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

The cell cycle, autophagy, and cell wall integrity pathway jointly governed by MoSwe1 in Magnaporthe oryzae

The cell cycle is pivotal to cellular differentiation in plant pathogenic fungi. Cell wall integrity (CWI) signaling plays an essential role in coping with cell wall stress. Autophagy is a degradation process in which cells decompose their components to recover macromolecules and provide energy under stress conditions. However, the specific association between cell cycle, autophagy and CWI pathway remains unclear in model pathogenic fungi Magnaporthe oryzae. Here, we have identified MoSwe1 as the conserved component of the cell cycle in the rice blast fungus. We have found that MoSwe1 targets MoMps1, a conserved critical MAP kinase of the CWI pathway, through protein phosphorylation that positively regulates CWI signaling. The CWI pathway is abnormal in the ΔMoswe1 mutant with cell cycle arrest. In addition, we provided evidence that MoSwe1 positively regulates autophagy by interacting with MoAtg17 and MoAtg18, the core autophagy proteins. Moreover, the S phase initiation was earlier, the morphology of conidia and appressoria was abnormal, and septum formation and glycogen degradation were impaired in the ΔMoswe1 mutant. Our research defines that MoSWE1 regulation of G1/S transition, CWI pathway, and autophagy supports its specific requirement for appressorium development and virulence in plant pathogenic fungi. Video Abstract.

S, Sharma VK. Genome-wide dissection of AT-hook motif nuclear-localized gene family and their expression profiling for drought and salt stress in rice (Oryza sativa)

AT-hook motif nuclear localized (AHL) genes are functionally very less explored, but their nature is very diverse. In the present study, we identified 20 AHL genes in rice. Phylogenetic analyses and evolutionary classification of AHL genes showed that they are conserved in plants, but the number of genes is still expanding in different crops and regulating new biological functions. Gene structure analysis showed that OsAHLs are with and without intron types of genes, suggesting that AHL genes added intron during evolution for neofunctionalization. The cis analysis of OsAHL genes suggested its motif diversity. In order to understand the function, 19 transcriptomes were identified from various tissues and different developmental stages of rice, and they were divided into eight groups by different temporal and spatial expression. Through co-expression analysis, 11 OsAHLs and 13 novel genes with intricate networks that control many biological pathways in rice were identified. The interactions of OsAHL proteins showed that they co-regulate important processes including flowering, reproductive organ development, and photosynthesis activity. The functionality of all 20 genes of OsAHL for drought and salt stress in leaf tissues of two contrasting genotypes (IR64 and NL44) of rice was studied using qRT-PCR. The result clearly showed significant upregulation of OsAHL genes under drought and salt conditions over the control. The differential expression between IR64 and NL44 showed a significant upregulation of OsAHL genes in NL44 as compared to the IR64 genotype under drought and salt stress. Overall, the result indicates that AHL genes might be involved in mediating drought and salt-signaling transduction pathways. The drought- and salt-tolerant nature of NL44 was also confirmed by expression profiling.

The CWI Pathway: A Versatile Toolbox to Arrest Cell-Cycle Progression

Cell-signaling pathways are essential for cells to respond and adapt to changes in their environmental conditions. The cell-wall integrity (CWI) pathway of Saccharomyces cerevisiae is activated by environmental stresses, compounds, and morphogenetic processes that compromise the cell wall, orchestrating the appropriate cellular response to cope with these adverse conditions. During cell-cycle progression, the CWI pathway is activated in periods of polarized growth, such as budding or cytokinesis, regulating cell-wall biosynthesis and the actin cytoskeleton. Importantly, accumulated evidence has indicated a reciprocal regulation of the cell-cycle regulatory system by the CWI pathway. In this paper, we describe how the CWI pathway regulates the main cell-cycle transitions in response to cell-surface perturbance to delay cell-cycle progression. In particular, it affects the Start transcriptional program and the initiation of DNA replication at the G1/S transition, and entry and progression through mitosis. We also describe the involvement of the CWI pathway in the response to genotoxic stress and its connection with the DNA integrity checkpoint, the mechanism that ensures the correct transmission of genetic material and cell survival. Thus, the CWI pathway emerges as a master brake that stops cell-cycle progression when cells are coping with distinct unfavorable conditions.

Protein phosphatase 2A (PP2A) promotes anaphase entry after DNA replication stress in budding yeast

DNA replication stress activates the S-phase checkpoint that arrests the cell cycle, but it is poorly understood how cells recover from this arrest. Cyclin-dependent kinase (CDK) and protein phosphatase 2A (PP2A) are key cell cycle regulators, and Cdc55 is a regulatory subunit of PP2A in budding yeast. We found that yeast cells lacking functional PP2ACdc55 showed slow growth in the presence of hydroxyurea (HU), a DNA synthesis inhibitor, without obvious viability loss. Moreover, PP2A mutants exhibited delayed anaphase entry and sustained levels of anaphase inhibitor Pds1 after HU treatment. A DNA damage checkpoint Chk1 phosphorylates and stabilizes Pds1. We show that chk1Δ and mutation of the Chk1 phosphorylation sites in Pds1 largely restored efficient anaphase entry in PP2A mutants after HU treatment. In addition, deletion of SWE1, which encodes the inhibitory kinase for CDK or mutation of the Swe1 phosphorylation site in CDK (cdc28F19), also suppressed the anaphase entry delay in PP2A mutants after HU treatment. Our genetic data suggest that Swe1/CDK acts upstream of Pds1. Surprisingly, cdc55Δ showed significant suppression to the viability loss of S-phase checkpoint mutants during DNA synthesis block. Together, our results uncover a PP2A-Swe1-CDK-Chk1-Pds1 axis that promotes recovery from DNA replication stress.

Cdc7-mediated phosphorylation of Cse4 regulates high-fidelity chromosome segregation in budding yeast

Faithful chromosome segregation maintains chromosomal stability as errors in this process contribute to chromosomal instability (CIN), which has been observed in many diseases including cancer. Epigenetic regulation of kinetochore proteins such as Cse4 (CENP-A in humans) plays a critical role in high-fidelity chromosome segregation. Here we show that Cse4 is a substrate of evolutionarily conserved Cdc7 kinase, and that Cdc7-mediated phosphorylation of Cse4 prevents CIN. We determined that Cdc7 phosphorylates Cse4 in vitro and interacts with Cse4 in vivo in a cell cycle-dependent manner. Cdc7 is required for kinetochore integrity as reduced levels of CEN-associated Cse4, a faster exchange of Cse4 at the metaphase kinetochores, and defects in chromosome segregation, are observed in a cdc7-7 strain. Phosphorylation of Cse4 by Cdc7 is important for cell survival as constitutive association of a kinase-dead variant of Cdc7 (cdc7-kd) with Cse4 at the kinetochore leads to growth defects. Moreover, phospho-deficient mutations of Cse4 for consensus Cdc7 target sites contribute to CIN phenotype. In summary, our results have defined a role for Cdc7-mediated phosphorylation of Cse4 in faithful chromosome segregation.

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.

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.

Pseudohyphal Growth of the Emerging Pathogen Candida auris Is Triggered by Genotoxic Stress through the S Phase Checkpoint

Candida auris is a newly emerged fungal pathogen of humans. This species was first reported in 2009 when it was identified in an ear infection of a patient in Japan. However, despite intense interest in this organism as an often multidrug-resistant fungus, there is little knowledge about its cellular biology. During infection of human patients, fungi are able to change cell shape from ellipsoidal yeast cells to elongated filaments to adapt to various conditions within the host organism. There are different types of filaments, which are triggered by reactions to different cues. Candida auris fails to form filaments when exposed to triggers that stimulate yeast filament morphogenesis in other fungi. Here, we show that it does form filaments when its DNA is damaged. These conditions might arise when Candida auris cells interact with host immune cells or during growth in certain host tissues (kidney or bladder) or during treatment with antifungal drugs.

The morphogenetic switching between yeast cells and filaments (true hyphae and pseudohyphae) is a key cellular feature required for full virulence in many polymorphic fungal pathogens, such as Candida albicans. In the recently emerged yeast pathogen Candida auris, occasional elongation of cells has been reported. However, environmental conditions and genetic triggers for filament formation have remained elusive. Here, we report that induction of DNA damage and perturbation of replication forks by treatment with genotoxins, such as hydroxyurea, methyl methanesulfonate, and the clinically relevant fungistatic 5-fluorocytosine, cause filamentation in C. auris. The filaments formed were characteristic of pseudohyphae and not parallel-sided true hyphae. Pseudohyphal growth is apparently signaled through the S phase checkpoint and, interestingly, is Tup1 independent in C. auris. Intriguingly, the morphogenetic switching capability is strain specific in C. auris, highlighting the heterogenous nature of the species as a whole.

IMPORTANCECandida auris is a newly emerged fungal pathogen of humans. This species was first reported in 2009 when it was identified in an ear infection of a patient in Japan. However, despite intense interest in this organism as an often multidrug-resistant fungus, there is little knowledge about its cellular biology. During infection of human patients, fungi are able to change cell shape from ellipsoidal yeast cells to elongated filaments to adapt to various conditions within the host organism. There are different types of filaments, which are triggered by reactions to different cues. Candida auris fails to form filaments when exposed to triggers that stimulate yeast filament morphogenesis in other fungi. Here, we show that it does form filaments when its DNA is damaged. These conditions might arise when Candida auris cells interact with host immune cells or during growth in certain host tissues (kidney or bladder) or during treatment with antifungal drugs.

Regulation of Mitotic Exit by Cell Cycle Checkpoints: Lessons From Saccharomyces cerevisiae

In order to preserve genome integrity and their ploidy, cells must ensure that the duplicated genome has been faithfully replicated and evenly distributed before they complete their division by mitosis. To this end, cells have developed highly elaborated checkpoints that halt mitotic progression when problems in DNA integrity or chromosome segregation arise, providing them with time to fix these issues before advancing further into the cell cycle. Remarkably, exit from mitosis constitutes a key cell cycle transition that is targeted by the main mitotic checkpoints, despite these surveillance mechanisms being activated by specific intracellular signals and acting at different stages of cell division. Focusing primarily on research carried out using Saccharomyces cerevisiae as a model organism, the aim of this review is to provide a general overview of the molecular mechanisms by which the major cell cycle checkpoints control mitotic exit and to highlight the importance of the proper regulation of this process for the maintenance of genome stability during the distribution of the duplicated chromosomes between the dividing cells.

Protein kinases in mitotic phosphorylation of budding yeast CENP-A

Centromere identity is specified epigenetically by specialized nucleosomes containing the evolutionarily conserved centromeric histone H3 variant (Cse4 in budding yeast, CENP-A in humans) which is essential for faithful chromosome segregation. However, the mechanisms of epigenetic regulation of Cse4 have not been clearly defined. We have identified two kinases, Cdc5 (Plk1 in humans) and Ipl1 (Aurora B kinase in humans) that phosphorylate Cse4 to prevent chromosomal instability (CIN). Cdc5 associates with Cse4 in mitosis and Cdc5-mediated phosphorylation of Cse4 is coincident with the centromeric enrichment of Cdc5 during metaphase. Defects in Cdc5-mediated Cse4 phosphorylation causes CIN, whereas constitutive association of Cdc5 with Cse4 results in lethality. Cse4 is also a substrate for Ipl1 and phospho-mimetic cse4 mutants suppress growth defects of ipl1 and Ipl1 kinetochore substrate mutants, namely dam1 spc34 and ndc80. Ipl1-mediated phosphorylation of Cse4 regulates kinetochore-microtubule interactions and chromosome biorientation. We propose that collaboration of Cdc5- and Ipl1-mediated phosphorylation of Cse4 modulates kinetochore structure and function, and chromosome biorientation. These findings demonstrate how phosphorylation of Cse4 regulates the integrity of the kinetochore, and acts as an epigenetic marker for mitotic control.

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

Cell cycle-dependent association of polo kinase Cdc5 with CENP-A contributes to faithful chromosome segregation in budding yeast

Evolutionarily conserved polo-like kinase, Cdc5 (Plk1 in humans), associates with kinetochores during mitosis; however, the role of cell cycle-dependent centromeric ( CEN) association of Cdc5 and its substrates that exclusively localize to the kinetochore have not been characterized. Here we report that evolutionarily conserved CEN histone H3 variant, Cse4 (CENP-A in humans), is a substrate of Cdc5, and that the cell cycle-regulated association of Cse4 with Cdc5 is required for cell growth. Cdc5 contributes to Cse4 phosphorylation in vivo and interacts with Cse4 in mitotic cells. Mass spectrometry analysis of in vitro kinase assays showed that Cdc5 phosphorylates nine serine residues clustered within the N-terminus of Cse4. Strains with cse4-9SA exhibit increased errors in chromosome segregation, reduced levels of CEN-associated Mif2 and Mcd1/Scc1 when combined with a deletion of MCM21. Moreover, the loss of Cdc5 from the CEN chromatin contributes to defects in kinetochore integrity and reduction in CEN-associated Cse4. The cell cycle-regulated association of Cdc5 with Cse4 is essential for cell viability as constitutive association of Cdc5 with Cse4 at the kinetochore leads to growth defects. In summary, our results have defined a role for Cdc5-mediated Cse4 phosphorylation in faithful chromosome segregation.

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.

Functions and regulation of the Polo-like kinase Cdc5 in the absence and presence of DNA damage

Polo-like kinases are essential cell cycle regulators that are conserved from yeast to humans. Unlike higher eukaryotes, who express multiple Polo-like kinase family members that perform many important functions, budding yeast express only a single Polo-like kinase, Cdc5, which is the homolog of mammalian cell cycle master regulator Polo-like kinase 1. Cdc5 is a fascinating multifaceted protein that is programmed to target its many substrates in a timely, sequential manner to ensure proper cell cycle progression. Over the years, many lessons about Polo-like kinase 1 have been learned by studying Cdc5 in budding yeast. Cdc5 has been well documented in regulating mitotic entry, chromosome segregation, mitotic exit, and cytokinesis. Cdc5 also plays important roles during cell division after DNA damage. Here, we briefly review the many functions of Cdc5 and its regulation in the absence and presence of DNA damage.

A complex molecular switch directs stress-induced cyclin C nuclear release through SCFGrr1-mediated degradation of Med13

In response to oxidative stress, cells must choose either to live or to die. Here we show that the E3 ligase SCFGrr1 mediates the destruction of Med13, which releases cyclin C into the cytoplasm and results in cell death. The Med13 SCF degron is most likely primed by the Cdk8 kinase and marked for destruction by the MAPK Slt2.

In response to oxidative stress, cells decide whether to mount a survival or cell death response. The conserved cyclin C and its kinase partner Cdk8 play a key role in this decision. Both are members of the Cdk8 kinase module, which, with Med12 and Med13, associate with the core mediator complex of RNA polymerase II. In Saccharomyces cerevisiae, oxidative stress triggers Med13 destruction, which thereafter releases cyclin C into the cytoplasm. Cytoplasmic cyclin C associates with mitochondria, where it induces hyperfragmentation and regulated cell death. In this report, we show that residues 742–844 of Med13’s 600–amino acid intrinsic disordered region (IDR) both directs cyclin C-Cdk8 association and serves as the degron that mediates ubiquitin ligase SCF^Grr1-dependent destruction of Med13 following oxidative stress. Here, cyclin C-Cdk8 phosphorylation of Med13 most likely primes the phosphodegron for destruction. Next, pro-oxidant stimulation of the cell wall integrity pathway MAP kinase Slt2 initially phosphorylates cyclin C to trigger its release from Med13. Thereafter, Med13 itself is modified by Slt2 to stimulate SCF^Grr1-mediated destruction. Taken together, these results support a model in which this IDR of Med13 plays a key role in controlling a molecular switch that dictates cell fate following exposure to adverse environments.

The Unsolved Problem of How Cells Sense Micron-Scale Curvature

Membrane curvature is a fundamental feature of cells and their organelles. Much of what we know about how cells sense curved surfaces comes from studies examining nanometer-sized molecules on nanometer-scale curvatures. We are only just beginning to understand how cells recognize curved topologies at the micron scale. In this review, we provide the reader with an overview of our current understanding of how cells sense and respond to micron-scale membrane curvature.

The budding yeast Polo-like kinase localizes to distinct populations at centrosomes during mitosis

The yeast Polo kinase Cdc5 changes its localization at centrosomes during the cell cycle. Cdc5 localizes to the nuclear centrosome surface in early mitosis and relocalizes to the cytoplasmic centrosome side in late anaphase. Cdc14 and Bfa1 play important roles in regulating Cdc5 centrosome localization.

The budding yeast Polo-like kinase Cdc5 is a key regulator of many mitotic events. Cdc5 coordinates its functions spatially and temporally by changing its localization during the cell cycle: Cdc5 is imported into the nucleus in G2 phase and released to the cytoplasm in anaphase, where it accumulates at the bud neck. Cdc5 also localizes to the spindle pole bodies (SPBs) from S phase until the end of mitosis. Whether Cdc5 changes its SPB population during the cell cycle is not known. We find that Cdc5 localizes to distinct SPB subpopulations, depending on the mitotic stage. Cdc5 localizes to the nuclear side of the SPBs during metaphase and early anaphase and to the cytoplasmic surface of the SPBs during late anaphase. Cdc14 is necessary to relocalize Cdc5 from the nuclear SPB plaque. Accumulation of Cdc5 at the daughter SPB in late anaphase is controlled by Bfa1. We also show that Cdc5 and Bfa1 are found in spatially distinct locations at the SPBs during G2/M arrest after DNA damage. Collectively our data reveal that Cdc5 is a dynamic component of the SPBs during mitosis and provide new insight into its regulation during both late mitotic events and DNA damage–induced G2/M arrest.

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.

Coordinate action of distinct sequence elements localizes checkpoint kinase Hsl1 to the septin collar at the bud neck in Saccharomyces cerevisiae

A long-standing conundrum is resolved about the underlying sequence determinants and molecular mechanism responsible for the recruitment of the protein kinase Hsl1 (an indispensable component of the so-called “morphogenesis checkpoint”) exclusively to the septin collar at the bud neck.

Passage through the eukaryotic cell cycle requires processes that are tightly regulated both spatially and temporally. Surveillance mechanisms (checkpoints) exert quality control and impose order on the timing and organization of downstream events by impeding cell cycle progression until the necessary components are available and undamaged and have acted in the proper sequence. In budding yeast, a checkpoint exists that does not allow timely execution of the G2/M transition unless and until a collar of septin filaments has properly assembled at the bud neck, which is the site where subsequent cytokinesis will occur. An essential component of this checkpoint is the large (1518-residue) protein kinase Hsl1, which localizes to the bud neck only if the septin collar has been correctly formed. Hsl1 reportedly interacts with particular septins; however, the precise molecular determinants in Hsl1 responsible for its recruitment to this cellular location during G2 have not been elucidated. We performed a comprehensive mutational dissection and accompanying image analysis to identify the sequence elements within Hsl1 responsible for its localization to the septins at the bud neck. Unexpectedly, we found that this targeting is multipartite. A segment of the central region of Hsl1 (residues 611–950), composed of two tandem, semiredundant but distinct septin-associating elements, is necessary and sufficient for binding to septin filaments both in vitro and in vivo. However, in addition to 611–950, efficient localization of Hsl1 to the septin collar in the cell obligatorily requires generalized targeting to the cytosolic face of the plasma membrane, a function normally provided by the C-terminal phosphatidylserine-binding KA1 domain (residues 1379–1518) in Hsl1 but that can be replaced by other, heterologous phosphatidylserine-binding sequences.

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.

The budding yeast Polo-like kinase Cdc5 is released from the nucleus during anaphase for timely mitotic exit

Polo-like kinases are important regulators of multiple mitotic events; however, how Polo-like kinases are spatially and temporally regulated to perform their many tasks is not well understood. Here, we examined the subcellular localization of the budding yeast Polo-like kinase Cdc5 using a functional Cdc5-GFP protein expressed from the endogenous locus. In addition to the well-described localization of Cdc5 at the spindle pole bodies (SPBs) and the bud neck, we found that Cdc5-GFP accumulates in the nucleus in early mitosis but is released to the cytoplasm in late mitosis in a manner dependent on the Cdc14 phosphatase. This Cdc5 release from the nucleus is important for mitotic exit because artificial sequestration of Cdc5 in the nucleus by addition of a strong nuclear localization signal (NLS) resulted in mitotic exit defects. We identified a key cytoplasmic target of Cdc5 as Bfa1, an inhibitor of mitotic exit. Our study revealed a novel layer of Cdc5 regulation and suggests the existence of a possible coordination between Cdc5 and Cdc14 activity.

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.

A novel role for the alcohol sensitive ring/PHD finger protein Asr1p in regulating cell cycle mediated by septin-dependent assembly in yeast

Septin is a conserved eukaryotic family of GTP-binding filament-forming proteins with functions in cytokinesis and other processes. It has been suggested that the dynamic assembly of septin, including the processes from septin initially localizing to the presumptive bud site to the septin collar finally splitting into two cells, coordinates closely with the checkpoint response of cell cycle. Here, we discovered that over-expression of Alcohol sensitive Ring/PHD finger 1 protein (Asr1p) in Saccharomyces cerevisiae triggered the Swe1p-dependent cell cycle checkpoint for a G2/M transition delay, and this G2/M transition delay was caused by the septin defect. Since it was shown that Asr1p affected actin dynamics through the interaction with Crn1p and crn1 should be epistatic to asr1 in the regulation of actin, the gene knockout of crn1 in the Asr1p over-expression strain restored the defects in septin and cell cycle along with the disordered actin dynamics. Our investigation further showed that the disturbed septin assembly caused by abnormal Asr1p lead to the abnormal localization of the checkpoint proteins such as Gla4/PAK and Cdc5/Polo, and finally triggered the Swe1p-dependent G2/M transition arrest. Additionally, the Ring finger/PHD domains of Asr1p were illustrated to be required but not sufficient for its role in septin. Taken together, our current data suggested a close relationship in the assembly between septin and actin cytoskeleton, which also partially explained how actin cytoskeleton participated in the regulation of the checkpoint of G2/M.

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.

Global analysis of cdc14 dephosphorylation sites reveals essential regulatory role in mitosis and cytokinesis

Degradation of the M phase cyclins triggers the exit from M phase. Cdc14 is the major phosphatase required for the exit from the M phase. One of the functions of Cdc14 is to dephosphorylate and activate the Cdh1/APC/C complex, resulting in the degradation of the M phase cyclins. However, other crucial targets of Cdc14 for mitosis and cytokinesis remain to be elucidated. Here we systematically analyzed the positions of dephosphorylation sites for Cdc14 in the budding yeast Saccharomyces cerevisiae. Quantitative mass spectrometry identified a total of 835 dephosphorylation sites on 455 potential Cdc14 substrates in vivo. We validated two events, and through functional studies we discovered that Cdc14-mediated dephosphorylation of Smc4 and Bud3 is essential for proper mitosis and cytokinesis, respectively. These results provide insight into the Cdc14-mediated pathways for exiting the M phase.

Budding yeast greatwall and endosulfines control activity and spatial regulation of PP2A(Cdc55) for timely mitotic progression

Entry into mitosis is triggered by cyclinB/Cdk1, whose activity is abruptly raised by a positive feedback loop. The Greatwall kinase phosphorylates proteins of the endosulfine family and allows them to bind and inhibit the main Cdk1-counteracting PP2A-B55 phosphatase, thereby promoting mitotic entry. In contrast to most eukaryotic systems, Cdc14 is the main Cdk1-antagonizing phosphatase in budding yeast, while the PP2A^Cdc55 phosphatase promotes, instead of preventing, mitotic entry by participating to the positive feedback loop of Cdk1 activation. Here we show that budding yeast endosulfines (Igo1 and Igo2) bind to PP2A^Cdc55 in a cell cycle-regulated manner upon Greatwall (Rim15)-dependent phosphorylation. Phosphorylated Igo1 inhibits PP2A^Cdc55 activity in vitro and induces mitotic entry in Xenopus egg extracts, indicating that it bears a conserved PP2A-binding and -inhibitory activity. Surprisingly, deletion of IGO1 and IGO2 in yeast cells leads to a decrease in PP2A phosphatase activity, suggesting that endosulfines act also as positive regulators of PP2A in yeast. Consistently, RIM15 and IGO1/2 promote, like PP2A^Cdc55, timely entry into mitosis under temperature-stress, owing to the accumulation of Tyr-phosphorylated Cdk1. In addition, they contribute to the nuclear export of PP2A^Cdc55, which has recently been proposed to promote mitotic entry. Altogether, our data indicate that Igo proteins participate in the positive feedback loop for Cdk1 activation. We conclude that Greatwall, endosulfines, and PP2A are part of a regulatory module that has been conserved during evolution irrespective of PP2A function in the control of mitosis. However, this conserved module is adapted to account for differences in the regulation of mitotic entry in different organisms.

In all eukaryotic cells chromosome partition during mitosis requires a number of processes, including the formation of the mitotic spindle, i.e. the machinery that drives chromosome segregation to the daughter cells. Mitotic entry requires a delicate balance between protein phosphorylation, driven by cyclin-dependent kinases (CDKs), and protein dephosphorylation, carried out by specific phosphatases that counteract CDK activity. A critical threshold in CDK activity is indeed required for mitotic entry. In the past few years the Greatwall kinase has also been implicated in mitotic entry through phosphorylation of proteins of the endosulfine family, which in turn inhibit the activity of the PP2A phosphatase that would otherwise dephosphorylate CDK targets. Whether Greatwall and endosulfines have a mitotic function in budding yeast, where PP2A promotes, rather than inhibits, mitotic entry has not been established. Here we show that the Greatwall-endosulfine-PP2A regulatory module is conserved also in budding yeast and that endosulfines from different species are interchangeable for their mitotic function. However, in budding yeast cells endosulfines contribute to full activation and proper localization of PP2A, suggesting that they act as both inhibitors and activators of PP2A. Our data emphasize how the same regulatory module is adapted to meet specific mitotic features in different organisms.

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.

Cla4, but not Rac1, regulates the filamentous response of Ustilago maydis to low ammonium conditions

Ustilago maydis, the fungal pathogen of maize, undergoes a dimorphic transition from budding yeast-like growth to filamentous growth, both as part of its program for pathogenesis and distinctly, in response to environmental cues, such as acid pH or low nitrogen availability. Smu1 is a p21-activated protein kinase (PAK) with roles in both the mating response required for the former function, as well as for the nutrient response. Hsl7 may be a negative regulator of Smu1 and appears to play a role in cell length and cell cycle.  Additional proteins that participate in cell polarity and filamentation pathways include the small G protein, Rac1, and its effector PAK kinase, Cla4. Here we describe further experiments that explore the roles of Cla4 and Rac1 in the response to nitrogen availability. While deletion of rac1severely delays filamentous growth on solid media low in ammonium (SLAD), we found that deletion of cla4 does not abolish filamentous cell morphology on solid SLAD. Unexpectedly, however, the Dcla4 mutants also filament in liquid SLAD. The filamentous cell morphology of the cla4 mutant in liquid SLAD has only been seen previously for one other mutant, a strain deleted for hsl7 that simultaneously over-expresses smu1. 

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.

A link between mitotic entry and membrane growth suggests a novel model for cell size control

Addition of new membrane to the cell surface by membrane trafficking is necessary for cell growth. In this paper, we report that blocking membrane traffic causes a mitotic checkpoint arrest via Wee1-dependent inhibitory phosphorylation of Cdk1. Checkpoint signals are relayed by the Rho1 GTPase, protein kinase C (Pkc1), and a specific form of protein phosphatase 2A (PP2A(Cdc55)). Signaling via this pathway is dependent on membrane traffic and appears to increase gradually during polar bud growth. We hypothesize that delivery of vesicles to the site of bud growth generates a signal that is proportional to the extent of polarized membrane growth and that the strength of the signal is read by downstream components to determine when sufficient growth has occurred for initiation of mitosis. Growth-dependent signaling could explain how membrane growth is integrated with cell cycle progression. It could also control both cell size and morphogenesis, thereby reconciling divergent models for mitotic checkpoint function.

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.

Phosphorylation of Cdc5 regulates its accumulation

Background

Cdc5 (polo kinase/Plk1) is a highly conserved key regulator of the S. cerevisiae cell cycle from S-phase until cytokinesis. However, much of the regulatory mechanisms that govern Cdc5 remain to be determined. Cdc5 is phosphorylated on up to 10 sites during mitosis. In this study, we investigated the function of phosphorylation site T23, the only full consensus Cdk1 (Cdc28) phosphorylation site present.

Findings

Cdc5^T23A introduces a degron that reduces its cellular amount to undetectable levels, which are nevertheless sufficient for normal cell proliferation. The degron acts in cis and is reversed by N-terminal GFP-tagging. Cdk1 kinase activity is required to maintain Cdc5 levels during G2. This, Cdk1 inhibited, Cdc5 degradation is APC/C^Cdh1 independent and requires new protein synthesis. Cdc5^T23E is hyperactive, and reduces the levels of Cdc5 (in trans) and drastically reduces Clb2 levels.

Conclusions

Phosphorylation of Cdc5 by Cdk1 is required to maintain Cdc5 levels during G2. However, phosphorylation of T23 (probably by Cdk1) caps Cdc5 and other CLB2 cluster protein accumulation, preventing potential protein toxicity, which may arise from their overexpression or from APC/C^Cdh1 inactivation.

14-3-3 proteins as signaling integration points for cell cycle control and apoptosis

14-3-3 proteins play critical roles in the regulation of cell fate through phospho-dependent binding to a large number of intracellular proteins that are targeted by various classes of protein kinases. 14-3-3 proteins play particularly important roles in coordinating progression of cells through the cell cycle, regulating their response to DNA damage, and influencing life-death decisions following internal injury or external cytokine-mediated cues. This review focuses on 14-3-3-dependent pathways that control cell cycle arrest and recovery, and the influence of 14-3-3 on the apoptotic machinery at multiple levels of regulation. Recognition of 14-3-3 proteins as signaling integrators that connect protein kinase signaling pathways to resulting cellular phenotypes, and their exquisite control through feedforward and feedback loops, identifies new drug targets for human disease, and highlights the emerging importance of using systems-based approaches to understand signal transduction events at the network biology level.

Dynamics of Cdk1 substrate specificity during the cell cycle

Cdk specificity is determined by the intrinsic selectivity of the active site and by substrate docking sites on the cyclin subunit. There is a long-standing debate about the relative importance of these factors in the timing of Cdk1 substrate phosphorylation. We analyzed major budding yeast cyclins (the G1/S-cyclin Cln2, S-cyclin Clb5, G2/M-cyclin Clb3, and M-cyclin Clb2) and found that the activity of Cdk1 toward the consensus motif increased gradually in the sequence Cln2-Clb5-Clb3-Clb2, in parallel with cell cycle progression. Further, we identified a docking element that compensates for the weak intrinsic specificity of Cln2 toward G1-specific targets. In addition, Cln2-Cdk1 showed distinct consensus site specificity, suggesting that cyclins do not merely activate Cdk1 but also modulate its active-site specificity. Finally, we identified several Cln2-, Clb3-, and Clb2-specific Cdk1 targets. We propose that robust timing and ordering of cell cycle events depend on gradual changes in the substrate specificity of Cdk1.

Molecular evolution of cyclin proteins in animals and fungi

BACKGROUND

The passage through the cell cycle is controlled by complexes of cyclins, the regulatory units, with cyclin-dependent kinases, the catalytic units. It is also known that cyclins form several families, which differ considerably in primary structure from one eukaryotic organism to another. Despite these lines of evidence, the relationship between the evolution of cyclins and their function is an open issue. Here we present the results of our study on the molecular evolution of A-, B-, D-, E-type cyclin proteins in animals and fungi.

RESULTS

We constructed phylogenetic trees for these proteins, their ancestral sequences and analyzed patterns of amino acid replacements. The analysis of infrequently fixed atypical amino acid replacements in cyclins evidenced that accelerated evolution proceeded predominantly during paralog duplication or after it in animals and fungi and that it was related to aromorphic changes in animals. It was shown also that evolutionary flexibility of cyclin function may be provided by consequential reorganization of regions on protein surface remote from CDK binding sites in animal and fungal cyclins and by functional differentiation of paralogous cyclins formed in animal evolution.

CONCLUSIONS

The results suggested that changes in the number and/or nature of cyclin-binding proteins may underlie the evolutionary role of the alterations in the molecular structure of cyclins and their involvement in diverse molecular-genetic events.

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.

Phosphorylation-dependent regulation of the F-BAR protein Hof1 during cytokinesis

Spatial and timely coordination of cytokinesis is crucial for the maintenance of organelle inheritance and genome integrity. The mitotic exit network (MEN) pathway controls both the timely initiation of mitotic exit and cytokinesis in budding yeast. Here we identified the conserved F-BAR protein Hof1 as a substrate of the MEN kinase complex Dbf2-Mob1 during cytokinesis. We show that polo-like kinase Cdc5 first phosphorylates Hof1 to allow subsequent phosphorylation by Dbf2-Mob1. This releases Hof1 from the septin ring and facilitates Hof1 binding to the medial actomyosin ring (AMR), where Hof1 promotes AMR contraction and membrane ingression. Domain structure analysis established that the central, unstructured, region of Hof1, named the ring localization sequence (RLS), is sufficient to mediate Hof1's binding to the medial ring in a cell cycle-dependent manner. Genetic and functional data support a model in which Dbf2-Mob1 regulates Hof1 by inducing domain rearrangements, leading to the exposure of the Hof1 RLS domain during telophase.

Role of Hsl7 in morphology and pathogenicity and its interaction with other signaling components in the plant pathogen Ustilago maydis

The phytopathogenic fungus Ustilago maydis undergoes a dimorphic transition in response to mating pheromone, host, and environmental cues. On a solid medium deficient in ammonium (SLAD [0.17% yeast nitrogen base without ammonium sulfate or amino acids, 2% dextrose, 50 μM ammonium sulfate]), U. maydis produces a filamentous colony morphology, while in liquid SLAD, the cells do not form filaments. The p21-activated protein kinases (PAKs) play a substantial role in regulating the dimorphic transition in fungi. The PAK-like Ste20 homologue Smu1 is required for a normal response to pheromone, via upregulation of pheromone expression, and virulence, and its disruption affects both processes. Our experiments suggest that Smu1 also regulates cell length and the filamentous response on solid SLAD medium. Yeast two-hybrid analysis suggested an Hsl7 homologue as a potential interacting partner of Smu1, and a unique open reading frame for such an arginine methyltransferase was detected in the U. maydis genome sequence. Hsl7 regulates cell length and the filamentous response to solid SLAD in a fashion opposite to that of Smu1, but neither overexpression nor disruption of hsl7 attenuates virulence. Simultaneous disruption of hsl7 and overexpression of smu1 lead to a hyperfilamentous response on solid SLAD. Moreover, only this double mutant strain forms filaments in liquid SLAD. The double mutant strain was also significantly reduced in virulence. A similar filamentous response in both solid and liquid SLAD was observed in strains lacking another PAK-like protein kinase involved in cytokinesis and polar growth, Cla4. Our data suggest that Hsl7 may regulate cell cycle progression, while both Smu1 and Cla4 appear to be involved in the filamentous response in U. maydis.

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.

Cdk1 and SUMO regulate Swe1 stability

The Swe1/Wee1 kinase phosphorylates and inhibits Cdk1-Clb2 and is a major mitotic switch. Swe1 levels are controlled by ubiquitin mediated degradation, which is regulated by interactions with various mitotic kinases. We have recently reported that Swe1 levels are capable of sensing the progress of the cell cycle by measuring the levels of Cdk1-Clb2, Cdc5 and Hsl1. We report here a novel mechanism that regulates the levels of Swe1. We show that S.cerevisiae Swe1 is modified by Smt3/SUMO on residue K594 in a Cdk1 dependant manner. A degradation of the swe1^K594R mutant that cannot be modified by Smt3 is considerably delayed in comparison to wild type Swe1. Swe1^K594R cells express elevated levels of Swe1 protein and demonstrate higher levels of Swe1 activity as manifested by Cdk1-Y19 phosphorylation. Interestingly this mutant is not targeted, like wild type Swe1, to the bud neck where Swe1 degradation takes place. We show that Swe1 is SUMOylated by the Siz1 SUMO ligase, and consequently siz1Δ cells express elevated levels of Swe1 protein and activity. Finally we show that swe1^K594R cells are sensitive to osmotic stress, which is in line with their compromised regulation of Swe1 degradation.

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.

Dbf4 regulates the Cdc5 Polo-like kinase through a distinct non-canonical binding interaction

Cdc7-Dbf4 is a conserved, two-subunit kinase required for initiating eukaryotic DNA replication. Recent studies have shown that Cdc7-Dbf4 also regulates the mitotic exit network (MEN) and monopolar homolog orientation in meiosis I (Matos, J., Lipp, J. J., Bogdanova, A., Guillot, S., Okaz, E., Junqueira, M., Shevchenko, A., and Zachariae, W. (2008) Cell 135, 662-678 and Miller, C. T., Gabrielse, C., Chen, Y. C., and Weinreich, M. (2009) PLoS Genet. 5, e1000498). Both activities likely involve a Cdc7-Dbf4 interaction with Cdc5, the single Polo-like kinase in budding yeast. We previously showed that Dbf4 binds the Cdc5 polo-box domain (PBD) via an ∼40-residue N-terminal sequence, which lacks a PBD consensus binding site (S(pS/pT)(P/X)), and that Dbf4 inhibits Cdc5 function during mitosis. Here we identify a non-consensus PBD binding site within Dbf4 and demonstrate that the PBD-Dbf4 interaction occurs via a distinct PBD surface from that used to bind phosphoproteins. Genetic and biochemical analysis of multiple dbf4 mutants indicate that Dbf4 inhibits Cdc5 function through direct binding. Surprisingly, mutation of invariant Cdc5 residues required for binding phosphorylated substrates has little effect on yeast viability or growth rate. Instead, cdc5 mutants defective for binding phosphoproteins exhibit enhanced resistance to microtubule disruption and an increased rate of spindle elongation. This study, therefore, details the molecular nature of a new type of PBD binding and reveals that Cdc5 targeting to phosphorylated substrates likely regulates spindle dynamics.