The role of the yeast spindle pole body and the mammalian centrosome in regulating late mitotic events

RNA and Protein Detection by Single-Molecule Fluorescent in Situ Hybridization (smFISH) Combined with Immunofluorescence in the Budding Yeast S. cerevisiae

The inherent stochastic processes governing gene expression give rise to heterogeneity across individual cells, highlighting the importance of single-cell studies. The emergence of single-molecule fluorescent in situ hybridization (smFISH) enabled gene expression analysis at the single-cell level while including the spatial dimension through the visualization and quantification of mRNAs in intact fixed cells. By combining smFISH with immunofluorescence (IF), a comprehensive approach takes shape facilitating the study of mRNAs and proteins to correlate gene expression profiles to different cellular states. This chapter serves as a comprehensive guide to a smFISH-IF protocol optimized for gene expression analysis in the budding yeast S. cerevisiae. We utilize smFISH to visualize the mRNA localization pattern of the CLB2 cyclin over the course of the cell cycle inferred by alpha-tubulin IF.

The signalling lipid PI3,5P2 is essential for timely mitotic exit

Coordination of mitotic exit with chromosome segregation is key for successful mitosis. Mitotic exit in budding yeast is executed by the mitotic exit network (MEN), which is negatively regulated by the spindle position checkpoint (SPOC). SPOC kinase Kin4 is crucial for SPOC activation in response to spindle positioning defects. Here, we report that the lysosomal signalling lipid phosphatidylinositol-3,5-bisphosphate (PI3,5P2) has an unanticipated role in the timely execution of mitotic exit. We show that the lack of PI3,5P2 causes a delay in mitotic exit, whereas elevated levels of PI3,5P2 accelerates mitotic exit in mitotic exit defective cells. Our data indicate that PI3,5P2 promotes mitotic exit in part through impairment of Kin4. This process is largely dependent on the known PI3,5P2 effector protein Atg18. Our work thus uncovers a novel link between PI3,5P2 and mitotic exit.

Moonlighting at the Poles: Non-Canonical Functions of Centrosomes

Centrosomes are best known as the microtubule organizing centers (MTOCs) of eukaryotic cells. In addition to their classic role in chromosome segregation, centrosomes play diverse roles unrelated to their MTOC activity during cell proliferation and quiescence. Metazoan centrosomes and their functional doppelgängers from lower eukaryotes, the spindle pole bodies (SPBs), act as important structural platforms that orchestrate signaling events essential for cell cycle progression, cellular responses to DNA damage, sensory reception and cell homeostasis. Here, we provide a critical overview of the unconventional and often overlooked roles of centrosomes/SPBs in the life cycle of eukaryotic cells.

Protein phosphatase 1 in association with Bud14 inhibits mitotic exit in Saccharomyces cerevisiae

Mitotic exit in budding yeast is dependent on correct orientation of the mitotic spindle along the cell polarity axis. When accurate positioning of the spindle fails, a surveillance mechanism named the spindle position checkpoint (SPOC) prevents cells from exiting mitosis. Mutants with a defective SPOC become multinucleated and lose their genomic integrity. Yet, a comprehensive understanding of the SPOC mechanism is missing. In this study, we identified the type 1 protein phosphatase, Glc7, in association with its regulatory protein Bud14 as a novel checkpoint component. We further showed that Glc7-Bud14 promotes dephosphorylation of the SPOC effector protein Bfa1. Our results suggest a model in which two mechanisms act in parallel for a robust checkpoint response: first, the SPOC kinase Kin4 isolates Bfa1 away from the inhibitory kinase Cdc5, and second, Glc7-Bud14 dephosphorylates Bfa1 to fully activate the checkpoint effector.

Simultaneous Detection of mRNA and Protein in S. cerevisiae by Single-Molecule FISH and Immunofluorescence

Single-molecule fluorescent in situ hybridization (smFISH) enables the detection and quantification of endogenous mRNAs within intact fixed cells. This method utilizes tens of singly labeled fluorescent DNA probes hybridized against the mRNA of interest, which can be detected by using standard wide-field fluorescence microscopy. This approach provides the means to generate absolute quantifications of gene expression within single cells, which can be used to link molecular fluctuations to phenotypes. To be able to correlate the expression of an mRNA and a protein of interest in individual cells, we combined smFISH with immunofluorescence (IF) in yeast cells. Here, we present our smFISH-IF protocol to visualize and quantify two cell cycle-controlled mRNAs (CLN2 and ASH1) and the cell cycle marker alpha-tubulin in S. cerevisiae. This protocol, which is performed over 2 days, can be used to visualize up to three colors at the time (i.e., two mRNAs, one protein). Even if the described protocol is designed for S. cerevisiae, we think that the considerations discussed here can be useful to develop and troubleshoot smFISH-IF protocols for other model organisms.

Crosstalk Between Pheromone Signaling and NADPH Oxidase Complexes Coordinates Fungal Developmental Processes

Sexual and asexual development in filamentous ascomycetes is controlled by components of conserved signaling pathways. Here, we investigated the development of mutant strains lacking genes for kinases MAK2, MEK2, and MIK2, as well as the scaffold protein HAM5 of the pheromone response (PR) pathway. All had a defect in fruiting body development and hyphal fusion. Another phenotype was a defect in melanin-dependent ascospore germination. However, this deficiency was observed only in kinase deletion mutants, but not in strains lacking HAM5. Notably, the same developmental phenotypes were previously described for nicotinamide adenine dinucleotide phosphate (NADPH) oxidase 1 (NOX1) mutants, but the germination defect was only seen in NOX2 mutants. These data suggest a molecular link between the pheromone signaling pathway and both NOX complexes. Using data from yeast two-hybrid (Y2H) analysis, we found that the scaffolding protein HAM5 interacts with NOR1, the regulator of NOX1 and NOX2 complexes. This interaction was further confirmed using differently fluorescent-labeled proteins to demonstrate that NOR1 and HAM5 co-localize at cytoplasmic spots and tips of mature hyphae. This observation was supported by phenotypic characterization of single and double mutants. The oxidative stress response and the initiation of fruiting bodies were similar in Δham5Δnor1 and Δham5, but distinctly reduced in Δnor1, indicating that the double deletion leads to a partial suppression of the Δnor1 phenotype. We conclude that the PR and NOX1 complexes are connected by direct interaction between HAM5 and NOR1. In contrast, PR kinases are linked to the NOX2 complex without participation of HAM5.

Single molecule mRNA fluorescent in situ hybridization combined with immunofluorescence in S. cerevisiae: Dataset and quantification

Single-molecule fluorescent in situ hybridization (smFISH) has emerged as a powerful technique that allows one to localize and quantify the absolute number of mRNAs in single cells. In combination with immunofluorescence (IF), smFISH can be used to correlate the expression of an mRNA and a protein of interest in single cells. Here, we provide and quantify an smFISH-IF dataset in S. cerevisiae. We measured the expression of the cell cycle-controlled mRNA CLN2 and the cell cycle marker alpha-tubulin. The smFISH-IF protocol describing the dataset generation is published in the accompanying article "Simultaneous detection of mRNA and protein in S. cerevisiae by single-molecule FISH and Immunofluorescence" [1]. Here, we analyze the smFISH data using the freely available software FISH-quant [2]. The provided datasets are intended to assist scientists interested in setting up smFISH-IF protocol in their laboratory. Furthermore, scientists interested in the generation of imaging analysis tools for single-cell approaches may find the provided dataset useful. To this end, we provide the differential interference contrast (DIC) channel, as well as multicolor, raw Z-stacks for smFISH, IF and DAPI.

Critical slowing down and attractive manifold: A mechanism for dynamic robustness in the yeast cell-cycle process

Biological processes that execute complex multiple functions, such as the cell cycle, must ensure the order of sequential events and maintain dynamic robustness against various fluctuations. Here, we examine the mechanisms and fundamental structure that achieve these properties in the cell cycle of the budding yeast Saccharomyces cerevisiae. We show that this process behaves like an excitable system containing three well-decoupled saddle-node bifurcations to execute DNA replication and mitosis events. The yeast cell-cycle regulatory network can be divided into three modules-the G1/S phase, early M phase, and late M phase-wherein both positive feedback loops in each module and interactions among modules play important roles. Specifically, when the cell-cycle process operates near the critical points of the saddle-node bifurcations, a critical slowing down effect takes place. Such interregnum then allows for an attractive manifold and sufficient duration for cell-cycle events, within which to assess the completion of DNA replication and mitosis, e.g., spindle assembly. Moreover, such arrangement ensures that any fluctuation in an early module or event will not transmit to a later module or event. Thus, our results suggest a possible dynamical mechanism of the cell-cycle process to ensure event order and dynamic robustness and give insight into the evolution of eukaryotic cell-cycle processes.

Cdc14 activation requires coordinated Cdk1-dependent phosphorylation of Net1 and PP2A-Cdc55 at anaphase onset

Exit from mitosis and completion of cytokinesis require the inactivation of mitotic cyclin-dependent kinase (Cdk) activity. In budding yeast, Cdc14 phosphatase is a key mitotic regulator that is activated in anaphase to counteract Cdk activity. In metaphase, Cdc14 is kept inactive in the nucleolus, where it is sequestered by its inhibitor, Net1. At anaphase onset, downregulation of PP2ACdc55 phosphatase by separase and Zds1 protein promotes Net1 phosphorylation and, consequently, Cdc14 release from the nucleolus. The mechanism by which PP2ACdc55 activity is downregulated during anaphase remains to be elucidated. Here, we demonstrate that Cdc55 regulatory subunit is phosphorylated in anaphase in a Cdk1-Clb2-dependent manner. Interestingly, cdc55-ED phosphomimetic mutant inactivates PP2ACdc55 phosphatase activity towards Net1 and promotes Cdc14 activation. Separase and Zds1 facilitate Cdk-dependent Net1 phosphorylation and Cdc14 release from the nucleolus by modulating PP2ACdc55 activity via Cdc55 phosphorylation. In addition, human Cdk1-CyclinB1 phosphorylates human B55, indicating that the mechanism is conserved in higher eukaryotes.

Breaking Bad: Uncoupling of Modularity in Centriole Biogenesis and the Generation of Excess Centrioles in Cancer

Centrosomes are tiny yet complex cytoplasmic structures that perform a variety of roles related to their ability to act as microtubule-organizing centers. Like the genome, centrosomes are single copy structures that undergo a precise semi-conservative replication once each cell cycle. Precise replication of the centrosome is essential for genome integrity, because the duplicated centrosomes will serve as the poles of a bipolar mitotic spindle, and any number of centrosomes other than two will lead to an aberrant spindle that mis-segregates chromosomes. Indeed, excess centrosomes are observed in a variety of human tumors where they generate abnormal spindles in situ that are thought to participate in tumorigenesis by driving genomic instability. At the heart of the centrosome is a pair of centrioles, and at the heart of centrosome duplication is the replication of this centriole pair. Centriole replication proceeds through a complex macromolecular assembly process. However, while centrosomes may contain as many as 500 proteins, only a handful of proteins have been shown to be essential for centriole replication. Our observations suggest that centriole replication is a modular, bottom-up process that we envision akin to building a house; the proper site of assembly is identified, a foundation is assembled at that site, and subsequent modules are added on top of the foundation. Here, we discuss the data underlying our view of modularity in the centriole assembly process, and suggest that non-essential centriole assembly factors take on greater importance in cancer cells due to their function in coordination between centriole modules, using the Monopolar spindles 1 protein kinase and its substrate Centrin 2 to illustrate our model.

Budding Yeast BFA1 Has Multiple Positive Roles in Directing Late Mitotic Events

The proper regulation of cell cycle transitions is paramount to the maintenance of cellular genome integrity. In Saccharomyces cerevisiae, the mitotic exit network (MEN) is a Ras-like signaling cascade that effects the transition from M phase to G1 during the cell division cycle in budding yeast. MEN activation is tightly regulated. It occurs during anaphase and is coupled to mitotic spindle position by the spindle position checkpoint (SPoC). Bfa1 is a key component of the SPoC and functions as part of a two-component GAP complex along with Bub2 The GAP activity of Bfa1-Bub2 keeps the MEN GTPase Tem1 inactive in cells with mispositioned spindles, thereby preventing inappropriate mitotic exit and preserving genome integrity. Interestingly, a GAP-independent role for Bfa1 in mitotic exit regulation has been previously identified. However the nature of this Bub2-independent role and its biological significance are not understood. Here we show that Bfa1 also activates the MEN by promoting the localization of Tem1 primarily to the daughter spindle pole body (dSPB). We demonstrate that the overexpression of BFA1 is lethal due to defects in Tem1 localization, which is required for its activity. In addition, our studies demonstrate a Tem1-independent role for Bfa1 in promoting proper cytokinesis. Cells lacking TEM1, in which the essential mitotic exit function is bypassed, exhibit cytokinesis defects. These defects are suppressed by the overexpression of BFA1 We conclude that Bfa1 functions to both inhibit and activate late mitotic events.

An improved MS2 system for accurate reporting of the mRNA life cycle

The MS2-MCP system enables researchers to image multiple steps of the mRNA life cycle with high temporal and spatial resolution. However, for short-lived mRNAs, the tight binding of the MS2 coat protein (MCP) to the MS2 binding sites (MBS) protects the RNA from being efficiently degraded, and this confounds the study of mRNA regulation. Here, we describe a reporter system (MBSV6) with reduced affinity for the MCP, which allows mRNA degradation while preserving single-molecule detection determined by single-molecule FISH (smFISH) or live imaging. Constitutive mRNAs (MDN1 and DOA1) and highly-regulated mRNAs (GAL1 and ASH1) endogenously tagged with MBSV6 in Saccharomyces cerevisiae degrade normally. As a result, short-lived mRNAs were imaged throughout their complete life cycle. The MBSV6 reporter revealed that, in contrast to previous findings, coordinated recruitment of mRNAs at specialized structures such as P-bodies during stress did not occur, and mRNA degradation was heterogeneously distributed in the cytoplasm.

Comparative Interactomes of VRK1 and VRK3 with Their Distinct Roles in the Cell Cycle of Liver Cancer

Vaccinia-related kinase 1 (VRK1) and VRK3 are members of the VRK family of serine/threonine kinases and are principally localized in the nucleus. Despite the crucial roles of VRK1/VRK3 in physiology and disease, the molecular and functional interactions of VRK1/VRK3 are poorly understood. Here, we identified over 200 unreported VRK1/VRK3-interacting candidate proteins by affinity purification and LC-MS/MS. The networks of VRK1 and VRK3 interactomes were found to be associated with important biological processes such as the cell cycle, DNA repair, chromatin assembly, and RNA processing. Interactions of interacting proteins with VRK1/VRK3 were confirmed by biochemical assays. We also found that phosphorylations of XRCC5 were regulated by both VRK1/VRK3, and that of CCNB1 was regulated by VRK3. In liver cancer cells and tissues, VRK1/VRK3 were highly upregulated and its depletion affected cell cycle progression in the different phases. VRK3 seemed to affect S phase progression and G2 or M phase entry and exit, whereas VRK1 affects G1/S transition in the liver cancer, which could be explained by different interacting candidate proteins. Thus, this study not only provides a resource for investigating the unidentified functions of VRK1/VRK3, but also an insight into the regulatory roles of VRK1/VRK3 in biological processes.

Cytoskeletal variations in an asymmetric cell division support diversity in nematode sperm size and sex ratios

Asymmetric partitioning is an essential component of many developmental processes. As spermatogenesis concludes, sperm are streamlined by discarding unnecessary cellular components into cellular wastebags called residual bodies (RBs). During nematode spermatogenesis, this asymmetric partitioning event occurs shortly after anaphase II, and both microtubules and actin partition into a central RB. Here, we use fluorescence and transmission electron microscopy to elucidate and compare the intermediate steps of RB formation in Caenorhabditis elegans, Rhabditis sp. SB347 (recently named Auanema rhodensis) and related nematodes. In all cases, intact microtubules reorganize and move from centrosomal to non-centrosomal sites at the RB-sperm boundary whereas actin reorganizes through cortical ring expansion and clearance from the poles. However, in species with tiny spermatocytes, these cytoskeletal changes are restricted to one pole. Consequently, partitioning yields one functional sperm with the X-bearing chromosome complement and an RB with the other chromosome set. Unipolar partitioning may not require an unpaired X, as it also occurs in XX spermatocytes. Instead, constraints related to spermatocyte downsizing may have contributed to the evolution of a sperm cell equivalent to female polar bodies.

Temporal and compartment-specific signals coordinate mitotic exit with spindle position

The spatiotemporal control of mitotic exit is crucial for faithful chromosome segregation during mitosis. In budding yeast, the mitotic exit network (MEN) drives cells out of mitosis, whereas the spindle position checkpoint (SPOC) blocks MEN activity when the anaphase spindle is mispositioned. How the SPOC operates at a molecular level remains unclear. Here, we report novel insights into how mitotic signalling pathways orchestrate chromosome segregation in time and space. We establish that the key function of the central SPOC kinase, Kin4, is to counterbalance MEN activation by the cdc fourteen early anaphase release (FEAR) network in the mother cell compartment. Remarkably, Kin4 becomes dispensable for SPOC function in the absence of FEAR. Cells lacking both FEAR and Kin4 show that FEAR contributes to mitotic exit through regulation of the SPOC component Bfa1 and the MEN kinase Cdc15. Furthermore, we uncover controls that specifically promote mitotic exit in the daughter cell compartment.

The mitotic exit network (MEN) triggers mitotic exit and can be blocked by the spindle position checkpoint (SPOC). Here the authors show that SPOC kinase Kin4 counterbalances MEN activation by the Cdc fourteen early anaphase release (FEAR) network in the mother cell and that in the absence of FEAR mitotic exit requires daughter cell-confined factors.

The final cut: cell polarity meets cytokinesis at the bud neck in S. cerevisiae

Cell division is a fundamental but complex process that gives rise to two daughter cells. It includes an ordered set of events, altogether called "the cell cycle", that culminate with cytokinesis, the final stage of mitosis leading to the physical separation of the two daughter cells. Symmetric cell division equally partitions cellular components between the two daughter cells, which are therefore identical to one another and often share the same fate. In many cases, however, cell division is asymmetrical and generates two daughter cells that differ in specific protein inheritance, cell size, or developmental potential. The budding yeast Saccharomyces cerevisiae has proven to be an excellent system to investigate the molecular mechanisms governing asymmetric cell division and cytokinesis. Budding yeast is highly polarized during the cell cycle and divides asymmetrically, producing two cells with distinct sizes and fates. Many components of the machinery establishing cell polarization during budding are relocalized to the division site (i.e., the bud neck) for cytokinesis. In this review we recapitulate how budding yeast cells undergo polarized processes at the bud neck for cell division.

Centrosomes as signalling centres

Centrosomes-as well as the related spindle pole bodies (SPBs) of yeast-have been extensively studied from the perspective of their microtubule-organizing roles. Moreover, the biogenesis and duplication of these organelles have been the subject of much attention, and the importance of centrosomes and the centriole-ciliary apparatus for human disease is well recognized. Much less developed is our understanding of another facet of centrosomes and SPBs, namely their possible role as signalling centres. Yet, many signalling components, including kinases and phosphatases, have been associated with centrosomes and spindle poles, giving rise to the hypothesis that these organelles might serve as hubs for the integration and coordination of signalling pathways. In this review, we discuss a number of selected studies that bear on this notion. We cover different processes (cell cycle control, development, DNA damage response) and organisms (yeast, invertebrates and vertebrates), but have made no attempt to be comprehensive. This field is still young and although the concept of centrosomes and SPBs as signalling centres is attractive, it remains primarily a concept-in need of further scrutiny. We hope that this review will stimulate thought and experimentation.

Asymmetry of the budding yeast Tem1 GTPase at spindle poles is required for spindle positioning but not for mitotic exit

The asymmetrically dividing yeast S. cerevisiae assembles a bipolar spindle well after establishing the future site of cell division (i.e., the bud neck) and the division axis (i.e., the mother-bud axis). A surveillance mechanism called spindle position checkpoint (SPOC) delays mitotic exit and cytokinesis until the spindle is properly positioned relative to the mother-bud axis, thereby ensuring the correct ploidy of the progeny. SPOC relies on the heterodimeric GTPase-activating protein Bub2/Bfa1 that inhibits the small GTPase Tem1, in turn essential for activating the mitotic exit network (MEN) kinase cascade and cytokinesis. The Bub2/Bfa1 GAP and the Tem1 GTPase form a complex at spindle poles that undergoes a remarkable asymmetry during mitosis when the spindle is properly positioned, with the complex accumulating on the bud-directed old spindle pole. In contrast, the complex remains symmetrically localized on both poles of misaligned spindles. The mechanism driving asymmetry of Bub2/Bfa1/Tem1 in mitosis is unclear. Furthermore, whether asymmetry is involved in timely mitotic exit is controversial. We investigated the mechanism by which the GAP Bub2/Bfa1 controls GTP hydrolysis on Tem1 and generated a series of mutants leading to constitutive Tem1 activation. These mutants are SPOC-defective and invariably lead to symmetrical localization of Bub2/Bfa1/Tem1 at spindle poles, indicating that GTP hydrolysis is essential for asymmetry. Constitutive tethering of Bub2 or Bfa1 to both spindle poles impairs SPOC response but does not impair mitotic exit. Rather, it facilitates mitotic exit of MEN mutants, likely by increasing the residence time of Tem1 at spindle poles where it gets active. Surprisingly, all mutant or chimeric proteins leading to symmetrical localization of Bub2/Bfa1/Tem1 lead to increased symmetry at spindle poles of the Kar9 protein that mediates spindle positioning and cause spindle misalignment. Thus, asymmetry of the Bub2/Bfa1/Tem1 complex is crucial to control Kar9 distribution and spindle positioning during mitosis.

The centrosome and its duplication cycle

The centrosome was discovered in the late 19th century when mitosis was first described. Long recognized as a key organelle of the spindle pole, its core component, the centriole, was realized more than 50 or so years later also to comprise the basal body of the cilium. Here, we chart the more recent acquisition of a molecular understanding of centrosome structure and function. The strategies for gaining such knowledge were quickly developed in the yeasts to decipher the structure and function of their distinctive spindle pole bodies. Only within the past decade have studies with model eukaryotes and cultured cells brought a similar degree of sophistication to our understanding of the centrosome duplication cycle and the multiple roles of this organelle and its component parts in cell division and signaling. Now as we begin to understand these functions in the context of development, the way is being opened up for studies of the roles of centrosomes in human disease.

Regulation of spindle pole body assembly and cytokinesis by the centrin-binding protein Sfi1 in fission yeast

A previous model suggested doubling of Sfi1 as the first step of SPB assembly. Here it is shown that Sfi1 is gradually recruited to SPBs throughout the cell cycle. Conserved tryptophans in Sfi1 are required for its equal partitioning during mitosis, and unequal partitioning of Sfi1 underlies SPB assembly and mitotic defects in the next cell cycle.

Centrosomes play critical roles in the cell division cycle and ciliogenesis. Sfi1 is a centrin-binding protein conserved from yeast to humans. Budding yeast Sfi1 is essential for the initiation of spindle pole body (SPB; yeast centrosome) duplication. However, the recruitment and partitioning of Sfi1 to centrosomal structures have never been fully investigated in any organism, and the presumed importance of the conserved tryptophans in the internal repeats of Sfi1 remains untested. Here we report that in fission yeast, instead of doubling abruptly at the initiation of SPB duplication and remaining at a constant level thereafter, Sfi1 is gradually recruited to SPBs throughout the cell cycle. Like an sfi1Δ mutant, a Trp-to-Arg mutant (sfi1-M46) forms monopolar spindles and exhibits mitosis and cytokinesis defects. Sfi1-M46 protein associates preferentially with one of the two daughter SPBs during mitosis, resulting in a failure of new SPB assembly in the SPB receiving insufficient Sfi1. Although all five conserved tryptophans tested are involved in Sfi1 partitioning, the importance of the individual repeats in Sfi1 differs. In summary, our results reveal a link between the conserved tryptophans and Sfi1 partitioning and suggest a revision of the model for SPB assembly.

Dual Regulation of the mitotic exit network (MEN) by PP2A-Cdc55 phosphatase

Exit from mitosis in budding yeast is triggered by activation of the key mitotic phosphatase Cdc14. At anaphase onset, the protease separase and Zds1 promote the downregulation of PP2A^Cdc55 phosphatase, which facilitates Cdk1-dependent phosphorylation of Net1 and provides the first wave of Cdc14 activity. Once Cdk1 activity starts to decline, the mitotic exit network (MEN) is activated to achieve full Cdc14 activation. Here we describe how the PP2A^Cdc55 phosphatase could act as a functional link between FEAR and MEN due to its action on Bfa1 and Mob1. We demonstrate that PP2A^Cdc55 regulates MEN activation by facilitating Cdc5- and Cdk1-dependent phosphorylation of Bfa1 and Mob1, respectively. Downregulation of PP2A^Cdc55 initiates MEN activity up to Cdc15 by Bfa1 inactivation. Surprisingly, the premature Bfa1 inactivation observed does not entail premature MEN activation, since an additional Cdk1-Clb2 inhibitory signal acting towards Dbf2-Mob1 activity restrains MEN activity until anaphase. In conclusion, we propose a clear picture of how PP2A^Cdc55 functions affect the regulation of various MEN components, contributing to mitotic exit.

Cell cycle studies over the years have tried to elucidate the molecular mechanisms behind cell division, one of the most highly regulated of all cell processes, which ensures life in all organisms. Protein phosphorylation emerged as a key regulatory mechanism in the cell cycle. The highly conserved family of cyclin-dependent kinases, the Cdks, are considered the main component of the cell cycle control system. However, it has become clear that opposing phosphatases also play a key role in determining the phosphorylation state of the proteins. Cells enter mitosis when mitotic Cdk activity increases, having its pick of activity during metaphase. To exit mitosis, cells must coordinate chromosome segregation with Cdk inactivation processes involving the activation of protein phosphatases. Here we show that the phosphatase PP2A regulates the mitotic exit network (MEN) by counteracting the phosphorylation of Bfa1 and Mob1. Our findings provide new insights into the mechanism by which PP2A-Cdc55 functions affect the regulation of various MEN components that contribute to mitotic exit. The core signalling elements of the MEN, SIN and Hippo pathways are highly conserved. Therefore, studies of MEN regulation will contribute to our understanding of MEN-related pathways in other organisms.

Mitotic exit and separation of mother and daughter cells

Productive cell proliferation involves efficient and accurate splitting of the dividing cell into two separate entities. This orderly process reflects coordination of diverse cytological events by regulatory systems that drive the cell from mitosis into G1. In the budding yeast Saccharomyces cerevisiae, separation of mother and daughter cells involves coordinated actomyosin ring contraction and septum synthesis, followed by septum destruction. These events occur in precise and rapid sequence once chromosomes are segregated and are linked with spindle organization and mitotic progress by intricate cell cycle control machinery. Additionally, critical paarts of the mother/daughter separation process are asymmetric, reflecting a form of fate specification that occurs in every cell division. This chapter describes central events of budding yeast cell separation, as well as the control pathways that integrate them and link them with the cell cycle.

Ultrastructural disorder of actin mutant suggests uncoupling of actin-dependent pathway from microtubule-dependent pathway in budding yeast

Temperature-sensitive actin mutant of Saccharomyces cerevisiae act1-1 was studied at a permissive temperature of 23°C by light, fluorescent and electron microscopy to elucidate the roles of actin cytoskeleton in the cycling eukaryotic cells. Mutant cells that grew slowly at the permissive temperature showed aberrations in the cytoskeleton and cell cycle. Mutant cells contained aberrant 'faint actin cables,' that failed in directing of mitochondria, vacuoles and secretory vesicles to the bud and the stray vesicles delivered their content to the mother wall instead of the bud. Bud growth was delayed. Spindle pole bodies and cytoplasmic microtubules did not direct to the bud, and nucleus failed to migrate to the bud. Repeated nuclear divisions produced multinucleated cells, indicating continued cycling of actin mutant cells that failed in the morphogenetic checkpoint, the spindle position checkpoint and cytokinesis. Thus, a single actin mutation appears to indicate uncoupling in space and time of the 'actin cytoskeleton-dependent cytoplasmic pathway of bud development and organelle positioning and inheritance' from the 'microtubule-dependent nuclear division pathway' in a budding yeast cell cycle.

An EF-hands protein, centrin-1, is an EGTA-sensitive SUMO-interacting protein in mouse testis

A multifunctional calcium-binding protein, centrin-1, is specifically expressed in male germ cells, certain neurons and ciliated cells. We identified centrin-1 as a protein interacting with SUMO-2/3 using yeast two-hybrid screening of a mouse testicular cDNA library. In bead halo assays, the interaction between centrin-1 and SUMO-2/3 was reduced in the presence of EGTA and facilitated by the addition of CaCl₂. immunostaining of seminiferous tubules in 35-day-old mouse testes revealed that cells in the layer containing spermatogonia showed colocalization of SUMO-2/3 with centrin-1 in cytoplasmic spots. Identification of centrin-1 as the EGTA-sensitive SUMO-2/3-interacting protein indicates the possible role of calcium in modulating the centrin-1-SUMO-2/3 interaction and suggests the importance of this interaction in mouse testis.

Elm1 kinase activates the spindle position checkpoint kinase Kin4

Elm1 phosphorylates a conserved residue within the Kin4 kinase domain to coordinate spindle position with cell cycle progression.

Budding yeast asymmetric cell division relies upon the precise coordination of spindle orientation and cell cycle progression. The spindle position checkpoint (SPOC) is a surveillance mechanism that prevents cells with misoriented spindles from exiting mitosis. The cortical kinase Kin4 acts near the top of this network. How Kin4 kinase activity is regulated and maintained in respect to spindle positional cues remains to be established. Here, we show that the bud neck–associated kinase Elm1 participates in Kin4 activation and SPOC signaling by phosphorylating a conserved residue within the activation loop of Kin4. Blocking Elm1 function abolishes Kin4 kinase activity in vivo and eliminates the SPOC response to spindle misalignment. These findings establish a novel function for Elm1 in the coordination of spindle positioning with cell cycle progression via its control of Kin4.

Targeted localization of Inn1, Cyk3 and Chs2 by the mitotic-exit network regulates cytokinesis in budding yeast

The mitotic-exit network (MEN) is a signaling pathway that is essential for the coordination of mitotic exit and cytokinesis. Whereas the role of the MEN in mitotic exit is well established, the molecular mechanisms by which MEN components regulate cytokinesis remain poorly understood. Here, we show that the MEN controls components involved in septum formation, including Inn1, Cyk3 and Chs2. MEN-deficient mutants, forced to exit mitosis as a result of Cdk1 inactivation, show defects in targeting Cyk3 and Inn1 to the bud-neck region. In addition, we found that the chitin synthase Chs2 did not efficiently localize at the bud neck in the absence of MEN activity. Ultrastructural analysis of the bud neck revealed that low MEN activity led to unilateral, uncoordinated extension of the primary and secondary septa. This defect was partially suppressed by increased levels of Cyk3. We therefore propose that the MEN directly controls cytokinesis via targeting of Inn1, Cyk3 and Chs2 to the bud neck.

Mutual regulation of cyclin-dependent kinase and the mitotic exit network

The mitotic exit network (MEN) is a spindle pole body (SPB)-associated, GTPase-driven signaling cascade that controls mitotic exit. The inhibitory Bfa1-Bub2 GTPase-activating protein (GAP) only associates with the daughter SPB (dSPB), raising the question as to how the MEN is regulated on the mother SPB (mSPB). Here, we show mutual regulation of cyclin-dependent kinase 1 (Cdk1) and the MEN. In early anaphase Cdk1 becomes recruited to the mSPB depending on the activity of the MEN kinase Cdc15. Conversely, Cdk1 negatively regulates binding of Cdc15 to the mSPB. In addition, Cdk1 phosphorylates the Mob1 protein to inhibit the activity of Dbf2-Mob1 kinase that regulates Cdc14 phosphatase. Our data revise the understanding of the spatial regulation of the MEN. Although MEN activity in the daughter cells is controlled by Bfa1-Bub2, Cdk1 inhibits MEN activity at the mSPB. Consistent with this model, only triple mutants that lack BUB2 and the Cdk1 phosphorylation sites in Mob1 and Cdc15 show mitotic exit defects.

Structural mutants of the spindle pole body cause distinct alteration of cytoplasmic microtubules and nuclear dynamics in multinucleated hyphae

To determine how microtubule (MT) nucleation and nuclear migration are controlled in multinucleated hyphae we deleted genes encoding MTOC subunits and AgStu2. The novel phenotypes we observed in these mutants compared with analogous deletions in budding yeast allowed us to assign functions to the two types of cMTs that we observe in A. gossypii.

In the multinucleate fungus Ashbya gossypii, cytoplasmic microtubules (cMTs) emerge from the spindle pole body outer plaque (OP) in perpendicular and tangential directions. To elucidate the role of cMTs in forward/backward movements (oscillations) and bypassing of nuclei, we constructed mutants potentially affecting cMT nucleation or stability. Hyphae lacking the OP components AgSpc72, AgNud1, AgCnm67, or the microtubule-stabilizing factor AgStu2 grew like wild- type but showed substantial alterations in the number, length, and/or nucleation sites of cMTs. These mutants differently influenced nuclear oscillation and bypassing. In Agspc72Δ, only long cMTs were observed, which emanate tangentially from reduced OPs; nuclei mainly moved with the cytoplasmic stream but some performed rapid bypassing. Agnud1Δ and Agcnm67Δ lack OPs; short and long cMTs emerged from the spindle pole body bridge/half-bridge structures, explaining nuclear oscillation and bypassing in these mutants. In Agstu2Δ only very short cMTs emanated from structurally intact OPs; all nuclei moved with the cytoplasmic stream. Therefore, long tangential cMTs promote nuclear bypassing and short cMTs are important for nuclear oscillation. Our electron microscopy ultrastructural analysis also indicated that assembly of the OP occurs in a stepwise manner, starting with AgCnm67, followed by AgNud1 and lastly AgSpc72.

Lte1 contributes to Bfa1 localization rather than stimulating nucleotide exchange by Tem1

Lte1 is a mitotic regulator long envisaged as a guanosine nucleotide exchange factor (GEF) for Tem1, the small guanosine triphosphatase governing activity of the Saccharomyces cerevisiae mitotic exit network. We demonstrate that this model requires reevaluation. No GEF activity was detectable in vitro, and mutational analysis of Lte1's putative GEF domain indicated that Lte1 activity relies on interaction with Ras for localization at the bud cortex rather than providing nucleotide exchange. Instead, we found that Lte1 can determine the subcellular localization of Bfa1 at spindle pole bodies (SPBs). Under conditions in which Lte1 is essential, Lte1 promoted the loss of Bfa1 from the maternal SPB. Moreover, in cells with a misaligned spindle, mislocalization of Lte1 in the mother cell promoted loss of Bfa1 from one SPB and allowed bypass of the spindle position checkpoint. We observed that lte1 mutants display aberrant localization of the polarity cap, which is the organizer of the actin cytoskeleton. We propose that Lte1's role in cell polarization underlies its contribution to mitotic regulation.

SUMO-dependent regulation of centrin-2

Centrins are multifunctional Ca(2+)-binding proteins that are highly conserved from yeast to humans. Centrin-2 is a core component of the centrosome of higher eukaryotes. In addition, it is present within the nucleus, in which it is part of the xeroderma pigmentosum group C (XPC) complex, which controls nucleotide excision repair (NER). Regulation of the subcellular distribution of centrin-2 has so far remained elusive. Here we show that centrin-2 is a substrate of SUMOylation in vitro and in vivo, and that it is preferentially modified by SUMO2/3. Moreover, we identify the SUMO E3-like ligase human polycomb protein 2 (PC2; also known as hPC2) as essential for centrin-2 modification. Interference with the SUMOylation pathway leads to a striking defect in nuclear localization of centrin-2 and accumulation in the cytoplasm, whereas centrosomal recruitment of centrin-2 is unaffected. Depletion of the XPC protein mimics this situation and we provide evidence that SUMO conjugation of centrin-2 enhances its binding to the XPC protein. These data show that the nucleocytoplasmic shuttling of centrin-2 depends on the SUMO system and indicates that localization of centrin-2 within the nucleus depends on its ability to bind to the XPC protein.

The SESA network links duplication of the yeast centrosome with the protein translation machinery

The yeast spindle pole body (SPB), the functional equivalent of mammalian centrosome, duplicates in G1/S phase of the cell cycle and then becomes inserted into the nuclear envelope. Here we describe a link between SPB duplication and targeted translation control. When insertion of the newly formed SPB into the nuclear envelope fails, the SESA network comprising the GYF domain protein Smy2, the translation inhibitor Eap1, the mRNA-binding protein Scp160 and the Asc1 protein, specifically inhibits initiation of translation of POM34 mRNA that encodes an integral membrane protein of the nuclear pore complex, while having no impact on other mRNAs. In response to SESA, POM34 mRNA accumulates in the cytoplasm and is not targeted to the ER for cotranslational translocation of the protein. Reduced level of Pom34 is sufficient to restore viability of mutants with defects in SPB duplication. We suggest that the SESA network provides a mechanism by which cells can regulate the translation of specific mRNAs. This regulation is used to coordinate competing events in the nuclear envelope.

Septins and the lateral compartmentalization of eukaryotic membranes

Eukaryotic cells from neurons and epithelial cells to unicellular fungi frequently rely on cellular appendages such as axons, dendritic spines, cilia, and buds for their biology. The emergence and differentiation of these appendages depend on the formation of lateral diffusion barriers at their bases to insulate their membranes from the rest of the cell. Here, we review recent progress regarding the molecular mechanisms and functions of such barriers. This overview underlines the importance and conservation of septin-dependent diffusion barriers, which coordinately compartmentalize both plasmatic and internal membranes. We discuss their role in memory establishment and the control of cellular aging.

The centrosome and asymmetric cell division

Asymmetric stem cell division is a mechanism widely employed by the cell to maintain tissue homeostasis, resulting in the production of one stem cell and one differentiating cell. However, asymmetric cell division is not limited to stem cells and is widely observed even in unicellular organisms as well as in cells that make up highly complex tissues. In asymmetric cell division, cells must organize their intracellular components along the axis of asymmetry (sometimes in the context of extracellular architecture). Recent studies have described cell asymmetry in many cell types and in many cases such asymmetry involves the centrosome (or spindle pole body in yeast) as the center of cytoskeleton organization. In this review, I summarize recent discoveries in cellular polarity that lead to an asymmetric outcome, with a focus on centrosome function.

Dbf2-Mob1 drives relocalization of protein phosphatase Cdc14 to the cytoplasm during exit from mitosis

Exit from mitosis is characterized by a precipitous decline in cyclin-dependent kinase (Cdk) activity, dissolution of mitotic structures, and cytokinesis. In Saccharomyces cerevisiae, mitotic exit is driven by a protein phosphatase, Cdc14, which is in part responsible for counteracting Cdk activity. Throughout interphase, Cdc14 is sequestered in the nucleolus, but successful anaphase activates the mitotic exit network (MEN), which triggers dispersal of Cdc14 throughout the cell by a mechanism that has remained unknown. In this study, we show that a MEN component, protein kinase Dbf2–Mob1, promotes transfer of Cdc14 to the cytoplasm and consequent exit from mitosis by direct phosphorylation of Cdc14 on serine and threonine residues adjacent to a nuclear localization signal (NLS), thereby abrogating its NLS activity. Our results define a mechanism by which the MEN promotes exit from mitosis.

Spindle alignment regulates the dynamic association of checkpoint proteins with yeast spindle pole bodies

In many polarized cells, the accuracy of chromosome segregation depends on the correct positioning of the mitotic spindle. In budding yeast, the spindle positioning checkpoint (SPOC) delays mitotic exit when the anaphase spindle fails to extend toward the mother-daughter axis. However it remains to be established how spindle orientation is translated to SPOC components at the yeast spindle pole bodies (SPB). Here, we used photobleaching techniques to show that the dynamics with which Bub2-Bfa1 turned over at SPBs significantly increased upon SPOC activation. A version of Bfa1 that was stably associated with SPBs rendered the cells SPOC deficient without affecting other Bub2-Bfa1 functions, demonstrating the functional importance of regulating the dynamics of Bfa1 SPB association. In addition, we established that the SPOC kinase Kin4 is the major regulator of Bfa1 residence time at SPBs. We suggest that upon SPOC activation Bfa1-Bub2 spreads throughout the cytoplasm, thereby inhibiting mitotic exit.

Regulation of asymmetric stem cell division: spindle orientation and the centrosome

Asymmetric stem cell division, as a means of maintaining adequate numbers of stem cells, has attracted widespread attention from researchers in the stem cell biology field. Yet, the molecular and cellular mechanisms that govern asymmetric stem cell division remain poorly understood. Stem cells are not the only cell population that divides asymmetrically, and fortunately, great progress has been made in the understanding of asymmetric cell division during development, providing insight into strategies that stem cells may employ to divide asymmetrically. This review will summarize the importance of stem cell function and the role of asymmetric division in controlling stem cell behavior.

Asymmetric centrosome behavior and the mechanisms of stem cell division

The ability of dividing cells to produce daughters with different fates is an important developmental mechanism conserved from bacteria to fungi, plants, and metazoan animals. Asymmetric outcomes of a cell division can be specified by two general mechanisms: asymmetric segregation of intrinsic fate determinants or asymmetric placement of daughter cells into microenvironments that provide extrinsic signals that direct cells to different states. For both, spindle orientation must be coordinated with the localization of intrinsic determinants or source of extrinsic signals to achieve the proper asymmetric outcome. Recent work on spindle orientation in Drosophila melanogaster male germline stem cells and neuroblasts has brought into sharp focus the key role of differential centrosome behavior in developmentally programmed asymmetric division (for reviews see Cabernard, C., and C.Q. Doe. 2007. Curr. Biol. 17:R465-R467; Gonzalez, C. 2007. Nat. Rev. Genet. 8:462-472). These findings provide new insights and suggest intriguing new models for how cells coordinate spindle orientation with their cellular microenvironment to regulate and direct cell fate decisions within tissues.

The yeast centrosome translates the positional information of the anaphase spindle into a cell cycle signal

The spindle orientation checkpoint (SPOC) of budding yeast delays mitotic exit when cytoplasmic microtubules (MTs) are defective, causing the spindle to become misaligned. Delay is achieved by maintaining the activity of the Bfa1-Bub2 guanosine triphosphatase-activating protein complex, an inhibitor of mitotic exit. In this study, we show that the spindle pole body (SPB) component Spc72, a transforming acidic coiled coil-like molecule that interacts with the gamma-tubulin complex, recruits Kin4 kinase to both SPBs when cytoplasmic MTs are defective. This allows Kin4 to phosphorylate the SPB-associated Bfa1, rendering it resistant to inactivation by Cdc5 polo kinase. Consistently, forced targeting of Kin4 to both SPBs delays mitotic exit even when the anaphase spindle is correctly aligned. Moreover, we present evidence that Spc72 has an additional function in SPOC regulation that is independent of the recruitment of Kin4. Thus, Spc72 provides a missing link between cytoplasmic MT function and components of the SPOC.

The yeast penta-EF protein Pef1p is involved in cation-dependent budding and cell polarization

Penta-EF-hand (PEF) proteins bind calcium and participate in a variety of calcium-dependent processes in vertebrates. In yeast, intracellular cations regulate processes like cell division and polarized growth. This study reports the identification of a unique PEF protein in Saccharomyces cerevisiae encoded by the uncharacterized open reading frame YGR058w. Pef1p has a long and unstructured N-terminal domain conserved in ascomycetes, and a highly conserved C-terminal calcium binding domain homologous to human ALG-2 and sorcin. Pef1p binds calcium and zinc and homodimerizes in vitro and in vivo like vertebrate homologues. Disruption of PEF1 induces defective growth in SDS and cation depletion conditions. Significantly, a critical substitution in the second EF hand (E218A) lowers the in vitro affinity for zinc and phenocopies growth defects. The dissection of protein-protein interactions and the cellular localization of Pef1p analogous to that of RAM pathway components controlling daughter-specific gene expression at the site of bud emergence bring out the importance of this novel protein. Our data suggest that cation homeostasis is involved in the control of polarized growth and in stress response in budding yeast.

Animal cytokinesis: from parts list to mechanisms

The mechanism underlying cytokinesis, the final step in cell division, remains one of the major unsolved questions in basic cell biology. Thanks to advances in functional genomics and proteomics, we are now able to assemble a "parts list" of proteins involved in cytokinesis. In this review, we discuss how to relate this parts list to biological mechanism. For easier analysis, we split cytokinesis into discrete steps: cleavage plane specification, rearrangement of microtubule structures, contractile ring assembly, ring ingression, and completion. We report on the advances that have been made to understand these steps and how they can be integrated into a global understanding of cytokinesis. We also discuss the extent to which classic questions have been answered and identify major outstanding questions.

The spindle position checkpoint in budding yeast: the motherly care of MEN

Mitotic exit and cytokinesis must be tightly coupled to nuclear division both in time and space in order to preserve genome stability and to ensure that daughter cells inherit the right set of chromosomes after cell division. This is achieved in budding yeast through control over a signal transduction cascade, the mitotic exit network (MEN), which is required for mitotic CDK inactivation in telophase and for cytokinesis. Current models of MEN activation emphasize on the bud as the place where most control is exerted. This review focuses on recent data that instead point to the mother cell as being the residence of key regulators of late mitotic events.

Kin4 kinase delays mitotic exit in response to spindle alignment defects

For many polarized cells, it is critical that the mitotic spindle becomes positioned relative to the polarity axis. This is especially important in yeast, where the site of cytokinesis is predetermined. The spindle position checkpoint (SPOC) therefore delays mitotic exit of cells with a mispositioned spindle. One component of the SPOC is the Bub2-Bfa1 complex, an inhibitor of the mitotic exit network (MEN). Here, we show that the Kin4 kinase is a component of the SPOC and as such is essential to delay cell cycle progression of cells with a misaligned spindle. When spindles are correctly oriented, Kin4 and Bub2-Bfa1 are asymmetrically localized to opposite spindle pole bodies (SPBs). Bub2-Bfa1 then becomes inhibited by Cdc5 polo kinase with anaphase onset, a prerequisite for mitotic exit. In response to spindle misalignment, Kin4 and Bub2-Bfa1 are brought together at both SPBs. Kin4 now maintains Bub2-Bfa1 activity by counteracting Cdc5, thereby inhibiting mitotic exit.

Severing all ties between mother and daughter: cell separation in budding yeast

At the end of nuclear division in the budding yeast, acto-myosin ring contraction and cytokinesis occur between mother and daughter cells. This is followed by cell separation, after which mother and daughter cells go their separate ways. While cell separation may be the last event that takes place between the two cells, it is nonetheless under tight regulation which ensures that both cells are viable upon separation. It is becoming increasingly clear that the components of the cell separation machinery are controlled at various levels, including the temporal and spatial regulation of the genes encoding for the components and the specific localization of the components to the neck. In addition, these regulatory controls are co-ordinated with exit from mitosis, thereby placing a mechanistic link between the end of mitosis and cell separation. More importantly, the success of the cell separation event is contingent upon the presence of a trilaminar septum, whose assembly is dependent on a host of proteins which localize to the neck over the span of one cell division cycle.

A novel mechanism of nuclear envelope break-down in a fungus: nuclear migration strips off the envelope

In animals, the nuclear envelope disassembles in mitosis, while budding and fission yeast form an intranuclear spindle. Ultrastructural data indicate that basidiomycetes, such as the pathogen Ustilago maydis, undergo an 'open mitosis'. Here we describe the mechanism of nuclear envelope break-down in U. maydis. In interphase, the nucleus resides in the mother cell and the spindle pole body is inactive. Prior to mitosis, it becomes activated and nucleates microtubules that reach into the daughter cell. Dynein appears at microtubule tips and exerts force on the spindle pole body, which leads to the formation of a long nuclear extension that reaches into the bud. Chromosomes migrate through this extension and together with the spindle pole bodies leave the old envelope, which remains in the mother cell until late telophase. Inhibition of nuclear migration or deletion of a Tem1p-like GTPase leads to a 'closed' mitosis, indicating that spindle pole bodies have to reach into the bud where MEN signalling participates in envelope removal. Our data indicate that dynein-mediated premitotic nuclear migration is essential for envelope removal in U. maydis.

Silencing of polo-like kinase (Plk) 1 via siRNA causes induction of apoptosis and impairment of mitosis machinery in human prostate cancer cells: implications for the treatment of prostate cancer

Prostate cancer (PCa) is one of the most common cancers in men. Each year approximately 543,000 new cases are reported worldwide, and the disease kills 200,000 (mostly older men) in developed countries. The existing treatment approaches and surgical intervention have not been able to effectively manage this dreaded cancer and, therefore, continuing efforts are ongoing to explore novel targets and strategies for the management of PCa. The activity of polo-like kinase 1 (Plk1) is elevated in tissues and cells with a high mitotic index, including cancer cells. An increasing body of evidence suggests that the level of Plk1 expression has prognostic value for predicting outcomes in patients with some cancers. A close correlation between Plk1 expression and carcinogenesis has been documented. However, the role of Plk1 in PCa is not known. We propagated a hypothesis that Plk1 inhibition will result in elimination of human PCa cells via a mitotic arrest followed by apoptosis (1). To define the role of Plk1 in PCa, we used the technique of RNA silencing via small interfering RNA (siRNA). First, using a series of human prostate carcinoma cells and normal human prostate epithelial (PrEC) cells, we assessed Plk1 levels in PCa. Immunoblot analyses clearly showed a significant expression of Plk1 in LNCaP, DU145, and PC3 human PCa cells. Interestingly, Plk1 was not detectable in normal PrEC cells. Next, we transfected the PCa cells with Plk 1 siRNA, which resulted in a significant inhibition in Plk1 protein in all PCa cells. Plk1 depletion resulted in a decrease in cell viability and induction of apoptosis in PCa cells but had no appreciable effect in normal PrEC cells. Our data also demonstrated that Plk1 siRNA transfection of PCa cells resulted in 1) a mitotic cell cycle arrest, 2) failure of cytokinesis, and 3) defects in centrosome integrity and maturation. Thus, our study suggested that 1) Plk1 plays a critical role in the process of PCa development and 2) gene therapeutic approaches aimed at Plk1 or the pharmacological inhibitors of Plk1 may be developed for the management of PCa.

SIN and the art of splitting the fission yeast cell

The septation initiation network (SIN) triggers the onset of cytokinesis in the fission yeast Schizosaccharomyces pombe by promoting contraction of the medially placed F-actin ring. SIN signaling is regulated by the polo-like kinase plo1p and by cdc2p, the initiator of mitosis, and its activation is co-ordinated with other events in mitosis to ensure that cytokinesis does not begin until chromosomes have been separated. Though the SIN controls the contractile ring, the signal originates from the poles of the mitotic spindle. Recent studies suggest that the spindle pole body may act as a dynamic assembly site for active SIN signaling complexes. In the budding yeast Saccharomyces cerevisiae the counterpart of the SIN, called the MEN, mediates both mitotic exit and cytokinesis, in part through regulating activation of the phosphoprotein phosphatase Cdc14p. Flp1p, the S. pombe ortholog of Cdc14p, is not essential for mitotic exit, but may contribute to an orderly mitosis-G1 transition by regulating the destruction of the mitotic inducer cdc25p.

Novel functional dissection of the localization-specific roles of budding yeast polo kinase Cdc5p

Budding yeast polo kinase Cdc5p localizes to the spindle pole body (SPB) and to the bud-neck and plays multiple roles during M-phase progression. To dissect localization-specific mitotic functions of Cdc5p, we tethered a localization-defective N-terminal kinase domain of Cdc5p (Cdc5pDeltaC) to the SPB or to the bud-neck with components specifically localizing to one of these sites and characterized these mutants in a cdc5Delta background. Characterization of a viable, SPB-localizing, CDC5DeltaC-CNM67 mutant revealed that it is defective in timely degradation of Swe1p, a negative regulator of Cdc28p. Loss of BFA1, a negative regulator of mitotic exit, rescued the lethality of a neck-localizing CDC5DeltaC-CDC12 or CDC5DeltaC-CDC3 mutant but yielded severe defects in cytokinesis. These data suggest that the SPB-associated Cdc5p activity is critical for both mitotic exit and cytokinesis, whereas the bud neck-localized Cdc5p is required for proper Swe1p regulation. Interestingly, a cdc5Delta bfa1Delta swe1Delta triple mutant is viable but grows slowly, whereas cdc5Delta cells bearing both CDC5DeltaC-CNM67 and CDC5DeltaC-CDC12 grow well with only a mild cell cycle delay. Thus, SPB- and the bud-neck-localized Cdc5p control most of the critical Cdc5p functions and downregulation of Bfa1p and Swe1p at the respective locations are two critical factors that require Cdc5p.