The osmotic integrity of the yeast cell requires a functional PKC1 gene product

TOR complex 2 is a master regulator of plasma membrane homeostasis

As first demonstrated in budding yeast (Saccharomyces cerevisiae), all eukaryotic cells contain two, distinct multi-component protein kinase complexes that each harbor the TOR (Target Of Rapamycin) polypeptide as the catalytic subunit. These ensembles, dubbed TORC1 and TORC2, function as universal, centrally important sensors, integrators, and controllers of eukaryotic cell growth and homeostasis. TORC1, activated on the cytosolic surface of the lysosome (or, in yeast, on the cytosolic surface of the vacuole), has emerged as a primary nutrient sensor that promotes cellular biosynthesis and suppresses autophagy. TORC2, located primarily at the plasma membrane, plays a major role in maintaining the proper levels and bilayer distribution of all plasma membrane components (sphingolipids, glycerophospholipids, sterols, and integral membrane proteins). This article surveys what we have learned about signaling via the TORC2 complex, largely through studies conducted in S. cerevisiae. In this yeast, conditions that challenge plasma membrane integrity can, depending on the nature of the stress, stimulate or inhibit TORC2, resulting in, respectively, up-regulation or down-regulation of the phosphorylation and thus the activity of its essential downstream effector the AGC family protein kinase Ypk1. Through the ensuing effect on the efficiency with which Ypk1 phosphorylates multiple substrates that control diverse processes, membrane homeostasis is maintained. Thus, the major focus here is on TORC2, Ypk1, and the multifarious targets of Ypk1 and how the functions of these substrates are regulated by their Ypk1-mediated phosphorylation, with emphasis on recent advances in our understanding of these processes.

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.

Cohesin dysfunction results in cell wall defects in budding yeast

Cohesin is a conserved chromatin-binding multisubunit protein complex involved in diverse chromosomal transactions such as sister-chromatid cohesion, chromosome condensation, regulation of gene expression, DNA replication, and repair. While working with a budding yeast temperature-sensitive mutant, mcd1-1, defective in a cohesin subunit, we observed that it was resistant to zymolyase, indicating an altered cell wall organization. The budding yeast cell wall is a strong but elastic structure essential for maintenance of cell shape and protection from extreme environmental challenges. Here, we show that the cohesin complex plays an important role in cell wall maintenance. Cohesin mutants showed high chitin content in the cell wall and sensitivity to multiple cell wall stress-inducing agents. Interestingly, temperature-dependent lethality of cohesin mutants was osmoremedial, in a HOG1-MAPK pathway-dependent manner, suggesting that the temperature sensitivity of these mutants may arise partially from cell wall defects. Moreover, Mpk1 hyper-phosphorylation indicated activation of the cell wall integrity (CWI) signaling pathway in cohesin mutants. Genetic interaction analysis revealed that the CWI pathway is essential for survival of mcd1-1 upon additional cell wall stress. The cell wall defect was independent of the cohesion function and accompanied by misregulation of expression of several genes having cell wall-related functions. Our findings reveal a requirement of cohesin in maintenance of CWI that is independent of the CWI pathway, and that may arise from cohesin's role in regulating the expression of multiple genes encoding proteins involved in cell wall organization and biosynthesis.

Intracellular mechanism by which genotoxic stress activates yeast SAPK Mpk1

Stress-activated MAP kinases (SAPKs) respond to a wide variety of stressors. In most cases, the pathways through which specific stress signals are transmitted to the SAPKs are not known. The Saccharomyces cerevisiae SAPK Mpk1 (Slt2) is a well-characterized component of the cell-wall integrity (CWI) signaling pathway, which responds to physical and chemical challenges to the cell wall. However, Mpk1 is also activated in response to genotoxic stress through an unknown pathway. We show that, in contrast to cell-wall stress, the pathway for Mpk1 activation by genotoxic stress does not involve the stimulation of the MAP kinase kinases (MEKs) that function immediately upstream of Mpk1. Instead, DNA damage activates Mpk1 through induction of proteasomal degradation of Msg5, the dual-specificity protein phosphatase principally responsible for maintaining Mpk1 in a low-activity state in the absence of stress. Blocking Msg5 degradation in response to genotoxic stress prevented Mpk1 activation. This work raises the possibility that other Mpk1-activating stressors act intracellularly at different points along the canonical Mpk1 activation pathway.

A MYST Histone Acetyltransferase Modulates Conidia Development and Secondary Metabolism in Pestalotiopsis microspora, a Taxol Producer

Reverse genetics is a promising strategy for elucidating the regulatory mechanisms involved in secondary metabolism and development in fungi. Previous studies have demonstrated the key role of histone acetyltransferases in transcriptional regulation. Here, we identified a MYST family histone acetyltransferase encoding gene, mst2, in the filamentous fungus Pestalotiopsis microspora NK17 and revealed its role in development and secondary metabolism. The gene mst2 showed temporal expression that corresponded to the conidiation process in the wild-type strain. Deletion of mst2 resulted in serious growth retardation and impaired conidial development, e.g., a delay and reduced capacity of conidiation and aberrant conidia. Overexpression of mst2 triggered earlier conidiation and higher conidial production. Additionally, deletion of mst2 led to abnormal germination of the conidia and caused cell wall defects. Most significantly, by HPLC profiling, we found that loss of mst2 diminished the production of secondary metabolites in the fungus. Our data suggest that mst2 may function as a general mediator in growth, secondary metabolism and morphological development.

The TORC2-Dependent Signaling Network in the Yeast Saccharomyces cerevisiae

To grow, eukaryotic cells must expand by inserting glycerolipids, sphingolipids, sterols, and proteins into their plasma membrane, and maintain the proper levels and bilayer distribution. A fungal cell must coordinate growth with enlargement of its cell wall. In Saccharomyces cerevisiae, a plasma membrane-localized protein kinase complex, Target of Rapamicin (TOR) complex-2 (TORC2) (mammalian ortholog is mTORC2), serves as a sensor and master regulator of these plasma membrane- and cell wall-associated events by directly phosphorylating and thereby stimulating the activity of two types of effector protein kinases: Ypk1 (mammalian ortholog is SGK1), along with a paralog (Ypk2); and, Pkc1 (mammalian ortholog is PKN2/PRK2). Ypk1 is a central regulator of pathways and processes required for plasma membrane lipid and protein homeostasis, and requires phosphorylation on its T-loop by eisosome-associated protein kinase Pkh1 (mammalian ortholog is PDK1) and a paralog (Pkh2). For cell survival under various stresses, Ypk1 function requires TORC2-mediated phosphorylation at multiple sites near its C terminus. Pkc1 controls diverse processes, especially cell wall synthesis and integrity. Pkc1 is also regulated by Pkh1- and TORC2-dependent phosphorylation, but, in addition, by interaction with Rho1-GTP and lipids phosphatidylserine (PtdSer) and diacylglycerol (DAG). We also describe here what is currently known about the downstream substrates modulated by Ypk1-mediated and Pkc1-mediated phosphorylation.

Methylglyoxal activates the target of rapamycin complex 2-protein kinase C signaling pathway in Saccharomyces cerevisiae

Methylglyoxal is a typical 2-oxoaldehyde derived from glycolysis. We show here that methylglyoxal activates the Pkc1-Mpk1 mitogen-activated protein (MAP) kinase cascade in a target of rapamycin complex 2 (TORC2)-dependent manner in the budding yeast Saccharomyces cerevisiae. We demonstrate that TORC2 phosphorylates Pkc1 at Thr(1125) and Ser(1143). Methylglyoxal enhanced the phosphorylation of Pkc1 at Ser(1143), which transmitted the signal to the downstream Mpk1 MAP kinase cascade. We found that the phosphorylation status of Pkc1(T1125) affected the phosphorylation of Pkc1 at Ser(1143), in addition to its protein levels. Methylglyoxal activated mammalian TORC2 signaling, which, in turn, phosphorylated Akt at Ser(473). Our results suggest that methylglyoxal is a conserved initiator of TORC2 signaling among eukaryotes.

Protein kinase C controls activation of the DNA integrity checkpoint

The protein kinase C (PKC) superfamily plays key regulatory roles in numerous cellular processes. Saccharomyces cerevisiae contains a single PKC, Pkc1, whose main function is cell wall integrity maintenance. In this work, we connect the Pkc1 protein to the maintenance of genome integrity in response to genotoxic stresses. Pkc1 and its kinase activity are necessary for the phosphorylation of checkpoint kinase Rad53, histone H2A and Xrs2 protein after deoxyribonucleic acid (DNA) damage, indicating that Pkc1 is required for activation of checkpoint kinases Mec1 and Tel1. Furthermore, Pkc1 electrophoretic mobility is delayed after inducing DNA damage, which reflects that Pkc1 is post-translationally modified. This modification is a phosphorylation event mediated by Tel1. The expression of different mammalian PKC isoforms at the endogenous level in yeast pkc1 mutant cells revealed that PKCδ is able to activate the DNA integrity checkpoint. Finally, downregulation of PKCδ activity in HeLa cells caused a defective activation of checkpoint kinase Chk2 when DNA damage was induced. Our results indicate that the control of the DNA integrity checkpoint by PKC is a mechanism conserved from yeast to humans.

Regulation of cell wall biogenesis in Saccharomyces cerevisiae: the cell wall integrity signaling pathway

The yeast cell wall is a strong, but elastic, structure that is essential not only for the maintenance of cell shape and integrity, but also for progression through the cell cycle. During growth and morphogenesis, and in response to environmental challenges, the cell wall is remodeled in a highly regulated and polarized manner, a process that is principally under the control of the cell wall integrity (CWI) signaling pathway. This pathway transmits wall stress signals from the cell surface to the Rho1 GTPase, which mobilizes a physiologic response through a variety of effectors. Activation of CWI signaling regulates the production of various carbohydrate polymers of the cell wall, as well as their polarized delivery to the site of cell wall remodeling. This review article centers on CWI signaling in Saccharomyces cerevisiae through the cell cycle and in response to cell wall stress. The interface of this signaling pathway with other pathways that contribute to the maintenance of cell wall integrity is also discussed.

Nutrients and the Pkh1/2 and Pkc1 protein kinases control mRNA decay and P-body assembly in yeast

Regulated mRNA decay is essential for eukaryotic survival but the mechanisms for regulating global decay and coordinating it with growth, nutrient, and environmental cues are not known. Here we show that a signal transduction pathway containing the Pkh1/Pkh2 protein kinases and one of their effector kinases, Pkc1, is required for and regulates global mRNA decay at the deadenylation step in Saccharomyces cerevisiae. Additionally, many stresses disrupt protein synthesis and release mRNAs from polysomes for incorporation into P-bodies for degradation or storage. We find that the Pkh1/2-Pkc1 pathway is also required for stress-induced P-body assembly. Control of mRNA decay and P-body assembly by the Pkh-Pkc1 pathway only occurs in nutrient-poor medium, suggesting a novel role for these processes in evolution. Our identification of a signaling pathway for regulating global mRNA decay and P-body assembly provides a means to coordinate mRNA decay with other cellular processes essential for growth and long-term survival. Mammals may use similar regulatory mechanisms because components of the decay apparatus and signaling pathways are conserved.

Asc1 supports cell-wall integrity near bud sites by a Pkc1 independent mechanism

Background

The yeast ribosomal protein Asc1 is a WD-protein family member. Its mammalian ortholog, RACK1 was initially discovered as a receptor for activated protein C kinase (PKC) that functions to maintain the active conformation of PKC and to support its movement to target sites. In the budding yeast though, a connection between Asc1p and the PKC signaling pathway has never been reported.

Methodology/Principal Findings

In the present study we found that asc1-deletion mutant (asc1Δ) presents some of the hallmarks of PKC signaling mutants. These include an increased sensitivity to staurosporine, a specific Pkc1p inhibitor, and susceptibility to cell-wall perturbing treatments such as hypotonic- and heat shock conditions and zymolase treatment. Microscopic analysis of asc1Δ cells revealed cell-wall invaginations near bud sites after exposure to hypotonic conditions, and the dynamic of cells' survival after this stress further supports the involvement of Asc1p in maintaining the cell-wall integrity during the mid-to late stages of bud formation. Genetic interactions between asc1 and pkc1 reveal synergistic sensitivities of a double-knock out mutant (asc1Δ/pkc1Δ) to cell-wall stress conditions, and high basal level of PKC signaling in asc1Δ. Furthermore, Asc1p has no effect on the cellular distribution or redistribution of Pkc1p at optimal or at cell-wall stress conditions.

Conclusions/Significance

Taken together, our data support the idea that unlike its mammalian orthologs, Asc1p acts remotely from Pkc1p, to regulate the integrity of the cell-wall. We speculate that its role is exerted through translation regulation of bud-site related mRNAs during cells' growth.

A fitness-based interferential genetics approach using hypertoxic/inactive gene alleles as references

Genetics, genomics, and biochemistry have all been of immense help in characterizing macromolecular cell entities and their interactions. Still, obtaining an overall picture of the functioning of even a simple unicellular species has remained a challenging task. One possible way to obtain a comprehensive picture has been described: by capitalizing on the observation that the overexpression on a multicopy plasmid of apparently any wild-type gene in yeast can lead to some negative effect on cell fitness (referring to the concept of "gene toxicity"), the FIG (fitness-based interferential genetics) approach was devised for selecting normal genes that are in antagonistic (and potentially also agonistic) relationship with a particular gene used as a reference. Herein, we take a complementary approach to FIG, by first selecting a "hypertoxic" allele of the reference gene--which easily provides the general possibility of obtaining gene products with the remarkable property of being inactive without altering their macromolecular interactivity--and then looking for the genes that interact functionally with this reference. Thus, FIG and the present approach (Trap-FIG), both taking advantage of the negative effects on cell fitness induced by various quantitative modulations in cellular networks, could potentially pave the way for the emergence of efficient in situ biochemistry.

Hos2p/Set3p deacetylase complex signals secretory stress through the Mpk1p cell integrity pathway

Perturbations in secretory function activate stress response pathways critical for yeast survival. Here we report the identification of the Hos2p/Set3p deacetylase complex (SET3C) as an essential component of the secretory stress response. Strains lacking core components of the Hos2p/Set3p complex exhibit hypersensitivity to secretory stress. Although not required for the unfolded protein response (UPR) and ribosomal gene repression, the Hos2p complex is required for proper activation of the Mpk1p/Slt2p cell integrity kinase cascade. Disruption of the Hos2p complex results in abrogated Mpk1p phosphorylation, whereas constitutive activation of the Mpk1p pathway rescues the hos2Delta mutant growth defect in response to secretory stress. Furthermore, Hos2p activity is required for the Mpk1p-mediated activation of stress-responsive transcription factor Rlm1p, but not for the stress-induced degradation of the C-type cyclin Ssn8p. Our results identify the Hos2p complex as a critical component of the secretory stress response and support the existence a coordinated stress response consisting of the UPR, ribosomal gene repression, and mitogen-activated protein kinase signaling in response to defects in secretory function.

The Hsp110 protein chaperone Sse1 is required for yeast cell wall integrity and morphogenesis

Molecular chaperones direct refolding and triage decisions and support signal transduction responses to cytotoxic stress. The eukaryotic chaperone Hsp110 is represented by the SSE1/2 genes in Saccharomyces cerevisiae, which act as nucleotide exchange factors (NEFs) for cognate cytosolic Hsp70 chaperones. In this report, we present evidence that Sse1 is required for signaling through the cell integrity pathway via partnership with Hsp90 and the terminal MAP kinase Slt2. We found that sse1Delta and sti1Delta mutant cells share the typical cell integrity mutant phenotypes of osmoremediated temperature-sensitive growth and sensitivity to cell wall-damaging agents. Sse1 binds to Slt2 in vivo and similar to Hsp90 mutants, Slt2 stability and phosphorylation is not compromised in sse1Delta cells, whereas activation of the downstream transcription factor Rlm1 is abolished. In addition to Rlm1, Slt2 activates the Swi4/Swi6 heterodimer SBF in response to cell wall damage. SSE1 displayed dramatic synthetic phenotypes when disrupted in combination with mutations in SBF and the related Mbp1/Swi6 heterodimer MBF, characterized by severe growth and morphological defects. These defects were reversed by restoration of Hsp70 NEF activity, providing a mechanistic model wherein Sse1 functionally partners with Hsp90 as an Hsp70 NEF to promote client protein maturation and interaction with downstream effectors.

The Aspergillus nidulans pkcA gene is involved in polarized growth, morphogenesis and maintenance of cell wall integrity

The protein kinase C (PKC) family participates in maintaining integrity and growth of fungal cell walls. However, the precise molecular role of these proteins in the filamentous fungi remains unknown. In this work, pkcA, the gene encoding the PKC homolog in the filamentous fungus Aspergillus nidulans, was cloned and its function analyzed using a conditional alcA-PKC mutant strain. Repression of pkcA expression resulted in increased conidial swelling, decreased rates of hyphal growth, changes in the ultrastructure of the cell wall and increased sensitivity to antifungal agents. These results suggest that the protein encoded by pkcA is involved in key aspects of cell morphogenesis and cell wall integrity.

Central roles of small GTPases in the development of cell polarity in yeast and beyond

SUMMARY

The establishment of cell polarity is critical for the development of many organisms and for the function of many cell types. A large number of studies of diverse organisms from yeast to humans indicate that the conserved, small-molecular-weight GTPases function as key signaling proteins involved in cell polarization. The budding yeast Saccharomyces cerevisiae is a particularly attractive model because it displays pronounced cell polarity in response to intracellular and extracellular cues. Cells of S. cerevisiae undergo polarized growth during various phases of their life cycle, such as during vegetative growth, mating between haploid cells of opposite mating types, and filamentous growth upon deprivation of nutrition such as nitrogen. Substantial progress has been made in deciphering the molecular basis of cell polarity in budding yeast. In particular, it becomes increasingly clear how small GTPases regulate polarized cytoskeletal organization, cell wall assembly, and exocytosis at the molecular level and how these GTPases are regulated. In this review, we discuss the key signaling pathways that regulate cell polarization during the mitotic cell cycle and during mating.

The Cdc34/SCF ubiquitination complex mediates Saccharomyces cerevisiae cell wall integrity

To identify novel functions for the Cdc34/SCF ubiquitination complex, we analyzed genomewide transcriptional profiles of cdc53-1 and cdc34-2 Saccharomyces cerevisiae mutants. This analysis revealed altered expression for several gene families, including genes involved in the regulation of cell wall organization and biosynthesis. This led us to uncover a role for the Cdc34/SCF complex in the regulation of cell wall integrity. In support of this, cdc53-1 and cdc34-2 mutants exhibit phenotypes characteristic of cell wall integrity mutants, such as SDS sensitivity and temperature-sensitive suppression by osmotic stabilizers. Examination of these mutants revealed defects in their induction of Slt2 phosphorylation, indicating defects in Pkc1-Slt2 MAPK signaling. Consistent with this, synthetic genetic interactions were observed between the genes encoding the Cdc34/SCF complex and key components of the Pck1-Slt2 MAPK pathway. Further analysis revealed that Cdc34/SCF mutants have reduced levels of active Rho1, suggesting that these defects stem from the deregulated activity of the Rho1 GTPase. Altering the activity of Rho1 via manipulation of the Rho1-GAPs LRG1 or SAC7 affected Cdc34/SCF mutant growth. Strikingly, however, deletion of LRG1 rescued the growth defects associated with Cdc34/SCF mutants, whereas deletion of SAC7 enhanced these defects. Given the differential roles that these GAPs play in the regulation of Rho1, these observations indicate the importance of coordinating Cdc34/SCF activity with specific Rho1 functions.

Rapid depletion of mutant eukaryotic initiation factor 5A at restrictive temperature reveals connections to actin cytoskeleton and cell cycle progression

Eukaryotic initiation factor 5A (eIF5A) is the only protein in nature that contains hypusine, an unusual amino acid derived from the modification of lysine by spermidine. Two genes, TIF51A and TIF51B, encode eIF5A in the yeast Saccharomyces cerevisiae. In an effort to understand the structure-function relationship of eIF5A, we have generated yeast mutants by introducing plasmid-borne tif51A into a double null strain where both TIF51A and TIF51B have been disrupted. One of the mutants, tsL102A strain (tif51A L102A tif51aDelta tif51bDelta) exhibits a strong temperature-sensitive growth phenotype. At the restrictive temperature, tsL102A strain also exhibits a cell shape change, a lack of volume change in response to temperature increase and becomes more sensitive to ethanol, a hallmark of defects in the PKC/WSC cell wall integrity pathway. In addition, a striking change in actin dynamics and a complete cell cycle arrest at G1 phase occur in tsL102A cells at restrictive temperature. The temperature-sensitivity of tsL102A strain is due to a rapid loss of mutant eIF5A with the half-life reduced from 6 h at permissive temperature to 20 min at restrictive temperature. Phenylmethyl sulfonylfluoride (PMSF), an irreversible inhibitor of serine protease, inhibited the degradation of mutant eIF5A and suppressed the temperature-sensitive growth arrest. Sorbitol, an osmotic stabilizer that complement defects in PKC/WSC pathways, stabilizes the mutant eIF5A and suppresses all the observed temperature-sensitive phenotypes.

Saccharomyces cerevisiae heat shock transcription factor regulates cell wall remodeling in response to heat shock

The heat shock transcription factor Hsf1 of the yeast Saccharomyces cerevisiae regulates expression of genes encoding heat shock proteins and a variety of other proteins as well. To better understand the cellular roles of Hsf1, we screened multicopy suppressor genes of a temperature-sensitive hsf1 mutation. The RIM15 gene, encoding a protein kinase that is negatively regulated by the cyclic AMP-dependent protein kinase, was identified as a suppressor, but Rim15-regulated stress-responsive transcription factors, such as Msn2, Msn4, and Gis1, were unable to rescue the temperature-sensitive growth phenotype of the hsf1 mutant. Another class of suppressors encoded cell wall stress sensors, Wsc1, Wsc2, and Mid2, and the GDP/GTP exchange factor Rom2 that interacts with these cell wall sensors. Activation of a protein kinase, Pkc1, which is induced by these cell wall sensor proteins upon heat shock, but not activation of the Pkc1-regulated mitogen-activated protein kinase cascade, was necessary for the hsf1 suppression. Like Wsc-Pkc1 pathway mutants, hsf1 cells exhibited an osmotic remedial cell lysis phenotype at elevated temperatures. Several of the other suppressors were found to encode proteins functioning in cell wall organization. These results suggest that Hsf1 in concert with Pkc1 regulates cell wall remodeling in response to heat shock.

Discovery of cercosporamide, a known antifungal natural product, as a selective Pkc1 kinase inhibitor through high-throughput screening

The Pkc1-mediated cell wall integrity-signaling pathway is highly conserved in fungi and is essential for fungal growth. We thus explored the potential of targeting the Pkc1 protein kinase for developing broad-spectrum fungicidal antifungal drugs through a Candida albicans Pkc1-based high-throughput screening. We discovered that cercosporamide, a broad-spectrum natural antifungal compound, but previously with an unknown mode of action, is actually a selective and highly potent fungal Pkc1 kinase inhibitor. This finding provides a molecular explanation for previous observations in which Saccharomyces cerevisiae cell wall mutants were found to be highly sensitive to cercosporamide. Indeed, S. cerevisiae mutant cells with reduced Pkc1 kinase activity become hypersensitive to cercosporamide, and this sensitivity can be suppressed under high-osmotic growth conditions. Together, the results demonstrate that cercosporamide acts selectively on Pkc1 kinase and, thus, they provide a molecular mechanism for its antifungal activity. Furthermore, cercosporamide and a beta-1,3-glucan synthase inhibitor echinocandin analog, by targeting two different key components of the cell wall biosynthesis pathway, are highly synergistic in their antifungal activities. The synergistic antifungal activity between Pkc1 kinase and beta-1,3-glucan synthase inhibitors points to a potential highly effective combination therapy to treat fungal infections.

Cell wall integrity signaling in Saccharomyces cerevisiae

The yeast cell wall is a highly dynamic structure that is responsible for protecting the cell from rapid changes in external osmotic potential. The wall is also critical for cell expansion during growth and morphogenesis. This review discusses recent advances in understanding the various signal transduction pathways that allow cells to monitor the state of the cell wall and respond to environmental challenges to this structure. The cell wall integrity signaling pathway controlled by the small G-protein Rho1 is principally responsible for orchestrating changes to the cell wall periodically through the cell cycle and in response to various forms of cell wall stress. This signaling pathway acts through direct control of wall biosynthetic enzymes, transcriptional regulation of cell wall-related genes, and polarization of the actin cytoskeleton. However, additional signaling pathways interface both with the cell wall integrity signaling pathway and with the actin cytoskeleton to coordinate polarized secretion with cell wall expansion. These include Ca(2+) signaling, phosphatidylinositide signaling at the plasma membrane, sphingoid base signaling through the Pkh1 and -2 protein kinases, Tor kinase signaling, and pathways controlled by the Rho3, Rho4, and Cdc42 G-proteins.

Molecular analysis reveals localization of Saccharomyces cerevisiae protein kinase C to sites of polarized growth and Pkc1p targeting to the nucleus and mitotic spindle

The catalytic activity and intracellular localization of protein kinase C (PKC) are both highly regulated in vivo. This family of kinases contains conserved regulatory motifs, i.e., the C1, C2, and HR1 domains, which target PKC isoforms to specific subcellular compartments and restrict their activity spatially. Saccharomyces cerevisiae contains a single PKC isozyme, Pkc1p, which contains all of the regulatory motifs found in mammalian PKCs. Pkc1p localizes to sites of polarized growth, consistent with its main function in maintaining cell integrity. We dissected the molecular basis of Pkc1p localization by expressing each of its domains individually and in combinations as green fluorescent protein fusions. We find that the Rho1p-binding domains, HR1 and C1, are responsible for targeting Pkc1p to the bud tip and cell periphery, respectively. We demonstrate that Pkc1p activity is required for its normal localization to the bud neck, which also depends on the integrity of the septin ring. In addition, we show for the first time that yeast protein kinase C can accumulate in the nucleus, and we identify a nuclear exit signal as well as nuclear localization signals within the Pkc1p sequence. Thus, we propose that Pkc1p shuttles in and out of the nucleus and consequently has access to nuclear substrates. Surprisingly, we find that deletion of the HR1 domain results in Pkc1p localization to the mitotic spindle and that the C2 domain is responsible for this targeting. This novel nuclear and spindle localization of Pkc1p may provide a molecular explanation for previous observations that suggest a role for Pkc1p in regulating microtubule function.

The Ras/protein kinase A pathway acts in parallel with the Mob2/Cbk1 pathway to effect cell cycle progression and proper bud site selection

In Saccharomyces cerevisiae, Ras proteins connect nutrient availability to cell growth through regulation of protein kinase A (PKA) activity. Ras proteins also have PKA-independent functions in mitosis and actin repolarization. We have found that mutations in MOB2 or CBK1 confer a slow-growth phenotype in a ras2Delta background. The slow-growth phenotype of mob2Delta ras2Delta cells results from a G1 delay that is accompanied by an increase in size, suggesting a G1/S role for Ras not previously described. In addition, mob2Delta strains have imprecise bud site selection, a defect exacerbated by deletion of RAS2. Mob2 and Cbk1 act to properly localize Ace2, a transcription factor that directs daughter cell-specific transcription of several genes. The growth and budding phenotypes of the double-deletion strains are Ace2 independent but are suppressed by overexpression of the PKA catalytic subunit, Tpk1. From these observations, we conclude that the PKA pathway and Mob2/Cbk1 act in parallel to determine bud site selection and promote cell cycle progression.

A novel phosphatidylinositol(3,4,5)P3 pathway in fission yeast

The mammalian tumor suppressor, phosphatase and tensin homologue deleted on chromosome 10 (PTEN), inhibits cell growth and survival by dephosphorylating phosphatidylinositol-(3,4,5)-trisphosphate (PI[3,4,5]P3). We have found a homologue of PTEN in the fission yeast, Schizosaccharomyces pombe (ptn1). This was an unexpected finding because yeast (S. pombe and Saccharomyces cerevisiae) lack the class I phosphoinositide 3-kinases that generate PI(3,4,5)P3 in higher eukaryotes. Indeed, PI(3,4,5)P3 has not been detected in yeast. Surprisingly, upon deletion of ptn1 in S. pombe, PI(3,4,5)P3 became detectable at levels comparable to those in mammalian cells, indicating that a pathway exists for synthesis of this lipid and that the S. pombe ptn1, like mammalian PTEN, suppresses PI(3,4,5)P3 levels. By examining various mutants, we show that synthesis of PI(3,4,5)P3 in S. pombe requires the class III phosphoinositide 3-kinase, vps34p, and the phosphatidylinositol-4-phosphate 5-kinase, its3p, but does not require the phosphatidylinositol-3-phosphate 5-kinase, fab1p. These studies suggest that a pathway for PI(3,4,5)P3 synthesis downstream of a class III phosphoinositide 3-kinase evolved before the appearance of class I phosphoinositide 3-kinases.

The PXL1 gene of Saccharomyces cerevisiae encodes a paxillin-like protein functioning in polarized cell growth

The Saccharomyces cerevisiae open reading frame YKR090w encodes a predicted protein displaying similarity in organization to paxillin, a scaffolding protein that organizes signaling and actin cytoskeletal regulating activities in many higher eucaryotic cell types. We found that YKR090w functions in a manner analogous to paxillin as a mediator of polarized cell growth; thus, we have named this gene PXL1 (Paxillin-like protein 1). Analyses of pxl1Delta strains show that PXL1 is required for the selection and maintenance of polarized growth sites during vegetative growth and mating. Genetic analyses of strains lacking both PXL1 and the Rho GAP BEM2 demonstrate that such cells display pronounced growth defects in response to different conditions causing Rho1 pathway activation. PXL1 also displays genetic interactions with the Rho1 effector FKS1. Pxl1p may therefore function as a modulator of Rho-GTPase signaling. A GFP::Pxl1 fusion protein localizes to sites of polarized cell growth. Experiments mapping the localization determinants of Pxl1p demonstrate the existence of localization mechanisms conserved between paxillin and Pxl1p and indicate an evolutionarily ancient and conserved role for LIM domain proteins in acting to modulate cell signaling and cytoskeletal organization during polarized growth.

Disruption of the Saccharomyces cerevisiae cell-wall pathway gene SLG1 causes hypersensitivity to the antitumor drug bleomycin

Bleomycin is an antitumor drug that damages DNA via a free radical-dependent mechanism, and yeast mutants defective in DNA repair are hypersensitive to the drug. To identify possible pathways that may contribute to bleomycin resistance in yeast, we characterized a panel of bleomycin-sensitive mutants that were previously isolated by insertion mutagenesis using the transposon miniTn3::Leu2::LacZ::AMP( R). One of these mutants harbored a single insertion in the SLG1 gene, which encodes a cell membrane protein that senses cell wall stress, and functions to maintain cell wall function by activating the protein kinase C signaling pathway. Deletion of the SLG1 gene in parental strains caused hypersensitivity to bleomycin, and this correlated with an accumulation of damaged DNA. A plasmid that expresses the native SLG1 gene or that increases PKC1 gene dosage restored bleomycin resistance to the slg1Delta mutant. Two-dimensional gel electrophoresis revealed that exposure to bleomycin triggered the expression of certain proteins, presumably to maintain cell wall function, in a Slg1-dependent manner. In addition, mutants lacking cell wall function were found to be hypersensitive to bleomycin. We conclude that mutants deficient in proteins that maintain cell wall function are severely compromised in their ability to limit bleomycin entry into the cell. Therefore, these mutants are burdened with increased genotoxicity upon exposure to bleomycin in the medium. Our results show that major mechanisms other than DNA repair are operating in yeast to mediate bleomycin resistance.

HTL1 encodes a novel factor that interacts with the RSC chromatin remodeling complex in Saccharomyces cerevisiae

RSC is an essential chromatin remodeling complex in Saccharomyces cerevisiae that performs central roles in transcriptional regulation and cell cycle progression. Here we identify Htl1 as a novel factor that associates with the RSC complex both physically and functionally. We isolated HTL1 through a genetic screen for mutants that displayed additive growth defects with a conditional mutation in the protein kinase C gene (PKC1), which has been suggested through genetic connections to interact functionally with RSC. Several lines of evidence connect HTL1 to RSC function. First, an htl1Delta mutant displayed temperature-sensitive growth and a G(2)/M cell cycle arrest at restrictive temperatures, a phenotype similar to that of strains with conditional mutations in essential RSC components. Second, we isolated RSC3, which encodes a component of the RSC complex, as a dosage suppressor of the htl1Delta growth arrest. Third, an htl1Delta mutant displayed additive growth defects with conditional rsc3 alleles. Fourth, overexpression of HTL1 suppressed the growth defect of a strain with a conditional mutation in another RSC component, RSC8. Finally, we demonstrate that Htl1 is a nuclear protein that can associate in vivo with a fraction of the RSC complex. We propose that an RSC-Htl1 complex acts coordinately with protein kinase C to regulate the G(2)/M transition.

Maintenance of mating cell integrity requires the adhesin Fig2p

Fungal adhesins represent a large family of serine/threonine-rich secreted glycoproteins. Adhesins have been shown to play roles in heterotypic and homotypic cell-cell adhesion processes, morphogenetic pathways and invasive/pseudohyphal growth, frequently in response to differentiation cues. Here we address the role of the Saccharomyces cerevisiae mating-specific adhesin Fig2p. Cells lacking FIG2 possess a variety of mating defects that relate to processes involving the cell wall, including morphogenetic defects, cell fusion defects, and alterations in agglutination activities. We found that mating-specific morphogenetic defects caused by the absence of FIG2 are suppressible by increased external osmolarity and that, during mating, fig2delta cells display reduced viability relative to wild-type cells. These defects result from alterations in signaling activated by the mating and cell integrity pathways. Finally, we show that fig2delta zygotes also have defects in zygotic spindle positioning that are osmoremedial, whereas the requirements for FIG2 in normal cell-cell agglutination and cell fusion during mating are insensitive to changes in the extracellular osmotic environment. We conclude that FIG2 performs distinct functions in the mating cell wall that are separable with respect to their ability to be suppressed by changes in external osmolarity and that a fundamental role of FIG2 in mating cells is the maintenance of cell integrity.

Osmotic stress signaling and osmoadaptation in yeasts

The ability to adapt to altered availability of free water is a fundamental property of living cells. The principles underlying osmoadaptation are well conserved. The yeast Saccharomyces cerevisiae is an excellent model system with which to study the molecular biology and physiology of osmoadaptation. Upon a shift to high osmolarity, yeast cells rapidly stimulate a mitogen-activated protein (MAP) kinase cascade, the high-osmolarity glycerol (HOG) pathway, which orchestrates part of the transcriptional response. The dynamic operation of the HOG pathway has been well studied, and similar osmosensing pathways exist in other eukaryotes. Protein kinase A, which seems to mediate a response to diverse stress conditions, is also involved in the transcriptional response program. Expression changes after a shift to high osmolarity aim at adjusting metabolism and the production of cellular protectants. Accumulation of the osmolyte glycerol, which is also controlled by altering transmembrane glycerol transport, is of central importance. Upon a shift from high to low osmolarity, yeast cells stimulate a different MAP kinase cascade, the cell integrity pathway. The transcriptional program upon hypo-osmotic shock seems to aim at adjusting cell surface properties. Rapid export of glycerol is an important event in adaptation to low osmolarity. Osmoadaptation, adjustment of cell surface properties, and the control of cell morphogenesis, growth, and proliferation are highly coordinated processes. The Skn7p response regulator may be involved in coordinating these events. An integrated understanding of osmoadaptation requires not only knowledge of the function of many uncharacterized genes but also further insight into the time line of events, their interdependence, their dynamics, and their spatial organization as well as the importance of subtle effects.

Yeast Rpi1 is a putative transcriptional regulator that contributes to preparation for stationary phase

The RPI1 gene of Saccharomyces cerevisiae was identified initially as a dosage suppressor of the heat shock sensitivity associated with overexpression of RAS2 (J. Kim and S. Powers, Mol. Cell. Biol. 11:3894-3904, 1991). Based on its failure to suppress mutationally activated RAS2, RPII was proposed to be a negative regulator of the Ras/cyclic AMP (cAMP) pathway that functions at a point upstream of Ras. We isolated RPI1 as a high-copy-number suppressor of the cell lysis defect associated with a null mutation in the MPK1 gene, which encodes the mitogen-activated protein kinase of the cell wall integrity-signaling pathway. Although the sequence of Rpil is not informative about its function, we present evidence that this protein resides in the nucleus, possesses a transcriptional activation domain, and affects the mRNA levels of several cell wall metabolism genes. In contrast to the previous report, we found that RPI1 overexpression suppresses defects associated with mutational hyperactivation of the Ras/cAMP pathway at all points including constitutive mutations in the cAMP-dependent protein kinase. We present additional genetic and biochemical evidence that Rpil functions independently of and in opposition to the Ras/cAMP pathway to promote preparations for the stationary phase. Among these preparations is a fortification of the cell wall that is antagonized by Ras pathway activity. This observation reveals a novel link between the Ras/cAMP pathway and cell wall integrity. Finally, we propose that inappropriate expression of RPI1 during log phase growth drives fortification of the cell wall and that this behavior is responsible for suppression of the mpkl cell lysis defect.

A protein interaction map for cell polarity development

Many genes required for cell polarity development in budding yeast have been identified and arranged into a functional hierarchy. Core elements of the hierarchy are widely conserved, underlying cell polarity development in diverse eukaryotes. To enumerate more fully the protein-protein interactions that mediate cell polarity development, and to uncover novel mechanisms that coordinate the numerous events involved, we carried out a large-scale two-hybrid experiment. 68 Gal4 DNA binding domain fusions of yeast proteins associated with the actin cytoskeleton, septins, the secretory apparatus, and Rho-type GTPases were used to screen an array of yeast transformants that express approximately 90% of the predicted Saccharomyces cerevisiae open reading frames as Gal4 activation domain fusions. 191 protein-protein interactions were detected, of which 128 had not been described previously. 44 interactions implicated 20 previously uncharacterized proteins in cell polarity development. Further insights into possible roles of 13 of these proteins were revealed by their multiple two-hybrid interactions and by subcellular localization. Included in the interaction network were associations of Cdc42 and Rho1 pathways with proteins involved in exocytosis, septin organization, actin assembly, microtubule organization, autophagy, cytokinesis, and cell wall synthesis. Other interactions suggested direct connections between Rho1- and Cdc42-regulated pathways; the secretory apparatus and regulators of polarity establishment; actin assembly and the morphogenesis checkpoint; and the exocytic and endocytic machinery. In total, a network of interactions that provide an integrated response of signaling proteins, the cytoskeleton, and organelles to the spatial cues that direct polarity development was revealed.

Wsc1 and Mid2 are cell surface sensors for cell wall integrity signaling that act through Rom2, a guanine nucleotide exchange factor for Rho1

Wsc1 and Mid2 are highly O-glycosylated cell surface proteins that reside in the plasma membrane of Saccharomyces cerevisiae. They have been proposed to function as mechanosensors of cell wall stress induced by wall remodeling during vegetative growth and pheromone-induced morphogenesis. These proteins are required for activation of the cell wall integrity signaling pathway that consists of the small G-protein Rho1, protein kinase C (Pkc1), and a mitogen-activated protein kinase cascade. We show here by two-hybrid experiments that the C-terminal cytoplasmic domains of Wsc1 and Mid2 interact with Rom2, a guanine nucleotide exchange factor (GEF) for Rho1. At least with regard to Wsc1, this interaction is mediated by the Rom2 N-terminal domain. This domain is distinct from the Rho1-interacting domain, suggesting that the GEF can interact simultaneously with a sensor and with Rho1. We also demonstrate that extracts from wsc1 and mid2 mutants are deficient in the ability to catalyze GTP loading of Rho1 in vitro, providing evidence that the function of the sensor-Rom2 interaction is to stimulate nucleotide exchange toward this G-protein. In a related line of investigation, we identified the PMT2 gene in a genetic screen for mutations that confer an additive cell lysis defect with a wsc1 null allele. Pmt2 is a member of a six-protein family in yeast that catalyzes the first step in O mannosylation of target proteins. We demonstrate that Mid2 is not mannosylated in a pmt2 mutant and that this modification is important for signaling by Mid2.

Distinct roles for the yeast phosphatidylinositol 4-kinases, Stt4p and Pik1p, in secretion, cell growth, and organelle membrane dynamics

The yeast Saccharomyces cerevisiae possesses two genes that encode phosphatidylinositol (PtdIns) 4-kinases, STT4 and PIK1. Both gene products phosphorylate PtdIns at the D-4 position of the inositol ring to generate PtdIns(4)P, which plays an essential role in yeast viability because deletion of either STT4 or PIK1 is lethal. Furthermore, although both enzymes have the same biochemical activity, increased expression of either kinase cannot compensate for the loss of the other, suggesting that these kinases regulate distinct intracellular functions, each of which is required for yeast cell growth. By the construction of temperature-conditional single and double mutants, we have found that Stt4p activity is required for the maintenance of vacuole morphology, cell wall integrity, and actin cytoskeleton organization. In contrast, Pik1p is essential for normal secretion, Golgi and vacuole membrane dynamics, and endocytosis. Strikingly, pik1(ts) cells exhibit a rapid defect in secretion of Golgi-modified secretory pathway cargos, Hsp150p and invertase, whereas stt4(ts) cells exhibit no detectable secretory defects. Both single mutants reduce PtdIns(4)P by approximately 50%; however, stt4(ts)/pik1(ts) double mutant cells produce more than 10-fold less PtdIns(4)P as well as PtdIns(4,5)P(2). The aberrant Golgi morphology found in pik1(ts) mutants is strikingly similar to that found in cells lacking the function of Arf1p, a small GTPase that is known to regulate multiple membrane trafficking events throughout the cell. Consistent with this observation, arf1 mutants exhibit reduced PtdIns(4)P levels. In contrast, diminished levels of PtdIns(4)P observed in stt4(ts) cells at restrictive temperature result in a dramatic change in vacuole size compared with pik1(ts) cells and persistent actin delocalization. Based on these results, we propose that Stt4p and Pik1p act as the major, if not the only, PtdIns 4-kinases in yeast and produce distinct pools of PtdIns(4)P and PtdIns(4,5)P(2) that act on different intracellular membranes to recruit or activate as yet uncharacterized effector proteins.

Association of the cell cycle transcription factor Mbp1 with the Skn7 response regulator in budding yeast

We previously isolated the SKN7 gene in a screen designed to isolate new components of the G1-S cell cycle transcription machinery in budding yeast. We have now found that Skn7 associates with Mbp1, the DNA-binding component of the G1-S transcription factor DSC1/MBF. SKN7 and MBP1 show several genetic interactions. Skn7 overexpression is lethal and is suppressed by a mutation in MBP1. Similarly, high overexpression of Mbp1 is lethal and can be suppressed by skn7 mutations. SKN7 is also required for MBP1 function in a mutant compromised for G1-specific transcription. Gel-retardation assays indicate that Skn7 is not an integral part of MBF. However, a physical interaction between Skn7 and Mbp1 was detected using two-hybrid assays and GST pulldowns. Thus, Skn7 and Mbp1 seem to form a transcription factor independent of MBF. Genetic data suggest that this new transcription factor could be involved in the bud-emergence process.

Mid2 is a putative sensor for cell integrity signaling in Saccharomyces cerevisiae

Hcs77 is a putative cell surface sensor for cell integrity signaling in Saccharomyces cerevisiae. Its loss of function results in cell lysis during growth at elevated temperatures (e.g., 39 degrees C) and impaired signaling to the Mpk1 mitogen-activated protein kinase in response to mild heat shock. We isolated the MID2 gene as a dosage suppressor of the cell lysis defect of an hcs77 null mutant. MID2 encodes a putative membrane protein whose function is required for survival of pheromone treatment. Mid2 possesses properties similar to those of Hcs77, including a single transmembrane domain and a long region that is rich in seryl and threonyl residues. We demonstrate that Mid2 is required for cell integrity signaling in response to pheromone. Additionally, we show that Mid2 and Hcs77 serve a redundant but essential function as cell surface sensors for cell integrity signaling during vegetative growth. Both proteins are uniformly distributed through the plasma membrane and are highly O-mannosylated on their extracellular domains. Finally, we identified a yeast homolog of MID2, designated MTL1, which provides a partially redundant function with MID2 for cell integrity signaling during vegetative growth at elevated temperature but not for survival of pheromone treatment. We conclude that Hcs77 is dedicated to signaling cell wall stress during vegetative growth and that Mid2 participates in this signaling, but its primary role is in signaling wall stress during pheromone-induced morphogenesis.

MCD4 encodes a conserved endoplasmic reticulum membrane protein essential for glycosylphosphatidylinositol anchor synthesis in yeast

Glycosylphosphatidylinositol (GPI)-anchored proteins are cell surface-localized proteins that serve many important cellular functions. The pathway mediating synthesis and attachment of the GPI anchor to these proteins in eukaryotic cells is complex, highly conserved, and plays a critical role in the proper targeting, transport, and function of all GPI-anchored protein family members. In this article, we demonstrate that MCD4, an essential gene that was initially identified in a genetic screen to isolate Saccharomyces cerevisiae mutants defective for bud emergence, encodes a previously unidentified component of the GPI anchor synthesis pathway. Mcd4p is a multimembrane-spanning protein that localizes to the endoplasmic reticulum (ER) and contains a large NH2-terminal ER lumenal domain. We have also cloned the human MCD4 gene and found that Mcd4p is both highly conserved throughout eukaryotes and has two yeast homologues. Mcd4p's lumenal domain contains three conserved motifs found in mammalian phosphodiesterases and nucleotide pyrophosphases; notably, the temperature-conditional MCD4 allele used for our studies (mcd4-174) harbors a single amino acid change in motif 2. The mcd4-174 mutant (1) is defective in ER-to-Golgi transport of GPI-anchored proteins (i.e., Gas1p) while other proteins (i.e., CPY) are unaffected; (2) secretes and releases (potentially up-regulated cell wall) proteins into the medium, suggesting a defect in cell wall integrity; and (3) exhibits marked morphological defects, most notably the accumulation of distorted, ER- and vesicle-like membranes. mcd4-174 cells synthesize all classes of inositolphosphoceramides, indicating that the GPI protein transport block is not due to deficient ceramide synthesis. However, mcd4-174 cells have a severe defect in incorporation of [3H]inositol into proteins and accumulate several previously uncharacterized [3H]inositol-labeled lipids whose properties are consistent with their being GPI precursors. Together, these studies demonstrate that MCD4 encodes a new, conserved component of the GPI anchor synthesis pathway and highlight the intimate connections between GPI anchoring, bud emergence, cell wall function, and feedback mechanisms likely to be involved in regulating each of these essential processes. A putative role for Mcd4p as participating in the modification of GPI anchors with side chain phosphoethanolamine is also discussed.

A complex containing RNA polymerase II, Paf1p, Cdc73p, Hpr1p, and Ccr4p plays a role in protein kinase C signaling

Yeast contains at least two complex forms of RNA polymerase II (Pol II), one including the Srbps and a second biochemically distinct form defined by the presence of Paf1p and Cdc73p (X. Shi et al., Mol. Cell. Biol. 17:1160-1169, 1997). In this work we demonstrate that Ccr4p and Hpr1p are components of the Paf1p-Cdc73p-Pol II complex. We have found many synthetic genetic interactions between factors within the Paf1p-Cdc73p complex, including the lethality of paf1Delta ccr4Delta, paf1Delta hpr1Delta, ccr4Delta hpr1Delta, and ccr4Delta gal11Delta double mutants. In addition, paf1Delta and ccr4Delta are lethal in combination with srb5Delta, indicating that the factors within and between the two RNA polymerase II complexes have overlapping essential functions. We have used differential display to identify several genes whose expression is affected by mutations in components of the Paf1p-Cdc73p-Pol II complex. Additionally, as previously observed for hpr1Delta, deleting PAF1 or CDC73 leads to elevated recombination between direct repeats. The paf1Delta and ccr4Delta mutations, as well as gal11Delta, demonstrate sensitivity to cell wall-damaging agents, rescue of the temperature-sensitive phenotype by sorbitol, and reduced expression of genes involved in cell wall biosynthesis. This unusual combination of effects on recombination and cell wall integrity has also been observed for mutations in genes in the Pkc1p-Mpk1p kinase cascade. Consistent with a role for this novel form of RNA polymerase II in the Pkc1p-Mpk1p signaling pathway, we find that paf1Delta mpk1Delta and paf1Delta pkc1Delta double mutants do not demonstrate an enhanced phenotype relative to the single mutants. Our observation that the Mpk1p kinase is fully active in a paf1Delta strain indicates that the Paf1p-Cdc73p complex may function downstream of the Pkc1p-Mpk1p cascade to regulate the expression of a subset of yeast genes.

POG1, a novel yeast gene, promotes recovery from pheromone arrest via the G1 cyclin CLN2

In the absence of a successful mating, pheromone-arrested Saccharomyces cerevisiae cells reenter the mitotic cycle through a recovery process that involves downregulation of the mating mitogen-activated protein kinase (MAPK) cascade. We have isolated a novel gene, POG1, whose promotion of recovery parallels that of the MAPK phosphatase Msg5. POG1 confers alpha-factor resistance when overexpressed and enhances alpha-factor sensitivity when deleted in the background of an msg5 mutant. Overexpression of POG1 inhibits alpha-factor-induced G1 arrest and transcriptional repression of the CLN1 and CLN2 genes. The block in transcriptional repression occurs at SCB/MCB promoter elements by a mechanism that requires Bck1 but not Cln3. Genetic tests strongly argue that POG1 promotes recovery through upregulation of the CLN2 gene and that the resulting Cln2 protein promotes recovery primarily through an effect on Ste20, an activator of the mating MAPK cascade. A pog1 cln3 double mutant displays synthetic mutant phenotypes shared by cell-wall integrity and actin cytoskeleton mutants, with no synthetic defect in the expression of CLN1 or CLN2. These and other results suggest that POG1 may regulate additional genes during vegetative growth and recovery.

MAP kinase pathways in the yeast Saccharomyces cerevisiae

A cascade of three protein kinases known as a mitogen-activated protein kinase (MAPK) cascade is commonly found as part of the signaling pathways in eukaryotic cells. Almost two decades of genetic and biochemical experimentation plus the recently completed DNA sequence of the Saccharomyces cerevisiae genome have revealed just five functionally distinct MAPK cascades in this yeast. Sexual conjugation, cell growth, and adaptation to stress, for example, all require MAPK-mediated cellular responses. A primary function of these cascades appears to be the regulation of gene expression in response to extracellular signals or as part of specific developmental processes. In addition, the MAPK cascades often appear to regulate the cell cycle and vice versa. Despite the success of the gene hunter era in revealing these pathways, there are still many significant gaps in our knowledge of the molecular mechanisms for activation of these cascades and how the cascades regulate cell function. For example, comparison of different yeast signaling pathways reveals a surprising variety of different types of upstream signaling proteins that function to activate a MAPK cascade, yet how the upstream proteins actually activate the cascade remains unclear. We also know that the yeast MAPK pathways regulate each other and interact with other signaling pathways to produce a coordinated pattern of gene expression, but the molecular mechanisms of this cross talk are poorly understood. This review is therefore an attempt to present the current knowledge of MAPK pathways in yeast and some directions for future research in this area.

Identification of functional connections between calmodulin and the yeast actin cytoskeleton

One of four intragenic complementing groups of temperature-sensitive yeast calmodulin mutations, cmd1A, results in a characteristic functional defect in actin organization. We report here that among the complementing mutations, a representative cmd1A mutation (cmd1-226: F92A) is synthetically lethal with a mutation in MYO2 that encodes a class V unconventional myosin with calmodulin-binding domains. Gel overlay assay shows that a mutant calmodulin with the F92A alteration has severely reduced binding affinity to a GST-Myo2p fusion protein. Random replacement and site-directed mutagenesis at position 92 of calmodulin indicate that hydrophobic and aromatic residues are allowed at this position, suggesting an importance of hydrophobic interaction between calmodulin and Myo2p. To analyze other components involved in actin organization through calmodulin, we isolated and characterized mutations that show synthetic lethal interaction with cmd1-226; these "cax" mutants fell into five complementation groups. Interestingly, all the mutations themselves affect actin organization. Unlike cax2, cax3, cax4, and cax5 mutations, cax1 shows allele-specific synthetic lethality with the cmd1A allele. CAX1 is identical to ANP1/GEM3/MCD2, which is involved in protein glycosylation. CAX4 is identical to the ORF YGR036c, and CAX5 is identical to MNN10/SLC2/BED1. We discuss possible roles for Cax proteins in the regulation of the actin cytoskeleton.

Signalling in the yeasts: an informational cascade with links to the filamentous fungi

All cells, from bacteria and yeasts to mammalian cells, respond to cues from their environment. A variety of mechanisms exist for the transduction of these external signals to the interior of the cell, resulting in altered patterns of protein activity. Eukaryotic cells commonly transduce external cues via a conserved module composed of three protein kinases, the mitogen-activated protein kinase (MAPK) cascade. This module can then activate substrates, some of which include transcriptional activators. Multiple MAPK signalling pathways coexist in a cell. This review considers different MAPK cascade signalling pathways that govern several aspects of the life cycle of budding and fission yeasts: conjugation and meiosis by the pheromone response pathway, stress response by the high-osmolarity sensing pathway, cell wall biosynthesis in response to activation of the low-osmolarity and heat-sensing pathway, and pseudohyphal growth in response to activation of a subset of the components of the pheromone response pathway. Because the MAPK cascade components are highly conserved, a key question in studies of these pathways is the mechanism by which specificity of response is achieved. Several other issues to be addressed in this review concern the nature of the receptors used to sense the external signals and the mechanism by which the receptors communicate with other components leading to activation of the MAPK cascade. Recently, it has become apparent that MAPK cascades are important in governing the pathogenicity of filamentous fungi.

Temperature-induced expression of yeast FKS2 is under the dual control of protein kinase C and calcineurin

FKS1 and FKS2 are alternative subunits of the glucan synthase complex, which is responsible for synthesizing 1,3-beta-glucan chains, the major structural polymer of the Saccharomyces cerevisiae cell wall. Expression of FKS1 predominates during growth under optimal conditions. In contrast, FKS2 expression is induced by mating pheromone, high extracellular [Ca2+], growth on poor carbon sources, or in an fks1 mutant. Induction of FKS2 expression in response to pheromone, CaCl2, or loss of FKS1 function requires the Ca2+/calmodulin-dependent protein phosphatase calcineurin. Therefore, a double mutant in calcineurin (CNB1) and FKS1 is inviable due to a deficiency in FKS2 expression. To identify novel regulators of FKS2 expression, we isolated genes whose overexpression obviates the calcineurin requirement for viability of an fks1 mutant. Two components of the cell integrity signaling pathway controlled by the RHO1 G protein (MKK1 and RLM1) were identified through this screen. This signaling pathway is activated during growth at moderately high temperatures. We demonstrate that calcineurin and the cell integrity pathway function in parallel, through separable promoter elements, to induce FKS2 expression during growth at 39 degrees C. Because RHO1 also serves as a regulatory subunit of the glucan synthase, our results define a regulatory circuit through which RHO1 controls both the activity of this enzyme complex and the expression of at least one of its components. We show also that FKS2 induction during growth on poor carbon sources is a response to glucose depletion and is under the control of the SNF1 protein kinase and the MIG1 transcriptional repressor. Finally, we show that FKS2 expression is induced as cells enter stationary phase through a SNF1-, calcineurin-, and cell integrity signaling-independent pathway.

A family of genes required for maintenance of cell wall integrity and for the stress response in Saccharomyces cerevisiae

The PKC1-MPK1 pathway in yeast functions in the maintenance of cell wall integrity and in the stress response. We have identified a family of genes that are putative regulators of this pathway. WSC1, WSC2, and WSC3 encode predicted integral membrane proteins with a conserved cysteine motif and a WSC1-green fluorescence protein fusion protein localizes to the plasma membrane. Deletion of WSC results in phenotypes similar to mutants in the PKC1-MPK1 pathway and an increase in the activity of MPK1 upon a mild heat treatment is impaired in a wscDelta mutant. Genetic analysis places the function of WSC upstream of PKC1, suggesting that they play a role in its activation. We also find a genetic interaction between WSC and the RAS-cAMP pathway. The RAS-cAMP pathway is required for cell cycle progression and for the heat shock response. Overexpression of WSC suppresses the heat shock sensitivity of a strain in which RAS is hyperactivated and the heat shock sensitivity of a wscDelta strain is rescued by deletion of RAS2. The functional characteristics and cellular localization of WSC suggest that they may mediate intracellular responses to environmental stress in yeast.

Coordination of the mating and cell integrity mitogen-activated protein kinase pathways in Saccharomyces cerevisiae

Mating pheromone stimulates a mitogen-activated protein (MAP) kinase activation pathway in Saccharomyces cerevisiae that induces cells to differentiate and form projections oriented toward the gradient of pheromone secreted by a mating partner. The polarized growth of mating projections involves new cell wall synthesis, a process that relies on activation of the cell integrity MAP kinase, Mpk1. In this report, we show that Mpk1 activation during pheromone induction requires the transcriptional output of the mating pathway and protein synthesis. Consequently, Mpk1 activation occurs subsequent to the activation of the mating pathway MAP kinase cascade. Additionally, Spa2 and Bni1, a formin family member, are two coil-coil-related proteins that are involved in the timing and other aspects of mating projection formation. Both proteins also affect the timing and extent of Mpk1 activation. This correlation suggests that projection formation comprises part of the pheromone-induced signal that coordinates Mpk1 activation with mating differentiation. Stimulation of Mpk1 activity occurs through the cell integrity phosphorylation cascade and depends on Pkc1 and the redundant MAP/Erk kinases (MEKs), Mkk1 and Mkk2. Surprisingly, Mpk1 activation by pheromone was only partially impaired in cells lacking the MEK kinase Bck1. This Bck1-independent mechanism reveals the existence of an alternative activator of Mkk1/Mkk2 in some strain backgrounds that at least functions under pheromone-induced conditions.

Osmotic balance regulates cell fusion during mating in Saccharomyces cerevisiae

Successful zygote formation during yeast mating requires cell fusion of the two haploid mating partners. To ensure that cells do not lyse as they remodel their cell wall, the fusion event is both temporally and spatially regulated: the cell wall is degraded only after cell-cell contact and only in the region of cell-cell contact. To understand how cell fusion is regulated, we identified mutants defective in cell fusion based upon their defect in mating to a fus1 fus2 strain (Chenevert, J., N. Valtz, and I. Herskowitz. 1994. Genetics 136:1287-1297). Two of these cell fusion mutants are defective in the FPS1 gene, which codes for a glycerol facilitator (Luyten, K., J. Albertyn, W.F. Skibbe, B.A. Prior, J. Ramos, J.M. Thevelein, and S. Hohmann. 1995. EMBO [Eur. Mol. Biol. Organ.] J. 14:1360-1371). To determine whether inability to maintain osmotic balance accounts for the defect in cell fusion in these mutants, we analyzed the behavior of an fps1Delta mutant with reduced intracellular glycerol levels because of a defect in the glycerol-3-phosphate dehydrogenase (GPD1) gene (Albertyn, J., S. Hohmann, J.M. Thevelein, and B.A. Prior. 1994. Mol. Cell. Biol. 14:4135-4144): deletion of GPD1 partially suppressed the cell fusion defect of fps1 mutants. In contrast, overexpression of GPD1 exacerbated the defect. The fusion defect could also be partially suppressed by 1 M sorbitol. These observations indicate that the fusion defect of fps1 mutants results from inability to regulate osmotic balance and provide evidence that the osmotic state of the cell can regulate fusion. We have also observed that mutants expressing hyperactive protein kinase C exhibit a cell fusion defect similar to that of fps1 mutants. We propose that Pkc1p regulates cell fusion in response to osmotic disequilibrium. Unlike fps1 mutants, fus1 and fus2 mutants are not influenced by expression of GPD1 or by 1 M sorbitol. Their fusion defect is thus unlikely to result from altered osmotic balance.

Mkh1, a MEK kinase required for cell wall integrity and proper response to osmotic and temperature stress in Schizosaccharomyces pombe

We have identified a Schizosaccharomyces pombe gene, mkh1, that encodes a MEK kinase (MEKK) homolog. The coding region of mkh1 is contained within a single exon encoding a 1,116-amino-acid protein. The putative catalytic domain of Mkh1 is 54% identical to the catalytic domain of S. cerevisiae Bck1, the most closely related protein. Deletion of mkh1 did not significantly affect cell growth or division under standard conditions. However, mkh1delta cell growth was inhibited by high KCl or NaCl concentrations. mkh1delta cells required a longer time to reenter the cell cycle after prolonged stationary-phase arrest. Also, mkh1delta cells exhibited a round cell shape, while overexpression of Mkh1 resulted in an elongated cell shape. mkh1delta cells exhibited a more dramatic phenotype when grown in nutrient-limiting conditions at high temperature or in hyperosmotic medium. In such conditions, completion of cytokinesis was inhibited, resulting in the growth of pseudohyphal filaments with multiple septa and nuclei. Also, mkh1delta cells were hypersensitive to beta-glucanase treatment. Together these results suggest that Mkh1 regulates cell morphology, cell wall integrity, salt resistance, cell cycle reentry from stationary-phase arrest, and filamentous growth in response to stress. These phenotypes are essentially identical to those exhibited by cells lacking Pmk1/Spm1, a recently identified mitogen-activated protein kinase. Our evidence suggests that Pmk1/Spm1 acts downstream from Mkh1 in a common pathway. Our results also suggest that Mkh1 and Pck2 act independently to maintain cell wall integrity, cell morphology, and salt resistance but act in opposition to regulate filamentous growth.

The Saccharomyces cerevisiae MADS-box transcription factor Rlm1 is a target for the Mpk1 mitogen-activated protein kinase pathway

Mutation of Saccharomyces cerevisiae RLM1, which encodes a MADS-box transcription factor, confers resistance to the toxic effects of constitutive activity of the Mpk1 mitogen-activated kinase (MAPK) pathway. The Rlm1 DNA-binding domain, which is similar to that of the metazoan MEF2 transcription factors, is also closely related to that of a second S. cerevisiae protein, Smp1 (second MEF2-like protein), encoded by the YBR182C open reading frame (N. Demolis et al., Yeast 10:1511-1525, 1994; H. Feldmann et al., EMBO J. 13:5795-5809, 1994). We show that Rlm1 and Smp1 have MEF2-related DNA-binding specificities: Rlm1 binds with the same specificity as MEF2, CTA(T/A)4TAG, while SMP1 binds a more extended consensus sequence, ACTACTA(T/A)4TAG. The two DNA-binding domains can heterodimerize with each other and with MEF2A. Deletion of RLM1 enhances resistance to cell wall disruptants, increases saturation density, reduces flocculation, and inactivates reporter genes controlled by the Rlm1 consensus binding site. Deletion of SMP1 neither causes these phenotypes nor enhances the Rlm1 deletion phenotype. However, overexpression of the DNA-binding domain of either protein causes an osmoremedial phenotype. Synthetic and naturally occurring MEF2 consensus sequences exhibit strong RLM1- and MPK1-dependent upstream activation sequence activity. Transcriptional activation by Rlm1 requires its C-terminal sequences, and Gal4 fusion proteins containing Rlm1 C-terminal sequences also act as MPK1-dependent transcriptional activators. These results establish the Rlm1 C-terminal sequences as a target for the Mpk1 MAPK pathway.

The fission yeast pmk1+ gene encodes a novel mitogen-activated protein kinase homolog which regulates cell integrity and functions coordinately with the protein kinase C pathway

We have isolated a gene, pmk1+, a third mitogen-activated protein kinase (MAPK) gene homolog from the fission yeast Schizosaccharomyces pombe. The predicted amino acid sequence shows the most homology (63 to 65% identity) to those of budding yeast Saccharomyces Mpk1 and Candida Mkc1. The Pmk1 protein contains phosphorylated tyrosines, and the level of tyrosine phosphorylation was increased in the dsp1 mutant which lacks an attenuating phosphatase for Pmk1. The level of tyrosine phosphorylation appears constant during hypotonic or heat shock treatment. The cells with pmk1 deleted (delta pmk1) are viable but show various defective phenotypes, including cell wall weakness, abnormal cell shape, a cytokinesis defect, and altered sensitivities to cations, such as hypersensitivity to potassium and resistance to sodium. Consistent with a high degree of conservation of amino acid sequence, multicopy plasmids containing the MPK1 gene rescued the defective phenotypes of the delta pmk1 mutant. The frog MAPK gene also suppressed the pmk1 disruptant. The results of genetic analysis indicated that Pmk1 lies on a novel MAPK pathway which does not overlap functionally with the other two MAPK pathways, the Spk1-dependent mating signal pathway and Sty1/Spc1/Phh1-dependent stress-sensing pathway. In Saccharomyces cerevisiae, Mpk1 is involved in cell wall integrity and functions downstream of the protein kinase C homolog. In contrast, in S. pombe, Pmk1 may not act in a linear manner with respect to fission yeast protein kinase C homologs. Interestingly, however, these two pathways are not independent; instead, they regulate cell integrity in a coordinate manner.

PWP2, a member of the WD-repeat family of proteins, is an essential Saccharomyces cerevisiae gene involved in cell separation

WD-repeat proteins contain four to eight copies of a conserved motif that usually ends with a tryptophan-aspartate (WD) dipeptide. The Saccharomyces cerevisiae PWP2 gene, identified by sequencing of chromosome III, is predicted to contain eight so-called WD-repeats, flanked by nonhomologous extensions. This gene is expressed as a 3.2-kb mRNA in all cell types and encodes a protein of 104 kDa. The PWP2 gene is essential for growth because spores carrying the pwp2 delta 1::HIS3 disruption germinate before arresting growth with one or two large buds. The growth defect of pwp2 delta 1::HIS3 cells was rescued by expression of PWP2 or epitope-tagged HA-PWP2 using the galactose-inducible GALI promoter. In the absence of galactose, depletion of Pwp2p resulted in multibudded cells with defects in bud site selection, cytokinesis, and hydrolysis of the septal junction between mother and daughter cells. In cell fractionation studies, HA-Pwp2p was localized in the particulate component of cell lysates, from which it would be solubulized by high salt and alkaline buffer but not by nonionic detergents or urea. Indirect immunofluorescence microscopy indicated that HA-Pwp2p was clustered at multiple points in the cytoplasm. These results suggest that Pwp2p exists in a proteinaceous complex, possibly associated with the cytoskeleton, where it functions in control of cell growth and separation.

ROM7/BEM4 encodes a novel protein that interacts with the Rho1p small GTP-binding protein in Saccharomyces cerevisiae

The RHO1 gene encodes a homolog of the mammalian RhoA small GTP-binding protein in the yeast Saccharomyces cerevisiae. Rho1p is localized at the growth site and is required for bud formation. The RHO1(G22S, D125N) mutation is a temperature-sensitive and dominant negative mutation of RHO1, and a multicopy suppressor of RHO1(G22S, D125N), ROM7, was isolated. Nucleotide sequencing of ROM7 revealed that it is identical to the BEM4 gene (GenBank accession number L27816), although its physiological function has not yet been reported. Disruption of BEM4 resulted in the cold- and temperature-sensitive growth phenotypes, and cells of the deltabem4 mutant showed abnormal morphology, suggesting that BEM4 is involved in the budding process. The temperature-sensitive growth phenotype was suppressed by overexpression of RHO1, ROM2, which encodes a Rho1p-specific GDP/GTP exchange factor, or PKC1, which encodes a target of Rho1p. Moreover, glucan synthase activity, which is activated by Rho1p, was significantly reduced in the deltabem4 mutant. Two-hybrid and biochemical experiments revealed that Bem4p directly interacts with the nucleotide-free form of Rho1p and, to lesser extents, with the GDP- and GTP-bound forms of Rho1p, although Bem4p showed neither GDP/GTP exchange factor, GDP dissociation inhibitor, nor GTPase-activating protein activity toward Rho1p. These results indicate that Bem4p is a novel protein directly interacting with Rho1p and is involved in the RHO1-mediated signaling pathway.