Two novel protein kinase C-related genes of fission yeast are essential for cell viability and implicated in cell shape control

Multiple polarity kinases inhibit phase separation of F-BAR protein Cdc15 and antagonize cytokinetic ring assembly in fission yeast

The F-BAR protein Cdc15 is essential for cytokinesis in Schizosaccharomyces pombe and plays a key role in attaching the cytokinetic ring (CR) to the plasma membrane (PM). Cdc15's abilities to bind to the membrane and oligomerize via its F-BAR domain are inhibited by phosphorylation of its intrinsically disordered region (IDR). Multiple cell polarity kinases regulate Cdc15 IDR phosphostate, and of these the DYRK kinase Pom1 phosphorylation sites on Cdc15 have been shown in vivo to prevent CR formation at cell tips. Here, we compared the ability of Pom1 to control Cdc15 phosphostate and cortical localization to that of other Cdc15 kinases: Kin1, Pck1, and Shk1. We identified distinct but overlapping cohorts of Cdc15 phosphorylation sites targeted by each kinase, and the number of sites correlated with each kinases' abilities to influence Cdc15 PM localization. Coarse-grained simulations predicted that cumulative IDR phosphorylation moves the IDRs of a dimer apart and toward the F-BAR tips. Further, simulations indicated that the overall negative charge of phosphorylation masks positively charged amino acids necessary for F-BAR oligomerization and membrane interaction. Finally, simulations suggested that dephosphorylated Cdc15 undergoes phase separation driven by IDR interactions. Indeed, dephosphorylated but not phosphorylated Cdc15 undergoes liquid-liquid phase separation to form droplets in vitro that recruit Cdc15 binding partners. In cells, Cdc15 phosphomutants also formed PM-bound condensates that recruit other CR components. Together, we propose that a threshold of Cdc15 phosphorylation by assorted kinases prevents Cdc15 condensation on the PM and antagonizes CR assembly.

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.

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.

Calcium signaling is involved in diverse cellular processes in fungi

Calcium (Ca²⁺) is a universal signalling molecule of life. The Ca²⁺ signalling is an evolutionarily conserved process from prokaryotes to eukaryotes. Ca²⁺ at high concentration is deleterious to the cell; therefore, cell maintains a low resting level of intracellular free Ca²⁺ concentration ([Ca²⁺]_c). The resting [Ca²⁺]_c is tightly regulated, and a transient increase of the [Ca²⁺]_c initiates a signalling cascade in the cell. Ca²⁺ signalling plays an essential role in various processes, including growth, development, reproduction, tolerance to stress conditions, and virulence in fungi. In this review, we describe the evolutionary aspects of Ca²⁺ signalling and cell functions of major Ca²⁺ signalling proteins in different fungi.

Fission yeast cell wall biosynthesis and cell integrity signalling

The cell wall is a structure external to the plasma membrane that is essential for the survival of the fungi. This polysaccharidic structure confers resistance to the cell internal turgor pressure and protection against mechanical injury. The fungal wall is also responsible for the shape of these organisms due to different structural polysaccharides, such as β-(1,3)-glucan, which form fibers and confer rigidity to the cell wall. These polysaccharides are not present in animal cells and therefore they constitute excellent targets for antifungal chemotherapies. Cell wall damage leads to the activation of MAPK signaling pathways, which respond to the damage by activating the repair of the wall and the maintenance of the cell integrity. Fission yeast Schizosaccharomyces pombe is a model organism for the study morphogenesis, cell wall, and how different inputs might regulate this structure. We present here a short overview of the fission yeast wall composition and provide information about the main biosynthetic activities that assemble this cell wall. Additionally, we comment the recent advances in the knowledge of the cell wall functions and discuss the role of the cell integrity MAPK signaling pathway in the regulation of fission yeast wall.

Fission yeast ceramide ts mutants cwh43 exhibit defects in G0 quiescence, nutrient metabolism, and lipid homeostasis

Cellular nutrient states control whether cells proliferate, or whether they enter or exit quiescence. Here, we report characterizations of fission yeast temperature-sensitive (ts) mutants of the evolutionarily conserved transmembrane protein, Cwh43, and explore its relevance to utilization of glucose, nitrogen-source, and lipids. GFP-tagged Cwh43 localizes at ER associated with the nuclear envelope and the plasma membrane, as in budding yeast. We found that cwh43 mutants failed to divide in low glucose and lost viability during quiescence under nitrogen starvation. In cwh43 mutant, comprehensive metabolome analysis demonstrated dramatic changes in marker metabolites that altered under low glucose and/or nitrogen starvation, although cwh43 apparently consumed glucose in the culture media. Furthermore, we found that cwh43 mutant had elevated levels of triacylglycerols (TGs) and coenzyme A, and that it accumulated lipid droplets. Notably, TG biosynthesis was required to maintain cell division in cwh43 mutant. Thus, Cwh43 affects utilization of glucose and nitrogen-sources, as well as storage lipid metabolism. These results may fit to a notion developed in budding yeast that Cwh43 conjugates ceramide to GPI (glycosylphosphatidylinositol)-anchored proteins and maintains integrity of membrane organization.

Protein kinase C is essential for viability of the rice blast fungus Magnaporthe oryzae

Protein kinase C constitutes a family of serine–threonine kinases found in all eukaryotes and implicated in a wide range of cellular functions, including regulation of cell growth, cellular differentiation and immunity. Here, we present three independent lines of evidence which indicate that protein kinase C is essential for viability of Magnaporthe oryzae. First, all attempts to generate a target deletion of PKC1, the single copy protein kinase C‐encoding gene, proved unsuccessful. Secondly, conditional gene silencing of PKC1 by RNA interference led to severely reduced growth of the fungus, which was reversed by targeted deletion of the Dicer2‐encoding gene, MDL2. Finally, selective kinase inhibition of protein kinase C by targeted allelic replacement with an analogue‐sensitive PKC1^AS allele led to specific loss of fungal viability in the presence of the PP1 inhibitor. Global transcriptional profiling following selective PKC inhibition identified significant changes in gene expression associated with cell wall re‐modelling, autophagy, signal transduction and secondary metabolism. When considered together, these results suggest protein kinase C is essential for growth and development of M. oryzae with extensive downstream targets in addition to the cell integrity pathway. Targeting protein kinase C signalling may therefore prove an effective means of controlling rice blast disease.

Cell migration and division in amoeboid-like fission yeast

Yeast cells are non-motile and are encased in a cell wall that supports high internal turgor pressure. The cell wall is also essential for cellular morphogenesis and cell division. Here, we report unexpected morphogenetic changes in a Schizosaccharomyces pombe mutant defective in cell wall biogenesis. These cells form dynamic cytoplasmic protrusions caused by internal turgor pressure and also exhibit amoeboid-like cell migration resulting from repeated protrusive cycles. The cytokinetic ring responsible for cell division in wild-type yeast often fails in these cells; however, they were still able to divide using a ring-independent alternative mechanism relying on extrusion of the cell body through a hole in the cell wall. This mechanism of cell division may resemble an ancestral mode of division in the absence of cytokinetic machinery. Our findings highlight how a single gene change can lead to the emergence of different modes of cell growth, migration and division.

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.

Actin filament severing by cofilin is more important for assembly than constriction of the cytokinetic contractile ring

When fission yeast express mutant cofilin that is inefficient at actin filament severing, cytokinetic contractile ring formation is severely impaired, but ring contraction proceeds efficiently.

We created two new mutants of fission yeast cofilin to investigate why cytokinesis in many organisms depends on this small actin-binding protein. These mutant cofilins bound actin monomers normally, but bound and severed ADP-actin filaments much slower than wild-type cofilin. Cells depending on mutant cofilins condensed nodes, precursors of the contractile ring, into clumps rather than rings. Starting from clumped nodes, mutant cells slowly assembled rings from diverse intermediate structures including spiral strands containing actin filaments and other contractile ring proteins. This process in mutant cells depended on α-actinin. These slowly assembled contractile rings constricted at a normal rate but with more variability, indicating ring constriction is not very sensitive to defects in severing by cofilin. Computer simulations of the search-capture-pull and release model of contractile ring formation predicted that nodes clump when the release step is slow, so cofilin severing of actin filament connections between nodes likely contributes to the release step.

Schizosaccharomyces pombe cell division cycle under limited glucose requires Ssp1 kinase, the putative CaMKK, and Sds23, a PP2A-related phosphatase inhibitor

Calcium/calmodulin-dependent protein kinase (CaMK) is required for diverse cellular functions, and similar kinases exist in fungi. Although mammalian CaMK kinase (CaMKK) activates CaMK and also evolutionarily-conserved AMP-activated protein kinase (AMPK), CaMKK is yet to be established in yeast. We here report that the fission yeast Schizosaccharomyces pombe Ssp1 kinase, which controls G2/M transition and response to stress, is the putative CaMKK. Ssp1 has a CaM binding domain (CBD) and associates with 14-3-3 proteins as mammalian CaMKK does. Temperature-sensitive ssp1 mutants isolated are defective in the tolerance to limited glucose, and this tolerance requires the conserved stretch present between the kinase domain and CBD. Sds23, multi-copy suppressor for mutants defective in type 1 phosphatase and APC/cyclosome, also suppresses the ssp1 phenotype, and is required for the tolerance to limited glucose. We demonstrate that Sds23 binds to type 2A protein phosphatases (PP2A) and PP2A-related phosphatase Ppe1, and that Sds23 inhibits Ppe1 phosphatase activity. Ssp1 and Ppe1 thus seem to antagonize in utilizing limited glucose. We also show that Ppk9 and Ssp2 are the catalytic subunits of AMPK and AMPK-related kinases, respectively, which bind to common beta-(Amk2) and gamma-(Cbs2) subunits.

The Rho1p exchange factor Rgf1p signals upstream from the Pmk1 mitogen-activated protein kinase pathway in fission yeast

The Schizosaccharomyces pombe exchange factor Rgf1p specifically regulates Rho1p during polarized growth. Rgf1p activates the beta-glucan synthase (GS) complex containing the catalytic subunit Bgs4p and is involved in the activation of growth at the second end, a transition that requires actin reorganization. In this work, we investigated Rgf1p signaling and observed that Rgf1p acted upstream from the Pck2p-Pmk1p MAPK signaling pathway. We noted that Rgf1p and calcineurin play antagonistic roles in Cl(-) homeostasis; rgf1Delta cells showed the vic phenotype (viable in the presence of immunosuppressant and chlorine ion) and were unable to grow in the presence of high salt concentrations, both phenotypes being characteristic of knockouts of the MAPK components. In addition, mutations that perturb signaling through the MAPK pathway resulted in defective cell integrity (hypersensitivity to caspofungin and beta-glucanase). Rgf1p acts by positively regulating a subset of stimuli toward the Pmk1p-cell integrity pathway. After osmotic shock and cell wall damage HA-tagged Pmk1p was phosphorylated in wild-type cells but not in rgf1Delta cells. Finally, we provide evidence to show that Rgf1p regulates Pmk1p activation in a process that involves the activation of Rho1p and Pck2p, and we demonstrate that Rgf1p is unique in this signaling process, because Pmk1p activation was largely independent of the other two Rho1p-specific GEFs, Rgf2p and Rgf3p.

Stress-regulated kinase pathways in the recovery of tip growth and microtubule dynamics following osmotic stress in S. pombe

The cell-integrity and stress-response MAP kinase pathways (CIP and SRP, respectively) are stimulated by various environmental stresses. Ssp1 kinase modulates actin dynamics and is rapidly recruited to the plasma membrane following osmotic stress. Here, we show that osmotic stress arrested tip growth, induced the deposition of abnormal cell-wall deposits at tips and led to disassociation of F-actin foci from cell tips together with a reduction in the amount of F-actin in these foci. Osmotic stress also ;froze' the dynamics of interphase microtubule bundles, with microtubules remaining static for approximately 38 minutes (at 30 degrees C) before fragmenting upon return to dynamic behaviour. The timing with which microtubules resumed dynamic behaviour relied upon SRP activation of Atf1-mediated transcription, but not on either CIP or Ssp1 signalling. Analysis of the recovery of tip growth showed that: (1) the timing of recovery was controlled by SRP-stimulated Atf1 transcription; (2) re-establishment of polarized tip growth was absolutely dependent upon SRP and partially dependent upon Ssp1 signalling; and (3) selection of the site for polarized tip extension required Ssp1 and the SRP-associated polarity factor Wsh3 (also known as Tea4). CIP signalling did not impact upon any aspect of recovery. The normal kinetics of tip growth following osmotic stress of plo1.S402A/E mutants established that SRP control over the resumption of tip growth after osmotic stress is distinct from its control of tip growth following heat or gravitational stresses.

Phosphoinositide Signaling in Unicellular Eukaryotes

The review considers the up to date achievements in the role of membrane phosphoinositides and keys enzymes of the lipid branch of the phosphoinositide signal pathway (PI-pathway) in unicellular eukaryotes. Particular attention is paid to mechanisms of phospholipase C (PLC) activation and the PLC interaction both with cell surface receptors and with the effector cytoplasm targets. The role of protein kinase C (PKC) in intracellular signaling and the relationship of the PI-pathway key enzymes with protein tyrosine kinases (PTK)-signaling and cAMP-protein kinase A (PKA) pathway are discussed.

Schizosaccharomyces pombe protein phosphatase 1 in mitosis, endocytosis and a partnership with Wsh3/Tea4 to control polarised growth

PP1 holoenzymes are composed of a small number of catalytic subunits and an array of regulatory, targeting, subunits. The Schizosaccharomyces pombe genome encodes two highly related catalytic subunits, Dis2 and Sds21. The gene for either protein can be individually deleted, however, simultaneous deletion of both is lethal. We fused enhanced green fluorescent protein (EGFP) coding sequences to the 5' end of the endogenous sds21(+) and dis2(+) genes. Dis2.NEGFP accumulated in nuclei, associated with centromeres, foci at cell tips and endocytic vesicles. This actin-dependent endocytosis occurred between nuclei and growing tips and was polarised towards growing tips. When dis2(+) was present, Sds21.NEGFP was predominantly a nuclear protein, greatly enriched in the nucleolus. When dis2(+) was deleted, Sds21.NEGFP levels increased and Sds21.NEGFP was then clearly detected at centromeres, endocytic vesicles and cell tips. Dis2.NEGFP was recruited to cell tips by the formin binding, stress pathway scaffold Wsh3 (also known as Tea4). Wsh3/Tea4 modulates polarised tip growth in unperturbed cell cycles and governs polarised growth following osmotic stress. Mutating the PP1 recruiting RVXF motif in Wsh3/Tea4 blocked PP1 binding, altered cell cycle regulated growth to induce branching, induced branching from existing tips in response to stress, and blocked the induction of actin filaments that would otherwise arise from Wsh3/Tea4 overproduction.

Atf1 is a target of the mitogen-activated protein kinase Pmk1 and regulates cell integrity in fission yeast

In fission yeast, knockout of the calcineurin gene resulted in hypersensitivity to Cl(-), and the overexpression of pmp1(+) encoding a dual-specificity phosphatase for Pmk1 mitogen-activated protein kinase (MAPK) or the knockout of the components of the Pmk1 pathway complemented the Cl(-) hypersensitivity of calcineurin deletion. Here, we showed that the overexpression of ptc1(+) and ptc3(+), both encoding type 2C protein phosphatase (PP2C), previously known to inactivate the Wis1-Spc1-Atf1 stress-activated MAPK signaling pathway, suppressed the Cl(-) hypersensitivity of calcineurin deletion. We also demonstrated that the mRNA levels of these two PP2Cs and pyp2(+), another negative regulator of Spc1, are dependent on Pmk1. Notably, the deletion of Atf1, but not that of Spc1, displayed hypersensitivity to the cell wall-damaging agents and also suppressed the Cl(-) hypersensitivity of calcineurin deletion, both of which are characteristic phenotypes shared by the mutation of the components of the Pmk1 MAPK pathway. Moreover, micafungin treatment induced Pmk1 hyperactivation that resulted in Atf1 hyperphosphorylation. Together, our results suggest that PP2C is involved in a negative feedback loop of the Pmk1 signaling, and results also demonstrate that Atf1 is a key component of the cell integrity signaling downstream of Pmk1 MAPK.

Splitting of the fission yeast septum

In cell-walled organisms, a cross wall (septum) is produced during cytokinesis, which then splits in certain organisms to allow the daughter cells to separate. The formation and the subsequent cleavage of the septum require wall synthesis and wall degradation, which need to be strictly coordinated in order to prevent cell lysis. The dividing fission yeast (Schizosaccharomyces) cell produces a three-layered septum in which the middle layer and a narrow band of the adjacent cell wall can be degraded without threatening the integrity of the separating daughter cells. This spatially very precise process requires the activity of the Agn1p 1,3-alpha-glucanase and the Eng1p 1,3-beta-glucanase, which are localized to the septum by a complex mechanism involving the formation of a septin ring and the directed activity of the exocyst system. The Sep1p-Ace2p transcription-factor cascade regulates the expression of many genes producing proteins for this complex process. Recent advances in research into the molecular mechanisms of separation and its regulation are discussed in this review.

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

We have previously demonstrated that knockout of the calcineurin gene or inhibition of calcineurin activity by immunosuppressants resulted in hypersensitivity to Cl- in fission yeast. We also demonstrated that knockout of the components of the Pmk1 mitogen-activated protein kinase (MAPK) pathway, such as Pmk1 or Pek1 complemented the hypersensitivity to Cl-. Using this interaction between calcineurin and Pmk1 MAPK, here we developed a genetic screen that aims to identify new regulators of the Pmk1 signaling and isolated vic (viable in the presence of immunosuppressant and chloride ion) mutants. One of the mutants, vic1-1, carried a missense mutation in the cpp1+ gene encoding a beta subunit of the protein farnesyltransferase, which caused an amino acid substitution of aspartate 155 of Cpp1 to asparagine (Cpp1(D155N)). Analysis of the mutant strain revealed that Rho2 is a novel target of Cpp1. Moreover, Cpp1 and Rho2 act upstream of Pck2-Pmk1 MAPK signaling pathway, thereby resulting in the vic phenotype upon their mutations. Interestingly, compared with other substrates of Cpp1, defects of Rho2 function were more phenotypically manifested by the Cpp1(D155N) mutation. Together, our results demonstrate that Cpp1 is a key component of the Pck2-Pmk1 signaling through the spatial control of the small GTPase Rho2.

Real-time monitoring of calcineurin activity in living cells: evidence for two distinct Ca2+-dependent pathways in fission yeast

In fission yeast, calcineurin dephosphorylates and activates the Prz1 transcription factor. Here, we identified the calcineurin-dependent response element (CDRE) in the promoter region of prz1(+) gene and monitored the calcineurin activity in living cells using a destabilized luciferase reporter gene fused to three tandem repeats of CDRE. Elevated extracellular CaCl(2) caused an increase in calcineurin activity with an initial peak and then approached a sustained constant level in a concentration-dependent manner. In CaCl(2)-sensitive mutants such as Deltapmc1, the response was markedly enhanced, reflecting its high intracellular Ca(2+). Agents expected to induce Ca(2+) influx showed distinct patterns of the CDRE-reporter activity, suggesting different mechanisms of calcineurin activation. Knockout of yam8(+) or cch1(+) encoding putative subunits of a Ca(2+) channel abolished the activation of calcineurin upon exposure to various stimuli, including high extracellular NaCl and cell wall-damaging agents. However, knockout of yam8(+) or cch1(+) did not affect the activation of calcineurin upon stimulation by elevated extracellular Ca(2+). The Pck2 protein kinase C-Pmk1 mitogen-activate protein kinase pathway was required for the stimulation of calcineurin via Yam8/Cch1-mediated Ca(2+) influx, but it was not required for the stimulation by elevated extracellular Ca(2+), suggesting two distinct pathways for calcineurin activation.

Protein kinase C (PkcA) of Aspergillus nidulans is involved in penicillin production

The biosynthesis of the beta-lactam antibiotic penicillin in the filamentous fungus Aspergillus nidulans is catalyzed by three enzymes that are encoded by the acvA, ipnA, and aatA genes. A variety of cis-acting DNA elements and regulatory factors form a complex regulatory network controlling these beta-lactam biosynthesis genes. Regulators involved include the CCAAT-binding complex AnCF and AnBH1. AnBH1 acts as a repressor of the penicillin biosynthesis gene aatA. Until now, however, little information has been available on the signal transduction cascades leading to the transcription factors. Here we show that inhibition of protein kinase C (Pkc) activity in A. nidulans led to cytoplasmic localization of an AnBH1-enhanced green fluorescent protein (EGFP) fusion protein. Computer analysis of the genome and screening of an A. nidulans gene library revealed that the fungus possesses two putative Pkc-encoding genes, which we designated pkcA and pkcB. Only PkcA showed all the characteristic features of fungal Pkc's. Production of pkcA antisense RNA in A. nidulans led to reduced growth and conidiation in Aspergillus minimal medium, while in fermentation medium it led to enhanced expression of an aatAp-lacZ gene fusion, reduced pencillin production, and predominantly cytoplasmic localization of AnBH1. These data agree with the finding that inhibition of Pkc activity prevented nuclear localization of AnBH1-EGFP. As a result, repression of aatA expression was relieved. The involvement of Pkc in penicillin biosynthesis is also interesting in light of the fact that in the yeast Saccharomyces cerevisiae, Pkc plays a major role in maintaining cell integrity.

Rgf1p is a specific Rho1-GEF that coordinates cell polarization with cell wall biogenesis in fission yeast

Rho1p regulates cell integrity by controlling the actin cytoskeleton and cell wall synthesis. We have identified a new GEF, designated Rgf1p, which specifically regulates Rho1p during polarized growth. The phenotype of rgf1 null cells was very similar to that seen after depletion of Rho1p, 30% of cells being lysed. In addition, rgf1(+) deletion caused hypersensitivity to the antifungal drug Caspofungin and defects in the establishment of bipolar growth. rho1(+), but none of the other GTPases of the Rho-family, suppressed the rgf1Delta phenotypes. Moreover, deletion of rgf1(+) suppressed the severe growth defect in rga1(+) null mutants (a Rho1-GAP, negative regulator). Rgf1p and Rho1p coimmunoprecipitated and overexpression of rgf1(+) specifically increased the GTP-bound Rho1p; it caused changes in cell morphology, and a large increase in beta(1,3)-glucan synthase activity. These effects were similar to those elicited when the hyperactive rho1-G15V allele was expressed. A genetic relationship was observed between Rgf1p, Bgs4p (beta[1,3]-glucan synthase), and Pck1p (protein kinase C [PKC] homologue); Bgs4p and Pck1p suppressed the hypersensitivity to Caspofungin in rgf1Delta mutants. Rgf1p localized to the growing ends and the septum, where Rho1, Pck1p, and Bgs4p are known to function. Our results suggest that Rgf1p probably activates the Rho functions necessary for coordinating actin deposition with cell wall biosynthesis during bipolar growth, allowing the cells to remodel their wall without risk of rupture.

Global Analysis of the Hortaea werneckii Proteome: Studying Steroid Response in Yeast

The response of the halophilic black yeast Hortaea werneckii to the steroid hormone progesterone has been studied at the protein level using fluorescent two-dimensional differential gel electrophoresis (2D-DIGE) technology in combination with mass spectrometry. Data on protein identification from this study reveal molecular mechanisms of the response to progesterone. In particular, the overexpression of Pck2 and Pac2 in the stimulated cells indicates the interactions of progesterone with the cell growth and reproduction signaling pathways.

Apoptosis and lipoapoptosis in the fission yeast Schizosaccharomyces pombe

Yeasts being simple eukaryotes are established genetic systems that are often employed to solve important biological questions. Recently, it has become evident that certain cell death programs exist in these unicellular organisms. For example, it has been shown recently that strains of the fission yeast Schizosaccharomyces pombe deficient in triacylglycerol synthesis undergo cell death with prominent apoptotic markers. This minireview is intended to discuss key developments that have rendered fission yeast useful both as a tool and as a model for apoptosis and lipoapoptosis research. It is attempted to delineate a putative signaling pathway leading to the execution of lipoapoptosis in the fission yeast. Although in its infancy, apoptosis research in the fission yeast promises exciting breakthroughs in the near future.

In vivo activation of protein kinase A in Schizosaccharomyces pombe requires threonine phosphorylation at its activation loop and is dependent on PDK1

Phosphoinositide-dependent protein kinase 1 (PDK1) plays a central role in cellular signaling by phosphorylating members of the AGC family of kinases. This family includes protein kinase C (PKC), protein kinase B (PKB), p70/p90 ribosomal S6 kinases (RSK and S6K), and the catalytic subunit of cAMP-dependent protein kinase (PKA). Although PDK1 phosphorylates and activates PKC, PKB, and RSK in vivo, PDK1 regulation of PKA remains controversial. We isolated ksg1, the fission yeast ortholog of mammalian PDK1, as a suppressor of growth defects caused by loss of the stress-activated MAP kinase, Spc1. Here, we demonstrate that Ksg1 is required for activation of PKA. Cells containing the ksg1.12 thermolabile allele exhibit pleiotropic phenotypes, including the failure to arrest in G(1) and an inability to conjugate. The ksg1.12 allele strongly suppresses defects associated with unregulated PKA. Pka1, the catalytic subunit of cAMP-dependent protein kinase, is phosphorylated in vivo at Thr-356, which is located in the activation loop of the kinase and corresponds to Thr-197 in mammalian PKA. Phosphorylation of Thr-356 is required for in vivo activation of Pka1 and is dependent upon Ksg1. These data provide experimental evidence that PKA is a physiological substrate for PDK1.

Phosphatidylinositol-4-phosphate 5-kinase regulates fission yeast cell integrity through a phospholipase C-mediated protein kinase C-independent pathway

Fission yeast its3-1 mutant is an allele of the essential gene its3+ that encodes a phosphatidylinositol-4-phosphate 5-kinase (PIP5K) that produces phosphatidylinositol 4,5-bisphosphate. We found that the its3-1 mutant is sensitive to micafungin, a (1,3)-beta-D-glucan synthase inhibitor, suggesting a cell wall integrity defect. Consistently, its3-1 mutation caused synthetic lethality with a (1,3)-beta-D-glucan synthase mutant, bgs1-i2, and its3-1 mutant cells showed aberrant localization of green fluorescent protein-Bgs1. Similar aberrant localization of green fluorescent protein-tagged Rgf1, a putative phosphatidylinositol 4,5-bisphosphate-binding guanine nucleotide exchange factor for Rho protein, in its3-1 mutants was observed, suggesting a defective Rgf1/Rho pathway. To unravel the molecular mechanism(s), putative downstream components of PIP5K signaling were analyzed. Unexpectedly, overexpression of phospholipase C (Plc1), but not that of protein kinase C (PKC; Pck1 and Pck2), suppressed the phenotypes of the its3-1 mutant. These findings indicate that PKCs are not involved in the suppression, and further analysis revealed that PKCs are not downstream of Plc1 in fission yeast. Also, the enzymatic activity of Plc1 is essential for the suppression of the phenotypes and for the viability of the its3-1 mutant. These findings suggest that Its3 PIP5K regulates cell integrity through a Plc1-mediated PKC-independent pathway, in addition to the Rho/PKC pathway.

Characterization of PKC2, a gene encoding a second protein kinase C isotype of Saccharomyces cerevisiae

BACKGROUND

Protein kinase C (PKC) has attracted considerable attention over the past decade, primarily because of its presumed role in cellular growth control and tumourigenesis. Mammalian cells express at least 10 different isozymes of PKC; it is this complexity that has made elucidating the precise functions of PKC: so difficult. The identification of PKC homologues in organisms such as Drosophila, Xenopus, Dictyostelium, Aplysia and Caenorhabditis indicates that the enzyme is evolutionarily conserved, and this has stimulated our search for counterparts in the yeast Saccharomyces cerevisiae, in which powerful genetic analyses can be used. To date, only one PKC homologue, PKC1, has been identified in yeast and no biochemical activity has been definitively ascribed to the encoded protein. This, and the inability to identify other PKC homologues in yeast by DNA hybridization, has led to doubts about the existence of PKC isozymes in yeast. We have taken the approach of screening yeast expression libraries with anti-PKC antibodies in an attempt to identify further homologues.

RESULTS

We have identified a novel PKC isozyme, Pkc2p, encoded by the gene PKC2. We report here the sequence of PKC2 and a comparison showing its similarity to other PKCs. Phylogenetic analysis suggests that all known PKC genes, including PKC2, originated from a common ancestor. Disruption of the PKC2 protein-coding region, deleting the entire catalytic domain of the encoded enzyme, is not lethal to yeast growing on rich media. However, the pkc2 mutant, unlike wild-type strains, fails to grow on minimal media containing limited concentrations of amino acids. This implicates Pkc2p in the response of yeast cells to amino-acid starvation.

CONCLUSION

We have shown that yeast cells do express more than one PKC isozyme, by identifying and characterizing a novel PKC gene PKC2, the product of which may be involved in the cellular response to amino-acid starvation.

Evolution, biochemistry and genetics of protein kinase C in fungi

From yeast to humans, protein kinase C is an enzyme of central importance in signal transduction processes. In baker's yeast, Saccharomyces cerevisiae, a single gene encoding Pkc1p has been identified. Mutant analyses revealed that Pkc1p function is essential to ensure cellular integrity through regulation of a specific MAP kinase cascade. Due to the involvement of different defective mammalian isozymes in various diseases and the model character of simple eukaryotes, increasing attention has been paid to the structure and regulation of the enzymes of fungal origin. This review summarizes the knowledge gathered so far.

Calcineurin phosphatase in signal transduction: lessons from fission yeast

Calcineurin (protein phosphatase 2B), the only serine/threonine phosphatase under the control of Ca2+/calmodulin, is an important mediator in signal transmission, connecting the Ca2+-dependent signalling to a wide variety of cellular responses. Furthermore, calcineurin is specifically inhibited by the immunosuppressant drugs cyclosporin A and tacrolimus (FK506), and these drugs have been a powerful tool for identifying many of the roles of calcineurin. Calcineurin is enriched in the neural tissues, and also distributes broadly in other tissues. The structure of the protein is highly conserved from yeast to man. The combined use of powerful genetics and of specific calcineurin inhibitors in fission yeast Schizosaccharomyces pombe (S. pombe) identified new components of the calcineurin pathway, and defined new roles of calcineurin in the regulation of the many cellular processes. Recent data has revealed functional interactions in which calcineurin phosphatase is involved, such as the cross-talk between the Pmk1 MAP kinase signalling, or the PI signalling. Calcineurin also participates in membrane traffic and cytokinesis of fission yeast through its functional connection with members of the small GTPase Rab/Ypt family, and Type II myosin, respectively. These findings highlight the potential of fission yeast genetic studies to elucidate conserved elements of signal transduction cascades.

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

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

Characterization of a protein kinase C gene in Sporothrix schenckii and its expression during the yeast-to-mycelium transition

The yeast-to-mycelium transition in Sporothrix schenckii has been shown to respond to protein kinase C (PKC) effectors, indicating the involvement of PKC in this regulation. In this study, we identified the presence of two pkcl-like genes in S. schenckii. Using fungal genomic DNA as template and primers targeted to conserved sequences in the Saccharomyces cerevisiae pkc1 gene, two partially overlapping extra long polymerase chain reaction (XL-PCR) products were obtained. These XL-PCR products were sequenced and found to encode part of the C3/C4 domains of two different PKC-like proteins. The presence of two different genes was confirmed by Southern blot analysis. These two genes were named pkcSs-1 and pkcSs-2. The sequence of the pkcSs-2 gene was completed and revealed an open reading frame of 3942 nucleotides interrupted by five introns. A transcript of 8.7 kb was detected in northern blot analysis of poly A+ RNA. The pkcSs-2 gene encodes a protein of 1194 amino acids and 132.84 kDa that contains the characteristic structure and domains of other fungal PKCs reported to date. Using reverse transcription-PCR (RT-PCR), the pkcSs-2 gene was found to be expressed at all intervals tested during the yeast-to-mycelium transition.

Pleckstrin homology domain interacts with Rkp1/Cpc2, a RACK1 homolog, to modulate Pck2-mediated signaling process in Schizosaccharomyces pombe

Rkp1/Cpc2, a fission yeast RACK1 homolog, interacts with Pck2, a PKC homolog, and is involved in the regulation of pck2-mediated signaling process. The N-terminal region of split pleckstrin homology domain (nPH) in human PLC-gamma1 bound to Rkp1/Cpc2 concomitantly with Pck2. nPH inhibited kinase activity of GST-Pck2 purified from Schizosaccharomyces pombe in vitro. The lethality induced by pck2(+) overexpression was suppressed by coexpression of either rkp1(+) or nPH domain. This result suggests that Rkp1/Cpc2 interacts with PH domain-containing protein and regulates the Pck2-mediated signaling process in S. pombe.

Characterization of GTPase-activating proteins for the function of the Rho-family small GTPases in the fission yeast Schizosaccharomyces pombe

BACKGROUND

The small GTPase Rho1 has been shown to regulate the organization of the actin cytoskeleton and formation of the cell wall in the fission yeast Schizosaccharomyces pombe. Activity of Rho1 must be precisely regulated in vivo, since both increases and decreases in its activity affect cell growth and shape. Thus, it is important to clarify the mechanism by which the activity of Rho1 is regulated in vivo.

RESULTS

Seven genes encoding putative GAPs, GTPase-activating proteins, for the function of the Rho-family proteins were isolated from S. pombe. After disruption of these genes, rga1+ was found to play important roles in cell growth and morphogenesis. In rga1 null cells, delocalized F-actin patches and extraordinary thickening of the cell wall and the septum were observed. On the other hand, over-expression of Rga1 produced shrunken or dumpy cells. The phenotype of the rga1 null cells or the Rga1-over-expressing cells was similar to that of cells containing abnormally high or low Rho1 activity, respectively. Moreover, direct association of Rga1 with Rho1 was shown. Rga1 was localized to the cell ends and septum where Rho1 is known to function.

CONCLUSIONS

In S. pombe, Rga1 is involved in the F-actin patch localization, cell morphogenesis, regulation of septation, and cell wall synthesis, probably functioning as a GAP for the function of Rho1.

Characterization of a fission yeast mutant which displays defects in cell wall integrity and cytokinesis

The fission yeast cps6-153 mutant was originally isolated based on its hypersensitivity to the spindle poison isopropyl N-3-chlorophenyl carbamate (CIPC). The mutant also shows defects in both cell wall integrity and cytokinesis, resulting in the accumulation of unseparated cells with weakened cell walls. The arrested cells display a disoriented alignment of cytoplasmic microtubules. When the mutant cells are cultivated at high temperature (35 degrees C), both cell walls and septa become very thick. Electron microscopy revealed the disorganized structure of the thickened cell walls and septa, in which fibrillar components were not completely masked with an amorphous matrix. rad25+ was cloned from a genomic library by complementation of the mutant phenotypes, suggesting the involvement of Rad25p, one of two 14-3-3 proteins in S. pombe, in the pathway of cell wall integrity and cytokinesis.

Characterization of a Schizosaccharomyces pombe mutant deficient in UDP-galactose transport activity

In fission yeast, Schizosaccharomyces pombe, the carbohydrate components of the cell wall consist of galactomannan, unlike in Saccharomyces cerevisiae. We previously found that the disruption of gms1+, a gene encoding the UDP-galactose transporter required for the synthesis of galactomannan, led to the complete defect of cell surface galactosylation in Sz. pombe. The Deltagms1 strain is therefore useful for the analysis of physiological properties of galactose residues in Sz. pombe. The deletion strain of gms1+ was viable; however, itshowed an aberrant cell morphology and increased sensitivities to digestion with beta-glucanase and to various drugs, such as hygromycin B, sodium orthovanadate and Calcofluor white. A reduction of galactomannan layers of the cell wall in the Deltagms1 strain was observed by scanning and transmission electron microscopic analyses. The addition of osmotic stabilizer suppressed the morphologic defect of the Deltagms1 cells, while other phenotypes were weakly suppressed. The Deltagms1 (h90) strain was incapable of sexual conjugation during nutritional starvation. These results suggest that the cell surface galactosylation is required not only for non-sexual flocculation but also for sexual conjugation in Sz. pombe.

Rkp1/Cpc2, a fission yeast RACK1 homolog, is involved in actin cytoskeleton organization through protein kinase C, Pck2, signaling

The Rkp1/Cpc2, a fission yeast RACK1 homolog, interacted with Pck2, one of the known PKC homologs, in vivo and in vitro. The rkp1-deletion mutants (Deltarkp1) are elongated and the pck2-deletion mutant (Deltapck2) showed abnormal morphology. The double-deletion mutant (Deltarkp1Deltapck2) showed more aberrant cell shapes and was sensitive to high salt concentration. Both Deltarkp1 and Deltapck2 cells were sensitive to latrunculin B (Lat B) which inhibits actin polymerization. The cells expressing the human RACK1 homolog complemented the latrunculin B sensitivity of Deltarkp1 indicating that human RACK1 is a functional homolog of Rkp1/Cpc2. We propose that Rkp1/Cpc2 may function as a receptor for Pck2 in the regulation of actin cytoskeleton organization during cell wall synthesis and morphogenesis of Schizosaccharomyces pombe.

Schizosaccharomyces pombe rho2p GTPase regulates cell wall alpha-glucan biosynthesis through the protein kinase pck2p

Schizosaccharomyces pombe rho1(+) and rho2(+) genes are involved in the control of cell morphogenesis, cell integrity, and polarization of the actin cytoskeleton. Although both GTPases interact with each of the two S. pombe protein kinase C homologues, Pck1p and Pck2p, their functions are distinct from each other. It is known that Rho1p regulates (1,3)beta-D-glucan synthesis both directly and through Pck2p. In this paper, we have investigated Rho2p signaling and show that pck2 delta and rho2 delta strains display similar defects with regard to cell wall integrity, indicating that they might be in the same signaling pathway. We also show that Rho2 GTPase regulates the synthesis of alpha-D-glucan, the other main structural polymer of the S. pombe cell wall, primarily through Pck2p. Although overexpression of rho2(+) in wild-type or pck1 delta cells is lethal and causes morphological alterations, actin depolarization, and an increase in alpha-D-glucan biosynthesis, all of these effects are suppressed in a pck2 delta strain. In addition, genetic interactions suggest that Rho2p and Pck2p are important for the regulation of Mok1p, the major (1-3)alpha-D-glucan synthase. Thus, a rho2 delta mutation, like pck2 delta, is synthetically lethal with mok1-664, and the mutant partially fails to localize Mok1p to the growing areas. Moreover, overexpression of mok1(+) in rho2 delta cells causes a lethal phenotype that is completely different from that of mok1(+) overexpression in wild-type cells, and the increase in alpha-glucan is considerably lower. Taken together, all of these results indicate the presence of a signaling pathway regulating alpha-glucan biosynthesis in which the Rho2p GTPase activates Pck2p, and this kinase in turn controls Mok1p.

Analysis of 114 kb of DNA sequence from fission yeast chromosome 2 immediately centromere-distal to his5

One hundred and fourteen kilobase pairs (kb) of contiguous genomic sequence have been determined immediately distal to the his5 genetic marker located about 0.9 Mb from the centromere on the long arm of Schizosaccharomyces pombe chromosome 2. The sequence is contained in overlapping cosmid clones c16H5, c12D12, c24C6 and c19G7, of which 20 kb are identical to previously reported sequence from clone c21H7. The remaining 93 781 bp of sequence contains 10 known genes (cdc14, cdm1, cps1, gpa1, msh2, pck2, rip1, rps30-2, sad1 and ubl1), 32 open reading frames (ORFs) capable of coding for proteins of at least 100 amino acid residues in length, one 5S rRNA gene, one tRNA(Pro) gene, one lone Tf1-type long terminal repeat (LTR) and one lone Tf2-type LTR. There is a density of one protein-coding gene per 2.2 kb and 22 of the 42 ORFs (52%) incorporate one or more introns. Twenty-one of the novel ORFs show sequence similarities which suggest functions of their products, including a cyclin C, a MADS box transcription factor, mad2-like protein, telomere binding protein, topoisomerase II-associated protein, ATP-dependent DEAH box RNA helicase, G10 protein, ubiquitin-activating e1-like enzyme, nucleoporin, prolyl-tRNA synthetase, peptidylprolyl isomerase, delta-1-pyrroline-5-carboxylate dehydrogenase, protein transport protein, coatomer epsilon, TCP-1 chaperonin, beta-subunit of 6-phosphofructokinase, aminodeoxychorismate lyase, a phosphate transport protein and a thioredoxin.

sn-1,2-diacylglycerol levels in the fungus Neurospora crassa display circadian rhythmicity

The fungus Neurospora crassa is a model organism for investigating the biochemical mechanism of circadian (daily) rhythmicity. When a choline-requiring strain (chol-1) is depleted of choline, the period of the conidiation rhythm lengthens. We have found that the levels of sn-1,2-diacylglycerol (DAG) increase in proportion to the increase in period. Other clock mutations that change the period do not affect the levels of DAG. Membrane-permeant DAGs and inhibitors of DAG kinase were found to further lengthen the period of choline-depleted cultures. The level of DAG oscillates with a period comparable to the rhythm of conidiation in wild-type strains, choline-depleted cultures, and frq mutants, including a null frq strain. The DAG rhythm is present at the growing margin and also persists in older areas that have completed development. The phase of the DAG rhythm can be set by the light-to-dark transition, but the level of DAG is not immediately affected by light. Our results indicate that rhythms in DAG levels in Neurospora are driven by a light-sensitive circadian oscillator that does not require the frq gene product. High levels of DAG may feed back on that oscillator to lengthen its period.

Cloning and characterization of PKC-homologous genes in the truffle species Tuber borchii and Tuber magnatum

The protein kinases C (PKCs) define a growing family of ubiquitous signal transducting serine/threonine kinases that control ion conductance channels, release of hormones and cell growth and proliferation. Degenerated oligonucleotides were used as primers for polymerase chain reactions to amplify PKC-related sequences from the white truffle species Tuber magnatum and Tuber borchii. The deduced amino acid sequences of cloned sequences reveal domains homologous to the regulatory and kinase domains of PKC-related proteins, but lack typical Ca(2+)-binding domain and therefore should be classified as nPKCs. Both contain a large extended N-terminus which is found exclusively in fungi PKCs. Phylogenetic analysis of the kinase domain demonstrates high homology with known filamentous fungi isoenzymes.

Human ERK1 induces filamentous growth and cell wall remodeling pathways in Saccharomyces cerevisiae

Expression of an activated extracellular signal-regulated kinase 1 (ERK1) construct in yeast cells was used to examine the conservation of function among mitogen-activated protein (MAP) kinases. Sequence alignment of the human MAP kinase ERK1 with all Saccharomyces cerevisiae kinases reveals a particularly strong kinship with Kss1p (invasive growth promoting MAP kinase), Fus3p (pheromone response MAP/ERK kinase), and Mpk1p (cell wall remodeling MAP kinase). A fusion protein of constitutively active human MAP/ERK kinase 1 (MEK) and human ERK1 was introduced under regulated expression into yeast cells. The fusion protein (MEK/ERK) induced a filamentation response element promoter and led to a growth retardation effect concomitant with a morphological change resulting in elongated cells, bipolar budding, and multicell chains. Induction of filamentous growth was also observed for diploid cells following MEK/ERK expression in liquid culture. Neither haploids nor diploids, however, showed marked penetration of agar medium. These effects could be triggered by either moderate MEK/ERK expression at 37 degrees C or by high level MEK/ERK expression at 30 degrees C. The combination of high level MEK/ERK expression and 37 degrees C resulted in cell death. The deleterious effects of MEK/ERK expression and high temperature were significantly mitigated by 1 m sorbitol, which also enhanced the filamentous phenotype. MEK/ERK was able to constitutively activate a cell wall maintenance reporter gene, suggesting misregulation of this pathway. In contrast, MEK/ERK effectively blocked expression from a pheromone-responsive element promoter and inhibited mating. These results are consistent with MEK/ERK promoting filamentous growth and altering the cell wall through its ability to partially mimic Kss1p and stimulate a pathway normally controlled by Mpk1p, while appearing to inhibit the normal functioning of the structurally related yeast MAP kinase Fus3p.

Mis3 with a conserved RNA binding motif is essential for ribosome biogenesis and implicated in the start of cell growth and S phase checkpoint

BACKGROUND

In normal somatic cell cycle, growth and cell cycle are properly coupled. Although CDK (cyclin-dependent kinase) activity is known to be essential for cell cycle control, the mechanism to ensure the coupling has been little understood.

RESULTS

We here show that fission yeast Mis3, a novel evolutionarily highly conserved protein with the RNA-interacting KH motif, is essential for ribosome RNA processing, and implicated in initiating the cell growth. Growth arrest of mis3-224, a temperature sensitive mutant at the restrictive temperature, coincides with the early G2 block in the complete medium or the G1/S block in the release from nitrogen starvation, reflecting coupling of cell growth and division. Genetic interactions indicated that Mis3 shares functions with cell cycle regulators and RNA processing proteins, and is under the control of Dsk1 kinase and PP1 phosphatase. Mis3 is needed for the formation of 18S ribosome RNA, and may hence direct the level of proteins required for the coupling. One such candidate is Mik1 kinase. mis3-224 is sensitive to hydroxyurea, and the level of Mik1 protein increases during replication checkpoint in a manner dependent upon the presence of Mis3 and Cds1.

CONCLUSIONS

Mis3 is essential for ribosome biogenesis, supports S phase checkpoint, and is needed for the coupling between growth and cell cycle. Whether Mis3 interacts solely with ribosomal precursor RNA remains to be determined.

Role of cell shape in determination of the division plane in Schizosaccharomyces pombe: random orientation of septa in spherical cells

The establishment of growth polarity in Schizosaccharomyces pombe cells is a combined function of the cytoplasmic cytoskeleton and the shape of the cell wall inherited from the mother cell. The septum that divides the cylindrical cell into two siblings is formed midway between the growing poles and perpendicularly to the axis that connects them. Since the daughter cells also extend at their ends and form their septa at right angles to the longitudinal axis, their septal (division) planes lie parallel to those of the mother cell. To gain a better understanding of how this regularity is ensured, we investigated septation in spherical cells that do not inherit morphologically predetermined cell ends to establish poles for growth. We studied four mutants (defining four novel genes), over 95% of whose cells displayed a completely spherical morphology and a deficiency in mating and showed a random distribution of cytoplasmic microtubules, Tea1p, and F-actin, indicating that the cytoplasmic cytoskeleton was poorly polarized or apolar. Septum positioning was examined by visualizing septa and division scars by calcofluor staining and by the analysis of electron microscopic images. Freeze-substitution, freeze-etching, and scanning electron microscopy were used. We found that the elongated bipolar shape is not essential for the determination of a division plane that can separate the postmitotic nuclei. However, it seems to be necessary for the maintenance of the parallel orientation of septa over the generations. In the spherical cells, the division scars and septa usually lie at angles to each other on the cell surface. We hypothesize that the shape of the cell indirectly affects the positioning of the septum by directing the extension of the spindle.

Large-scale screening of intracellular protein localization in living fission yeast cells by the use of a GFP-fusion genomic DNA library

BACKGROUND

Intracellular localization is an important part of the characterization of a gene product. In an attempt to search for genes based on the intracellular localization of their products, we constructed a green fluorescent protein (GFP)-fusion genomic DNA library of S. pombe.

RESULTS

We constructed the S. pombe GFP-fusion genomic DNA library by fusing, in all three reading frames, random fragments of genomic DNA to the 5' end of the GFP gene in such a way that expression of potential GFP-fusion proteins would be under the control of the own promoters contained in the genomic DNA fragments. Fission yeast cells were transformed with this plasmid library, and microscopic screening of 49 845 transformants yielded 6954 transformants which exhibited GFP fluorescence, of which 728 transformants showed fluorescence localized to distinct intracellular structures such as the nucleus, the nuclear membrane, and cytoskeletal structures. Plasmids were isolated from 516 of these transformants, and a determination of their DNA sequences identified 250 independent genes. The intracellular localizations of the 250 GFP-fusion constructs was categorized as an image database; using this database, DNA sequences can be searched for based on the localizations of their products.

CONCLUSIONS

A number of new intracellular structural components were found in this library. The library of GFP-fusion constructs also provides useful fluorescent markers for various intracellular structures and cellular activities, which can be readily used for microscopic observation in living cells.

The MAP kinase cascade that includes MAPKKK-related protein kinase NPK1 controls a mitotic proces in plant cells

The tobacco NPK1 cDNA was the first-isolated plant cDNA for a homolog of mitogen-activated protein kinase kinase kinases (MAPKKKs). The kinase domain of the NPK1 protein can replace the functions of MAPKKKs in yeasts, while the amino acid sequence of the kinase-unrelated region does not have any homology to those of MAPKKKs from other organisms. Transcription of the NPK1 gene takes place in meristematic tissues or immature organs in a tobacco plant. During a tobacco cell cycle, transcriptional and translational products of NPK1 are present from S to M phase and decrease after the M phase. Expression of the NACK1 gene, which is predicted to encode a novel kinesin-like microtubule-based motor protein capable of activating NPK1, is specific to M phase, suggesting that activation of NPK1 occurs in M phase. Characterization of cDNAs for a MAPKK and a MAPK which can act downstream of NPK1 makes a proposition that the MAP kinase pathway involving NPK1 regulates a mitotic process associated with microtubules.

Blue light signaling chains in Phycomyces: phototransduction of carotenogenesis and morphogenesis involves distinct protein kinase/phosphatase elements

Carotenogenesis and morphogenesis represent two of the several responses sensitive to blue light which characterize the lower eukaryote Phycomyces blakesleeanus. Speculating that reversible phosphorylation may be an intracellular event beyond the photoperception step, we resorted to the use of first-choice inhibitors of protein phosphatases and protein kinases. The mycelial beta-carotene content of dark-grown cultures was induced by all agents administered, while the morphogenic output showed the typical trend effected by light only with one of the protein kinase inhibitors. Our data provide convincing evidence that protein phosphorylation plays a regulatory role in photocarotenogenesis and photomorphogenesis of Phycomyces. According to the model we propose, the putative signaling elements involved are anticipated to have a repressive function in the dark so that the responses are maintained in the "off" mode until the moment photon information has to flow through the regulatory circuit.

Ultrastructure of cell wall of the cps8 actin mutant cell in Schizosaccharomyces pombe

A Schizosaccharomyces pombe cps8 mutant, of which the gene encodes a mutant actin with an amino acid substitution of Asp for Gly(273) [J. Ishiguro and W. Kobayashi (1996) FEBS Lett. 392, 237-241], was used to determine the role of the actin cytoskeleton in cell wall formation. In the cps8 mutant cells, atomic force microscopic and scanning electron microscopic images showed abnormal depolarized and branched morphology. Fibrous material covered a part of the surface of growing cps8 cells. Transmission electron microscopic images showed variable thickness of the cell wall due to multilayering of cell wall materials, and aberrant multisepta due to diagonal growth of the primary septum, whereas the normal primary septum grows at a right angle from the cortex. This abnormal septum formation may induce abnormality of the cell with multinuclei and/or multisepta, caused by non-separation of daughter cells. These results indicate that actin plays an important role in cell wall and septum formation.

Schizosaccharomyces pombe protein kinase C homologues, pck1p and pck2p, are targets of rho1p and rho2p and differentially regulate cell integrity

Schizosaccharomyces pombe rho1(+) is required for maintenance of cell integrity and polarization of the actin cytoskeleton. However, no other effector besides the (1,3)beta-D-glucan synthase enzyme has been identified in S. pombe. We have further investigated if rho1(+)signalling could be also mediated by the two protein kinase C homologues, pck1p and pck2p. We show in this study that both kinases interact with rho1p and rho2p only when bound to GTP, as most GTPase effectors do. Interestingly, the interaction was mapped in a different part of the proteins than in Saccharomyces cerevisiae Pkc1p. Thus, active rho1p binds to the amino-terminal region of the pcks where two HR1 motifs are located, and binding to the GTPase dramatically stabilizes the kinases. Detailed biochemical analysis suggests that pck2p is more important in the regulation of the enzyme (1–3)beta-D-glucan synthase. Thus, overexpression of pck2(+), but not pck1(+), caused a general increase in cell wall biosynthesis, mainly in beta-glucan, and (1–3)beta-D-glucan synthase activity was considerably augmented. When this activity was separated into soluble and membrane fractions and reconstituted, the increase caused by pck2(+) overexpression was exclusively detected in the membrane component. We also show that both protein kinase C homologues are required for the maintenance of cell integrity. pck1delta and pck2delta strains present a number of defects related to the cell wall, indicating that this structure might be co-ordinately regulated by both kinases. In addition, pck2p, but not pck1p, seems to be involved in keeping cell polarity. Genetic evidence indicates that both pck1(+) and pck2(+) interact with cps1(+) and gls2(+), two genes similar to S. cerevisiae FKS1 and FKS2 that encode membrane subunits of the (1–3)beta-D-glucan synthase. pck1(+)also showed a genetic interaction with ras1(+) and ral1(+) suggesting the existence of a functional link between both signalling pathways.

RHO GTPases in the control of cell morphology, cell polarity, and actin localization in fission yeast

The fission yeast Schizosaccharomyces pombe undergoes morphogenetic changes during both vegetative and sexual cell cycles that require asymmetric cell growth and actin cytoskeleton reorganisations. Different complex signal transduction pathways participate in S. pombe morphogenesis. The Rho family of GTPases are present in all eukaryotic cells, from yeast to mammals, and their role as key regulators in the signalling pathways that control actin organisation and morphogenetic processes is well known. In this review, we will briefly summarize the role of the Rho GTPases in the establishment and maintenance of cell polarity and growth of S. pombe. As in other fungi, S. pombe morphogenesis is closely related to cell wall biosynthesis, and Rho GTPases are critical modulators of this process. They provide the coordinated regulation of cell wall biosynthetic enzymes and actin organisation required to maintain cell integrity and polarised growth.

Cloning and characterization of psu1(+), a new essential fission yeast gene involved in cell wall synthesis

We have isolated a new gene, psu1(+), from the fission yeast Schizosaccharomyces pombe. The predicted amino acid sequences shows that this protein has striking homology to the SUN family of the budding yeast, hence designated Psu1 (S. pombe homologue of the SUN family). Disruption of the psu1(+) gene revealed that it is essential for growth, and the null phenotype showed the swelling of cells followed by eventual lysis. We introduced psu1(+) gene in the disruptant strain and repressed it giving resistance to 1, 3-beta-glucanase digestion. Our results suggest that Psu1 plays an essential role in cell wall synthesis in S. pombe.