Multiple protein tyrosine phosphatase-encoding genes in the yeast Saccharomyces cerevisiae

Distinct and redundant roles of protein tyrosine phosphatases Ptp1 and Ptp2 in governing the differentiation and pathogenicity of Cryptococcus neoformans

Protein tyrosine phosphatases (PTPs) serve as key negative-feedback regulators of mitogen-activated protein kinase (MAPK) signaling cascades. However, their roles and regulatory mechanisms in human fungal pathogens remain elusive. In this study, we characterized the functions of two PTPs, Ptp1 and Ptp2, in Cryptococcus neoformans, which causes fatal meningoencephalitis. PTP1 and PTP2 were found to be stress-inducible genes, which were controlled by the MAPK Hog1 and the transcription factor Atf1. Ptp2 suppressed the hyperphosphorylation of Hog1 and was involved in mediating vegetative growth, sexual differentiation, stress responses, antifungal drug resistance, and virulence factor regulation through the negative-feedback loop of the HOG pathway. In contrast, Ptp1 was not essential for Hog1 regulation, despite its Hog1-dependent induction. However, in the absence of Ptp2, Ptp1 served as a complementary PTP to control some stress responses. In differentiation, Ptp1 acted as a negative regulator, but in a Hog1- and Cpk1-independent manner. Additionally, Ptp1 and Ptp2 localized to the cytosol but were enriched in the nucleus during the stress response, affecting the transient nuclear localization of Hog1. Finally, Ptp1 and Ptp2 played minor and major roles, respectively, in the virulence of C. neoformans. Taken together, our data suggested that PTPs could be exploited as novel antifungal targets.

Formylchromone derivatives as a novel class of protein tyrosine phosphatase 1B inhibitors

Formylchromone inhibits a human protein tyrosine phosphatase PTP1B with a IC(50) value of 73 microM. The chemical reactivity of formylchromone was adjusted by substitution at various positions of the formylchromone skeleton. In an initial assessment of the structure-activity relationship, the most potent inhibitor showed an IC(50) of 4.3 microM against PTP1B and strong or medium selectivity against other human PTPases, LAR and TC-PTP. This compound, however, was not selective against microbial PTPases, YPTP1 and YOP. The potency and selectivity of the formylchromone derivatives expecting further improvements provides a novel pharmacophore for the design of drugs for the treatment of type 2 diabetes and obesity.

A series of double disruptants for protein phosphatase genes in Saccharomyces cerevisiae and their phenotypic analysis

Thirty-two protein phosphatase (PPase) genes were identified in Saccharomyces cerevisiae based on the nucleotide sequences of the entire genome. In an effort to understand the role of PPases and their functional redundancy in the cellular physiology of one of the reference eukaryotic organisms, a series of single and double PPase gene disruptants were constructed in the W303 strain background. Two single disruptants for the CDC14 and GLC7 genes were lethal. Double disruptants for 30 non-essential PPase genes were constructed in all possible 435 combinations. No double disruptant showed synthetic lethality. Several phenotypes of the viable 30 single and 435 double disruptants were examined; temperature-sensitive growth, utilization of carbon sources and sensitivity to cations and drugs. Four double disruptants exhibited synthetic phenotypes in addition to eight single ones: the pph21 pph22 double disruptant showed slow growth on complete medium, as did the sit4 and yvh1 single ones. In addition to the ptc1, ynr022c and ycr079w single disruptants, the ppz1 ppz2 double disruptant showed temperature-sensitive slow growth. The msg5 ptp2 double disruptant, like the ynr022c single one, did not grow on complete medium containing 0.3 M CaCl(2). The double msg5 ptc2 disruptant failed to grow on medium containing 1.0 M NaCl and, like the ynr022c single deletion, also could not grow on medium containing 0.3 M CaCl(2). The synthetic phenotypes in the two latter cases where each of the PPases is categorized in a different phosphatase family led us to discuss the novel mechanism involved in the functional redundancy of the PPases.

MAP kinase pathways in the yeast Saccharomyces cerevisiae

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

Two protein-tyrosine phosphatases inactivate the osmotic stress response pathway in yeast by targeting the mitogen-activated protein kinase, Hog1

Protein phosphatases inactivate mitogen-activated protein kinase (MAPK) signaling pathways by dephosphorylating components of the MAPK cascade. Two genes encoding protein-tyrosine phosphatases, PTP2, and a new phosphatase, PTP3, have been isolated in a genetic selection for negative regulators of an osmotic stress response pathway called HOG, for high osmolarity glycerol, in budding yeast. PTP2 and PTP3 were isolated as multicopy suppressors of a severe growth defect due to hyperactivation of the HOG pathway. Phosphatase activity is required for suppression since mutation of the catalytic Cys residue in Ptp2 and Ptp3, destroys suppressor function and biochemical activity. The substrate of these phosphatases is likely to be the MAPK, Hog1. Catalytically inactive Ptp2 and Ptp3 coprecipitate with Hog1 from yeast extracts. In addition, strains lacking PTP2 and PTP3 do not dephosphorylate Hog1-phosphotyrosine as well as wild type. The latter suggests that PTP2 and PTP3 play a role in adaptation. Consistent with this role, osmotic stress induces expression of PTP2 and PTP3 transcripts in a Hog1-dependent manner. Thus Ptp2 and Ptp3 likely act in a negative feedback loop to inactivate Hog1.

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.

Yeast protein serine/threonine phosphatases: multiple roles and diverse regulation

Since the isolation of the first yeast protein phosphatase genes in 1989, much progress has been made in understanding this important group of proteins. Yeast contain genes encoding all the major types of protein phosphatase found in higher eukaryotes and the ability to use genetic approaches will complement the wealth of biochemical information available from other systems. This review will summarize recent progress in understanding the structure, function and regulation of the PPP family of protein serine-threonine phosphatases, concentrating on the budding yeast Saccharomyces cerevisiae.

The role of the Src homology-2 domain in the lethal effect of Src expression in the yeast Saccharomyces cerevisiae

Expression of the retroviral transforming gene v-src arrests the proliferation of the yeast Saccharomyces cerevisiae. A functional Src SH2 (Src homology 2) domain is required for this arrest. To examine the mechanism by which Src blocks yeast cell proliferation, and to determine the role of the Src SH2 domain in the growth arrest, src variants were expressed in yeast under the control of the galactose-inducible GAL1 promoter. Following galactose induction of Src expression, phosphotyrosyl-proteins were isolated by immunoprecipitation with beads coupled to either anti-phosphotyrosine antibody or to a recombinant fusion protein containing the Src SH2 domain. A group of SH2-binding phosphotyrosyl proteins was detected in cells expressing toxic forms of Src, but were not detected in cells expressing non-toxic variants. This group of phosphotyrosyl-proteins represents a minor subset of the proteins phosphorylated by v-Src. The lethality of v-Src and the phosphorylation of SH2-binding proteins were co-ordinately affected by alterations in phosphotyrosine-phosphatase activity. These observations indicate that the lethality of Src is correlated with the phosphorylation of proteins that bind to the Src SH2 domain. The role of the SH2 domain in determining the lethal effects of Src in yeast may be similar to its role in targeting Src to substrates necessary for its biological effects in vertebrate cells.

Regulation of the Saccharomyces cerevisiae HOG1 mitogen-activated protein kinase by the PTP2 and PTP3 protein tyrosine phosphatases

In response to increases in extracellular osmolarity, Saccharomyces cerevisiae activates the HOG1 mitogen-activated protein kinase (MAPK) cascade, which is composed of a pair of redundant MAPK kinase kinases, namely, Ssk2p and Ssk22p, the MAPK kinase Pbs2p, and the MAPK Hog1p. Hog1p is activated by Pbs2p through phosphorylation of specific threonine and tyrosine residues. Activated Hog1p is essential for survival of yeast cells at high osmolarity. However, expression of constitutively active mutant kinases, such as those encoded by SSK2deltaN and PBS2(DD), is toxic and results in a lethal level of Hog1p activation. Overexpression of the protein tyrosine phosphatase Ptp2p suppresses the lethality of these mutations by dephosphorylating Hog1p. A catalytically inactive Cys-to-Ser Ptp2p mutant (Ptp2(C/S)p) is tightly bound to tyrosine-phosphorylated Hog1p in vivo. Disruption of PTP2 leads to elevated levels of tyrosine-phosphorylated Hog1p following exposure of cells to high osmolarity. Disruption of both PTP2 and another protein tyrosine phosphatase gene, PTP3, results in constitutive Hog1p tyrosine phosphorylation even in the absence of increased osmolarity. Thus, Ptp2p and Ptp3p are the major phosphatases responsible for the tyrosine dephosphorylation of Hog1p. When catalytically inactive Hog1(K/N)p is expressed in hog1delta cells, it is constitutively tyrosine phosphorylated. In contrast, Hog1(K/N)p, expressed together with wild-type Hog1p, is tyrosine phosphorylated only when cells are exposed to high osmolarity. Thus, the kinase activity of Hog1p is required for its own tyrosine dephosphorylation. Northern blot analyses suggest that Hog1p regulates Ptp2p and/or Ptp3p activity at the posttranscriptional level.

Yeast protein serine/threonine phosphatases: multiple roles and diverse regulation

Since the isolation of the first yeast protein phosphatase genes in 1989, much progress has been made in understanding this important group of proteins. Yeast contain genes encoding all the major types of protein phosphatase found in higher eukaryotes and the ability to use genetic approaches will complement the wealth of biochemical information available from other systems. This review will summarize recent progress in understanding the structure, function and regulation of the PPP family of protein serine-threonine phosphatases, concentrating on the budding yeast Saccharomyces cerevisiae.

The S. cerevisiae nitrogen starvation-induced Yvh1p and Ptp2p phosphatases play a role in control of sporulation

Starvation for nitrogen in the absence of a fermentable carbon source causes diploid Saccharomyces cerevisiae cells to leave vegetative growth, enter meiosis, and sporulare; the former nutritional condition also induces expression of the YVH1 gene that encodes a protein phosphatase. This correlation prompted us to determine whether the Yvh1p phosphatase was a participant in the network that controls the onset of meiosis and sporulation. We found that expression of the IME2 gene, encoding a protein kinase homologue required for meiosis- and sporulation-specific gene expression, is decreased in a yvh1 disrupted strain. We also observed a decrease, albeit a smaller one, in the expression of IME1 which encodes an activator protein required for IME2 expression. Under identical experimental conditions, expression of the MCKI and IME4 genes (which promote sporulation but do not require Ime1p for expression) was not affected. These results demonstrate the specificity of the yvh1 disruption phenotype. They suggest that decreased steady-state levels of IME1 and IME2 mRNA were not merely the result of non-specific adverse affects on nucleic acid metabolism caused by the yvh1 disruption. Sporulation of a homozygous yvh1 disruption mutant was delayed and less efficient overall compared to an isogenic wild-type strain, a result which correlates with decreased IME1 and IME2 gene expression. We also observed that expression of the PTP2 tyrosine phosphatase gene (a negative regulator of the osmosensing MAP kinase cascade), but not the PTP1 gene (also encoding a tyrosine phosphatase) was induced by nitrogen-starvation. Although disruption of PTP2 alone did not demonstrably affect sporulation or IME2 gene expression, sporulation was decreased more in a yvh1, ptp2 double mutant than in a yvh1 single mutant; it was nearly abolished in the double mutant. These data suggest that the YVH1 and PTP2 encoded phosphatases likely participate in the control network regulating meiosis and sporulation. Expression of YVH1 and PTP2 was not affected by nitrogen source quality (asparagine compared to proline) suggesting that nitrogen starvation-induced YVH1 and PTP2 expression and sensitivity to nitrogen catabolite repression are on two different branches of the nitrogen regulatory network.

The yeast immunophilin Fpr3 is a physiological substrate of the tyrosine-specific phosphoprotein phosphatase Ptp1

The tyrosine-specific phosphoprotein phosphatase encoded by the Saccharomyces cerevisiae PTP1 gene dephosphorylates artificial substrates in vitro, but little is known about its functions and substrates in vivo. The presence of Ptp1 resulted in dephosphorylation of multiple tyrosine-phosphorylated proteins in yeast expressing a heterologous tyrosine-specific protein kinase, indicating that Ptp1 can dephosphorylate a broad range of substrates in vivo. Correspondingly, several proteins phosphorylated at tyrosine by endogenous protein kinases exhibited a marked increase in tyrosine phosphorylation in ptp1 mutant cells. One of these phosphotyrosyl proteins (p70) was also dephosphorylated in vitro when incubated with recombinant Ptp1. p70 was purified to homogeneity; analysis of four tryptic peptides revealed that p70 is identical to the recently described FPR3 gene product, a nucleolarly localized proline rotamase of the FK506- and rapamycin-binding family. The identity of p70 with Fpr3 was confirmed in the demonstration that the abundance of tyrosine-phosphorylated p70 in ptp1 mutants was strictly correlated with the level of FPR3 expression; immobilized phosphotyrosyl Fpr3 was directly dephosphorylated by recombinant Ptp1. Site-directed mutagenesis demonstrated that the site of tyrosine phosphorylation is Tyr-184, which resides within the nucleolin-like amino-terminal domain of Fpr3. Protein kinase activities from yeast cell extracts can bind to and phosphorylate the immobilized amino-terminal domain of Fpr3 on serine, threonine, and tyrosine. Fpr3 represents the first phosphotyrosyl protein identified in S. cerevisiae that is not itself a protein kinase and is as yet the only known physiological substrate of Ptp1.

Cloning and characterization of a Saccharomyces cerevisiae gene encoding the low molecular weight protein-tyrosine phosphatase

The low molecular weight protein-tyrosine phosphatase (low M(r) PTPase) is an 18-kDa cytoplasmic enzyme of unknown function that has been previously found in several vertebrates. Using an oligonucleotide probe derived from the active site sequence of the mammalian low M(r) PTPases, a Saccharomyces cerevisiae gene that encodes a homolog of this enzyme was cloned by low stringency hybridization. This gene, LTP1, together with a neighboring gene, TKL1, is shown to be located on the right arm of chromosome XVI. The deduced amino acid sequence of its 161-amino acid residue product shows a 39% average identity with that of the mammalian enzymes. The yeast Ltp1 protein was expressed in Escherichia coli, purified to homogeneity, and shown to possess PTPase activity. The recombinant Ltp1 efficiently hydrolyzes phosphotyrosine and a phosphotyrosine-containing peptide, Tyr531-fyn, but it shows low activity toward phosphoserine and phosphothreonine. The catalytic activity of Ltp1 toward a number of substrates was approximately 30-fold lower than the corresponding values measured for the bovine low M(r) PTPase. However, the yeast enzyme was markedly activated by adenine and some purine nucleosides and nucleotides, including cAMP and cGMP. In the case of adenine, the activity of Ltp1 was increased by approximately 30-fold. The high degree of evolutionary conservation of the low M(r) PTPases implies a significant role for this enzyme. However, neither the disruption of the LTP1 gene nor an approximately 10-fold overexpression of its product in S. cerevisiae caused any apparent phenotypic changes under the conditions tested. No proteins related to Ltp1 could be detected in extracts of the ltp1 null mutant, either by immunoblotting or by gel-filtration analysis accompanied by extended kinetic assays, consistent with the conclusion that LTP1 is the only low M(r) PTPase-encoding gene in S. cerevisiae.

Regulation by protein-tyrosine phosphatase PTP2 is distinct from that by PTP1 during Dictyostelium growth and development

We have cloned a gene encoding a second Dictyostelium discoideum protein-tyrosine phosphatase (PTP2) whose catalytic domain has approximately 30 to 39% amino acid identity with those of other PTPs and a 41% amino acid identity with D. discoideum PTP1. Like PTP1, PTP2 is a nonreceptor PTP with the catalytic domain located at the C terminus of the protein. PTP2 has a predicted molecular weight of 43,000 and possesses an acidic 58-amino-acid insertion 24 amino acids from the N terminus of the conserved catalytic domain. PTP2 transcripts are expressed at moderate levels in vegetative cells and are induced severalfold at the onset of development. Studies with a PTP2-lacZ reporter gene fusion indicate that PTP2, like PTP1, is preferentially expressed in prestalk and anterior-like cell types during the multicellular stages of development. PTP2 gene disruptants (ptp2 null cells) are not detectably altered in growth and show a temporal pattern of development similar to that of wild-type cells. ptp2 null slugs and fruiting bodies, however, are significantly larger than those of wild-type slugs, suggesting a role for PTP2 in regulating multicellular structures. D. discoideum strains overexpressing PTP2 from the PTP2 promoter exhibit growth rate and developmental abnormalities, the severity of which corresponds to the level of PTP2 overexpression. Strains with high overexpression of the PTP2 gene grow slowly on bacterial lawns and produce small cells in axenic medium. When development is initiated in these strains, cells are able to aggregate but then stop further morphogenesis for 6 to 8 h, after which time a variable fraction of these aggregates continue with normal timing, producing diminutive fruiting bodies. These disruption and overexpression phenotypes for PTP2 are distinct from the corresponding mutant PTP1 phenotypes. Immunoprobing PTP2 mutant strains during growth and development with antiphosphotyrosine antibodies reveals several changes in the tyrosine phosphorylation of proteins in PTP2 mutant strains compared with that in wild-type cells. These changes are different from those identified in the previously characterized corresponding PTP1 disruption and overexpression mutant strains. Thus, although PTP2 and PTP1 are nonreceptor PTPs with similar spatial patterns of expression, our findings suggest that they possess distinct regulatory functions in controlling D. discoideum growth and development.

Regulation by protein-tyrosine phosphatase PTP2 is distinct from that by PTP1 during Dictyostelium growth and development

We have cloned a gene encoding a second Dictyostelium discoideum protein-tyrosine phosphatase (PTP2) whose catalytic domain has approximately 30 to 39% amino acid identity with those of other PTPs and a 41% amino acid identity with D. discoideum PTP1. Like PTP1, PTP2 is a nonreceptor PTP with the catalytic domain located at the C terminus of the protein. PTP2 has a predicted molecular weight of 43,000 and possesses an acidic 58-amino-acid insertion 24 amino acids from the N terminus of the conserved catalytic domain. PTP2 transcripts are expressed at moderate levels in vegetative cells and are induced severalfold at the onset of development. Studies with a PTP2-lacZ reporter gene fusion indicate that PTP2, like PTP1, is preferentially expressed in prestalk and anterior-like cell types during the multicellular stages of development. PTP2 gene disruptants (ptp2 null cells) are not detectably altered in growth and show a temporal pattern of development similar to that of wild-type cells. ptp2 null slugs and fruiting bodies, however, are significantly larger than those of wild-type slugs, suggesting a role for PTP2 in regulating multicellular structures. D. discoideum strains overexpressing PTP2 from the PTP2 promoter exhibit growth rate and developmental abnormalities, the severity of which corresponds to the level of PTP2 overexpression. Strains with high overexpression of the PTP2 gene grow slowly on bacterial lawns and produce small cells in axenic medium. When development is initiated in these strains, cells are able to aggregate but then stop further morphogenesis for 6 to 8 h, after which time a variable fraction of these aggregates continue with normal timing, producing diminutive fruiting bodies. These disruption and overexpression phenotypes for PTP2 are distinct from the corresponding mutant PTP1 phenotypes. Immunoprobing PTP2 mutant strains during growth and development with antiphosphotyrosine antibodies reveals several changes in the tyrosine phosphorylation of proteins in PTP2 mutant strains compared with that in wild-type cells. These changes are different from those identified in the previously characterized corresponding PTP1 disruption and overexpression mutant strains. Thus, although PTP2 and PTP1 are nonreceptor PTPs with similar spatial patterns of expression, our findings suggest that they possess distinct regulatory functions in controlling D. discoideum growth and development.

A two-component system that regulates an osmosensing MAP kinase cascade in yeast

In the prokaryotic two-component signal transduction systems, recognition of an environmental stimulus by a sensor molecule results in the activation of its histidine kinase domain and phosphorylation of a histidine residue within that domain. This phosphate group is then transferred to an aspartate residue in the receiver domain of a cognate response regulator molecule, resulting in the activation of its output function. Although a few eukaryotic proteins were identified recently that show sequence similarity to the prokaryotic sensors or response regulators, it has not been clear whether they constituted a part of a 'two-component' system. Here we describe a two-component system in Saccharomyces cerevisiae that regulates an osmosensing MAP kinase cascade.

Aberrant protein phosphorylation at tyrosine is responsible for the growth-inhibitory action of pp60v-src expressed in the yeast Saccharomyces cerevisiae

Expression of pp60v-src, the transforming protein of Rous sarcoma virus, arrests the growth of the yeast Saccharomyces cerevisiae. To determine the basis of this growth arrest, yeast strains were constructed that expressed either wild-type v-src or various mutant v-src genes under the control of the galactose-inducible, glucose repressible GAL1 promoter. When shifted to galactose medium, cells expressing wild-type v-src ceased growth immediately and lost viability, whereas cells expressing a catalytically inactive mutant (K295M) continued to grow normally, indicating that the kinase activity of pp60v-src is required for its growth inhibitory effect. Mutants of v-src altered in the SH2/SH3 domain (XD4, XD6, SPX1, and SHX13) and a mutant lacking a functional N-terminal myristoylation signal (MM4) caused only a partial inhibition of growth, indicating that complete growth inhibition requires either targeting of the active kinase or binding of the kinase to phosphorylated substrates, or both. Cells arrested by v-src expression displayed aberrant microtubule structures, alterations in DNA content and elevated p34CDC28 kinase activity. Immunoblotting with antiphosphotyrosine antibody showed that many yeast proteins, including the p34CDC28 kinase, became phosphorylated at tyrosine in cells expressing v-src. Both the growth inhibition and the tyrosine-specific protein phosphorylation observed following v-src expression were reversed by co-expression of a mammalian phosphotyrosine-specific phosphoprotein phosphatase (PTP1B). However a v-src mutant with a small insertion in the catalytic domain (SRX5) had the same lethal effect as wild-type v-src, yet induced only very low levels of protein-tyrosine phosphorylation. These results indicate that inappropriate phosphorylation at tyrosine is the primary cause of the lethal effect of pp60v-src expression but suggest that only a limited subset of the phosphorylated proteins are involved in this effect.

Mutations in a protein tyrosine phosphatase gene (PTP2) and a protein serine/threonine phosphatase gene (PTC1) cause a synthetic growth defect in Saccharomyces cerevisiae

Two protein tyrosine phosphatase genes, PTP1 and PTP2, are known in Saccharomyces cerevisiae. However, the functions of these tyrosine phosphatases are unknown, because mutations in either or both phosphatase genes have no clear phenotypic effects. In this report, we demonstrate that although ptp2 has no obvious phenotype by itself, it has a profound effect on cell growth when combined with mutations in a novel protein phosphatase gene. Using a colony color sectoring assay, we isolated 25 mutants in which the expression of PTP1 or PTP2 is required for growth. Complementation tests of the mutants showed that they have a mutation in one of three genes. Cloning and sequence determination of one of these gene, PTC1, indicated that it encodes a homolog of the mammalian protein serine/threonine phosphatase 2C (PP2C). The amino acid sequence of the PTC1 product is approximately 35% identical to PP2C. Disruption of PTC1 indicated that the PTC1 function is nonessential. In contrast, ptc1 ptp2 double mutants showed a marked growth defect. To examine whether PTC1 encodes an active protein phosphatase, a glutathione S-transferase (GST)-PTC1 fusion gene was constructed and expressed in Escherichia coli. Purified GST-PTC1 fusion protein hydrolyzed a serine phosphorylated substrate in the presence of the divalent cation Mg2+ or Mn2+. GST-PTC1 also had weak (approximately 0.5% of its serine phosphatase activity) protein tyrosine phosphatase activity.

Mutations in a protein tyrosine phosphatase gene (PTP2) and a protein serine/threonine phosphatase gene (PTC1) cause a synthetic growth defect in Saccharomyces cerevisiae

Two protein tyrosine phosphatase genes, PTP1 and PTP2, are known in Saccharomyces cerevisiae. However, the functions of these tyrosine phosphatases are unknown, because mutations in either or both phosphatase genes have no clear phenotypic effects. In this report, we demonstrate that although ptp2 has no obvious phenotype by itself, it has a profound effect on cell growth when combined with mutations in a novel protein phosphatase gene. Using a colony color sectoring assay, we isolated 25 mutants in which the expression of PTP1 or PTP2 is required for growth. Complementation tests of the mutants showed that they have a mutation in one of three genes. Cloning and sequence determination of one of these gene, PTC1, indicated that it encodes a homolog of the mammalian protein serine/threonine phosphatase 2C (PP2C). The amino acid sequence of the PTC1 product is approximately 35% identical to PP2C. Disruption of PTC1 indicated that the PTC1 function is nonessential. In contrast, ptc1 ptp2 double mutants showed a marked growth defect. To examine whether PTC1 encodes an active protein phosphatase, a glutathione S-transferase (GST)-PTC1 fusion gene was constructed and expressed in Escherichia coli. Purified GST-PTC1 fusion protein hydrolyzed a serine phosphorylated substrate in the presence of the divalent cation Mg2+ or Mn2+. GST-PTC1 also had weak (approximately 0.5% of its serine phosphatase activity) protein tyrosine phosphatase activity.

Architecture of a yeast U6 RNA gene promoter

The promoters of vertebrate and yeast U6 small nuclear RNA genes are structurally dissimilar, although both are recognized by RNA polymerase III. Vertebrate U6 RNA genes have exclusively upstream promoters, while the U6 RNA gene from the yeast Saccharomyces cerevisiae (SNR6) has internal and downstream promoter elements that match the tRNA gene intragenic A- and B-block elements, respectively. Substitution of the SNR6 A or B block greatly diminished U6 RNA accumulation in vivo, and a subcellular extract competent for RNA polymerase III transcription generated nearly identical DNase I protection patterns over the SNR6 downstream B block and a tRNA gene intragenic B block. We conclude that the SNR6 promoter is functionally similar to tRNA gene promoters, although the effects of extragenic deletion mutations suggest that the downstream location of the SNR6 B block imposes unique positional constraints on its function. Both vertebrate and yeast U6 RNA genes have an upstream TATA box element not normally found in tRNA genes. Substitution of the SNR6 TATA box altered the site of transcription initiation in vivo, while substitution of sequences further upstream had no effect on SNR6 transcription. We present a model for the SNR6 transcription complex that explains these results in terms of their effects on the binding of transcription initiation factor TFIIIB.

Architecture of a yeast U6 RNA gene promoter

The promoters of vertebrate and yeast U6 small nuclear RNA genes are structurally dissimilar, although both are recognized by RNA polymerase III. Vertebrate U6 RNA genes have exclusively upstream promoters, while the U6 RNA gene from the yeast Saccharomyces cerevisiae (SNR6) has internal and downstream promoter elements that match the tRNA gene intragenic A- and B-block elements, respectively. Substitution of the SNR6 A or B block greatly diminished U6 RNA accumulation in vivo, and a subcellular extract competent for RNA polymerase III transcription generated nearly identical DNase I protection patterns over the SNR6 downstream B block and a tRNA gene intragenic B block. We conclude that the SNR6 promoter is functionally similar to tRNA gene promoters, although the effects of extragenic deletion mutations suggest that the downstream location of the SNR6 B block imposes unique positional constraints on its function. Both vertebrate and yeast U6 RNA genes have an upstream TATA box element not normally found in tRNA genes. Substitution of the SNR6 TATA box altered the site of transcription initiation in vivo, while substitution of sequences further upstream had no effect on SNR6 transcription. We present a model for the SNR6 transcription complex that explains these results in terms of their effects on the binding of transcription initiation factor TFIIIB.