A morphogenesis checkpoint monitors the actin cytoskeleton in yeast

Rac1 promotes kidney collecting duct repair by mechanically coupling cell morphology to mitotic entry

Prolonged obstruction of the ureter, which leads to injury of the kidney collecting ducts, results in permanent structural damage, while early reversal allows for repair. Cell structure is defined by the actin cytoskeleton, which is dynamically organized by small Rho guanosine triphosphatases (GTPases). In this study, we identified the Rho GTPase, Rac1, as a driver of postobstructive kidney collecting duct repair. After the relief of ureteric obstruction, Rac1 promoted actin cytoskeletal reconstitution, which was required to maintain normal mitotic morphology allowing for successful cell division. Mechanistically, Rac1 restricted excessive actomyosin activity that stabilized the negative mitotic entry kinase Wee1. This mechanism ensured mechanical G2-M checkpoint stability and prevented premature mitotic entry. The repair defects following injury could be rescued by direct myosin inhibition. Thus, Rac1-dependent control of the actin cytoskeleton integrates with the cell cycle to mediate kidney tubular repair by preventing dysmorphic cells from entering cell division.

Haspin Modulates the G2/M Transition Delay in Response to Polarization Failures in Budding Yeast

Symmetry breaking by cellular polarization is an exquisite requirement for the cell-cycle of Saccharomyces cerevisiae cells, as it allows bud emergence and growth. This process is based on the formation of polarity clusters at the incipient bud site, first, and the bud tip later in the cell-cycle, that overall promote bud emission and growth. Given the extreme relevance of this process, a surveillance mechanism, known as the morphogenesis checkpoint, has evolved to coordinate the formation of the bud and cell cycle progression, delaying mitosis in the presence of morphogenetic problems. The atypical protein kinase haspin is responsible for histone H3-T3 phosphorylation and, in yeast, for resolution of polarity clusters in mitosis. Here, we report a novel role for haspin in the regulation of the morphogenesis checkpoint in response to polarity insults. Particularly, we show that cells lacking the haspin ortholog Alk1 fail to achieve sustained checkpoint activation and enter mitosis even in the absence of a bud. In alk1Δ cells, we report a reduced phosphorylation of Cdc28-Y19, which stems from a premature activation of the Mih1 phosphatase. Overall, the data presented in this work define yeast haspin as a novel regulator of the morphogenesis checkpoint in Saccharomyces cerevisiae, where it monitors polarity establishment and it couples bud emergence to the G2/M cell cycle transition.

A cell cycle checkpoint for the endoplasmic reticulum

The generation of new cells is one of the most fundamental aspects of cell biology. Proper regulation of the cell cycle is critical for human health, as underscored by many diseases associated with errors in cell cycle regulation, including both cancer and hereditary diseases. A large body of work has identified regulatory mechanisms and checkpoints that ensure accurate and timely replication and segregation of chromosomal DNA. However, few studies have evaluated the extent to which similar checkpoints exist for the division of cytoplasmic components, including organelles. Such checkpoint mechanisms might be crucial for compartments that cannot be generated de novo, such as the endoplasmic reticulum (ER). In this review, we highlight recent work in the model organism Saccharomyces cerevisiae that led to the discovery of such a checkpoint that ensures that cells inherit functional ER into the daughter cell.

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.

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.

•

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

•

LxF interaction enhances mitotic CDK substrate phosphorylation

•

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

•

Cyclin-specific targeting enables finetuning of CDK function

A role for Gic1 and Gic2 in Cdc42 polarization at elevated temperature

The conserved Rho-family GTPase Cdc42 is a master regulator of polarity establishment in many cell types. Cdc42 becomes activated and concentrated in a region of the cell cortex, and recruits a variety of effector proteins to that site. In turn, many effectors participate in regulation of cytoskeletal elements in order to remodel the cytoskeleton in a polarized manner. The budding yeast Saccharomyces cerevisiae has served as a tractable model system for studies of cell polarity. In yeast cells, Cdc42 polarization involves a positive feedback loop in which effectors called p21-activated kinases (PAKs) act to recruit a Cdc42-directed guanine nucleotide exchange factor (GEF), generating more GTP-Cdc42 in areas that already have GTP-Cdc42. The GTPase-interacting components (GICs) Gic1 and Gic2 are also Cdc42 effectors, and have been implicated in regulation of the actin and septin cytoskeleton. However, we report that cells lacking GICs are primarily defective in polarizing Cdc42 itself, suggesting that they act upstream as well as downstream of Cdc42 in yeast. Our findings suggest that feedback pathways involving GTPase effectors may be more prevalent than had been appreciated.

Multiple inputs ensure yeast cell size homeostasis during cell cycle progression

Coordination of cell growth with division is essential for proper cell function. In budding yeast, although some molecular mechanisms responsible for cell size control during G1 have been elucidated, the mechanism by which cell size homeostasis is established remains to be discovered. Here, we developed a new technique based on quantification of histone levels to monitor cell cycle progression in individual cells with unprecedented accuracy. Our analysis establishes the existence of a mechanism controlling bud size in G2/M that prevents premature onset of anaphase, and controls the overall size variability. While most G1 mutants do not display impaired size homeostasis, mutants in which cyclin B-Cdk regulation is altered display large size variability. Our study thus demonstrates that size homeostasis is not controlled by a G1-specific mechanism alone but is likely to be an emergent property resulting from the integration of several mechanisms that coordinate cell and bud growth with division.

Prolonged cyclin-dependent kinase inhibition results in septin perturbations during return to growth and mitosis

By investigating how yeast cells coordinate polarity and division in a special type of cell division called return to growth, Gihana et al. discover that although checkpoints are normally beneficial, prolonged activation of the morphogenesis checkpoint is instead detrimental to the cell.

We investigated how Saccharomyces cerevisiae coordinate polarization, budding, and anaphase during a unique developmental program called return to growth (RTG) in which cells in meiosis return to mitosis upon nutrient shift. Cells reentering mitosis from prophase I deviate from the normal cell cycle by budding in G2 instead of G1. We found that cells do not maintain the bipolar budding pattern, a characteristic of diploid cells. Furthermore, strict temporal regulation of M-phase cyclin-dependent kinase (CDK; M-CDK) is important for polarity establishment and morphogenesis. Cells with premature M-CDK activity caused by loss of checkpoint kinase Swe1 failed to polarize and underwent anaphase without budding. Mutants with increased Swe1-dependent M-CDK inhibition showed additional or more penetrant phenotypes in RTG than mitosis, including elongated buds, multiple buds, spindle mispositioning, and septin perturbation. Surprisingly, the enhanced and additional phenotypes were not exclusive to RTG but also occurred with prolonged Swe1-dependent CDK inhibition in mitosis. Our analysis reveals that prolonged activation of the Swe1-dependent checkpoint can be detrimental instead of beneficial.

Many roads to symmetry breaking: molecular mechanisms and theoretical models of yeast cell polarity

Mathematical modeling has been instrumental in identifying common principles of cell polarity across diverse systems. These principles include positive feedback loops that are required to destabilize a spatially uniform state of the cell. The conserved small G-protein Cdc42 is a master regulator of eukaryotic cellular polarization. Here we discuss recent developments in studies of Cdc42 polarization in budding and fission yeasts and demonstrate that models describing symmetry-breaking polarization can be classified into six minimal classes based on the structure of positive feedback loops that activate and localize Cdc42. Owing to their generic system-independent nature, these model classes are also likely to be relevant for the G-protein–based symmetry-breaking systems of higher eukaryotes. We review experimental evidence pro et contra different theoretically plausible models and conclude that several parallel and non–mutually exclusive mechanisms are likely involved in cellular polarization of yeasts. This potential redundancy needs to be taken into consideration when interpreting the results of recent cell-rewiring studies.

Fungal Cell Cycle: A Unicellular versus Multicellular Comparison

All cells must accurately replicate DNA and partition it to daughter cells. The basic cell cycle machinery is highly conserved among eukaryotes. Most of the mechanisms that control the cell cycle were worked out in fungal cells, taking advantage of their powerful genetics and rapid duplication times. Here we describe the cell cycles of the unicellular budding yeast Saccharomyces cerevisiae and the multicellular filamentous fungus Aspergillus nidulans. We compare and contrast morphological landmarks of G1, S, G2, and M phases, molecular mechanisms that drive cell cycle progression, and checkpoints in these model unicellular and multicellular fungal systems.

Inositol phosphate multikinase dependent transcriptional control

Production of lipid-derived inositol phosphates including IP4 and IP5 is an evolutionarily conserved process essential for cellular adaptive responses that is dependent on both phospholipase C and the inositol phosphate multikinase Ipk2 (also known as Arg82 and IPMK). Studies of Ipk2, along with Arg82 prior to demonstrating its IP kinase activity, have provided an important link between control of gene expression and IP metabolism as both kinase dependent and independent functions are required for proper transcriptional complex function that enables cellular adaptation in response to extracellular queues such as nutrient availability. Here we define a promoter sequence cis-element, 5'-CCCTAAAAGG-3', that mediates both kinase-dependent and independent functions of Ipk2. Using a synthetic biological strategy, we show that proper gene expression in cells lacking Ipk2 may be restored through add-back of two components: IP4/IP5 production and overproduction of the MADS box DNA binding protein, Mcm1. Our results are consistent with a mechanism by which Ipk2 harbors a dual functionality that stabilizes transcription factor levels and enzymatically produces a small molecule code, which together coordinate control of biological processes and gene expression.

Septin-Associated Protein Kinases in the Yeast Saccharomyces cerevisiae

Septins are a family of eukaryotic GTP-binding proteins that associate into linear rods, which, in turn, polymerize end-on-end into filaments, and further assemble into other, more elaborate super-structures at discrete subcellular locations. Hence, septin-based ensembles are considered elements of the cytoskeleton. One function of these structures that has been well-documented in studies conducted in budding yeast Saccharomyces cerevisiae is to serve as a scaffold that recruits regulatory proteins, which dictate the spatial and temporal control of certain aspects of the cell division cycle. In particular, septin-associated protein kinases couple cell cycle progression with cellular morphogenesis. Thus, septin-containing structures serve as signaling platforms that integrate a multitude of signals and coordinate key downstream networks required for cell cycle passage. This review summarizes what we currently understand about how the action of septin-associated protein kinases and their substrates control information flow to drive the cell cycle into and out of mitosis, to regulate bud growth, and especially to direct timely and efficient execution of cytokinesis and cell abscission. Thus, septin structures represent a regulatory node at the intersection of many signaling pathways. In addition, and importantly, the activities of certain septin-associated protein kinases also regulate the state of organization of the septins themselves, creating a complex feedback loop.

Cell-cycle involvement in autophagy and apoptosis in yeast

Regulation of the cell cycle and apoptosis are two eukaryotic processes required to ensure maintenance of genomic integrity, especially in response to DNA damage. The ease with which yeast, amongst other eukaryotes, can switch from cellular proliferation to cell death may be the result of a common set of biochemical factors which play dual roles depending on the cell's physiological state. A wide variety of homologues are shared between different yeasts and metazoans and this conservation confirms their importance. This review gives an overview of key molecular players involved in yeast cell-cycle regulation, and those involved in mechanisms which are induced by cell-cycle dysregulation. One such mechanism is autophagy which, depending on the severity and type of DNA damage, may either contribute to the cell's survival or death. Cell-cycle dysregulation due to checkpoint deficiency leads to mitotic catastrophe which in turn leads to programmed cell death. Molecular players implicated in the yeast apoptotic pathway were shown to play important roles in the cell cycle. These include the metacaspase Yca1p, the caspase-like protein Esp1p, the cohesin subunit Mcd1p, as well as the inhibitor of apoptosis protein Bir1p. The roles of these molecular players are discussed.

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.

Regulation of Sphingolipid Biosynthesis by the Morphogenesis Checkpoint Kinase Swe1

Sphingolipid (SL) biosynthesis is negatively regulated by the highly conserved endoplasmic reticulum-localized Orm family proteins. Defective SL synthesis in Saccharomyces cerevisiae leads to increased phosphorylation and inhibition of Orm proteins by the kinase Ypk1. Here we present evidence that the yeast morphogenesis checkpoint kinase, Swe1, regulates SL biosynthesis independent of the Ypk1 pathway. Deletion of the Swe1 kinase renders mutant cells sensitive to serine palmitoyltransferase inhibition due to impaired sphingoid long-chain base synthesis. Based on these data and previous results, we suggest that Swe1 kinase perceives alterations in SL homeostasis, activates SL synthesis, and may thus represent the missing regulatory link that controls the SL rheostat during the cell cycle.

Rio1 promotes rDNA stability and downregulates RNA polymerase I to ensure rDNA segregation

The conserved protein kinase Rio1 localizes to the cytoplasm and nucleus of eukaryotic cells. While the roles of Rio1 in the cytoplasm are well characterized, its nuclear function remains unknown. Here we show that nuclear Rio1 promotes rDNA array stability and segregation in Saccharomyces cerevisiae. During rDNA replication in S phase, Rio1 downregulates RNA polymerase I (PolI) and recruits the histone deacetylase Sir2. Both interventions ensure rDNA copy-number homeostasis and prevent the formation of extrachromosomal rDNA circles, which are linked to accelerated ageing in yeast. During anaphase, Rio1 downregulates PolI by targeting its subunit Rpa43, causing PolI to dissociate from the rDNA. By stimulating the processing of PolI-generated transcripts at the rDNA, Rio1 allows for rDNA condensation and segregation in late anaphase. These events finalize the genome transmission process. We identify Rio1 as an essential nucleolar housekeeper that integrates rDNA replication and segregation with ribosome biogenesis.

Morphogenesis checkpoint kinase Swe1 is the executor of lipolysis-dependent cell-cycle progression

Cell growth and division requires the precise duplication of cellular DNA content but also of membranes and organelles. Knowledge about the cell-cycle-dependent regulation of membrane and storage lipid homeostasis is only rudimentary. Previous work from our laboratory has shown that the breakdown of triacylglycerols (TGs) is regulated in a cell-cycle-dependent manner, by activation of the Tgl4 lipase by the major cyclin-dependent kinase Cdc28. The lipases Tgl3 and Tgl4 are required for efficient cell-cycle progression during the G1/S (Gap1/replication phase) transition, at the onset of bud formation, and their absence leads to a cell-cycle delay. We now show that defective lipolysis activates the Swe1 morphogenesis checkpoint kinase that halts cell-cycle progression by phosphorylation of Cdc28 at tyrosine residue 19. Saturated long-chain fatty acids and phytosphingosine supplementation rescue the cell-cycle delay in the Tgl3/Tgl4 lipase-deficient strain, suggesting that Swe1 activity responds to imbalanced sphingolipid metabolism, in the absence of TG degradation. We propose a model by which TG-derived sphingolipids are required to activate the protein phosphatase 2A (PP2A(Cdc55)) to attenuate Swe1 phosphorylation and its inhibitory effect on Cdc28 at the G1/S transition of the cell cycle.

Actin and endocytosis in budding yeast

Endocytosis, the process whereby the plasma membrane invaginates to form vesicles, is essential for bringing many substances into the cell and for membrane turnover. The mechanism driving clathrin-mediated endocytosis (CME) involves > 50 different protein components assembling at a single location on the plasma membrane in a temporally ordered and hierarchal pathway. These proteins perform precisely choreographed steps that promote receptor recognition and clustering, membrane remodeling, and force-generating actin-filament assembly and turnover to drive membrane invagination and vesicle scission. Many critical aspects of the CME mechanism are conserved from yeast to mammals and were first elucidated in yeast, demonstrating that it is a powerful system for studying endocytosis. In this review, we describe our current mechanistic understanding of each step in the process of yeast CME, and the essential roles played by actin polymerization at these sites, while providing a historical perspective of how the landscape has changed since the preceding version of the YeastBook was published 17 years ago (1997). Finally, we discuss the key unresolved issues and where future studies might be headed.

Cell and molecular biology of septins

Septins are a family of GTP-binding proteins that assemble into cytoskeletal filaments. Unlike other cytoskeletal components, septins form ordered arrays of defined stoichiometry that can polymerize into long filaments and bundle laterally. Septins associate directly with membranes and have been implicated in providing membrane stability and serving as diffusion barriers for membrane proteins. In addition, septins bind other proteins and have been shown to function as multimolecular scaffolds by recruiting components of signaling pathways. Remarkably, septins participate in a spectrum of cellular processes including cytokinesis, ciliogenesis, cell migration, polarity, and cell-pathogen interactions. Given their breadth of functions, it is not surprising that septin abnormalities have also been linked to human diseases. In this review, we discuss the current knowledge of septin structure, assembly and function, and discuss these in the context of human disease.

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.

Yeast RAD2, a homolog of human XPG, plays a key role in the regulation of the cell cycle and actin dynamics

Mutations in the human XPG gene cause Cockayne syndrome (CS) and xeroderma pigmentosum (XP). Transcription defects have been suggested as the fundamental cause of CS; however, defining CS as a transcription syndrome is inconclusive. In particular, the function of XPG in transcription has not been clearly demonstrated. Here, we provide evidence for the involvement of RAD2, the Saccharomyces cerevisiae counterpart of XPG, in cell cycle regulation and efficient actin assembly following ultraviolet irradiation. RAD2 C-terminal deletion, which resembles the XPG mutation found in XPG/CS cells, caused cell growth arrest, the cell cycle stalling, a defective α-factor response, shortened lifespan, cell polarity defect, and misregulated actin-dynamics after DNA damage. Overexpression of the C-terminal 65 amino acids of Rad2p was sufficient to induce hyper-cell polarization. In addition, RAD2 genetically interacts with TPM1 during cell polarization. These results provide insights into the role of RAD2 in post-UV irradiation cell cycle regulation and actin assembly, which may be an underlying cause of XPG/CS.

Control analysis of the eukaryotic cell cycle using gene copy-number series in yeast tetraploids

Background

In the model eukaryote, Saccharomyces cerevisiae, previous experiments have identified those genes that exert the most significant control over cell growth rate. These genes are termed HFC for high flux control. Such genes are overrepresented within pathways controlling the mitotic cell cycle.

Results

We postulated that the increase/decrease in growth rate is due to a change in the rate of progression through specific cell cycle steps. We extended and further developed an existing logical model of the yeast cell cycle in order elucidate how the HFC genes modulated progress through the cycle. This model can simulate gene dosage-variation and calculate the cycle time, determine the order and relative speed at which events occur, and predict arrests and failures to correctly execute a step. To experimentally test our model’s predictions, we constructed a tetraploid series of deletion mutants for a set of eight genes that control the G2/M transition. This system allowed us to vary gene copy number through more intermediate levels than previous studies and examine the impact of copy-number variation on growth, cell-cycle phenotype, and response to different cellular stresses.

Conclusions

For the majority of strains, the predictions agreed with experimental observations, validating our model and its use for further predictions. Where simulation and experiment diverged, we uncovered both novel tetraploid-specific phenotypes and a switch in the determinative execution point of a key cell-cycle regulator, the Cdc28 kinase, from the G1/S to the S/G2 boundaries.

A Wee1 checkpoint inhibits anaphase onset

The budding yeast Wee1 kinase Swe1 restrains the metaphase-to-anaphase transition by preventing the Cdk1-dependent phosphorylation and activation of APC^Cdc20.

Cdk1 drives both mitotic entry and the metaphase-to-anaphase transition. Past work has shown that Wee1 inhibition of Cdk1 blocks mitotic entry. Here we show that the budding yeast Wee1 kinase, Swe1, also restrains the metaphase-to-anaphase transition by preventing Cdk1 phosphorylation and activation of the mitotic form of the anaphase-promoting complex/cyclosome (APC^Cdc20). Deletion of SWE1 or its opposing phosphatase MIH1 (the budding yeast cdc25⁺) altered the timing of anaphase onset, and activation of the Swe1-dependent morphogenesis checkpoint or overexpression of Swe1 blocked cells in metaphase with reduced APC activity in vivo and in vitro. The morphogenesis checkpoint also depended on Cdc55, a regulatory subunit of protein phosphatase 2A (PP2A). cdc55Δ checkpoint defects were rescued by mutating 12 Cdk1 phosphorylation sites on the APC, demonstrating that the APC is a target of this checkpoint. These data suggest a model in which stepwise activation of Cdk1 and inhibition of PP2A^Cdc55 triggers anaphase onset.

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.

The role of autophagy in genome stability through suppression of abnormal mitosis under starvation

The coordination of subcellular processes during adaptation to environmental change is a key feature of biological systems. Starvation of essential nutrients slows cell cycling and ultimately causes G1 arrest, and nitrogen starvation delays G2/M progression. Here, we show that budding yeast cells can be efficiently returned to the G1 phase under starvation conditions in an autophagy-dependent manner. Starvation attenuates TORC1 activity, causing a G2/M delay in a Swe1-dependent checkpoint mechanism, and starvation-induced autophagy assists in the recovery from a G2/M delay by supplying amino acids required for cell growth. Persistent delay of the cell cycle by a deficiency in autophagy causes aberrant nuclear division without sufficient cell growth, leading to an increased frequency in aneuploidy after refeeding the nitrogen source. Our data establish the role of autophagy in genome stability through modulation of cell division under conditions that repress cell growth.

Diversity, classification and function of the plant protein kinase superfamily

Eukaryotic protein kinases belong to a large superfamily with hundreds to thousands of copies and are components of essentially all cellular functions. The goals of this study are to classify protein kinases from 25 plant species and to assess their evolutionary history in conjunction with consideration of their molecular functions. The protein kinase superfamily has expanded in the flowering plant lineage, in part through recent duplications. As a result, the flowering plant protein kinase repertoire, or kinome, is in general significantly larger than other eukaryotes, ranging in size from 600 to 2500 members. This large variation in kinome size is mainly due to the expansion and contraction of a few families, particularly the receptor-like kinase/Pelle family. A number of protein kinases reside in highly conserved, low copy number families and often play broadly conserved regulatory roles in metabolism and cell division, although functions of plant homologues have often diverged from their metazoan counterparts. Members of expanded plant kinase families often have roles in plant-specific processes and some may have contributed to adaptive evolution. Nonetheless, non-adaptive explanations, such as kinase duplicate subfunctionalization and insufficient time for pseudogenization, may also contribute to the large number of seemingly functional protein kinases in plants.

The Mck1 GSK-3 kinase inhibits the activity of Clb2-Cdk1 post-nuclear division

The glycogen synthase kinase-3 homolog, Mck1, has been implicated in many cellular functions, from sporulation to calcium stress response in budding yeast. Here, we report a novel function for Mck1 in the inhibition of Clb2-Cdk1 activity post nuclear division. Clb2-Cdk1, the major mitotic cyclin-Cdk complex in yeast, accumulates before anaphase and must be inhibited in telophase for cells to exit mitosis and enter into the next cell cycle. We show that the mck1Δ mutant is highly sensitive to increased Clb2-Cdk1 activity caused either by overexpression of Clb2 or the Cdk1-activating phosphatase Mih1. Deletion of the Cdk1 inhibitory kinase, SWE1, in combination with a mck1Δ mutant results in a synthetic growth defect, suggesting that Mck1 and Swe1 function in parallel pathways to inhibit Clb2-Cdk1. We find that mck1Δ strains have a delay in mitotic exit as well as elevated levels of Clb2-Cdk1 activity post-nuclear division. Using a co-immunoprecipitation assay, we identify a physical interaction between Mck1 and both Clb2 and Mih1. Finally, we demonstrate that phosphorylation of purified Clb2 by Cdk1 is inhibited by catalytically active Mck1 but not catalytically inactive Mck1 in vitro. We propose that Mck1 inhibits the activity of Clb2-Cdk1 via interaction with Clb2. The mammalian glycogen synthase kinase-3 homolog has been implicated in cyclin inhibition, suggesting a conserved cell cycle function for both yeast and mammalian glycogen synthase kinases.

Morphogenesis and the cell cycle

Studies of the processes leading to the construction of a bud and its separation from the mother cell in Saccharomyces cerevisiae have provided foundational paradigms for the mechanisms of polarity establishment, cytoskeletal organization, and cytokinesis. Here we review our current understanding of how these morphogenetic events occur and how they are controlled by the cell-cycle-regulatory cyclin-CDK system. In addition, defects in morphogenesis provide signals that feed back on the cyclin-CDK system, and we review what is known regarding regulation of cell-cycle progression in response to such defects, primarily acting through the kinase Swe1p. The bidirectional communication between morphogenesis and the cell cycle is crucial for successful proliferation, and its study has illuminated many elegant and often unexpected regulatory mechanisms. Despite considerable progress, however, many of the most puzzling mysteries in this field remain to be resolved.

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.

And the dead shall rise: actin and myosin return to the spindle

The spindle directs chromosome partitioning in eukaryotes and, for the last three decades, has been considered primarily a structure based on microtubules, microtubule motors, and other microtubule binding proteins. However, a surprisingly large body of both old and new studies suggests roles for actin filaments (F-actin) and myosins (F-actin-based motor proteins) in spindle assembly and function. Here we review these data and conclude that in several cases the evidence for the participation of F-actin and myosins in spindle function is very strong, and in the situations where it is less strong, there is nevertheless enough evidence to warrant further investigation.

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.

Budding yeast Dma1 and Dma2 participate in regulation of Swe1 levels and localization

Timely down-regulation of the evolutionarily conserved protein kinase Swe1 plays an important role in cell cycle control, as Swe1 can block nuclear division through inhibitory phosphorylation of the catalytic subunit of cyclin-dependent kinase. In particular, Swe1 degradation is important for budding yeast cell survival in case of DNA replication stress, whereas it is inhibited by the morphogenesis checkpoint in response to alterations in actin cytoskeleton or septin structure. We show that the lack of the Dma1 and Dma2 ubiquitin ligases, which moderately affects Swe1 localization and degradation during an unperturbed cell cycle with no apparent phenotypic effects, is toxic for cells that are partially defective in Swe1 down-regulation. Moreover, Swe1 is stabilized, restrained at the bud neck, and hyperphosphorylated in dma1Δ dma2Δ cells subjected to DNA replication stress, indicating that the mechanism stabilizing Swe1 under these conditions is different from the one triggered by the morphogenesis checkpoint. Finally, the Dma proteins are required for proper Swe1 ubiquitylation. Taken together, the data highlight a previously unknown role of these proteins in the complex regulation of Swe1 and suggest that they might contribute to control, directly or indirectly, Swe1 ubiquitylation.

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.

Knr4 N-terminal domain controls its localization and function during sexual differentiation and vegetative growth

The Saccharomyces cerevisiae protein Knr4 is composed of a globular central core flanked by two natively disordered regions. Although the central part of the protein holds most of its biological function, the N-terminal domain (amino acids 1-80) is essential in the absence of a functional CWI pathway. We show that this specific protein domain is required for the proper cellular localization of Knr4 at sites of polarized growth during vegetative growth and sexual differentiation (bud tip and 'shmoo' tip). Moreover, Knr4 N-terminal domain is also necessary for cell cycle arrest and shmoo formation in response to pheromone to occur at the correct speed. Thus, the presence of Knr4 at the incipient mating projection site seems important for the establishment of the following polarized growth. Cell wall integrity (CWI) and calcineurin pathways are known to share a common essential function, for which they can substitute for one another. Searching for Knr4 partners responsible for survival in a CWI-defective background, we found that the catalytic subunit of calcineurin Cna1 physically interacts with Knr4 in the yeast two-hybrid assay, in a manner dependent on the presence of the Knr4 N-terminal domain. In addition, we present evidence that Knr4 protein participates in the morphogenesis checkpoint, a safety mechanism that holds the cell cycle in response to bud formation defects or insults in cytoskeleton organization, and in which both the CWI pathway and calcineurin are involved.

Actin cytoskeleton dynamics and the cell division cycle

The network of actin filaments is one of the crucial cytoskeletal structures contributing to the morphological framework of a cell and which participates in the dynamic regulation of cellular functions. In adherent cell types, cells adhere to the substratum during interphase and spread to assume their characteristic shape supported by the actin cytoskeleton. This actin cytoskeleton is reorganized during mitosis to form rounded cells with increased cortical rigidity. The actin cytoskeleton is re-established after mitosis, allowing cells to regain their extended shape and attachment to the substratum. The modulation of such drastic changes in cell shape in coordination with cell cycle progression suggests a tight regulatory interaction between cytoskeleton signalling, cell-cell/cell-matrix adhesions and mitotic events. Here, we review the contribution of the actin cytoskeleton to cell cycle progression with an emphasis on the effectors responsible for the regulation of the actin cytoskeleton and integration of their activities with the cell cycle machinery.

VopF, a type III effector protein from a non-O1, non-O139 Vibrio cholerae strain, demonstrates toxicity in a Saccharomyces cerevisiae model

VopF, a type III effector protein, has been identified as a contributory factor to the intestinal colonization of type III secretion system-positive, non-O1, non-O139 Vibrio cholerae strains. To gain more insight into the function of VopF, a yeast model was developed. Using this model, it was found that ectopic expression of VopF conferred toxicity in yeast.

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.

SLA2 mutations cause SWE1-mediated cell cycle phenotypes in Candida albicans and Saccharomyces cerevisiae

The early endocytic patch protein Sla2 is important for morphogenesis and growth rates in Saccharomyces cerevisiae and Candida albicans, but the mechanism that connects these processes is not clear. Here we report that growth defects in cells lacking CaSLA2 or ScSLA2 are associated with a cell cycle delay that is influenced by Swe1, a morphogenesis checkpoint kinase. To establish how Swe1 monitors Sla2 function, we compared actin organization and cell cycle dynamics in strains lacking other components of early endocytic patches (Sla1 and Abp1) with those in strains lacking Sla2. Only sla2 strains had defects in actin cables, a known trigger of the morphogenesis checkpoint, yet all three strains exhibited Swe1-dependent phenotypes. Thus, Swe1 appears to monitor actin patch in addition to actin cable function. Furthermore, Swe1 contributed to virulence in a mouse model of disseminated candidiasis, implying a role for the morphogenesis checkpoint during the pathogenesis of C. albicans infections.

Cdc14 inhibition by the spindle assembly checkpoint prevents unscheduled centrosome separation in budding yeast

The spindle assembly checkpoint (SAC) is an evolutionarily conserved surveillance mechanism that delays anaphase onset and mitotic exit in response to the lack of kinetochore attachment. The target of the SAC is the E3 ubiquitin ligase anaphase-promoting complex (APC) bound to its Cdc20 activator. The Cdc20/APC complex is in turn required for sister chromatid separation and mitotic exit through ubiquitin-mediated proteolysis of securin, thus relieving inhibition of separase that unties sister chromatids. Separase is also involved in the Cdc-fourteen early anaphase release (FEAR) pathway of nucleolar release and activation of the Cdc14 phosphatase, which regulates several microtubule-linked processes at the metaphase/anaphase transition and also drives mitotic exit. Here, we report that the SAC prevents separation of microtubule-organizing centers (spindle pole bodies [SPBs]) when spindle assembly is defective. Under these circumstances, failure of SAC activation causes unscheduled SPB separation, which requires Cdc20/APC, the FEAR pathway, cytoplasmic dynein, and the actin cytoskeleton. We propose that, besides inhibiting sister chromatid separation, the SAC preserves the accurate transmission of chromosomes also by preventing SPBs to migrate far apart until the conditions to assemble a bipolar spindle are satisfied.

Cell polarity determinants establish asymmetry in MEN signaling

Components of the mitotic exit network (MEN), a signaling pathway that triggers exit from mitosis, localize to the spindle pole body (SPB) that migrates into the daughter cell during anaphase but are largely absent from the SPB that remains in the mother cell. Through the analysis of one of the determinants of this asymmetry, Bfa1, we find that the machinery responsible for establishing cell polarity and cytoplasmic microtubules collaborate to establish MEN asymmetry. In cells defective in the Cdc42 signaling pathway or the formin Bni1, Bfa1 localizes to both SPBs. The quantitative analysis of Bfa1 localization further shows that Bfa1 can associate with both SPBs in a transient and highly dynamic fashion, but the protein is stabilized on the SPB that migrates into the daughter cell during anaphase through microtubule-bud cortex interactions. Our results indicate that mother-daughter cell asymmetry determinants establish MEN signaling asymmetry through microtubule-bud cortex interactions.

The yeast LATS/Ndr kinase Cbk1 regulates growth via Golgi-dependent glycosylation and secretion

Saccharomyces cerevisiae Cbk1 is a LATS/Ndr protein kinase and a downstream component of the regulation of Ace2 and morphogenesis (RAM) signaling network. Cbk1 and the RAM network are required for cellular morphogenesis, cell separation, and maintenance of cell integrity. Here, we examine the phenotypes of conditional cbk1 mutants to determine the essential function of Cbk1. Cbk1 inhibition severely disrupts growth and protein secretion, and triggers the Swe1-dependent morphogenesis checkpoint. Cbk1 inhibition also delays the polarity establishment of the exocytosis regulators Rab-GTPase Sec4 and its exchange factor Sec2, but it does not interfere with actin polarity establishment. Cbk1 binds to and phosphorylates Sec2, suggesting that it regulates Sec4-dependent exocytosis. Intriguingly, Cbk1 inhibition causes a >30% decrease in post-Golgi vesicle accumulation in late secretion mutants, indicating that Cbk1 also functions upstream of Sec2-Sec4, perhaps at the level of the Golgi. In agreement, conditional cbk1 mutants mislocalize the cis-Golgi mannosyltransferase Och1, are hypersensitive to the aminoglycoside hygromycin B, and exhibit diminished invertase and Sim1 glycosylation. Significantly, the conditional lethality and hygromycin B sensitivity of cbk1 mutants are suppressed by moderate overexpression of several Golgi mannosyltransferases. These data suggest that an important function for Cbk1 and the RAM signaling network is to regulate growth and secretion via Golgi and Sec2/Sec4-dependent processes.

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.

Ca2+ signals coordinate zygotic polarization and cell cycle progression in the brown alga Fucus serratus

Zygotes of the fucoid brown algae provide excellent models for addressing fundamental questions about zygotic symmetry breaking. Although the acquisition of polarity is tightly coordinated with the timing and orientation of the first asymmetric division--with zygotes having to pass through a G1/S-phase checkpoint before the polarization axis can be fixed--the mechanisms behind the interdependence of polarization and cell cycle progression remain unclear. In this study, we combine in vivo Ca2+ imaging, single cell monitoring of S-phase progression and multivariate analysis of high-throughput intracellular Ca2+ buffer loading to demonstrate that Ca2+ signals coordinate polarization and cell cycle progression in the Fucus serratus zygote. Consistent with earlier studies on this organism, and in contrast to animal models, we observe no fast Ca2+ wave following fertilization. Rather, we show distinct slow localized Ca2+ elevations associated with both fertilization and S-phase progression, and we show that both S-phase and zygotic polarization are dependent on pre-S-phase Ca2+ increases. Surprisingly, this Ca2+ requirement cannot be explained by co-dependence on a single G1/S-phase checkpoint, as S phase and zygotic polarization are differentially sensitive to pre-S-phase Ca2+ elevations and can be uncoupled. Furthermore, subsequent cell cycle progression through M phase is independent of localized actin polymerization and zygotic polarization. This absence of a morphogenesis checkpoint, together with the observed Ca2+-dependences of S phase and polarization, show that the regulation of zygotic division in the brown algae differs from that in other eukaryotic model systems, such as yeast and Drosophila.

Inheritance of cortical ER in yeast is required for normal septin organization

How cells monitor the distribution of organelles is largely unknown. In budding yeast, the largest subdomain of the endoplasmic reticulum (ER) is a network of cortical ER (cER) that adheres to the plasma membrane. Delivery of cER from mother cells to buds, which is termed cER inheritance, occurs as an orderly process early in budding. We find that cER inheritance is defective in cells lacking Scs2, a yeast homologue of the integral ER membrane protein VAP (vesicle-associated membrane protein–associated protein) conserved in all eukaryotes. Scs2 and human VAP both target yeast bud tips, suggesting a conserved action of VAP in attaching ER to sites of polarized growth. In addition, the loss of either Scs2 or Ice2 (another protein involved in cER inheritance) perturbs septin assembly at the bud neck. This perturbation leads to a delay in the transition through G2, activating the Saccharomyces wee1 kinase (Swe1) and the morphogenesis checkpoint. Thus, we identify a mechanism involved in sensing the distribution of ER.

A novel pathway that coordinates mitotic exit with spindle position

In budding yeast, the spindle position checkpoint (SPC) delays mitotic exit until the mitotic spindle moves into the neck between the mother and bud. This checkpoint works by inhibiting the mitotic exit network (MEN), a signaling cascade initiated and controlled by Tem1, a small GTPase. Tem1 is regulated by a putative guanine exchange factor, Lte1, but the function and regulation of Lte1 remains poorly understood. Here, we identify novel components of the checkpoint that operate upstream of Lte1. We present genetic evidence in agreement with existing biochemical evidence for the molecular mechanism of a pathway that links microtubule-cortex interactions with Lte1 and mitotic exit. Each component of this pathway is required for the spindle position checkpoint to delay mitotic exit until the spindle is positioned correctly.