Requirement of either of a pair of ras-related genes of Saccharomyces cerevisiae for spore viability

Interaction of TOR and PKA Signaling in S. cerevisiae

TOR and PKA signaling are the major growth-regulatory nutrient-sensing pathways in S. cerevisiae. A number of experimental findings demonstrated a close relationship between these pathways: Both are responsive to glucose availability. Both regulate ribosome production on the transcriptional level and repress autophagy and the cellular stress response. Sch9, a major downstream effector of TORC1 presumably shares its kinase consensus motif with PKA, and genetic rescue and synthetic defects between PKA and Sch9 have been known for a long time. Further, studies in the first decade of this century have suggested direct regulation of PKA by TORC1. Nonetheless, the contribution of a potential direct cross-talk vs. potential sharing of targets between the pathways has still not been completely resolved. What is more, other findings have in contrast highlighted an antagonistic relationship between the two pathways. In this review, I explore the association between TOR and PKA signaling, mainly by focusing on proteins that are commonly referred to as shared TOR and PKA targets. Most of these proteins are transcription factors which to a large part explain the major transcriptional responses elicited by TOR and PKA upon nutrient shifts. I examine the evidence that these proteins are indeed direct targets of both pathways and which aspects of their regulation are targeted by TOR and PKA. I further explore if they are phosphorylated on shared sites by PKA and Sch9 or when experimental findings point towards regulation via the PP2ASit4/PP2A branch downstream of TORC1. Finally, I critically review data suggesting direct cross-talk between the pathways and its potential mechanism.

The Yeast Saccharomyces cerevisiae as a Model for Understanding RAS Proteins and their Role in Human Tumorigenesis

The exploitation of the yeast Saccharomyces cerevisiae as a biological model for the investigation of complex molecular processes conserved in multicellular organisms, such as humans, has allowed fundamental biological discoveries. When comparing yeast and human proteins, it is clear that both amino acid sequences and protein functions are often very well conserved. One example of the high degree of conservation between human and yeast proteins is highlighted by the members of the RAS family. Indeed, the study of the signaling pathways regulated by RAS in yeast cells led to the discovery of properties that were often found interchangeable with RAS proto-oncogenes in human pathways, and vice versa. In this work, we performed an updated critical literature review on human and yeast RAS pathways, specifically highlighting the similarities and differences between them. Moreover, we emphasized the contribution of studying yeast RAS pathways for the understanding of human RAS and how this model organism can contribute to unveil the roles of RAS oncoproteins in the regulation of mechanisms important in the tumorigenic process, like autophagy.

Ras GTPases modulate morphogenesis, sporulation and cellulase gene expression in the cellulolytic fungus Trichoderma reesei

Background

The model cellulolytic fungus Trichoderma reesei (teleomorph Hypocrea jecorina) is capable of responding to environmental cues to compete for nutrients in its natural saprophytic habitat despite its genome encodes fewer degradative enzymes. Efficient signalling pathways in perception and interpretation of environmental signals are indispensable in this process. Ras GTPases represent a kind of critical signal proteins involved in signal transduction and regulation of gene expression. In T. reesei the genome contains two Ras subfamily small GTPases TrRas1 and TrRas2 homologous to Ras1 and Ras2 from S. cerevisiae, but their functions remain unknown.

Methodology/Principal Findings

Here, we have investigated the roles of GTPases TrRas1 and TrRas2 during fungal morphogenesis and cellulase gene expression. We show that both TrRas1 and TrRas2 play important roles in some cellular processes such as polarized apical growth, hyphal branch formation, sporulation and cAMP level adjustment, while TrRas1 is more dominant in these processes. Strikingly, we find that TrRas2 is involved in modulation of cellulase gene expression. Deletion of TrRas2 results in considerably decreased transcription of cellulolytic genes upon growth on cellulose. Although the strain carrying a constitutively activated TrRas2^G16V allele exhibits increased cellulase gene transcription, the cbh1 and cbh2 expression in this mutant still strictly depends on cellulose, indicating TrRas2 does not directly mediate the transmission of the cellulose signal. In addition, our data suggest that the effect of TrRas2 on cellulase gene is exerted through regulation of transcript abundance of cellulase transcription factors such as Xyr1, but the influence is independent of cAMP signalling pathway.

Conclusions/Significance

Together, these findings elucidate the functions for Ras signalling of T. reesei in cellular morphogenesis, especially in cellulase gene expression, which contribute to deciphering the powerful competitive ability of plant cell wall degrading fungi in nature.

Ras1 and Ras2 play antagonistic roles in regulating cellular cAMP level, stationary-phase entry and stress response in Candida albicans

The GTPase Ras1 activates the yeast-to-hypha transition in Candida albicans by activating cAMP synthesis. Here, we have characterized Ras2. Ras2 belongs to a group of atypical Ras proteins in some fungal species that share poor identity with other Ras GTPases with many variations in conserved motifs thought to be crucial for Ras-associated activities. We find that recombinant Ras2 is enzymatically as active as Ras1. However, only RAS1 can rescue the lethality of the Saccharomyces cerevisiae ras1 ras2 mutant, suggesting functional divergence of the two genes. ras2Delta is normal in hyphal growth, but deleting RAS2 in the ras1Delta background greatly aggravates the hyphal defect, indicating that Ras2 also has a role in hyphal development. Strikingly, while RAS1 deletion causes a approximately 20-fold decrease in cellular cAMP, further deletion of RAS2 restores it to approximately 30% of the wild-type level. Consistently, while the ras1Delta mutant enters the stationary phase prematurely, the double mutant does so normally. Moreover, ras1Delta cells exhibit increased resistance to H(2)O(2) and higher sensitivity to the heavy metal Co(2+), whereas ras2Delta cells show the opposite phenotypes. Together, our data reveal a novel regulatory mechanism by which two antagonizing Ras GTPases balance each other in regulating multiple cellular processes in C. albicans.

The Stress-Activated Signaling (SAS) Pathways of a Human Fungal Pathogen, Cryptococcus neoformans

Cryptococcus neoformans is a basidiomycete human fungal pathogen that causes meningoencephalitis in both immunocompromised and immunocompetent individuals. The ability to sense and respond to diverse extracellular signals is essential for the pathogen to infect and cause disease in the host. Four major stress-activated signaling (SAS) pathways have been characterized in C. neoformans, including the HOG (high osmolarity glycerol response), PKC/Mpk1 MAPK (mitogen-activated protein kinase), calcium-dependent calcineurin, and RAS signaling pathways. The HOG pathway in C. neoformans not only controls responses to diverse environmental stresses, including osmotic shock, UV irradiation, oxidative stress, heavy metal stress, antifungal drugs, toxic metabolites, and high temperature, but also regulates ergosterol biosynthesis. The PKC (Protein kinase C)/Mpk1 pathway in C. neoformans is involved in a variety of stress responses, including osmotic, oxidative, and nitrosative stresses and breaches of cell wall integrity. The Ca(2+)/calmodulin- and Ras-signaling pathways also play critical roles in adaptation to certain environmental stresses, such as high temperature and sexual differentiation. Perturbation of the SAS pathways not only impairs the ability of C. neoformans to resist a variety of environmental stresses during host infection, but also affects production of virulence factors, such as capsule and melanin. A drug(s) capable of targeting signaling components of the SAS pathway will be effective for treatment of cryptococcosis.

Elevated Ras/protein kinase A activity in Saccharomyces cerevisiae reduces proliferation rate and lifespan by two different reactive oxygen species-dependent routes

Cells with overactive RAS/protein kinase A (PKA) signaling, such as RAS2(Val19) cells, exhibit reduced proliferation rates and accelerated replicative senescence. We show here that the extended generation time of RAS2(Val19)cells is the result of abrogated ATP/ADP carrier activity of the mitochondria. Both PKA-dependent and independent routes are responsible for inhibiting ATP/ADP exchange in the RAS-overactive cells. The reduced carrier activity is due, at least in part, to elevated levels of reactive oxygen species (ROS), which also cause a proteolysis-dependent fragmentation of the Aac2p carrier both in vivo and on isolated mitochondria. Attenuated carrier activity is suppressed by overproducing the superoxide dismutase, Sod1p, and this enhances both the proliferation rate and the replicative longevity of RAS2(Val19) cells. In contrast, overproducing functional Aac2p restored proliferation but not longevity of RAS2(Val19) cells. Thus, Ras signaling affects proliferation rate and replicative lifespan by two different, ROS-dependent, routes. While the reduction in generation time is linked to the inactivation, specifically, of the mitochondrial nucleotide carrier, longevity is affected by other, and hitherto unknown, target(s) of ROS attack.

RAS function and protein kinase cascades

This paper reviews recent progress in understanding the function of RAS in three systems: the budding yeast (Saccharomyces cerevisiae), the fission yeast (Schizosaccharomyces pombe) and Xenopus laevis oocytes. One of the functions of RAS in S. cerevisiae is the stimulation of adenylate cyclase. This leads to the activation of the cAMP-dependent protein kinases--a function that has probably not been conserved in evolution. The immediate function of RAS in S. pombe is not known, but it may lead to the activation of a protein kinase cascade. This cascade has likely been conserved in evolution and linkage between it and RAS can be demonstrated in cell-free extracts from Xenopus oocytes. The Xenopus cell-free system provides a means to test specific hypotheses about RAS function and to isolate targets of RAS.

Central roles of small GTPases in the development of cell polarity in yeast and beyond

SUMMARY

The establishment of cell polarity is critical for the development of many organisms and for the function of many cell types. A large number of studies of diverse organisms from yeast to humans indicate that the conserved, small-molecular-weight GTPases function as key signaling proteins involved in cell polarization. The budding yeast Saccharomyces cerevisiae is a particularly attractive model because it displays pronounced cell polarity in response to intracellular and extracellular cues. Cells of S. cerevisiae undergo polarized growth during various phases of their life cycle, such as during vegetative growth, mating between haploid cells of opposite mating types, and filamentous growth upon deprivation of nutrition such as nitrogen. Substantial progress has been made in deciphering the molecular basis of cell polarity in budding yeast. In particular, it becomes increasingly clear how small GTPases regulate polarized cytoskeletal organization, cell wall assembly, and exocytosis at the molecular level and how these GTPases are regulated. In this review, we discuss the key signaling pathways that regulate cell polarization during the mitotic cell cycle and during mating.

Regulation of respiratory growth by Ras: the glyoxylate cycle mutant, cit2Delta, is suppressed by RAS2

In Saccharomyces cerevisiae the Ras/cAMP/PKA signalling pathway controls multiple metabolic pathways, and alterations in the intracellular concentrations of cAMP through modification of signalling pathway factors can be lethal or result in severe growth defects. In this work, the important role of Ras2p in metabolic regulation during growth on the non-fermentable carbon source glycerol is further investigated. The data show that the overexpression of RAS2 suppresses the growth defect of the glyoxylate cycle citrate synthase mutant, cit2Delta. The overexpression results in enhanced proliferation and biomass yield when cells are grown on glycerol as sole carbon source, and increases citrate synthase activity and intracellular citrate concentration. Interestingly, the suppression of cit2Delta and the enhanced proliferation and biomass yield are only observed when RAS2 is overexpressed and not in strains containing the constitutively active allele RAS2(val19). However, both RAS2 and RAS2(val19)upregulated citrate synthase activity. We propose that the RAS2 overexpression results in a combination of general upregulation of respiratory growth capacity and an increase in mitochondrial citrate/citrate synthases, which together, complement the metabolic requirements of the cit2Delta mutant. The data therefore provide new evidence for the role of Ras2p as a powerful modulator of metabolism during growth on a non-fermentable carbon source.

Glucose signaling in Saccharomyces cerevisiae

Eukaryotic cells possess an exquisitely interwoven and fine-tuned series of signal transduction mechanisms with which to sense and respond to the ubiquitous fermentable carbon source glucose. The budding yeast Saccharomyces cerevisiae has proven to be a fertile model system with which to identify glucose signaling factors, determine the relevant functional and physical interrelationships, and characterize the corresponding metabolic, transcriptomic, and proteomic readouts. The early events in glucose signaling appear to require both extracellular sensing by transmembrane proteins and intracellular sensing by G proteins. Intermediate steps involve cAMP-dependent stimulation of protein kinase A (PKA) as well as one or more redundant PKA-independent pathways. The final steps are mediated by a relatively small collection of transcriptional regulators that collaborate closely to maximize the cellular rates of energy generation and growth. Understanding the nuclear events in this process may necessitate the further elaboration of a new model for eukaryotic gene regulation, called "reverse recruitment." An essential feature of this idea is that fine-structure mapping of nuclear architecture will be required to understand the reception of regulatory signals that emanate from the plasma membrane and cytoplasm. Completion of this task should result in a much improved understanding of eukaryotic growth, differentiation, and carcinogenesis.

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.

Guanyl nucleotide exchange factor Sql2 and Ras2 regulate filamentous growth in Ustilago maydis

The cyclic AMP (cAMP)-signaling pathway regulates cell morphology and plays a crucial role during pathogenic development of the plant-pathogenic fungus Ustilago maydis. Strains lacking components of this signaling pathway, such as the Galpha-subunit Gpa3 or the adenylyl cyclase Uac1, are nonpathogenic and grow filamentously. On the other hand, strains exhibiting an activated cAMP pathway due to a dominant-active allele of gpa3 display a glossy colony phenotype and are unable to proliferate in plant tumors. Here we present the identification of sql2 as a suppressor of the glossy colony phenotype of a gpa3(Q206L) strain. sql2 encodes a protein with similarity to CDC25-like guanine nucleotide exchange factors, which are known to act on Ras proteins. Overexpression of sql2 leads to filamentous growth that cannot be suppressed by exogenous cAMP, suggesting that Sql2 does not act upstream of Uac1. To gain more insight in signaling processes regulated by Sql2, we isolated two genes encoding Ras proteins. Expression of dominant active alleles of ras1 and ras2 showed that Ras2 induces filamentous growth while Ras1 does not affect cell morphology but elevates pheromone gene expression. These results indicate that Ras1 and Ras2 fulfill different functions in U. maydis. Moreover, observed similarities between the filaments induced by sql2 and ras2 suggest that Sql2 is an activator of Ras2. Interestingly, sql2 deletion mutants are affected in pathogenic development but not in mating, indicating a specific function of sql2 during pathogenesis.

Ras1 and Ras2 contribute shared and unique roles in physiology and virulence of Cryptococcus neoformans

The Ras1 signal transduction pathway controls the ability of the pathogenic fungus Cryptococcus neoformans to grow at high temperatures and to mate. A second RAS gene was identified in this organism. RAS2 is expressed at a very low level compared to RAS1, and a ras2 mutation caused no alterations in vegetative growth rate, differentiation or virulence factor expression. The ras2 mutant strain was equally virulent to the wild-type strain in the murine inhalational model of cryptococcosis. Although a ras1 ras2 double mutant strain is viable, mutation of both RAS genes results in a decreased growth rate at all temperatures compared to strains with either single mutation. Overexpression of the RAS2 gene completely suppressed the ras1 mutant mating defect and partially suppressed its high temperature growth defect. After prolonged incubation at a restrictive temperature, the ras1 mutant demonstrated actin polarity defects that were also partially suppressed by RAS2 overexpression. These studies indicate that the C. neoformans Ras1 and Ras2 proteins share overlapping functions, but also play distinct signalling roles. Our findings also suggest a mechanism by which Ras1 controls growth of this pathogenic fungus at 37 degrees C, supporting a conserved role for Ras homologues in microbial cellular differentiation, morphogenesis and virulence.

Ras links cellular morphogenesis to virulence by regulation of the MAP kinase and cAMP signalling pathways in the pathogenic fungus Candida albicans

The pathogenic fungus Candida albicans is capable of responding to a wide variety of environmental cues with a morphological transition from a budding yeast to a polarized filamentous form. We demonstrate that the Ras homologue of C. albicans, CaRas1p, is required for this morphological transition and thereby contributes to the development of pathogenicity. However, CaRas1p is not required for cellular viability. Deletion of both alleles of the CaRAS1 gene caused in vitro defects in morphological transition that were reversed by either supplementing the growth media with cAMP or overexpressing components of the filament-inducing mitogen-activated protein (MAP) kinase cascade. The induction of filament-specific secreted aspartyl proteinases encoded by the SAP4-6 genes was blocked in the mutant cells. The defects in filament formation were also observed in situ after phagocytosis of C. albicans cells in a macrophage cell culture assay and, in vivo, after infection of kidneys in a mouse model for systemic candidiasis. In the macrophage assay, the mutant cells were less resistant to phagocytosis. Moreover, the defects in filament formation were associated with reduced virulence in the mouse model. These results indicate that, in response to environmental cues, CaRas1p is required for the regulation of both a MAP kinase signalling pathway and a cAMP signalling pathway. CaRas1p-dependent activation of these pathways contributes to the pathogenicity of C. albicans cells through the induction of polarized morphogenesis. These findings elucidate a new medically relevant role for Ras in cellular morphogenesis and virulence in an important human infectious disease.

Conserved cAMP signaling cascades regulate fungal development and virulence

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

RAS1 regulates filamentation, mating and growth at high temperature of Cryptococcus neoformans

Cryptococcus neoformans is a basidiomycete yeast and opportunistic human pathogen of increasing clinical importance due to the increasing population of immunocompromised patients. To further investigate signal transduction cascades regulating fungal pathogenesis, we have identified the gene encoding a RAS homologue in this organism. The RAS1 gene was disrupted by transformation and homologous recombination. The resulting ras1 mutant strain was viable, but failed to grow at 37 degrees C, and exhibited significant defects in mating and agar adherence. The ras1 mutant strain was also avirulent in an animal model of cryptococcal meningitis. Reintroduction of the wild-type RAS1 gene complemented these ras1 mutant phenotypes and restored virulence in animals. A dominantly active RAS1 mutant allele, RAS1Q67L, induced a differentiation phenotype known as haploid fruiting, which involves filamentation, agar invasion and sporulation in response to nitrogen deprivation. The ras1 mutant mating defect was suppressed by overexpression of MAP kinase signalling elements and partially suppressed by exogenous cAMP. Additionally, cAMP also suppressed the agar adherence defect of the ras1 mutant. However, the ability of the ras1 mutant strain to grow at elevated temperature was not restored by cAMP or MAP kinase overexpression. Our findings support a model in which RAS1 signals in C. neoformans through cAMP-dependent, MAP kinase, and RAS-specific signalling cascades to regulate mating and filamentation, as well as growth at high temperature which is necessary for maintenance of infection.

Preservation of frozen yeast cells by trehalose

Two different methods commonly used to preserve intact yeast cells-freezing and freeze-drying-were compared. Different yeast cells submitted to these treatments were stored for 28 days and cell viability assessed during this period. Intact yeast cells showed to be less tolerant to freeze-drying than to freezing. The rate of survival for both treatments could be enhanced by exogenous trehalose (10%) added during freezing and freeze-drying treatments or by a combination of two procedures: a pre-exposure of cells to 40 degrees C for 60 min and addition of trehalose. A maximum survival level of 71.5 +/- 6.3% after freezing could be achieved at the end of a storage period of 28 days, whereas only 25.0 +/- 1.4% showed the ability to tolerate freeze-drying treatment, if both low-temperature treatments were preceded by a heat exposure and addition of trehalose to yeast cells. Increased survival ability was also obtained when the pre-exposure treatment of yeast cells was performed at 10 degrees C for 3 h and trehalose was added: these treatments enhanced cell survival following freezing from 20.5 +/- 7. 7% to 60.0 +/- 3.5%. Although both mild cold and heat shock treatments could enhance cell tolerance to low temperature, only the heat treatment was able to increase the accumulation of intracellular trehalose whereas, during cold shock exposure, the intracellular amount of trehalose remained unaltered. Intracellular trehalose levels seemed not to be the only factor contributing to cell tolerance against freezing and freeze-drying treatments; however, the protection that this sugar confers to cells can be exerted only if it is to be found on both sides of the plasma membrane.

The yeast ras/cyclic AMP pathway induces invasive growth by suppressing the cellular stress response

Haploid yeast cells are capable of invading agar when grown on rich media. Cells of the Sigma1278b genetic background manifest this property, whereas other laboratory strains are incapable of invasive growth. We show that disruption of the RAS2 gene in the Sigma1278b background significantly reduces invasive growth but that expression of a constitutively active Ras2p (Ras2(Val19)p) in this strain has a minimal effect on its invasiveness. On the other hand, expression of Ras2(Val19)p in another laboratory strain, SP1, rendered it invasive. These results suggest that a hyperactive Ras2 pathway induces invasive growth and that this pathway might be overactive in the Sigma1278b genetic background. Indeed, cells of the Sigma1278b are defective in the induction of stress-responsive genes, while their Gcn4 target genes are constitutively transcribed. This pattern of gene expression was previously shown to be associated with an active Ras/cyclic AMP (cAMP) pathway. We show that suppression of stress-related genes in Sigma1278b cells is a result of their inability to activate transcription through the stress response element (STRE). Disruption of RAS2, which abolished invasiveness, induced an increase in STRE activity. Further, in the SP1 genetic background, disruption of either the MSN2/4 genes (encoding activators of STRE) or the yAP-1 gene was sufficient to restore invasive growth in ras2Delta cells. We conclude that Ras2-mediated suppression of the stress response is sufficient to induce invasiveness. Accordingly, the fact that the stress response is suppressed in Sigma1278b background explains its invasiveness. It seems that invasiveness is a phenotype related to unregulated growth and is therefore manifested by cells harboring an overactive Ras/cAMP cascade. In this respect, invasiveness in yeast is reminiscent of the property of ras-transformed fibroblasts to invade soft agar.

Interorganelle signaling is a determinant of longevity in Saccharomyces cerevisiae

Replicative capacity, which is the number of times an individual cell divides, is the measure of longevity in the yeast Saccharomyces cerevisiae. In this study, a process that involves signaling from the mitochondrion to the nucleus, called retrograde regulation, is shown to determine yeast longevity, and its induction resulted in postponed senescence. Activation of retrograde regulation, by genetic and environmental means, correlated with increased replicative capacity in four different S. cerevisiae strains. Deletion of a gene required for the retrograde response, RTG2, eliminated the increased replicative capacity. RAS2, a gene previously shown to influence longevity in yeast, interacts with retrograde regulation in setting yeast longevity. The molecular mechanism of aging elucidated here parallels the results of genetic studies of aging in nematodes and fruit flies, as well as the caloric restriction paradigm in mammals, and it underscores the importance of metabolic regulation in aging, suggesting a general applicability.

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.

Respiration and low cAMP-dependent protein kinase activity are required for high-level expression of the peroxisomal thiolase gene in Saccharomyces cerevisiae

Transcription of genes for peroxisomal proteins is repressed by glucose and induced by oleate. At least for the peroxisomal thiolase gene (POT1) there is a third regulatory mechanism, mediated by the transcription factor Adr1p, which is responsible for the high-level expression of the gene in stationary phase. Here we show that a region in the POT1 promoter that extends from positions -238 to -152 mediates this mechanism, and we suggest that Adr1p acts indirectly on POT1. We have also analyzed the role of the cAMP-dependent protein kinase (PKA) in the transcriptional regulation of POT1. PKA exerts a negative control: the high, unregulated PKA activity in a bcy1 mutant maintains POT1 transcription at the repressed level. In a ras2 mutant, which has low PKA activity, glucose repression is not alleviated but in non-repressing conditions POT1 regulation is perturbed and expression prematurely increases during exponential phase. This suggests that the PKA signalling pathway controls the regulation of POT1 in stationary phase. Finally, we have found that Adr1p-dependent expression in stationary phase and induction by oleate are both abolished when respiration is blocked. Utilization of fatty acids as carbon source requires respiration. Our result points to the existence of mechanisms that co-ordinate the level of expression of thiolase and the functional state of the mitochondria.

Redundant control of Ultrabithorax by zeste involves functional levels of zeste protein binding at the Ultrabithorax promoter

Many biological processes appear to be controlled by functionally redundant genes or pathways, but it has proven difficult to understand the nature of this redundancy. Here, we have analyzed a redundant regulatory interaction between the Drosophila transcription factor zeste and the homeotic gene Ultrabithorax. Mutations in zeste do not affect the cis-regulation of the endogenous Ultrabithorax gene; however, the expression of small Ultrabithorax promoter constructs is strongly dependent upon zeste. We show that this difference is due to redundant cis-regulatory elements in the Ultrabithorax gene, which presumably contain binding sites for factors that share the function of zeste. We also provide evidence suggesting that zeste and the gene encoding the GAGA factor have an overlapping function in regulating Ultrabithorax. Furthermore, we show that the zeste protein is bound at equal levels in vivo to a Ultrabithorax promoter construct, which zeste strongly activates, and to the identical promoter region in the endogenous Ultrabithorax gene, which zeste redundantly regulates. These results suggest that zeste is significantly active in the wild-type animal and not simply a factor that is induced as a back-up when other activators fail.

Differential activation of yeast adenylyl cyclase by Ras1 and Ras2 depends on the conserved N terminus

Although both Ras1 and Ras2 activate adenylyl cyclase in yeast, a number of differences can be observed regarding their function in the cAMP pathway. To explore the relative contribution of conserved and variable domains in determining these differences, chimeric RAS1-RAS2 or RAS2-RAS1 genes were constructed by swapping the sequences encoding the variable C-terminal domains. These constructs were expressed in a cdc25ts ras1 ras2 strain. Biochemical data show that the difference in efficacy of adenylyl cyclase activation between the two Ras proteins resides in the highly conserved N-terminal domain. This finding is supported by the observation that Ras2 delta, in which the C-terminal domain of Ras2 has been deleted, is a more potent activator of the yeast adenylyl cyclase than Ras1 delta, in which the C-terminal domain of Ras1 has been deleted. These observations suggest that amino acid residues other than the highly conserved residues of the effector domain within the N terminus may determine the efficiency of functional interaction with adenylyl cyclase. Similar levels of intracellular cAMP were found in Ras1, Ras1-Ras2, Ras1 delta, Ras2, and Ras2-Ras1 strains throughout the growth curve. This was found to result from the higher expression of Ras1 and Ras1-Ras2, which compensate for their lower efficacy in activating adenylyl cyclase. These results suggest that the difference between the Ras1 and the Ras2 phenotype is not due to their different efficacy in activating the cAMP pathway and that the divergent C-terminal domains are responsible for these differences, through interaction with other regulatory elements.

Response of a yeast glycogen synthase gene to stress

In the yeast Saccharomyces cerevisiae, glycogen synthase is encoded by two genes: GSY1 and GSY2. The activity of the enzymes increases as cultures enter the stationary phase of growth. Using a GSY2::lacZ fusion gene, we have demonstrated that the increase in glycogen synthase activity resulted, at least in part, from an increase in the level of the protein rather than simply from a change in its phosphorylation state. Northern blot analysis showed a parallel increase in the level of the GSY2 mRNA, which is consistent with transcriptional activation of GSY2. Deletion analysis identified three regions upstream of GSY2 which are involved in GSY2 expression: regions A (-390 to -347 relative to the start of translation), B (-252 to -209) and C (-209 to -167). Region A or C independently activated expression of GSY2. In contrast, region B alone yielded only modest expression. Expression of GSY2 is induced by growth to stationary phase, heat shock or nitrogen starvation. Response to these stressors is mediated by elements within regions A and C. These elements appear to be related to the stress-response elements found in other stress-responsive genes.

Two distinct regions of Ras participate in functional interaction with GDP-GTP exchangers

We have previously implemented a combined genetic/biochemical approach, for analysis of insertion-deletion mutants, to identify sites of Harvey-Ras participating in the interaction with guanine nucleotide exchangers, using the yeast Cdc25 as a model exchanger. We showed that positions 101-106 may be required for catalyzed exchange. We here present a further improved strategy to define more precisely the residues on Ras participating in this interaction. Non-conservative replacements at positions 103 or 105 abolished response to Cdc25 while substitutions at positions 102 or 104 were partially affected. The same substitutions had no effect on coupling to adenylyl cyclase. Since the strategy enables us to assess Ras functional interaction with both the exchanger and effector simultaneously, we have also examined the effect of substitutions in the distal part of the switch II region (amino acids 69-78). In contrast to other reports, substitutions at positions 69 or 73 prevented Cdc25 response while mutations at position 74 did not prevent this interaction. However, all these substitutions partly affected cyclase activation. These findings establish the crucial role of the 102-105 region in the catalyzed exchange reaction and suggest that the 69-74 area would be required for the functional interaction with both exchangers and effector molecules.

Concerted action of RAS and G proteins in the sexual response pathways of Schizosaccharomyces pombe

We have shown that the expression of mam2, the gene encoding the Schizosaccharomyces pombe P-factor pheromone receptor, is dependent upon components of the pheromone signal transduction pathway, including Ras1, Gpa1, Byr1 and Byr2, each of which is required for both conjugation and sporulation. Studies of the expression of mam2 in mutant S. pombe cells confirm previous conclusions, based on the ability of cells to sporulate, that the Byr1 protein kinase acts downstream of the Byr2 protein kinase and that both act downstream of Ras1, the S. pombe RAS homolog, and Gpa1, the G alpha component that mediates the occupancy of the mam2 receptor. In addition, our present studies show that Ras1 and Gpa1 each act downstream from the other and hence act in concert. The Spk1 kinase, which is required for conjugation and sporulation and which is a structural and functional homolog of the vertebrate MAP kinases, is not required for mam2 expression.

Concerted action of RAS and G proteins in the sexual response pathways of Schizosaccharomyces pombe

We have shown that the expression of mam2, the gene encoding the Schizosaccharomyces pombe P-factor pheromone receptor, is dependent upon components of the pheromone signal transduction pathway, including Ras1, Gpa1, Byr1 and Byr2, each of which is required for both conjugation and sporulation. Studies of the expression of mam2 in mutant S. pombe cells confirm previous conclusions, based on the ability of cells to sporulate, that the Byr1 protein kinase acts downstream of the Byr2 protein kinase and that both act downstream of Ras1, the S. pombe RAS homolog, and Gpa1, the G alpha component that mediates the occupancy of the mam2 receptor. In addition, our present studies show that Ras1 and Gpa1 each act downstream from the other and hence act in concert. The Spk1 kinase, which is required for conjugation and sporulation and which is a structural and functional homolog of the vertebrate MAP kinases, is not required for mam2 expression.

Schizosaccharomyces pombe Spk1 is a tyrosine-phosphorylated protein functionally related to Xenopus mitogen-activated protein kinase

Mitogen-activated protein kinase (MAPK) and its direct activator, MAPK kinase (MAPKK), have been suggested to play a pivotal role in a variety of signal transduction pathways in higher eukaryotes. The fission yeast Schizosaccharomyces pombe carries a gene, named spk1, whose product is structurally related to vertebrate MAPK. Here we show that Spk1 is functionally related to Xenopus MAPK. (i) Xenopus MAPK partially complemented a defect in the spk1- mutant. An spk1- diploid strain could not sporulate, but one carrying Xenopus MAPK could. (ii) Both Spk1 and Xenopus MAPK interfered with sporulation if overexpressed in S. pombe cells. (iii) Spk1 underwent tyrosine phosphorylation as does Xenopus MAPK. Tyrosine phosphorylation of Spk1 appeared to be dependent upon mating signals because it occurred in homothallic cells but not in heterothallic cells. Furthermore, this phosphorylation was diminished in a byr1 disruptant strain, suggesting that spk1 lies downstream of byr1, which encodes a MAPKK homolog in S. pombe. Taken together, the MAPKK-MAPK cascade may be evolutionarily conserved in signaling pathways in yeasts and vertebrates.

Schizosaccharomyces pombe Spk1 is a tyrosine-phosphorylated protein functionally related to Xenopus mitogen-activated protein kinase

Mitogen-activated protein kinase (MAPK) and its direct activator, MAPK kinase (MAPKK), have been suggested to play a pivotal role in a variety of signal transduction pathways in higher eukaryotes. The fission yeast Schizosaccharomyces pombe carries a gene, named spk1, whose product is structurally related to vertebrate MAPK. Here we show that Spk1 is functionally related to Xenopus MAPK. (i) Xenopus MAPK partially complemented a defect in the spk1- mutant. An spk1- diploid strain could not sporulate, but one carrying Xenopus MAPK could. (ii) Both Spk1 and Xenopus MAPK interfered with sporulation if overexpressed in S. pombe cells. (iii) Spk1 underwent tyrosine phosphorylation as does Xenopus MAPK. Tyrosine phosphorylation of Spk1 appeared to be dependent upon mating signals because it occurred in homothallic cells but not in heterothallic cells. Furthermore, this phosphorylation was diminished in a byr1 disruptant strain, suggesting that spk1 lies downstream of byr1, which encodes a MAPKK homolog in S. pombe. Taken together, the MAPKK-MAPK cascade may be evolutionarily conserved in signaling pathways in yeasts and vertebrates.

Synthetic enhancement in gene interaction: a genetic tool come of age

Synthetic enhancement, in which one mutation exacerbates the severity of another, is a genetic interaction of increasing importance in biology. This review focuses on the logic of interpreting synthetic enhancement, and begins by comparing the phenomenon with genetic suppression.

Genetic evidence for in vivo cross-specificity of the CaaX-box protein prenyltransferases farnesyltransferase and geranylgeranyltransferase-I in Saccharomyces cerevisiae

Two protein prenyltransferase enzymes, farnesyltransferase (FTase) and geranylgeranyltransferase-I (GGTase-I), catalyze the covalent attachment of a farnesyl or geranylgeranyl lipid group to the cysteine of a CaaX sequence (cysteine [C], two aliphatic amino acids [aa], and any amino acid [X]. In vitro studies reported here confirm previous reports that CaaX proteins with a C-terminal serine are farnesylated by FTase and those with a C-terminal leucine are geranylgeranylated by GGTase-I. In addition, we found that FTase can farnesylate CaaX proteins with a C-terminal leucine and can transfer a geranylgeranyl group to some CaaX proteins. Genetic data indicate that FTase and GGTase-I have the same substrate preferences in vivo as in vitro and also show that each enzyme can prenylate some of the preferred substrates of the other enzyme in vivo. Specifically, the viability of yeast cells lacking FTase is due to prenylation of Ras proteins by GGTase-I. Although this GGTase-I dependent prenylation of Ras is sufficient for growth, it is not sufficient for mutationally activated Ras proteins to exert deleterious effects on growth. The dependence of the activated Ras phenotype on FTase can be bypassed by replacing the C-terminal serine with leucine. This altered form of Ras appears to be prenylated by both GGTase-I and FTase, since it produces an activated phenotype in a strain lacking either FTase or GGTase-I. Yeast cells can grow in the absence of GGTase-I as long as two essential substrates are overexpressed, but their growth is slow. Such strains are dependent on FTase for viability and are able to grow faster when FTase is overproduced, suggesting that FTase can prenylate the essential substrates of GGTase-I when they are overproduced.

Genetic evidence for in vivo cross-specificity of the CaaX-box protein prenyltransferases farnesyltransferase and geranylgeranyltransferase-I in Saccharomyces cerevisiae

Two protein prenyltransferase enzymes, farnesyltransferase (FTase) and geranylgeranyltransferase-I (GGTase-I), catalyze the covalent attachment of a farnesyl or geranylgeranyl lipid group to the cysteine of a CaaX sequence (cysteine [C], two aliphatic amino acids [aa], and any amino acid [X]. In vitro studies reported here confirm previous reports that CaaX proteins with a C-terminal serine are farnesylated by FTase and those with a C-terminal leucine are geranylgeranylated by GGTase-I. In addition, we found that FTase can farnesylate CaaX proteins with a C-terminal leucine and can transfer a geranylgeranyl group to some CaaX proteins. Genetic data indicate that FTase and GGTase-I have the same substrate preferences in vivo as in vitro and also show that each enzyme can prenylate some of the preferred substrates of the other enzyme in vivo. Specifically, the viability of yeast cells lacking FTase is due to prenylation of Ras proteins by GGTase-I. Although this GGTase-I dependent prenylation of Ras is sufficient for growth, it is not sufficient for mutationally activated Ras proteins to exert deleterious effects on growth. The dependence of the activated Ras phenotype on FTase can be bypassed by replacing the C-terminal serine with leucine. This altered form of Ras appears to be prenylated by both GGTase-I and FTase, since it produces an activated phenotype in a strain lacking either FTase or GGTase-I. Yeast cells can grow in the absence of GGTase-I as long as two essential substrates are overexpressed, but their growth is slow. Such strains are dependent on FTase for viability and are able to grow faster when FTase is overproduced, suggesting that FTase can prenylate the essential substrates of GGTase-I when they are overproduced.

Residues crucial for Ras interaction with GDP-GTP exchangers

Cdc25 is essential for Ras-mediated activation of adenylyl cyclase in the yeast Saccharomyces cerevisiae. This protein acts by catalyzing GDP-GTP exchange on yeast Ras. Harvey (Ha) ras expressed in S. cerevisiae is also recognized by both Cdc25 and Sdc25, a yeast homolog of Cdc25. Thus it is feasible to examine molecular aspects of mammalian Ras modulation by Cdc25 using the RAS/cAMP pathway in yeast as a model system. Here, we describe mutational analysis of Ha-ras for the identification of residues critical for the ability of Ras to interact with Cdc25 and related guanine nucleotide-release proteins. Mutations within codons 97-108 impaired Ras-mediated activation of adenylyl cyclase in the presence but not in the absence of mammalian GTPase-activating protein. Such mutations, therefore, affected the ability of Ras to undergo GDP-GTP exchange catalyzed by the guanine nucleotide exchanger without preventing Ras activation of the effector. Similar mutations were previously shown to impair the ability of c-ras to transform mammalian cells while having a less drastic effect on v-ras.

Activation of adenylate cyclase in cdc25 mutants of Saccharomyces cerevisiae

The activation of adenylate cyclase by guanine nucleotides and 6-deoxyglucose was studied in membrane preparations from S. cerevisiae mutants lacking the CDC25 gene product. Adenylate cyclase from cdc25 ts membranes was activated by GTP and GppNHp in membranes from cells collected after glucose was exhausted from the medium. The activation was also observed in membranes from repressed cells at 2.5 mM Mg2+. It is also shown that 6-deoxyglucose can activate adenylate cyclase in the absence of CDC25 gene product. The relative amount of membrane-bound adenylate cyclase was drastically reduced in cdc25 ts membranes when subjected to the restrictive temperature, while no significant change was observed in the wild type. These data suggest that Cdc25 might not be required in certain conditions for the guanine nucleotide exchange reaction in Ras and that it might be implicated in anchoring the Ras/adenylate cyclase system to the plasma membrane.

A yeast GTPase-activating protein that interacts specifically with a member of the Ypt/Rab family

Members of the Ras superfamily of GTP-binding proteins are involved in a variety of cellular processes, including signal transduction, cytoskeletal organization and protein transport. GTP-binding proteins of the Ypt/Rab family direct vesicular protein transport in the secretory and endocytic pathways in the yeast Saccharomyces cerevisiae (Ypt proteins) and in mammalian systems (Rab proteins). The cellular activity of monomeric GTP-binding proteins is influenced by proteins that regulate GDP/GTP exchange and GTP hydrolysis. GTPase-activating proteins (GAPs) can increase the slow intrinsic GTPase activity of GTP-binding proteins by several orders of magnitude. As GAPs modulate the activity of GTP-binding proteins, they are thought to give a biochemical handle on the functioning of Ypt/Rab proteins in transport vesicle budding and docking or fusion at donor and acceptor membranes. We report here the first cloned GTPase-activating protein for the Ypt/Rab protein family. The gene, GYP6 (GAP of Ypt6 protein), encodes a protein of 458 amino acids which is highly specific for the Ypt6 protein and shows little or no cross-reactivity with other Ypt/Rab family members or with H-Ras p21.

The RAS-adenylate cyclase pathway and cell cycle control in Saccharomyces cerevisiae

The cell cycle of Saccharomyces cerevisiae contains a decision point in G1 called 'start', which is composed of two specific sites. Nutrient-starved cells arrest at the first site while pheromone-treated cells arrest at the second site. Functioning of the RAS-adenylate cyclase pathway is required for progression over the nutrient-starvation site while overactivation of the pathway renders the cells unable to arrest at this site. However, progression of cycling cells over the nutrient-starvation site does not appear to be triggered by the RAS-adenylate cyclase pathway in response to a specific stimulus, such as an exogenous nutrient. The essential function of the pathway appears to be limited to provision of a basal level of cAMP. cAMP-dependent protein kinase rather than cAMP might be the universal integrator of nutrient availability in yeast. On the other hand stimulation of the pathway in glucose-derepressed yeast cells by rapidly-fermented sugars, such as glucose, is well documented and might play a role in the control of the transition from gluconeogenic growth to fermentative growth. The initial trigger of this signalling pathway is proposed to reside in a 'glucose sensing complex' which has both a function in controlling the influx of glucose into the cell and in activating in addition to the RAS-adenylate cyclase pathway all other glucose-induced regulatory pathways in yeast. Two crucial problems remaining to be solved with respect to cell cycle control are the nature of the connection between the RAS-adenylate cyclase pathway and nitrogen-source induced progression over the nutrient-starvation site of 'start' and second the nature of the downstream processes linking the RAS-adenylate cyclase pathway to Cyclin/CDC28 controlled progression over the pheromone site of 'start'.

zeste, a nonessential gene, potently activates Ultrabithorax transcription in the Drosophila embryo

The GAGA, NTF-1, and zeste proteins have been purified previously from Drosophila embryo extracts and shown to activate the Ultrabithorax (Ubx) promoter in vitro. Here, differently mutated Ubx-promoter constructs containing binding sites for none, one, or all three of these transcription factors have been introduced into Drosophila by P-element transformation. Binding sites for each factor activate dramatically different patterns of transcription. In zeste mutant embryos, the activation by zeste protein-binding sites is essentially abolished. These genetic data, when considered with our earlier biochemical experiments, demonstrate that zeste directly and potently activates Ubx transcription in vivo. Surprisingly, previous genetic experiments indicate that zeste is a nonessential gene shown only to act in a dispensable regulatory process termed transvection. In our transgenic experiments, zeste is not activating transcription by transvection. We propose that the function of zeste in Drosophila is much broader than assumed previously, and that it is a member of a redundant system of transcription factors that regulate and maintain the expression of Ubx and other Drosophila genes.

Subtype determination of Drosophila embryonic external sensory organs by redundant homeo box genes BarH1 and BarH2

BarH1 and BarH2 are two closely related homeo box genes that form a small complex at the Bar locus on the X chromosome of Drosophila. By immunostaining, we showed that BarH1 and BarH2 proteins are coexpressed in cells belonging to the central and peripheral nervous systems in embryos. In external sensory (es) organs, their expression was particularly apparent in thecogens (glial cells) and neurons at late development. Although deletion of BarH2 caused no appreciable morphological change in es organs, the simultaneous deletion of BarH1 and BarH2 led to a homeotic change in these organs with consequent conversion from campaniform-like sensilla to trichoid sensilla. In contrast, the overexpression of either BarH1 or BarH2 resulted in opposite morphological change. It would thus follow that BarH1 and BarH2 are a pair of redundant homeo box genes required for the subtype specification of es organs.

Unipolar cell divisions in the yeast S. cerevisiae lead to filamentous growth: regulation by starvation and RAS

Diploid S. cerevisiae strains undergo a dimorphic transition that involves changes in cell shape and the pattern of cell division and results in invasive filamentous growth in response to starvation for nitrogen. Cells become long and thin and form pseudohyphae that grow away from the colony and invade the agar medium. Pseudohyphal growth allows yeast cells to forage for nutrients. Pseudohyphal growth requires the polar budding pattern of a/alpha diploid cells; haploid axially budding cells of identical genotype cannot undergo this dimorphic transition. Constitutive activation of RAS2 or mutation of SHR3, a gene required for amino acid uptake, enhance the pseudohyphal phenotype; a dominant mutation in RSR1/BUD1 that causes random budding suppresses pseudohyphal growth.

The plasma membrane ferrireductase activity of Saccharomyces cerevisiae is partially controlled by cyclic AMP

The plasma-membrane-bound ferrireductase activity of ras1 and ras2 mutants of Saccharomyces cerevisiae is not induced in response to iron limitation. This phenotype was suppressed by the bcy1 mutation in ras2 but not in ras1 mutants. The cellular haem content of ras-1-bearing strains decreased dramatically when cells were grown in semi-synthetic medium (low yeast extract content), which could account for their very low ferrireductase activity. The ferrireductase activity of cdc25 and cdc35 mutants dropped when the cells were shifted to a non-permissive temperature. This drop was prevented in the double mutant cdc35 sra5 by adding cyclic AMP to the growth medium. We propose that ferrireductase activity is under the control of a cyclic AMP-dependent protein phosphorylation.

TFS1: a suppressor of cdc25 mutations in Saccharomyces cerevisiae

The TFS1 gene of Saccharomyces cerevisiae is a dosage-dependent suppressor of cdc25 mutations. Overexpression of TFS1 does not alleviate defects of temperature-sensitive adenylyl cyclase (cdc35) or ras2 disruption mutations. The ability of TFS1 to suppress cdc25 is allele specific: the temperature-sensitive cdc25-1 mutation is suppressed efficiently but the cdc25-5 mutation and two disruption mutations are only partially suppressed. TFS1 maps to a previously undefined locus on chromosome XII between RDN1 and CDC42. The DNA sequence of TFS1 contains a single long open reading frame encoding a 219 amino acid polypeptide that is similar in sequence to two mammalian brain proteins. Insertion and deletion mutations in TFS1 are haploviable, indicating that TFS1 is not essential for growth.

Glycogen hyperaccumulation in Saccharomyces cerevisiae ras2 mutant. A biochemical study

The mechanism by which yeast ras2 mutant hyperaccumulates glycogen has been investigated. Total glycogen synthase activity was between 2.5 and 1.3 times higher in the ras2 mutant than in an isogenic strain. In addition, while in the normal strain the glycogen synthase activation state decreased along the exponential phase, in the mutant strain the opposite behaviour was observed: glycogen synthase activation state rose continuously reaching full activation at the beginning of the stationary phase. Glycogen phosphorylase a activity was up to 40 times higher in the mutant than in the normal strain. Glucose 6-phosphate and fructose 2,6-bisphosphate levels were slightly more elevated in the mutants. The increase in total glycogen synthase and, particularly, the full activation of this enzyme may explain glycogen hyperaccumulation in the ras2 mutant even in the presence of elevated levels of glycogen phosphorylase a.

Identification of a GTPase-activating protein homolog in Schizosaccharomyces pombe

Loss of function of the Schizosaccharomyces pombe gap1 gene results in the same phenotypes as those caused by an activated ras1 mutation, i.e., hypersensitivity to the mating factor and inability to perform efficient mating. Sequence analysis of gap1 indicates that it encodes a homolog of the mammalian Ras GTPase-activating protein (GAP). The predicted gap1 gene product has 766 amino acids with relatively short N- and C-terminal regions flanking the conserved core sequence of GAP. Genetic analysis suggests that S. pombe Gap1 functions primarily as a negative regulator of Ras1, like S. cerevisiae GAP homologs encoded by IRA1 and IRA2, but is unlikely to be a downstream effector of the Ras protein, a role proposed for mammalian GAP. Thus, Gap1 and Ste6, a putative GDP-GTP-exchanging protein for Ras1 previously identified, appear to play antagonistic roles in the Ras-GTPase cycle in S. pombe. Furthermore, we suggest that this Ras-GTPase cycle involves the ra12 gene product, another positive regulator of Ras1 whose homologs have not been identified in other organisms, which could function either as a second GDP-GTP-exchanging protein or as a factor that negatively regulates Gap1 activity.