A pair of functionally redundant yeast genes (PPZ1 and PPZ2) encoding type 1-related protein phosphatases function within the PKC1-mediated pathway

When Phosphatases Go Mad: The Molecular Basis for Toxicity of Yeast Ppz1

The fact that overexpression of the yeast Ser/Thr protein phosphatase Ppz1 induces a dramatic halt in cell proliferation was known long ago, but only work in the last few years has provided insight into the molecular basis for this toxicity. Overexpression of Ppz1 causes abundant changes in gene expression and modifies the phosphorylation state of more than 150 proteins, including key signaling protein kinases such as Hog1 or Snf1. Diverse cellular processes are altered: halt in translation, failure to properly adapt to low glucose supply, acidification of the cytosol, or depletion of intracellular potassium content are a few examples. Therefore, the toxicity derived from an excess of Ppz1 appears to be multifactorial, the characteristic cell growth blockage thus arising from the combination of various altered processes. Notably, overexpression of the Ppz1 regulatory subunit Hal3 fully counteracts the toxic effects of the phosphatase, and this process involves intracellular relocation of the phosphatase to internal membranes.

The Toxic Effects of Ppz1 Overexpression Involve Nha1-Mediated Deregulation of K+ and H+ Homeostasis

The alteration of the fine-tuned balance of phospho/dephosphorylation reactions in the cell often results in functional disturbance. In the yeast Saccharomyces cerevisiae, the overexpression of Ser/Thr phosphatase Ppz1 drastically blocks cell proliferation, with a profound change in the transcriptomic and phosphoproteomic profiles. While the deleterious effect on growth likely derives from the alteration of multiple targets, the precise mechanisms are still obscure. Ppz1 is a negative effector of potassium influx. However, we show that the toxic effect of Ppz1 overexpression is unrelated to the Trk1/2 high-affinity potassium importers. Cells overexpressing Ppz1 exhibit decreased K⁺ content, increased cytosolic acidification, and fail to properly acidify the medium. These effects, as well as the growth defect, are counteracted by the deletion of NHA1 gene, which encodes a plasma membrane Na⁺, K⁺/H⁺ antiporter. The beneficial effect of a lack of Nha1 on the growth vanishes as the pH of the medium approaches neutrality, is not eliminated by the expression of two non-functional Nha1 variants (D145N or D177N), and is exacerbated by a hyperactive Nha1 version (S481A). All our results show that high levels of Ppz1 overactivate Nha1, leading to an excessive entry of H⁺ and efflux of K⁺, which is detrimental for growth.

The N-Terminal Region of Yeast Protein Phosphatase Ppz1 Is a Determinant for Its Toxicity

The Ppz enzymes are Ser/Thr protein phosphatases present only in fungi that are characterized by a highly conserved C-terminal catalytic region, related to PP1c phosphatases, and a more divergent N-terminal extension. In Saccharomyces cerevisiae, Ppz phosphatases are encoded by two paralog genes, PPZ1 and PPZ2. Ppz1 is the most toxic protein when overexpressed in budding yeast, halting cell proliferation, and this effect requires its phosphatase activity. We show here that, in spite of their conserved catalytic domain, Ppz2 was not toxic when tested under the same conditions as Ppz1, albeit Ppz2 levels were somewhat lower. Remarkably, a hybrid protein composed of the N-terminal extension of Ppz1 and the catalytic domain of Ppz2 was as toxic as Ppz1, even if its expression level was comparable to that of Ppz2. Similar amounts of yeast PP1c (Glc7) produced an intermediate effect on growth. Mutation of the Ppz1 myristoylable Gly2 to Ala avoided the localization of the phosphatase at the cell periphery but only slightly attenuated its toxicity. Therefore, the N-terminal extension of Ppz1 plays a key role in defining Ppz1 toxicity. This region is predicted to be intrinsically disordered and contains several putative folding-upon-binding regions which are absent in Ppz2 and might be relevant for toxicity.

Yeast Ppz1 protein phosphatase toxicity involves the alteration of multiple cellular targets

Control of the protein phosphorylation status is a major mechanism for regulation of cellular processes, and its alteration often lead to functional disorders. Ppz1, a protein phosphatase only found in fungi, is the most toxic protein when overexpressed in Saccharomyces cerevisiae. To investigate the molecular basis of this phenomenon, we carried out combined genome-wide transcriptomic and phosphoproteomic analyses. We have found that Ppz1 overexpression causes major changes in gene expression, affecting ~ 20% of the genome, together with oxidative stress and increase in total adenylate pools. Concurrently, we observe changes in the phosphorylation pattern of near 400 proteins (mainly dephosphorylated), including many proteins involved in mitotic cell cycle and bud emergence, rapid dephosphorylation of Snf1 and its downstream transcription factor Mig1, and phosphorylation of Hog1 and its downstream transcription factor Sko1. Deletion of HOG1 attenuates the growth defect of Ppz1-overexpressing cells, while that of SKO1 aggravates it. Our results demonstrate that Ppz1 overexpression has a widespread impact in the yeast cells and reveals new aspects of the regulation of the cell cycle.

Protein Phosphatase Ppz1 Is Not Regulated by a Hal3-Like Protein in Plant Pathogen Ustilago maydis

Ppz enzymes are type-1 related Ser/Thr protein phosphatases that are restricted to fungi. In S. cerevisiae and other fungi, Ppz1 is involved in cation homeostasis and is regulated by two structurally-related inhibitory subunits, Hal3 and Vhs3, with Hal3 being the most physiologically relevant. Remarkably, Hal3 and Vhs3 have moonlighting properties, as they participate in an atypical heterotrimeric phosphopantothenoyl cysteine decarboxylase (PPCDC), a key enzyme for Coenzyme A biosynthesis. Here we identify and functionally characterize Ppz1 phosphatase (UmPpz1) and its presumed regulatory subunit (UmHal3) in the plant pathogen fungus Ustilago maydis. UmPpz1 is not an essential protein in U. maydis and, although possibly related to the cell wall integrity pathway, is not involved in monovalent cation homeostasis. The expression of UmPpz1 in S. cerevisiae Ppz1-deficient cells partially mimics the functions of the endogenous enzyme. In contrast to what was found in C. albicans and A. fumigatus, UmPpz1 is not a virulence determinant. UmHal3, an unusually large protein, is the only functional PPCDC in U. maydis and, therefore, an essential protein. However, when overexpressed in U. maydis or S. cerevisiae, UmHal3 does not reproduce Ppz1-inhibitory phenotypes. Indeed, UmHal3 does not inhibit UmPpz1 in vitro (although ScHal3 does). Therefore, UmHal3 might not be a moonlighting protein.

Ser/Thr protein phosphatases in fungi: structure, regulation and function

Reversible phospho-dephosphorylation of proteins is a major mechanism for the control of cellular functions. By large, Ser and Thr are the most frequently residues phosphorylated in eukar-yotes. Removal of phosphate from these amino acids is catalyzed by a large family of well-conserved enzymes, collectively called Ser/Thr protein phosphatases. The activity of these enzymes has an enormous impact on cellular functioning. In this work we pre-sent the members of this family in S. cerevisiae and other fungal species, and review the most recent findings concerning their regu-lation and the roles they play in the most diverse aspects of cell biology.

Proteomic analysis of protein phosphatase Z1 from Candida albicans

Protein phosphatase Z is a "novel type" fungus specific serine/threonine protein phosphatase. Previously our research group identified the CaPPZ1 gene in the opportunistic pathogen Candida albicans and reported that the gene deletion had several important physiological consequences. In order to reveal the protein targets and the associated mechanisms behind the functions of the phosphatase a proteomic method was adopted for the comparison of the cappz1 deletion mutant and the genetically matching QMY23 control strain. Proteins extracted from the control and deletion mutant strains were separated by two-dimensional gel electrophoresis and the protein spots were stained with RuBPS and Pro-Q Diamond in order to visualize the total proteome and the phosphoproteome, respectively. The alterations in spot intensities were determined by densitometry and were analysed with the Delta2D (Decodon) software. Spots showing significantly different intensities between the mutant and control strains were excised from the gels and were digested with trypsin. The resulting peptides were identified by LC-MS/MS mass spectrometry. As many as 15 protein spots were found that exhibited significant changes in their intensity upon the deletion of the phosphatase and 20 phosphoproteins were identified in which the level of phosphorylation was modified significantly in the mutant. In agreement with previous findings we found that the affected proteins function in protein synthesis, oxidative stress response, regulation of morphology and metabolism. Among these proteins we identified two potential CaPpz1 substrates (Eft2 and Rpp0) that may regulate the elongation step of translation. RT-qPCR experiments revealed that the expression of the genes coding for the affected proteins was not altered significantly. Thus, the absence of CaPpz1 exerted its effects via protein synthesis/degradation and phosphorylation/dephosphorylation. In addition, our proteomics data strongly suggested a role for CaPpz1 in biofilm formation, was confirmed experimentally. Thus our unbiased proteomic approach lead to the discovery of a novel function for this phosphatase in C. albicans.

The inhibitory mechanism of Hal3 on the yeast Ppz1 phosphatase: A mutagenesis analysis

The Ser/Thr protein phosphatase (PPase) Ppz1 is an enzyme related to the ubiquitous type-1 PPases (PP1c) but found only in fungi. It is regulated by an inhibitory subunit, Hal3, which binds to its catalytic domain. Overexpression of Ppz1 is highly toxic for yeast cells, so its de-regulation has been proposed as a target for novel antifungal therapies. While modulation of PP1c by its many regulatory subunits has been extensively characterized, the manner by which Hal3 controls Ppz1 remains unknown. We have used error-prone PCR mutagenesis to construct a library of Ppz1 variants and developed a functional assay to identify mutations affecting the binding or/and the inhibitory capacity of Hal3. We have characterized diverse Ppz1 mutated versions in vivo and in vitro and found that, although they were clearly refractory to Hal3 inhibition, none of them exhibited significant reduction in Hal3 binding. Mapping the mutations strengthened the notion that Hal3 does not interact with Ppz1 through its RVxF-like motif (found in most PP1c regulators). In contrast, the most relevant mutations mapped to a conserved α-helix region used by mammalian Inhibitor-2 to regulate PP1c. Therefore, modulation of PP1c and Ppz1 by their subunits likely differs, but could share some structural features.

PP2A Controls Genome Integrity by Integrating Nutrient-Sensing and Metabolic Pathways with the DNA Damage Response

Mec1^ATR mediates the DNA damage response (DDR), integrating chromosomal signals and mechanical stimuli. We show that the PP2A phosphatases, ceramide-activated enzymes, couple cell metabolism with the DDR. Using genomic screens, metabolic analysis, and genetic and pharmacological studies, we found that PP2A attenuates the DDR and that three metabolic circuits influence the DDR by modulating PP2A activity. Irc21, a putative cytochrome b5 reductase that promotes the condensation reaction generating dihydroceramides (DHCs), and Ppm1, a PP2A methyltransferase, counteract the DDR by activating PP2A; conversely, the nutrient-sensing TORC1-Tap42 axis sustains DDR activation by inhibiting PP2A. Loss-of-function mutations in IRC21, PPM1, and PP2A and hyperactive tap42 alleles rescue mec1 mutants. Ceramides synergize with rapamycin, a TORC1 inhibitor, in counteracting the DDR. Hence, PP2A integrates nutrient-sensing and metabolic pathways to attenuate the Mec1^ATR response. Our observations imply that metabolic changes affect genome integrity and may help with exploiting therapeutic options and repositioning known drugs.

Zds1/Zds2-PP2ACdc55 complex specifies signaling output from Rho1 GTPase

Zds1/Zds2–PP2A^Cdc55 forms a complex with Rho1 GTPase and specifies Rho1 signaling outcome by regulating Rho1 GAPs in budding yeast.

Budding yeast Rho1 guanosine triphosphatase (GTPase) plays an essential role in polarized cell growth by regulating cell wall glucan synthesis and actin organization. Upon cell wall damage, Rho1 blocks polarized cell growth and repairs the wounds by activating the cell wall integrity (CWI) Pkc1–mitogen-activated protein kinase (MAPK) pathway. A fundamental question is how active Rho1 promotes distinct signaling outputs under different conditions. Here we identified the Zds1/Zds2–protein phosphatase 2A^Cdc55 (PP2A^Cdc55) complex as a novel Rho1 effector that regulates Rho1 signaling specificity. Zds1/Zds2–PP2A^Cdc55 promotes polarized growth and cell wall synthesis by inhibiting Rho1 GTPase-activating protein (GAP) Lrg1 but inhibits CWI pathway by stabilizing another Rho1 GAP, Sac7, suggesting that active Rho1 is biased toward cell growth over stress response. Conversely, upon cell wall damage, Pkc1–Mpk1 activity inhibits cortical PP2A^Cdc55, ensuring that Rho1 preferentially activates the CWI pathway for cell wall repair. We propose that PP2A^Cdc55 specifies Rho1 signaling output and that reciprocal antagonism between Rho1–PP2A^Cdc55 and Rho1–Pkc1 explains how only one signaling pathway is robustly activated at a time.

Wide-Ranging Effects of the Yeast Ptc1 Protein Phosphatase Acting Through the MAPK Kinase Mkk1

The Saccharomyces cerevisiae type 2C protein phosphatase Ptc1 is required for a wide variety of cellular functions, although only a few cellular targets have been identified. A genetic screen in search of mutations in protein kinase-encoding genes able to suppress multiple phenotypic traits caused by the ptc1 deletion yielded a single gene, MKK1, coding for a MAPK kinase (MAPKK) known to activate the cell-wall integrity (CWI) Slt2 MAPK. In contrast, mutation of the MKK1 paralog, MKK2, had a less significant effect. Deletion of MKK1 abolished the increased phosphorylation of Slt2 induced by the absence of Ptc1 both under basal and CWI pathway stimulatory conditions. We demonstrate that Ptc1 acts at the level of the MAPKKs of the CWI pathway, but only the Mkk1 kinase activity is essential for ptc1 mutants to display high Slt2 activation. We also show that Ptc1 is able to dephosphorylate Mkk1 in vitro. Our results reveal the preeminent role of Mkk1 in signaling through the CWI pathway and strongly suggest that hyperactivation of Slt2 caused by upregulation of Mkk1 is at the basis of most of the phenotypic defects associated with lack of Ptc1 function.

Bck2 acts through the MADS box protein Mcm1 to activate cell-cycle-regulated genes in budding yeast

The Bck2 protein is a potent genetic regulator of cell-cycle-dependent gene expression in budding yeast. To date, most experiments have focused on assessing a potential role for Bck2 in activation of the G1/S-specific transcription factors SBF (Swi4, Swi6) and MBF (Mbp1, Swi6), yet the mechanism of gene activation by Bck2 has remained obscure. We performed a yeast two-hybrid screen using a truncated version of Bck2 and discovered six novel Bck2-binding partners including Mcm1, an essential protein that binds to and activates M/G1 promoters through Early Cell cycle Box (ECB) elements as well as to G2/M promoters. At M/G1 promoters Mcm1 is inhibited by association with two repressors, Yox1 or Yhp1, and gene activation ensues once repression is relieved by an unknown activating signal. Here, we show that Bck2 interacts physically with Mcm1 to activate genes during G1 phase. We used chromatin immunoprecipitation (ChIP) experiments to show that Bck2 localizes to the promoters of M/G1-specific genes, in a manner dependent on functional ECB elements, as well as to the promoters of G1/S and G2/M genes. The Bck2-Mcm1 interaction requires valine 69 on Mcm1, a residue known to be required for interaction with Yox1. Overexpression of BCK2 decreases Yox1 localization to the early G1-specific CLN3 promoter and rescues the lethality caused by overexpression of YOX1. Our data suggest that Yox1 and Bck2 may compete for access to the Mcm1-ECB scaffold to ensure appropriate activation of the initial suite of genes required for cell cycle commitment.

Cell-cycle perturbations suppress the slow-growth defect of spt10Δ mutants in Saccharomyces cerevisiae

Spt10 is a putative acetyltransferase of Saccharomyces cerevisiae that directly activates the transcription of histone genes. Deletion of SPT10 causes a severe slow growth phenotype, showing that Spt10 is critical for normal cell division. To gain insight into the function of Spt10, we identified mutations that impair or improve the growth of spt10 null (spt10Δ) mutants. Mutations that cause lethality in combination with spt10Δ include particular components of the SAGA complex as well as asf1Δ and hir1Δ. Partial suppressors of the spt10Δ growth defect include mutations that perturb cell-cycle progression through the G1/S transition, S phase, and G2/M. Consistent with these results, slowing of cell-cycle progression by treatment with hydroxyurea or growth on medium containing glycerol as the carbon source also partially suppresses the spt10Δ slow-growth defect. In addition, mutations that impair the Lsm1-7-Pat1 complex, which regulates decapping of polyadenylated mRNAs, also partially suppress the spt10Δ growth defect. Interestingly, suppression of the spt10Δ growth defect is not accompanied by a restoration of normal histone mRNA levels. These findings suggest that Spt10 has multiple roles during cell division.

Rho signaling participates in membrane fluidity homeostasis

Preservation of both the integrity and fluidity of biological membranes is a critical cellular homeostatic function. Signaling pathways that govern lipid bilayer fluidity have long been known in bacteria, yet no such pathways have been identified in eukaryotes. Here we identify mutants of the yeast Saccharomyces cerevisiae whose growth is differentially influenced by its two principal unsaturated fatty acids, oleic and palmitoleic acid. Strains deficient in the core components of the cell wall integrity (CWI) pathway, a MAP kinase pathway dependent on both Pkc1 (yeast's sole protein kinase C) and Rho1 (the yeast RhoA-like small GTPase), were among those inhibited by palmitoleate yet stimulated by oleate. A single GEF (Tus1) and a single GAP (Sac7) of Rho1 were also identified, neither of which participate in the CWI pathway. In contrast, key components of the CWI pathway, such as Rom2, Bem2 and Rlm1, failed to influence fatty acid sensitivity. The differential influence of palmitoleate and oleate on growth of key mutants correlated with changes in membrane fluidity measured by fluorescence anisotropy of TMA-DPH, a plasma membrane-bound dye. This work provides the first evidence for the existence of a signaling pathway that enables eukaryotic cells to control membrane fluidity, a requirement for division, differentiation and environmental adaptation.

Conserved Ser/Arg-rich motif in PPZ orthologs from fungi is important for its role in cation tolerance

PPZ1 orthologs, novel members of a phosphoprotein phosphatase family of phosphatases, are found only in fungi. They regulate diverse physiological processes in fungi e.g. ion homeostasis, cell size, cell integrity, etc. Although they are an important determinant of salt tolerance in fungi, their physiological role remained unexplored in any halotolerant species. In this context we report here molecular and functional characterization of DhPPZ1 from Debaryomyces hansenii, which is one of the most halotolerant and osmotolerant species of yeast. Our results showed that DhPPZ1 knock-out strain displayed higher tolerance to toxic cations, and unlike in Saccharomyces cerevisiae, Na(+)/H(+) antiporter appeared to have an important role in this process. Besides salt tolerance, DhPPZ1 also had role in cell wall integrity and growth in D. hansenii. We have also identified a short, serine-arginine-rich sequence motif in DhPpz1p that is essential for its role in salt tolerance but not in other physiological processes. Taken together, these results underscore a distinct role of DhPpz1p in D. hansenii and illustrate an example of how organisms utilize the same molecular tool box differently to garner adaptive fitness for their respective ecological niches.

Cbk1 kinase and Bck2 control MAP kinase activation and inactivation during heat shock

Cbk1 kinase was previously implicated in regulating polarized morphogenesis, gene expression, and cell integrity. This study reveals that Cbk1 regulates heat shock signaling and stress adaptation by modulating Mpk1 activity and MAPK phosphatase localization. A model for Cbk1 and its putative substrate for these functions is presented.

Saccharomyces cerevisiae Cbk1 kinase is a LATS/NDR tumor suppressor orthologue and component of the Regulation of Ace2 and Morphogenesis signaling network. Cbk1 was previously implicated in regulating polarized morphogenesis, gene expression, and cell integrity. Here we establish that Cbk1 is critical for heat shock and cell wall stress signaling via Bck2, a protein associated with the Pkc1-Mpk1 cell integrity pathway. We demonstrate that cbk1 and bck2 loss-of-function mutations prevent Mpk1 kinase activation and Mpk1-dependent gene expression but do not disrupt Mpk1 Thr-190/Tyr-192 phosphorylation. Bck2 overexpression partially restores Mpk1-dependent Rlm1 transcription factor activity in cbk1 mutants, suggesting that Bck2 functions downstream of Cbk1. We demonstrate that Bck2 precisely colocalizes with the mitogen-activated protein kinase (MAPK) phosphatase Sdp1. During heat shock, Bck2 and Sdp1 transiently redistribute from nuclei and the cytosol to mitochondria and other cytoplasmic puncta before returning to their pre-stressed localization patterns. Significantly, Cbk1 inhibition delays the return of Bck2 and Sdp1 to their pre-stressed localization patterns and delays Mpk1 Thr-190/Tyr-192 dephosphorylation upon heat shock adaptation. We conclude that Cbk1 and Bck2 are required for Mpk1 activation during heat shock and cell wall stress and for Mpk1 dephosphorylation during heat shock adaptation. These data provide the first evidence that Cbk1 kinase regulates MAPK-dependent stress signaling and provide mechanistic insight into Sdp1 phosphatase regulation.

Alkali metal cation transport and homeostasis in yeasts

The maintenance of appropriate intracellular concentrations of alkali metal cations, principally K(+) and Na(+), is of utmost importance for living cells, since they determine cell volume, intracellular pH, and potential across the plasma membrane, among other important cellular parameters. Yeasts have developed a number of strategies to adapt to large variations in the concentrations of these cations in the environment, basically by controlling transport processes. Plasma membrane high-affinity K(+) transporters allow intracellular accumulation of this cation even when it is scarce in the environment. Exposure to high concentrations of Na(+) can be tolerated due to the existence of an Na(+), K(+)-ATPase and an Na(+), K(+)/H(+)-antiporter, which contribute to the potassium balance as well. Cations can also be sequestered through various antiporters into intracellular organelles, such as the vacuole. Although some uncertainties still persist, the nature of the major structural components responsible for alkali metal cation fluxes across yeast membranes has been defined within the last 20 years. In contrast, the regulatory components and their interactions are, in many cases, still unclear. Conserved signaling pathways (e.g., calcineurin and HOG) are known to participate in the regulation of influx and efflux processes at the plasma membrane level, even though the molecular details are obscure. Similarly, very little is known about the regulation of organellar transport and homeostasis of alkali metal cations. The aim of this review is to provide a comprehensive and up-to-date vision of the mechanisms responsible for alkali metal cation transport and their regulation in the model yeast Saccharomyces cerevisiae and to establish, when possible, comparisons with other yeasts and higher plants.

Moonlighting proteins Hal3 and Vhs3 form a heteromeric PPCDC with Ykl088w in yeast CoA biosynthesis

Unlike most other organisms, the essential five-step coenzyme A biosynthetic pathway has not been fully resolved in yeast. Specifically, the genes encoding the phosphopantothenoylcysteine decarboxylase (PPCDC) activity still remain unidentified. Sequence homology analyses suggest three candidates-Ykl088w, Hal3 and Vhs3-as putative PPCDC enzymes in Saccharomyces cerevisiae. Notably, Hal3 and Vhs3 have been characterized as negative regulatory subunits of the Ppz1 protein phosphatase. Here we show that YKL088w does not encode a third Ppz1 regulatory subunit, and that the essential roles of Ykl088w and the Hal3 and Vhs3 pair are complementary, cannot be interchanged and can be attributed to PPCDC-related functions. We demonstrate that while known eukaryotic PPCDCs are homotrimers, the active yeast enzyme is a heterotrimer that consists of Ykl088w and Hal3/Vhs3 monomers that separately provides two essential catalytic residues. Our results unveil Hal3 and Vhs3 as moonlighting proteins involved in both CoA biosynthesis and protein phosphatase regulation.

Regulation of the cell cycle by protein phosphatase 2A in Saccharomyces cerevisiae

Protein phosphatase 2A (PP2A) has long been implicated in cell cycle regulation in many different organisms. In the yeast Saccharomyces cerevisiae, PP2A controls cell cycle progression mainly through modulation of cyclin-dependent kinase (CDK) at the G(2)/M transition. However, CDK does not appear to be a direct target of PP2A. PP2A affects CDK activity through its roles in checkpoint controls. Inactivation of PP2A downregulates CDK by activating the morphogenesis checkpoint and, consequently, delays mitotic entry. Defects in PP2A also compromise the spindle checkpoint and predispose the cell to an error-prone mitotic exit. In addition, PP2A is involved in controlling the G(1)/S transition and cytokinesis. These findings suggest that PP2A functions in many stages of the cell cycle and its effect on cell cycle progression is pleiotropic.

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.

Response of the Saccharomyces cerevisiae Mpk1 mitogen-activated protein kinase pathway to increases in internal turgor pressure caused by loss of Ppz protein phosphatases

The Mpk1 pathway of Saccharomyces cerevisiae is a key determinant of cell wall integrity. A genetic link between the Mpk1 kinase and the Ppz phosphatases has been reported, but the nature of this connection was unclear. Recently, the Ppz phosphatases were shown to be regulators of K+ and pH homeostasis. Here, we demonstrate that Ppz-deficient strains display increased steady-state K+ levels and sensitivity to increased KCl concentrations. Given these observations and the fact that K+ is the major determinant of intracellular turgor pressure, we reasoned that the connection between PPZ1 and -2 and MPK1 was due to the combination of increased internal turgor pressure in Ppz-deficient strains and cell wall instability observed in strains lacking MPK1. Accordingly, the MPK1 gene was up-regulated, the Mpk1 protein was overexpressed, and the phosphorylated active form was more abundant in Ppz-deficient strains. Moreover, the expression of genes previously identified as targets of the Mpk1 pathway are also up-regulated in strains lacking PPZ1 and -2. The transcriptional and posttranslational modifications of Mpk1 were not observed when the internal K+ concentration (and thus turgor pressure) was lowered by disrupting the TRK1 and -2 K+ transporter genes or when the cell wall was stabilized by the addition of sorbitol. Moreover, we present genetic evidence showing that both the Wsc1 and Mid2 branches of the Mpk1 pathway contribute to this response. Finally, despite its role in G1/S transition, increased levels of activated Mpk1 do not appear to be responsible for the cell cycle phenotype observed in Ppz-deficient strains.

Regulation of ENA1 Na(+)-ATPase gene expression by the Ppz1 protein phosphatase is mediated by the calcineurin pathway

Saccharomyces cerevisiae strains lacking the Ppz1 protein phosphatase are salt tolerant and display increased expression of the ENA1 Na(+)-ATPase gene, a major determinant for sodium extrusion, while cells devoid of the similar Ppz2 protein do not show these phenotypes. However, a ppz1 ppz2 mutant displays higher levels of ENA1 expression than the ppz1 strain. We show here that the increased activity of the ENA1 promoter in a ppz1 ppz2 mutant maps to two regions: one region located at -751 to -667, containing a calcineurin-dependent response element (CDRE), and one downstream region (-573 to -490) whose activity responds to intracellular alkalinization. In contrast, the increased ENA1 expression in a ppz1 mutant is mediated solely by an intact calcineurin/Crz1 signaling pathway, on the basis that (i) this effect maps to a single region that contains the CDRE and (ii) it is blocked by the calcineurin inhibitor FK506, as well as by deletion of the CNB1 or CRZ1 gene. The calcineurin dependence of the increased ENA1 expression of a ppz1 mutant would suggest that Ppz1 could negatively regulate calcineurin activity. In agreement with this notion, a ppz1 strain is calcium sensitive, and this mutation does not result in a decrease in the calcium hypertolerance of a cnb1 mutant. It has been shown that ENA1 can be induced by alkalinization of the medium and that a ppz1 ppz2 strain has a higher intracellular pH. However, we present several lines of evidence that show that the gene expression profile of a ppz1 mutant does not involve an alkalinization effect. In conclusion, we have identified a novel role for calcineurin, but not alkalinization, in the control of ENA1 expression in ppz1 mutants.

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.

The Ppz protein phosphatases are key regulators of K+ and pH homeostasis: implications for salt tolerance, cell wall integrity and cell cycle progression

The yeast Ppz protein phosphatases and the Hal3p inhibitory subunit are important determinants of salt tolerance, cell wall integrity and cell cycle progression. We present several lines of evidence showing that these disparate phenotypes are connected by the fact that Ppz regulates K+ transport. First, salt tolerance, cell wall integrity and cell cycle phenotypes of Ppz mutants are dependent on the Trk K+ transporters. Secondly, Ppz mutants exhibit altered activity of the Trk system, as measured by rubidium uptake. Thirdly, Ppz mutants exhibit altered intracellular K+ and pH, as expected from H+ efflux providing electrical balance during K+ uptake. Our unifying picture of Ppz phenotypes contends that activation of Trk by decreased Ppz activity results in plasma membrane depolarization (reducing uptake of toxic cations), increased intracellular K+ and turgor (compromising cell integrity), and increased intracellular pH (augmenting the expression of pH-regulated genes and facilitating alpha-factor recovery). In addition to providing a coherent explanation for all Ppz-dependent phenotypes, our results provide evidence for a causal relationship between intracellular cation homeostasis and a potential cell cycle checkpoint.

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.

The yeast ser/thr phosphatases sit4 and ppz1 play opposite roles in regulation of the cell cycle

Yeast cells overexpressing the Ser/Thr protein phosphatase Ppz1 display a slow-growth phenotype. These cells recover slowly from alpha-factor or nutrient depletion-induced G1 arrest, showing a considerable delay in bud emergence as well as in the expression of the G1 cyclins Cln2 and Clb5. Therefore, an excess of the Ppz1 phosphatase interferes with the normal transition from G1 to S phase. The growth defect is rescued by overexpression of the HAL3/SIS2 gene, encoding a negative regulator of Ppz1. High-copy-number expression of HAL3/SIS2 has been reported to improve cell growth and to increase expression of G1 cyclins in sit4 phosphatase mutants. We show here that the described effects of HAL3/SIS2 on sit4 mutants are fully mediated by the Ppz1 phosphatase. The growth defect caused by overexpression of PPZ1 is intensified in strains with low G1 cyclin levels (such as bck2Delta or cln3Delta mutants), whereas mutation of PPZ1 rescues the synthetic lethal phenotype of sit4 cln3 mutants. These results reveal a role for Ppz1 as a regulatory component of the yeast cell cycle, reinforce the notion that Hal3/Sis2 serves as a negative modulator of the biological functions of Ppz1, and indicate that the Sit4 and Ppz1 Ser/Thr phosphatases play opposite roles in control of the G1/S transition.

SWM1, a developmentally regulated gene, is required for spore wall assembly in Saccharomyces cerevisiae

Meiosis in Saccharomyces cerevisiae is followed by encapsulation of haploid nuclei within multilayered spore walls. Formation of this spore-specific wall requires the coordinated activity of enzymes involved in the biosynthesis of its components. Completion of late events in the sporulation program, leading to spore wall formation, requires the SWM1 gene. SWM1 is expressed at low levels during vegetative growth but its transcription is strongly induced under sporulating conditions, with kinetics similar to those of middle sporulation-specific genes. Homozygous swm1Delta diploids proceed normally through both meiotic divisions but fail to produce mature asci. Consistent with this finding, swm1Delta mutant asci display enhanced sensitivity to enzymatic digestion and heat shock. Deletion of SWM1 specifically affects the expression of mid-late and late sporulation-specific genes. All of the phenotypes observed are similar to those found for the deletion of SPS1 or SMK1, two putative components of a sporulation-specific MAP kinase cascade. However, epistasis analyses indicate that Swm1p does not form part of the Sps1p-Smk1p-MAP kinase pathway. We propose that Swm1p, a nuclear protein, would participate in a different signal transduction pathway that is also required for the coordination of the biochemical and morphological events occurring during the last phase of the sporulation program.

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.

The yeast histidine protein kinase, Sln1p, mediates phosphotransfer to two response regulators, Ssk1p and Skn7p

The Saccharomyces cerevisiae Sln1 protein is a 'two-component' regulator involved in osmotolerance. Two-component regulators are a family of signal-transduction molecules with histidine kinase activity common in prokaryotes and recently identified in eukaryotes. Phosphorylation of Sln1p inhibits the HOG1 MAP kinase osmosensing pathway via a phosphorelay mechanism including Ypd1p and the response regulator, Ssk1p. SLN1 also activates an MCM1-dependent reporter gene, P-lacZ, but this function is independent of Ssk1p. We present genetic and biochemical evidence that Skn7p is the response regulator for this alternative Sln1p signaling pathway. Thus, the yeast Sln1 phosphorelay is actually more complex than appreciated previously; the Sln1 kinase and Ypd1 phosphorelay intermediate regulate the activity of two distinct response regulators, Ssk1p and Skn7p. The established role of Skn7p in oxidative stress is independent of the conserved receiver domain aspartate, D427. In contrast, we show that Sln1p activation of Skn7p requires phosphorylation of D427. The expression of TRX2, previously shown to exhibit Skn7p-dependent oxidative-stress activation, is also regulated by the SLN1 phosphorelay functions of Skn7p. The identification of genes responsive to both classes of Skn7p function suggests a central role for Skn7p and the SLN1-SKN7 pathway in integrating and coordinating cellular response to various types of environmental stress.

The yeast halotolerance determinant Hal3p is an inhibitory subunit of the Ppz1p Ser/Thr protein phosphatase

Components of cellular stress responses can be identified by correlating changes in stress tolerance with gain or loss of function of defined genes. Previous work has shown that yeast cells deficient in Ppz1 protein phosphatase or overexpressing Hal3p, a novel regulatory protein of unknown function, exhibit increased resistance to sodium and lithium, whereas cells lacking Hal3p display increased sensitivity. These effects are largely a result of changes in expression of ENA1, encoding the major cation extrusion pump of yeast cells. Disruption or overexpression of HAL3 (also known as SIS2) has no effect on salt tolerance in the absence of PPZ1, suggesting that Hal3p might function upstream of Ppz1p in a novel signal transduction pathway. Hal3p is recovered from crude yeast homogenates by using immobilized, bacterially expressed Ppz1p fused to glutathione S-transferase, and it also copurifies with affinity-purified glutathione S-transferase-Ppz1p from yeast extracts. In both cases, the interaction is stronger when only the carboxyl-terminal catalytic phosphatase domain of Ppz1p is expressed. In vitro experiments reveal that the protein phosphatase activity of Ppz1p is inhibited by Hal3p. Overexpression of Hal3p suppresses the reduced growth rate because of the overexpression of Ppz1p and aggravates the lytic phenotype of a slt2/mpk1 mitogen-activated protein kinase mutant (thus mimicking the deletion of PPZ1). Therefore, Hal3p might modulate diverse physiological functions of the Ppz1 phosphatase, such as salt stress tolerance and cell cycle progression, by acting as a inhibitory subunit.

Identification and analysis of Mot3, a zinc finger protein that binds to the retrotransposon Ty long terminal repeat (delta) in Saccharomyces cerevisiae

Spt3 and Mot1 are two transcription factors of Saccharomyces cerevisiae that are thought to act in a related fashion to control the function of TATA-binding protein (TBP). Current models suggest that while Spt3 and Mot1 do not directly interact, they do function in a related fashion to stabilize the TBP-TATA interaction at particular promoters. Consistent with this model, certain combinations of spt3 and mot1 mutations are inviable. To identify additional proteins related to Spt3 and Mot1 functions, we screened for high-copy-number suppressors of the mot1 spt3 inviability. This screen identified a previously unstudied gene, MOT3, that encodes a zinc finger protein. We show that Mot3 binds in vitro to three sites within the retrotransposon Ty long terminal repeat (delta) sequence. One of these sites is immediately 5' of the delta TATA region. Although a mot3 null mutation causes no strong phenotypes, it does cause some mild phenotypes, including a very modest increase in Ty mRNA levels, partial suppression of transcriptional defects caused by a mot1 mutation, and partial suppression of an spt3 mutation. These results, in conjunction with those of an independent study of Mot3 (A. Grishin, M. Rothenberg, M. A. Downs, and K. J. Blumer, Genetics, in press), suggest that this protein plays a varied role in gene expression that may be largely redundant with other factors.

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.

The Cln3 cyclin is down-regulated by translational repression and degradation during the G1 arrest caused by nitrogen deprivation in budding yeast

Nutrients are among the most important trophic factors in all organisms. When deprived of essential nutrients, yeast cells use accumulated reserves to complete the current cycle and arrest in the following G1 phase. We show here that the Cln3 cyclin, which has a key role in the timely activation of SBF (Swi4-Swi6)- and MBF (Mbp1-Swi6)-dependent promoters in late G1, is down-regulated rapidly at a post-transcriptional level in cells deprived of the nitrogen source. In addition to the fact that Cln3 is degraded faster by ubiquitin-dependent mechanisms, we have found that translation of the CLN3 mRNA is repressed approximately 8-fold under nitrogen deprivation conditions. As a consequence, both SBF- and MBF-dependent expression is strongly down-regulated. Mainly because of their transcriptional dependence on SBF, and perhaps with the contribution of similar post-transcriptional mechanisms to those found for Cln3, the G1 cyclins Cln1 and 2 become undetectable in starved cells. The complete loss of Cln cyclins and the sustained presence of the Clb-cyclin kinase inhibitor Sic1 in starved cells may provide the molecular basis for the G1 arrest caused by nitrogen deprivation.

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.

DBF2, a cell cycle-regulated protein kinase, is physically and functionally associated with the CCR4 transcriptional regulatory complex

CCR4, a general transcriptional regulator affecting the expression of a number of genes in yeast, forms a multi-subunit complex in vivo. Using the yeast two-hybrid screen, we have identified DBF2, a cell cycle-regulated protein kinase, as a CCR4-associated protein. DBF2 is required for cell cycle progression at the telophase to G1 cell cycle transition. DBF2 co-immunoprecipitated with CCR4 and CAF1/POP2, a CCR4-associated factor, and co-purified with the CCR4 complex. Moreover, a dbf2 disruption resulted in phenotypes and transcriptional defects similar to those observed in strains deficient for CCR4 or CAF1. ccr4 and caf1 mutations, on the other hand, were found to affect cell cycle progression in a manner similar to that observed for dbf2 defects. These data indicate that DBF2 is involved in the control of gene expression and suggest that the CCR4 complex regulates transcription during the late mitotic part of the cell cycle.

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.

BRO1, a novel gene that interacts with components of the Pkc1p-mitogen-activated protein kinase pathway in Saccharomyces cerevisiae

Yeast cells with mutations in BRO1 display phenotypes similar to those caused by deletion of BCK1, a gene encoding a MEK kinase that functions in a mitogen-activated protein kinase pathway mediating maintenance of cell integrity. bro1 cells exhibit a temperature-sensitive growth defect that is suppressed by the addition of osmotic stabilizers or Ca2+ to the growth medium or by additional copies of the BCK1 gene. At permissive temperatures, bro1 mutants are sensitive to caffeine and respond abnormally to nutrient limitation. A null mutation in BRO1 is synthetically lethal with null mutations in BCK1, MPK1, which encodes a mitogen-activated protein kinase that functions downstream of Bck1p, or PKC1, a gene encoding a protein kinase C homolog that activates Bck1p. Analysis of the isolated BRO1 gene revealed that it encodes a novel, 97-kDa polypeptide which contains a putative SH3 domain-binding motif and is homologous to a protein of unknown function in Caenorhabditis elegans.

Yeast RLM1 encodes a serum response factor-like protein that may function downstream of the Mpk1 (Slt2) mitogen-activated protein kinase pathway

The MPK1 (SLT2) gene of Saccharomyces cerevisiae encodes a mitogen-activated protein kinase that is regulated by a kinase cascade whose known elements are Pkc1 (a homolog of protein kinase C), Bck1 (Slk1) (a homolog of MEK kinase), and the functionally redundant Mpk1 activators Mkk1 and Mkk2 (homologs of MEK). An activated mutation of MKK1, MKK1P386, inhibits growth when overexpressed. This growth-inhibitory effect was suppressed by the mpk1 delta mutation, suggesting that hyperactivation of the Mpk1 pathway is toxic to cells. To search for genes that interact with the Mpk1 pathway, we isolated both chromosomal mutations and dosage suppressor genes that ameliorate the growth-inhibitory effect of overexpressed Mkk1P386. One of the genes identified by the analysis of chromosomal mutations is RLM1 (resistance to lethality of MKK1P386 overexpression), which encodes a protein homologous to a conserved domain of the MADS (Mcm1, Agamous, Deficiens, and serum response factor) box family of transcription factors. Although rlm1 delta cells grow normally at any temperature, they display a caffeine-sensitive phenotype similar to that observed in mutants defective in BCK1, MKK1/MKK2, or MPK1. A gene fusion that provides Rlm1 with a transcriptional activation domain of Gal4 suppresses bck1 delta and mpk1 delta. A screening for dosage suppressors yielded the MSG5 genes, which encode a dual-specificity protein phosphatase. Our results suggest that Rlm1 functions as a transcription factor downstream of Mpk1 that is subject to activation by the Mpk1 mitogen-activated protein kinase pathway.

Functional characterization of the MKC1 gene of Candida albicans, which encodes a mitogen-activated protein kinase homolog related to cell integrity

Mitogen-activated protein (MAP) kinases represent a group of serine/threonine protein kinases playing a central role in signal transduction processes in eukaryotic cells. Using a strategy based on the complementation of the thermosensitive autolytic phenotype of slt2 null mutants, we have isolated a Candida albicans homolog of Saccharomyces cerevisiae MAP kinase gene SLT2 (MPK1), which is involved in the recently outlined PKC1-controlled signalling pathway. The isolated gene, named MKC1 (MAP kinase from C. albicans), coded for a putative protein, Mkc1p, of 58,320 Da that displayed all the characteristic domains of MAP kinases and was 55% identical to S. cerevisiae Slt2p (Mpk1p). The MKC1 gene was deleted in a diploid Candida strain, and heterozygous and homozygous strains, in both Ura+ and Ura- backgrounds, were obtained to facilitate the analysis of the function of the gene. Deletion of the two alleles of the MKC1 gene gave rise to viable cells that grew at 28 and 37 degrees C but, nevertheless, displayed a variety of phenotypic traits under more stringent conditions. These included a low growth yield and a loss of viability in cultures grown at 42 degrees C, a high sensitivity to thermal shocks at 55 degrees C, an enhanced susceptibility to caffeine that was osmotically remediable, and the formation of a weak cell wall with a very low resistance to complex lytic enzyme preparations. The analysis of the functions downstream of the MKC1 gene should contribute to understanding of the connection of growth and morphogenesis in pathogenic fungi.

Activation of CLN1 and CLN2 G1 cyclin gene expression by BCK2

The Saccharomyces cerevisiae CLN3 protein, a G1 cyclin, positively regulates the expression of CLN1 and CLN2, two additional G1 cyclins whose expression during late G1 is activated, in part, by the transcription factors SWI4 and SWI6. We isolated 12 complementation groups of mutants that require CLN3. The members of one of these complementation groups have mutations in the BCK2 gene. In a wild-type CLN3 genetic background, bck2 mutants have a normal growth rate but have a larger cell size, are more sensitive to alpha-factor, and have a modest defect in the accumulation of CLN1 and CLN2 RNA. In the absence of CLN3, bck2 mutations cause an extremely slow growth rate: the cells accumulate in late G1 with very low levels of CLN1 and CLN2 RNA. The slow growth rate and long G1 delay of bck2 cln3 mutants are cured by heterologous expression of CLN2. Moreover, overexpression of BCK2 induces very high levels of CLN1, CLN2, and HCS26 RNAs. The results suggest that BCK2 and CLN3 provide parallel activation pathways for the expression of CLN1 and CLN2 during late G1.

Suppressors of nmtl-181, a conditional lethal allele of the Saccharomyces cerevisiae myristoyl-CoA:protein N-myristoyltransferase gene, reveal proteins involved in regulating protein N-myristoylation

Several essential Saccharomyces cerevisiae proteins require myristate to be covalently bound to their amino-terminal glycine for biological activity. Protein N-myristoylation is catalyzed by myristoyl-CoA:protein N-myristoyl-transferase, Nmt1p. nmt1-181 encodes a mutant enzyme with a Gly451-->Asp substitution. nmt181p has a reduced affinity for myristoyl-CoA and produces global defects in protein N-myristoylation at > or = 30 degrees C. nmt1-181 results in growth arrest at various stages of the cell cycle within 1 hr after cells are shifted to > or = 30 degrees C and lethality within 8 hr. The growth-arrest phenotype and loss of viability do not require components of the mating pathway and are associated with lysis sensitivity that may be related to undermyristoylation of two protein phosphatases, Ppz1p and Ppz2p. Growth can be rescued at 30 degrees C by adding myristate or sorbitol to the medium or by removing inosine. Cells can be rescued at 37 degrees C by overexpressing nmt1-181p or Nmt1p or by adding myristate to the medium. Selection of high-copy suppressors of the myristate auxotrophy and lethality observed at 37 degrees C yielded only NMT1, whereas six unlinked suppressors of the myristoylation defect (SMD1-6) were obtained when the screen was conducted at 30 degrees C. The protein products of three SMD loci were identified: (i) cdc39-delta 1.7p, which transactivates NMT1; (ii) Fas1p, the beta subunit of the fatty acid synthetase complex, activates FAS2's promoter and increases myristoylation of Gpa1p; and (iii) Pho5p, the major secreted acid phosphatase produced by this yeast. PHO5 is normally induced when yeast are grown in phosphate-depleted medium. Removal of inorganic phosphate from the medium also rescues nmt1-181 cells at 30 degrees C. PHO5's mechanism of suppression of nmt1-181 appears to involve, at least in part, activation of FAS2 transcription and a resulting effect on FAS1 expression. There is an inverse relationship between cellular N-myristoyltransferase and secreted acid phosphatase activities. These observations provide a potential mechanism for coupling phosphate metabolism with the regulation of myristoyl-CoA synthesis and protein N-myristoylation.

Characterization of the yeast (1-->6)-beta-glucan biosynthetic components, Kre6p and Skn1p, and genetic interactions between the PKC1 pathway and extracellular matrix assembly

A characterization of the S. cerevisiae KRE6 and SKN1 gene products extends previous genetic studies on their role in (1-->6)-beta-glucan biosynthesis (Roemer, T., and H. Bussey. 1991. Yeast beta-glucan synthesis: KRE6 encodes a predicted type II membrane protein required for glucan synthesis in vivo and for glucan synthase activity in vitro. Proc. Natl. Acad. Sci. USA. 88:11295-11299; Roemer, T., S. Delaney, and H. Bussey. 1993. SKN1 and KRE6 define a pair of functional homologs encoding putative membrane proteins involved in beta-glucan synthesis. Mol. Cell. Biol. 13:4039-4048). KRE6 and SKN1 are predicted to encode homologous proteins that participate in assembly of the cell wall polymer (1-->6)-beta-glucan. KRE6 and SKN1 encode phosphorylated integral-membrane glycoproteins, with Kre6p likely localized within a Golgi subcompartment. Deletion of both these genes is shown to result in a dramatic disorganization of cell wall ultrastructure. Consistent with their direct role in the assembly of this polymer, both Kre6p and Skn1p possess COOH-terminal domains with significant sequence similarity to two recently identified glucan-binding proteins. Deletion of the yeast protein kinase C homolog, PKC1, leads to a lysis defect (Levin, D. E., and E. Bartlett-Heubusch. 1992. Mutants in the S. cerevisiae PKC1 gene display a cell cycle-specific osmotic stability defect. J. Cell Biol. 116:1221-1229). Kre6p when even mildly overproduced, can suppress this pkc1 lysis defect. When mutated, several KRE pathway genes and members of the PKC1-mediated MAP kinase pathway have synthetic lethal interactions as double mutants. These suppression and synthetic lethal interactions, as well as reduced beta-glucan and mannan levels in the pkc1 null wall, support a role for the PKC1 pathway functioning in cell wall assembly. PKC1 potentially participates in cell wall assembly by regulating the synthesis of cell wall components, including (1-->6)-beta-glucan.

Mutation of the gene encoding protein kinase C 1 stimulates mitotic recombination in Saccharomyces cerevisiae

We have isolated a recessive allele of the yeast protein kinase C gene (PKC1) which promotes an elevated rate of mitotic recombination and confers a temperature-sensitive growth defect. The rate of recombination was increased between genes in direct repeat and at a series of heteroalleles and was dependent upon the RAD52 gene product. The mutant pkc1 allele was sequenced and found to encode a single amino acid change within the catalytic domain. Osmotic stabilizing agents rescued the temperature-sensitive growth defect but not the hyperrecombination phenotype, indicating that the two traits are separable. This separability suggests that the PKC1 gene product (Pkc1p) regulates DNA metabolism by an alternate pathway to that used in the regulation of cell lysis. The regulation of recombination is a previously unidentified role for Pkc1p.

Mutation of the gene encoding protein kinase C 1 stimulates mitotic recombination in Saccharomyces cerevisiae

We have isolated a recessive allele of the yeast protein kinase C gene (PKC1) which promotes an elevated rate of mitotic recombination and confers a temperature-sensitive growth defect. The rate of recombination was increased between genes in direct repeat and at a series of heteroalleles and was dependent upon the RAD52 gene product. The mutant pkc1 allele was sequenced and found to encode a single amino acid change within the catalytic domain. Osmotic stabilizing agents rescued the temperature-sensitive growth defect but not the hyperrecombination phenotype, indicating that the two traits are separable. This separability suggests that the PKC1 gene product (Pkc1p) regulates DNA metabolism by an alternate pathway to that used in the regulation of cell lysis. The regulation of recombination is a previously unidentified role for Pkc1p.

Growth site localization of Rho1 small GTP-binding protein and its involvement in bud formation in Saccharomyces cerevisiae

The Rho small GTP-binding protein family regulates various actomyosin-dependent cell functions, such as cell morphology, locomotion, cytokinesis, membrane ruffling, and smooth muscle contraction. In the yeast Saccharomyces cerevisiae, there is a homologue of mammalian RhoA, RHO1, which is essential for vegetative growth of yeast cells. To explore the function of the RHO1 gene, we isolated a recessive temperature-sensitive mutation of RHO1, rho1-104. The rho1-104 mutation caused amino acid substitutions of Asp 72 to Asn and Cys 164 to Tyr of Rho1p. Strains bearing the rho1-104 mutation accumulated tiny- or small-budded cells in which cortical actin patches were clustered to buds at the restrictive temperature. Cell lysis and cell death were also seen with the rho1-104 mutant. Indirect immunofluorescence microscopic study demonstrated that Rho1p was concentrated to the periphery of the cells where cortical actin patches were clustered, including the site of bud emergence, the tip of the growing buds, and the mother-bud neck region of cells prior to cytokinesis. Indirect immunofluorescence study with cells overexpressing RHO1 suggested that the Rho1p-binding site was saturable. A mutant Rho1p with an amino acid substitution at the lipid modification site remained in the cytoplasm. These results suggest that Rho1 small GTP-binding protein binds to a specific site at the growth region of cells, where Rho1p exerts its function in controlling cell growth.

Induction of pseudohyphal growth by overexpression of PHD1, a Saccharomyces cerevisiae gene related to transcriptional regulators of fungal development

When starved for nitrogen, MATa/MAT alpha cells of the budding yeast Saccharomyces cerevisiae undergo a dimorphic transition to pseudohyphal growth. A visual genetic screen, called PHD (pseudohyphal determinant), for S. cerevisiae pseudohyphal growth mutants was developed. The PHD screen was used to identify seven S. cerevisiae genes that when overexpressed in MATa/MAT alpha cells growing on nitrogen starvation medium cause precocious and unusually vigorous pseudohyphal growth. PHD1, a gene whose overexpression induced invasive pseudohyphal growth on a nutritionally rich medium, was characterized. PHD1 maps to chromosome XI and is predicted to encode a 366-amino-acid protein. PHD1 has a SWI4- and MBP1-like DNA binding motif that is 73% identical over 100 amino acids to a region of Aspergillus nidulans StuA. StuA regulates two pseudohyphal growth-like cell divisions during conidiophore morphogenesis. Epitope-tagged PHD1 was localized to the nucleus by indirect immunofluorescence. These facts suggest that PHD1 may function as a transcriptional regulatory protein. Overexpression of PHD1 in wild-type haploid strains does not induce pseudohyphal growth. Interestingly, PHD1 overexpression enhances pseudohyphal growth in a haploid strain that has the diploid polar budding pattern because of a mutation in the BUD4 gene. In addition, wild-type diploid strains lacking PHD1 undergo pseudohyphal growth when starved for nitrogen. The possible functions of PHD1 in pseudohyphal growth and the uses of the PHD screen to identify morphogenetic regulatory genes from heterologous organisms are discussed.

Genes that can bypass the CLN requirement for Saccharomyces cerevisiae cell cycle START

Cell cycle START in Saccharomyces cerevisiae requires at least one of the three CLN genes (CLN1, CLN2, or CLN3). A total of 12 mutations bypassing this requirement were found to be dominant mutations in a single gene that we named BYC1 (for bypass of CLN requirement). We also isolated a plasmid that had cln bypass activity at a low copy number; the gene responsible was distinct from BYC1 and was identical to the recently described BCK2 gene. Strains carrying bck2::ARG4 disruption alleles were fully viable, but bck2::ARG4 completely suppressed the cln bypass activity of BYC1. swi4 and swi6 deletion alleles also efficiently suppressed BYC1 cln bypass activity; Swi4 and Swi6 are components of a transcription factor previously implicated in control of CLN1 and CLN2 expression. bck2::ARG4 was synthetically lethal with cln3 deletion, suggesting that CLN1 and CLN2 cannot function in the simultaneous absence of BCK2 and CLN3; this observation correlates with low expression of CLN1 and CLN2 in bck2 strains deprived of CLN3 function. Thus, factors implicated in CLN1 and CLN2 expression and/or function are also required for BYC1 function in the absence of all three CLN genes; this may suggest the involvement of other targets of Swi4, Swi6, and Bck2 in START.

Genes that can bypass the CLN requirement for Saccharomyces cerevisiae cell cycle START

Cell cycle START in Saccharomyces cerevisiae requires at least one of the three CLN genes (CLN1, CLN2, or CLN3). A total of 12 mutations bypassing this requirement were found to be dominant mutations in a single gene that we named BYC1 (for bypass of CLN requirement). We also isolated a plasmid that had cln bypass activity at a low copy number; the gene responsible was distinct from BYC1 and was identical to the recently described BCK2 gene. Strains carrying bck2::ARG4 disruption alleles were fully viable, but bck2::ARG4 completely suppressed the cln bypass activity of BYC1. swi4 and swi6 deletion alleles also efficiently suppressed BYC1 cln bypass activity; Swi4 and Swi6 are components of a transcription factor previously implicated in control of CLN1 and CLN2 expression. bck2::ARG4 was synthetically lethal with cln3 deletion, suggesting that CLN1 and CLN2 cannot function in the simultaneous absence of BCK2 and CLN3; this observation correlates with low expression of CLN1 and CLN2 in bck2 strains deprived of CLN3 function. Thus, factors implicated in CLN1 and CLN2 expression and/or function are also required for BYC1 function in the absence of all three CLN genes; this may suggest the involvement of other targets of Swi4, Swi6, and Bck2 in START.