Complex formation, phosphorylation, and localization of protein kinase A of Schizosaccharomyces pombe upon glucose starvation

Mutational analyses of the interacting domains of Schizosaccharomyces pombe Byr2 with 14-3-3s

The Byr2 kinase of fission yeast Schizosaccharomyces pombe is recruited to the membrane with the assistance of Ras1. Byr2 is also negatively regulated by 14-3-3 proteins encoded by rad24 and rad25. We conducted domain and mutational analysis of Byr2 to determine which region is critical for its binding to 14-3-3 proteins. Rad24 and Rad25 bound to both the Ras interaction domain in the N-terminus and to the C-terminal catalytic domain of Byr2. When amino acid residues S87 and T94 of the Ras-interacting domain of Byr2 were mutated to alanine, Rad24 could no longer bind to Byr2. S402, S566, S650, and S654 mutations in the C-terminal domain of Byr2 also abolished its interaction with Rad24 and Rad25. More than three mutations in the C-terminal domain were required to abolish completely its interaction with 14-3-3 protein, suggesting that multiple residues are involved in this interaction. Expression of the N-terminal domain of Byr2 in wild-type cells lowered the mating ratio, because it likely blocked the interaction of Byr2 with Ste4 and Ras1, whereas expression of the catalytic domain of Byr2 increased the mating ratio as a result of freeing from intramolecular regulation by the N-terminal domain of Byr2. The S87A and T94A mutations of Byr2 increased the mating ratio and attenuated inhibition of Byr2 by Rad24; therefore, these two amino acids are critical for its regulation by Rad24. S566 of Byr2 is critical for activity of Byr2 but not for its interaction with 14-3-3 proteins. In this study, we show that 14-3-3 proteins interact with two separate domains in Byr2 as negative regulators.

Live-cell fluorescence imaging and optogenetic control of PKA kinase activity in fission yeast Schizosaccharomyces pombe

The cAMP-PKA signaling pathway plays a crucial role in sensing and responding to nutrient availability in the fission yeast Schizosaccharomyces pombe. This pathway monitors external glucose levels to control cell growth and sexual differentiation. However, the temporal dynamics of the cAMP-PKA pathway in response to external stimuli remains unclear mainly due to the lack of tools to quantitatively visualize the activity of the pathway. Here, we report the development of the kinase translocation reporter (KTR)-based biosensor spPKA-KTR1.0, which allows us to measure the dynamics of PKA activity in fission yeast cells. The spPKA-KTR1.0 is derived from the transcription factor Rst2, which translocates from the nucleus to the cytoplasm upon PKA activation. We found that spPKA-KTR1.0 translocates between the nucleus and cytoplasm in a cAMP-PKA pathway-dependent manner, indicating that the spPKA-KTR1.0 is a reliable indicator of the PKA activity in fission yeast cells. In addition, we implemented a system that simultaneously visualizes and manipulates the cAMP-PKA signaling dynamics by introducing bPAC, a photoactivatable adenylate cyclase, in combination with spPKA-KTR1.0. This system offers an opportunity for investigating the role of the signaling dynamics of the cAMP-PKA pathway in fission yeast cells with higher temporal resolution.

Regulation of sexual differentiation initiation in Schizosaccharomyces pombe

The fission yeast Schizosaccharomyces pombe is an excellent model organism to explore cellular events owing to rich tools in genetics, molecular biology, cellular biology, and biochemistry. Schizosaccharomyces pombe proliferates continuously when nutrients are abundant but arrests in G1 phase upon depletion of nutrients such as nitrogen and glucose. When cells of opposite mating types are present, cells conjugate, fuse, undergo meiosis, and finally form 4 spores. This sexual differentiation process in S. pombe has been studied extensively. To execute sexual differentiation, the glucose-sensing cAMP-PKA (cyclic adenosine monophosphate-protein kinase A) pathway, nitrogen-sensing TOR (target of rapamycin) pathway, and SAPK (stress-activating protein kinase) pathway are crucial, and the MAPK (mitogen-activating protein kinase) cascade is essential for pheromone sensing. These signals regulate ste11 at the transcriptional and translational levels, and Ste11 is modified in multiple ways. This review summarizes the initiation of sexual differentiation in S. pombe based on results I have helped to obtain, including the work of many excellent researchers.

Pps1, phosphatidylserine synthase, regulates the salt stress response in Schizosaccharomyces pombe

Phosphatidylserine (PS) is important for maintaining growth, cytoskeleton, and various functions in yeast; however, its role in stress responses is poorly understood. In Schizosaccharomyces pombe, the PS synthase deletion (pps1∆) mutant shows defects in growth, morphology, cytokinesis, actin cytoskeleton, and cell wall integrity, and these phenotypes are rescued by ethanolamine supplementation. Here, we evaluated the role of Pps1 in the salt stress response in S. pombe. We found that pps1∆ cells are sensitive to salt stresses such as KCl and CaCl2 even in the presence of ethanolamine. Loss of the functional cAMP-dependent protein kinase (git3∆ or pka1∆) or phospholipase B Plb1 (plb1∆) enhanced the salt stress-sensitive phenotype in pps1∆ cells. Green fluorescent protein (GFP)-Pps1 was localized at the plasma membrane and endoplasmic reticulum regardless of the stress conditions. In pka1∆ cells, GFP-Pps1 was accumulated around the nucleus under the KCl stress. Pka1 was localized in the nucleus and the cytoplasm under normal conditions and transferred from the nucleus to the cytoplasm under salt-stress conditions. Pka1 translocated from the nucleus to the cytoplasm during CaCl2 stress in the wild-type cells, while it remained localized in the nucleus in pps1∆ cells. Expression and phosphorylation of Pka1-GFP were not changed in pps1∆ cells. Our results demonstrate that Pps1 plays an important role in the salt stress response in S. pombe.

A dominant negative 14-3-3 mutant in Schizosaccharomyces pombe distinguishes the binding proteins involved in sexual differentiation and check point

The homothallic fission yeast Schizosaccharomyces pombe undergoes sexual differentiation when starved, but sam (skips the requirement of starvation for mating) mutants such as those carrying mutations in adenylate cyclase (cyr1) or protein kinase A (pka1) mate without starvation. Here, we identified sam3, a dominant negative allele of rad24, encoding one of two 14-3-3 proteins. Genetic mapping and whole-genome sequencing showed that the sam3 mutation comprises a change in nucleotide at position 959 from guanine to adenine, which switches the amino acid at position 185 from glutamic acid to lysine (E185K). We generated the rad24-E185K integrated mutant and its phenotype was similar to that of the sam3 mutant, including calcium sensitivity and UV non-sensitivity, but the phenotype is different from that of the Δrad24 strain. While the UV-sensitive phenotype was observed in the Δrad24 mutant, it was not observed in the sam3 and rad24-E185K mutants. The expression of the rad24-E185K gene in wild type cells induced spore formation in the nutrient rich medium, confirming rad24-E185K is dominant. This dominant effect of rad24-E185K was also observed in Δras1 and Δbyr2 diploid mutants, indicating that rad24-E185K generate stronger phenotype than rad24 null mutants. Ste11, the key transcription factor for sexual differentiation was expressed in sam3 mutants without starvation and it predominantly localized to the nucleus. The Rad24-E185K mutant protein retained its interaction with Check point kinase1 (Chk1), whereas it reduced interaction with Ste11, an RNA binding protein Mei2, and a MAPKKK Byr2, freeing these proteins from negative regulation by Rad24, that account for the sam phenotype and UV non-sensitive phenotype. Glucose depletion in rad24-E185K or Δpka1 Δrad24 double mutation induced haploid meiosis, leading to the formation of spores in haploid. The position of glutamic acid 185 is conserved in all major 14-3-3s; hence, our finding of a dominant negative allele of 14-3-3 is useful for understanding 14-3-3s in other organisms.

Tfs1, transcription elongation factor TFIIS, has an impact on chromosome segregation affected by pka1 deletion in Schizosaccharomyces pombe

The cAMP-dependent protein kinase (PKA) pathway in Schizosaccharomyces pombe plays an important role in microtubule organization and chromosome segregation. Typically, loss of functional Pka1 induces sensitivity to the microtubule-destabilizing drug thiabendazole (TBZ) and chromosome mis-segregation. To determine the mechanism via which Pka1 is involved in these events, we explored the relevance of transcription factors by creating a double-deletion strain of pka1 and 102 individual genes encoding transcription factors. We found that rst2∆, tfs1∆, mca1∆, and moc3∆ suppressed the TBZ-sensitive phenotype of the pka1∆ strain, among which tfs1∆ was the strongest suppressor. All single mutants (rst2∆, tfs1∆, mca1∆, and moc3∆) showed a TBZ-tolerant phenotype. Tfs1 has two transcriptional domains (TFIIS and Zn finger domains), both of which contributed to the suppression of the pka1∆-induced TBZ-sensitive phenotype. pka1∆-induced chromosome mis-segregation was rescued by tfs1∆ in the presence of TBZ. tfs1 overexpression induced the TBZ-sensitive phenotype and a high frequency of chromosome mis-segregation, suggesting that the amount of Tfs1 must be strictly controlled. However, Tfs1-expression levels did not differ between the wild-type and pka1∆ strains, and the Tfs1-GFP protein was localized to the nucleus and cytoplasm in both strains, which excludes the direct regulation of expression and localization of Tfs1 by Pka1. Growth inhibition by TBZ in pka1∆ strains was notably rescued by double deletion of rst2 and tfs1 rather than single deletion of rst2 or tfs1, indicating that Rst2 and Tfs1 contribute independently to counteract TBZ toxicity. Our findings highlight Tfs1 as a key transcription factor for proper chromosome segregation.

Expression of Mug14 is regulated by the transcription factor Rst2 through the cAMP-dependent protein kinase pathway in Schizosaccharomyces pombe

The cAMP-dependent protein kinase (Pka1) regulates many cellular events, including sexual development and glycogenesis, and response to the limitation of glucose, in Schizosaccharomyces pombe. Despite its importance in many cellular events, the targets of the cAMP/PKA pathway have not been fully investigated. Here, we demonstrate that the expression of mug14 is induced by downregulation of the cAMP/PKA pathway and limitation of glucose. This regulation is dependent on the function of Rst2, a transcription factor that regulates transition from mitosis to meiosis. The loss of the C2H2-type zinc finger domain in Rst2, termed Rst2 (C2H2∆), abolished the induction of Mug14 expression. Upon deletion of the stress starvation response element of the S. pombe (STREP: CCCCTC) sequence, which is a potential binding site of Rst2 on mug14, in the pka1∆ strain, its induction was abolished. The expression of Mug14 was significantly reduced and delayed by the limitation of glucose and also by nitrogen starvation in the rst2∆ strain. Mug14 is known to share a common function with Mde1 and Mta3 in the methionine salvage pathway, but the expression of mde1 and mta3 mRNAs was not enhanced by pka1 deletion and limitation of glucose. We conclude that the expression of Mug14 is upregulated by Rst2 under the control of the cAMP/PKA signaling pathway, which senses the limitation of glucose.

Glucose limitation and pka1 deletion rescue aberrant mitotic spindle formation induced by Mal3 overexpression in Schizosaccharomyces pombe

The cAMP-dependent protein kinase Pka1 is known as a regulator of glycogenesis, transition into meiosis, proper chromosome segregation, and stress responses in Schizosaccharomyces pombe. We demonstrated that both the cAMP/PKA pathway and glucose limitation play roles in appropriate spindle formation. Overexpression of Mal3 (1-308), an EB1 family protein, caused growth defects, increased 4C DNA content, and induced monopolar spindle formation. Overproduction of a high-affinity microtubule binding mutant (Q89R) and a recombinant protein possessing the CH and EB1 domains (1-241) both resulted in more severe phenotypes than Mal3 (1-308). Loss of functional Pka1 and glucose limitation rescued the phenotypes of Mal3-overexpressing cells, whereas deletion of Tor1 or Ssp2 did not. Growth defects and monopolar spindle formation in a kinesin-5 mutant, cut7-446, was partially rescued by pka1 deletion or glucose limitation. These findings suggest that Pka1 and glucose limitation regulate proper spindle formation in Mal3-overexpressing cells and the cut7-446 mutant.

CoQ10 production in Schizosaccharomyces pombe is increased by reduction of glucose levels or deletion of pka1

Coenzyme Q (CoQ) is an essential component of the electron transport system that produces ATP in nearly all living cells. CoQ₁₀ is a popular commercial food supplement around the world, and demand for efficient production of this molecule has increased in recent years. In this study, we explored CoQ₁₀ production in the fission yeast Schizosaccharomyces pombe. We found that CoQ₁₀ level was higher in stationary phase than in log phase, and that it increased when the cells were grown in a low concentration of glucose, in maltose, or in glycerol/ethanol medium. Because glucose signaling is mediated by cAMP, we evaluated the involvement of this pathway in CoQ biosynthesis. Loss of Pka1, the catalytic subunit of cAMP-dependent protein kinase, increased production of CoQ₁₀, whereas loss of the regulatory subunit Cgs1 decreased production. Manipulation of other components of the cAMP-signaling pathway affected CoQ₁₀ production in a consistent manner. We also found that glycerol metabolism was controlled by the cAMP/PKA pathway. CoQ₁₀ production by the S. pombe ∆pka1 reached 0.98 mg/g dry cell weight in medium containing a non-fermentable carbon source [2% glycerol (w/v) and 1% ethanol (w/v) supplemented with 0.5% casamino acids (w/v)], twofold higher than the production in wild-type cells under normal growth conditions. These findings demonstrate that carbon source, growth phase, and the cAMP-signaling pathway are important factors in CoQ₁₀ production in S. pombe.

A genome-wide analysis of carbon catabolite repression in Schizosaccharomyces pombe

BACKGROUND

Optimal glucose metabolism is central to the growth and development of cells. In microbial eukaryotes, carbon catabolite repression (CCR) mediates the preferential utilization of glucose, primarily by repressing alternate carbon source utilization. In fission yeast, CCR is mediated by transcriptional repressors Scr1 and the Tup/Ssn6 complex, with the Rst2 transcription factor important for activation of gluconeogenesis and sexual differentiation genes upon derepression. Through genetic and genome-wide methods, this study aimed to comprehensively characterize CCR in fission yeast by identifying the genes and biological processes that are regulated by Scr1, Tup/Ssn6 and Rst2, the core CCR machinery.

RESULTS

The transcriptional response of fission yeast to glucose-sufficient or glucose-deficient growth conditions in wild type and CCR mutant cells was determined by RNA-seq and ChIP-seq. Scr1 was found to regulate genes involved in carbon metabolism, hexose uptake, gluconeogenesis and the TCA cycle. Surprisingly, a role for Scr1 in the suppression of sexual differentiation was also identified, as homothallic scr1 deletion mutants showed ectopic meiosis in carbon and nitrogen rich conditions. ChIP-seq characterised the targets of Tup/Ssn6 and Rst2 identifying regulatory roles within and independent of CCR. Finally, a subset of genes bound by all three factors was identified, implying that regulation of certain loci may be modulated in a competitive fashion between the Scr1, Tup/Ssn6 repressors and the Rst2 activator.

CONCLUSIONS

By identifying the genes directly and indirectly regulated by Scr1, Tup/Ssn6 and Rst2, this study comprehensively defined the gene regulatory networks of CCR in fission yeast and revealed the transcriptional complexities governing this system.

The RHEB-mTOR axis regulates expression of Tf2 transposons in fission yeast

The human TSC2 gene, mutations in which predispose individuals to the disease tuberous sclerosis complex (TSC), encodes a GTPase-activating protein for the GTPase RHEB. Loss of TSC2 results in constitutive activation of RHEB and its target mammalian target of rapamycin (mTOR). We have previously reported that fission yeast (Schizosaccharomyces pombe) Tf2 retrotransposons (hereafter Tf2s) are abnormally induced upon nitrogen starvation in cells lacking the tsc2⁺ gene (Δtsc2), a homolog of the human TSC2 gene, and in cells with a dominant-active mutation in the fission yeast RHEB GTPase (rhb1-DA4). We report here that induction of Tf2s in these mutants is suppressed upon overexpression of the cgs2⁺ gene, which encodes a cAMP-specific phosphodiesterase, or upon deletion of components in the glucose/cAMP signaling pathway, namely Cyr1, Pka1, Tor1 and the stress-activated transcription factor Atf1. The results suggest that the glucose/cAMP signaling pathway is downregulated when cells are starved for nitrogen. We also show that Tf2 proteins are degraded via autophagy, which is under control of Tor2, a homolog of human mTOR. It appears that failure in the two processes, downregulation of the glucose/cAMP signaling pathway and induction of autophagy, allows abnormal induction of Tf2s upon nitrogen starvation in Δtsc2 and rhb1-DA4 cells.

Genes Controlling 2-deoxyglucose Induced Lysis and Formation of Reactive Oxygen Species in Schizosaccharomyces pombe

Schizosaccharomyces pombe cells of strains each carrying a deletion of one of the genes snf5, ypa1, pho7 and pas1 and of a strain overexpressing gene odr1, have been previously shown to grow in presence of the toxic glucose analogue 2-deoxyglucose (2-DG). Here we report that these genes control 2-DG induced lysis and are, with the exception of odr1, also involved in control of formation of reactive oxygen species (ROS) upon exposure of cells to H2O2. Lysis of deletion strains, but not of strain overexpressing odr1, is dependent on glucose concentration of the medium whereas ROS formation is glucose independent.

Schizosaccharomyces pombe rsv1 Transcription Factor and its Putative Homologues Preserved their Functional Homology and are Evolutionarily Conserved

Environmental glucose is an important regulator of biological processes, as it can launch different cell processes depending on its concentration. Thus, low glucose concentration can induce entry into quiescence, which ensures long-term viability for the cells or in other cases mycelial growth in the dimorphic species, which, in turn, provides the cells with fresh nutrients. Several genes, such as the genes of cAMP cascade, are involved in glucose sensing and response. Since this signal transduction pathway seemed to be an evolutionarily conserved process, we assumed that its genes were also conserved and preserved their functional homology. To obtain evidence, Schizosaccharomyces pombe rsv1 and its orthologous genes were investigated using in silico and experimental approaches. Our results supported that the Rsv1 zinc-finger transcription factors of Schizosaccharomyces japonicus and Schizosaccharomyces octosporus and the Candida albicans cas5p were really functional homologues of the S. pombe Rsv1. Namely, the homologous proteins were able to restore mutant phenotype of the S. pombe rsv1-deleted cells. Bioinformatic anaysis revealed that the most conserved parts of the proteins always contained the C2H2 domains and the complementation abilities of the counterpart genes were not uniform regarding the investigated features, which can be in connection with the conserved regions outside C2H2.

A Novel Phosphoregulatory Switch Controls the Activity and Function of the Major Catalytic Subunit of Protein Kinase A in Aspergillus fumigatus

Invasive aspergillosis (IA), caused by the filamentous fungal pathogen Aspergillus fumigatus, is a major cause of death among immunocompromised patients. The cyclic AMP/protein kinase A (PKA) signaling pathway is essential for hyphal growth and virulence of A. fumigatus, but the mechanism of regulation of PKA remains largely unknown. Here, we discovered a novel mechanism for the regulation of PKA activity in A. fumigatus via phosphorylation of key residues within the major catalytic subunit, PkaC1. Phosphopeptide enrichment and tandem mass spectrometry revealed the phosphorylation of PkaC1 at four sites (S175, T331, T333, and T337) with implications for important and diverse roles in the regulation of A. fumigatus PKA. While the phosphorylation at one of the residues (T333) is conserved in other species, the identification of three other residues represents previously unknown PKA phosphoregulation in A. fumigatus Site-directed mutagenesis of the phosphorylated residues to mimic or prevent phosphorylation revealed dramatic effects on kinase activity, growth, conidiation, cell wall stress response, and virulence in both invertebrate and murine infection models. Three-dimensional structural modeling of A. fumigatus PkaC1 substantiated the positive or negative regulatory roles for specific residues. Suppression of PKA activity also led to downregulation of PkaC1 protein levels in an apparent novel negative-feedback mechanism. Taken together, we propose a model in which PkaC1 phosphorylation both positively and negatively modulates its activity. These findings pave the way for future discovery of fungus-specific aspects of this key signaling network.

IMPORTANCE

Our understanding of signal transduction networks in pathogenic fungi is limited, despite the increase in invasive fungal infections and rising mortality rates in the immunosuppressed patient population. Because PKA is known to be essential for hyphal growth and virulence of A. fumigatus, we sought to identify fungus-specific regulatory mechanisms governing PKA activity. In this study, we identify, for the first time, a novel mechanism for the regulation of PKA signaling in which differential phosphorylation of the PkaC1 catalytic subunit can lead to either positive or negative regulation of activity. Furthermore, we show that inactivation of PKA signaling leads to downregulation of catalytic subunit protein levels in a negative-feedback mechanism distinct from expression patterns previously reported in the yeasts. Our findings represent a divergence in the regulation of PKA signaling in A. fumigatus, which could potentially be exploited as a target and also open the avenue for discovery of fungus-specific downstream effectors of PKA.

cAMP-dependent protein kinase involves calcium tolerance through the regulation of Prz1 in Schizosaccharomyces pombe

The cAMP-dependent protein kinase Pka1 is known as a regulator of glycogenesis, meiosis, and stress responses in Schizosaccharomyces pombe. We demonstrated that Pka1 is responsible for calcium tolerance. Loss of functional components of the PKA pathway such as Git3, Gpa2, Cyr1, and Pka1 yields a CaCl2-sensitive phenotype, while loss of Cgs1, a regulatory subunit of PKA, results in CaCl2 tolerance. Cytoplasmic distribution of Cgs1 and Pka1 is increased by the addition of CaCl2, suggesting that CaCl2 induces dissociation of Cgs1 and Pka1. The expression of Prz1, a transcriptional regulator in calcium homeostasis, is elevated in a pka1∆ strain and in a wild type strain under glucose-limited conditions. Accordingly, higher expression of Prz1 in the wild type strain results in a CaCl2-sensitive phenotype. These findings suggest that Pka1 is essential for tolerance to exogenous CaCl2, probably because the expression level of Prz1 needs to be properly regulated by Pka1.

Expression pattern and phenotypic characterization of the mutant strain reveals target genes and processes regulated by pka1 in the dimorphic fission yeast Schizosaccharomyces japonicus

The cAMP cascade plays an important role in several biological processes. Thus, study of its molecular details can contribute to a better understanding of these processes, treatment of diseases, or even finding antifungal drug targets. To gain further information about the PKA pathway, and its evolutionarily conserved and species-specific features, the central regulator pka1 gene, which encodes the cAMP-dependent protein kinase catalytic subunit, was studied in the less known haplontic, dimorphic fission yeast Schizosaccharomyces japonicus. Namely, this species belongs to a highly divergent phylogenetic branch of fungi. Furthermore, S. japonicus had only a single copy pka1 gene in contrast to the budding yeasts. Therefore, the pka1 deleted mutant was created, whose RNA sequencing and phenotypic studies revealed that the Pka1 regulated at least 373 genes, among them further kinases, phosphatases and transcriptional regulators. It regulated elongation of hyphae, cell size, aging and stress response. Furthermore, half of the pka1 target genes seemed to be conserved in Schizosaccharomyces pombe and S. japonicus. However, there were oppositely regulated genes in the two closely related species. The target genes suggest that this single gene must be able to fulfill all the functions of TPK1-3 of Saccharomyces cerevisiae. Thus, our results shed light on certain similarities and differences of the PKA pathway of S. japonicus compared to the budding yeasts and confirmed the multifunctionality of the pka1 gene, but further experiments are needed to prove its involvement in the metabolic processes and transport.

Identification of nuclear genes affecting 2-Deoxyglucose resistance in Schizosaccharomyces pombe

2-Deoxyglucose (2-DG) is a toxic glucose analog. To identify genes involved in 2-DG toxicity in Schizosaccharomyces pombe, we screened a wild-type overexpression library for genes which render cells 2-DG resistant. A gene we termed odr1, encoding an uncharacterized hydrolase, led to strong resistance and altered invertase expression when overexpressed. We speculate that Odr1 neutralizes the toxic form of 2-DG, similar to the Saccharomyces cerevisiae Dog1 and Dog2 phosphatases which dephosphorylate 2-DG-6-phosphate synthesized by hexokinase. In a complementary approach, we screened a haploid deletion library to identify 2-DG-resistant mutants. This screen identified the genes snf5, ypa1, pas1 and pho7. In liquid medium, deletions of these genes conferred 2-DG resistance preferentially under glucose-repressed conditions. The deletion mutants expressed invertase activity more constitutively than the control strain, indicating defects in the control of glucose repression. No S. cerevisiae orthologs of the pho7 gene is known, and no 2-DG resistance has been reported for any of the deletion mutants of the other genes identified here. Moreover, 2-DG leads to derepressed invertase activity in S. pombe, while in S. cerevisiae it becomes repressed. Taken together, these findings suggest that mechanisms involved in 2-DG resistance differ between budding and fission yeasts.

Fission Yeast CSL Transcription Factors: Mapping Their Target Genes and Biological Roles

BACKGROUND

Cbf11 and Cbf12, the fission yeast CSL transcription factors, have been implicated in the regulation of cell-cycle progression, but no specific roles have been described and their target genes have been only partially mapped.

METHODOLOGY/PRINCIPAL FINDINGS

Using a combination of transcriptome profiling under various conditions and genome-wide analysis of CSL-DNA interactions, we identify genes regulated directly and indirectly by CSL proteins in fission yeast. We show that the expression of stress-response genes and genes that are expressed periodically during the cell cycle is deregulated upon genetic manipulation of cbf11 and/or cbf12. Accordingly, the coordination of mitosis and cytokinesis is perturbed in cells with genetically manipulated CSL protein levels, together with other specific defects in cell-cycle progression. Cbf11 activity is nutrient-dependent and Δcbf11-associated defects are mitigated by inactivation of the protein kinase A (Pka1) and stress-activated MAP kinase (Sty1p38) pathways. Furthermore, Cbf11 directly regulates a set of lipid metabolism genes and Δcbf11 cells feature a stark decrease in the number of storage lipid droplets.

CONCLUSIONS/SIGNIFICANCE

Our results provide a framework for a more detailed understanding of the role of CSL proteins in the regulation of cell-cycle progression in fission yeast.

The zinc finger protein Gsf1 regulates Gsf2-dependent flocculation in fission yeast

Fission yeast flocculates nonsexually by induction of the flocculin encoded by gsf2(+) which is controlled by the positive regulator Mbx2. Here, we report a novel gene designated gsf1(+) found to be a negative regulator of nonsexual flocculation. We identified gsf1(+) as a multicopy suppressor of a sam2 mutation, which caused growth sensitivity to Ca(2+) and also found a nonsense mutation in gsf1(+) in a previously isolated gsf1 mutant. The gsf1(+) gene encodes a 547-aa protein containing a Zn(2)-Cys(6) binuclear cluster-type zinc finger motif. The Gsf1 protein localized in the nucleus, consistent with a role as a transcription factor. Deletion of gsf1(+) resulted in nonsexual flocculation inducible by CaCl2 , which was suppressed by the addition of EDTA or galactose. Both gsf2(+) and mbx2(+) were highly expressed in the gsf1 mutant. gsf1∆ gsf2∆ and gsf1∆ mbx2∆ double mutants did not flocculate, suggesting that gsf1(+) is an upstream regulator. In addition, the gsf1 mutant was sensitive to CaCl2 , KCl, HU, and TBZ, consistent with the possibility that gsf1(+) plays a role in functions unrelated to flocculation. Taken together, these results suggest that nonsexual flocculation in fission yeast is negatively controlled by Gsf1, which controls expression of mbx2(+) and gsf2(+) .

Regulation of autophagy by glucose in Mammalian cells

Autophagy is an evolutionarily conserved process that contributes to maintain cell homeostasis. Although it is strongly regulated by many extracellular factors, induction of autophagy is mainly produced by starvation of nutrients. In mammalian cells, the regulation of autophagy by amino acids, and also by the hormone insulin, has been extensively investigated, but knowledge about the effects of other autophagy regulators, including another nutrient, glucose, is more limited. Here we will focus on the signalling pathways by which environmental glucose directly, i.e., independently of insulin and glucagon, regulates autophagy in mammalian cells, but we will also briefly mention some data in yeast. Although glucose deprivation mainly induces autophagy via AMPK activation and the subsequent inhibition of mTORC1, we will also comment other signalling pathways, as well as evidences indicating that, under certain conditions, autophagy can be activated by glucose. A better understanding on how glucose regulates autophagy not only will expand our basic knowledge of this important cell process, but it will be also relevant to understand common human disorders, such as cancer and diabetes, in which glucose levels play an important role.

Multistep regulation of protein kinase A in its localization, phosphorylation and binding with a regulatory subunit in fission yeast

The cAMP-PKA is the major glucose-sensing pathway that controls sexual differentiation in Schizosaccharomyces pombe. Sequencing from the pka1 locus of recessive sam mutants, in which cells are highly inclined to sexual differentiation, led to the identification of mutations in the pka1 locus in sam5 (pka1-G441E) and sam7 (pka1-G441R). Rst2 and Ste11 proteins were induced and localized to the nucleus of sam5 and sam7 mutants even under rich glucose conditions, indicating that the function of Pka1 was completely abolished by mutations. Pka1-G441E and Pka1-G441R mutant proteins reside in the cytoplasm, even under glucose-rich conditions, while wild-type Pka1 resides in the nucleus, indicating that the functionality of Pka1 is important for its nuclear localization. This is supported by the observation that the Pka1-T356A mutant, which partially lacks Pka1 function, was localized to both the cytoplasm and the nucleus, but an active phosphomimetic Pka1-T356D mutant prtotein was localized to the nucleus under glucose-rich conditions. In addition to the basal phosphorylation of Pka1 at T356, hyperphosphorylation of Pka1 was observed under glucose-starved conditions, and such hyperphosphorylation was not observed in pka1-G441E, pka1-G441R, pka1-T356A or pka1-T356D mutants. As these mutant proteins failed to interact with a regulatory subunit Cgs1, hyperphosphorylation of Pka1 mutant proteins was considered to be dependent on Cgs1 interaction. Consistent with a role for Cgs1 in Pka1 phosphorylation, we detected the formation of a Cgs1-Pka1 complex prior to Pka1 hyperphosphorylation. Together, these results indicate that nuclear localization of Pka1 depends on its activity and hyperphosphorylation of Pka1 depends on Cgs1 interaction.