Rho2 is a target of the farnesyltransferase Cpp1 and acts upstream of Pmk1 mitogen-activated protein kinase signaling in fission yeast

Atg1, a key regulator of autophagy, functions to promote MAPK activation and cell death upon calcium overload in fission yeast

Autophagy promotes or inhibits cell death depending on the environment and cell type. Our previous findings suggested that Atg1 is genetically involved in the regulation of Pmk1 MAPK in fission yeast. Here, we showed that Δatg1 displays lower levels of Pmk1 MAPK phosphorylation than did the wild-type (WT) cells upon treatment with a 1,3-β-D-glucan synthase inhibitor micafungin or CaCl₂, both of which activate Pmk1. Moreover, the overproduction of Atg1, but not that of the kinase inactivating Atg1^D193A activates Pmk1 without any extracellular stimuli, suggesting that Atg1 may promote Pmk1 MAPK signaling activation. Notably, the overproduction of Atg1 induces a toxic effect on the growth of WT cells and the deletion of Pmk1 failed to suppress the cell death induced by Atg1, indicating that the Atg1-mediated cell death requires additional mechanism(s) other than Pmk1 activation. Moreover, atg1 gene deletion induces tolerance to micafungin and CaCl₂, whereas pmk1 deletion induces severe sensitivities to these compounds. The Δatg1Δpmk1 double mutants display intermediate sensitivities to these compounds, showing that atg1 deletion partly suppressed growth inhibition induced by Δpmk1. Thus, Atg1 may act to promote cell death upon micafungin and CaCl₂ stimuli regardless of Pmk1 MAPK activity. Since micafungin and CaCl₂ are intracellular calcium inducers, our data reveal a novel role of the autophagy regulator Atg1 to induce cell death upon calcium overload independent of its role in Pmk1 MAPK activation.

Fission Yeast Rho1p-GEFs: From Polarity and Cell Wall Synthesis to Genome Stability

Rho1p is a membrane-associated protein that belongs to the Rho family of small GTPases. These proteins coordinate processes such as actin remodelling and polarised secretion to maintain the shape and homeostasis of yeast cells. In response to extracellular stimuli, Rho1p undergoes conformational switching between a guanosine triphosphate (GTP)-bound active state and a guanosine diphosphate (GDP)-bound inactive state. Cycling is improved with guanine nucleotide exchange factor (GEF) activity necessary to activate signalling and GTPase activating protein (GAP) activity required for subsequent signal depletion. This review focuses on fission yeast Rho1p GEFs, Rgf1p, Rgf2p, and Rgf3p that belong to the family of DH-PH domain-containing Dbl-related GEFs. They are multi-domain proteins that detect biological signals that induce or inhibit their catalytic activity over Rho1p. Each of them activates Rho1p in different places and times. Rgf1p acts preferentially during polarised growth. Rgf2p is required for sporulation, and Rgf3p plays an essential function in septum synthesis. In addition, we outline the noncanonical roles of Rho1p-GEFs in genomic instability.

The Fission Yeast Cell Integrity Pathway: A Functional Hub for Cell Survival upon Stress and Beyond

The survival of eukaryotic organisms during environmental changes is largely dependent on the adaptive responses elicited by signal transduction cascades, including those regulated by the Mitogen-Activated Protein Kinase (MAPK) pathways. The Cell Integrity Pathway (CIP), one of the three MAPK pathways found in the simple eukaryote fission of yeast Schizosaccharomyces pombe, shows strong homology with mammalian Extracellular signal-Regulated Kinases (ERKs). Remarkably, studies over the last few decades have gradually positioned the CIP as a multi-faceted pathway that impacts multiple functional aspects of the fission yeast life cycle during unperturbed growth and in response to stress. They include the control of mRNA-stability through RNA binding proteins, regulation of calcium homeostasis, and modulation of cell wall integrity and cytokinesis. Moreover, distinct evidence has disclosed the existence of sophisticated interplay between the CIP and other environmentally regulated pathways, including Stress-Activated MAP Kinase signaling (SAPK) and the Target of Rapamycin (TOR). In this review we present a current overview of the organization and underlying regulatory mechanisms of the CIP in S. pombe, describe its most prominent functions, and discuss possible targets of and roles for this pathway. The evolutionary conservation of CIP signaling in the dimorphic fission yeast S. japonicus will also be addressed.

Identification of ksg1 mutation showing long-lived phenotype in fission yeast

Fission yeast is a good model organism for the study of lifespan. To elucidate the mechanism, we screened for long-lived mutants. We found a nonsense mutation in the ksg1⁺ gene, which encodes an ortholog of mammalian PDK1 (phosphoinositide-dependent protein kinase). The mutation was in the PH domain of Ksg1 and caused defect in membrane localization and protein stability. Analysis of the ksg1 mutant revealed that the reduced amounts and/or activity of the Ksg1 protein are responsible for the increased lifespan. Ksg1 is essential for growth and known to phosphorylate multiple substrates, but the substrate responsible for the long-lived phenotype of ksg1 mutation is not yet known. Genetic analysis showed that deletion of pck2 suppressed the long-lived phenotype of ksg1 mutant, suggesting that Pck2 might be involved in the lifespan extension caused by ksg1 mutation.

The Multiple Functions of Rho GTPases in Fission Yeasts

The Rho family of GTPases represents highly conserved molecular switches involved in a plethora of physiological processes. Fission yeast Schizosaccharomyces pombe has become a fundamental model organism to study the functions of Rho GTPases over the past few decades. In recent years, another fission yeast species, Schizosaccharomyces japonicus, has come into focus offering insight into evolutionary changes within the genus. Both fission yeasts contain only six Rho-type GTPases that are spatiotemporally controlled by multiple guanine-nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs), and whose intricate regulation in response to external cues is starting to be uncovered. In the present review, we will outline and discuss the current knowledge and recent advances on how the fission yeasts Rho family GTPases regulate essential physiological processes such as morphogenesis and polarity, cellular integrity, cytokinesis and cellular differentiation.

Sequestration of the PKC ortholog Pck2 in stress granules as a feedback mechanism of MAPK signaling in fission yeast

Protein kinase C (PKC) signaling is a highly conserved signaling module that plays a central role in a myriad of physiological processes, ranging from cell proliferation to cell death, via various signaling pathways, including MAPK signaling. Stress granules (SGs) are non-membranous cytoplasmic foci that aggregate in cells exposed to environmental stresses. Here, we explored the role of SGs in PKC/MAPK signaling activation in fission yeast. High-heat stress (HHS) induced Pmk1 MAPK activation and Pck2 translocation from the cell tips into poly(A)-binding protein (Pabp)-positive SGs. Pck2 dispersal from the cell tips required Pck2 kinase activity, and constitutively active Pck2 exhibited increased translocation to SGs. Importantly, Pmk1 deletion impaired Pck2 recruitment to SGs, indicating that MAPK activation stimulates Pck2 SG translocation. Consistently, HHS-induced SGs delayed Pck2 relocalization at the cell tips, thereby blocking subsequent Pmk1 reactivation after recovery from HHS. HHS partitioned Pck2 into the Pabp-positive SG-containing fraction, which resulted in reduced Pck2 abundance and kinase activity in the soluble fraction. Taken together, these results indicate that MAPK-dependent Pck2 SG recruitment serves as a feedback mechanism to intercept PKC/MAPK activation induced by HHS, which might underlie PKC-related diseases.

A novel checkpoint pathway controls actomyosin ring constriction trigger in fission yeast

In fission yeast, the septation initiation network (SIN) ensures temporal coordination between actomyosin ring (CAR) constriction with membrane ingression and septum synthesis. However, questions remain about CAR regulation under stress conditions. We show that Rgf1p (Rho1p GEF), participates in a delay of cytokinesis under cell wall stress (blankophor, BP). BP did not interfere with CAR assembly or the rate of CAR constriction, but did delay the onset of constriction in the wild type cells but not in the rgf1Δ cells. This delay was also abolished in the absence of Pmk1p, the MAPK of the cell integrity pathway (CIP), leading to premature abscission and a multi-septated phenotype. Moreover, cytokinesis delay correlates with maintained SIN signaling and depends on the SIN to be achieved. Thus, we propose that the CIP participates in a checkpoint, capable of triggering a CAR constriction delay through the SIN pathway to ensure that cytokinesis terminates successfully.

Rho Family GTPases in Fission Yeast Cytokinesis

During cytokinesis, actomyosin ring constriction drives furrow formation. In animal cells, Rho GTPases drive this process through the positioning and assembly of the actomyosin ring, and through extracellular matrix remodeling within the furrow. In the fission yeast S. pombe, actomyosin ring constriction and septum formation are concurrent processes. While S. pombe is the primary source from which the mechanics of ring assembly and constriction stem, much less is known about the regulation of Rho GTPases that control these processes. Of the six Rho GTPases encoded in S. pombe, only Rho1, the RhoA homologue, has been shown to be essential for cytokinesis. While Rho3, Rho4, and Cdc42 have defined roles in cytokinesis, Rho2 and Rho5 play minor to no roles in this process. Here we review the roles of the Rho GTPases during cytokinesis, with a focus on their regulation, and discuss whether crosstalk between GTPases, as has been reported in other organisms, exists during cytokinesis in S. pombe.

[Fission Yeast as a Model System for Studying Cancer Signaling and Drug Discovery: Discovery of ACA-28 as a Novel Inducer of ERK-dependent Apoptosis Reveals a New Cancer Therapy]

Mitogen-activated protein kinase (MAPK) pathways are evolutionarily conserved kinase modules that link extracellular signals to the machinery that controls fundamental cellular processes such as growth, proliferation, differentiation, and apoptosis. The Ras/Raf/MEK/ERK MAPK pathway is one of the most studied of the mammalian MAPK pathways and has attracted intense research interest because of its critical involvement in the regulation of cell proliferation. The mutational activation of upstream signaling components that constitutively activate ERK MAPKs as seen in various primary tumor samples has validated this pathway for drug discovery. The fission yeast Schizosaccharomyces pombe is an important tool for cancer research. This well-studied model organism has enabled groundbreaking, Nobel Prize-winning discoveries and has provided insights into how both normal and cancerous cells grow and divide. We performed chemical genetic screening using a fission yeast phenotypic assay and demonstrated that ACA-28, a synthetic derivative of 1'-acetoxychavicol acetate (ACA), effectively inhibited the growth of melanoma cancer cells wherein ERK MAPK signaling is hyperactivated due to mutations in the upstream activating regulators. Importantly, the growth of normal human epidermal melanocytes was less affected by ACA-28. In addition, ACA-28 specifically induced apoptosis in NIH/3T3 cells oncogenically transformed with HER2/ErbB2 but not in the parental cells. Notably, the ACA-28-induced apoptosis was abrogated when ERK activation was blocked with the specific MEK inhibitor U0126. Consistently, ACA-28 more strongly stimulated ERK phosphorylation in melanoma cells as compared with normal human epidermal melanocytes. ACA-28 might serve as a promising seed compound to combat ERK-dependent cancers by stimulating oncogenic signaling.

Functional interaction between Cdc42 and the stress MAPK signaling pathway during the regulation of fission yeast polarized growth

Cell polarization can be defined as the generation and maintenance of directional cellular organization. The spatial distribution and protein or lipid composition of the cell are not symmetric but organized in specialized domains which allow cells to grow and acquire a certain shape that is closely linked to their physiological function. The establishment and maintenance of polarized growth requires the coordination of diverse processes including cytoskeletal dynamics, membrane trafficking, and signaling cascade regulation. Some of the major players involved in the selection and maintenance of sites for polarized growth are Rho GTPases, which recognize the polarization site and transmit the signal to regulatory proteins of the cytoskeleton. Additionally, cytoskeletal organization, polarized secretion, and endocytosis are controlled by signaling pathways including those mediated by mitogen-activated protein kinases (MAPKs). Rho GTPases and the MAPK signaling pathways are strongly conserved from yeast to mammals, suggesting that the basic mechanisms of polarized growth have been maintained throughout evolution. For this reason, the study of how polarized growth is established and regulated in simple organisms such as the fission yeast Schizosaccharomyces pombe has contributed to broaden our knowledge about these processes in multicellular organisms. We review here the function of the Cdc42 GTPase and the stress activated MAPK (SAPK) signaling pathways during fission yeast polarized growth, and discuss the relevance of the crosstalk between both pathways.

Rgf1p (Rho1p GEF) is required for double-strand break repair in fission yeast

Rho GTPases are conserved molecules that control cytoskeletal dynamics. These functions are expedited by Rho GEFs that stimulate the release of GDP to enable GTP binding, thereby allowing Rho proteins to initiate intracellular signaling. How Rho GEFs and Rho GTPases protect cells from DNA damage is unknown. Here, we explore the extreme sensitivity of a deletion mutation in the Rho1p exchange factor Rgf1p to the DNA break/inducing antibiotic phleomycin (Phl). The Rgf1p mutant cells are defective in reentry into the cell cycle following the induction of severe DNA damage. This phenotype correlates with the inability of rgf1Δ cells to efficiently repair fragmented chromosomes after Phl treatment. Consistent with this observation Rad11p (ssDNA binding protein, RPA), Rad52p, Rad54p and Rad51p, which facilitate strand invasion in the process of homology-directed repair (HDR), are permanently stacked in Phl-induced foci in rgf1Δ cells. These phenotypes are phenocopied by genetic inhibition of Rho1p. Our data provide evidence that Rgf1p/Rho1p activity positively controls a repair function that confers resistance against the anti-cancer drug Phl.

Rae1-mediated nuclear export of Rnc1 is an important determinant in controlling MAPK signaling

In eukaryotic cells, RNA binding proteins (RBPs) play critical roles in regulating almost every aspect of gene expression, often shuttling between the nucleus and the cytoplasm. They are also key determinants in cell fate via controlling the target mRNAs under the regulation of various signaling pathways in response to environmental stresses. Therefore, understanding the mechanisms that couple the location of mRNA and RBPs is a major challenge in the field of gene expression and signal responses. In fission yeast, a KH-type RBP Rnc1 negatively regulates MAPK signaling activation via mRNA stabilization of the dual-specificity MAPK phosphatase Pmp1, which dephosphorylates MAPK Pmk1. Rnc1 also serves as a target of MAPK phosphorylation, which makes a feedback loop mediated by an RBP. We recently discovered that the nuclear export of Rnc1 requires mRNA-binding ability and the mRNA export factor Rae1. This strongly suggested the presence of an mRNA-export system, which recognizes the mRNA/RBP complex and dictates the location and post-transcriptional regulation of mRNA cargo. Here, we briefly review the known mechanisms of general nuclear transporting systems, with an emphasis on our recent findings on the spatial regulation of Rnc1 and its impact on the regulation of the MAPK signal transduction cascade.

Distinct functional relevance of dynamic GTPase cysteine methylation in fission yeast

The final step in post-translational processing of Ras and Rho GTPases involves methylation of the prenylated cysteine residue by an isoprenylcysteine-O-carboxyl methyltransferase (ICMT). ICMT activity is essential for cell growth and development in higher eukaryotes, and inhibition of GTPase methylation has become an attractive target in cancer therapy to inactivate prenylated oncoproteins. However, the specificity and dynamics of the GTPase methylation process remain to be fully clarified. Notably, cells lacking Mam4, the ICMT ortholog in the fission yeast Schizosaccharomyces pombe, are viable. We have exploited this feature to analyze the role of methylation on GTPase localization and function. We show that methylation differentially affects GTPase membrane localization, being particularly relevant for plasma membrane tethering and downstream signaling of palmitoylated and farnesylated GTPases Ras1 and Rho2 lacking C-terminal polybasic motifs. Indeed, Ras1 and Rho2 cysteine methylation is required for proper regulation of differentiation elicited by MAPK Spk1 and for stress-dependent activation of the cell integrity pathway (CIP) and its main effector MAPK Pmk1. Further, Mam4 negatively regulates TORC2 signaling by a cross-inhibitory mechanism relying on Rho GTPase methylation. These results highlight the requirement for a tight control of GTPase methylation in vivo to allow adequate GTPase function.

Methionine synthase is localized to the nucleus in Pichia pastoris and Candida albicans and to the cytoplasm in Saccharomyces cerevisiae

Methionine synthase (MS) catalyzes methylation of homocysteine, the last step in the biosynthesis of methionine, which is essential for the regeneration of tetrahydrofolate and biosynthesis of S-adenosylmethionine. Here, we report that MS is localized to the nucleus of Pichia pastoris and Candida albicans but is cytoplasmic in Saccharomyces cerevisiae The P. pastoris strain carrying a deletion of the MET6 gene encoding MS (Ppmet6) exhibits methionine as well as adenine auxotrophy indicating that MS is required for methionine as well as adenine biosynthesis. Nuclear localization of P. pastoris MS (PpMS) was abrogated by the deletion of 107 C-terminal amino acids or the R742A mutation. In silico analysis of the PpMS structure indicated that PpMS may exist in a dimer-like configuration in which Arg-742 of a monomer forms a salt bridge with Asp-113 of another monomer. Biochemical studies indicate that R742A as well as D113R mutations abrogate nuclear localization of PpMS and its ability to reverse methionine auxotrophy of Ppmet6 Thus, association of two PpMS monomers through the interaction of Arg-742 and Asp-113 is essential for catalytic activity and nuclear localization. When PpMS is targeted to the cytoplasm employing a heterologous nuclear export signal, it is expressed at very low levels and is unable to reverse methionine and adenine auxotrophy of Ppmet6 Thus, nuclear localization is essential for the stability and function of MS in P. pastoris. We conclude that nuclear localization of MS is a unique feature of respiratory yeasts such as P. pastoris and C. albicans, and it may have novel moonlighting functions in the nucleus.

Differential functional regulation of protein kinase C (PKC) orthologs in fission yeast

The two PKC orthologs Pck1 and Pck2 in the fission yeast Schizosaccharomyces pombe operate in a redundant fashion to control essential functions, including morphogenesis and cell wall biosynthesis, as well as the activity of the cell integrity pathway and its core element, the MAPK Pmk1. We show here that, despite the strong structural similarity and functional redundancy of these two enzymes, the mechanisms regulating their maturation, activation, and stabilization have a remarkably distinct biological impact on both kinases. We found that, in contrast to Pck2, putative in vivo phosphorylation of Pck1 within the conserved activation loop, turn, and hydrophobic motifs is essential for Pck1 stability and biological functions. Constitutive Pck activation promoted dephosphorylation and destabilization of Pck2, whereas it enhanced Pck1 levels to interfere with proper downstream signaling to the cell integrity pathway via Pck2. Importantly, although catalytic activity was essential for Pck1 function, Pck2 remained partially functional independent of its catalytic activity. Our findings suggest that early divergence from a common ancestor in fission yeast involved important changes in the mechanisms regulating catalytic activation and stability of PKC family members to allow for flexible and dynamic control of downstream functions, including MAPK signaling.

Spatial regulation of the KH domain RNA-binding protein Rnc1 mediated by a Crm1-independent nuclear export system in Schizosaccharomyces pombe

RNA-binding proteins (RBPs) play important roles in the posttranscriptional regulation of gene expression, including mRNA stability, transport and translation. Fission yeast rnc1+ encodes a K Homology (KH)-type RBP, which binds and stabilizes the Pmp1 MAPK phosphatase mRNA thereby suppressing the Cl- hypersensitivity of calcineurin deletion and MAPK signaling mutants. Here, we analyzed the spatial regulation of Rnc1 and discovered a putative nuclear export signal (NES)Rnc1 , which dictates the cytoplasmic localization of Rnc1 in a Crm1-independent manner. Notably, mutations in the NESRnc1 altered nucleocytoplasmic distribution of Rnc1 and abolished its function to suppress calcineurin deletion, although the Rnc1 NES mutant maintains the ability to bind Pmp1 mRNA. Intriguingly, the Rnc1 NES mutant destabilized Pmp1 mRNA, suggesting the functional importance of the Rnc1 cytoplasmic localization. Mutation in Rae1, but not Mex67 deletion or overproduction, induced Rnc1 accumulation in the nucleus, suggesting that Rnc1 is exported from the nucleus to the cytoplasm via the mRNA export pathway involving Rae1. Importantly, mutations in the Rnc1 KH-domains abolished the mRNA-binding ability and induced nuclear localization, suggesting that Rnc1 may be exported from the nucleus together with its target mRNAs. Collectively, the functional Rae1-dependent mRNA export system may influence the cytoplasmic localization and function of Rnc1.

Skb5, an SH3 adaptor protein, regulates Pmk1 MAPK signaling by controlling the intracellular localization of Mkh1 MAPKKK

The MAPK cascade is a highly conserved signaling module composed of MAPK/MAPKK/MAPKKK. MAPKKK Mkh1 is an initiating kinase in Pmk1 MAPK signaling, which regulates cell integrity in fission yeast. Our genetic screen for regulators of Pmk1 signaling identified Skb5 (Shk1 kinase binding protein 5), an SH3 domain-containing adaptor protein. Here, we showed that Skb5 serves as an inhibitor of Pmk1 MAPK signaling activation by downregulating Mkh1 localization to cell tips via its interaction with the SH3 domain. Consistently, the Mkh13PA mutant protein, with impaired Skb5 binding, remained in the cell tips, even when Skb5 was overproduced. Intriguingly, Skb5 needs Mkh1 to localize to the growing ends as Mkh1 deletion and disruption of Mkh1 binding impairs Skb5 localization. Deletion of Pck2, an upstream activator of Mkh1, impaired the cell tip localization of Mkh1 and Skb5 as well as Mkh1/Skb5 interaction. Interestingly, both Pck2 and Mkh1 localized to the cell tips at the G1/S phase, which coincided with Pmk1 MAPK activation. Altogether, Mkh1 localization to cell tips is important for transmitting upstream signaling to Pmk1 and Skb5 spatially regulates this process.

Eisosomes Regulate Phosphatidylinositol 4,5-Bisphosphate (PI(4,5)P2) Cortical Clusters and Mitogen-activated Protein (MAP) Kinase Signaling upon Osmotic Stress

Eisosomes are multiprotein structures that generate linear invaginations at the plasma membrane of yeast cells. The core component of eisosomes, the BAR domain protein Pil1, generates these invaginations through direct binding to lipids including phosphoinositides. Eisosomes promote hydrolysis of phosphatidylinositol 4,5 bisphosphate (PI(4,5)P2) by functioning with synaptojanin, but the cellular processes regulated by this pathway have been unknown. Here, we found that PI(4,5)P2 regulation by eisosomes inhibits the cell integrity pathway, a conserved MAPK signal transduction cascade. This pathway is activated by multiple environmental conditions including osmotic stress in the fission yeast Schizosaccharomyces pombe. Activation of the MAPK Pmk1 was impaired by mutations in the phosphatidylinositol (PI) 5-kinase Its3, but this defect was suppressed by removal of eisosomes. Using fluorescent biosensors, we found that osmotic stress induced the formation of PI(4,5)P2 clusters that were spatially organized by eisosomes in both fission yeast and budding yeast cells. These cortical clusters contained the PI 5-kinase Its3 and did not assemble in the its3-1 mutant. The GTPase Rho2, an upstream activator of Pmk1, also co-localized with PI(4,5)P2 clusters under osmotic stress, providing a molecular link between these novel clusters and MAPK activation. Our findings have revealed that eisosomes regulate activation of MAPK signal transduction through the organization of cortical lipid-based microdomains.

Mechanics and morphogenesis of fission yeast cells

The integration of biochemical and biomechanical elements is at the heart of morphogenesis. While animal cells are relatively soft objects which shape and mechanics is mostly regulated by cytoskeletal networks, walled cells including those of plants, fungi and bacteria are encased in a rigid cell wall which resist high internal turgor pressure. How these particular mechanical properties may influence basic cellular processes, such as growth, shape and division remains poorly understood. Recent work using the model fungal cell fission yeast, Schizosaccharomyces pombe, highlights important contribution of cell mechanics to various morphogenesis processes. We envision this genetically tractable system to serve as a novel standard for the mechanobiology of walled cell.

Rho2 palmitoylation is required for plasma membrane localization and proper signaling to the fission yeast cell integrity mitogen- activated protein kinase pathway

The fission yeast small GTPase Rho2 regulates morphogenesis and is an upstream activator of the cell integrity pathway, whose key element, mitogen-activated protein kinase (MAPK) Pmk1, becomes activated by multiple environmental stimuli and controls several cellular functions. Here we demonstrate that farnesylated Rho2 becomes palmitoylated in vivo at cysteine-196 within its carboxyl end and that this modification allows its specific targeting to the plasma membrane. Unlike that of other palmitoylated and prenylated GTPases, the Rho2 control of morphogenesis and Pmk1 activity is strictly dependent upon plasma membrane localization and is not found in other cellular membranes. Indeed, artificial plasma membrane targeting bypassed the Rho2 need for palmitoylation in order to signal. Detailed functional analysis of Rho2 chimeras fused to the carboxyl end from the essential GTPase Rho1 showed that GTPase palmitoylation is partially dependent on the prenylation context and confirmed that Rho2 signaling is independent of Rho GTP dissociation inhibitor (GDI) function. We further demonstrate that Rho2 is an in vivo substrate for DHHC family acyltransferase Erf2 palmitoyltransferase. Remarkably, Rho3, another Erf2 target, negatively regulates Pmk1 activity in a Rho2-independent fashion, thus revealing the existence of cross talk whereby both GTPases antagonistically modulate the activity of this MAPK cascade.

Balance between exocytosis and endocytosis determines the efficacy of sterol-targeting antibiotics

Antifungals targeting membrane ergosterol are longstanding, yet indispensable drugs in clinical use. However, the mechanisms by which the cellular membrane domains recognized by these antibiotics are generated remain largely unknown. Here, we demonstrate that the balance between endocytosis and exocytosis in membrane trafficking is a critical factor in the action of sterol-targeting antibiotics. When fission yeast cells were treated with manumycin A, cellular binding and the action of the antifungals filipin, amphotericin B, and theonellamides, all of which are ergosterol-binders, were abolished. Additionally, manumycin A treatment attenuated Cdc42 activity and inhibited exocytosis, while endocytosis was only moderately suppressed. Similar defects in membrane trafficking could be reproduced by heat shock and genetic perturbation, which also abolished the action of the antibiotics. We propose that exocytosis and endocytosis respectively supply and internalize the specific plasma membrane domains recognized by sterol-targeting antibiotics.

An isoprenylation and palmitoylation motif promotes intraluminal vesicle delivery of proteins in cells from distant species

The C-terminal ends of small GTPases contain hypervariable sequences which may be posttranslationally modified by defined lipid moieties. The diverse structural motifs generated direct proteins towards specific cellular membranes or organelles. However, knowledge on the factors that determine these selective associations is limited. Here we show, using advanced microscopy, that the isoprenylation and palmitoylation motif of human RhoB (–CINCCKVL) targets chimeric proteins to intraluminal vesicles of endolysosomes in human cells, displaying preferential co-localization with components of the late endocytic pathway. Moreover, this distribution is conserved in distant species, including cells from amphibians, insects and fungi. Blocking lipidic modifications results in accumulation of CINCCKVL chimeras in the cytosol, from where they can reach endolysosomes upon release of this block. Remarkably, CINCCKVL constructs are sorted to intraluminal vesicles in a cholesterol-dependent process. In the lower species, neither the C-terminal sequence of RhoB, nor the endosomal distribution of its homologs are conserved; in spite of this, CINCCKVL constructs also reach endolysosomes in Xenopus laevis and insect cells. Strikingly, this behavior is prominent in the filamentous ascomycete fungus Aspergillus nidulans, in which GFP-CINCCKVL is sorted into endosomes and vacuoles in a lipidation-dependent manner and allows monitoring endosomal movement in live fungi. In summary, the isoprenylated and palmitoylated CINCCKVL sequence constitutes a specific structure which delineates an endolysosomal sorting strategy operative in phylogenetically diverse organisms.

Glucose activates TORC2-Gad8 protein via positive regulation of the cAMP/cAMP-dependent protein kinase A (PKA) pathway and negative regulation of the Pmk1 protein-mitogen-activated protein kinase pathway

The target of rapamycin (TOR) kinase belongs to the highly conserved eukaryotic family of phosphatidylinositol 3-kinase-related kinases. TOR proteins are found at the core of two evolutionary conserved complexes, known as TORC1 and TORC2. In fission yeast, TORC2 is dispensable for proliferation under optimal growth conditions but is required for starvation and stress responses. TORC2 has been implicated in a wide variety of functions; however, the signals that regulate TORC2 activity have so far remained obscure. TORC2 has one known direct substrate, the AGC kinase Gad8, which is related to AKT in human cells. Gad8 is phosphorylated by TORC2 at Ser-546 (equivalent to AKT Ser-473), leading to its activation. Here, we show that glucose is necessary and sufficient to induce Gad8 Ser-546 phosphorylation in vivo and Gad8 kinase activity in vitro. The glucose signal that activates TORC2-Gad8 is mediated via the cAMP/PKA pathway, a major glucose-sensing pathway. By contrast, Pmk1, similar to human extracellular signal-regulated kinases and a major stress-induced mitogen activated protein kinase (MAPK) in fission yeast, inhibits TORC2-dependent Gad8 phosphorylation and activation. Inhibition of TORC2-Gad8 also occurs in response to ionic or osmotic stress, in a manner dependent on the cAMP/PKA and Pmk1-MAPK signaling pathways. Our findings highlight the significance of glucose availability in regulation of TORC2-Gad8 and indicate a novel link between the cAMP/PKA, Pmk1/MAPK, and TORC2-Gad8 signaling.

Rho1 GTPase and PKC ortholog Pck1 are upstream activators of the cell integrity MAPK pathway in fission yeast

In the fission yeast Schizosaccharomyces pombe the cell integrity pathway (CIP) orchestrates multiple biological processes like cell wall maintenance and ionic homeostasis by fine tuning activation of MAPK Pmk1 in response to various environmental conditions. The small GTPase Rho2 positively regulates the CIP through protein kinase C ortholog Pck2. However, Pmk1 retains some function in mutants lacking either Rho2 or Pck2, suggesting the existence of additional upstream regulatory elements to modulate its activity depending on the nature of the environmental stimulus. The essential GTPase Rho1 is a candidate to control the activity of the CIP by acting upstream of Pck2, whereas Pck1, a second PKC ortholog, appears to negatively regulate Pmk1 activity. However, the exact regulatory nature of these two proteins within the CIP has remained elusive. By exhaustive characterization of strains expressing a hypomorphic Rho1 allele (rho1-596) in different genetic backgrounds we show that both Rho1 and Pck1 are positive upstream regulatory members of the CIP in addition to Rho2 and Pck2. In this new model Rho1 and Rho2 control Pmk1 basal activity during vegetative growth mainly through Pck2. Notably, whereas Rho2-Pck2 elicit Pmk1 activation in response to most environmental stimuli, Rho1 drives Pmk1 activation through either Pck2 or Pck1 exclusively in response to cell wall damage. Our study reveals the intricate and complex functional architecture of the upstream elements participating in this signaling pathway as compared to similar routes from other simple eukaryotic organisms.

The checkpoint-dependent nuclear accumulation of Rho1p exchange factor Rgf1p is important for tolerance to chronic replication stress

Guanine nucleotide exchange factors control many aspects of cell morphogenesis by turning on Rho-GTPases. The fission yeast exchange factor Rgf1p (Rho gef1) specifically regulates Rho1p during polarized growth and localizes to cortical sites. Here we report that Rgf1p is relocalized to the cell nucleus during the stalled replication caused by hydroxyurea (HU). Import to the nucleus is mediated by a nuclear localization sequence at the N-terminus of Rgf1p, whereas release into the cytoplasm requires two leucine-rich nuclear export sequences at the C-terminus. Moreover, Rgf1p nuclear accumulation during replication arrest depends on the 14-3-3 chaperone Rad24p and the DNA replication checkpoint kinase Cds1p. Both proteins control the nuclear accumulation of Rgf1p by inhibition of its nuclear export. A mutant, Rgf1p-9A, that substitutes nine serine potential phosphorylation Cds1p sites for alanine fails to accumulate in the nucleus in response to replication stress, and this correlates with a severe defect in survival in the presence of HU. In conclusion, we propose that the regulation of Rgf1p could be part of the mechanism by which Cds1p and Rad24p promote survival in the presence of chronic replication stress. It will be of general interest to understand whether the same is true for homologues of Rgf1p in budding yeast and higher eukaryotes.

Multiple regulatory levels influence cell integrity control by PKC ortholog Pck2 in fission yeast

Fission yeast PKC ortholog Pck2 controls cell wall synthesis and is a major upstream activator of the cell integrity pathway (CIP) and its core component, MAP kinase Pmk1, in response to environmental stimuli. We show that in vivo phosphorylation of Pck2 at the conserved T842 activation loop during growth and in response to different stresses is mediated by the PDK ortholog Ksg1 and an autophosphorylation mechanism. However, T842 phosphorylation is not essential for Pmk1 activation, and putative phosphorylation at T846 might play an additional role for Pck2 catalytic activation and downstream signaling. These events together with turn motif autophosphorylation at T984 and binding to small GTPases Rho1 and/or Rho2 stabilize and render Pck2 competent to exert its biological functions. Remarkably, the TORC2 complex does not participate in catalytic activation of Pck2, but instead contributes to de novo Pck2 synthesis which is essential to activate the CIP in response to cell wall damage or glucose exhaustion. These results unveil a novel mechanism whereby TOR regulates PKC function at a translational level and add a new regulatory layer to MAPK signaling cascades.

Hsp70-Hsp40 chaperone complex functions in controlling polarized growth by repressing Hsf1-driven heat stress-associated transcription

How the molecular mechanisms of stress response are integrated at the cellular level remains obscure. Here we show that the cellular polarity machinery in the fission yeast Schizosaccharomyces pombe undergoes dynamic adaptation to thermal stress resulting in a period of decreased Cdc42 activity and altered, monopolar growth. Cells where the heat stress-associated transcription was genetically upregulated exhibit similar growth patterning in the absence of temperature insults. We identify the Ssa2-Mas5/Hsp70-Hsp40 chaperone complex as repressor of the heat shock transcription factor Hsf1. Cells lacking this chaperone activity constitutively activate the heat-stress-associated transcriptional program. Interestingly, they also exhibit intermittent monopolar growth within a physiological temperature range and are unable to adapt to heat stress. We propose that by negatively regulating the heat stress-associated transcription, the Ssa2-Mas5 chaperone system could optimize cellular growth under different temperature regiments.

Heat stress, caused by fluctuations in ambient temperature, occurs frequently in nature. How organisms adapt and maintain regular patterns of growth over a range of environmental conditions remain poorly understood. Our work in the simple unicellular yeast Schizosaccharomyces pombe suggests that the heat stress-associated transcription must be repressed by the evolutionary conserved Hsp70-Hsp40 chaperone complex to allow cells to adapt the polarized growth machinery to elevated temperature.

Negative functional interaction between cell integrity MAPK pathway and Rho1 GTPase in fission yeast

Rho1 GTPase is the main activator of cell wall glucan biosynthesis and regulates actin cytoskeleton in fungi, including Schizosaccharomyces pombe. We have obtained a fission yeast thermosensitive mutant strain carrying the rho1-596 allele, which displays reduced Rho1 GTPase activity. This strain has severe cell wall defects and a thermosensitive growth, which is partially suppressed by osmotic stabilization. In a global screening for rho1-596 multicopy suppresors the pmp1+ gene was identified. Pmp1 is a dual specificity phosphatase that negatively regulates the Pmk1 mitogen-activated protein kinase (MAPK) cell integrity pathway. Accordingly, elimination of Pmk1 MAPK partially rescued rho1-596 thermosensitivity, corroborating the unexpected antagonistic functional relationship of these genes. We found that rho1-596 cells displayed increased basal activation of the cell integrity MAPK pathway and therefore were hypersensitive to MgCl2 and FK506. Moreover, the absence of calcineurin was lethal for rho1-596. We found a higher level of calcineurin activity in rho1-596 than in wild-type cells, and overexpression of constitutively active calcineurin partially rescued rho1-596 thermosensitivity. All together our results suggest that loss of Rho1 function causes an increase in the cell integrity MAPK activity, which is detrimental to the cells and turns calcineurin activity essential.

The fission yeast cell wall stress sensor-like proteins Mtl2 and Wsc1 act by turning on the GTPase Rho1p but act independently of the cell wall integrity pathway

Sensing stressful conditions that affect the cell wall reorganization is important for yeast survival. Here, we studied two proteins SpWsc1p and SpMtl2p with structural features indicative of plasma membrane-associated cell wall sensors. We found that Mtl2p and Wsc1p act by turning on the Rho1p GTPase. Each gene could be deleted individually without affecting viability, but the deletion of both was lethal and this phenotype was rescued by overexpression of the genes encoding either Rho1p or its GDP/GTP exchange factors (GEFs). In addition, wsc1Δ and mtl2Δ cells showed a low level of Rho1p-GTP under cell wall stress. Mtl2p-GFP (green fluorescent protein) localized to the cell periphery and was necessary for survival under different types of cell wall stress. Wsc1p-GFP was concentrated in patches at the cell tips, it interacted with the Rho-GEF Rgf2p, and its overexpression activated cell wall biosynthesis. Our results are consistent with the notion that cell wall assembly is regulated by two different networks involving Rho1p. One includes signaling from Mtl2p through Rho1p to Pck1p, while the second one implicates signaling from Wsc1p and Rgf2p through Rho1p to activate glucan synthase (GS). Finally, signaling through the mitogen-activated protein kinase (MAPK) Pmk1p remained active in mtl2Δ and wsc1Δ disruptants exposed to cell wall stress, suggesting that the cell wall stress-sensing spectrum of Schizosaccharomyces pombe sensor-like proteins differs from that of Saccharomyces cerevisiae.

Genome-wide screening for genes associated with valproic acid sensitivity in fission yeast

We have been studying the action mechanisms of valproic acid (VPA) in fission yeast Schizosaccharomyces pombe by developing a genetic screen for mutants that show hypersensitivity to VPA. In the present study, we performed a genome-wide screen of 3004 haploid deletion strains and confirmed 148 deletion strains to be VPA sensitive. Of the 148 strains, 93 strains also showed sensitivity to another aliphatic acids HDAC inhibitor, sodium butyrate (SB), and 55 strains showed sensitivity to VPA but not to SB. Interestingly, we found that both VPA and SB treatment induced a marked increase in the transcription activity of Atf1 in wild-type cells. However, in clr6-1, a mutant allele the clr6⁺ gene encoding class I HDAC, neither VPA- nor SB induced the activation of Atf1 transcription activity. We also found that VPA, but not SB, caused an increase in cytoplasmic Ca²⁺ level. We further found that the cytoplasmic Ca²⁺ increase was caused by Ca²⁺ influx from extracellular medium via Cch1-Yam8 channel complex. Altogether, our present study indicates that VPA and SB play similar but distinct roles in multiple physiological processes in fission yeast.

A genome-wide screening of potential target genes to enhance the antifungal activity of micafungin in Schizosaccharomyces pombe

Micafungin is a non-reversible inhibitor of 1, 3-β-D-glucan synthase and interferes with fungal cell wall synthesis. Clinically, micafungin has been shown to be efficacious for the treatment of invasive candidiasis and invasive aspergillosis. However, considering its relatively restricted antifungal spectrum, combination therapy with micafungin plus other agents should be considered in critically ill patients. To identify potential therapeutic targets for syncretic drug combinations that potentiate micafungin action, we carried out a genome-wide screen for altered sensitivity to micafungin by using the model yeast Schizosaccharomyces pombe mutant library. We confirmed that 159 deletion strains in the library are micafungin sensitive and classified them into various functional categories, including cell wall biosynthesis, gene expression and chromatin remodeling, membrane trafficking, signaling transduction, ubiquitination, ergosterol biosynthetic process and a variety of other known functions or still unknown functions. On the other hand, we also investigated the growth inhibitory activities of some well-known drugs in combination with micafungin including antifungal drug amphotericin B, fluconazole and immunosuppressive drug FK506. We found that amphotericin B in combination with micafungin showed a more potent inhibitory activity against wild-type cells than that of micafungin alone, whereas fluconazole in combination with micafungin did not. Also, the immunosuppressive drug FK506 showed synergistic inhibitory effect with micafungin on the growth of wild-type cells, whereas it decreased the inhibitory effect of micafungin in Δpmk1 cells, a deletion mutant of the cell wall integrity mitogen-activated protein kinase (MAPK) Pmk1. Altogether, our findings provide useful information for new potential drug combinations in the treatment of fungal infections.

Role of the fission yeast cell integrity MAPK pathway in response to glucose limitation

Background

Glucose is a signaling molecule which regulates multiple events in eukaryotic organisms and the most preferred carbon source in the fission yeast Schizosaccharomyces pombe. The ability of this yeast to grow in the absence of glucose becomes strongly limited due to lack of enzymes of the glyoxylate cycle that support diauxic growth. The stress-activated protein kinase (SAPK) pathway and its effectors, Sty1 MAPK and transcription factor Atf1, play a critical role in the adaptation of fission yeast to grow on alternative non-fermentable carbon sources by inducing the expression of fbp1⁺ gene, coding for the gluconeogenic enzyme fructose-1,6-bisphosphatase. The cell integrity Pmk1 pathway is another MAPK cascade that regulates various processes in fission yeast, including cell wall construction, cytokinesis, and ionic homeostasis. Pmk1 pathway also becomes strongly activated in response to glucose deprivation but its role during glucose exhaustion and ensuing adaptation to respiratory metabolism is currently unknown.

Results

We found that Pmk1 activation in the absence of glucose takes place only after complete depletion of this carbon source and that such activation is not related to an endogenous oxidative stress. Notably, Pmk1 MAPK activation relies on de novo protein synthesis, is independent on known upstream activators of the pathway like Rho2 GTPase, and involves PKC ortholog Pck2. Also, the Glucose/cAMP pathway is required operative for full activation of the Pmk1 signaling cascade. Mutants lacking Pmk1 displayed a partial growth defect in respiratory media which was not observed in the presence of glucose. This phenotype was accompanied by a decreased and delayed expression of transcription factor Atf1 and target genes fbp1⁺ and pyp2⁺. Intriguingly, the kinetics of Sty1 activation in Pmk1-less cells was clearly altered during growth adaptation to non-fermentable carbon sources.

Conclusions

Unknown upstream elements mediate Pck2-dependent signal transduction of glucose withdrawal to the cell integrity MAPK pathway. This signaling cascade reinforces the adaptive response of fission yeast to such nutritional stress by enhancing the activity of the SAPK pathway.

Biological significance of nuclear localization of mitogen-activated protein kinase Pmk1 in fission yeast

Mitogen-activated protein kinase (MAPK) signaling pathways play a fundamental role in the response of eukaryotic cells to environmental changes. Also, much evidence shows that the stimulus-dependent nuclear targeting of this class of regulatory kinases is crucial for adequate regulation of distinct cellular events. In the fission yeast Schizosaccharomyces pombe, the cell integrity MAPK pathway, whose central element is the MAPK Pmk1, regulates multiple processes such as cell wall integrity, vacuole fusion, cytokinesis, and ionic homeostasis. In non-stressed cells Pmk1 is constitutively localized in both cytoplasm and nucleus, and its localization pattern appears unaffected by its activation status or in response to stress, thus questioning the biological significance of the presence of this MAPK into the nucleus. We have addressed this issue by characterizing mutants expressing Pmk1 versions excluded from the cell nucleus and anchored to the plasma membrane in different genetic backgrounds. Although nuclear Pmk1 partially regulates cell wall integrity at a transcriptional level, membrane-tethered Pmk1 performs many of the biological functions assigned to wild type MAPK like regulation of chloride homeostasis, vacuole fusion, and cellular separation. However, we found that down-regulation of nuclear Pmk1 by MAPK phosphatases induced by the stress activated protein kinase pathway is important for the fine modulation of extranuclear Pmk1 activity. These results highlight the importance of the control of MAPK activity at subcellular level.

A genomewide screen in Schizosaccharomyces pombe for genes affecting the sensitivity of antifungal drugs that target ergosterol biosynthesis

We performed a genomewide screen for altered sensitivity to antifungal drugs, including clotrimazole and terbinafine, that target ergosterol biosynthesis using a Schizosaccharomyces pombe gene deletion library consisting of 3,004 nonessential haploid deletion mutants. We identified 109 mutants that were hypersensitive and 11 mutants that were resistant to these antifungals. Proteins whose absence rendered cells sensitive to these antifungals were classified into various functional categories, including ergosterol biosynthesis, membrane trafficking, histone acetylation and deacetylation, ubiquitination, signal transduction, ribosome biosynthesis and assembly, regulation of transcription and translation, cell wall organization and biogenesis, mitochondrion function, amino acid metabolism, nucleic acid metabolism, lipid metabolism, meiosis, and other functions. Also, proteins whose absence rendered cells resistant to these antifungals were classified into functional categories including mitochondrion function, ubiquitination, membrane trafficking, cell polarity, chromatin remodeling, and some unknown functions. Furthermore, the 109 sensitive mutants were tested for sensitivity to micafungin, another antifungal drug that inhibits (1,3)-β-D-glucan synthase, and 57 hypersensitive mutants were identified, suggesting that these mutants were defective in cell wall integrity. Altogether, our findings in fission yeast have shed light on molecular pathways associated with the cellular response to ergosterol biosynthesis inhibitors and may provide useful information for developing strategies aimed at sensitizing cells to these drugs.

Genome-wide screening for genes associated with FK506 sensitivity in fission yeast

We have been studying calcineurin signal transduction pathway in fission yeast Schizosaccharomyces pombe (S. pombe) by developing a genetic screen for mutants that show hypersensitivity to the immunosuppressive calcineurin inhibitor FK506 (tacrolimus). In the present study, to identify nonessential genes that are functionally related to the calcineurin signaling pathway, we performed a genome-wide screen of 3004 haploid deletion strains and confirmed 72 deletion strains to be FK506 sensitive. These 72 genes are classified into nine functional groups to include membrane trafficking (16 genes), signal transduction (10 genes), ubiquitination (8 genes), chromatin remodeling (6 genes), cytokinesis (4 genes), ribosomal protein (3 genes), RNA binding protein (3 genes), and a variety of other known functions (17 genes) or still unknown functions (5 genes) in the biological system. In our previous screening of FK506-sensitive mutants we isolated several membrane-trafficking mutants showing defective cell wall integrity. Here, we further examined the vacuolar fusion, the v-SNARE synaptobrevin Syb1 localization, and the sensitivity to the β-glucan synthase inhibitor micafungin in these 72 FK506-sensitive strains. Results showed that 25 deletion strains exhibited abnormal vacuole fusion, 19 deletion strains exhibited Syb1 mislocalization, and 14 deletion strains exhibited both abnormal vacuole fusion and Syb1 mislocalization, while 42 deletion strains showed both normal vacuole fusion and Syb1 localization. Likewise, 16 deletion strains showed sensitivity to micafungin. Altogether, our present study indicates that calcineurin mediates a plethora of physiological processes in fission yeast, and that calcineurin is extensively involved in cross-talk between signaling pathways.

Transient receptor potential (TRP) and Cch1-Yam8 channels play key roles in the regulation of cytoplasmic Ca2+ in fission yeast

The regulation of cytoplasmic Ca²⁺ is crucial for various cellular processes. Here, we examined the cytoplasmic Ca²⁺ levels in living fission yeast cells by a highly sensitive bioluminescence resonance energy transfer-based assay using GFP-aequorin fusion protein linked by 19 amino acid. We monitored the cytoplasmic Ca²⁺ level and its change caused by extracellular stimulants such as CaCl₂ or NaCl plus FK506 (calcineurin inhibitor). We found that the extracellularly added Ca²⁺ caused a dose-dependent increase in the cytoplasmic Ca²⁺ level and resulted in a burst-like peak. The overexpression of two transient receptor potential (TRP) channel homologues, Trp1322 or Pkd2, markedly enhanced this response. Interestingly, the burst-like peak upon TRP overexpression was completely abolished by gene deletion of calcineurin and was dramatically decreased by gene deletion of Prz1, a downstream transcription factor activated by calcineurin. Furthermore, 1 hour treatment with FK506 failed to suppress the burst-like peak. These results suggest that the burst-like Ca²⁺ peak is dependent on the transcriptional activity of Prz1, but not on the direct TRP dephosphorylation. We also found that extracellularly added NaCl plus FK506 caused a synergistic cytosolic Ca²⁺ increase that is dependent on the inhibition of calcineurin activity, but not on the inhibition of Prz1. The synergistic Ca²⁺ increase is abolished by the addition of the Ca²⁺ chelator BAPTA into the media, and is also abolished by deletion of the gene encoding a subunit of the Cch1-Yam8 Ca²⁺ channel complex, indicating that the synergistic increase is caused by the Ca²⁺ influx from the extracellular medium via the Cch1-Yam8 complex. Furthermore, deletion of Pmk1 MAPK abolished the Ca²⁺ influx, and overexpression of the constitutively active Pek1 MAPKK enhanced the influx. These results suggest that Pmk1 MAPK and calcineurin positively and negatively regulate the Cch1-Yam8 complex, respectively, via modulating the balance between phosphorylation and dyphosphorylation state.

Rga4, a Rho-GAP from fission yeast: Finding specificity within promiscuity

Regulation by signaling molecules of pathways involved in determining cell size and shape is fundamental to understand morphogenesis. In eukaryotic cells, Rho GTPases modulate cellular events by acting as molecular switches. GTPase Activating Proteins (GAPs) control the fine-tuning of Rho GTPase activity as downregulators that promote their inactive state. We use Schizosaccharomyces pombe as a model to unveil key mechanisms underlying processes of general significance. Rga4, one of the nine RhoGAPs present in the fission yeast, is a key factor in the control of cell polarity and morphogenesis by negatively regulating the activity of the essential Rho GTPase Cdc42. We have demonstrated that Rga4 is also a GAP for Rho2 GTPase, which acts upstream of the Pmk1 cell integrity MAP kinase pathway and positively regulates cell integrity and cell separation. Our findings suggest that Rga4 control of both Cdc42 and Rho2 function is rather independent, thus providing a good example of regulatory specificity. Additionally, we describe multiple GAPs that can downregulate Pmk1 activity in a Rho2-dependent and independent fashion. These studies corroborate the existence of a sophisticated regulatory network by which different RhoGAPs modulate differentially the activity of Rho GTPases, and the existence of different inputs for the Pmk1 cell integrity MAP kinase pathway.

Kin1 is a plasma membrane-associated kinase that regulates the cell surface in fission yeast

Cell morphogenesis is a complex process that depends on cytoskeleton and membrane organization, intracellular signalling and vesicular trafficking. The rod shape of the fission yeast Schizosaccharomyces pombe and the availability of powerful genetic tools make this species an excellent model to study cell morphology. Here we have investigated the function of the conserved Kin1 kinase. Kin1-GFP associates dynamically with the plasma membrane at sites of active cell surface remodelling and is present in the membrane fraction. Kin1Δ null cells show severe defects in cell wall structure and are unable to maintain a rod shape. To explore Kin1 primary function, we constructed an ATP analogue-sensitive allele kin1-as1. Kin1 inhibition primarily promotes delocalization of plasma membrane-associated markers of actively growing cell surface regions. Kin1 itself is depolarized and its mobility is strongly reduced. Subsequently, amorphous cell wall material accumulates at the cell surface, a phenotype that is dependent on vesicular trafficking, and the cell wall integrity mitogen-activated protein kinase pathway is activated. Deletion of cell wall integrity mitogen-activated protein kinase components reduces kin1Δ hypersensitivity to stresses such as those induced by Calcofluor white and SDS. We propose that Kin1 is required for a tight link between the plasma membrane and the cell wall.

Cooperation between the septins and the actomyosin ring and role of a cell-integrity pathway during cell division in fission yeast

A major question about cytokinesis concerns the role of the septin proteins, which localize to the division site in all animal and fungal cells but are essential for cytokinesis only in some cell types. For example, in Schizosaccharomyces pombe, four septins localize to the division site, but deletion of the four genes produces only a modest delay in cell separation. To ask if the S. pombe septins function redundantly in cytokinesis, we conducted a synthetic-lethal screen in a septin-deficient strain and identified seven mutations. One mutation affects Cdc4, a myosin light chain that is an essential component of the cytokinetic actomyosin ring. Five others cause frequent cell lysis during cell separation and map to two loci. These mutations and their dosage suppressors define a signaling pathway (including Rho1 and a novel arrestin) for repairing cell-wall damage. The seventh mutation affects the poorly understood RNA-binding protein Scw1 and severely delays cell separation when combined either with a septin mutation or with a mutation affecting the septin-interacting, anillin-like protein Mid2, suggesting that Scw1 functions in a pathway parallel to that of the septins. Taken together, our results suggest that the S. pombe septins participate redundantly in one or more pathways that cooperate with the actomyosin ring during cytokinesis and that a septin defect causes septum defects that can be repaired effectively only when the cell-integrity pathway is intact.

Rho GTPases: regulation of cell polarity and growth in yeasts

Eukaryotic cells display a wide range of morphologies important for cellular function and development. A particular cell shape is made via the generation of asymmetry in the organization of cytoskeletal elements, usually leading to actin localization at sites of growth. The Rho family of GTPases is present in all eukaryotic cells, from yeast to mammals, and their role as key regulators in the signalling pathways that control actin organization and morphogenetic processes is well known. In the present review we will discuss the role of Rho GTPases as regulators of yeasts' polarized growth, their mechanism of activation and signalling pathways in Saccharomyces cerevisiae and Schizosaccharomyces pombe. These two model yeasts have been very useful in the study of the molecular mechanisms responsible for cell polarity. As in other organisms with cell walls, yeast's polarized growth is closely related to cell-wall biosynthesis, and Rho GTPases are critical modulators of this process. They provide the co-ordinated regulation of cell-wall biosynthetic enzymes and actin organization required to maintain cell integrity during vegetative growth.

Rga4 modulates the activity of the fission yeast cell integrity MAPK pathway by acting as a Rho2 GTPase-activating protein

Rho GTPase-activating proteins (GAPs) are responsible for the inactivation of Rho GTPases, which are involved in the regulation of critical biological responses in eukaryotic cells, ranging from cell cycle control to cellular morphogenesis. The genome of fission yeast Schizosaccharomyces pombe contains six genes coding for putative Rho GTPases, whereas nine genes code for predicted Rho GAPs (Rga1 to Rga9). One of them, Rga4, has been recently described as a Cdc42 GAP, involved in the control of cell diameter and symmetry in fission yeast. In this work we show that Rga4 is also a Rho2 GAP that negatively modulates the activity of the cell integrity pathway and its main effector, MAPK Pmk1. The DYRK-type protein kinase Pom1, which regulates both the localization and phosphorylation state of Rga4, is also a negative regulator of the Pmk1 pathway, but this control is not dependent upon the Rga4 role as a Rho2-GAP. Hence, two subsets of Rga4 negatively regulate Cdc42 and Rho2 functions in a specific and unrelated way. Finally, we show that Rga7, another Rho2 GAP, down-regulates the Pmk1 pathway in addition to Rga4. These results reinforce the notion of the existence of complex mechanisms determining the selectivity of Rho GAPs toward Rho GTPases and their functions.

The cell surface protein gene ecm33+ is a target of the two transcription factors Atf1 and Mbx1 and negatively regulates Pmk1 MAPK cell integrity signaling in fission yeast

The highly conserved fission yeast Pmk1 MAPK pathway plays a key role in cell integrity by regulating Atf1, which belongs to the ATF/cAMP-responsive element-binding (CREB) protein family. We identified and characterized ecm33(+), which encodes a glycosyl-phosphatidylinositol (GPI)-anchored cell surface protein as a transcriptional target of Pmk1 and Atf1. We demonstrated that the gene expression of Ecm33 is regulated by two transcription factors Atf1 and a MADS-box-type transcription factor Mbx1. We identified a putative ATF/CREB-binding site and an RLM1-binding site in the ecm33(+) promoter region and monitored the transcriptional activity of Atf1 or Mbx1 in living cells using a destabilized luciferase reporter gene fused to three tandem repeats of the CRE and six tandem repeats of the Rlm1-binding sequence, respectively. These reporter genes reflect the activation of the Pmk1 pathway by various stimuli, thereby enabling the real-time monitoring of the Pmk1 cell integrity pathway. Notably, the Deltaecm33 cells displayed hyperactivation of the Pmk1 signaling together with hypersensitivity to Ca(2+) and an abnormal morphology, which were almost abolished by simultaneous deletion of the components of the Rho2/Pck2/Pmk1 pathway. Our results suggest that Ecm33 is involved in the negative feedback regulation of Pmk1 cell integrity signaling and is linked to cellular Ca(2+) signaling.