Altered phosphotransfer in an activated mutant of the Saccharomyces cerevisiae two-component osmosensor Sln1p.

The SLN1 two-component signaling pathway of Saccharomyces cerevisiae utilizes a multistep phosphorelay mechanism to control osmotic stress responses via the HOG1 mitogen-activated protein kinase pathway and the transcription factor Skn7p. Sln1p consists of a sensor kinase module that undergoes histidine autophosphorylation and a receiver module that autocatalytically transfers the phosphoryl group from histidine to aspartate. The Sln1p aspartyl phosphate is then transferred to Ypd1p, which in turn transfers the phosphoryl group to a conserved aspartate on one of two response regulators, Ssk1p and Skn7p. Activated alleles of SLN1 (sln1*) were previously identified that appear to increase the level of phosphorylation of downstream targets Ssk1p and Skn7p. In principle, the phenotype of sln1* alleles could arise from an increase in autophosphorylation or phosphotransfer activities or a decrease in an intrinsic or extrinsic dephosphorylation activity. Genetic analysis of the activated mutants has been unable to distinguish between these possibilities. In this report, we address this issue by analyzing phosphorelay and phosphohydrolysis reactions involving the Sln1p-associated receiver. The results are consistent with a model in which the activated phenotype of the sln1* allele, sln-22, arises from a shift in the phosphotransfer equilibrium from Sln1p to Ypd1p, rather than from impaired dephosphorylation of the system in response to osmotic stress.

Essential functions of protein tyrosine phosphatases PTP2 and PTP3 and RIM11 tyrosine phosphorylation in Saccharomyces cerevisiae meiosis and sporulation.

Tyrosine phosphorylation plays a central role in eukaryotic signal transduction. In yeast, MAP kinase pathways are regulated by tyrosine phosphorylation, and it has been speculated that other biochemical processes may also be regulated by tyrosine phosphorylation. Previous genetic and biochemical studies demonstrate that protein tyrosine phosphatases (PTPases) negatively regulate yeast MAP kinases. Here we report that deletion of PTP2 and PTP3 results in a sporulation defect, suggesting that tyrosine phosphorylation is involved in regulation of meiosis and sporulation. Deletion of PTP2 and PTP3 blocks cells at an early stage of sporulation before premeiotic DNA synthesis and induction of meiotic-specific genes. We observed that tyrosine phosphorylation of several proteins, including 52-, 43-, and 42-kDa proteins, was changed in ptp2Deltaptp3Delta homozygous deletion cells under sporulation conditions. The 42-kDa tyrosine-phosphorylated protein was identified as Mck1, which is a member of the GSK3 family of protein kinases and previously known to be phosphorylated on tyrosine. Mutation of MCK1 decreases sporulation efficiency, whereas mutation of RIM11, another GSK3 member, specifically abolishes sporulation; therefore, we investigated regulation of Rim11 by Tyr phosphorylation during sporulation. We demonstrated that Rim11 is phosphorylated on Tyr-199, and the Tyr phosphorylation is essential for its in vivo function, although Rim11 appears not to be directly regulated by Ptp2 and Ptp3. Biochemical characterizations indicate that tyrosine phosphorylation of Rim11 is essential for the activity of Rim11 to phosphorylate substrates. Our data demonstrate important roles of protein tyrosine phosphorylation in meiosis and sporulation

Erf2, a novel gene product that affects the localization and palmitoylation of Ras2 in Saccharomyces cerevisiae.

Plasma membrane localization of Ras requires posttranslational addition of farnesyl and palmitoyl lipid moieties to a C-terminal CaaX motif (C is cysteine, a is any aliphatic residue, X is the carboxy terminal residue). To better understand the relationship between posttranslational processing and the subcellular localization of Ras, a yeast genetic screen was undertaken based on the loss of function of a palmitoylation-dependent RAS2 allele. Mutations were identified in an uncharacterized open reading frame (YLR246w) that we have designated ERF2 and a previously described suppressor of hyperactive Ras, SHR5. ERF2 encodes a 41-kDa protein with four predicted transmembrane (TM) segments and a motif consisting of the amino acids Asp-His-His-Cys (DHHC) within a cysteine-rich domain (CRD), called DHHC-CRD. Mutations within the DHHC-CRD abolish Erf2 function. Subcellular fractionation and immunolocalization experiments reveal that Erf2 tagged with a triply iterated hemagglutinin epitope is an integral membrane protein that colocalizes with the yeast endoplasmic reticulum marker Kar2. Strains lacking ERF2 are viable, but they have a synthetic growth defect in the absence of RAS2 and partially suppress the heat shock sensitivity resulting from expression of the hyperactive RAS2(V19) allele. Ras2 proteins expressed in an erf2Delta strain have a reduced level of palmitoylation and are partially mislocalized to the vacuole. Based on these observations, we propose that Erf2 is a component of a previously uncharacterized Ras subcellular localization pathway. Putative members of an Erf2 family of proteins have been uncovered in yeast, plant, worm, insect, and mammalian genome databases, suggesting that Erf2 plays a role in Ras localization in all eucaryotes.

Antifungal properties and target evaluation of three putative bacterial histidine kinase inhibitors.

Histidine protein kinases have been explored as potential antibacterial drug targets. The recent identification of two-component histidine kinases in fungi has led us to investigate the antifungal properties of three bacterial histidine kinase inhibitors (RWJ-49815, RWJ-49968, and RWJ-61907). All three compounds were found to inhibit growth of the Saccharomyces cerevisiae and Candida albicans strains, with MICs ranging from 1 to 20 microg/ml. However, deletion of SLN1, the only histidine kinase in S. cerevisiae, did not alter drug efficacy. In vitro kinase assays were performed by using the Sln1 histidine kinase purified from bacteria as a fusion protein to glutathione S-transferase. RWJ-49815 and RWJ-49968 inhibited kinase a 50% inhibitory concentration of 10 microM, whereas RWJ-61907 failed to inhibit at concentrations up to 100 microM. Based on these results, we conclude that these compounds have antifungal properties; however, their mode of action appears to be independent of histidine kinase inhibition.

Expression of MFA1 and STE6 is sufficient for mating type-independent secretion of yeast a-factor, but not mating competence.

The yeast a-factor mating peptide and its transporter Ste6 are normally expressed only in MATa haploid cells. The a-factor is initially produced as a 36- or 38-residue peptide precursor and must undergo extensive post-translational processing to produce an active 12 amino-acid lipopeptide. To better understand the steps required for Ste6-dependent a-factor transport, we have reconstituted a-factor synthesis and transport in MATalpha haploids and MATa/alpha diploids. Ste6 and a-factor were stably expressed in MATalpha and MATa/alpha cells and the ectopically expressed a-factor was correctly processed. In addition, Ste6 was able to transport a-factor from all cell types, indicating that once expressed no other MATa-specific functions are required. However, despite significant levels of a-factor secretion, MATalpha cells are unable to support efficient mating.

Intracellular glycerol levels modulate the activity of Sln1p, a Saccharomyces cerevisiae two-component regulator.

The HOG mitogen-activated protein kinase pathway mediates the osmotic stress response in Saccharomyces cerevisiae, activating genes like GPD1 (glycerol phosphate dehydrogenase), required for survival under hyperosmotic conditions. Activity of this pathway is regulated by Sln1p, a homolog of the "two-component" histidine kinase family of signal transduction molecules prominent in bacteria. Sln1p also regulates the activity of a Hog1p-independent pathway whose transcriptional output can be monitored using an Mcm1p-dependent lacZ reporter gene. The relationship between the two Sln1p branches is unclear, however, the requirement for unphosphorylated pathway intermediates in Hog1p pathway activation and for phosphorylated intermediates in the activation of the Mcm1p reporter suggests that the two Sln1p branches are reciprocally regulated. To further investigate the signals and molecules involved in modulating Sln1p activity, we have screened for new mutations that elevate the activity of the Mcm1p-dependent lacZ reporter gene. We find that loss of function mutations in FPS1, a gene encoding the major glycerol transporter in yeast activates the reporter in a SLN1-dependent fashion. We propose that elevated intracellular glycerol levels in the fps1 mutant shift Sln1p to the phosphorylated state and trigger the Sln1-dependent activity of the Mcm1 reporter. These observations are consistent with a model in which Sln1p autophosphorylation is triggered by a hypo-osmotic stimulus and indicate that the Sln1p osmosensor is tied generally to osmotic balance, and may not specifically sense an external osmolyte.

The yeast histidine protein kinase, Sln1p, mediates phosphotransfer to two response regulators, Ssk1p and Skn7p.

The Saccharomyces cerevisiae Sln1 protein is a 'two-component' regulator involved in osmotolerance. Two-component regulators are a family of signal-transduction molecules with histidine kinase activity common in prokaryotes and recently identified in eukaryotes. Phosphorylation of Sln1p inhibits the HOG1 MAP kinase osmosensing pathway via a phosphorelay mechanism including Ypd1p and the response regulator, Ssk1p. SLN1 also activates an MCM1-dependent reporter gene, P-lacZ, but this function is independent of Ssk1p. We present genetic and biochemical evidence that Skn7p is the response regulator for this alternative Sln1p signaling pathway. Thus, the yeast Sln1 phosphorelay is actually more complex than appreciated previously; the Sln1 kinase and Ypd1 phosphorelay intermediate regulate the activity of two distinct response regulators, Ssk1p and Skn7p. The established role of Skn7p in oxidative stress is independent of the conserved receiver domain aspartate, D427. In contrast, we show that Sln1p activation of Skn7p requires phosphorylation of D427. The expression of TRX2, previously shown to exhibit Skn7p-dependent oxidative-stress activation, is also regulated by the SLN1 phosphorelay functions of Skn7p. The identification of genes responsive to both classes of Skn7p function suggests a central role for Skn7p and the SLN1-SKN7 pathway in integrating and coordinating cellular response to various types of environmental stress.

Differential regulation of FUS3 MAP kinase by tyrosine-specific phosphatases PTP2/PTP3 and dual-specificity phosphatase MSG5 in Saccharomyces cerevisiae.

The Saccharomyces cerevisiae mating pheromone response is mediated by activation of a MAP kinase (Fus3p and Kss1p) signaling pathway. Pheromone stimulation causes cell cycle arrest. Therefore, inactivation of the Fus3p and Kss1p MAP kinases is required during recovery phase for the resumption of cell growth. We have isolated a novel protein tyrosine phosphatase gene, PTP3, as a negative regulator of this pathway. Ptp3p directly dephosphorylates and inactivates Fus3p MAP kinase in vitro. Multicopy PTP3 represses pheromone-induced transcription and promotes recovery. In contrast, disruption of PTP3 in combination with its homolog PTP2 results in constitutive tyrosine phosphorylation, enhanced kinase activity of Fus3p MAP kinase on stimulation, and delayed recovery from the cell cycle arrest. Both tyrosine phosphorylation and kinase activity of Fus3p are further increased by disruption of PTP3 and PTP2 in combination with MSG5, which encodes a dual-specificity phosphatase. Cells deleted for all three of the phosphatases (ptp2delta ptp3delta msg5delta) are hypersensitive to pheromone and exhibit a severe defect in recovery from pheromone-induced growth arrest. Our data indicate that Ptp3p is the major phosphatase responsible for tyrosine dephosphorylation of Fus3p to maintain a low basal activity; it also has important roles, along with Msg5p, in inactivation of Fus3p following pheromone stimulation. These data present the first evidence for a coordinated regulation of MAP kinase function through differential actions of protein tyrosine phosphatases and a dual-specificity phosphatase.

Activated alleles of yeast SLN1 increase Mcm1-dependent reporter gene expression and diminish signaling through the Hog1 osmosensing pathway.

Two-component signal transduction systems involving histidine autophosphorylation and phosphotransfer to an aspartate residue on a receiver molecule have only recently been discovered in eukaryotes, although they are well studied in prokaryotes. The Sln1 protein of Saccharomyces cerevisiae is a two-component regulator involved in osmotolerance. Phosphorylation of Sln1p leads to inhibition of the Hog1 mitogen-activated protein kinase osmosensing pathway. We have discovered a second function of Sln1p by identifying recessive activated alleles (designated nrp2) that regulate the essential transcription factor Mcm1. nrp2 alleles cause a 5-fold increase in the activity of an Mcm1-dependent reporter, whereas deletion of SLN1 causes a 10-fold decrease in reporter activity and a corresponding decrease in expression of Mcm1-dependent genes. In addition to activating Mcm1p, nrp2 mutants exhibit reduced phosphorylation of Hog1p and increased osmosensitivity suggesting that nrp2 mutations shift the Sln1p equilibrium toward the phosphorylated state. Two nrp2 mutations map to conserved residues in the receiver domain (P1148S and P1196L) and correspond to residues implicated in bacterial receivers to control receiver phosphorylation state. Thus, it appears that increased Sln1p phosphorylation both stimulates Mcm1p activity and diminishes signaling through the Hog1 osmosensing pathway.

An amino terminal prosequence is required for efficient synthesis of S. cerevisiae a-factor.

The Saccharomyces cerevisiae a-mating pheromones are 12 amino acid lipopeptides whose secretion is dependent on the ABC transporter, Ste6p. The pheromones are synthesized as 36 and 38 amino acid precursors that terminate in a CaaX box (C is Cys, a is an aliphatic residue, and X is the C-terminal amino acid). Posttranslational processing of the a-factor precursors includes at least 5 events. C-terminal processing of the CaaX box includes farnesylation of Cys, removal of the -aaX residues, and methylation of the cysteine alpha-carboxyl group. The N-terminal steps involve proteolytic cleavages that remove the prosequences. In this report, we have investigated the role of posttranslational modification in the generation of functional a-factor. Wild type, mutant and chimeric forms of a-factor have been expressed in yeast and assessed for their abilities to serve as sources of functional a-factor. We have found that although modification of the CaaX box is necessary, it is not sufficient to generate bioactive a-factor. The amino terminal prosequences are also required. Deletion of these sequences reduces intracellular levels of a-factor resulting in sterility. Glutathione-S-transferase (GST)-a-factor fusions undergo CaaX box processing and membrane localization, but are not substrates for the N-terminal proteases and fail to interact with Ste6p. These results suggest that the amino terminal precursor sequences play a direct role in the generation of functional a-factor.

Functional consequence of mutating conserved residues of the yeast farnesyl-protein transferase beta-subunit Ram1(Dpr1).

Ras proteins, fungal mating pheromones, and other proteins terminating in the sequence CaaX (where C is Cys, a is any aliphatic amino acid, and X is the C-terminal residue) are posttranslationally prenylated. Farnesyl-protein transferase (FPTase) transfers the farnesyl moiety of farnesyl pyrophosphate (FPP) to the thiol of the CaaX box cysteine in a reaction that requires Zn2+ and Mg2+. We have created mutations in conserved amino acids of the yeast Ram1 protein to identify residues important for Zn2+-dependent FPTase activity. Wild-type and mutant Ram1 proteins were expressed as operon fusions in bacteria, and FPTase activity was measured. Mutations in conserved residues Glu256, His258, Asp307, Cys309, Asp360, and His363 reduce FPTase activity. Asp307, Cys309, and His363 correspond to the residues that have been shown to coordinate Zn2+ in mammalian FPTase. The H258N mutant enzyme exhibited an increased sensitivity to the Zn2+ chelator 1,10-phenanthroline, required higher concentrations of Zn2+ to restore activity to the apoenzyme, and had a 10-fold reduction in catalytic efficiency. The decreases in FPTase activity observed do not appear to be caused by major structural perturbations because the mutants were stably expressed and retained the ability to interact with Ram2p during purification. The FPTase activity of the mutants measured in vitro correlated well with their ability to complement the mating and growth defects of a ram1Delta strain in vivo.

Farnesylation and proteolysis are sequential, but distinct steps in the CaaX box modification pathway.

Membrane localization of Ras proteins requires posttranslational modification of a conserved C-terminal sequence motif known as the CaaX box (C is Cys, a is any aliphatic amino acid, and X is the carboxyl terminal residue). The modification steps include farnesylation, removal of the three C-terminal amino acids, carboxyl-methylation, and palmitoylation. In yeast, the farnesyltransferase (FTase) is encoded by the RAM1(DPR1) and RAM2 genes, and the methyltransferase is the product of STE14. The gene encoding the protease(s) that is responsible for modification of the CaaX has not been identified. We have used in vitro-synthesized Ras2p and synthetic peptide substrates to investigate the relationship between farnesylation and proteolysis. Addition of yeast cytosolic extracts to rabbit reticulocyte extracts programmed to synthesize Ras2p led to prenylation of Ras2p and a change in electrophoretic mobility similar to that observed during Ras maturation in vivo. However, it was not possible to determine if the mobility shift is the result of prenylation, proteolysis or a combination of both steps. Therefore, we examined the relationship between farnesylation and proteolysis directly using extracts prepared from bacteria overexpressing the genes for the yeast FTase (RAM1 and RAM2) and synthetic CaaX box peptides. Extracts from bacteria expressing RAM1/RAM2 efficiently prenylate CaaX box peptides, but do not proteolyze the -aaX residues. However, addition of yeast extracts from wild type, ram1, or ste14 mutants resulted in the removal of the -aaX residues from prenylated CaaX box peptides.

The essential transcription factor, Mcm1, is a downstream target of Sln1, a yeast "two-component" regulator.

In a search for mutants exhibiting altered activity of the yeast transcription factor, Mcm1, we have identified the SLN1 gene, whose product is highly related to bacterial two-component sensor-regulator proteins. sln1 alleles identified in our screen increased Mcm1p-mediated transcriptional activation, while deletion of the SLN1 locus severely reduced Mcm1p activity. Our data establish that Mcm1p is a downstream target of the Sln1 signaling pathway. Yeast Sln1p was recently shown to be involved in osmoregulation and to depend on the Hog1 MAP kinase (Maeda, T., Wurgler-Murphy, S., and Saito, H. (1994) Nature 369, 242-245). We show that SLN1-mediated regulation of Mcm1p activity is independent of the Hog1 MAP kinase, and suggest that the role of SLN1 is not restricted to osmoregulation.

A polybasic domain allows nonprenylated Ras proteins to function in Saccharomyces cerevisiae.

Ras proteins undergo a series of posttranslational modifications prior to association with the cytoplasmic surface of the plasma membrane. The modification steps include farnesylation, proteolysis, methylesterification, and palmitoylation. A 4-amino acid residue motif known as the CaaX box (C is cysteine, a is generally aliphatic, and X is the carboxyl-terminal residue) is the sequence recognized by the prenyl transferase that initiates the modification pathway. As part of our studies to define the requirements for Ras membrane association, we directed mutagenesis to the yeast Ras2 protein CaaX box to assess the relative importance of prenylation, palmitoylation, and stretches of basic amino acids on the function of the protein. The wild type yeast Ras2 protein terminates in the sequence Cys-Cys-Ile-Ile-Ser. We have identified mutations that do not contain a CaaX box but still encode functional Ras proteins. These mutations replace the terminal serine of the CaaX box with the sequence -Lys-Leu-Ile-Lys-Arg-Lys. Three mutants have been analyzed in detail. Ras2(CCIIKLIKRK) functions at a level similar to wild type Ras2, whereas cells expressing only Ras2(SCIIKLIKRK) and Ras2(SSIIKLIKRK) forms of Ras2 protein grow more slowly at 30 degrees C. In addition, strains expressing only Ras2(SSIIKLIKRK) protein fail to grow at 37 degrees C. Replacement of the basic residues with neutral amino acids (Ras2(CCIISIIS)) completely abolishes their ability to support Ras-dependent growth. The extension mutants are not prenylated, but Ras2(CCIIKLIKRK) and Ras2(SCIIKLIKRK) are palmitoylated. These results demonstrate that a diverse set of carboxyl-terminal sequence motifs and posttranslational modifications lead to functional Ras proteins in yeast.

Vectors for the inducible overexpression of glutathione S-transferase fusion proteins in yeast.

A rapid and convenient method of protein purification involves creating a fusion protein with glutathione S-transferase (GST) (Smith and Johnson, Gene 67, 31-40, 1988). In this report, we describe two vectors for the conditional expression of GST fusions in Saccharomyces cerevisiae. The parent plasmid is based on a high-copy, galactose-inducible shuttle vector previously described (Baldari et al., EMBO J. 6, 229-243, 1987). We have demonstrated the use of this system by creating fusions between GST and the yeast RAS2 gene. GST-Ras2 fusion proteins undergo the post-translational modifications required for Ras2p to become membrane localized. These vectors provide a useful system for the expression and purification of eukaryotic proteins requiring post-translational modification.

Normal mitochondrial structure and genome maintenance in yeast requires the dynamin-like product of the MGM1 gene.

The isolation and characterization of MGM1, an yeast gene with homology to members of the dynamin gene family, is described. The MGM1 gene is located on the right arm of chromosome XV between STE4 and PTP2. Sequence analysis revealed a single open reading frame of 902 residues capable of encoding a protein with an approximate molecular mass of 101 kDa. Loss of MGM1 resulted in slow growth on rich medium, failure to grow on non-fermentable carbon sources, and loss of mitochondrial DNA. The mitochondria also appeared abnormal when visualized with an antibody to a mitochondrial-matrix marker. MGM1 encodes a dynamin-like protein involved in the propagation of functional mitochondria in yeast.

Isolation and characterization of a second protein tyrosine phosphatase gene, PTP2, from Saccharomyces cerevisiae.

A putative protein tyrosine phosphatase (PTPase) gene, PTP2, was cloned from Saccharomyces cerevisiae. The complete yeast PTP2 gene encodes a 750-amino acid residue protein with a predicted mass of 86 kDa. The conserved PTPase domain was localized in the C-terminal half of the protein. Amino acid sequence alignment of the yeast PTPase domain with other phosphatases indicated approximately 20-25% sequence identity with the mammalian PTPase and a similar degree of identity with the PTPase encoded by the yeast PTP1 gene. The PTP2 gene is closely linked to the yeast RET1 and STE4 genes and is localized on the right arm of chromosome 15. Gene disruption experiments demonstrated that neither PTP2 alone nor PTP2 in combination with PTP1 was essential for growth under the conditions tested. The ability of PTP2 to complement the cdc25-22 mutant of Schizosaccharomyces pombe was also examined, and unlike the human T-cell PTPase, which was able to complement the cdc25-22 mutant, the S. cerevisiae PTP2 was unable to complement the cdc25-22 mutant of S. pombe.

Cloning and expression of a yeast protein tyrosine phosphatase.

To study the regulation of tyrosine phosphorylation/dephosphorylation in Saccharomyces cerevisiae, a protein tyrosine phosphatase (PTPase) was cloned by the polymerase chain reaction (PCR). Conserved amino acid sequences within the mammalian PTPases were used to design primers which generated a yeast PCR fragment. The sequence of the PCR fragment encoded a protein with homology to the mammalian PTPases. The PCR fragment was used to identify the yeast PTP1 gene which has an open reading frame encoding a 335-amino acid residue protein. This yeast PTPase shows 26% sequence identity to the rat PTPase, although highly conserved residues within the mammalian enzymes are invariant in the yeast protein. The yeast PTP1 is physicallt linked to the 5'-end of a heat shock gene SSB1. This yeast PTP1 gene was expressed in Escherichia coli and obtained in a highly purified form by a single affinity chromatography step. The recombinant yeast PTPase hydrolyzed phosphotyrosine containing substrates approximately 1000 times faster than a phosphoserine containing substrate. Gene disruption of yeast PTP1 has no visible effect on vegetative growth.