Yeast pseudohyphal growth is regulated by GPA2, a G protein alpha homolog

Novel sensing mechanisms and targets for the cAMP-protein kinase A pathway in the yeast Saccharomyces cerevisiae

The cAMP-protein kinase A (PKA) pathway in the yeast Saccharomyces cerevisiae plays a major role in the control of metabolism, stress resistance and proliferation, in particular in connection with the available nutrient conditions. Extensive information has been obtained on the core section of the pathway, i.e. Cdc25, Ras, adenylate cyclase, PKA, and on components interacting directly with this core section, such as the Ira proteins, Cap/Srv2 and the two cAMP phosphodiesterases. Recent work has now started to reveal upstream regulatory components and downstream targets of the pathway. A G-protein-coupled receptor system (Gpr1-Gpa2) acts upstream of adenylate cyclase and is required for glucose activation of cAMP synthesis in concert with a glucose phosphorylation-dependent mechanism. Although a genuine signalling role for the Ras proteins remains unclear, they appear to mediate at least part of the potent stimulation of cAMP synthesis by intracellular acidification. Recently, several new targets of the PKA pathway have been discovered. These include the Msn2 and Msn4 transcription factors mediating part of the induction of STRE-controlled genes by a variety of stress conditions, the Rim15 protein kinase involved in stationary phase induction of a similar set of genes and the Pde1 low-affinity cAMP phosphodiesterase, which specifically controls agonist-induced cAMP signalling. A major issue that remains to be resolved is the precise connection between the cAMP-PKA pathway and other nutrient-regulated components involved in the control of growth and of phenotypic characteristics correlated with growth, such as the Sch9 and Yak1 protein kinases. Cln3 appears to play a crucial role in the connection between the availability of certain nutrients and Cdc28 kinase activity, but it remains to be clarified which nutrient-controlled pathways control Cln3 levels.

The yeast trimeric guanine nucleotide-binding protein alpha subunit, Gpa2p, controls the meiosis-specific kinase Ime2p activity in response to nutrients

Saccharomyces cerevisiae Gpa2p, the alpha subunit of a heterotrimeric guanine nucleotide-binding protein (G protein), is involved in the regulation of vegetative growth and pseudohyphal development. Here we report that Gpa2p also controls sporulation by interacting with the regulatory domain of Ime2p (Sme1p), a protein kinase essential for entrance of meiosis and sporulation. Protein-protein interactions between Gpa2p and Ime2p depend on the GTP-bound state of Gpa2p and correlate with down-regulation of Ime2p kinase activity in vitro. Overexpression of Ime2p inhibits pseudohyphal development and enables diploid cells to sporulate even in the presence of glucose or nitrogen. In contrast, overexpression of Gpa2p in cells simultaneously overproducing Ime2p results in a drastic reduction of sporulation efficiency, demonstrating an inhibitory effect of Gpa2p on Ime2p function. Furthermore, deletion of GPA2 accelerates sporulation on low-nitrogen medium. These observations are consistent with the following model. In glucose-containing medium, diploid cells do not sporulate because Ime2p is inactive or expressed at low levels. Upon starvation, expression of Gpa2p and Ime2p is induced but sporulation is prevented as long as nitrogen is present in the medium. The negative control of Ime2p kinase activity is exerted at least in part through the activated form of Gpa2p and is released as soon as nutrients are exhausted. This model attributes a switch function to Gpa2p in the meiosis-pseudohyphal growth decision.

Cloning and characterisation of PGA1 and PGA2: two G protein alpha-subunits from pea that promote growth in the yeast Saccharomyces cerevisiae

We report here on the cloning and characterization of two G protein alpha-subunits from pea: PGA1 and PGA2. Based on DNA gel blot analysis, PGA1 and PGA2 are the only Galpha homologous sequences in pea. RT-PCR analysis reveals that PGA1 and PGA2 transcripts are present in a variety of adult pea tissues. However, PGA2 mRNA is consistently detected at a lower level than PGA1 and demonstrates some degree of tissue specificity relative to PGA1. In the apical bud of pea seedlings, PGA1 and PGA2 transcripts decrease in response to 24 h of white light following growth for 6 days in darkness. The G protein mediated, yeast mating pathway was used to analyse the function of PGA1 and PGA2 in vivo. PGA1 downregulates the mating pathway, but through a mechanism that is independent of Gbetagamma sequestration. Unexpectedly, both PGA1 and PGA2 promote growth through a mating pathway independent mechanism.

Chromosome separation and exit from mitosis in budding yeast: dependence on growth revealed by cAMP-mediated inhibition

Cell cycle progression of somatic cells depends on net mass accumulation. In Saccharomyces cerevisiae the cAMP-dependent kinases (PKAs) promote cytoplasmic growth and modulate the growth-regulated mechanism triggering the begin of DNA synthesis. By altering the cAMP signal in budding yeast cells we show here that mitotic events can also depend on growth. In fact, the hyperactivation of PKAs permanently inhibited both anaphase and exit from mitosis when cell growth was repressed. In S. cerevisiae the anaphase promoting complex (APC) triggers entry into anaphase by mediating the degradation of Pds1p. The cAMP pathway activation was lethal together with a partial impairment of the Cdc16p APC subunit, causing a preanaphase arrest, and conversely low PKA activity suppressed the lethality of cdc16-1 cells. Deregulated PKAs partially prevented the decrease of Pds1p intracellular levels concomitantly with the anaphase inhibition, and the PKA-dependent preanaphase arrest could be suppressed in pds1(-) cells. Thus, the cAMP pathway and APC functionally interact in S. cerevisiae and Pds1p is required for the cAMP-mediated inhibition of chromosome separation. Exit from mitosis requires APC, Cdc15p, and the polo-like Cdc5p kinase. PKA hyperactivation and a cdc15 mutation were synthetically lethal and brought to a telophase arrest. Finally, a low cAMP signal allowed cell division at a small cell size and suppressed the lethality of cdc15-2 or cdc5-1 cells. We propose that mitosis progression and the M/G1 phase transition specifically depend on cell growth through a mechanism modulated by PKAs and interacting with the APC/CDC15/CDC5 mitotic system. A possible functional antagonism between PKAs and the mitosis promoting factor is also discussed.

Signal transduction cascades regulating mating, filamentation, and virulence in Cryptococcus neoformans

Cryptococcus neoformans is a basidiomycetous fungal pathogen that infects the central nervous system. The organism has a defined sexual cycle involving mating between haploid MATalpha and MATa cells. Recent studies have revealed signaling cascades that coordinately regulate differentiation and virulence of C. neoformans. One signaling cascade involves a conserved G-protein alpha subunit and cAMP, and senses nutrients during mating and virulence. The second is a conserved mitogen activated protein (MAP) kinase cascade that senses pheromone during mating, and also regulates haploid fruiting and virulence. Interestingly, some of the MAP kinase components are encoded by the MAT locus itself, which may explain the unique association of the MATalpha locus with physiology and virulence.

Regulatory networks controlling Candida albicans morphogenesis

Candida albicans undergoes reversible morphogenetic transitions between budding, pseudohyphal and hyphal growth forms that promote the virulence of this pathogenic fungus. The regulatory networks that control morphogenesis are being elucidated; however, the primary signals that trigger morphogenesis remain obscure, and the physiological outputs of these networks are complex.

Cyclic AMP-dependent protein kinase regulates pseudohyphal differentiation in Saccharomyces cerevisiae

In response to nitrogen starvation, diploid cells of the yeast Saccharomyces cerevisiae differentiate to a filamentous growth form known as pseudohyphal differentiation. Filamentous growth is regulated by elements of the pheromone mitogen-activated protein (MAP) kinase cascade and a second signaling cascade involving the receptor Gpr1, the Galpha protein Gpa2, Ras2, and cyclic AMP (cAMP). We show here that the Gpr1-Gpa2-cAMP pathway signals via the cAMP-dependent protein kinase, protein kinase A (PKA), to regulate pseudohyphal differentiation. Activation of PKA by mutation of the regulatory subunit Bcy1 enhances filamentous growth. Mutation and overexpression of the PKA catalytic subunits reveal that the Tpk2 catalytic subunit activates filamentous growth, whereas the Tpk1 and Tpk3 catalytic subunits inhibit filamentous growth. The PKA pathway regulates unipolar budding and agar invasion, whereas the MAP kinase cascade regulates cell elongation and invasion. Epistasis analysis supports a model in which PKA functions downstream of the Gpr1 receptor and the Gpa2 and Ras2 G proteins. Activation of filamentous growth by PKA does not require the transcription factors Ste12 and Tec1 of the MAP kinase cascade, Phd1, or the PKA targets Msn2 and Msn4. PKA signals pseudohyphal growth, in part, by regulating Flo8-dependent expression of the cell surface flocculin Flo11. In summary, the cAMP-dependent protein kinase plays an intimate positive and negative role in regulating filamentous growth, and these findings may provide insight into the roles of PKA in mating, morphogenesis, and virulence in other yeasts and pathogenic fungi.

A G1 cyclin is necessary for maintenance of filamentous growth in Candida albicans

Candida albicans undergoes a dramatic morphological transition in response to various growth conditions. This ability to switch from a yeast form to a hyphal form is required for its pathogenicity. The intractability of Candida to traditional genetic approaches has hampered the study of the molecular mechanism governing this developmental switch. Our approach is to use the more genetically tractable yeast Saccharomyces cerevisiae to yield clues about the molecular control of filamentation for further studies in Candida. G1 cyclins Cln1 and Cln2 have been implicated in the control of morphogenesis in S. cerevisiae. We show that C. albicans CLN1 (CaCLN1) has the same cell cycle-specific expression pattern as CLN1 and CLN2 of S. cerevisiae. To investigate whether G1 cyclins are similarly involved in the regulation of cell morphogenesis during the yeast-to-hypha transition of C. albicans, we mutated CaCLN1. Cacln1/Cacln1 cells were found to be slower than wild-type cells in cell cycle progression. The Cacln1/Cacln1 mutants were also defective in hyphal colony formation on several solid media. Furthermore, while mutant strains developed germ tubes under several hypha-inducing conditions, they were unable to maintain the hyphal growth mode in a synthetic hypha-inducing liquid medium and were deficient in the expression of hypha-specific genes in this medium. Our results suggest that CaCln1 may coordinately regulate hyphal development with signal transduction pathways in response to various environmental cues.

MOD-D, a Galpha subunit of the fungus Podospora anserina, is involved in both regulation of development and vegetative incompatibility

Cell death via vegetative incompatibility is widespread in fungi but molecular mechanism and biological function of the process are poorly understood. One way to investigate this phenomenon was to study genes named mod that modified incompatibility reaction. In this study, we cloned the mod-D gene that encodes a Galpha protein. The mod-D mutant strains present developmental defects. Previously, we showed that the mod-E gene encodes an HSP90. The mod-E1 mutation suppresses both vegetative incompatibility and developmental defects due to the mod-D mutation. Moreover, we isolated the PaAC gene, which encodes an adenylate cyclase, as a partial suppressor of the mod-D1 mutation. Our previous results showed that the molecular mechanisms involved in vegetative incompatibility and developmental pathways are connected, suggesting that vegetative incompatibility may result from disorders in some developmental steps. Our new result corroborates the involvement of mod genes in signal transduction pathways. As expected, we showed that an increase in the cAMP level is able to suppress the defects in vegetative growth due to the mod-D1 mutation. However, cAMP increase has no influence on the suppressor effect of the mod-D1 mutation on vegetative incompatibility, suggesting that this suppressor effect is independent of the cAMP pathway.

In budding yeast, reactive oxygen species induce both RAS-dependent and RAS-independent cell cycle-specific arrest

The role of mild oxidative stresses elicited by diethylmaleate (DEM)-induced glutathione depletion in the progression of the yeast cell cycle has been investigated. We found that different wild-type strains are sensitive to oxidative stresses induced by similar DEM doses: approximately 1 mM on YPD plates, 5-10 mM in shaken flasks. At lower doses, DEM caused a transient decrease in growth rate, largely because of a decreased G1-to-S transition. Treatment with higher DEM doses leads to complete growth arrest, with most cells found in the unbudded G1 phase of the cell cycle. DEM treatment resulted in transcriptional induction of stress-responsive element (STRE)-controlled genes and was relieved by treatment with the antioxidant N-acetyl cysteine. Reciprocal shift experiments with cdc25 and cdc28 mutants showed that the major cell cycle arrest point was located in the Start area, at or near the CDC25-mediated step, before the step mediated by the CDC28 cyclin-dependent kinase. The DEM-induced G1 arrest requires a properly regulated RAS pathway and can be bypassed by overexpressing the G1-specific cyclin CLN2. However, cells with either a deregulated RAS pathway or overexpressing CLN2 failed to grow and arrested as budded cells, indicating that a second DEM-sensitive cell cycle step exists.

Crosstalk between the Ras2p-controlled mitogen-activated protein kinase and cAMP pathways during invasive growth of Saccharomyces cerevisiae

The two highly conserved RAS genes of the budding yeast Saccharomyces cerevisiae are redundant for viability. Here we show that haploid invasive growth development depends on RAS2 but not RAS1. Ras1p is not sufficiently expressed to induce invasive growth. Ras2p activates invasive growth using either of two downstream signaling pathways, the filamentation MAPK (Cdc42p/Ste20p/MAPK) cascade or the cAMP-dependent protein kinase (Cyr1p/cAMP/PKA) pathway. This signal branch point can be uncoupled in cells expressing Ras2p mutant proteins that carry amino acid substitutions in the adenylyl cyclase interaction domain and therefore activate invasive growth solely dependent on the MAPK cascade. Both Ras2p-controlled signaling pathways stimulate expression of the filamentation response element-driven reporter gene depending on the transcription factors Ste12p and Tec1p, indicating a crosstalk between the MAPK and the cAMP signaling pathways in haploid cells during invasive growth.

The WD repeat: a common architecture for diverse functions

Our knowledge of the large family of proteins that contain the WD repeat continues to accumulate. The WD-repeat proteins are found in all eukaryotes and are implicated in a wide variety of crucial functions. The solution of the three-dimensional structure of one WD-repeat protein and the assumption that the structure will be common to all members of this family has allowed subfamilies of WD-repeat proteins to be defined on the basis of probable surface similarity. Proteins that have very similar surfaces are likely to have common binding partners and similar functions.

MAP kinase and cAMP filamentation signaling pathways converge on the unusually large promoter of the yeast FLO11 gene

In Saccharomyces cerevisiae, two major signal transduction pathways, the Kss1 MAPK pathway and the cAMP-regulated pathway, are critical for the differentiation of round yeast form cells to multicellular, invasive pseudohyphae. Here we report that these parallel pathways converge on the promoter of a gene, FLO11, which encodes a cell surface protein required for pseudohyphal formation. The FLO11 promoter is unusually large, containing at least four upstream activation sequences (UASs) and nine repression elements which together span at least 2.8 kb. Several lines of evidence indicate that the MAPK and cAMP signals are received by distinct transcription factors and promoter elements. First, regulation via the MAPK pathway requires the transcription factors Ste12p/Tec1p, whereas cAMP-mediated activation requires a distinct factor, Flo8p. Secondly, mutations in either pathway block FLO11 transcription. Overexpression of STE12 can suppress the loss of FLO8, and overexpression of FLO8 can suppress the loss of STE12. Finally, multiple distinct promoter regions of the FLO11 promoter are required for its activation by either Flo8p or Ste12p/ Tec1p. Thus, like the promoters of the key developmental genes, HO and IME1, the FLO11 promoter is large and complex, endowing it with the ability to integrate multiple inputs.

Positive regulation of adenylyl cyclase activity by a galphai homolog in Neurospora crassa

GNA-1 and GNA-2 are two G protein alpha subunits from the filamentous fungus Neurospora crassa. Loss of gna-1 leads to multiple phenotypes, while Deltagna-2 strains do not exhibit visible defects. However, Deltagna-1Deltagna-2 mutants are more affected in Deltagna-1 phenotypes. Here we report a biochemical investigation of the roles of GNA-1 and GNA-2 in cAMP metabolism. Assays of Mg2+ ATP-dependent adenylyl cyclase activity (+/-GppNHp) in extracts from submerged cultures indicated that Deltagna-2 strains were normal, whereas Deltagna-1 and Deltagna-1Deltagna-2 strains had only 10-15% the activity of the wild-type control. Levels of the Gbeta protein, GNB-1, were normal in Deltagna-1 strains, excluding altered GNB-1 production as a factor in loss of adenylyl cyclase activity. Steady-state cAMP levels in Deltagna-1 and Deltagna-1Deltagna-2 mutants were reduced relative to wild-type under conditions that result in morphological abnormalities (solid medium), while levels in submerged culture were normal. cAMP phosphodiesterase activities in submerged cultures of Deltagna-1 and/or Deltagna-2 strains were lower than in wild-type; the individual deletions were additive in decreasing activity. These results suggest that in submerged culture, N. crassa, like mammalian systems, possesses compensatory mechanisms that maintain cAMP at relatively constant levels. Furthermore, the finding that Mg2+ATP-dependent adenylyl cyclase activity in wild-type cell extracts could be inhibited using anti-GNA-1 IgG suggests that GNA-1 directly interacts with adenylyl cyclase in N. crassa.

Control of Saccharomyces cerevisiae filamentous growth by cyclin-dependent kinase Cdc28

The ascomycete Saccharomyces cerevisiae exhibits alternative vegetative growth states referred to as the yeast form and the filamentous form, and it switches between the two morphologies depending on specific environmental signals. To identify molecules involved in control of morphologic differentiation, this study characterized mutant S. cerevisiae strains that exhibit filamentous growth in the absence of the normal external signals. A specific amino acid substitution in the cyclin-dependent protein kinase Cdc28 was found to cause constitutive expression of most filamentous growth characteristics. These effects include specifically modified cell polarity characteristics in addition to the defined shape and division cycle alterations typical of the filamentous form. Several other mutations affecting Cdc28 function also had specific effects on filamentous growth. Constitutive filamentous growth resulting from deletion of the protein kinase Elm1 was prevented by modification of Cdc28 such that it could not be phosphorylated on tyrosine residue 19. In addition, various mutations affecting Hsl1 or Swe1, known or presumed components of a protein kinase cascade that mediates Cdc28 phosphorylation on Y19, either prevented or enhanced filamentous growth. The data suggest that a protein kinase cascade involving Elm1, Hsl1, and Swe1 can modulate Cdc28 activity and that Cdc28 in turn exerts global effects that cause filamentous growth.

Msn1p/Mss10p, Mss11p and Muc1p/Flo11p are part of a signal transduction pathway downstream of Mep2p regulating invasive growth and pseudohyphal differentiation in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, a network of signal transduction pathways governs the switch from yeast-type growth to pseudohyphal and invasive growth that occurs in response to nutrient limitation. Important elements of this network have been identified, including nutrient signal receptors, GTP-binding proteins, components of the pheromone-dependent MAP kinase cascade and several transcription factors. However, the structural and functional mapping of these pathways is far from complete. Here, we present data regarding three genes, MSN1/MSS10, MSS11 and MUC1/FLO11, which form an essential part of the signal transduction network establishing invasive growth. Both MSN1 and MSS11 are involved in the co-regulation of starch degradation and invasive growth. Msn1p and Mss11p act downstream of Mep2p and Ras2p and regulate the transcription of both STA2 and MUC1. We show that MUC1 mediates the effect of Msn1p and Mss11p on invasive growth. In addition, our results suggest that the activity of Msn1p is independent of the invasive growth MAP kinase cascade, but the Mss11p is required for the activation of pseudohyphal and invasive growth by Ste12p. We also show that starch metabolism in S. cerevisiae is subject to regulation by components of the MAP kinase cascade.

MAP kinase pathways in the yeast Saccharomyces cerevisiae

A cascade of three protein kinases known as a mitogen-activated protein kinase (MAPK) cascade is commonly found as part of the signaling pathways in eukaryotic cells. Almost two decades of genetic and biochemical experimentation plus the recently completed DNA sequence of the Saccharomyces cerevisiae genome have revealed just five functionally distinct MAPK cascades in this yeast. Sexual conjugation, cell growth, and adaptation to stress, for example, all require MAPK-mediated cellular responses. A primary function of these cascades appears to be the regulation of gene expression in response to extracellular signals or as part of specific developmental processes. In addition, the MAPK cascades often appear to regulate the cell cycle and vice versa. Despite the success of the gene hunter era in revealing these pathways, there are still many significant gaps in our knowledge of the molecular mechanisms for activation of these cascades and how the cascades regulate cell function. For example, comparison of different yeast signaling pathways reveals a surprising variety of different types of upstream signaling proteins that function to activate a MAPK cascade, yet how the upstream proteins actually activate the cascade remains unclear. We also know that the yeast MAPK pathways regulate each other and interact with other signaling pathways to produce a coordinated pattern of gene expression, but the molecular mechanisms of this cross talk are poorly understood. This review is therefore an attempt to present the current knowledge of MAPK pathways in yeast and some directions for future research in this area.

Sex and crime: heterotrimeric G proteins in fungal mating and pathogenesis

Heterotrimeric G proteins act as signal transducers that couple cell-surface receptors to cytoplasmic effector proteins. In fungi, G proteins play essential roles during sexual and pathogenic development. They are part of the pheromone signaling cascade in both ascomycetes and basidiomycetes, which is crucial for the recognition and fusion of cells of opposite mating type. In addition, G proteins affect a number of developmental and morphogenetic processes which determine the virulence of plant and human fungal pathogens. Cloning and targeted disruption of genes encoding alpha subunits of G proteins allowed the attribution of specific functions to these signal transducing molecules. Several lines of evidence indicate that many of the known fungal G proteins influence the intracellular level of cAMP by either stimulating or inhibiting adenylyl cyclase.

Efficient transition to growth on fermentable carbon sources in Saccharomyces cerevisiae requires signaling through the Ras pathway

Strains carrying ras2(318S) as their sole RAS gene fail to elicit a transient increase in cAMP levels following addition of glucose to starved cells but maintain normal steady-state levels of cAMP under a variety of growth conditions. Such strains show extended delays in resuming growth following transition from a quiescent state to glucose-containing growth media, either in emerging from stationary phase or following inoculation as spores onto fresh media. Otherwise, growth of such strains is indistinguishable from that of RAS2(+) strains. ras2(318S) strains also exhibit a delay in glucose-stimulated phosphorylation and turnover of fructose-1,6-bisphosphatase, a substrate of the cAMP-dependent protein kinase A (PKA) and a key component of the gluconeogenic branch of the glycolytic pathway. Finally Tpk(w) strains, which fail to modulate PKA in response to fluctuations in cAMP levels, show the same growth delay phenotypes, as do ras2(318S) strains. These observations indicate that the glucose-induced cAMP spike results in a transient activation of PKA, which is required for efficient transition of yeast cells from a quiescent state to resumption of rapid growth. This represents the first demonstration that yeast cells use the Ras pathway to transmit a signal to effect a biological change in response to an upstream stimulus.

Regulators of pseudohyphal differentiation in Saccharomyces cerevisiae identified through multicopy suppressor analysis in ammonium permease mutant strains

Nitrogen-starved diploid cells of the yeast Saccharomyces cerevisiae differentiate into a filamentous, pseudohyphal growth form. Recognition of nitrogen starvation is mediated, at least in part, by the ammonium permease Mep2p and the Galpha subunit Gpa2p. Genetic activation of the pheromone-responsive MAP kinase cascade, which is also required for filamentous growth, only weakly suppresses the filamentation defect of Deltamep2/Deltamep2 and Deltagpa2/Deltagpa2 strain. Surprisingly, deletion of Mep1p, an ammonium permease not previously thought to regulate differentiation, significantly enhances the potency of MAP kinase activation, such that the STE11-4 allele induces filamentation to near wild-type levels in Deltamep1/Deltamep1 Deltamep2/Deltamep2 and Deltamep1/Deltamep1 Deltagpa2/Deltagpa2 strains. To identify additional regulatory components, we isolated high-copy suppressors of the filamentation defect of the Deltamep1/Deltamep1 Deltamep2/Deltamep2 mutant. Multicopy expression of TEC1, PHD1, PHD2 (MSS10/MSN1/FUP4), MSN5, CDC6, MSS11, MGA1, SKN7, DOT6, HMS1, HMS2, or MEP2 each restored filamentation in a Deltamep1/Deltamep1 Deltamep2/Deltamep2 strain. Overexpression of SRK1 (SSD1), URE2, DAL80, MEP1, or MEP3 suppressed only the growth defect of the Deltamep1/Deltamep1 Deltamep2/Deltamep2 mutant strain. Characterization of these genes through deletion analysis and epistasis underscores the complexity of this developmental pathway and suggests that stress conditions other than nitrogen deprivation may also promote filamentous growth.

Gpr1p, a putative G-protein coupled receptor, regulates glucose-dependent cellular cAMP level in yeast Saccharomyces cerevisiae

How cells monitor the availability of nutrition and transduce signals is a fundamental, unanswered question. We have found that Gpr1p, a recently identified G-protein (Gpa2p) coupled receptor in yeast Saccharomyces cerevisiae, regulate the cellular cAMP level in response to glucose. The glucose-induced higher cAMP level found in the strain with GPA2 in multicopy plasmid decreased by deletion of GPR1 gene. A transient increase of cAMP in response to glucose was not observed in a Deltagpr1 mutant strain and this defect was complemented and restored by introducing GPR1 gene with YCp vector. Gpr1p was also required for the increase of cAMP in response to other fermentable sugars. Both membrane proximal regions o the third cytosolic loop in Gpr1p, which has been shown to be important for coupling to G-proteins, were also required for glucose-induced transient increase of cAMP. Our findings suggest that Gpr1p is part of the nutrition sensing machinery most likely acting as a receptor to monitor glucose as well as other fermentable sugars and regulate cellular cAMP levels.

A Candida albicans chaperonin subunit (CaCct8p) as a suppressor of morphogenesis and Ras phenotypes in C. albicans and Saccharomyces cerevisiae

Saccharomyces cerevisiae and the pathogen Candida albicans can be induced to undergo morphogenesis from a yeast to a filamentous form. A C. albicans gene (CaCCT8) was identified encoding a subunit of the Cct chaperonin complex, whose expression prevents filament formation in both fungi without interfering with growth of the yeast form. In S. cerevisiae, pseudohyphal growth induced by Ras2Val19, by overproduction of Phd1p or by expression of the C. albicans EFG1 gene, was blocked by CaCct8p and its N-terminally deleted derivative CaCct8-delta1p; in contrast, pseudohyphal induction by other components (Cph1p, Cdc42p) could not be suppressed, indicating that morphogenesis per se is not inhibited. CaCCT8 expression also interfered with other Ras2pVal19 phenotypes, including heat sensitivity, lack of glycogen accumulation and lack of sporulation. In C. albicans, overproduction of CaCct8p effectively blocked hyphal morphogenesis induced by starvation conditions and by serum. The results suggest that the activity of a component in the Ras2p signal transduction pathway is suppressed by excess chaperonin subunits. This component may be a novel folding target for the Cct complex. In agreement with this hypothesis, disruption of one of the two CaCCT8 alleles in C. albicans led to defective hyphal morphogenesis.

Signal transduction pathways regulating differentiation and pathogenicity of Cryptococcus neoformans

The basidiomycetous yeast Cryptococcus neoformans is a human pathogen. Several phenotypes of this organism are defined as virulence traits including the polysaccharide capsule, melanin, and the ability to grow at 37 degreesC. The signaling pathways regulating the expression of these phenotypes and other important cellular processes are being defined on a molecular level. For example, the highly conserved signaling molecule calcineurin regulates high temperature growth in C. neoformans. A cryptococcal homolog of Saccharomyces cerevisiae STE12, the gene for a transcriptional regulator activated by the MAP kinase cascade, has also been identified. Additionally, the C. neoformans Galpha protein GPA1 and cAMP regulate mating, melanin production, encapsulation, and pathogenicity. This fungus is an excellent model to further dissect virulence-associated signaling pathways. The conserved role of Galpha proteins and cAMP-associated signaling pathways in fungal differentiation and pathogenicity is also reviewed.

The control of filamentous differentiation and virulence in fungi

Many members of the fungal kingdom have a distinguishing feature, dimorphism, which is the ability to switch between two morphological forms: a cellular yeast form and a multicellular invasive filamentous form. At least three pathways are involved in regulating the transition between these two forms in the budding yeast Saccharomyces cerevisiae, and evidence is now emerging that homologous signalling modules are involved in regulating filament formation and virulence in a range of human and plant fungal pathogens. Strikingly, components used to signal sexual differentiation in the response to mating pheromones are often reutilized to regulate dimorphic development, suggesting an ancient link between these processes.

Divergent cAMP signaling pathways regulate growth and pathogenesis in the rice blast fungus Magnaporthe grisea

cAMP is involved in signaling appressorium formation in the rice blast fungus Magnaporthe grisea. However, null mutations in a protein kinase A (PKA) catalytic subunit gene, CPKA, do not block appressorium formation, and mutations in the adenylate cyclase gene have pleiotropic effects on growth, conidiation, sexual development, and appressorium formation. Thus, cAMP signaling plays roles in both growth and morphogenesis as well as in appressorium formation. To clarify cAMP signaling in M. grisea, we have identified strains in which a null mutation in the adenylate cyclase gene (MAC1) has an unstable phenotype such that the bypass suppressors of the Mac1(-) phenotype (sum) could be identified. sum mutations completely restore growth and sexual and asexual morphogenesis and lead to an ability to form appressoria under conditions inhibitory to the wild type. PKA assays and molecular cloning showed that one suppressor mutation (sum1-99) alters a conserved amino acid in cAMP binding domain A of the regulatory subunit gene of PKA (SUM1), whereas other suppressor mutations act independently of PKA activity. PKA assays demonstrated that the catalytic subunit gene, CPKA, encodes the only detectable PKA activity in M. grisea. Because CPKA is dispensable for growth, morphogenesis, and appressorium formation, divergent catalytic subunit genes must play roles in these processes. These results suggest a model in which both saprophytic and pathogenic growth of M. grisea is regulated by adenylate cyclase but different effectors of cAMP mediate downstream effects specific for either cell morphogenesis or pathogenesis.

Gene disruption to evaluate the role of fungal candidate virulence genes

Gene disruption is a powerful genetic tool that can define pathogenic or virulence factors. In the past two years gene disruption approaches have been used to identify fungal virulence genes. The capsule genes, an alpha subunit of G protein and certain kinases of Cryptococcus neoformans have clearly been demonstrated to be associated with pathogenicity. In Candida albicans at least four genes involved in hyphal formation have been disrupted and tested for virulence. In other fungi, such as Histoplasma capsulatum, however, more efficient gene disruption methods need to be developed before such approaches can be regularly used for identifying virulence genes.

Involvement of distinct G-proteins, Gpa2 and Ras, in glucose- and intracellular acidification-induced cAMP signalling in the yeast Saccharomyces cerevisiae

Adenylate cyclase activity in Saccharomyces cerevisiae is dependent on Ras proteins. Both addition of glucose to glucose-deprived (derepressed) cells and intracellular acidification trigger an increase in the cAMP level in vivo. We show that intracellular acidification, but not glucose, causes an increase in the GTP/GDP ratio on the Ras proteins independent of Cdc25 and Sdc25. Deletion of the GTPase-activating proteins Ira1 and Ira2, or expression of the RAS2(val19) allele, causes an enhanced GTP/GDP basal ratio and abolishes the intracellular acidification-induced increase. In the ira1Delta ira2Delta strain, intracellular acidification still triggers a cAMP increase. Glucose also did not cause an increase in the GTP/GDP ratio in a strain with reduced feedback inhibition of cAMP synthesis. Further investigation indicated that feedback inhibition by cAPK on cAMP synthesis acts independently of changes in the GTP/GDP ratio on Ras. Stimulation by glucose was dependent on the Galpha-protein Gpa2, whose deletion confers the typical phenotype associated with a reduced cAMP level: higher heat resistance, a higher level of trehalose and glycogen and elevated expression of STRE-controlled genes. However, the typical fluctuation in these characteristics during diauxic growth on glucose was still present. Overexpression of Ras2(val19) inhibited both the acidification- and glucose-induced cAMP increase even in a protein kinase A-attenuated strain. Our results suggest that intracellular acidification stimulates cAMP synthesis in vivo at least through activation of the Ras proteins, while glucose acts through the Gpa2 protein. Interaction of Ras2(val19) with adenylate cyclase apparently prevents its activation by both agonists.

The mating factor response pathway regulates transcription of TEC1, a gene involved in pseudohyphal differentiation of Saccharomyces cerevisiae

The transcription factor Tec1 is involved in pseudohyphal differentiation and agar-invasive growth of Saccharomyces cerevisiae cells. The sole element in the TEC1 promoter that has thus far been shown to control Tec1 function is the filament response element. We find that the TEC1 promoter also contains several pheromone response element sequences which are likely to be functional: TEC1 transcription is induced by mating factor, cell cycle regulated and dependent on the Ste4, Ste18 and Ste5 components of the mating factor signal transduction pathway. Using alleles of the transcription factor Ste12 that are defective in DNA binding, transcriptional induction or cooperativity with other transcription factors, we find little correlation between TEC1 transcript levels and agar-invasive growth.

Sequence elements that contribute to the degradation of yeast G alpha

BACKGROUND

Gpa1 is the alpha subunit of the yeast G-protein that regulates signal transduction during mating. The stability of Galpha/Gpa1 is influenced by the ubiquitin-dependent N-end rule pathway, suggesting that the regulation of G alpha levels may be important for effective mating response and recovery.

RESULTS

The G alpha sequences that confer sensitivity to degradation by the N-end rule pathway were identified. The insertion of this degradation signal (G1-Deg) into the ordinarily stable Gpa2 protein conferred proteolytic targeting. We examined G alpha degradation under different conditions and found that it was efficiently degraded in haploid and diploid cells, but was stable if it was synthesized prior to expression of the N-end rule pathway. Interestingly, a specific mutation in G alpha that is believed to promote the GTP-bound form (N388K) caused accelerated degradation.

CONCLUSION

A region encompassing a putative effector-binding domain (G1-Deg) is required for G alpha degradation via the N-end rule pathway. Our studies have shown that G alpha is susceptible to proteolysis soon after synthesis. These results are in agreement with the idea that G alpha is more unstable in the GTP-bound form, which is the predominant state of monomeric/free G alpha soon after synthesis. It is likely that the signal transduced by Gbetagamma can be regulated by adjusting the levels of G alpha through proteolysis.

Ash1, a daughter cell-specific protein, is required for pseudohyphal growth of Saccharomyces cerevisiae

Ash1 (for asymmetric synthesis of HO) was first uncovered in genetic screens that revealed its role in mating-type switching. Ash1 prevents HO expression in daughter cells. Because Ash1 has a zinc finger-like domain related to that of the GATA family of transcription factors, it presumably acts by repressing HO transcription. Nonswitching diploid cells also express Ash1, suggesting it could have functions in addition to regulation of HO expression. We show here that Ash1 has an essential function for pseudohyphal growth. Our epistasis analyses are consistent with the deduction that Ash1 acts separately from the mitogen-activated protein kinase cascade and Ste12. Similarly to the case in yeast form cells, Ash1 is asymmetrically localized to the nuclei of daughter cells during pseudohyphal growth. This asymmetric localization reveals that there is a previously unsuspected daughter cell-specific function necessary for pseudohyphal growth.

GPR1 encodes a putative G protein-coupled receptor that associates with the Gpa2p Galpha subunit and functions in a Ras-independent pathway

The yeast RAS1 and RAS2 genes appear to be involved in control of cell growth in response to nutrients. Here we show that this growth control also involves a signal mediated by the heterotrimeric G protein alpha subunit homolog encoded by GPA2. A GPA2 null allele conferred a severe growth defect on cells containing a null allele of RAS2, although either mutation alone had little effect on growth rate. A constitutive allele of GPA2 could stimulate growth of a strain lacking both RAS genes. Constitutive GPA2 conferred heat shock sensitivity on both wild-type cells and cells lacking RAS function, but had no effect in a strain containing a null allele of SCH9, which encodes a kinase related to protein kinase A. The GPR1 gene was isolated and was found to encode a protein with the characteristics of a G protein-coupled receptor. Double Deltagpr1 Deltaras2 mutants displayed a severe growth defect that was suppressed by expression of the constitutive allele of GPA2, confirming that GPR1 acts upstream of GPA2. Gpr1p is expressed on the cell surface and requires sequences in the membrane-proximal region of its third cytoplasmic loop for function, as expected for a G protein-coupled receptor. GPR1 RNA was induced when cells were starved for nitrogen and amino acids. These results are consistent with a model in which the GPR1/GPA2 pathway activates the Sch9p kinase to generate a response that acts in parallel with that generated by the Ras/cAMP pathway, resulting in the integration of nutrient signals.

The MEP2 ammonium permease regulates pseudohyphal differentiation in Saccharomyces cerevisiae

In response to nitrogen starvation, diploid cells of the budding yeast Saccharomyces cerevisiae differentiate into a filamentous, pseudohyphal growth form. This dimorphic transition is regulated by the Galpha protein GPA2, by RAS2, and by elements of the pheromone-responsive MAP kinase cascade, yet the mechanisms by which nitrogen starvation is sensed remain unclear. We have found that MEP2, a high affinity ammonium permease, is required for pseudohyphal differentiation in response to ammonium limitation. In contrast, MEP1 and MEP3, which are lower affinity ammonium permeases, are not required for filamentous growth. Deltamep2 mutant strains had no defects in growth rates or ammonium uptake, even at limiting ammonium concentrations. The pseudohyphal defect of Deltamep2/Deltamep2 strains was suppressed by dominant active GPA2 or RAS2 mutations and by addition of exogenous cAMP, but was not suppressed by activated alleles of the MAP kinase pathway. Analysis of MEP1/MEP2 hybrid proteins identified a small intracellular loop of MEP2 involved in the pseudohyphal regulatory function. In addition, mutations in GLN3, URE2 and NPR1, which abrogate MEP2 expression or stability, also conferred pseudohyphal growth defects. We propose that MEP2 is an ammonium sensor, generating a signal to regulate filamentous growth in response to ammonium starvation.

Cryptococcus neoformans mating and virulence are regulated by the G-protein alpha subunit GPA1 and cAMP

This study explores signal transduction pathways that function during mating and infection in the opportunistic, human fungal pathogen Cryptococcus neoformans. The gene encoding a G-protein alpha subunit homolog, GPA1, was disrupted by homologous recombination. The gpa1 mutant strain was viable but exhibited a defect in mating in response to nitrogen starvation. Additionally, the gpa1 mutant strain failed to induce two well-established virulence factors-melanin synthesis, in response to glucose starvation; and capsule production, in response to iron limitation. As a consequence, virulence of the gpa1 mutant strain was significantly attenuated in an animal model of cryptococcal meningitis. Reintroduction of the wild-type GPA1 gene complemented the gpa1 mutant phenotypes and restored mating, melanin and capsule production, and virulence. Similarly, exogenous cAMP also suppressed the gpa1 mutant phenotypes, restoring mating and production of melanin and capsule. These observations support a model in which GPA1 has a role in sensing diverse environmental signals required for mating and virulence by regulating cAMP metabolism in C. neoformans.