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

Galpha subunit Gpa2 recruits kelch repeat subunits that inhibit receptor-G protein coupling during cAMP-induced dimorphic transitions in Saccharomyces cerevisiae

All eukaryotic cells sense extracellular stimuli and activate intracellular signaling cascades via G protein-coupled receptors (GPCR) and associated heterotrimeric G proteins. The Saccharomyces cerevisiae GPCR Gpr1 and associated Galpha subunit Gpa2 sense extracellular carbon sources (including glucose) to govern filamentous growth. In contrast to conventional Galpha subunits, Gpa2 forms an atypical G protein complex with the kelch repeat Gbeta mimic proteins Gpb1 and Gpb2. Gpb1/2 negatively regulate cAMP signaling by inhibiting Gpa2 and an as yet unidentified target. Here we show that Gpa2 requires lipid modifications of its N-terminus for membrane localization but association with the Gpr1 receptor or Gpb1/2 subunits is dispensable for membrane targeting. Instead, Gpa2 promotes membrane localization of its associated Gbeta mimic subunit Gpb2. We also show that the Gpa2 N-terminus binds both to Gpb2 and to the C-terminal tail of the Gpr1 receptor and that Gpb1/2 binding interferes with Gpr1 receptor coupling to Gpa2. Our studies invoke novel mechanisms involving GPCR-G protein modules that may be conserved in multicellular eukaryotes.

Gpr1, a putative G-protein-coupled receptor, regulates morphogenesis and hypha formation in the pathogenic fungus Candida albicans

In response to various extracellular signals, the morphology of the human fungal pathogen Candida albicans switches from yeast to hypha form. Here, we report that GPR1 encoding a putative G-protein-coupled receptor and GPA2 encoding a Galpha subunit are required for hypha formation and morphogenesis in C. albicans. Mutants lacking Gpr1 (gpr1/gpr1) or Gpa2 (gpa2/gpa2) are defective in hypha formation and morphogenesis on solid hypha-inducing media. These phenotypic defects in solid cultures are suppressed by exogenously added dibutyryl-cyclic AMP (dibutyryl-cAMP). Biochemical studies also reveal that GPR1 and GPA2 are required for a glucose-dependent increase in cellular cAMP. An epistasis analysis indicates that Gpr1 functions upstream of Gpa2 in the same signaling pathway, and a two-hybrid assay reveals that the carboxyl-terminal tail of Gpr1 interacts with Gpa2. Moreover, expression levels of HWP1 and ECE1, which are cAMP-dependent hypha-specific genes, are reduced in both mutant strains. These findings support a model that Gpr1, as well as Gpa2, regulates hypha formation and morphogenesis in a cAMP-dependent manner. In contrast, GPR1 and GPA2 are not required for hypha formation in liquid fetal bovine serum (FBS) medium. Furthermore, the gpr1 and the gpa2 mutant strains are fully virulent in a mouse infection. These findings suggest that Gpr1 and Gpa2 are involved in the glucose-sensing machinery that regulates morphogenesis and hypha formation in solid media via a cAMP-dependent mechanism, but they are not required for hypha formation in liquid medium or during invasive candidiasis.

The heterotrimeric G-protein GanB(alpha)-SfaD(beta)-GpgA(gamma) is a carbon source sensor involved in early cAMP-dependent germination in Aspergillus nidulans

The role of heterotrimeric G-proteins in cAMP-dependent germination of conidia was investigated in the filamentous ascomycete Aspergillus nidulans. We demonstrate that the G alpha-subunit GanB mediates a rapid and transient activation of cAMP synthesis in response to glucose during the early period of germination. Moreover, deletion of individual G-protein subunits resulted in defective trehalose mobilization and altered germination kinetics, indicating that GanB(alpha)-SfaD(beta)-GpgA(gamma) constitutes a functional heterotrimer and controls cAMP/PKA signaling in response to glucose as well as conidial germination. Further genetic analyses suggest that GanB plays a primary role in cAMP/PKA signaling, whereas the SfaD-GpgA (G betagamma) heterodimer is crucial for proper activation of GanB signaling sensitized by glucose. In addition, the RGS protein RgsA is also involved in regulation of the cAMP/PKA pathway and germination via attenuation of GanB signaling. Genetic epistatic analyses led us to conclude that all controls exerted by GanB(alpha)-SfaD(beta)-GpgA(gamma) on conidial germination are mediated through the cAMP/PKA pathway. Furthermore, GanB may function in sensing various carbon sources and subsequent activation of downstream signaling for germination.

Tor and cyclic AMP-protein kinase A: two parallel pathways regulating expression of genes required for cell growth

In the budding yeast Saccharomyces cerevisiae, the Tor and cyclic AMP-protein kinase A (cAMP-PKA) signaling cascades respond to nutrients and regulate coordinately the expression of genes required for cell growth, including ribosomal protein (RP) and stress-responsive (STRE) genes. The inhibition of Tor signaling by rapamycin results in repression of the RP genes and induction of the STRE genes. Mutations that hyperactivate PKA signaling confer resistance to rapamycin and suppress the repression of RP genes imposed by rapamycin. By contrast, partial inactivation of PKA confers rapamycin hypersensitivity but only modestly affects RP gene expression. Complete inactivation of PKA impairs RP gene expression and concomitantly enhances STRE gene expression; remarkably, this altered transcriptional pattern is still sensitive to rapamycin and thus subject to Tor control. These findings illustrate how the Tor and cAMP-PKA signaling pathways respond to nutrient signals to govern gene expression required for cell growth via two parallel routes, and they have broad implication for our understanding of analogous regulatory networks in normal and neoplastic mammalian cells.

The molecular chaperone Sse1 and the growth control protein kinase Sch9 collaborate to regulate protein kinase A activity in Saccharomyces cerevisiae

The Sch9 protein kinase regulates Hsp90-dependent signal transduction activity in the budding yeast Saccharomyces cerevisiae. Hsp90 functions in concert with a number of cochaperones, including the Hsp110 homolog Sse1. In this report, we demonstrate a novel synthetic genetic interaction between SSE1 and SCH9. This interaction was observed specifically during growth at elevated temperature and was suppressed by decreased signaling through the protein kinase A (PKA) signal transduction pathway. Correspondingly, sse1Delta sch9Delta cells were shown by both genetic and biochemical approaches to have abnormally high levels of PKA activity and were less sensitive to modulation of PKA by glucose availability. Growth defects of an sse1Delta mutant were corrected by reducing PKA signaling through overexpression of negative regulators or growth on nonoptimal carbon sources. Hyperactivation of the PKA pathway through expression of a constitutive RAS2 allele likewise resulted in temperature-sensitive growth, suggesting that modulation of PKA activity during thermal stress is required for adaptation and viability. Together these results demonstrate that the Sse1 chaperone and the growth control kinase Sch9 independently contribute to regulation of PKA signaling.

Genome-wide survey of non-essential genes required for slowed DNA synthesis-induced filamentous growth in yeast

We recently discovered that slowed DNA synthesis induces filamentous differentiation in S. cerevisiae. We screened the BY yeast deletion strains and identified four classes of non-essential genes that are required for both slowed DNA-induced filamentous growth and classic forms of filamentous growth: (a) genes encoding regulators of the actin cytoskeleton and cell polarity, ABP1, CAP2 and HUF1 (=YOR300W), in addition to the previously known BNI1, BUD2, PEA2, SPA2 and TPM1; (b) genes that are likely involved in cell wall biosynthesis, ECM25, GAS1 and PRS3; (c) genes encoding possible regulators of protein secretion, SEC66, RPL21A and RPL34B; (d) genes encoding factors for normal mitochondrial function, IML1 and UGO1. These results showed that pseudohyphal formation involves not the only previously known regulation of the actin cytoskeleton/cell polarity but also regulation of cell wall synthesis, protein secretion and mitochondrial function. Identification of multiple classes of genes that are required for both slowed DNA synthesis-induced and classic forms of filamentous growth confirms that slowed DNA synthesis-induced filamentous growth is bone fide filamentous differentiation.

Glucose-dependent cell size is regulated by a G protein-coupled receptor system in yeastSaccharomyces cerevisiae

In the yeast, Saccharomyces cerevisiae, cell size is affected by the kind of carbon source in the medium. Here, we present evidence that the Gpr1 receptor and Gpa2 Galpha subunit are required for both maintenance and modulation of cell size in response to glucose. In the presence of glucose, mutants lacking GPR1 or GPA2 gene showed smaller cells than the wild-type strain. Physiological studies revealed that protein synthesis rate was reduced in the mutant strains indicating that reduced growth rate, while the level of mRNAs for CLN1, 2 and 3 was not affected in all strains. Gene chip analysis also revealed a down-regulation in the expression of genes related to biosynthesis of not only protein but also other cellular component in the mutant strains. We also show that GPR1 and GPA2 are required for a rapid increase in cell size in response to glucose. Wild-type cells grown in ethanol quickly increased in size by addition of glucose, while little change was observed in the mutant strains, in which glucose-dependent cell cycle arrest caused by CLN1 repression was somewhat alleviated. Our study indicates that the yeast G-protein coupled receptor system consisting of Gpr1 and Gpa2 regulates cell size by affecting both growth rate and cell division.

The G protein-coupled receptor Gpr1 and the Galpha protein Gpa2 act through the cAMP-protein kinase A pathway to induce morphogenesis in Candida albicans

We investigated the role in cell morphogenesis and pathogenicity of the Candida albicans GPR1 gene, encoding the G protein-coupled receptor Gpr1. Deletion of C. albicans GPR1 has only minor effects in liquid hypha-inducing media but results in strong defects in the yeast-to-hypha transition on solid hypha-inducing media. Addition of cAMP, expression of a constitutively active allele of the Galpha protein Gpa2 or of the catalytic protein kinase A subunit TPK1 restores the wild-type phenotype of the CaGPR1-deleted strain. Overexpression of HST7, encoding a component of the mitogen-activated protein kinase pathway, does not suppress the defect in filamentation. These results indicate that CaGpr1 functions upstream in the cAMP-protein kinase A (PKA) pathway. We also show that, in the presence of glucose, CaGpr1 is important for amino acid-induced transition from yeast to hyphal cells. Finally, as opposed to previous reports, we show that CaGpa2 acts downstream of CaGpr1 as activator of the cAMP-PKA pathway but that deletion of neither CaGpr1 nor CaGpa2 affects glucose-induced cAMP signaling. In contrast, the latter is abolished in strains lacking CaCdc25 or CaRas1, suggesting that the CaCdc25-CaRas1 rather than the CaGpr1-CaGpa2 module mediates glucose-induced cAMP signaling in C. albicans.

Glucose and sucrose act as agonist and mannose as antagonist ligands of the G protein-coupled receptor Gpr1 in the yeast Saccharomyces cerevisiae

Several examples of G protein-coupled receptors have recently been suggested to respond to common sugars in millimolar concentrations. This low affinity has made it difficult to demonstrate direct receptor-ligand interaction. In the yeast Saccharomyces cerevisiae, rapid activation of the cAMP pathway by glucose and sucrose requires the GPCR Gpr1. Our results obtained by cysteine scanning mutagenesis and SCAM (substituted cysteine accessibility method) of residues in TMD VI provide strong evidence that glucose and sucrose directly interact as ligands with Gpr1. The affinity for sucrose is much higher. Structurally similar sugars such as galactose, mannose, and fructose do not act as agonists, but mannose acts as an antagonist for both sucrose and glucose. These results support the idea that Gpr1 directly senses sugars and that sugars can effectively bind GPCRs with a low affinity in a binding pocket formed by the transmembrane domains. The ligand repertoire of GPCRs can thus be extended to common sugars in millimolar concentrations.

TOR controls transcriptional and translational programs via Sap-Sit4 protein phosphatase signaling effectors

The Tor kinases are the targets of the immunosuppressive drug rapamycin and couple nutrient availability to cell growth. In the budding yeast Saccharomyces cerevisiae, the PP2A-related phosphatase Sit4 together with its regulatory subunit Tap42 mediates several Tor signaling events. Sit4 interacts with other potential regulatory proteins known as the Saps. Deletion of the SAP or SIT4 genes confers increased sensitivity to rapamycin and defects in expression of subsets of Tor-regulated genes. Sap155, Sap185, or Sap190 can restore these responses. Strains lacking Sap185 and Sap190 are hypersensitive to rapamycin, and this sensitivity is Gcn2 dependent and correlated with a defect in translation, constitutive eukaryotic initiation factor 2alpha hyperphosphorylation, induction of GCN4 translation, and hypersensitivity to amino acid starvation. We conclude that Tor signals via Sap-Sit4 complexes to control both transcriptional and translational programs that couple cell growth to amino acid availability.

Mss11p is a central element of the regulatory network that controls FLO11 expression and invasive growth in Saccharomyces cerevisiae

The invasive and filamentous growth forms of Saccharomyces cerevisiae are adaptations to specific environmental conditions, under particular conditions of limited nutrient availability. Both growth forms are dependent on the expression of the FLO11 gene, which encodes a cell-wall-associated glycoprotein involved in cellular adhesion. A complex regulatory network consisting of signaling pathways and transcription factors has been associated with the regulation of FLO11. Mss11p has been identified as a transcriptional activator of this gene, and here we present an extensive genetic analysis to identify functional relationships between Mss11p and other FLO11 regulators. The data show that Mss11p is absolutely required for the activation of FLO11 by most proteins that have previously been shown to affect FLO11 expression, including the signaling proteins Ras2p, Kss1p, and Tpk2p, the activators Tec1p, Flo8p, and Phd1p, and the repressors Nrg1p, Nrg2p, Sok2p, and Sfl1p. The genetic evidence furthermore suggests that Mss11p activity is not dependent on the presence of any of the above-mentioned factors and that the protein also regulates other genes involved in cellular adhesion phenotypes. Taken together, the data strongly suggest a central role for Mss11p in the regulatory network controlling FLO11 expression, invasive growth, and pseudohyphal differentiation.

Increased high-affinity phosphodiesterase PDE2 gene expression in germ tubes counteracts CAP1-dependent synthesis of cyclic AMP, limits hypha production and promotes virulence of Candida albicans

Frequent interconversion between yeasts, pseudohyphae and true hyphae is a hallmark of Candida albicans growth in mammalian tissues. The requirement for transient CAP1-dependent pulses of cAMP for generating true hyphae, Hwp1 and virulence raises questions about the role of yeast and pseudohyphal forms in the pathogenesis of candidiasis. In this study, hyperfilamentous mutants, limited in their capacity to produce buds, were generated by disrupting the high-affinity phosphodiesterase gene PDE2. Degradation of cAMP by the PDE2 gene product was confirmed by higher basal cAMP levels in the pde2/pde2 mutant and by accumulation of cAMP to levels permitting germ tube formation upon disrupting PDE2 in the cap1/cap1 mutant. Similar phenotypes of the C. albicans and Saccharomyces cerevisiae pde2/pde2 mutants were found, including sensitivity to nutritional starvation and exogenous cAMP and defective entry into stationary phase. Importantly, the hyperfilamentous mutants were as avirulent as hypofilamentous mutants in a systemic model of candidiasis. Growth in a multiplicity of forms appears to be a virulence attribute that is controlled by tight coupling of cAMP synthesis and degradation. Delayed increases in PDE2 mRNA in cAMP-deficient cap1/cap1 mutants during germ tube-inducing conditions suggested a mechanism of control involving cAMP-dependent induction of PDE2 mRNA.

Lipid-induced filamentous growth in Ustilago maydis

The phytopathogenic fungus Ustilago maydis is obligately dependent on infection of maize to complete the sexual phase of its life cycle. Mating interactions between haploid, budding cells establish an infectious filamentous cell type that invades the host, induces large tumours and eventually forms large masses of black spores. The ability to switch from budding to filamentous growth is therefore critical for infection and completion of the life cycle, although the signals that influence the transition have not been identified from the host or the environment. We have found that growth in the presence of lipids promotes a filamentous phenotype that resembles the infectious cell type found in planta. In addition, the ability of the fungus to respond to lipids is dependent on both the cAMP signalling pathway and a Ras/MAPK pathway; these pathways are known to regulate mating, filamentous growth and pathogenesis in U. maydis. Overall, these results lead us to hypothesize that lipids may represent one of the signals that promote and maintain the filamentous growth of the fungus in the host environment.

Structure, function and evolution of microbial adenylyl and guanylyl cyclases

Cells respond to signals of both environmental and biological origin. Responses are often receptor mediated and result in the synthesis of so-called second messengers that then provide a link between extracellular signals and downstream events, including changes in gene expression. Cyclic nucleotides (cAMP and cGMP) are among the most widely studied of this class of molecule. Research on their function and mode of action has been a paradigm for signal transduction systems and has shaped our understanding of this important area of biology. Cyclic nucleotides have diverse regulatory roles in both unicellular and multicellular organisms, highlighting the utility and success of this system of molecular communication. This review will examine the structural diversity of microbial adenylyl and guanylyl cyclases, the enzymes that synthesize cAMP and cGMP respectively. We will address the relationship of structure to biological function and speculate on the complex origin of these crucial regulatory molecules. A review is timely because the explosion of data from the various genome projects is providing new and exciting insights into protein function and evolution.

Ras and Gpa2 mediate one branch of a redundant glucose signaling pathway in yeast

Addition of glucose to starved yeast cells elicits a dramatic restructuring of the transcriptional and metabolic state of the cell. While many components of the signaling network responsible for this response have been identified, a comprehensive view of this network is lacking. We have used global analysis of gene expression to assess the roles of the small GTP-binding proteins, Ras2 and Gpa2, in mediating the transcriptional response to glucose. We find that 90% of the transcriptional changes in the cell attendant on glucose addition are recapitulated by activation of Ras2 or Gpa2. In addition, we find that protein kinase A (PKA) mediates all of the Ras2 and Gpa2 transcriptional effects. However, we also find that most of the transcriptional effects of glucose addition to wild-type cells are retained in strains containing a PKA unresponsive to changes in cAMP levels. Thus, most glucose-responsive genes are regulated redundantly by a Ras/PKA-dependent pathway and by one or more PKA-independent pathways. Computational analysis extracted RRPE/PAC as the major response element for Ras and glucose regulation and revealed additional response elements mediating glucose and Ras regulation. These studies provide a paradigm for extracting the topology of signal transduction pathways from expression data.

Cyclic AMP-dependent protein kinase regulates the subcellular localization of Snf1-Sip1 protein kinase

The Snf1/AMP-activated protein kinase family has diverse roles in cellular responses to metabolic stress. In Saccharomyces cerevisiae, Snf1 protein kinase has three isoforms of the beta subunit that confer versatility on the kinase and that exhibit distinct patterns of subcellular localization. The Sip1 beta subunit resides in the cytosol in glucose-grown cells and relocalizes to the vacuolar membrane in response to carbon stress. We show that translation of Sip1 initiates at the second ATG of the open reading frame, yielding a potential site for N myristoylation, and that mutation of the critical glycine abolishes relocalization. We further show that the cyclic AMP-dependent protein kinase (protein kinase A [PKA]) pathway maintains the cytoplasmic localization of Sip1 in glucose-grown cells. The Snf1 catalytic subunit also exhibits aberrant localization to the vacuolar membrane in PKA-deficient cells, indicating that PKA regulates the localization of Snf1-Sip1 protein kinase. These findings establish a novel mechanism of regulation of Snf1 protein kinase by the PKA pathway.

Cyclic AMP signaling in Cryptococcus neoformans

As pathogenic microorganisms establish an infection, they must be able to sense host-specific signals and respond by elaborating determinants that allow for survival in these hostile conditions. Pathogen cell surface proteins detect these signals and activate signal transduction cascades that ultimately alter gene expression resulting in an adaptive cellular response. Here we review the mechanisms by which a pathogenic fungus uses the highly conserved cAMP signal transduction pathway to regulate cellular differentiation as well as its virulence potential.

Cyclic AMP-dependent protein kinase catalytic subunits have divergent roles in virulence factor production in two varieties of the fungal pathogen Cryptococcus neoformans

Our earlier findings established that cyclic AMP-dependent protein kinase functions in a signaling cascade that regulates mating and virulence of Cryptococcus neoformans var. grubii (serotype A). Mutants lacking the serotype A protein kinase A (PKA) catalytic subunit Pka1 are unable to mate, fail to produce melanin or capsule, and are avirulent in animal models, whereas mutants lacking the PKA regulatory subunit Pkr1 overproduce capsule and are hypervirulent. Because other mutations have been observed to confer different phenotypes in two diverged varieties of C. neoformans (grubii variety [serotype A] and neoformans variety [serotype D]), we analyzed the functions of the PKA genes in the serotype D neoformans variety. Surprisingly, the Pka1 catalytic subunit was not required for mating, haploid fruiting, or melanin or capsule production of serotype D strains. Here we identify a second PKA catalytic subunit gene, PKA2, that is present in both serotype A and D strains of C. neoformans. The divergent Pka2 catalytic subunit was found to regulate mating, haploid fruiting, and virulence factor production in serotype D strains. In contrast, Pka2 has no role in mating, melanin production, or capsule formation in serotype A strains. Our studies illustrate how different components of signaling pathways can be co-opted and functionally specialized during the evolution of related but distinct varieties or subspecies of a human fungal pathogen.

TOR regulates the subcellular localization of Ime1, a transcriptional activator of meiotic development in budding yeast

The transcriptional activator Ime1 is a key regulator of meiosis and sporulation in budding yeast. Ime1 is controlled at different levels by nutrients and cell-type signals. Previously, we have proposed that G(1) cyclins would transmit nutritional signals to the Ime1 pathway by preventing the accumulation of Ime1 within the nucleus. We show here that nutritional signals regulate the subcellular localization of Ime1 through the TOR pathway. The inactivation of TOR with rapamycin promotes the nuclear accumulation and stabilization of Ime1, with consequent induction of early meiotic genes. On the contrary, the activation of TOR by glutamine induces the relocalization of Ime1 to the cytoplasm. Thus, TOR may sense optimal nitrogen- and carbon-limiting conditions to modulate Ime1 function. Besides TOR, ammonia induces an independent mechanism that prevents the accumulation of Ime1 in the nucleus. Both TOR and ammonia regulate Ime1 localization in the absence of Cdk1 activity and therefore use mechanisms different from those exerted by G(1) cyclins. Integration of independent mechanisms into a single early controlling step, such as the nuclear accumulation of Ime1, may help explain why yeast cells execute the meiotic program only when the appropriate internal and external conditions are met together.

Induction of S. cerevisiae filamentous differentiation by slowed DNA synthesis involves Mec1, Rad53 and Swe1 checkpoint proteins

A key question in eukaryotic differentiation is whether there are common regulators or biochemical events that are required for diverse types of differentiation or whether there is a core mechanism for differentiation. The unicellular model organism Saccharomyces cerevisiae undergoes filamentous differentiation in response to environmental cues. Because conserved cell cycle regulators, the mitotic cyclin-dependent kinase Clb2/Cdc28, and its inhibitor Swe1 were found to be involved in both nitrogen starvation- and short chain alcohol-induced filamentous differentiation, they were identified as components of the core mechanism for filamentous differentiation. We report here that slowed DNA synthesis also induces yeast filamentous differentiation through conserved checkpoint proteins Mec1 and Rad53. Swe1 and Clb2 are also involved in this form of differentiation, and the core status of Swe1/Clb2/Cdc28 in the mechanism of filamentous differentiation has therefore been confirmed. Because the cAMP and filamentous growth mitogen-activated protein kinase pathways that mediate nitrogen starvation-induced filamentous differentiation are not required for slowed DNA synthesis-induced filamentous growth, they can therefore be excluded from the core mechanism. More significantly, slowed DNA synthesis also induces differentiation in mammalian cancer cells, and such stimulus conservation may indicate that the core mechanism for yeast filamentous differentiation is conserved in mammalian differentiation.

Roles played by Ras subfamily proteins in the cell and developmental biology of microorganisms

The Ras subfamily proteins are monomeric GTPases that function as molecular switches in cellular signal transduction pathways. This review describes our current knowledge of the roles that these proteins play in the growth and differentiation of single celled microorganisms.

Mos10 (Vps60) is required for normal filament maturation in Saccharomyces cerevisiae

Early pseudohyphal growth of Saccharomyces cerevisiae is well described, and is known to be subject to a complex web of developmental regulation. In maturing filaments, young cells differ significantly from their pseudohyphal progenitors, in their shape, and in their timing and direction of cell division. The changes that occur during filament maturation result in round and oval cells surrounding and covering the pseudohyphal filament. In a screen for mutants that affect this process, a vacuolar protein sorting gene, MOS10 (VPS60), and a gene encoding an alpha subunit of the proteasome core, PRE9, were isolated. Characterization of the mos10/mos10 phenotype showed that the process of filament maturation is regulated differently from early filamentous growth, and that the requirement for Mos10 is limited to the maturation stage of pseudohyphal development. The mos10/mos10 phenotype is unlikely to be an unspecific effect of disruption of endocytosis or vacuolar protein sorting, because it is not recapitulated by mutants in other genes required for these processes. Disruption of homologues of MOS10, which act as components of the ESCRT-III complex in targeting proteins for vacuolar degradation, results in abnormal early pseudohyphal growth, not in the filament maturation defect seen in mos10/mos10. Thus, Mos10 may function in targeting of specific cargo proteins for degradation, under conditions particular to maturing filaments.

The essential transcription factor Reb1p interacts with the CLB2 UAS outside of the G2/M control region

Regulation of CLB2 is important both for completion of the normal vegetative cell cycle in Saccharomyces cerevisiae and for departure from the vegetative cell cycle upon nitrogen deprivation. Cell cycle-regulated transcription of CLB2 in the G2/M phase is known to be brought about by a set of proteins including Mcm1p, Fkh2/1p and Ndd1p that associate with a 35 bp G2/M-specific sequence common to a set of co-regulated genes. CLB2 transcription is regulated by additional signals, including by nitrogen levels, by positive feedback from the Clb2-Cdc28 kinase, and by osmotic stress, but the corresponding regulatory sequences and proteins have not been identified. We have found that the essential Reb1 transcription factor binds with high affinity to a sequence upstream of CLB2, within a region implicated previously by others in regulated expression, but upstream of the known G2/M-specific site. CLB2 sequence from the region around the Reb1p site blocks activation by the Gal4 protein when positioned downstream of the Gal4-binding site. Since a mutation in the Reb1p site abrogates this effect, we suggest that Reb1p is likely to occupy this site in vivo.

Genetics of Cryptococcus neoformans

Cryptococcus neoformans is a pathogenic fungus that primarily afflicts immunocompromised patients, infecting the central nervous system to cause meningoencephalitis that is uniformly fatal if untreated. C. neoformans is a basidiomycetous fungus with a defined sexual cycle that has been linked to differentiation and virulence. Recent advances in classical and molecular genetic approaches have allowed molecular descriptions of the pathways that control cell type and virulence. An ongoing genome sequencing project promises to reveal much about the evolution of this human fungal pathogen into three distinct varieties or species. C. neoformans shares features with both model ascomycetous yeasts (Saccharomyces cerevisiae, Schizosaccharomyces pombe) and basidiomycetous pathogens and mushrooms (Ustilago maydis, Coprinus cinereus, Schizophyllum commune), yet ongoing studies reveal unique features associated with virulence and the arrangement of the mating type locus. These advances have catapulted C. neoformans to center stage as a model of both fungal pathogenesis and the interesting approaches to life that the kingdom of fungi has adopted.

A kelch beta propeller featuring as a G beta structural mimic: reinventing the wheel?

New genetic and protein interaction data suggest that G protein alpha subunits may have partners with primary sequences that are quite divergent. How this is achieved may be through the adoption of similar structures, the beta propeller, by both proteins containing WD-40 repeats and kelch domains. Gettemans et al. describe results in yeast that suggest that kelch-domain proteins may serve as previously unrecognized beta subunits in the heterotrimeric G protein complex.

A novel Ras inhibitor, Eri1, engages yeast Ras at the endoplasmic reticulum

Ras oncoproteins are monomeric GTPases that link signals from the cell surface to pathways that regulate cell proliferation and differentiation. Constitutively active mutant forms of Ras are found in ca. 30% of human tumors. Here we report the isolation of a novel gene from Saccharomyces cerevisiae, designated ERI1 (for endoplasmic reticulum-associated Ras inhibitor 1), which behaves genetically as an inhibitor of Ras signaling. ERI1 encodes a 68-amino-acid protein that associates in vivo with GTP-bound Ras in a manner that requires an intact Ras-effector loop, suggesting that Eri1 competes for the same binding site as Ras target proteins. We show that Eri1 localizes primarily to the membrane of the endoplasmic reticulum (ER), where it engages Ras. The recent demonstration that signaling from mammalian Ras is not restricted to the cell surface but can also proceed from the cytoplasmic face of the ER suggests a regulatory function for Eri1 at that membrane.

Haploinsufficiency-based large-scale forward genetic analysis of filamentous growth in the diploid human fungal pathogen C.albicans

Candida albicans is the most prevalent human fungal pathogen. Here, we take advantage of haploinsufficiency and transposon mutagenesis to perform large-scale loss-of-function genetic screen in this organism. We identified mutations in 146 genes that affect the switch between its single-cell (yeast) form and filamentous forms of growth; this switch appears central to the virulence of C.albicans. The encoded proteins include those involved in nutrient sensing, signal transduction, transcriptional control, cytoskeletal organization and cell wall construction. Approximately one-third of the genes identified in the screen lack homologs in Saccharomyces cerevisiae and other model organisms and thus constitute candidate antifungal drug targets. These results illustrate the value of performing forward genetic studies in bona fide pathogens.

The G-protein alpha-subunit GasC plays a major role in germination in the dimorphic fungus Penicillium marneffei

The opportunistic human pathogen Penicillium marneffei exhibits a temperature-dependent dimorphic switch. At 25 degrees, multinucleate, septate hyphae that can undergo differentiation to produce asexual spores (conidia) are produced. At 37 degrees hyphae undergo arthroconidiation to produce uninucleate yeast cells that divide by fission. This work describes the cloning of the P. marneffei gasC gene encoding a G-protein alpha-subunit that shows high homology to members of the class III fungal Galpha-subunits. Characterization of a DeltagasC mutant and strains carrying a dominant-activating gasC(G45R) or a dominant-interfering gasC(G207R) allele show that GasC is a crucial regulator of germination. A DeltagasC mutant is severely delayed in germination, whereas strains carrying a dominant-activating gasC(G45R) allele show a significantly accelerated germination rate. Additionally, GasC signaling positively affects the production of the red pigment by P. marneffei at 25 degrees and negatively affects the onset of conidiation and the conidial yield, showing that GasC function overlaps with functions of the previously described Galpha-subunit GasA. In contrast to the S. cerevisiae ortholog Gpa2, our data indicate that GasC is not involved in carbon or nitrogen source sensing and plays no major role in either hyphal or yeast growth or in the switch between these two forms.

The G protein beta subunit FGB1 regulates development and pathogenicity in Fusarium oxysporum

The cloning of fgb1, the gene encoding a heterotrimeric G protein beta subunit FGB1 in Fusarium oxysporum, was performed by standard PCR techniques to evaluate the role of G protein signaling in this fungus. The full-length open reading frame spanned 1,077 nucleotides and the deduced primary structure of the protein (359 amino acid residues) showed high identity with G beta subunits from other organisms. Disruption of fgb1 led to decreased intracellular cAMP levels, reduced pathogenicity, and alterations in physiological characteristics, including heat resistance, colony morphology, conidia formation and germination frequency. We previously showed that most of these alterations (except germination frequency) were also observed in the disruptants of fga1, the gene for G alpha subunit FGA1 in F. oxysporum. These results suggest that FGA1 and FGB1 have partially overlapping functions in the regulation of development and pathogenicity in F. oxysporum.

Translational control by TOR and TAP42 through dephosphorylation of eIF2alpha kinase GCN2

Yeast protein kinase GCN2 stimulates the translation of transcriptional activator GCN4 by phosphorylating eIF2alpha in response to amino acid starvation. Kinase activation requires binding of uncharged tRNA to a histidyl tRNA synthetase-related domain in GCN2. Phosphorylation of serine 577 (Ser 577) in GCN2 by another kinase in vivo inhibits GCN2 function in rich medium by reducing tRNA binding activity. We show that rapamycin stimulates eIF2alpha phosphorylation by GCN2, with attendant induction of GCN4 translation, while reducing Ser 577 phosphorylation in nonstarved cells. The alanine 577 (Ala 577) mutation in GCN2 (S577A) dampened the effects of rapamycin on eIF2alpha phosphorylation and GCN4 translation, suggesting that GCN2 activation by rapamycin involves Ser 577 dephosphorylation. Rapamycin regulates the phosphorylation of Ser 577 and eIF2alpha by inhibiting the TOR pathway. Rapamycin-induced dephosphorylation of Ser 577, eIF2alpha phosphorylation, and induction of GCN4 all involve TAP42, a regulator of type 2A-related protein phosphatases. Our results add a new dimension to the regulation of protein synthesis by TOR proteins and demonstrate cross-talk between two major pathways for nutrient control of gene expression in yeast.

The Colletotrichum lagenariu Ste12-like gene CST1 is essential for appressorium penetration

Colletotrichum lagenarium is the causal agent of anthracnose of cucumber. This fungus produces a darkly melanized infection structure, appressoria, to penetrate the host leaves. The C. lagenarium CMK1 gene, a homologue of the Saccharomyces cerevisiae FUS3/KSS1 mitogen-activated protein (MAP) kinase genes, was shown to regulate conidial germination, appressorium formation, and invasive growth. In S. cerevisiae, Ste12p is known to be a transcriptional factor downstream of Fus3p/Kss1p MAP kinases. To evaluate the CMK1 MAP kinase pathway, we isolated the Ste12 homologue CST1 gene from C. lagenarium and characterized. The cst1delta strains were nonpathogenic on intact host leaves, but could form lesions when inoculated on wounded leaves. Conidia of the cst1delta strains could germinate and form melanized appressoria on both host leaf surface and artificial cellulose membrane, but could not produce infectious hyphae from appressoria, suggesting that CST1 is essential for appressorium penetration in C. lagenarium. In addition, matured appressoria of the cst1delta strains contained an extremely low level of lipid droplets compared with that of the wild-type strain. Lipid droplets were abundant in conidia of the cst1delta strains, but rapidly disappeared during appressorium formation. This misscheduled lipid degradation might be related to the failure of appressorium penetration in the cst1delta strain.

Krh1p and Krh2p act downstream of the Gpa2p G(alpha) subunit to negatively regulate haploid invasive growth

The yeast G(alpha) subunit Gpa2p and its coupled receptor Gpr1p function in a signaling pathway that is required for the transition to pseudohyphal and invasive growth. A two-hybrid screen using a constitutively active allele of GPA2 identified the KRH1 gene as encoding a potential binding partner of Gpa2p. Strains containing deletions of KRH1 and its homolog KRH2 were hyper-invasive and displayed a high level of expression of FLO11, a gene involved in pseudohyphal and invasive growth. Therefore, KRH1 and KRH2 encode negative regulators of the invasive growth pathway. Cells containing krh1Delta krh2Delta mutations also displayed increased sensitivity to heat shock and decreased sporulation efficiency, indicating that Krh1p and Krh2p regulate multiple processes controlled by the cAMP/PKA pathway. The krh1Delta krh2Delta mutations suppressed the effect of a gpa2Delta mutation on FLO11 expression and eliminated the effect of a constitutively active GPA2 allele on induction of FLO11 and heat shock sensitivity, suggesting that Krh1p and Krh2p act downstream of Gpa2p. The Sch9p kinase was not required for the signal generated by deletion of KRH1 and KRH2; however, the cAMP-dependent kinase Tpk2p was required for generation of this signal. These results support a model in which activation of Gpa2p relieves the inhibition exerted by Krh1p and Krh2p on components of the cAMP/PKA signaling pathway.

Cdc24, the GDP-GTP exchange factor for Cdc42, is required for invasive hyphal growth of Candida albicans

Candida albicans, the most common human fungal pathogen, is particularly problematic for immunocompromised individuals. The reversible transition of this fungal pathogen to a filamentous form that invades host tissue is important for its virulence. Although different signaling pathways such as a mitogen-activated protein kinase and a protein kinase A cascade are critical for this morphological transition, the function of polarity establishment proteins in this process has not been determined. We examined the role of four different polarity establishment proteins in C. albicans invasive growth and virulence by using strains in which one copy of each gene was deleted and the other copy expressed behind the regulatable promoter MET3. Strikingly, mutants with ectopic expression of either the Rho G-protein Cdc42 or its exchange factor Cdc24 are unable to form invasive hyphal filaments and germ tubes in response to serum or elevated temperature and yet grow normally as a budding yeast. Furthermore, these mutants are avirulent in a mouse model for systemic infection. This function of the Cdc42 GTPase module is not simply a general feature of polarity establishment proteins. Mutants with ectopic expression of the SH3 domain containing protein Bem1 or the Ras-like G-protein Bud1 can grow in an invasive fashion and are virulent in mice, albeit with reduced efficiency. These results indicate that a specific regulation of Cdc24/Cdc42 activity is required for invasive hyphal growth and suggest that these proteins are required for pathogenicity of C. albicans.

The sensing of nutritional status and the relationship to filamentous growth in Saccharomyces cerevisiae

Heterotrophic organisms rely on the ingestion of organic molecules or nutrients from the environment to sustain energy and biomass production. Non-motile, unicellular organisms have a limited ability to store nutrients or to take evasive action, and are therefore most directly dependent on the availability of nutrients in their immediate surrounding. Such organisms have evolved numerous developmental options in order to adapt to and to survive the permanently changing nutritional status of the environment. The phenotypical, physiological and molecular nature of nutrient-induced cellular adaptations has been most extensively studied in the yeast Saccharomyces cerevisiae. These studies have revealed a network of sensing mechanisms and of signalling pathways that generate and transmit the information on the nutritional status of the environment to the cellular machinery that implements specific developmental programmes. This review integrates our current knowledge on nutrient sensing and signalling in S. cerevisiae, and suggests how an integrated signalling network may lead to the establishment of a specific developmental programme, namely pseudohyphal differentiation and invasive growth.

Gpa2, a G-protein alpha subunit required for hyphal development in Candida albicans

Candida albicans is able to respond to environmental changes by inducing a distinct morphological program, which is related to the ability to infect mammalian hosts. Although some of the signal transduction pathways involved in this response are known, it is not clear how the environmental signals are sensed and transmitted to these transduction cascades. In this work, we have studied the function of GPA2, a new gene from C. albicans, which encodes a G-protein alpha-subunit homologue. We demonstrate that Gpa2 plays an important role in the yeast-hypha dimorphic transition in the response of C. albicans to some environmental inducers. Deletion of both alleles of the GPA2 gene causes in vitro defects in morphological transitions in Spider medium and SLAD medium and in embedded conditions but not in medium containing serum. These defects cannot be reversed by exogenous addition of cyclic AMP. However, overexpression of HST7, which encodes a component of the filament-inducing mitogen-activated protein kinase (MAPK) cascade, bypasses the Gpa2 requirement. We have obtained different gain-of-function and loss-of-function mutant alleles of the GPA2 gene, which we have introduced in several C. albicans genetic backgrounds. Our results indicate that, in response to environmental cues, Gpa2 is required for the regulation of a MAPK signaling pathway.

The adenylate cyclase (BAC) in Botrytis cinerea is required for full pathogenicity

SUMMARY The grey mould Botrytis cinerea is an economically important plant pathogen. Previously we found that null mutants of bcg1 encoding one of the two Galpha subunits of heterotrimeric GTP-binding proteins differed in colony morphology and showed reduced pathogenicity. To further understand the mechanisms involved in infection, we cloned the bac gene encoding adenylate cyclase, the enzyme that catalyses production of cAMP from ATP. The deduced protein sequence consists of 2300 amino acids, the ORF is interrupted by three conserved introns, and there is a high degree of similarity with the catalytic domains of other fungal adenylate cyclases. Gene replacement resulted in reduced vegetative growth and a morphology similar to that of bcg1 mutants. The wild-type (WT) colony morphology was partially restored by feeding exogenous cAMP. These bac mutants still had a low but constant level of cAMP, despite deletion of the complete catalytic domain of the enzyme. Conidia from bac mutants germinated, penetrated the leaves of Phaseolus vulgaris and caused spreading soft rot lesions (in contrast to bcg1 mutants), although these were slower to develop than in WT controls. Compared to the latter, the most striking difference was that no sporulation occurred on leaves inoculated with bac mutant conidia. These results confirm that the cAMP signalling pathway plays an important role in vegetative growth and pathogenicity in B. cinerea. On the other hand, a much stronger effect of bcg1 mutation on pathogenicity in comparison to the effects of bac mutations suggests that BCG1 controls at least one more signalling component other than adenylate cyclase, and that the cAMP signalling pathway is not the only one responsible for pathogenicity.

Control of morphogenesis in the human fungal pathogen Penicillium marneffei

Fungal pathogens are an increasing threat to human health due to the increasing population of immunocompromised individuals and the increased incidence of treatment-derived infections. Penicillium marneffei is an emerging fungal pathogen endemic to South-east Asia, where it is AIDS defining. Like many other fungal pathogens, P. marneffei is capable of alternating between a filamentous and a yeast growth form, known as dimorphic switching, in response to environmental stimuli. P. marneffei grows in the filamentous form at 25 degrees C and in the yeast form at 37 degrees C. During filamentous growth and in response to environmental cues, P. marneffei undergoes asexual development to form complex multicellular structures from which the infectious agents, the conidia, are produced. At 37 degrees C, P. marneffei undergoes the dimorphic switching program to produce the pathogenic yeast cells. These yeast cells are found intracellularly in the mononuclear phagocyte system of the host and divide by fission, in contrast to the budding mode of division exhibited by most other fungal pathogens. In addition, P. marneffei is evolutionarily distinct from most other dimorphic fungal pathogens and is the only known Penicillium species which exhibits dimorphic growth. The unique evolutionary history of P. marneffei and the rapidly increasing incidence of infection, coupled with the presence of both complex asexual development and dimorphic switching programs in one organism, makes this system a valuable one for the study of morphogenesis and pathogenicity. Recent development of molecular genetic techniques for P. marneffei, including DNA-mediated transformation, have greatly facilitated the study of these two important morphogenetic programs, asexual development and dimorphic switching, and we are beginning to uncover important determinants which control these events. Understand these programs is providing insights into the biology of P. marneffei and its pathogenic capacity.

Trichoderma atroviride G-protein alpha-subunit gene tga1 is involved in mycoparasitic coiling and conidiation

The soil fungus Trichoderma atroviride, a mycoparasite, responds to a number of external stimuli. In the presence of a fungal host, T. atroviride produces hydrolytic enzymes and coils around the host hyphae. In response to light or nutrient depletion, asexual sporulation is induced. In a biomimetic assay, different lectins induce coiling around nylon fibers; coiling in the absence of lectins can be induced by applying cyclic AMP (cAMP) or the heterotrimeric G-protein activator mastoparan. We isolated a T. atroviride G-protein alpha-subunit (Galpha) gene (tgal) belonging to the fungal subfamily with the highest similarity to the Galpha1 class. Generated transgenic lines that overexpress Galpha show very delayed sporulation and coil at a higher frequency. Furthermore, transgenic lines that express an activated mutant protein with no GTPase activity do not sporulate and coil at a higher frequency. Lines that express an antisense version of the gene are hypersporulating and coil at a much lower frequency in the biomimetic assay. The loss of Tgal in these mutants correlates with the loss of GTPase activity stimulated by the peptide toxin Mas-7. The application of Mas-7 to growing mycelial colonies raises intracellular cAMP levels, suggesting that Tgal can activate adenylyl cyclase. In contrast, cAMP levels and cAMP-dependent protein kinase activity drop when diffusible host signals are encountered and the mycoparasitism-related genes ech42 and prb1 are highly expressed. Mycoparasitic signaling is unlikely to be a linear pathway from host signals to increased cAMP levels. Our results demonstrate that the product of the tga1 gene is involved in both coiling and conidiation.

The Galpha protein Gpa2 controls yeast differentiation by interacting with kelch repeat proteins that mimic Gbeta subunits

G protein coupled receptors (GPCR) sense diverse ligands and signal via heterotrimeric G proteins. The Saccharomyces cerevisiae GPCR Gpr1 senses glucose and controls filamentous growth via an unusual Galpha protein, Gpa2, which lacks any known Gbetagamma subunits. Our genetic and biochemical studies identify Gpa2 interaction partners (Gpb1/2, Gpg1) and provide evidence that these proteins function as G protein subunit mimics and signaling effectors. Gpb1 and Gpb2 lack the seven WD-40 repeats found in Gbeta subunits and instead contain seven kelch repeats implicated in protein-protein interactions. Gbeta subunits and the kelch repeat protein galactose oxidase fold into strikingly similar seven-bladed beta propellers. Our studies demonstrate that Gpa2 signals in conjunction with Gbeta structural mimics and that homologous G protein subunits or effectors may be conserved in multicellular eukaryotes.

Amino acids control ammonia pulses in yeast colonies

Individual yeast colonies produce pulses of volatile ammonia separated by phases of medium acidification. Colonies of Saccharomyces cerevisiae mutant defective in the general amino acid permease, Gap1p, exhibit decreased ammonia production. Mutations in the S. cerevisiae amino acid sensor SPS completely abolish the colony ammonia pulses. In contrast, the ammonia pulse production is independent of external concentrations of ammonium and of its uptake by the ammonium permeases Mep1p, Mep2p, and Mep3p. It is concluded that in S. cerevisiae colonies, the extracellular amino acids, but not the extracellular ammonium, serve as a source for volatile ammonia production. These phenomena are not restricted to S. cerevisiae, since we observe that extracellular levels of 8 out of the 20 tested amino acids are necessary for ammonia pulses produced by Candida mogii colonies.

Protein kinase A operates a molecular switch that governs yeast pseudohyphal differentiation

The yeast Saccharomyces cerevisiae undergoes a dimorphic filamentous transition in response to nutrient cues that is affected by both mitogen-activated protein kinase and cyclic AMP-protein kinase A signaling cascades. Here two transcriptional regulators, Flo8 and Sfl1, are shown to be the direct molecular targets of protein kinase A. Flo8 and Sfl1 antagonistically control expression of the cell adhesin Flo11 via a common promoter element. Phosphorylation by the protein kinase A catalytic subunit Tpk2 promotes Flo8 binding and activation of the Flo11 promoter and relieves repression by prohibiting dimerization and DNA binding by Sfl1. Our studies illustrate in molecular detail how protein kinase A combinatorially effects a key developmental switch. Similar mechanisms may operate in pathogenic fungi and more complex multicellular eukaryotic organisms.

G-protein signaling mediates asexual development at 25 degrees C but has no effect on yeast-like growth at 37 degrees C in the dimorphic fungus Penicillium mameffei

The ascomycete Penicillium marneffei is an opportunistic human pathogen exhibiting a temperature-dependent dimorphic switch. At 25 degrees C, P. marneffei grows as filamentous multinucleate hyphae and undergoes asexual development, producing uninucleate spores. At 37 degrees C, it forms uninucleate yeast cells which divide by fission. We have cloned a gene encoding a G alpha subunit of a heterotrimeric G protein from P. marneffei named gasA with high similarity to fadA in Aspergillus nidulans. Through the characterization of a delta gasA strain and mutants carrying a dominant activating or a dominant interfering gasA allele, we show that GasA is a key regulator of asexual development but seems to play no role in the regulation of growth. A dominant activating gasA mutant whose mutation results in a G42-to-R change (gasA(G42R)) does not express brlA, the conidiation-specific regulatory gene, and is locked in vegetative growth, while a dominant interfering gasA(G203R) mutant shows inappropriate brlA expression and conidiation. Interestingly, the gasA mutants have no apparent defect in dimorphic switching or yeast-like growth at 37 degrees C. Growth tests on dibutyryl cyclic AMP (dbcAMP) and theophylline suggest that a cAMP-protein kinase A cascade may be involved in the GasA signaling pathway.

A G-protein beta subunit required for sexual and vegetative development and maintenance of normal G alpha protein levels in Neurospora crassa

The genome of the filamentous fungus Neurospora crassa contains a single gene encoding a heterotrimeric G-protein beta subunit, gnb-1. The predicted GNB-1 protein sequence is most identical to G beta proteins from the filamentous fungi Cryphonectria parasitica and Aspergillus nidulans. N. crassa GNB-1 is also 65% identical to the human GNB-1 protein but only 38 and 45% identical to G beta proteins from budding and fission yeasts. Previous studies in animal and fungal systems have elucidated phenotypes of G beta null mutants, but little is known about the effects of G beta loss on G alpha levels. In this study, we analyzed a gnb-1 deletion mutant for cellular phenotypes and levels of the three G alpha proteins. Delta gnb-1 strains are female-sterile, with production of aberrant fertilized reproductive structures. Delta gnb-1 strains conidiate more profusely and have altered mass on solid medium. Loss of gnb-1 leads to inappropriate conidiation and expression of a conidiation-specific gene during growth in submerged culture. Intracellular cyclic AMP levels are reduced by 60% in vegetative plate cultures of delta gnb-1 mutants. Loss of gnb-1 leads to lower levels of the three G alpha proteins under a variety of conditions. Analysis of transcript levels for the gna-1 and gna-2 G alpha genes in submerged cultures indicates that regulation of G alpha protein levels by gnb-1 is posttranscriptional. The results suggest that GNB-1 directly regulates apical extension rate and mass accumulation. In contrast, many other delta gnb-1 phenotypes, including female sterility and defective conidiation, can be explained by altered levels of the three N. crassa G alpha proteins.

G proteins and pheromone signaling

All cells have the capacity to respond to chemical and sensory stimuli. Central to many such signaling pathways is the heterotrimeric G protein, which transmits a signal from cell surface receptors to intracellular effectors. Recent studies using the yeast Saccharomyces cerevisiae have produced important advances in our understanding of G protein activation and inactivation. This review focuses on the mechanisms by which G proteins transmit a signal from peptide pheromone receptors to the mating response in yeast and how mechanisms elucidated in yeast can provide insights to signaling events in more complex organisms.

cAMP and ras signalling independently control spore germination in the filamentous fungus Aspergillus nidulans

The role of cAMP signalling during germination of asexual spores (conidia) of the filamentous fungus Aspergillus nidulans was investigated. A. nidulans strains defective for adenylate cyclase (CyaA) or for the functionally overlapping cAMP-dependent protein kinase (PkaA) and newly characterized SchA protein kinase, homologous to Saccharomyces cerevisiae Sch9, show altered trehalose mobilization and kinetics of germ tube outgrowth, in addition to other defects in colony formation. cAMP-dependent trehalose breakdown is triggered by the addition of a carbon source independently of further catabolism, suggesting that cAMP signalling controls early events of conidial germination in response to carbon source sensing. Additional results suggest that cAMP has targets other than PkaA and SchA and that PkaA retains activity in the absence of cAMP. Conversely, PkaA regulates cAMP levels in A. nidulans because these are elevated by approximately 250-fold in a strain that lacks PkaA. Furthermore, analysis of mutant strains impaired in both adenylate cyclase and RasA GTPase previously implicated in the control of A. nidulans spore germination suggested that RasA and cAMP signalling proceed independently during germination in A. nidulans.

Kap121p-mediated nuclear import is required for mating and cellular differentiation in yeast

To further our understanding of how the nucleocytoplasmic transport machinery interfaces with its cargoes and how this affects cellular physiology, we investigated the molecular mechanisms of phenotypes associated with mutations in karyopherin Kap121p. Two previously unreported phenotypes of kap121 cells were observed: defects in mating and in the transition from the normal yeast form to the pseudohyphal, invasive form. In parallel, we searched for Kap121p cargoes by using Kap121p as a probe in overlay assays of yeast nuclear proteins. One of the major interacting proteins identified by this procedure was Ste12p, a transcription factor central to both the mating response and the pseudohyphal transition. We therefore investigated whether defects in these differentiation processes were due to an inability to import Ste12p. Both immunopurification and in vitro binding studies demonstrated that Ste12p interacted specifically with Kap121p in a Ran-GTP-sensitive manner and that Ste12p was mislocalized to the cytoplasm by inactivation of Kap121p in a temperature-sensitive mutant. The Kap121p-specific nuclear localization signal (NLS) of Ste12p was determined to reside within a C-terminal region of Ste12p. Furthermore, by overexpression of STE12 or expression of a STE12-cNLS fusion in kap121 cells, the invasive-growth defect and the mating defect were both suppressed. Together these data demonstrate that Ste12p is imported into nuclei by Kap121p and that mating and differentiation defects associated with kap121 mutants are primarily attributable to the mislocalization of Ste12p.