Protein kinase A and mitogen-activated protein kinase pathways antagonistically regulate fission yeast fbp1 transcription by employing different modes of action at two upstream activation sites

Role of fission yeast Tup1-like repressors and Prr1 transcription factor in response to salt stress

In Schizosaccharomyces pombe, the Sty1 mitogen-activated protein kinase and the Atf1 transcription factor control transcriptional induction in response to elevated salt concentrations. Herein, we demonstrate that two repressors, Tup11 and Tup12, and the Prr1 transcription factor also function in the response to salt shock. We find that deletion of both tup genes together results in hypersensitivity to elevated cation concentrations (K(+) and Ca(2+)) and we identify cta3(+), which encodes an intracellular cation transporter, as a novel stress gene whose expression is positively controlled by the Sty1 pathway and negatively regulated by Tup repressors. The expression of cta3(+) is maintained at low levels by the Tup repressors, and relief from repression requires the Sty1, Atf1, and Prr1. Prr1 is also required for KCl-mediated induction of several other Sty1-dependent genes such as gpx1(+) and ctt1(+). Surprisingly, the KCl-mediated induction of cta3(+) expression occurs independently of Sty1 in a tup11Delta tup12Delta mutant and so the Tup repressors link induction to the Sty1 pathway. We also report that in contrast to a number of other Sty1- and Atf1-dependent genes, the expression of cta3(+) is induced only by high salt concentrations. However, in the absence of the Tup repressors this specificity is lost and a range of stresses induces cta3(+) expression.

Schizosaccharomyces pombe Git7p, a member of the Saccharomyces cerevisiae Sgtlp family, is required for glucose and cyclic AMP signaling, cell wall integrity, and septation

The Schizosaccharomyces pombe fbp1 gene, encoding fructose-1,6-bisphosphatase, is transcriptionally repressed by glucose. Mutations that confer constitutive fbp1 transcription identify git (glucose-insensitive transcription) genes that encode components of a cyclic AMP (cAMP) signaling pathway required for adenylate cyclase activation. Four of these genes encode the three subunits of a heterotrimeric G protein (gpa2, git5, and git11) and a G protein-coupled receptor (git3). Three additional genes, git1, git7, and git10, act in parallel to or downstream from the G protein genes. Here, we describe the cloning and characterization of the git7 gene. The Git7p protein is a member of the Saccharomyces cerevisiae Sgtlp protein family. In budding yeast, Sgtlp associates with Skplp and plays an essential role in kinetochore assembly, while in Arabidopsis, a pair of SGT1 proteins have been found to be involved in plant disease resistance through an interaction with RAR1. Like S. cerevisiae Sgtlp, Git7p is essential, but this requirement appears to be due to roles in septation and cell wall integrity, which are unrelated to cAMP signaling, as S. pombe cells lacking either adenylate cyclase or protein kinase A are viable. In addition, git7 mutants are sensitive to the microtubule-destabilizing drug benomyl, although they do not display a chromosome stability defect. Two alleles of git7 that are functional for cell growth and septation but defective for glucose-triggered cAMP signaling encode proteins that are altered in the highly conserved carboxy terminus. The S. cerevisiae and human SGT1 genes both suppress git7-93 but not git7-235 for glucose repression of fbp1 transcription and benomyl sensitivity. This allele-specific suppression indicates that the Git7p/Sgtlp proteins may act as multimers, such that Git7-93p but not Git7-235p can deliver the orthologous proteins to species-specific targets. Our studies suggest that members of the Git7p/Sgt1p protein family may play a conserved role in the regulation of adenylate cyclase activation in S. pombe, S. cerevisiae, and humans.

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.

Protein kinase A regulates sexual development and gluconeogenesis through phosphorylation of the Zn finger transcriptional activator Rst2p in fission yeast

Protein kinase A (PKAi a cyclic AMP-dependent protein kinase) negatively regulates sexual development and gluconeogenesis in fission yeast by suppressing the transcription of ste11 required for the former and the transcription of fbp1 required for the latter. Here we show that Rst2p, a zinc finger protein that can bind to the upstream region of ste11 and fbp1 via the STREP motif, mediates the activity of PKA to transcription of these genes. A simple reporter system confirmed that PKA could cause its negative effect on transcription through the combination of Rst2p and STREP. Rst2p was phosphorylated by PKA in vitro at two consensus sequences on it. Substitution of the target threonine residues by alanine made the protein active even in the presence of high PKA activity. Rst2p underwent hyperphosphorylation in the medium lacking glucose, and PKA inhibited this hyperphosphorylation. Rst2p was mainly cytoplasmic under high PKA activity but was concentrated in the nucleus when this activity was lowered, suggesting that PKA might regulate ste11 and fbp1 negatively by excluding Rst2p from the nucleus. However, the shift of Rst2p localization was not perfect under physiological conditions, leaving the possibility that PKA inhibits Rst2p function in another way as well. Although the PKA-Rst2p-STREP pathway is apparently central to the regulation of ste11 and fbp1 transcription in accordance with nutritional conditions, some additional paths are likely to connect nitrogen to repression of ste11 and glucose to repression of fbp1. These paths may ensure the specificity between the type of nutrients in shortage and the type of genes to be expressed.

Counteracting regulation of chromatin remodeling at a fission yeast cAMP response element-related recombination hotspot by stress-activated protein kinase, cAMP-dependent kinase and meiosis regulators

In fission yeast, an ATF/CREB-family transcription factor Atf1-Pcr1 plays important roles in the activation of early meiotic processes via the stress-activated protein kinase (SAPK) and the cAMP-dependent protein kinase (PKA) pathways. In addition, Atf1-Pcr1 binds to a cAMP responsive element (CRE)-like sequence at the site of the ade6-M26 mutation, which results in local enhancement of meiotic recombination and chromatin remodeling. Here we studied the roles of meiosis-inducing signal transduction pathways in M26 chromatin remodeling. Chromatin analysis revealed that persistent activation of PKA in meiosis inhibited M26 chromatin remodeling, suggesting that the PKA pathway represses M26 chromatin remodeling. The SAPK pathway activated M26 chromatin remodeling, since mutants lacking a component of this pathway, the Wis1 or Spc1/Sty1 kinases, had no M26 chromatin remodeling. M26 chromatin remodeling also required the meiosis regulators Mei2 and Mei3 but not the subsequently acting regulators Sme2 and Mei4, suggesting that induction of M26 chromatin remodeling needs meiosis-inducing signals before premeiotic DNA replication. Similar meiotic chromatin remodeling occurred meiotically around natural M26 heptamer sequences. These results demonstrate the coordinated action of genetic and physiological factors required to remodel chromatin in preparation for high levels of meiotic recombination and eukaryotic cellular differentiation.

Elements from the cAMP signaling pathway are involved in the control of expression of the yeast gluconeogenic gene FBP1

cAMP represses the transcription of some Saccharomyces cerevisiae genes sensitive to catabolite repression. The effect of cAMP on the expression of FBP1, encoding fructose-1,6-bisphosphatase (FbPase), has been further investigated. In yeast cells shifted to a derepressing medium, synthesis of FbPase was delayed if the strong decrease in intracellular cAMP, which occurs during the shift, was prevented. A similar delay occurred in a RAS2val19 strain, while in a tpk1w strain, with weak protein kinase A activity, induction of FbPase occurred earlier than in a TPK1 strain. In the tpk1w strain, proteins which bind the UAS1 element of FBP1 were present during growth on glucose but they were only weakly operative. Expression of CAT8 and SIP4, encoding proteins which bind the UAS2 element, was blocked by a high concentration of cAMP, but catabolite repression of these genes was not much relieved in a tpk1w strain. We conclude that in S. cerevisiae, as reported for Schizosaccharomyces pombe, control of FBP1 requires both cAMP-dependent and independent pathways; however, the mechanisms operating in the two yeasts are different.

Conserved cAMP signaling cascades regulate fungal development and virulence

Two well characterized signal transduction cascades regulating fungal development and virulence are the MAP kinase and cAMP signaling cascades. Here we review the current state of knowledge on cAMP signaling cascades in fungi. While the processes regulated by cAMP signaling in fungi are as diverse as the fungi themselves, the components involved in signal transduction are remarkably conserved. Fungal cAMP signaling cascades are also quite versatile, which is apparent from the differential regulation of similar biological processes. In this review we compare and contrast cAMP signaling pathways that regulate development in the budding yeast Saccharomyces cerevisiae, the fission yeast Schizosaccharomyces pombe, and differentiation and virulence in the human pathogen Cryptococcus neoformans and the plant pathogen Ustilago maydis. We also present examples of interaction between the cAMP and MAP kinase signaling cascades in the regulation of fungal development and virulence.

Transcriptional regulators of the Schizosaccharomyces pombe fbp1 gene include two redundant Tup1p-like corepressors and the CCAAT binding factor activation complex

The Schizosaccharomyces pombe fbp1 gene, which encodes fructose-1,6-bis-phosphatase, is transcriptionally repressed by glucose through the activation of the cAMP-dependent protein kinase A (PKA) and transcriptionally activated by glucose starvation through the activation of a mitogen-activated protein kinase (MAPK). To identify transcriptional regulators acting downstream from or in parallel to PKA, we screened an adh-driven cDNA plasmid library for genes that increase fbp1 transcription in a strain with elevated PKA activity. Two such clones express amino-terminally truncated forms of the S. pombe tup12 protein that resembles the Saccharomyces cerevisiae Tup1p global corepressor. These clones appear to act as dominant negative alleles. Deletion of both tup12 and the closely related tup11 gene causes a 100-fold increase in fbp1-lacZ expression, indicating that tup11 and tup12 are redundant negative regulators of fbp1 transcription. In strains lacking tup11 and tup12, the atf1-pcr1 transcriptional activator continues to play a central role in fbp1-lacZ expression; however, spc1 MAPK phosphorylation of atf1 is no longer essential for its activation. We discuss possible models for the role of tup11- and tup12-mediated repression with respect to signaling from the MAPK and PKA pathways. A third clone identified in our screen expresses the php5 protein subunit of the CCAAT-binding factor (CBF). Deletion of php5 reduces fbp1 expression under both repressed and derepressed conditions. The CBF appears to act in parallel to atf1-pcr1, although it is unclear whether or not CBF activity is regulated by PKA.

Hyperthermotolerant fission yeast mutations, sow1 and sow2, suppress the cell cycle defect and stress sensitivity of MAP kinase kinase wis1Delta

Wis1 is a mitogen-activated protein kinase kinase (MAPKK) that regulates mitosis and mediates stress responses in the fission yeast, Schizosaccharomyces pombe. wis1Delta strains are viable but stress-sensitive and show a mitotic delay. At high temperatures, wis1Delta cells cease division but cellular growth continues. Mutations that suppress the heat sensitivity of a wis1Delta strain were isolated and map to two apparently novel loci, sow1 (for suppressor of wis1Delta) and sow2. In addition to suppressing wis1Delta heat sensitivity, sow1 and sow2 can suppress wis1Delta osmosensitivity and cell cycle defects. sow1 and sow2 mutants in a wis1+ background were able to grow at higher temperatures than wild-type and sow1 showed a mitotic advance. The sow genes may therefore define a novel connection between stress tolerance and cell cycle control.