The protein kinase C-mediated MAP kinase pathway involved in the maintenance of cellular integrity in Saccharomyces cerevisiae

Reciprocal regulation between Slt2 MAPK and isoforms of Msg5 dual-specificity protein phosphatase modulates the yeast cell integrity pathway

Dual-specificity protein phosphatases (DSPs) are involved in the negative regulation of mitogen-activated protein kinases (MAPKs) by dephosphorylating both threonine- and tyrosine-conserved residues located at the activation loop. Here we show that Msg5 DSP activity is essential for maintaining a low level of signaling through the cell integrity pathway in Saccharomyces cerevisiae. Consistent with a role of this phosphatase on cell wall physiology, cells lacking Msg5 displayed an increased sensitivity to the cell wall-interfering compound Congo Red. We have observed that the N-terminal non-catalytic region of this phosphatase was responsible for binding to the kinase domain of Slt2, the MAPK that operates in this pathway. In vivo and in vitro experiments revealed that both proteins act on each other. Msg5 bound and dephosphorylated activated Slt2. Reciprocally, Slt2 phosphorylated Msg5 as a consequence of the activation of the cell integrity pathway. In addition, alternative use of translation initiation sites at MSG5 resulted in two protein forms that are functional on Slt2 and became equally phosphorylated following activation of this MAPK. Under activating conditions, a decrease in the affinity between Msg5 and Slt2 was observed, leading us to suggest that the mechanism by which Slt2 controls the action of Msg5 was via the modulation of protein-protein interactions. Our results indicate the existence of posttranscriptional mechanisms of regulation of DSPs in yeast and provide new insights into the negative control of the cell integrity pathway.

Genetic, biochemical, and morphological evidence for the involvement of N-glycosylation in biosynthesis of the cell wall beta1,6-glucan of Saccharomyces cerevisiae

Recent evidence indicates that Stt3p plays a central role in the recognition and/or catalytic step in N-glycosylation (asparagine-linked glycosylation) in the lumen of the endoplasmic reticulum. It is known that stt3 mutants exhibit certain phenotypic features that are suggestive of a cell wall defect. To understand the basis of these phenotypes, we devised a genetic screen to isolate strains bearing mutations that lead to synthetic lethality in combination with the stt3-1 mutation. Using this screen, we were surprised to identify two KRE genes (KRE5 and KRE9) that are involved in the biosynthesis of the cell wall beta1,6-glucan. This finding led us to propose that the N-glycosylation process is essential in the biosynthesis of cell wall beta1,6-glucan. This proposal was supported by the observation that several stt3 mutants exhibited a 60-70% reduction in the content of cell wall beta1,6-glucan as compared with WT cells. Transmission electron microscopy revealed that the stt3 mutant strains exhibit a diffused cell wall with loss of the outer mannoprotein layer as compared with the WT cells. Thus, we provide genetic, morphological, and biochemical evidence for the critical involvement of N-glycosylation in some step in assembly of the cell wall beta1,6-glucan in Saccharomyces cerevisiae.

Insight into Functional Aspects of Stt3p, a Subunit of the Oligosaccharyl Transferase

Over a decade ago, the gene STT3 was identified in a staurosporine and temperature sensitivity screen of yeast. Subsequently the product of this gene was shown to be a subunit of the endoplasmic reticulum-localized oligosaccharyl transferase (OT) complex. Although stt3 mutants are known to be staurosporine-sensitive, we found that mutants of other OT subunits (except ost4 Delta) are staurosporine-resistant, which indicates that this phenotype of stt3 mutants is not simply a consequence of their defect in glycosylation, as previously speculated. Staurosporine sensitivity was found to be an allele-specific phenotype restricted to cells harboring mutations in highly conserved residues in the N-terminal domain of the STT3 protein. Cells bearing mutations in one of the cytosolic-oriented loops (amino acids 158-168) in the N terminus of Stt3p were found to be specifically susceptible to staurosporine. Staurosporine is a specific inhibitor of Pkc1p, and a genetic link had previously been suggested between PKC1 and STT3. It is known that overexpression of PKC1 suppresses the staurosporine sensitivity of the stt3 mutants in an allele-specific manner, which is typical of mutants of Pkc1p cascade. It has been shown that the pkc1 null mutant exhibits lowered OT activity. Our results combined with these previous observations indicate that the N-terminal domain of Stt3p may interact with members of the Pkc1p cascade and consequently mutations in this domain result in staurosporine sensitivity. We further speculate that the Pkc1p regulates OT activity through the N-terminal domain of Stt3p, the C-terminal domain of which possesses the recognition and/or catalytic site of the OT complex.

Evidence for control of nitrogen metabolism by a START-dependent mechanism in Saccharomyces cerevisiae

It is generally thought that cell growth and metabolism regulate cell division and not vice versa. Here, we examined Saccharomyces cerevisiae cells growing under conditions of continuous culture in a chemostat. We found that loss of G1 cyclins, or inactivation of the cyclin-dependent kinase Cdc28p, reduced the activity of glutamate synthase (Glt1p), a key enzyme in nitrogen assimilation. We also present evidence indicating that the G1 cyclin-dependent control of Glt1p may involve Jem1p, a DnaJ-type chaperone. Our results suggest that completion of START may be linked to nitrogen metabolism.

Lipid hydroperoxides activate the mitogen-activated protein kinase Mpk1p in Saccharomyces cerevisiae

Saccharomyces cerevisiae is capable of responding to oxidants, including lipid peroxidation products. We investigate here the role of the mitogen-activated protein kinase Mpk1p in protection against linoleic acid hydroperoxide (LoaOOH), a product of radical attack on an unsaturated lipid. MPK1 was found to be required for resistance to LoaOOH. Furthermore, Mpk1p was rapidly and transiently phosphorylated in response to LoaOOH. This phosphorylation was dose-dependent and stimulated by sublethal concentrations as low as 1 mum in the external medium. Such low doses have been shown to result in resistance to subsequent challenge with a higher dose through the process of adaptation. However MPK1 was not essential for this adaptive response. MPK1 was also not involved in cell cycle modulation and acted independently of the cell cycle-regulating Oca1p. Transcriptional profiling of the mpk1Delta cells during LoaOOH stress indicated that Mpk1p may be important in effecting changes to the cell surface and metabolism during LoaOOH exposure. Furthermore, it revealed that Mpk1p is required for the regulation of 97 LoaOOH-responsive transcripts. Evidence is presented that the activation of Mpk1p may be caused by the activation of protein kinase C by LoaOOH.

The G1 Cyclin Cln3p Controls Vacuolar Biogenesis in Saccharomyces cerevisiae

How organelle biogenesis and inheritance is linked to cell division is poorly understood. In the budding yeast Saccharomyces cerevisiae the G1 cyclins Cln1,2,3p control initiation of cell division. Here we show that Cln3p controls vacuolar (lysosomal) biogenesis and segregation. First, loss of Cln3p, but not Cln1p or Cln2p, resulted in vacuolar fragmentation. Although the vacuoles of cln3Δ cells were fragmented, together they occupied a large space, which accounted for a significant fraction of the overall cell size increase in cln3Δ cells. Second, cytosol prepared from cells lacking Cln3p had reduced vacuolar homotypic fusion activity in cell-free assays. Third, vacuolar segregation was perturbed in cln3Δ cells. Our findings reveal a novel role for a eukaryotic G1 cyclin in cytoplasmic organelle biogenesis and segregation.

The yeasts Rho1p and Pkc1p regulate the transport of chitin synthase III (Chs3p) from internal stores to the plasma membrane

During cell stress, Saccharomyces cerevisiae increases the synthesis of chitin and glucans to strengthen and repair the cell wall. In this study, we show that under conditions of cell stress, the steady-state localization of chitin synthase III (Chs3p) shifts from internal stores (chitosomes) to the plasma membrane (PM). This redistribution occurs rapidly and requires the activators of the cell wall stress response signaling pathway, the G protein Rho1p, and the protein kinase Pkc1p, but not the cell integrity response mitogen-activated protein kinase cascade. Furthermore, expression of activated forms of Rho1p or Pkc1p, in the absence of cell stress, is sufficient to redistribute Chs3p to the PM. In cells deficient for both the clathrin adaptor complex 1 and Chs6p, where Chs3p is transported to the PM by an alternative bypass pathway, cell wall stress did not cause mobilization of Chs3p, suggesting that Rho1p/Pkc1p regulate Chs3p exit from the trans-Golgi network. The mobilization of an intracellular reservoir of Chs3p presents a novel opportunity to investigate the genetic basis of regulated vesicular traffic.

Actin remodeling to facilitate membrane fusion

Actin and its associated proteins participate in several intracellular trafficking mechanisms. This review assesses recent work that shows how actin participates in the terminal trafficking event of membrane bilayer fusion. A recent flurry of reports defines a role for Rho proteins in membrane fusion and also demonstrates that this role is distinct from any vesicle transport mechanism. Rho proteins are well known to govern actin remodeling, which implicates this process as a condition of membrane fusion. A small but significant body of work examines actin-regulated events of intracellular membrane fusion, exocytosis and endocytosis. In general, actin has been shown to act as a negative regulator of exocytosis. Cortical actin filaments act as a barrier that requires transient removal to allow vesicles to undergo docking at the plasma membrane. However, once docked, F-actin synthesis may act as a positive regulator to give the final stimulus to drive membrane fusion. F-actin synthesis is clearly needed for endocytosis and intracellular membrane fusion events. What may seem like dissimilar results are perhaps snapshots of a single mechanism of membranous actin remodeling (i.e. dynamic disassembly and reassembly) that is universally needed for all membrane fusion events.

The interaction of Slt2 MAP kinase with Knr4 is necessary for signalling through the cell wall integrity pathway in Saccharomyces cerevisiae

In budding yeast, PKC1 plays an essential role in cell integrity and proliferation through a linear MAP (Mitogen Activated Protein) kinase phosphorylation cascade, which ends up with the activation of the Slt2-MAP kinase by dual phosphorylation on two conserved threonine and tyrosine residues. In this phosphorylated form, Slt2p kinase activates by phosphorylation at least two known downstream targets: Rlm1p, which is implicated in the expression of cell wall-related genes, and SBF, required for transcription activation of cell cycle-regulated genes at the G1 to S transition. In this paper, we demonstrate by two-hybrid, in vitro immunoprecipitation and tandem affinity purification (TAP) methods that Knr4p physically interacts with Slt2p. Moreover, we show that the absence of Knr4p alters proper signalling of Slt2p to its two known downstream targets. In a knr4 null mutant, the SLT2-dependent activation of Rlm1p is strongly reduced and the transcriptional activity of Rlm1p is decreased, although the phosphorylated form of Slt2p is more abundant than in wild-type cells. On the contrary, SBF is abnormally activated in this mutant, as shown by a more abundant phosphorylated form of Swi6p, by higher beta-galactosidase levels from a SCB-lacZ gene fusion, and by deregulation of the cyclic behaviour of several cell cycle-regulated genes. These results, taken together with our recent finding that Bck2p requires Knr4p to activate additively with Cln3-Cdc28p SBF target genes, lead to a model in which Knr4p is involved in co-ordinating the Slt2p-mediated cell wall integrity pathway with progression of the cell cycle.

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

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

The Cryptococcus neoformans MAP kinase Mpk1 regulates cell integrity in response to antifungal drugs and loss of calcineurin function

Cell wall integrity is crucial for fungal growth, development and stress survival. In the model yeast Saccharomyces cerevisiae, the cell integrity Mpk1/Slt2 MAP kinase and calcineurin pathways monitor cell wall integrity and promote cell wall remodelling under stress conditions. We have identified the Cryptococcus neoformans homologue of the S. cerevisiae Mpk1/Slt2 MAP kinase and have characterized its role in the maintenance of cell integrity in response to elevated growth temperature and in the presence of cell wall synthesis inhibitors. C. neoformans Mpk1 is required for growth at 37 degrees C in vitro, and this growth defect is suppressed by osmotic stabilization. C. neoformans mutants lacking Mpk1 are attenuated for virulence in the mouse model of cryptococcosis. Phosphorylation of Mpk1 is induced in response to perturbations of cell wall biosynthesis by the antifungal drugs nikkomycin Z (a chitin synthase inhibitor), caspofungin (a beta-1,3-glucan synthase inhibitor), or FK506 (a calcineurin inhibitor), and mutants lacking Mpk1 display enhanced sensitivity to nikkomycin Z and caspofungin. Lastly, we show that calcineurin and Mpk1 play complementing roles in regulating cell integrity in C. neoformans. Our studies demonstrate that pharmacological inhibition of the cell integrity pathway would enhance the activity of antifungal drugs that target the cell wall.

Genome-wide analysis of the response to cell wall mutations in the yeast Saccharomyces cerevisiae

Perturbations of the yeast cell wall trigger a repair mechanism that reconfigures its molecular structure to preserve cell integrity. To investigate this mechanism, we compared the global gene expression in five mutant strains, each bearing a mutation (i.e. fks1, kre6, mnn9, gas1, and knr4 mutants) that affects in a different manner the cell wall construction. Altogether, 300 responsive genes were kept based on high stringency criteria during data processing. Functional classification of these differentially expressed genes showed a substantial subset of induced genes involved in cell wall construction and an enrichment of metabolic, energy generation, and cell defense categories, whereas families of genes belonging to transcription, protein synthesis, and cellular growth were underrepresented. Clustering methods isolated a single group of approximately 80 up-regulated genes that could be considered as the stereotypical transcriptional response of the cell wall compensatory mechanism. The in silico analysis of the DNA upstream region of these co-regulated genes revealed pairwise combinations of DNA-binding sites for transcriptional factors implicated in stress and heat shock responses (Msn2/4p and Hsf1p) with Rlm1p and Swi4p, two PKC1-regulated transcription factors involved in the activation genes related to cell wall biogenesis and G1/S transition. Moreover, this computational analysis also uncovered the 6-bp 5'-AGCCTC-3' CDRE (calcineurin-dependent response element) motif in 40% of the co-regulated genes. This motif was recently shown to be the DNA binding site for Crz1p, the major effector of calcineurin-regulated gene expression in yeast. Taken altogether, the data presented here lead to the conclusion that the cell wall compensatory mechanism, as triggered by cell wall mutations, integrates three major regulatory systems: namely the PKC1-SLT2 mitogen-activated protein kinase-signaling module, the "global stress" response mediated by Msn2/4p, and the Ca2+/calcineurin-dependent pathway. The relative importance of these regulatory systems in the cell wall compensatory mechanism is discussed.

Role for lipid signaling and the cell integrity MAP kinase cascade in yeast septum biogenesis

Polarized deposition of chitin at the bud neck is essential for cell separation in yeast. Chitin septum biogenesis is catalyzed by two distinct chitin synthase activities encoded by the CHS2 and CHS3 genes. The phosphoinositide phosphatase Sac1p is required for proper trafficking of the Chs3p chitin synthase. sac1 mutants also display a severe synthetic growth defect, with mutations in the SLT2 gene which encodes a MAP kinase involved in cell integrity. We characterized the defect that underlies this genetic interaction and found that sac1 Delta slt2 Delta cells arrest as large-budded cells because they fail to separate at the end of mitosis. This inability to complete cell division appears to be caused by an increased deposition of chitin at the septum area and correlates with a mislocalized accumulation of the Chs2p chitin synthase at the cell periphery. Our data therefore indicate that Sac1p and Slt2p have synergistic roles in regulating chitin septum biogenesis.

KlROM2 encodes an essential GEF homologue in Kluyveromyces lactis

Cellular integrity in yeasts is ensured by a rigid cell wall whose synthesis is controlled by a MAP kinase signal transduction cascade. In Saccharomyces cerevisiae upstream regulatory components of this MAP kinase pathway involve a single protein kinase C, which is regulated in part by interaction with the small GTPase Rho1p. This small G protein is in turn rendered inactive (GDP-bound) or is activated (GTP-bound) by the influence of GTPase activating proteins (GAPs) and the GDP/GTP exchange factors (GEFs), respectively. We report here on the isolation of a gene from Kluyveromyces lactis, KlROM2, which encodes a member of the latter protein family. The nucleotide sequence contains an open reading frame of 1227 amino acids, with an overall identity of 57% to the Rom2 protein of S. cerevisiae. Four conserved sequence motifs could be identified: a RhoGEF domain, a DEP sequence, a CNH domain and a less conserved pleckstrin homology (PH) sequence. Klrom2 null mutants show a lethal phenotype, which indicates that the gene may encode the only functional GEF regulating the cellular integrity pathway in K. lactis. Conditional genomic expression of KlROM2 resulted in sensitivity towards caffeine and Calcofluor white as typical phenotypes of mutants defective in this pathway. Overexpression of KIROM2 from multicopy plasmids under the control of the ScGAL1 promoter severely impaired growth in both S. cerevisiae and in K. lactis. The fact that the lethal phenotype was not prevented in mpk1 deletion mutants indicates that growth inhibition is not simply caused by hyperactivation of the Pkc1p signal transduction pathway.

Rga5p is a specific Rho1p GTPase-activating protein that regulates cell integrity in Schizosaccharomyces pombe

Schizosaccharomyces pombe Rho1p regulates (1,3)beta-d-glucan synthesis and is required for cell integrity maintenance and actin cytoskeleton organization, but nothing is known about the regulation of this protein. At least nine different S. pombe genes code for proteins predicted to act as Rho GTPase-activating proteins (GAPs). The results shown in this paper demonstrate that the protein encoded by the gene named rga5+ is a GAP specific for Rho1p. rga5+ overexpression is lethal and causes morphological alterations similar to those reported for Rho1p inactivation. rga5+ deletion is not lethal and causes a mild general increase in cell wall biosynthesis and morphological alterations when cells are grown at 37 degrees C. Upon mild overexpression, Rga5p localizes to growth areas and possesses both in vivo and in vitro GAP activity specific for Rho1p. Overexpression of rho1+ in rga5Delta cells is lethal, with a morphological phenotype resembling that of the overexpression of the constitutively active allele rho1G15V. In addition (1,3)beta-d-glucan synthase activity, regulated by Rho1p, is increased in rga5Delta cells and decreased in rga5-overexpressing cells. Moreover, the increase in (1,3)beta-d-glucan synthase activity caused by rho1+ overexpression is considerably higher in rga5Delta than in wild-type cells. Genetic interactions suggest that Rga5p is also important for the regulation of the other known Rho1p effectors, Pck1p and Pck2p.

The mitogen-activated protein kinase gene MAF1 is essential for the early differentiation phase of appressorium formation in Colletotrichum lagenarium

Colletotrichum lagenarium, the causal agent of cucumber anthracnose, invades host plants by forming a specialized infection structure called an appressorium. In this fungus, the mitogen-activated protein kinase (MAPK) gene CMK1 is involved in several steps of the infection process, including appressorium formation. In this study, the goal was to investigate roles of other MAPKs in C. lagenarium. The MAPK gene MAF1, related to Saccharomyces cerevisiae MPK1 and Magnaporthe grisea MPS1, was isolated and functionally characterized. The maf1 gene replacement mutants grew normally, but there was a significant reduction in conidiation and fungal pathogenicity. The M. grisea mps1 mutant forms appressoria, but conidia of the C. lagenarium maf1 mutants produced elongated germ tubes without appressoria on both host plant and glass, on which the wild type forms appressoria, suggesting that MAF1 has an essential role in appressorium formation on inductive surfaces. On a nutrient agar, wild-type conidia produced elongated germ tubes without appressoria. The morphological phenotype of the wild type on the nutrient agar was similar to that of the maf1 mutants on inductive surfaces, suggesting repression of the MAF1-mediated appressorium differentiation on the nutrient agar. The cmk1 mutants failed to form normal appressoria but produced swollen, appressorium-like structures on inductive surfaces, which is morphologically different from the maf1 mutants. These findings suggest that MAF1 is required for the early differentiation phase of appressorium formation, whereas CMK1 is involved in the maturation of appressoria.

Regulation of subtelomeric silencing during stress response

Sir proteins play a critical role in silent chromatin domains. While mutations can cause derepression of heterochromatin, it remains unclear whether silencing is actively involved in transcriptional control under changing environmental conditions. We find that TOR inhibits Sir3 phosphorylation. Rapamycin or stress induced by chlorpromazine leads to activation of MAP kinase Mpk1/Slt2, which phosphorylates Sir3. Sir3 hyperphosphorylation is correlated with reduced subtelomeric silencing, increased subtelomeric cell wall gene expression, and stress resistance to chlorpromazine, but does not affect the silent HML and rDNA loci. Based on these observations, we propose that regulation of silencing may be used to control gene expression at specific silent chromatin domains in response to stress and possibly other environmental changes.

Stationary phase in yeast

Eukaryotic cell proliferation is controlled by specific growth factors and the availability of essential nutrients. If either of these signals is lacking, cells may enter into a specialized nondividing resting state, known as stationary phase or G(0). The entry into such resting states is typically accompanied by a dramatic decrease in the overall growth rate and an increased resistance to a variety of environmental stresses. Since most cells spend most of their life in these quiescent states, it is important that we develop a full understanding of the biology of the stationary phase/G(0) cell. This knowledge would provide important insights into the control of two of the most fundamental aspects of eukaryotic cell biology: cell proliferation and long-term cell survival. This review will discuss some recent advances in our understanding of the stationary phase of growth in the budding yeast, Saccharomyces cerevisiae.

Regulation of the cell integrity pathway by rapamycin-sensitive TOR function in budding yeast

The TOR (target of rapamycin) pathway controls cell growth in response to nutrient availability in eukaryotic cells. Inactivation of TOR function by rapamycin or nutrient exhaustion is accompanied by triggering various cellular mechanisms aimed at overcoming the nutrient stress. Here we report that in Saccharomyces cerevisiae the protein kinase C (PKC)-mediated mitogen-activated protein kinase pathway is regulated by TOR function because upon specific Tor1 and Tor2 inhibition by rapamycin, Mpk1 is activated rapidly in a process mediated by Sit4 and Tap42. Osmotic stabilization of the plasma membrane prevents both Mpk1 activation by rapamycin and the growth defect that occurs upon the simultaneous absence of Tor1 and Mpk1 function, suggesting that, at least partially, TOR inhibition is sensed by the PKC pathway at the cell envelope. This process involves activation of cell surface sensors, Rom2, and downstream elements of the mitogen-activated protein kinase cascade. Rapamycin also induces depolarization of the actin cytoskeleton through the TOR proteins, Sit4 and Tap42, in an osmotically suppressible manner. Finally, we show that entry into stationary phase, a physiological situation of nutrient depletion, also leads to the activation of the PKC pathway, and we provide further evidence demonstrating that Mpk1 is essential for viability once cells enter G(0).

Two homologous genes, DCW1 (YKL046c) and DFG5, are essential for cell growth and encode glycosylphosphatidylinositol (GPI)-anchored membrane proteins required for cell wall biogenesis in Saccharomyces cerevisiae

The cell wall of Saccharomyces cerevisiae consists of glucan, chitin and various kinds of mannoproteins. Major parts of mannoproteins are synthesized as glycosylphosphatidylinositol (GPI)-anchored proteins and are then transferred to cell wall beta-1,6-glucan. A glycosyltransferase has been hypothesized to catalyse this transfer reaction. A database search revealed that the products of YKL046c and DFG5 are homologous to bacterial mannosidase. These genes are homologous to each other and have primary structures characteristic of GPI-anchored proteins. Although single disruptants of ykl046c and dfg5 were viable, ykl046cDelta was hypersensitive to a cell wall-digesting enzyme (zymolyase), suggesting that this gene is involved in cell wall biosynthesis. We therefore designated this gene as DCW1 (defective cell wall). A double disruptant of dcw1 and dfg5 was synthetically lethal, indicating that the functions of these gene products are redundant, and at least one of them is required for cell growth. Cells deficient in both Dcw1p and Dfg5p were round and large, had cell walls that contained an increased amount of chitin and secreted a major cell wall protein, Cwp1p, into the medium. Biochemical analyses showed that epitope-tagged Dcw1p is an N-glycosylated, GPI-anchored membrane protein and is localized in the membrane fraction including the cell surface. These results suggest that both Dcw1p and Dfg5p are GPI-anchored membrane proteins and are required for normal biosynthesis of the cell wall.

csmA, a gene encoding a class V chitin synthase with a myosin motor-like domain of Aspergillus nidulans, is translated as a single polypeptide and regulated in response to osmotic conditions

The csmA gene of Aspergillus nidulans encodes a polypeptide that consists of an N-terminal myosin motor-like domain and a C-terminal chitin synthase domain. csmA null mutants showed marked abnormalities in polarized growth, hyphal wall integrity, and conidiophore development. Furthermore, the growth of the csmA null mutants was sensitive to low osmotic conditions. In an effort to investigate the intracellular behavior of the csmA product (CsmA) and the regulation of its production, we constructed strains that produced CsmA tagged with nine repeats of the hemagglutinin A (HA) epitope at its COOH terminus (CsmA-HA) instead of CsmA. Western blot analysis with anti-HA antibody showed that the entire coding region of csmA was translated as a single polypeptide with an approximate molecular mass of 210kDa. CsmA-HA was produced during vegetative growth; however, its yield was significantly reduced under high osmotic conditions, suggesting that the role of CsmA in growth and morphogenesis is particularly important under low osmotic conditions.

Osmotic adaptation in yeast--control of the yeast osmolyte system

The yeast Saccharomyces cerevisiae (baker's yeast or budding yeast) is an excellent eukaryotic model system for cellular biology with a well-explored, completely sequenced genome. Yeast cells possess robust systems for osmotic adaptation. Central to the response to high osmolarity is the HOG pathway, one of the best-explored MAP kinase pathways. This pathway controls via different transcription factors the expression of more than 150 genes. In addition, osmotic responses are also controlled by protein kinase A via a general stress response pathway and by presently unknown signaling systems. The HOG pathway partially controls expression of genes encoding enzymes in glycerol production. Glycerol is the main yeast osmolyte, and its production is essential for growth in a high osmolarity medium. Upon hypo-osmotic shock, yeast cells transiently stimulate another MAP kinase pathway, the so-called PKC pathway, which appears to orchestrate the assembly of the cell surface and the cell wall. In addition, yeast cells show signs of a regulated volume decrease by rapidly exporting glycerol through Fps1p. This unusual MIP channel is gated by osmotic changes and thereby plays a key role in controlling the intracellular osmolyte content. Yeast cells also possess two aquaporins, Aqy1p and Aqy2p. The production of both proteins is strictly regulated, suggesting that these water channels play very specific roles in yeast physiology. Aqy1p appears to be developmentally regulated. Given the strong yeast research community and the excellent tools of genetics and functional genomics available, we expect yeast to be the best-explored cellular organism for several years ahead, and osmotic responses are a focus of interest for numerous yeast researchers.

Mutational analysis reveals a role for the C terminus of the proteasome subunit Rpt4p in spindle pole body duplication in Saccharomyces cerevisiae

The ubiquitin/proteasome pathway plays a key role in regulating cell cycle progression. Previously, we reported that a conditional mutation in the Saccharomyces cerevisiae gene RPT4/PCS1, which encodes one of six ATPases in the proteasome 19S cap complex/regulatory particle (RP), causes failure of spindle pole body (SPB) duplication. To improve our understanding of Rpt4p, we created 58 new mutations, 53 of which convert clustered, charged residues to alanine. Virtually all mutations that affect the N-terminal region, which contains a putative nuclear localization signal and coiled-coil motif, result in a wild-type phenotype. Nine mutations that affect the central ATPase domain and the C-terminal region confer recessive lethality. The two conditional mutations identified, rpt4-145 and rpt4-150, affect the C terminus. After shift to high temperature, these mutations generally cause cells to progress slowly through the first cell cycle and to arrest in the second cycle with large buds, a G2 content of DNA, and monopolar spindles, although this phenotype can vary depending on the medium. Additionally, we describe a genetic interaction between RPT4 and the naturally polymorphic gene SSD1, which in wild-type form modifies the rpt4-145 phenotype such that cells arrest in G2 of the first cycle with complete bipolar spindles.

Eng1p, an endo-1,3-beta-glucanase localized at the daughter side of the septum, is involved in cell separation in Saccharomyces cerevisiae

ENG1 (YNR067c), a gene encoding a new endo-1,3-beta-glucanase, was cloned by screening a genomic library with a DNA probe obtained by PCR with synthetic oligonucleotides designed according to conserved regions found between yeast exo-1,3-beta-glucanases (Exglp, Exg2p, and Ssglp). Eng1p shows strong sequence similarity to the product of the Saccharomyces cerevisiae ACF2 gene, involved in actin assembly "in vitro," and to proteins present in other yeast and fungal species. It is also related to plant glucan-binding elicitor proteins, which trigger the onset of a defense response upon fungal infection. Eng1p and Acf2p/Eng2p are glucan-hydrolyzing proteins that specifically act on 1,3-beta linkages, with an endolytic mode of action. Eng1p is an extracellular, heavily glycosylated protein, while Acf2p/Eng2p is an intracellular protein with no carbohydrate linked by N-glycosidic bonds. ENG1 transcription fluctuates periodically during the cell cycle; maximal accumulation occurs during the M/G1 transition and is dependent on the transcription factor Ace2p. Interestingly, eng1 deletion mutants show defects in cell separation, and Eng1p localizes asymmetrically to the daughter side of the septum, suggesting that this protein is involved, together with chitinase, in the dissolution of the mother-daughter septum.

Pkh1 and Pkh2 differentially phosphorylate and activate Ypk1 and Ykr2 and define protein kinase modules required for maintenance of cell wall integrity

Saccharomyces cerevisiae Pkh1 and Pkh2 are functionally redundant homologs of mammalian protein kinase, phosphoinositide-dependent protein kinase-1. They activate two closely related, functionally redundant enzymes, Ypk1 and Ykr2 (homologs of mammalian protein kinase, serum- and glucocorticoid-inducible protein kinase). We found that Ypk1 has a more prominent role than Ykr2 in mediating their shared essential function. Considerable evidence demonstrated that Pkh1 preferentially activates Ypk1, whereas Pkh2 preferentially activates Ykr2. Loss of Pkh1 (but not Pkh2) reduced Ypk1 activity; conversely, Pkh1 overexpression increased Ypk1 activity more than Pkh2 overexpression. Loss of Pkh2 reduced Ykr2 activity; correspondingly, Pkh2 overexpression increased Ykr2 activity more than Pkh1 overexpression. When overexpressed, a catalytically active C-terminal fragment (kinase domain) of Ypk1 was growth inhibitory; loss of Pkh1 (but not Pkh2) alleviated toxicity. Loss of Pkh2 (but not Pkh1) exacerbated the slow growth phenotype of a ypk1Delta strain. This Pkh1-Ypk1 and Pkh2-Ykr2 dichotomy is not absolute because all double mutants (pkh1Delta ypk1Delta, pkh2Delta ypk1Delta, pkh1Delta ykr2Delta, and pkh2Delta ykr2Delta) were viable. Compartmentation contributes to selectivity because Pkh1 and Ypk1 were located exclusively in the cytosol, whereas Pkh2 and Ykr2 entered the nucleus. At restrictive temperature, ypk1-1(ts) ykr2Delta cells lysed rapidly, but not in medium containing osmotic support. Dosage and extragenic suppressors were selected. Overexpression of Exg1 (major exoglucanase), or loss of Kex2 (endoprotease involved in Exg1 processing), rescued growth at high temperature. Viability was also maintained by PKC1 overexpression or an activated allele of the downstream protein kinase (BCK1-20). Conversely, absence of Mpk1 (distal mitogen-activated protein kinase of the PKC1 pathway) was lethal in ypk1-1(ts) ykr2Delta cells. Thus, Pkh1-Ypk1 and Pkh2-Ykr2 function in a novel pathway for cell wall integrity that acts in parallel with the Pkc1-dependent pathway.

Rho5p downregulates the yeast cell integrity pathway

The Rho family of proteins and their effectors are key regulators involved in many eukaryotic cell functions. In Saccharomyces cerevisiae the family consists of six members, Rho1p to Rho5p and Cdc42p. With the exception of Rho5p, these enzymes have been assigned different biological functions,including the regulation of polar growth, morphogenesis, actin cytoskeleton,budding and secretion. Here we show that a rho5 deletion results in an increased activity of the protein kinase C (Pkc1p)-dependent signal transduction pathway. Accordingly, the deletion shows an increased resistance to drugs such as caffeine, Calcofluor white and Congo red, which indicates activation of the pathway. In contrast, overexpression of an activated RHO5Q91H mutant renders cells more sensitive to these drugs. We conclude that Rho5p acts as an off-switch for the MAP-kinase cascade, which differentiates between MAP-kinase-dependent and -independent functions of Pkc1p. Kinetics of actin depolarisation and repolarisation after heat treatment of rho5 deletions as well as strains overexpressing the activated RHO5Q91H allele provide further evidence for such a function.

Dynamics of cell wall structure in Saccharomyces cerevisiae

The cell wall of Saccharomyces cerevisiae is an elastic structure that provides osmotic and physical protection and determines the shape of the cell. The inner layer of the wall is largely responsible for the mechanical strength of the wall and also provides the attachment sites for the proteins that form the outer layer of the wall. Here we find among others the sexual agglutinins and the flocculins. The outer protein layer also limits the permeability of the cell wall, thus shielding the plasma membrane from attack by foreign enzymes and membrane-perturbing compounds. The main features of the molecular organization of the yeast cell wall are now known. Importantly, the molecular composition and organization of the cell wall may vary considerably. For example, the incorporation of many cell wall proteins is temporally and spatially controlled and depends strongly on environmental conditions. Similarly, the formation of specific cell wall protein-polysaccharide complexes is strongly affected by external conditions. This points to a tight regulation of cell wall construction. Indeed, all five mitogen-activated protein kinase pathways in bakers' yeast affect the cell wall, and additional cell wall-related signaling routes have been identified. Finally, some potential targets for new antifungal compounds related to cell wall construction are discussed.

A novel connection between the yeast Cdc42 GTPase and the Slt2-mediated cell integrity pathway identified through the effect of secreted Salmonella GTPase modulators

Modulation of host cellular GTPases through the injection of the effector proteins SopE2 and SptP is essential for Salmonella typhimurium to enter into non-phagocytic cells. Here we show that expression of the guanine nucleotide exchange factor for Cdc42 SopE2 in Saccharomyces cerevisiae leads to the activation of Fus3 and Kss1 MAPKs, which operate in the mating and filamentation pathways, causing filamentous growth in haploid yeast cells. Furthermore, it promotes the activation of the cell integrity MAPK Slt2. Cdc42 activation by removal of its putative intrinsic GTPase-activating proteins (GAPs), Rga1, Rga2, and Bem3, also results in the phosphorylation of Kss1, Fus3, and Slt2 MAPKs. These data support the role of these GAP proteins as negative regulators of Cdc42, confirm the modulating effect of this GTPase on the filamentation and mating pathways and point to a novel connection between Cdc42 and the cell integrity pathway. Cdc42-induced activation of Slt2 occurs in a mating and filamentation pathway-dependent manner, but it does not require the function of Rho1, which is the GTPase that operates in the cell integrity pathway. Moreover, we report that Salmonella SptP can act as a GAP for Cdc42 in S. cerevisiae, down-regulating MAPK-mediated signaling. Thus, yeast provides a useful system to study the interaction of bacterial pathogenic proteins with eukaryotic signaling pathways. Furthermore, these proteins can be used as a tool to gain insight into the mechanisms that regulate MAPK-mediated signaling in eukaryotes.

Regulation of the Saccharomyces cerevisiae Slt2 kinase pathway by the stress-inducible Sdp1 dual specificity phosphatase

The Slt2/Mpk1 mitogen-activated protein kinase (MAPK) cell integrity pathway is involved in maintenance of cell shape and integrity during vegetative growth and mating in Saccharomyces cerevisiae. Slt2 is activated by dual phosphorylation of a threonine and tyrosine residue in response to several environmental stresses that perturb cell integrity. Negative regulation of Slt2 is achieved via dephosphorylation by two protein-tyrosine phosphatases, Ptp2 and Ptp3, and a dual specificity phosphatase, Msg5. In this study, we provide genetic and biochemical evidence that the stress-inducible dual specificity phosphatase, Sdp1, negatively regulates Slt2 by direct dephosphorylation. Deletion of SDP1 exacerbated growth defects due to overexpression of Mkk1(p386), a constitutively active mutant of Slt2 MAPK kinase, whereas overexpression of Sdp1 suppressed lethality caused by Mkk1(p386) overexpression. The heat shock-induced phosphorylation level of Slt2 was elevated in an sdp1Delta strain compared with that of the wild type, and heat shock-activated phospho-Slt2 was dephosphorylated by recombinant Sdp1 in vitro. Under normal growth conditions, an Sdp1-GFP fusion protein was localized to both the nucleus and cytoplasm. However, the Sdp1-GFP protein translocated to punctate spots throughout the cell after heat shock. SDP1 transcription was induced by several stress conditions in an Msn2/4-dependent manner but independent of the Rlm1 transcription factor, a downstream target activated by Slt2. Induction of SLT2 by high osmolarity was dependent on Rlm1 transcription factor and Hog1 kinase, suggesting cross-talk between Slt2 and Hog1 MAPK pathways. These studies demonstrate regulation of Slt2 activity and gene expression in coordination with other stress signaling pathways.

Phospholipase C is required for glucose-induced calcium influx in budding yeast

Intracellular calcium is a second messenger involved in several processes in yeast, such as mating, nutrient sensing, stress response and cell cycle events. It was reported that glucose addition stimulates a rapid increase in free calcium level in yeast. To investigate the calcium level variations induced by different stimuli we used a reporter system based on the photoprotein aequorin. Glucose addition (50 mM) to nutrient-starved cells induced an increase in free intracellular calcium concentration, mainly due to an influx from external medium. The increase of calcium reached its maximum 100-120 s after the stimulus. A concentration of about 20 mM glucose was required for a 50% increase in intracellular calcium. This response was completely abolished in strain plc1 Delta and in the isogenic wild-type strain treated with 3-nitrocoumarin, a phosphatidylinositol-specific phospholipase C inhibitor, suggesting that Plc1p is essential for glucose-induced calcium increase. This suggests that Plc1p should have a significant role in transducing glucose signal. The calcium influx induced by addition of high glucose on cells previously stimulated with low glucose levels was inhibited in strains with a deletion in the GPR1 or GPA2 genes, which suggests that glucose would be detected through the Gpr1p/Gpa2p receptor/G protein-coupled (GPCR) complex. Moreover, the signal was completely abolished in a strain unable to phosphorylate glucose, which is consistent with the reported requirement of glucose phosphorylation for GPCR complex activation.

Yeast RSC function is required for organization of the cellular cytoskeleton via an alternative PKC1 pathway

RSC is a 15-protein ATP-dependent chromatin-remodeling complex related to Snf-Swi, the prototypical ATP-dependent nucleosome remodeler in budding yeast. Despite insight into the mechanism by which purified RSC remodels nucleosomes, little is known about the chromosomal targets or cellular pathways in which RSC acts. To better understand the cellular function of RSC, a screen was undertaken for gene dosage suppressors of sth1-3ts, a temperature-sensitive mutation in STH1, which encodes the essential ATPase subunit. Slg1p and Mid2p, two type I transmembrane stress sensors of cell wall integrity that function upstream of protein kinase C (Pkc1p), were identified as multicopy suppressors of sth1-3ts cells. Although the sth1-3ts mutant exhibits defects characteristic of PKC1 pathway mutants (caffeine and staurosporine sensitivities and an osmoremedial phenotype), only upstream components and not downstream effectors of the PKC1-MAP kinase pathway can suppress defects conferred by sth1-3ts, suggesting that RSC functions in an alternative PKC1-dependent pathway. Moreover, sth1-3ts cells display defects in actin cytoskeletal rearrangements and are hypersensitive to the microtubule depolymerizing drug, TBZ; both of these defects can be corrected by the high-copy suppressors. Together, these data reveal an important functional connection between the RSC remodeler and PKC1-dependent signaling in regulating the cellular architecture.

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.

Evidence for involvement of Saccharomyces cerevisiae protein kinase C in glucose induction of HXT genes and derepression of SUC2

The PKC1 gene in the yeast Saccharomyces cerevisiae encodes protein kinase C that is known to control a mitogen-activated protein (MAP) kinase cascade consisting of Bck1, Mkk1 and Mkk2, and Mpk1. This cascade affects the cell wall integrity but the phenotype of Pkc1 mutants suggests additional targets which have not yet been identified. We show that a pkc1Delta mutant, as opposed to mutants in the MAP kinase cascade, displays two major defects in the control of carbon metabolism. It shows a delay in the initiation of fermentation upon addition of glucose and a defect in derepression of SUC2 gene after exhaustion of glucose from the medium. After addition of glucose the production of both ethanol and glycerol started very slowly. The V(max) of glucose transport dropped considerably and Northern blot analysis showed that induction of the HXT1, HXT2 and HXT4 genes was strongly reduced. Growth of the pkc1Delta mutant was absent on glycerol and poor on galactose and raffinose. Oxygen uptake was barely present. Derepression of invertase activity and SUC2 transcription upon transfer of cells from glucose to raffinose was deficient in the pkc1Delta mutant as opposed to the wild-type. Our results suggest an involvement of Pkc1p in the control of carbon metabolism which is not shared by the downstream MAP kinase cascade.

Regulation of yeast protein kinase C activity by interaction with the small GTPase Rho1p through its amino-terminal HR1 domain

Protein kinase C from Saccharomyces cerevisiae (Pkc1p) constitutes a prototypic member of the protein kinase C superfamily, as it shares all the conserved regions scattered among the isoenzymes of higher eukaryotes. The functional significance of some of the conserved domains in the yeast enzyme has not yet been investigated. We examined strains carrying a partial deletion in the amino-terminal region of the enzyme, which is homologous to the HR1 of the protein kinase C-related kinases. This strain was sensitive to the presence of caffeine, Calcofluor white and Congo red, all drugs known to affect mutants defective in the signal transduction pathway ensuring cellular integrity in which Pkc1p is a central component. Isolation of a single point mutation in HR1A, which shares the sensitivity to the drugs mentioned, confirmed the importance of this region for proper regulation of protein kinase C activity in vivo. Two-hybrid analysis provided evidence for an interaction of the small GTPase Rho1p with the HR1A region, in addition to the reported interaction of this protein with the C1 region of Pkc1p. MAP kinase phosphorylation assays indicate that this Rho1p-Pkc1p/HR1A interaction does not result in an activation of the kinase cascade. The intragenic lethality of mutants affected in both HR1A and the C1 domain reported in this work implies an essential role for Rho1p-Pkc1p interaction in yeast.

Stt4 PI 4-kinase localizes to the plasma membrane and functions in the Pkc1-mediated MAP kinase cascade

Production of the essential phospholipid PI4P at the Golgi by the Pik1 kinase is required for protein secretion, while a distinct pool of PI4P generated by the Stt4 kinase is critical for normal actin cytoskeleton organization. We identify a transmembrane protein that stabilizes Stt4 at the plasma membrane where it directs synthesis of PI4P, which is required for activation of the Rho1/Pkc1-mediated MAP kinase cascade. Inactivation of Stt4 or the PI4P 5-kinase Mss4 results in mislocalization of the Rho-GTPase GEF Rom2. Rom2 binds PI4,5P(2) through its PH domain and represents the first identified effector in the Stt4-Mss4 pathway. Based on these results, we propose that Stt4-Mss4 generates PI4,5P(2) at the plasma membrane, required to recruit/activate effector proteins such as Rom2.

The protein kinase C pathway is required for viability in quiescence in Saccharomyces cerevisiae

Protein kinase C, encoded by PKC1, regulates construction of the cell surface in vegetatively growing yeast cells. Pkc1 in part acts by regulating Mpk1, a MAP kinase. Mutants lacking Bck1, a component of the MAP kinase branch of the pathway, fail to respond normally to nitrogen starvation, which causes entry into quiescence. Given that the Tor1 and Tor2 proteins are key inhibitors of entry into quiescence, the Pkc1 pathway may regulate these proteins. We find that pkc1Delta and mpk1Delta mutants rapidly die by cell lysis upon carbon or nitrogen starvation. The Pkc1 pathway does not regulate the TOR proteins: transcriptional changes dependent on inhibition of the TORs occur normally in pkc1Delta and mpk1Delta mutants when starved for nitrogen; pkc1Delta and mpk1Delta mutants die rapidly upon treatment with rapamycin, an inhibitor of the TORs. We find that Mpk1 is transiently activated by rapamycin treatment via a novel mechanism. Finally, we find that rapamycin treatment or nitrogen starvation induces resistance to the cell wall-digesting enzyme zymolyase by a Pkc1-dependent mechanism. Thus, the Pkc1 pathway is not a nutrient sensor but acts downstream of TOR inhibition to maintain cell integrity in quiescence.

Yeast protein kinases and the RHO1 exchange factor TUS1 are novel components of the cell integrity pathway in yeast

The PKC1-associated mitogen-activated protein (MAP) kinase pathway of Saccharomyces cerevisiae regulates cell integrity by controlling the actin cytoskeleton and cell wall synthesis. Activation of PKC1 occurs via the GTPase RHO1 and the kinase pair PKH1 and PKH2. Here we report that YPK1 and YPK2, an essential pair of homologous kinases and proposed downstream effectors of PKH and sphingolipids, are also regulators of the PKC1-controlled MAP kinase cascade. ypk mutants display random distribution of the actin cytoskeleton and severely reduced activation of the MAP kinase MPK1. Upregulation of the RHO1 GTPase switch or the PKC1 effector MAP kinase pathway suppresses the growth and actin defects of ypk cells. ypk lethality is also suppressed by overexpression of an uncharacterized gene termed TUS1. TUS1 is a novel RHO1 exchange factor that contributes to cell wall integrity-mediated modulation of RHO1 activity. Thus, TUS1 and the YPKs add to the growing complexity of RHO1 and PKC1 regulation in the cell integrity signaling pathway. Furthermore, our findings suggest that the YPKs are a missing link between sphingolipid signaling and the cell integrity pathway.

Involvement of GFA1, which encodes glutamine-fructose-6-phosphate amidotransferase, in the activation of the chitin synthesis pathway in response to cell-wall defects in Saccharomyces cerevisiae

Cell-wall damage caused by mutations of cell-wall-related genes triggers a compensatory mechanism which eventually results in hyperaccumulation of chitin reaching 20% of the cell-wall dry mass. We show that activation of chitin synthesis is accompanied by a rise, from 1.3-fold to 3.5-fold according to the gene mutation, in the expression of most of the genes encoding enzymes of the chitin metabolic pathways. Evidence that GFA1, which encodes glutamine-fructose-6-Phosphate amidotransferase (Gfa1p), the first committed enzyme of this pathway, plays a major role in this process was as follows. Activation of chitin synthesis in the cell-wall mutants correlated with activation of GFA1 and with a proportional increase in Gfa1p activity. Overexpression of GFA1 caused an approximately threefold increase in chitin in the transformed cells, whereas chitin content was barely affected by the joint overexpression of CHS3 and CHS7. Introduction of a gfa1-97 allele mutation in the cell-wall-defective gas1Delta mutant or cultivation of this mutant in a hyperosmotic medium resulted in reduction in chitin synthesis that was proportional to the decrease in Gfa1p activity. Finally, the stimulation of chitin production was also accompanied by an increase in pools of fructose 6-Phosphate, a substrate of Gfa1p. In quantitative terms, we estimated the flux-coefficient control of Gfa1p to be in the range of 0.90, and found that regulation of the chitin metabolic pathway was mainly hierarchical, i.e. dominated by regulation of the amount of newly synthesized GFA1 protein. In the search for the mechanism by which GFA1 is activated in response to cell-wall perturbations, we could only show that neither MCM1 nor RLM1, which encode two transcriptional factors of the MADS box family that are required for expression of cell-cycle and cell-wall-related genes, was involved in this process.

Novel protein phosphatases in yeast

During the last decade several novel yeast genes encoding proteins related to the PPP family of Ser/Thr protein phosphatases have been discovered and their functional characterization initiated. Most of these novel phosphatases display intriguing structural features and/or are involved in a number of important functions, such as cell cycle regulation, protein synthesis and maintenance of cellular integrity. While in some cases these genes appear to be restricted to fungi, in others similar proteins can be found in higher eukaryotes. This review will summarize the latest advances in our understanding about how these phosphatases are regulated and fulfil their functions in the yeast cell.

Genetic interactions of yeast eukaryotic translation initiation factor 5A (eIF5A) reveal connections to poly(A)-binding protein and protein kinase C signaling

The highly conserved eukaryotic translation initiation factor eIF5A has been proposed to have various roles in the cell, from translation to mRNA decay to nuclear protein export. To further our understanding of this essential protein, three temperature-sensitive alleles of the yeast TIF51A gene have been characterized. Two mutant eIF5A proteins contain mutations in a proline residue at the junction between the two eIF5A domains and the third, strongest allele encodes a protein with a single mutation in each domain, both of which are required for the growth defect. The stronger tif51A alleles cause defects in degradation of short-lived mRNAs, supporting a role for this protein in mRNA decay. A multicopy suppressor screen revealed six genes, the overexpression of which allows growth of a tif51A-1 strain at high temperature; these genes include PAB1, PKC1, and PKC1 regulators WSC1, WSC2, and WSC3. Further results suggest that eIF5A may also be involved in ribosomal synthesis and the WSC/PKC1 signaling pathway for cell wall integrity or related processes.

Saccharomyces cerevisiae MPT5 and SSD1 function in parallel pathways to promote cell wall integrity

Yeast MPT5 (UTH4) is a limiting component for longevity. We show here that MPT5 also functions to promote cell wall integrity. Loss of Mpt5p results in phenotypes associated with a weakened cell wall, including sorbitol-remedial temperature sensitivity and sensitivities to calcofluor white and sodium dodecyl sulfate. Additionally, we find that mutation of MPT5, in the absence of SSD1-V, is lethal in combination with loss of either Ccr4p or Swi4p. These synthetic lethal interactions are suppressed by the SSD1-V allele. Furthermore, we have provided evidence that the short life span caused by loss of Mpt5p is due to a weakened cell wall. This cell wall defect may be the result of abnormal chitin biosynthesis or accumulation. These analyses have defined three genetic pathways that function in parallel to promote cell integrity: an Mpt5p-containing pathway, an Ssd1p-containing pathway, and a Pkc1p-dependent pathway. This work also provides evidence that post-transcriptional regulation is likely to be important both for maintaining cell integrity and for promoting longevity.

Overactivation of the protein kinase C-signaling pathway suppresses the defects of cells lacking the Rho3/Rho4-GAP Rgd1p in Saccharomyces cerevisiae

The nonessential RGD1 gene encodes a Rho-GTPase activating protein for the Rho3 and Rho4 proteins in Saccharomyces cerevisiae. Previous studies have revealed genetic interactions between RGD1 and the SLG1 and MID2 genes, encoding two putative sensors for cell integrity signaling, and VRP1 encoding an actin and myosin interacting protein involved in polarized growth. To better understand the role of Rgd1p, we isolated multicopy suppressor genes of the cell lethality of the double mutant rgd1Delta mid2Delta. RHO1 and RHO2 encoding two small GTPases, MKK1 encoding one of the MAP-kinase kinases in the protein kinase C (PKC) pathway, and MTL1, a MID2-homolog, were shown to suppress the rgd1Delta defects strengthening the functional links between RGD1 and the cell integrity pathway. Study of the transcriptional activity of Rlm1p, which is under the control of Mpk1p, the last kinase of the PKC pathway, and follow-up of the PST1 transcription, which is positively regulated by Rlm1p, indicate that the lack of RGD1 function diminishes the PKC pathway activity. We hypothesize that the rgd1Delta inactivation, at least through the hyperactivation of the small GTPases Rho3p and Rho4p, alters the secretory pathway and/or the actin cytoskeleton and decreases activity of the PKC pathway.

Comparison of morphogenetic networks of filamentous fungi and yeast

Fungi generally display either of two growth modes, yeast-like or filamentous, whereas dimorphic fungi, upon environmental stimuli, are able to switch between the yeast-like and the filamentous growth mode. Signal transduction pathways have been elucidated in the budding yeast Saccharomyces cerevisiae, establishing a morphogenetic network that links cell-cycle events with cellular morphogenesis. Recent molecular genetic studies in several filamentous fungal model systems revealed key components required for distinct steps from fungal spore germination to the maintenance of polar hyphal growth, mycelium formation, and nuclear division. This allows a mechanistic comparison of yeast-like and hyphal growth and the establishment of a core model morphogenetic network for filamentous growth including signaling via the cAMP pathway, Rho modules, and cell cycle kinases. Appreciating similarities between morphogenetic networks of the unicellular yeasts and the multicellular filamentous fungi will open new research directions, help in isolating the central network components, and ultimately pave the way to elucidate the central differences (of many) that distinguish, e.g., the growth mode of filamentous fungi from that of their yeast-like relatives, the role of cAMP signaling, and nuclear division.

Transcriptional coregulation by the cell integrity mitogen-activated protein kinase Slt2 and the cell cycle regulator Swi4

In Saccharomyces cerevisiae, the heterodimeric transcription factor SBF (for SCB binding factor) is composed of Swi4 and Swi6 and activates gene expression at the G(1)/S-phase transition of the mitotic cell cycle. Cell cycle commitment is associated not only with major alterations in gene expression but also with highly polarized cell growth; the mitogen-activated protein kinase (MAPK) Slt2 is required to maintain cell wall integrity during periods of polarized growth and cell wall stress. We describe experiments aimed at defining the regulatory pathway involving the cell cycle transcription factor SBF and Slt2-MAPK. Gene expression assays and chromatin immunoprecipitation experiments revealed Slt2-dependent recruitment of SBF to the promoters of the G(1) cyclins PCL1 and PCL2 after activation of the Slt2-MAPK pathway. We performed DNA microarray analysis and identified other genes whose expression was reduced in both SLT2 and SWI4 deletion strains. Genes that are sensitive to both Slt2 and Swi4 appear to be uniquely regulated and reveal a role for Swi4, the DNA-binding component of SBF, which is independent of the regulatory subunit Swi6. Some of the Swi4- and Slt2-dependent genes do not require Swi6 for either their expression or for Swi4 localization to their promoters. Consistent with these results, we found a direct interaction between Swi4 and Slt2. Our results establish a new Slt2-dependent mode of Swi4 regulation and suggest roles for Swi4 beyond its prominent role in controlling cell cycle transcription.

Regulation of organelle membrane fusion by Pkc1p

Membrane fusion relies on complex protein machineries, which act in sequence to catalyze the fusion of bilayers. The fusion of endoplasmic reticulum membranes requires the t-SNARE Ufe1p, and the AAA ATPase p97/Cdc48p. While the mechanisms of membrane fusion events have begun to emerge, little is known about how this fusion process is regulated. We provide first evidence that endoplasmic reticulum membrane fusion in yeast is regulated by the action of protein kinase C. Specifically, Pkc1p kinase activity is needed to protect the fusion machinery from ubiquitin-mediated degradation.

The phosphoinositide phosphatase Sac1p controls trafficking of the yeast Chs3p chitin synthase

Phosphoinositide phosphatases play an essential but as yet not well-understood role in lipid-based signal transduction. Members of a subfamily of these enzymes share a specific domain that was first identified in the yeast Sac1 protein [1]. Sac1 homology domains were shown to exhibit 3- and 4-phosphatase activity in vitro [2, 3] and were also found, in addition to rat and yeast Sac1p, in yeast Inp/Sjl proteins [4, 5] and mammalian synaptojanins [6]. Despite the detailed in vitro characterization of the enzymatic properties of yeast Sac1p, the exact cellular function of this protein has remained obscure. We report here that Sac1p has a specific role in secretion and acts as an antagonist of the phosphatidylinositol 4-kinase Pik1p in Golgi trafficking. Elimination of Sac1p leads to excessive forward transport of chitin synthases and thus causes specific cell wall defects. Similar defects in membrane trafficking are caused by the overexpression of PIK1. Taken together, these findings provide strong evidence that the generation of PtdIns(4)P is sufficient to trigger forward transport from the Golgi to the plasma membrane and that Sac1p is critically required for the termination of this signal.

Domain shuffling as a tool for investigation of protein function: substitution of the cysteine-rich region of Raf kinase and PKC eta for that of yeast Pkc1p

With the completion of the sequences of entire genomes, the need for functional characterisation of proteins and their domains is becoming acute. Conserved regions within proteins often share overlapping functions but despite this conservation may fulfil quite different tasks in different species. In this work, we investigated the cysteine-rich motif (C1 domain) of yeast protein kinase C (Pkc1p) as a model to establish a test system for domain function. C1 domains activate kinases through binding of either diacylglycerol and/or phosphatidylserine, as in many members of the protein kinase C (PKC) family, or by binding small GTPases, as in Raf kinase. In contrast to other members of the protein kinase C superfamily, Pkc1p of Saccharomyces cerevisiae is activated via binding of the small G-protein Rho1p to its C1 domain. We developed a system for domain shuffling to establish the function of C1 domains from human Raf kinase and rat PKC eta in yeast. Only the C1 domain from Raf kinase enabled the chimeric enzyme to bind Rho1p when substituted for the native yeast domain. Accordingly, a chimeric Pkc1p carrying the C1 from Raf kinase, but not that from PKC eta, was able to partially complement the phenotypes of a yeast pkc1 deletion mutant. We interpret these data as further evidence that interaction with a small GTPase is the main regulatory function of the C1 domain in yeast.

The arrest of secretion response in yeast: signaling from the secretory path to the nucleus via Wsc proteins and Pkc1p

The arrest of secretion response (ASR) in sec mutants reversibly inhibits nuclear import and relocates nuclear proteins to the cytoplasm. sec mutants also relocate nucleoporins; however, endocytic and Golgi-to-vacuole transport mutants do not cause relocation. The ASR requires Wsc membrane proteins that are trapped along the secretory path, rather than those which are at the plasma membrane. The activity of the downstream kinase, Pkc1p, is also required; however, the Pkc1p MAP kinase cascade is not. sec mutants initiate compensatory transcriptional changes distinct from those of the unfolded protein response.

The eukaryotic response regulator Skn7p regulates calcineurin signaling through stabilization of Crz1p

To survive ionic, pH and pheromone stress, the yeast Saccharomyces cerevisiae activates signaling through the Ca2+-activated phosphatase calcineurin to the transcription factor Crz1p/Tcn1p. We show that the overexpression of SKN7, a response-regulator transcription factor, activates transcription from a calcineurin/Crz1p-dependent response element (CDRE). Ca2+-induced, calcineurin/Crz1p-dependent activation of several genes is reduced in skn7 mutants. Skn7p modulates CDRE-dependent transcription by affecting Crz1p protein levels. Specifically, the rate of Crz1p turnover is increased in skn7 mutants. Calcineurin, but not its phosphatase activity, is required for Skn7p-mediated Crz1p stabilization. Skn7p binds to both calcineurin and Crz1p in vitro, and we suggest that this interaction is required for Skn7p regulation of Crz1p. The DNA-binding and internal coiled-coil domains, but not the response- regulator phosphorylation of Skn7p, are necessary for Crz1p-dependent transcriptional activation and Crz1p stabilization by Skn7 in vivo. The DNA-binding domain of Skn7p is also required for binding to Crz1p and calcineurin in vitro. Thus, we propose that Skn7p protects Crz1p from degradation by binding to it and calcineurin through its DNA-binding domain.