Phosphorylation-independent inhibition of Cdc28p by the tyrosine kinase Swe1p in the morphogenesis checkpoint

Self-assembled thiophanate-methyl/star polycation complex prevents plant cell-wall penetration and fungal carbon utilization during cotton infection by Verticillium dahliae

No effective fungicides are available for the management of Verticillium dahliae, which causes vascular wilt disease. In this study, a star polycation (SPc)-based nanodelivery system was used for the first time to develop a thiophanate-methyl (TM) nanoagent for the management of V. dahliae. SPc spontaneously assembled with TM through hydrogen bonding and Van der Waals forces to decrease the particle size of TM from 834 to 86 nm. Compared to TM alone, the SPc-loaded TM further reduced the colony diameter of V. dahliae to 1.12 and 0.64 cm, and the spore number to 1.13 × 10⁸ and 0.72 × 10⁸ cfu/mL at the concentrations of 3.77 and 4.71 mg/L, respectively. The TM nanoagents disturbed the expression of various crucial genes in V. dahliae, and contributed to preventing plant cell-wall degradation and carbon utilization by V. dahliae, which mainly impaired the infective interaction between pathogens and plants. TM nanoagents remarkably decreased the plant disease index and the fungal biomass in the root compared to TM alone, and its control efficacy was the best (61.20 %) among the various formulations tested in the field. Furthermore, SPc showed negligible acute toxicity toward cotton seeds. To the best of our knowledge, this study is the first to design a self-assembled nanofungicide that efficiently inhibits V. dahliae growth and protects cotton from the destructive Verticillium wilt.

Methylglyoxal inhibits nuclear division through alterations in vacuolar morphology and accumulation of Atg18 on the vacuolar membrane in Saccharomyces cerevisiae

Methylglyoxal (MG) is a natural metabolite derived from glycolysis, and it inhibits the growth of cells in all kinds of organisms. We recently reported that MG inhibits nuclear division in Saccharomyces cerevisiae. However, the mechanism by which MG blocks nuclear division remains unclear. Here, we show that increase in the levels of phosphatidylinositol 3,5-bisphosphate (PtdIns(3,5)P₂) is crucial for the inhibitory effects of MG on nuclear division, and the deletion of PtdIns(3,5)P₂-effector Atg18 alleviated the MG-mediated inhibitory effects. Previously, we reported that MG altered morphology of the vacuole to a single swelling form, where PtdIns(3,5)P₂ accumulates. The changes in the vacuolar morphology were also needed by MG to exert its inhibitory effects on nuclear division. The known checkpoint machinery, including the spindle assembly checkpoint and morphological checkpoint, are not involved in the blockade of nuclear division by MG. Our results suggest that both the accumulation of Atg18 on the vacuolar membrane and alterations in vacuolar morphology are necessary for the MG-induced inhibition of nuclear division.

The regulation of Net1/Cdc14 by the Hog1 MAPK upon osmostress unravels a new mechanism regulating mitosis

During evolution, cells have developed a plethora of mechanisms to optimize survival in a changing and unpredictable environment. In this regard, they have evolved networks that include environmental sensors, signaling transduction molecules and response mechanisms. Hog1 (yeast) and p38 (mammals) stress-activated protein kinases (SAPKs) are activated upon stress and they drive a full collection of cell adaptive responses aimed to maximize survival. SAPKs are extensively used to learn about the mechanisms through which cells adapt to changing environments. In addition to regulating gene expression and metabolism, SAPKs control cell cycle progression. In this review, we will discuss the latest findings related to the SAPK-driven regulation of mitosis upon osmostress in yeast.

Cyclin-Specific Docking Mechanisms Reveal the Complexity of M-CDK Function in the Cell Cycle

Cyclin-dependent kinases (CDKs) coordinate hundreds of molecular events during the cell cycle. Multiple cyclins are involved, but the global role of cyclin-specific phosphorylation has remained unsolved. We uncovered a cyclin docking motif, LxF, that mediates binding of replication factor Cdc6 to mitotic cyclin. This interaction leads to phospho-adaptor Cks1-mediated inhibition of M-CDK to facilitate Cdc6 accumulation and sequestration in mitosis. The LxF motif and Cks1 also mediate the mutual inhibition between M-CDK and the tyrosine kinase Swe1. Additionally, the LxF motif is critical for targeting M-CDK to phosphorylate several mitotic regulators; for example, Spo12 is targeted via LxF to release the phosphatase Cdc14. The results complete the full set of G1, S, and M-CDK docking mechanisms and outline the unified role of cyclin specificity and CDK activity thresholds. Cooperation of cyclin and Cks1 docking creates a variety of CDK thresholds and switching orders, including combinations of last in, first out (LIFO) and first in, first out (FIFO) ordering.

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Mitotic cyclin Clb2 binds a specific linear motif, LxF, in targets or inhibitors

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LxF interaction enhances mitotic CDK substrate phosphorylation

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Phospho-adaptor Cks1 and the LxF docking mediate CDK inhibition by Cdc6 and Swe1

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Cyclin-specific targeting enables finetuning of CDK function

Swe1 and Mih1 regulate mitotic spindle dynamics in budding yeast via Bik1

The mitotic spindle is a very dynamic structure that is built de novo and destroyed at each round of cell division. In order to perform its fundamental function during chromosome segregation, mitotic spindle dynamics must be tightly coordinated with other cell cycle events. These changes are driven by several protein kinases, phosphatases and microtubule-associated proteins. In budding yeast, the kinase Swe1 and the phosphatase Mih1 act in concert in controlling the phosphorylation state of Cdc28, the catalytic subunit of Cdk1, the major regulator of the cell cycle. In this study we show that Swe1 and Mih1 are also involved in the control of mitotic spindle dynamics. Our data indicate that Swe1 and the Polo-like kinase Cdc5 control the balance between phosphorylated and unphosphorylated forms of Mih1, which is, in turn, important for mitotic spindle elongation. Moreover, we show that the microtubule-associated protein Bik1 is a phosphoprotein, and that Swe1 and Mih1 are both involved in controlling phosphorylation of Bik1. These results uncover new players and provide insights into the complex regulation of mitotic spindle dynamics.

Shrinking Daughters: Rlm1-Dependent G1/S Checkpoint Maintains Saccharomyces cerevisiae Daughter Cell Size and Viability

The Rlm1 transcription factor is a target of the cell wall integrity pathway. We report that an rlm1Δ mutant grown on a nonfermentable carbon source at low osmolarity forms cell groups in which a mother cell is surrounded by smaller "satellite-daughter" cells. Mother cells in these groups progressed through repeated rounds of cell division with normal rates of bud growth and genetic stability; however, these cells underwent precocious START relative to wild-type mothers. Thus, once activated, Rlm1 delays the transition from G₁ to S, a mechanism we term the cell wall/START (CW/START) checkpoint. The rlm1Δ satellite-cell phenotype is suppressed by deletion of either SLT2, which encodes the kinase that activates Rlm1, or SWI4, which is also activated by Slt2; suggesting that Slt2 can have opposing roles in regulating the START transition. Consistent with an Rlm1-dependent CW/START checkpoint, rlm1Δ satellite daughters were unable to grow or divide further even after transfer to rich medium, but UV irradiation in G₁ could partially rescue rlm1Δ satellite daughters in the next division. Indeed, after cytokinesis, these satellite daughters shrank rapidly, displayed amorphous actin staining, and became more permeable. As a working hypothesis, we propose that duplication of an "actin-organizing center" in late G₁ may be required both to progress through START and to reestablish the actin cytoskeleton in daughter cells.

Kinetic partitioning during de novo septin filament assembly creates a critical G1 "window of opportunity" for mutant septin function

Septin proteins form highly conserved cytoskeletal filaments composed of hetero-oligomers with strict subunit stoichiometry. Mutations within one hetero-oligomerization interface (the "G" interface) bias the mutant septin toward conformations that are incompatible with filament assembly, causing disease in humans and, in budding yeast cells, temperature-sensitive defects in cytokinesis. We previously found that, when the amount of other hetero-oligomerization partners is limiting, wild-type and G interface-mutant alleles of a given yeast septin "compete" along parallel but distinct folding pathways for occupancy of a limited number of positions within septin hetero-octamers. Here, we synthesize a mathematical model that outlines the requirements for this phenomenon: if a wild-type septin traverses a folding pathway that includes a single rate-limiting folding step, the acquisition by a mutant septin of additional slow folding steps creates an initially large disparity between wild-type and mutant in the cellular concentrations of oligomerization-competent monomers. When the 2 alleles are co-expressed, this kinetic disparity results in mutant exclusion from hetero-oligomers, even when the folded mutant monomer is oligomerization-competent. To test this model experimentally, we first visualize the kinetic delay in mutant oligomerization in living cells, and then narrow or widen the "window of opportunity" for mutant septin oligomerization by altering the length of the G1 phase of the yeast cell cycle, and observe the predicted exacerbation or suppression, respectively, of mutant cellular phenotypes. These findings reveal a fundamental kinetic principle governing in vivo assembly of multiprotein complexes, independent of the ability of the subunits to associate with each other.

Sensing a bud in the yeast morphogenesis checkpoint: a role for Elm1

Yeast cells know whether or not they have a bud. The kinase Elm1 and the septin cytoskeleton are key transducers of cell shape information.

Bud formation by Saccharomyces cerevisiae must be coordinated with the nuclear cycle to enable successful proliferation. Many environmental stresses temporarily disrupt bud formation, and in such circumstances, the morphogenesis checkpoint halts nuclear division until bud formation can resume. Bud emergence is essential for degradation of the mitotic inhibitor, Swe1. Swe1 is localized to the septin cytoskeleton at the bud neck by the Swe1-binding protein Hsl7. Neck localization of Swe1 is required for Swe1 degradation. Although septins form a ring at the presumptive bud site before bud emergence, Hsl7 is not recruited to the septins until after bud emergence, suggesting that septins and/or Hsl7 respond to a “bud sensor.” Here we show that recruitment of Hsl7 to the septin ring depends on a combination of two septin-binding kinases: Hsl1 and Elm1. We elucidate which domains of these kinases are needed and show that artificial targeting of those domains suffices to recruit Hsl7 to septin rings even in unbudded cells. Moreover, recruitment of Elm1 is responsive to bud emergence. Our findings suggest that Elm1 plays a key role in sensing bud emergence.

Budding yeast Swe1 is involved in the control of mitotic spindle elongation and is regulated by Cdc14 phosphatase during mitosis

Cyclin-dependent kinase (Cdk1) activity is required for mitotic entry, and this event is restrained by an inhibitory phosphorylation of the catalytic subunit Cdc28 on a conserved tyrosine (Tyr(19)). This modification is brought about by the protein kinase Swe1 that inhibits Cdk1 activation thus blocking mitotic entry. Swe1 levels are regulated during the cell cycle, and they decrease during G2/M concomitantly to Cdk1 activation, which drives entry into mitosis. However, after mitotic entry, a pool of Swe1 persists, and we collected evidence that it is involved in controlling mitotic spindle elongation. We also describe that the protein phosphatase Cdc14 is implicated in Swe1 regulation; in fact, we observed that Swe1 dephosphorylation in vivo depends on Cdc14 that, in turn, is able to control its subcellular localization. In addition we show that the lack of Swe1 causes premature mitotic spindle elongation and that high levels of Swe1 block mitotic spindle elongation, indicating that Swe1 inhibits this process. Importantly, these effects are not dependent upon the role of in Cdk1 inhibition. These data fit into a model in which Cdc14 binds and inhibits Swe1 to allow timely mitotic spindle elongation.

Physiological Roles of Calcineurin inSaccharomyces cerevisiaewith Special Emphasis on Its Roles in G2/M Cell-Cycle Regulation

Calcineurin, a highly conserved Ca(2+)/CaM-dependent protein phosphatase, plays key regulatory roles in diverse biological processes from yeast to humans. Genetic and molecular analyses of the yeast model system have proved successful in dissecting complex regulatory pathways mediated by calcineurin. Saccharomyces cerevisiae calcineurin is not essential for growth under laboratory conditions, but becomes essential for survival under certain stress conditions, and is required for stress-induced expression of the genes for ion transporters and cell-wall synthesis. Yeast calcineurin, in collaboration with a Mpk1 MAP kinase cascade, is also important in G(2) cell-cycle regulation due to its action in a checkpoint-like mechanism. Genetic and molecular analysis of the Ca(2+)-dependent cell-cycle regulation has revealed an elaborate mechanism for the calcineurin-dependent regulation of the G(2)/M transition, in which calcineurin multilaterally activates Swe1, a negative regulator of the Cdc28/Clb complex, at the transcriptional, posttranslational, and degradation levels.

Comparative genetic analysis of PP2A-Cdc55 regulators in budding yeast

Cdc55, a regulatory B subunit of the protein phosphatase 2A (PP2A) complex, plays various functions during mitosis. Sequestration of Cdc55 from the nucleus by Zds1 and Zds2 is important for robust activation of mitotic Cdk1 and mitotic progression in budding yeast. However, Zds1-family proteins are found only in fungi but not in higher eukaryotes. In animal cells, highly conserved ENSA/ARPP-19 family proteins bind and inhibit PP2A-B55 activity for mitotic entry.   In this study, we compared the relative contribution of Zds1/Zds2 and ENSA-family proteins Igo1/Igo2 on Cdc55 functions in budding yeast mitosis. We confirmed that Igo1/Igo2 can inhibit Cdc55 in early mitosis, but their contribution to Cdc55 regulation is relatively minor compared with the role of Zds1/Zds2. In contrast to Zds1, which primarily localized to the sites of cell polarity and in the cytoplasm, Igo1 is localized in the nucleus, suggesting that Igo1/Igo2 inhibit Cdc55 in a manner distinct from Zds1/Zds2. Our analysis confirmed an evolutionarily conserved function of ENSA-family proteins in inhibiting PP2A-Cdc55, and we propose that Zds1-dependent sequestration of PP2A-Cdc55 from the nucleus is uniquely evolved to facilitate closed mitosis in fungal species.

The transmission of nuclear pore complexes to daughter cells requires a cytoplasmic pool of Nsp1

Nuclear pore complexes (NPCs) are essential protein assemblies that span the nuclear envelope and establish nuclear-cytoplasmic compartmentalization. We have investigated mechanisms that control NPC number in mother and daughter cells during the asymmetric division of budding yeast. By simultaneously tracking existing NPCs and newly synthesized NPC protomers (nups) through anaphase, we uncovered a pool of the central channel nup Nsp1 that is actively targeted to the bud in association with endoplasmic reticulum. Bud targeting required an intact actin cytoskeleton and the class V myosin, Myo2. Selective inhibition of cytoplasmic Nsp1 or inactivation of Myo2 reduced the inheritance of NPCs in daughter cells, leading to a daughter-specific loss of viability. Our data are consistent with a model in which Nsp1 releases a barrier that otherwise prevents NPC passage through the bud neck. It further supports the finding that NPC inheritance, not de novo NPC assembly, is primarily responsible for controlling NPC number in daughter cells.

Nuclear PP2A-Cdc55 prevents APC-Cdc20 activation during the spindle assembly checkpoint

Cdc55, a regulatory B-subunit of protein phosphatase 2A (PP2A) complex, is essential for the spindle assembly checkpoint (SAC) in budding yeast, but the regulation and molecular targets of PP2A-Cdc55 have not been clearly defined or are controversial. Here, we show that an important target of Cdc55 in the SAC is the anaphase-promoting complex (APC) coupled with Cdc20 and that APC-Cdc20 is kept inactive by dephosphorylation by nuclear PP2A-Cdc55 when spindle is damaged. By isolating a new class of Cdc55 mutants specifically defective in the SAC and by artificially manipulating nucleocytoplasmic distribution of Cdc55, we further show that nuclear Cdc55 is essential for the SAC. Because the Cdc55-binding proteins Zds1 and Zds2 inhibit both nuclear accumulation of Cdc55 and SAC activity, we propose that spatial control of PP2A by Zds1 family proteins is important for tight control of SAC and mitotic progression.

Cell cycle checkpoint regulators reach a zillion

Entry into mitosis is regulated by a checkpoint at the boundary between the G2 and M phases of the cell cycle (G2/M). In many organisms, this checkpoint surveys DNA damage and cell size and is controlled by both the activation of mitotic cyclin-dependent kinases (Cdks) and the inhibition of an opposing phosphatase, protein phosphatase 2A (PP2A). Misregulation of mitotic entry can often lead to oncogenesis or cell death. Recent research has focused on discovering the signaling pathways that feed into the core checkpoint control mechanisms dependent on Cdk and PP2A. Herein, we review the conserved mechanisms of the G2/M transition, including recently discovered upstream signaling pathways that link cell growth and DNA replication to cell cycle progression. Critical consideration of the human, frog and yeast models of mitotic entry frame unresolved and emerging questions in this field, providing a prediction of signaling molecules and pathways yet to be discovered.

Feedback control of Swe1p degradation in the yeast morphogenesis checkpoint

The morphogenesis checkpoint stabilizes the mitotic inhibitor Swe1p and prevents mitosis following stresses that affect bud formation. It is shown that, following some stresses, Swe1p stabilization is an indirect effect of cyclin-dependent kinase inhibition.

Saccharomyces cerevisiae cells exposed to a variety of physiological stresses transiently delay bud emergence or bud growth. To maintain coordination between bud formation and the cell cycle in such circumstances, the morphogenesis checkpoint delays nuclear division via the mitosis-inhibitory Wee1-family kinase, Swe1p. Swe1p is degraded during G2 in unstressed cells but is stabilized and accumulates following stress. Degradation of Swe1p is preceded by its recruitment to the septin scaffold at the mother-bud neck, mediated by the Swe1p-binding protein Hsl7p. Following osmotic shock or actin depolymerization, Swe1p is stabilized, and previous studies suggested that this was because Hsl7p was no longer recruited to the septin scaffold following stress. However, we now show that Hsl7p is in fact recruited to the septin scaffold in stressed cells. Using a cyclin-dependent kinase (CDK) mutant that is immune to checkpoint-mediated inhibition, we show that Swe1p stabilization following stress is an indirect effect of CDK inhibition. These findings demonstrate the physiological importance of a positive-feedback loop in which Swe1p activity inhibits the CDK, which then ceases to target Swe1p for degradation. They also highlight the difficulty in disentangling direct checkpoint pathways from the effects of positive-feedback loops active at the G2/M transition.

Roles of Hsl1p and Hsl7p in Swe1p degradation: beyond septin tethering

The morphogenesis checkpoint in Saccharomyces cerevisiae couples bud formation to the cell division cycle by delaying nuclear division until cells have successfully constructed a bud. The cell cycle delay is due to the mitosis-inhibitory kinase Swe1p, which phosphorylates the cyclin-dependent kinase Cdc28p. In unperturbed cells, Swe1p is degraded via a mechanism thought to involve its tethering to a cortical scaffold of septin proteins at the mother-bud neck. In cells that experience stresses that delay bud formation, Swe1p is stabilized, accumulates, and promotes a G(2) delay. The tethering of Swe1p to the neck requires two regulators, called Hsl1p and Hsl7p. Hsl1p interacts with septins, and Hsl7p interacts with Swe1p; tethering occurs when Hsl1p interacts with Hsl7p. Here we created a version of Swe1p that is artificially tethered to the neck by fusion to a septin so that Swe1p no longer requires Hsl1p or Hsl7p for its localization to the neck. We show that the interaction between Hsl1p and Hsl7p, required for normal Swe1p degradation, is no longer needed for septin-Swe1p degradation, supporting the idea that the Hsl1p-Hsl7p interaction serves mainly to tether Swe1p to the neck. However, both Hsl1p and Hsl7p are still required for Swe1p degradation, implying that these proteins play additional roles beyond localizing Swe1p to the neck.

Zds1 regulates PP2A(Cdc55) activity and Cdc14 activation during mitotic exit through its Zds_C motif

At anaphase onset, highly active mitotic cyclin-dependent kinase (Cdk) is inactivated to promote exit from mitosis and completion of cytokinesis. The budding yeast Cdc14p phosphatase is a key mitotic regulator that counteracts cyclin-dependent kinase (Cdk) activity during mitotic exit. Separase, together with Zds1p, promotes the downregulation of the protein phosphatase 2A in conjunction with its Cdc55p regulatory subunit (PP2A(Cdc55)) in early anaphase, enabling accumulation of phosphorylated forms of Net1p and release of Cdc14p from the nucleolus. Here we show that the C-terminal domain of Zds1p, called the Zds_C motif, is required for Zds1-induced release of Cdc14p, and the N-terminal domain of the protein might be involved in regulating this activity. More interestingly, Zds1p physically interacts with Cdc55p, and regulates its localization through the Zds_C motif. Nevertheless, expression of the Zds_C motif at endogenous levels cannot induce timely release of Cdc14p from the nucleolus, despite the proper (nucleolar) localization of Cdc55p. Our results suggest that the activity of PP2A(Cdc55) cannot be modulated solely through regulation of its localization, and that an additional regulatory step is probably required. These results suggest that Zds1p recruits PP2A(Cdc55) to the nucleolus and induces its inactivation by an unknown mechanism.

Cellular morphogenesis under stress is influenced by the sphingolipid pathway gene ISC1 and DNA integrity checkpoint genes in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, replication stress induced by hydroxyurea (HU) and methyl methanesulfonate (MMS) activates DNA integrity checkpoints; in checkpoint-defective yeast strains, HU treatment also induces morphological aberrations. We find that the sphingolipid pathway gene ISC1, the product of which catalyzes the generation of bioactive ceramides from complex sphingolipids, plays a novel role in determining cellular morphology following HU/MMS treatment. HU-treated isc1Δ cells display morphological aberrations, cell-wall defects, and defects in actin depolymerization. Swe1, a morphogenesis checkpoint regulator, and the cell cycle regulator Cdk1 play key roles in these morphological defects of isc1Δ cells. A genetic approach reveals that ISC1 interacts with other checkpoint proteins to control cell morphology. That is, yeast carrying deletions of both ISC1 and a replication checkpoint mediator gene including MRC1, TOF1, or CSM3 display basal morphological defects, which increase following HU treatment. Interestingly, strains with deletions of both ISC1 and the DNA damage checkpoint mediator gene RAD9 display reduced morphological aberrations irrespective of HU treatment, suggesting a role for RAD9 in determining the morphology of isc1Δ cells. Mechanistically, the checkpoint regulator Rad53 partially influences isc1Δ cell morphology in a dosage-dependent manner.

The budding yeast polo-like kinase Cdc5 regulates the Ndt80 branch of the meiotic recombination checkpoint pathway

Meiosis is a specialized cell division that generates haploid gametes. Accurate distribution of genetic information to the meiotic progeny is ensured by the action of the meiotic recombination checkpoint. The function of the evolutionarily conserved polo-like kinase in this meiotic surveillance mechanism is described.

Defects in chromosome synapsis and/or meiotic recombination activate a surveillance mechanism that blocks meiotic cell cycle progression to prevent anomalous chromosome segregation and formation of aberrant gametes. In the budding yeast zip1 mutant, which lacks a synaptonemal complex component, the meiotic recombination checkpoint is triggered, resulting in extremely delayed meiotic progression. We report that overproduction of the polo-like kinase Cdc5 partially alleviates the meiotic prophase arrest of zip1, leading to the formation of inviable meiotic products. Unlike vegetative cells, we demonstrate that Cdc5 overproduction does not stimulate meiotic checkpoint adaptation because the Mek1 kinase remains activated in zip1 2μ-CDC5 cells. Inappropriate meiotic divisions in zip1 promoted by high levels of active Cdc5 do not result from altered function of the cyclin-dependent kinase (CDK) inhibitor Swe1. In contrast, CDC5 overexpression leads to premature induction of the Ndt80 transcription factor, which drives the expression of genes required for meiotic divisions, including CLB1. We also show that depletion of Cdc5 during meiotic prophase prevents the production of Ndt80 and that CDK activity contributes to the induction of Ndt80 in zip1 cells overexpressing CDC5. Our results reveal a role for Cdc5 in meiotic checkpoint control by regulating Ndt80 function.

Spatial regulation of Cdc55-PP2A by Zds1/Zds2 controls mitotic entry and mitotic exit in budding yeast

Zds1/2 regulate mitotic progression by directing the nucleocytoplasmic distribution of Cdc55–PP2A.

Budding yeast CDC55 encodes a regulatory B subunit of the PP2A (protein phosphatase 2A), which plays important roles in mitotic entry and mitotic exit. The spatial and temporal regulation of PP2A is poorly understood, although recent studies demonstrated that the conserved proteins Zds1 and Zds2 stoichiometrically bind to Cdc55–PP2A and regulate it in a complex manner. Zds1/Zds2 promote Cdc55–PP2A function for mitotic entry, whereas Zds1/Zds2 inhibit Cdc55–PP2A function during mitotic exit. In this paper, we propose that Zds1/Zds2 primarily control Cdc55 localization. Cortical and cytoplasmic localization of Cdc55 requires Zds1/Zds2, and Cdc55 accumulates in the nucleus in the absence of Zds1/Zds2. By genetically manipulating the nucleocytoplasmic distribution of Cdc55, we showed that Cdc55 promotes mitotic entry when in the cytoplasm. On the other hand, nuclear Cdc55 prevents mitotic exit. Our analysis defines the long-sought molecular function for the zillion different screens family proteins and reveals the importance of the regulation of PP2A localization for proper mitotic progression.

Zds2p regulates Swe1p-dependent polarized cell growth in Saccharomyces cerevisiae via a novel Cdc55p interaction domain

Deletion of the paralogs ZDS1 and ZDS2 in the budding yeast Saccharomyces cerevisiae causes a mis-regulation of polarized cell growth. Here we show a function for these genes as regulators of the Swe1p (Wee1p) kinase-dependent G2/M checkpoint. We identified a conserved domain in the C-terminus of Zds2p consisting of amino acids 813-912 (hereafter referred to as ZH4 for Zds homology 4) that is required for regulation of Swe1p-dependent polarized bud growth. ZH4 is shown by protein affinity assays to be necessary and sufficient for interaction with Cdc55p, a regulatory subunit of protein phosphatase 2A (PP2A). We hypothesized that the Zds proteins are in a pathway that negatively regulates the Swe1p-dependent G2/M checkpoint via Cdc55p. Supporting this model, deletion of CDC55 rescues the aberrant bud morphology of a zds1Δzds2Δ strain. We also show that expression of ZDS1 or ZDS2 from a strong galactose-inducible promoter can induce mitosis even when the Swe1p-dependent G2/M checkpoint is activated by mis-organization of the actin cytoskeleton. This negative regulation requires the CDC55 gene. Together these data indicate that the Cdc55p/Zds2p module has a function in the regulation of the Swe1p-dependent G2/M checkpoint.

The Zds proteins control entry into mitosis and target protein phosphatase 2A to the Cdc25 phosphatase

The Wee1 kinase restrains entry into mitosis by phosphorylating and inhibiting cyclin-dependent kinase 1 (Cdk1). The Cdc25 phosphatase promotes entry into mitosis by removing Cdk1 inhibitory phosphorylation. Experiments in diverse systems have established that Wee1 and Cdc25 are regulated by protein phosphatase 2A (PP2A), but a full understanding of the function and regulation of PP2A in entry into mitosis has remained elusive. In budding yeast, entry into mitosis is controlled by a specific form of PP2A that is associated with the Cdc55 regulatory subunit (PP2A(Cdc55)). We show here that related proteins called Zds1 and Zds2 form a tight stoichiometric complex with PP2A(Cdc55) and target its activity to Cdc25 but not to Wee1. Conditional inactivation of the Zds proteins revealed that their function is required primarily at entry into mitosis. In addition, Zds1 undergoes cell cycle-dependent changes in phosphorylation. Together, these observations define a role for the Zds proteins in controlling specific functions of PP2A(Cdc55) and suggest that upstream signals that regulate PP2A(Cdc55) may play an important role in controlling entry into mitosis.

Hsp90 phosphorylation, Wee1 and the cell cycle

Heat Shock Protein 90 (Hsp90) is an essential molecular chaperone in eukaryotic cells, and it maintains the functional conformation of a subset of proteins that are typically key components of multiple regulatory and signaling networks mediating cancer cell proliferation, survival, and metastasis. It is possible to selectively inhibit Hsp90 using natural products such as geldanamycin (GA) or radicicol (RD), which have served as prototypes for development of synthetic Hsp90 inhibitors. These compounds bind within the ADP/ATP-binding site of the Hsp90 N-terminal domain to inhibit its ATPase activity. As numerous N-terminal domain inhibitors are currently undergoing extensive clinical evaluation, it is important to understand the factors that may modulate in vivo susceptibility to these drugs. We recently reported that Wee1Swe1-mediated, cell cycle-dependent, tyrosine phosphorylation of Hsp90 affects GA binding and impacts cancer cell sensitivity to Hsp90 inhibition. This phosphorylation also affects Hsp90 ATPase activity and its ability to chaperone a selected group of clients, comprised primarily of protein kinases. Wee1 regulates the G2/M transition. Here we present additional data demonstrating that tyrosine phosphorylation of Hsp90 by Wee1Swe1 is important for Wee1Swe1 association with Hsp90 and for Wee1Swe1 stability. Yeast expressing non-phosphorylatable yHsp90-Y24F, like swe1∆ yeast, undergo premature nuclear division that is insensitive to G2/M checkpoint arrest. These findings demonstrate the importance of Hsp90 phosphorylation for proper cell cycle regulation.

Control of cell proliferation, organ growth, and DNA damage response operate independently of dephosphorylation of the Arabidopsis Cdk1 homolog CDKA;1

Entry into mitosis is universally controlled by cyclin-dependent kinases (CDKs). A key regulatory event in metazoans and fission yeast is CDK activation by the removal of inhibitory phosphate groups in the ATP binding pocket catalyzed by Cdc25 phosphatases. In contrast with other multicellular organisms, we show here that in the flowering plant Arabidopsis thaliana, cell cycle control does not depend on sudden changes in the phosphorylation pattern of the PSTAIRE-containing Cdk1 homolog CDKA;1. Consistently, we found that neither mutants in a previously identified CDC25 candidate gene nor plants in which it is overexpressed display cell cycle defects. Inhibitory phosphorylation of CDKs is also the key event in metazoans to arrest cell cycle progression upon DNA damage. However, we show here that the DNA damage checkpoint in Arabidopsis can also operate independently of the phosphorylation of CDKA;1. These observations reveal a surprising degree of divergence in the circuitry of highly conserved core cell cycle regulators in multicellular organisms. Based on biomathematical simulations, we propose a plant-specific model of how progression through the cell cycle could be wired in Arabidopsis.

Identification of Clb2 residues required for Swe1 regulation of Clb2-Cdc28 in Saccharomyces cerevisiae

Wee1 kinases regulate the cell cycle through inhibitory phosphorylation of cyclin-dependent kinases (CDKs). Eukaryotic cells express multiple CDKs, each having a kinase subunit (Cdk) and a regulatory "cyclin" subunit that function at different stages of the cell cycle to regulate distinct processes. The cyclin imparts specificity to CDK-substrate interactions and also determines whether a particular CDK is subject to Wee1 regulation. Saccharomyces Wee1 (Swe1) inhibits Cdc28 (Cdk1) associated with the mitotic cyclin, Clb2, but not with the G(1) (Cln1, -2, and -3) or the S-phase (Clb5 and -6) cyclins. Here, we show that this specificity depends on two amino acids associated with a conserved "hydrophobic patch" (HP) motif on the cyclin surface, which mediates specificity of CDK-substrate interactions. Mutation of Clb2 residues N260 and K270 largely abrogates Clb2-Cdc28 regulation by Swe1, and reciprocal mutation of the corresponding residues in Clb5 can subject Clb5-Cdc28 to regulation by Swe1. Swe1 phosphorylation by Clb2-Cdc28, which is thought to activate Swe1 kinase, depends on N260 and K270, suggesting that specific regulation of Clb2-Cdc28 by Swe1 derives from the specific ability of Clb2 to target Swe1 for activating phosphorylation. The stable association of Swe1 with Clb2-Cdc28 also depends on these residues, suggesting that Swe1 may competitively inhibit Clb2-Cdc28 interactions with substrates, in addition to its well-known function as a regulator of CDK activity through tyrosine phosphorylation.

Nucleocytoplasmic trafficking of G2/M regulators in yeast

Nucleocytoplasmic shuttling is prevalent among many cell cycle regulators controlling the G2/M transition. Shuttling of cyclin/cyclin-dependent kinase (CDK) complexes is thought to provide access to substrates stably located in either compartment. Because cyclin/CDK shuttles between cellular compartments, an upstream regulator that is fixed in one compartment could in principle affect the entire cyclin/CDK pool. Alternatively, the regulators themselves may need to shuttle to effectively regulate their moving target. Here, we identify localization motifs in the budding yeast Swe1p (Wee1) and Mih1p (Cdc25) cell cycle regulators. Replacement of endogenous Swe1p or Mih1p with mutants impaired in nuclear import or export revealed that the nuclear pools of Swe1p and Mih1p were more effective in CDK regulation than were the cytoplasmic pools. Nevertheless, shuttling of cyclin/CDK complexes was sufficiently rapid to coordinate nuclear and cytoplasmic events even when Swe1p or Mih1p were restricted to one compartment. Additionally, we found that Swe1p nuclear export was important for its degradation. Because Swe1p degradation is regulated by cytoskeletal stress, shuttling of Swe1p between nucleus and cytoplasm serves to couple cytoplasmic stress to nuclear cyclin/CDK inhibition.

Multiple levels of cyclin specificity in cell-cycle control

Cyclins regulate the cell cycle by binding to and activating cyclin-dependent kinases (Cdks). Phosphorylation of specific targets by cyclin-Cdk complexes sets in motion different processes that drive the cell cycle in a timely manner. In budding yeast, a single Cdk is activated by multiple cyclins. The ability of these cyclins to target specific proteins and to initiate different cell-cycle events might, in some cases, reflect the timing of the expression of the cyclins; in others, it might reflect intrinsic properties of the cyclins that render them better suited to target particular proteins.

AgSwe1p regulates mitosis in response to morphogenesis and nutrients in multinucleated Ashbya gossypii cells

Nuclei in the filamentous, multinucleated fungus Ashbya gossypii divide asynchronously. We have investigated what internal and external signals spatially direct mitosis within these hyphal cells. Mitoses are most common near cortical septin rings found at growing tips and branchpoints. In septin mutants, mitoses are no longer concentrated at branchpoints, suggesting that the septin rings function to locally promote mitosis near new branches. Similarly, cells lacking AgSwe1p kinase (a Wee1 homologue), AgHsl1p (a Nim1-related kinase), and AgMih1p phosphatase (the Cdc25 homologue that likely counteracts AgSwe1p activity) also have mitoses distributed randomly in the hyphae as opposed to at branchpoints. Surprisingly, however, no phosphorylation of the CDK tyrosine 18 residue, the conserved substrate of Swe1p kinases, was detected in normally growing cells. In contrast, abundant CDK tyrosine phosphorylation was apparent in starving cells, resulting in diminished nuclear density. This starvation-induced CDK phosphorylation is AgSwe1p dependent, and overexpressed AgSwe1p is sufficient to delay nuclei even in rich nutrient conditions. In starving cells lacking septins or AgSwe1p negative regulators, the nuclear density is further diminished compared with wild type. We have generated a model in which AgSwe1p may regulate mitosis in response to cell intrinsic morphogenesis cues and external nutrient availability in multinucleated cells.

ELM1 is required for multidrug resistance in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, transcription of several drug transporter genes, including the major transporter gene PDR5, has been shown to peak during mitosis. The significance of this observation, however, remains unclear. PDR1 encodes the primary transcription activator of multiple drug transporter genes in S. cerevisiae, including PDR5. Here, we show that in synchronized PDR1 and pdr1-3 (multidrug resistant) strains, cellular efflux of a known substrate of ATP-binding-cassette transporters, doxorubicin (a fluorescent anticancer drug), is highest during mitosis when PDR5 transcription peaks. A genetic screen performed to identify regulators of multidrug resistance revealed that a truncation mutation in ELM1 (elm1-300) suppressed the multidrug resistance of pdr1-3. ELM1 encodes a serine/threonine protein kinase required for proper regulation of multiple cellular kinases, including those involved in mitosis, cytokinesis, and cellular morphogenesis. elm1-300 as well as elm1Delta mutations in a pdr1-3 strain also caused elongated bud morphology (indicating a G2/M delay) and reduction of PDR5 transcription under induced and noninduced conditions. Interestingly, mutations in several genes functionally related to ELM1, including cla4Delta, gin4Delta, and cdc28-C127Y, also caused drastic reductions in drug resistance and PDR5 transcription. Collectively, these data show that ELM1, and genes encoding related serine/threonine protein kinases, are required for regulation of multidrug resistance involving, at least in part, control of PDR5 transcription.

Downregulation of PP2A(Cdc55) phosphatase by separase initiates mitotic exit in budding yeast

After anaphase, the high mitotic cyclin-dependent kinase (Cdk) activity is downregulated to promote exit from mitosis. To this end, in the budding yeast S. cerevisiae, the Cdk counteracting phosphatase Cdc14 is activated. In metaphase, Cdc14 is kept inactive in the nucleolus by its inhibitor Net1. During anaphase, Cdk- and Polo-dependent phosphorylation of Net1 is thought to release active Cdc14. How Net1 is phosphorylated specifically in anaphase, when mitotic kinase activity starts to decline, has remained unexplained. Here, we show that PP2A(Cdc55) phosphatase keeps Net1 underphosphorylated in metaphase. The sister chromatid-separating protease separase, activated at anaphase onset, interacts with and downregulates PP2A(Cdc55), thereby facilitating Cdk-dependent Net1 phosphorylation. PP2A(Cdc55) downregulation also promotes phosphorylation of Bfa1, contributing to activation of the "mitotic exit network" that sustains Cdc14 as Cdk activity declines. These findings allow us to present a new quantitative model for mitotic exit in budding yeast.

LAS24/KOG1, a component of the TOR complex 1 (TORC1), is needed for resistance to local anesthetic tetracaine and normal distribution of actin cytoskeleton in yeast

It is known that some local anesthetics inhibit the growth of budding yeast cells. To investigate the pathway of local anesthetics' action, we isolated and characterized mutants that were hyper-sensitive to tetracaine, and at the same time, temperature-sensitive for growth. They were collectively called las (local anesthetic sensitive) mutants. One of the LAS genes, LAS24, was found to be identical to KOG1, which had been independently discovered as a member of the TOR complex 1 (TORC1). Las24p/Kog1p is a widely conserved TOR binding protein containing the NRC domain, HEAT repeats and WD-40 repeats, but its function remains unknown. Like the tor mutants, the las24 mutants were found to have a defect in cell wall integrity and to show sensitivity to rapamycin. Furthermore, Las24p is required not only in TORC1-mediated (rapamycin-sensitive) pathways such as translation initiation control and phosphorylation of Npr1p and Gln3p, but also for the normal distribution of the actin cytoskeleton, which has been regarded as a TORC2-mediated event. Intriguingly, the temperature-sensitivity of the las24 mutant was suppressed by either activation of Tap42/PPase or by down-regulation of the RAS/cAMP pathway. Suppressors of the temperature-sensitivity of the las24-1 mutant were found not to be effective for suppression of the tetracaine-sensitivity of the same mutant. These observations along with the facts that tetracaine and high temperature differentially affected the las24-1 mutant suggest that Las24p/Kog1p is not a target of tetracaine and that the tetracaine-sensitive step may be one of downstream branches of the TORC1 pathway. Consistent with the broad cellular functions exerted by the TOR pathway, we found that Las24p was associated with membranes and was localized at vacuoles, the plasma membrane and small vesicles.

Cdk1-dependent regulation of the mitotic inhibitor Wee1

The Wee1 kinase phosphorylates and inhibits cyclin-dependent kinase 1 (Cdk1), thereby delaying entry into mitosis until appropriate conditions have been met. An understanding of the mechanisms that regulate Wee1 should provide new insight into how cells make the decision to enter mitosis. We report here that Swe1, the budding-yeast homolog of Wee1, is directly regulated by Cdk1. Phosphorylation of Swe1 by Cdk1 activates Swe1 and is required for formation of a stable Swe1-Cdk1 complex that maintains Cdk1 in the inhibited state. Dephosphorylation of Cdk1 leads to further phosphorylation of Swe1 and release of Cdk1. Thus, Cdk1 both positively and negatively regulates its own inhibitor. Regulation of the Swe1-Cdk1 complex is likely to play a critical role in controlling the transition into mitosis.

Swe1 regulation and transcriptional control restrict the activity of mitotic cyclins toward replication proteins in Saccharomyces cerevisiae

Cyclin-dependent kinases (CDKs) drive the cell cycle through the phosphorylation of substrates that function in genome duplication and cell division. The existence of multiple cyclin subunits and their distinct cell cycle-regulated expression suggests that cyclins impart unique specificities to CDK-substrate interactions that are critical for normal cellular function. This study shows that the combination of early cell cycle expression and deletion of the CDK inhibitor Saccharomyces Wee1 (Swe1) enables the mitotic B-type (Clb) cyclins Clb2, Clb3, and Clb4 of Saccharomyces cerevisiae to initiate S phase with similar effectiveness as the S-phase cyclin Clb5. Although in vivo analysis indicates preferential phosphorylation of a replication substrate by Clb5-Cdk1, this difference is relatively minor compared with the impact of transcriptional control and Swe1 regulation. Indeed, early expressed Clb2-Cdk1 can activate all essential Clb-Cdk substrates in a strain lacking all other Clbs and Swe1. Thus, Swe1 regulation and expression timing are key mechanisms that sequester the broad activity of Clb2-Cdk1 from critical substrates. Furthermore, the ability of Swe1 to inhibit the activity of different B-type cyclins in replication initiation correlates with the normal expression timing of those cyclins, with no apparent in vivo inhibition of Clb5 and Clb6, moderate inhibition of Clb3 and Clb4, and strong inhibition of Clb2. Hence, Swe1 appears to reinforce the temporal activity of cyclins established through transcriptional control. The conserved nature of CDK function suggests that similar mechanisms regulate CDK specificity in multicellular organisms.

Concerted mechanism of Swe1/Wee1 regulation by multiple kinases in budding yeast

In eukaryotes, entry into mitosis is induced by cyclin B-bound Cdk1, which is held in check by the protein kinase, Wee1. In budding yeast, Swe1 (Wee1 ortholog) is targeted to the bud neck through Hsl1 (Nim1-related kinase) and its adaptor Hsl7, and is hyperphosphorylated prior to ubiquitin-mediated degradation. Here, we show that Hsl1 and Hsl7 are required for proper localization of Cdc5 (Polo-like kinase homolog) to the bud neck and Cdc5-dependent Swe1 phosphorylation. Mitotic cyclin (Clb2)-bound Cdc28 (Cdk1 homolog) directly phosphorylated Swe1 and this modification served as a priming step to promote subsequent Cdc5-dependent Swe1 hyperphosphorylation and degradation. Clb2-Cdc28 also facilitated Cdc5 localization to the bud neck through the enhanced interaction between the Clb2-Cdc28-phosphorylated Swe1 and the polo-box domain of Cdc5. We propose that the concerted action of Cdc28/Cdk1 and Cdc5/Polo on their common substrates is an evolutionarily conserved mechanism that is crucial for effectively triggering mitotic entry and other critical mitotic events.

Identification of residues in the WD-40 repeat motif of the F-box protein Met30p required for interaction with its substrate Met4p

The SCF family of ubiquitin-ligases consists of a common core machinery, namelySkp1p, Cdc53p, Hrt1p, and a variable component, the F-box protein that is responsible for substrate recognition. The F-box motif, which consists of approximately 40 amino acids, connects the F-box protein to the core ubiquitin-ligase machinery. Distinct SCF complexes, defined by distinct F-box proteins, target different substrate proteins for proteasome-dependent degradation. As part of the SCF(Met30p) complex, the F-box protein Met30p selects the substrate Met4p, a transcriptional activator for MET biosynthetic genes that mediate sulfur uptake and biosynthesis of sulfur containing compounds. When cells are grown in the absence of methionine, Met4p evades degradation by the SCF(Met30p) complex and activates the MET biosynthetic pathway. However, overproduction of Met30p represses MET gene expression and induces methionine auxotrophy in an otherwise methionine prototrophic strain. Here we demonstrate that overproduction of the C-terminal portion of Met30p, which is composed almost entirely of seven WD-40 repeat motifs, is necessary and sufficient to induce methionine auxotrophy and complement the temperature sensitive (ts) met30-6 mutation. Furthermore, we show that this region of Met30p is important for binding Met4p and that mutations that disrupt this interaction prevent both the induction of methionine auxotrophy and complementation of the met30-6 mutation. These assays have been exploited to identify residues that are important for the interaction of Met30p with its substrate. Since the C-terminal domain of Met30p lacks the F-box and cannot support the ubiquitination of Met4p, our results indicate that the recruitment of Met4p to the SCF(Met30p) complex itself results in inactivation of Met4p, independently of its ubiquitination.

Interplay between septin organization, cell cycle and cell shape in yeast

Septins are conserved filament-forming proteins that assemble into cortical cytoskeletal structures in animal and fungal cells. Although rapid progress has been made into the functions of septins, the mechanisms governing their localization and organization remain mysterious. In Saccharomyces cerevisiae, Cdc42p organizes the septin cytoskeleton into a ring in preparation for bud formation, following which septins remain as a collar at the mother-bud neck. We have dissected the phenotype of cdc42(V36T,K94E) cells that display an aberrant cell shape correlated with the development of ectopic septin caps and rings within the bud. The results suggest that a well-assembled septin cortex plays a novel role in directing growth to shape the nascent bud, and that a disorganized septin cortex directs improper growth generating an aberrant neck. Conversely, we found that the elongated bud shape arising as a result of the morphogenesis checkpoint cell cycle delay that accompanies septin perturbation can feed back to exacerbate minor defects in septin organization, by maintaining a bud-tip-localized septin assembly activity that competes with the neck-localized septin cortex. Using this exacerbation as a tool, we uncovered septin organization defects in many mutants not previously known to display such defects, expanding the cast of characters involved in proper assembly of the septin cortex to include CLN1, CLN2, BNI1, BNI4, BUD3, BUD4 and BUD5.

Opposing roles for actin in Cdc42p polarization

In animal and fungal cells, the monomeric GTPase Cdc42p is a key regulator of cell polarity that itself exhibits a polarized distribution in asymmetric cells. Previous work showed that in budding yeast, Cdc42p polarization is unaffected by depolymerization of the actin cytoskeleton (Ayscough et al., J. Cell Biol. 137, 399-416, 1997). Surprisingly, we now report that unlike complete actin depolymerization, partial actin depolymerization leads to the dispersal of Cdc42p from the polarization site in unbudded cells. We provide evidence that dispersal is due to endocytosis associated with cortical actin patches and that actin cables are required to counteract the dispersal and maintain Cdc42p polarity. Thus, although Cdc42p is initially polarized in an actin-independent manner, maintaining that polarity may involve a reinforcing feedback between Cdc42p and polarized actin cables to counteract the dispersing effects of actin-dependent endocytosis. In addition, we report that once a bud has formed, polarized Cdc42p becomes more resistant to dispersal, revealing an unexpected difference between unbudded and budded cells in the organization of the polarization site.

Mathematical model of the morphogenesis checkpoint in budding yeast

The morphogenesis checkpoint in budding yeast delays progression through the cell cycle in response to stimuli that prevent bud formation. Central to the checkpoint mechanism is Swe1 kinase: normally inactive, its activation halts cell cycle progression in G2. We propose a molecular network for Swe1 control, based on published observations of budding yeast and analogous control signals in fission yeast. The proposed Swe1 network is merged with a model of cyclin-dependent kinase regulation, converted into a set of differential equations and studied by numerical simulation. The simulations accurately reproduce the phenotypes of a dozen checkpoint mutants. Among other predictions, the model attributes a new role to Hsl1, a kinase known to play a role in Swe1 degradation: Hsl1 must also be indirectly responsible for potent inhibition of Swe1 activity. The model supports the idea that the morphogenesis checkpoint, like other checkpoints, raises the cell size threshold for progression from one phase of the cell cycle to the next.

Genetic interactions amongZDS1,2,CDC37, and protein kinase CK2 inSaccharomyces cerevisiae

We report here the identification of the homologous gene pair ZDS1,2 as multicopy suppressors of a temperature-sensitive allele (cka2-13(ts)) of the CKA2 gene encoding the alpha' catalytic subunit of protein kinase CK2. Overexpression of ZDS1,2 suppressed the temperature sensitivity, geldanamycin (GA) sensitivity, slow growth, and flocculation of multiple cka2 alleles and enhanced CK2 activity in vivo toward a known physiological substrate, Fpr3. Consistent with the existence of a recently described positive feedback loop between CK2 and Cdc37, overexpression of ZDS1,2 also suppressed the temperature sensitivity, abnormal morphology, and GA sensitivity of a CK2 phosphorylation-deficient mutant of CDC37, cdc37-S14A, as well as the GA sensitivity of a cdc37-1 allele. A likely basis for all of these effects is our observation that ZDS1,2 overexpression enhances Cdc37 protein levels. Activation of the positive feedback loop between CK2 and Cdc37 likely contributes to the pleiotropic nature of ZDS1,2, as both CK2 and Cdc37 regulate diverse cellular functions.

A monitor for bud emergence in the yeast morphogenesis checkpoint

Cell cycle transitions are subject to regulation by both external signals and internal checkpoints that monitor satisfactory progression of key cell cycle events. In budding yeast, the morphogenesis checkpoint arrests the cell cycle in response to perturbations that affect the actin cytoskeleton and bud formation. Herein, we identify a step in this checkpoint pathway that seems to be directly responsive to bud emergence. Activation of the kinase Hsl1p is dependent upon its recruitment to a cortical domain organized by the septins, a family of conserved filament-forming proteins. Under conditions that delayed or blocked bud emergence, Hsl1p recruitment to the septin cortex still took place, but hyperphosphorylation of Hsl1p and recruitment of the Hsl1p-binding protein Hsl7p to the septin cortex only occurred after bud emergence. At this time, the septin cortex spread to form a collar between mother and bud, and Hsl1p and Hsl7p were restricted to the bud side of the septin collar. We discuss models for translating cellular geometry (in this case, the emergence of a bud) into biochemical signals regulating cell proliferation.

Assembly interdependence among the S. cerevisiae bud neck ring proteins Elm1p, Hsl1p and Cdc12p

In Saccharomyces cerevisiae, a complex comprising more than 20 different polypeptides assembles in a ring at the neck between the mother cell and the bud. This complex functions to coordinate cell morphology with cell division. Relatively little is known about this control system, including the physical relationships between the components of the neck ring. This study addressed the assembly interactions of three components of the ring, specifically the protein kinases Elm1p and Hsl1p and the septin Cdc12p. Specific amino acid substitutions in each of these three proteins were identified that either cause or suppress a characteristic phenotype of abnormally elongated cells and delay in the G(2)-M transition. Each protein was fused to green fluorescent protein, and its ability to localize at the neck was monitored in vivo in cells of various genotypes. Localization of Hsl1p to the neck requires Elm1p function. Elm1p localized normally in the absence of Hsl1p, although a specific point mutation in Hsl1p clearly affected Elm1p localization. The cdc12-122 mutation prevented assembly of Elm1p or Hsl1p into the neck ring. Normal assembly of Cdc12p at the neck was dependent upon Elm1p and also, to a smaller extent, on Hsl1p. Ectopic localization of Cdc12p at the bud tip was observed frequently in elm1 mutants and also, to a lesser extent, in hsl1 mutants. Thus, Elm1p is a key factor in the assembly and/or maintenance of Hsl1p, as well as at least one septin, into the bud neck ring.

Mitogen-activated protein kinase stimulation of Ca(2+) signaling is required for survival of endoplasmic reticulum stress in yeast

Endoplasmic reticulum (ER) stress in the budding yeast Saccharomyces cerevisiae triggers Ca2+ influx through a plasma membrane channel composed of Cch1 and Mid1. This response activates calcineurin, which helps to prevent cell death during multiple forms of ER stress, including the response to azole-class antifungal drugs. Herein, we show that ER stress activates the cell integrity mitogen-activate protein kinase cascade in yeast and that the activation of Pkc1 and Mpk1 is necessary for stimulation of the Cch1-Mid1 Ca2+ channel independent of many known targets of Mpk1 (Rlm1, Swi4, Swi6, Mih1, Hsl1, and Swe1). ER stress generated in response to miconazole, tunicamycin, or other inhibitors also triggered a transient G2/M arrest that depended upon the Swe1 protein kinase. Calcineurin played little role in the Swe1-dependent cell cycle arrest and Swe1 had little effect on calcineurin-dependent avoidance of cell death. These findings help to clarify the interactions between Mpk1, calcineurin, and Swe1 and suggest that the calcium cell survival pathway promotes drug resistance independent of both the unfolded protein response and the G2/M cell cycle checkpoint.

Feedback inhibition on cell wall integrity signaling by Zds1 involves Gsk3 phosphorylation of a cAMP-dependent protein kinase regulatory subunit

We report here that budding yeast cAMP-dependent protein kinase (cAPK) is controlled by heat stress. A rise in temperature from 30 to 37 degrees C was found to result in both a higher expression and an increased cytoplasmic localization of its regulatory subunit Bcy1. Both of these effects required phosphorylation of serines located in its localization domain. Surprisingly, classic cAPK-controlled processes were found to be independent of Bcy1 phosphorylation, indicating that these modifications do not affect cAPK activity as such. Alternatively, phosphorylation may recruit cAPK to, and thereby control, a specific subset of (perhaps novel) cAPK targets that are presumably localized extranuclearly. Zds1 and Zds2 may play a role in this process, since these were found required to retain hyperphosphorylated Bcy1 in the cytoplasm at 37 degrees C. Mck1, a homologue of mammalian glycogen synthase kinase 3 and a downstream component of the heat-activated Pkc1-Slt2/Mpk1 cell wall integrity pathway, is partly responsible for hyperphosphorylations of Bcy1. Remarkably, Zds1 appears to act as a negative regulator of cell wall integrity signaling, and this activity is dependent in part on the phosphorylation status of Bcy1. Thus, Mck1 phosphorylation of Bcy1 and Zds1 may constitute an unprecedented negative feedback control on the cell wall integrity-signaling pathway.

The Saccharomyces cerevisiae bud-neck proteins Kcc4 and Gin4 have distinct but partially-overlapping cellular functions

In the budding yeast S. cerevisiae, Swe1 delays the onset of mitosis by phosphorylation and inactivation of the cyclin-dependent kinase Cdc28, thereby relaying the morphogenetic signal to the cell cycle. Hsl1/Nik1, Kcc4 and Gin4 are structurally homologous protein kinases that localize to the bud neck and negatively regulate Swe1 by phosphorylation. We report here that Kcc4 and Gin4 have partially overlapping but essentially distinct cellular functions. Deletion of KCC4 had a similar effect to GIN4 deletion, causing moderate defects in bud formation at stationary phase; overexpression of Kcc4 inhibited cell growth. KCC4 showed functional interaction with GIN4 in cdc28 mutants, and both Kcc4 and Gin4 proteins physically interacted with Swe1 in vitro. However, unlike gin4delta cells, kcc4Delta cells were not elongated but multi-budded at stationary phase, and showed resistance to 0.04% SDS and 0.003% calcofluor white. In light of the observation that Kcc4 and Gin4 specifically associate with distinct septin proteins, we propose that the observed functional distinction between Kcc4 and Gin4 is due to differences in septin association partners.

Determinants of Swe1p degradation in Saccharomyces cerevisiae

Swe1p, the sole Wee1-family kinase in Saccharomyces cerevisiae, is synthesized during late G1 and is then degraded as cells proceed through the cell cycle. However, Swe1p degradation is halted by the morphogenesis checkpoint, which responds to insults that perturb bud formation. The Swe1p stabilization promotes cell cycle arrest through Swe1p-mediated inhibitory phosphorylation of Cdc28p until the cells can recover from the perturbation and resume bud formation. Swe1p degradation involves the relocalization of Swe1p from the nucleus to the mother-bud neck, and neck targeting requires the Swe1p-interacting protein Hsl7p. In addition, Swe1p degradation is stimulated by its substrate, cyclin/Cdc28p, and Swe1p is thought to be a target of the ubiquitin ligase SCF(Met30) acting with the ubiquitin-conjugating enzyme Cdc34p. The basis for regulation of Swe1p degradation by the morphogenesis checkpoint remains unclear, and in order to elucidate that regulation we have dissected the Swe1p degradation pathway in more detail, yielding several novel findings. First, we show here that Met30p (and by implication SCF(Met30)) is not, in fact, required for Swe1p degradation. Second, cyclin/Cdc28p does not influence Swe1p neck targeting, but can directly phosphorylate Swe1p, suggesting that it acts downstream of neck targeting in the Swe1p degradation pathway. Third, a screen for functional but nondegradable mutants of SWE1 identified two small regions of Swe1p that are key to its degradation. One of these regions mediates interaction of Swe1p with Hsl7p, showing that the Swe1p-Hsl7p interaction is critical for Swe1p neck targeting and degradation. The other region did not appear to affect interactions with known Swe1p regulators, suggesting that other as-yet-unknown regulators exist.

Triggering the all-or-nothing switch into mitosis

The past decade of cell cycle investigations has identified many roads not taken. The kinase that drives mitosis can be modulated by cyclins, by activating phosphorylation, by inhibitory phosphorylation and by binding of inhibitors, but one of these regulatory options controls the transition from G2 phase to mitosis in most circumstances. A switch-like mechanism integrates signals of cellular status and commits the cell to mitosis by abruptly removing inhibitory phosphate from preformed cyclin:Cdk1 complexes. The pathways that flip this switch alter the balance of modifying reactions to favor dephosphorylation, thereby generating a flood of mitotic kinase.

Enhanced cell polarity in mutants of the budding yeast cyclin-dependent kinase Cdc28p

The yeast cyclin-dependent kinase Cdc28p regulates bud morphogenesis and cell cycle progression via the antagonistic activities of Cln and Clb cyclins. Cln G1 cyclins direct polarized growth and bud emergence, whereas Clb G2 cyclins promote isotropic growth of the bud and chromosome segregation. Using colony morphology as a screen to dissect regulation of polarity by Cdc28p, we identified nine point mutations that block the apical-isotropic switch while maintaining other functions. Like a clb2 Delta mutation, each confers tubular bud shape, apically polarized actin distribution, unipolar budding, and delayed anaphase. The mutations are all suppressed by CLB2 overexpression and are synthetically lethal with a CLB2 deletion. However, defects in multiple independent pathways may underlie their common phenotype, because the mutations are scattered throughout the CDC28 sequence, complement each other, and confer diverse biochemical properties. Glu12Gly, a mutation that alters a residue involved in Swe1p inhibition of Cdc28p, was unique in being suppressed by deficiency of SWE1 or CLN1. With wild-type CDC28, filament formation induced by CLN1 overexpression was markedly decreased in a SWE1 deletion. These results suggest that Swe1p, via inhibition of Clb2p/Cdc28p, may mediate much of the effect of Cln1p on filamentous morphogenesis.

A role for the Swe1 checkpoint kinase during filamentous growth of Saccharomyces cerevisiae

In this study we show that inactivation of Hsl1 or Hsl7, negative regulators of the Swe1 kinase, enhances the invasive behavior of haploid and diploid cells. The enhancement of filamentous growth caused by inactivation of both genes is mediated via the Swe1 protein kinase. Whereas Swe1 contributes noticeably to the effectiveness of haploid invasive growth under all conditions tested, its contribution to pseudohyphal growth is limited to the morphological response under standard assay conditions. However, Swe1 is essential for pseudohyphal differentiation under a number of nonstandard assay conditions including altered temperature and increased nitrogen. Swe1 is also required for pseudohyphal growth in the absence of Tec1 and for the induction of filamentation by butanol, a related phenomenon. Although inactivation of Hsl1 is sufficient to suppress the defect in filamentous growth caused by inactivation of Tec1 or Flo8, it is insufficient to promote filamentous growth in the absence of both factors. Moreover, inactivation of Hsl1 will not bypass the requirement for nitrogen starvation or growth on solid medium for pseudohyphal differentiation. We conclude that the Swe1 kinase modulates filamentous development under a broad spectrum of conditions and that its role is partially redundant with the Tec1 and Flo8 transcription factors.