Vacuole fusion regulated by protein phosphatase 2C in fission yeast

Ypt4 and lvs1 regulate vacuolar size and function in Schizosaccharomyces pombe

The yeast vacuole plays key roles in cellular stress responses. Here, we show that deletion of lvs1, the fission yeast homolog of the Chediak-Higashi Syndrome CHS1/LYST gene, increases vacuolar size, similar to deletion of the Rab4 homolog ypt4. Overexpression of lvs1-YFP rescued vacuolar size in ypt4Δ cells, but ypt4-YFP did not rescue lvs1Δ, suggesting that lvs1 may act downstream of ypt4. Vacuoles were capable of hypotonic shock-induced fusion and recovery in both ypt4Δ and lvs1Δ cells, although recovery may be slightly delayed in ypt4Δ. Endocytic and secretory trafficking were not affected, but ypt4Δ and lvs1Δ strains were sensitive to neutral pH and CaCl₂, consistent with vacuolar dysfunction. In addition to changes in vacuolar size, deletion of ypt4 also dramatically increased cell size, similar to tor1 mutants. These results implicate ypt4 and lvs1 in maintenance of vacuolar size and suggest that ypt4 may link vacuolar homeostasis to cell cycle progression.

H₂O₂ stress-specific regulation of S. pombe MAPK Sty1 by mitochondrial protein phosphatase Ptc4

In fission yeast, the stress-activated MAP kinase, Sty1, is activated via phosphorylation upon exposure to stress and orchestrates an appropriate response. Its activity is attenuated by either serine/threonine PP2C or tyrosine phosphatases. Here, we found that the PP2C phosphatase, Ptc4, plays an important role in inactivating Sty1 specifically upon oxidative stress. Sty1 activity remains high in a ptc4 deletion mutant upon H(2)O(2) but not under other types of stress. Surprisingly, Ptc4 localizes to the mitochondria and is targeted there by an N-terminal mitochondrial targeting sequence (MTS), which is cleaved upon import. A fraction of Sty1 also localizes to the mitochondria suggesting that Ptc4 attenuates the activity of a mitochondrial pool of this MAPK. Cleavage of the Ptc4 MTS is greatly reduced specifically upon H(2)O(2), resulting in the full-length form of the phosphatase; this displays a stronger interaction with Sty1, thus suggesting a novel mechanism by which the negative regulation of MAPK signalling is controlled and providing an explanation for the oxidative stress-specific nature of the regulation of Sty1 by Ptc4.

A type 2C protein phosphatase FgPtc3 is involved in cell wall integrity, lipid metabolism, and virulence in Fusarium graminearum

Type 2C protein phosphatases (PP2Cs) play important roles in regulating many biological processes in eukaryotes. Currently, little is known about functions of PP2Cs in filamentous fungi. The causal agent of wheat head blight, Fusarium graminearum, contains seven putative PP2C genes, FgPTC1, -3, -5, -5R, -6, -7 and -7R. In order to investigate roles of these PP2Cs, we constructed deletion mutants for all seven PP2C genes in this study. The FgPTC3 deletion mutant (ΔFgPtc3-8) exhibited reduced aerial hyphae formation and deoxynivalenol (DON) production, but increased production of conidia. The mutant showed increased resistance to osmotic stress and cell wall-damaging agents on potato dextrose agar plates. Pathogencity assays showed that ΔFgPtc3-8 is unable to infect flowering wheat head. All of the defects were restored when ΔFgPtc3-8 was complemented with the wild-type FgPTC3 gene. Additionally, the FgPTC3 partially rescued growth defect of a yeast PTC1 deletion mutant under various stress conditions. Ultrastructural and histochemical analyses showed that conidia of ΔFgPtc3-8 contained an unusually high number of large lipid droplets. Furthermore, the mutant accumulated a higher basal level of glycerol than the wild-type progenitor. Quantitative real-time PCR assays showed that basal expression of FgOS2, FgSLT2 and FgMKK1 in the mutant was significantly higher than that in the wild-type strain. Serial analysis of gene expression in ΔFgPtc3-8 revealed that FgPTC3 is associated with various metabolic pathways. In contrast to the FgPTC3 mutant, the deletion mutants of FgPTC1, FgPTC5, FgPTC5R, FgPTC6, FgPTC7 or FgPTC7R did not show aberrant phenotypic features when grown on PDA medium or inoculated on wheat head. These results indicate FgPtc3 is the key PP2C that plays a critical role in a variety of cellular and biological functions, including cell wall integrity, lipid and secondary metabolisms, and virulence in F. graminearum.

Type 2C protein phosphatases in fungi

Type 2C Ser/Thr phosphatases are a remarkable class of protein phosphatases, which are conserved in eukaryotes and involved in a large variety of functional processes. Unlike in other Ser/Thr phosphatases, the catalytic polypeptide is not usually associated with regulatory subunits, and functional specificity is achieved by encoding multiple isoforms. For fungi, most information comes from the study of type 2C protein phosphatase (PP2C) enzymes in Saccharomyces cerevisiae, where seven PP2C-encoding genes (PTC1 to -7) with diverse functions can be found. More recently, data on several Candida albicans PP2C proteins became available, suggesting that some of them can be involved in virulence. In this work we review the available literature on fungal PP2Cs and explore sequence databases to provide a comprehensive overview of these enzymes in fungi.

Identification and characterization of the type 2C protein phosphatase Ptc4p in the human fungal pathogen Candida albicans

Type 2C protein phosphatases (PP2C) are monomeric enzymes and their activities require the presence of magnesium or manganese ions. There are seven PP2C genes, named from PTC1 to PTC7, in Saccharomyces cerevisiae. In the current study we identified the CaPTC4 gene in Candida albicans and demonstrated that the CaPtc4p protein is a typical PP2C enzyme, which is highly conserved in fungal species. Deletion of CaPTC4 renders Candida cells sensitive to sodium and potassium ions as well as to antifungal azole drugs. In addition, we have shown that CaPtc4p is localized in the mitochondrion, suggesting that CaPtc4p is likely to be involved in the regulation of a mitochondrial function related to ion homeostasis.

Candida albicans CaPTC6 is a functional homologue for Saccharomyces cerevisiae ScPTC6 and encodes a type 2C protein phosphatase

Type 2C protein phosphatases (PP2C) are monomeric enzymes and their activities require the presence of magnesium or manganese ions. There are seven PP2C genes, ScPTC1, ScPTC2, ScPTC3, ScPTC4, ScPTC5, ScPTC6 and ScPTC7, in Saccharomyces cerevisiae. PTC6 is highly conserved in pathogenic and nonpathogenic yeasts. In the current study we have demonstrated that the Candida albicans CaPTC6 gene could complement the functions of ScPTC6 in the rapamycin and caffeine sensitivities of S. cerevisiae cells, indicating that they are functional homologues. We have also demonstrated that the CaPTC6-encoded protein is a typical PP2C enzyme and that CaPtc6p is localized in the mitochondrion of yeast-form and hyphal cells. However, deletion of CaPTC6 neither affects cell and hyphal growth nor renders Candida cells sensitive to rapamycin and caffeine. Therefore, possibly with a functional redundancy to other mitochondrial phosphatases, CaPtc6p is likely to be involved in the regulation of a mitochondrial physiology.

S. pombe btn1, the orthologue of the Batten disease gene CLN3, is required for vacuole protein sorting of Cpy1p and Golgi exit of Vps10p

Batten disease is characterised by lysosomal dysfunction. The most common type of the disease is caused by mutations in the membrane protein CLN3, whose function is unknown. We show that the fission yeast orthologue Btn1p, previously implicated in vacuole function, is required for correct sorting of the vacuole hydrolase carboxypeptidase Y (Cpy1p). This is, in part, due to a defect in trafficking of Vps10p, the sorting receptor for Cpy1p, from the Golgi to the trans-Golgi network in btn1Delta cells. Our data also implicate btn1 in other Vps10-independent Cpy1-sorting pathways. Furthermore, btn1 affects the number, intracellular location and structure of Golgi compartments. We show that the prevacuole location of Btn1p is at the Golgi, because Btn1p colocalises predominantly with the Golgi marker Gms1p in compartments that are sensitive to Brefeldin A. Btn1p function might be linked to that of Vps34p, a phosphatidylinositol 3-kinase, because Btn1p acts as a multicopy suppressor of the severe Cpy1p vacuole protein-sorting defect of vps34Delta cells. Together, these results indicate an important role for Btn1p in the Golgi complex, which affects Golgi homeostasis and vacuole protein sorting. We propose a similar role for CLN3 in mammalian cells.

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.

Expression ofCaPTC7is developmentally regulated during serum-induced morphogenesis in the human fungal pathogen Candida albicans

Type 2C protein phosphatases (PP2C) represent a diversified protein phosphatase family and play various roles in cells. We previously identified and characterized a novel PP2C phosphatase encoded by the CaPTC7 gene in the human fungal pathogen Candida albicans . The CaPtc7p has 365 amino acids with a PP2C core domain at the C terminus and an additional 116-residue N-terminal sequence containing a mitochondrion-targeting sequence. Here, we show that CaPtc7p is indeed localized in the mitochondrion, the only eukaryotic PP2C phosphatase that has been directly shown to reside in the mitochondrion, suggesting its potential role in the regulation of mitochondrial physiology. Furthermore, we show that the expression of CaPTC7 at both transcriptional and protein levels is developmentally regulated during the serum-induced morphogenesis of C. albicans cells. However, disruption of the two alleles of CaPTC7 does not affect cell viability or filamentous development in C. albicans.

Transcriptional Profiling of the Protein Phosphatase 2C Family in Yeast Provides Insights into the Unique Functional Roles of Ptc1

Type 2C protein phosphatases are encoded in Saccharomyces cerevisiae by several related genes (PTC1-5 and PTC7). To gain insight into the functions attributable to specific members of this gene family, we have investigated the transcriptional profiles of ptc1-5 mutants. Two main patterns were obtained as follows: the one generated by the ptc1 mutation and the one resulting from the lack of Ptc2-5. ptc4 and ptc5 profiles were quite similar, whereas that of ptc2 was less related to this group. Mutation of PTC1 resulted in increased expression of numerous genes that are also induced by cell wall damage, such as YKL161c, SED1, or CRH1, as well as in higher amounts of active Slt2 mitogen-activated protein kinase, indicating that lack of the phosphatase activates the cell wall integrity pathway. ptc1 cells were even more sensitive than slt2 mutants to a number of cell wall-damaging agents, and both mutations had additive effects. The sensitivity of ptc1 cells was not dependent on Hog1. Besides these phenotypes, we observed that calcineurin was hyperactivated in ptc1 cells, which were also highly sensitive to calcium ions, heavy metals, and alkaline pH, and exhibited a random haploid budding pattern. Remarkably, many of these traits are found in certain mutants with impaired vacuolar function. As ptc1 cells also display fragmented vacuoles, we hypothesized that lack of Ptc1 would primarily cause vacuolar malfunction, from which other phenotypes would derive. In agreement with this scenario, overexpression of VPS73, a gene of unknown function involved in vacuolar protein sorting, largely rescues not only vacuolar fragmentation but also sensitivity to cell wall damage, high calcium, alkaline pH, as well as other ptc1-specific phenotypes.

Bem1p Is a Positive Regulator of the Homotypic Fusion of Yeast Vacuoles

Docked vacuoles are believed to undergo rapid lipid mixing during hemifusion and then a slow, rate-limiting completion of fusion and mixing of lumenal contents. Previous genomic analysis has suggested that Bem1p, a scaffold protein critical for cell polarity, may support vacuole fusion. We now report that bem1Delta strains have fragmented vacuoles (vps class B and C). During in vitro fusion reactions, vacuoles from bem1Delta strains showed a strong reduction in the rate of lipid mixing when compared with vacuoles from the BEM1 parent. The reduction in the overall rate of fusion with bem1Delta vacuoles was modest, consistent with lipid mixing as a non-rate-limiting step in the pathway. Although the fusion of either BEM1 (wild-type) or bem1Delta vacuoles is stimulated by recombinant Bem1p, the lipid mixing of docked bem1Delta vacuoles is highly dependent on rBem1p under certain reaction conditions. Bem1p-stimulated lipid mixing is blocked by well characterized fusion inhibitors including lipid ligands and antibodies to Ypt7p, Vps33p, and Vam3p. Although full-length Bem1p is required for maximal stimulation, a truncation mutant comprising the SH3 domains and the Phox homology (PX) domain retains modest stimulatory activity. In contrast to an earlier report (Han, B. K., Bogomolnaya, L. M., Totten, J. M., Blank, H. M., Dangott, L. J., and Polymenis, M. (2005) Genes Dev. 19, 2606-2618), we did not find phosphorylation of Bem1p at Ser-72 to be required for Bem1p-stimulated fusion. Taken together, Bem1p is a positive regulator of lipid mixing during vacuole hemifusion and fusion.

btn1, theSchizosaccharomyces pombehomologue of the human Batten disease geneCLN3, regulates vacuole homeostasis

We have cloned the Schizosaccharomyces pombe homologue of the human Batten disease gene, CLN3. This gene, btn1, encodes a predicted transmembrane protein that is 30% identical and 48% similar to its human counterpart. Cells deleted for btn1 were viable but had enlarged and more alkaline vacuoles. Conversely overexpression of Btn1p reduced both vacuole diameter and pH. Thus Btn1p regulates vacuole homeostasis. The vacuolar defects of btn1Δ cells were rescued by heterologous expression of CLN3, proving that Btn1p and CLN3 are functional homologues. The disease severity of Batten disease-causing mutations (G187A, E295K and V330F), when expressed in btn1 appeared to correlate with their effect on vacuolar pH, suggesting that elevated lysosomal pH contributes to the disease process. In fission yeast, both Btn1p and CLN3 trafficked to the vacuole membrane via early endocytic and pre-vacuolar compartments, and localisation of Btn1p to the vacuole membrane was dependent on the Ras GTPase Ypt7p. Importantly, vacuoles in cells deleted for both ypt7 and btn1 were larger and more alkaline than those of cells deleted for ypt7 alone, indicating that Btn1p has a functional role prior to reaching the vacuole. Consistently, btn1 and vma1, the gene encoding subunit A of the V1 portion of vATPase, showed conditional synthetic lethality, and in cells deleted for vma1 (a subunit of the vacuolar ATPase) Btn1p was essential for septum deposition during cytokinesis.

Endocytosis in fission yeast is spatially associated with the actin cytoskeleton during polarised cell growth and cytokinesis

In the fission yeast, Schizosaccharomyces pombe, uptake of the fluorescent styryl dye FM4-64 via the endocytic pathway to the vacuole was localised to the poles of growing, interphase cells and to the cell equator during cell division, regions of cell wall deposition that are rich in actin. When the pattern of growth or the plane of cytokinesis was altered, the relationship between the actin cytoskeleton and the site of endocytosis was maintained. Transfer of the label to the vacuolar membrane was dependent upon the Rab GTPase Ypt7 and, hence, vesicle fusion. Endocytic vesicles transiently colocalised with actin patches and endocytosis was inhibited in mutants that affected actin patch integrity and by the actin inhibitor latrunculin A. Concentrations of latrunculin that removed actin cables but left patches unaffected had no effect on endocytosis at the poles, but abolished endocytosis at the cell equator. Equatorial, but not polar, endocytosis was also inhibited in cells lacking the formin For3 (which have selectively destabilised actin cables), in mutants of the exocyst complex and in cells treated with brefeldin A. Differential effects on endocytosis at the cell poles and equator were also observed in the actin mutant cps8 and the Arp2/3 complex mutant arp2. The redirection of endocytosis from the cell poles to the cell equator in M phase coincided with the anaphase separation of sister chromatids and was abolished in the septation initiation network (SIN) mutants cdc7, sid1 and sid2, demonstrating that the spatial reorganisation of the endocytic pathway in the S. pombe cell cycle requires a functional SIN pathway. We conclude that endocytosis in fission yeast has two distinct components, both of which are actin-based, but which are mechanistically distinct, as well as being spatially and temporally separated in the S. pombe cell cycle.

Ctr6, a vacuolar membrane copper transporter in Schizosaccharomyces pombe

Aerobic organisms possess efficient systems for the transport of copper. This involves transporters that mediate the passage of copper across biological membranes to reach essential intracellular copper-requiring enzymes. In this report, we identify a new copper transporter in Schizosaccharomyces pombe, encoded by the ctr6(+) gene. The transcription of ctr6(+) is induced under copper-limiting conditions. This regulation is mediated by the cis-acting promoter element CuSE (copper-signaling element) through the copper-sensing transcription factor Cuf1. An S. pombe strain bearing a disrupted ctr6Delta allele displays a strong reduction of copper,zinc superoxide dismutase activity. When the ctr6+ gene is overexpressed from the thiamine-inducible nmt1(+) promoter, the cells are unable to grow on medium containing exogenous copper. Surprisingly, this copper-sensitive growth phenotype is not due to an increase of copper uptake at the cell surface. Instead, copper delivery across the plasma membrane is reduced. Consistently, this results in repressing ctr4(+) gene expression. By using a functional ctr6(+) epitope-tagged allele expressed under the control of its own promoter, we localize the Ctr6 protein on the membrane of vacuoles. Furthermore, we demonstrate that Ctr6 is an integral membrane protein that can trimerize. Moreover, we show that Ctr6 harbors a putative copper-binding Met-X-His-Cys-X-Met-X-Met motif in the amino terminus, which is essential for its function. Our findings suggest that under conditions in which copper is scarce, Ctr6 is required as a means to mobilize stored copper from the vacuole to the cytosol.

Osmotic stress signaling and osmoadaptation in yeasts

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