyte lysate system. The rates of basal and rhodopsin-catalyzed nucleotide exchange were determined using a trypsin digestion assay specifically adapted for kinetic measurements. Charge-altering substitutions of two residues at the interdomain interface, Lys(273) and Lys(276), increased basal nucleotide exchange rates modestly (5-10-fold). However, we found no evidence that interactions spanning the two domains in Galpha(t) significantly affected either basal or rhodopsin-catalyzed nucleotide exchange rates. These results suggest that opening of the interdomain cleft is not an energetic barrier to nucleotide exchange in Galpha(t). Experiments with Galpha(i1) suggest by comparison that the organization and function of the interdomain region differ among various G protein subtypes.

A protein kinase C site highly conserved in P2X subunits controls the desensitization kinetics of P2X(2) ATP-gated channels

P2X receptors are nonselective cation channels gated by extracellular ATP. Recombinant mammalian P2X subunits assemble in homomeric ionotropic ATP receptors that differ by their agonist sensitivity and desensitization rate in heterologous expression systems. Using site-directed mutagenesis and voltage clamp recording in Xenopus oocytes, we identified the highly conserved protein kinase C site TX(K/R) located in the intracellular N terminus of P2X subunits as a critical determinant of kinetics in slowly desensitizing (time constant, >1 min) rat P2X(2) receptors. Mutant receptors P2X(2)T18A, T18N, and K20T devoid of this consensus site exhibited quickly desensitizing properties (time constant, <1 s). In contrast with wild-type receptors, mutant P2X(2) receptors with truncated C terminus exhibited variable cell-specific kinetics with quickly desensitizing currents converted to slowly desensitizing currents by phorbol ester-mediated stimulation of protein kinase C. Phosphorylation of Thr(18) was demonstrated directly by immunodetection using specific monoclonal antibodies directed against the phosphothreonine-proline motif. Our data indicate that both phosphorylation of the conserved threonine residue in the N-terminal domain by protein kinase C and interaction between the two cytoplasmic domains of P2X(2) subunits are necessary for the full expression of slowly desensitizing ATP-gated channels.

Functional and biochemical evidence for heteromeric ATP-gated channels composed of P2X1 and P2X5 subunits

The mammalian P2X receptor gene family encodes two-transmembrane domain nonselective cation channels gated by extracellular ATP. Anatomical localization data obtained by in situ hybridization and immunocytochemistry have shown that neuronal P2X subunits are expressed in specific but overlapping distribution patterns. Therefore, the native ionotropic ATP receptors diversity most likely arises from interactions between different P2X subunits that generate hetero-multimers phenotypically distinct from homomeric channels. Rat P2X1 and P2X5 mRNAs are localized within common subsets of peripheral and central sensory neurons as well as spinal motoneurons. The present study demonstrates a functional association between P2X1 and P2X5 subunits giving rise to hybrid ATP-gated channels endowed with the pharmacology of P2X1 and the kinetics of P2X5. When expressed in Xenopus oocytes, hetero-oligomeric P2X1+5 ATP receptors were characterized by slowly desensitizing currents highly sensitive to the agonist alpha,beta-methylene ATP (EC50 = 1.1 microM) and to the antagonist trinitrophenyl ATP (IC50 = 64 nM), observed with neither P2X1 nor P2X5 alone. Direct physical evidence for P2X1+5 co-assembly was provided by reciprocal subunit-specific co-purifications between epitope-tagged P2X1 and P2X5 subunits transfected in HEK-293A cells.