Protein phosphatase 2C, encoded by ptc1+, is important in the heat shock response of Schizosaccharomyces pombe

A phospho-switch at Acinus-Serine437 controls autophagic responses to Cadmium exposure and neurodegenerative stress

Neuronal health depends on quality control functions of autophagy, but mechanisms regulating neuronal autophagy are poorly understood. Previously, we showed that in Drosophila starvation-independent quality control autophagy is regulated by acinus (acn) and the Cdk5-dependent phosphorylation of its serine⁴³⁷ (Nandi et al., 2017). Here, we identify the phosphatase that counterbalances this activity and provides for the dynamic nature of acinus-serine⁴³⁷ (acn-S437) phosphorylation. A genetic screen identified six phosphatases that genetically interacted with an acn gain-of-function model. Among these, loss of function of only one, the PPM-type phosphatase Nil (CG6036), enhanced pS437-acn levels. Cdk5-dependent phosphorylation of acn-S437 in nil¹ animals elevates neuronal autophagy and reduces the accumulation of polyQ proteins in a Drosophila Huntington’s disease model. Consistent with previous findings that Cd²⁺ inhibits PPM-type phosphatases, Cd²⁺ exposure elevated acn-S437 phosphorylation which was necessary for increased neuronal autophagy and protection against Cd²⁺-induced cytotoxicity. Together, our data establish the acn-S437 phosphoswitch as critical integrator of multiple stress signals regulating neuronal autophagy.

Direct and indirect regulation of Pom1 cell size pathway by the protein phosphatase 2C Ptc1

The fission yeast cells Schizosaccharomyces pombe divide at constant cell size regulated by environmental stimuli. An important pathway of cell size control involves the membrane-associated DYRK-family kinase Pom1, which forms decreasing concentration gradients from cell poles and inhibits mitotic inducers at midcell. Here, we identify the phosphatase 2C Ptc1 as negative regulator of Pom1. Ptc1 localizes to cell poles in a manner dependent on polarity and cell-wall integrity factors. We show that Ptc1 directly binds Pom1 and can dephosphorylate it in vitro but modulates Pom1 localization indirectly upon growth in low-glucose conditions by influencing microtubule stability. Thus, Ptc1 phosphatase plays both direct and indirect roles in the Pom1 cell size control pathway.

Type 2C protein phosphatases in fungi

Type 2C Ser/Thr phosphatases are a remarkable class of protein phosphatases, which are conserved in eukaryotes and involved in a large variety of functional processes. Unlike in other Ser/Thr phosphatases, the catalytic polypeptide is not usually associated with regulatory subunits, and functional specificity is achieved by encoding multiple isoforms. For fungi, most information comes from the study of type 2C protein phosphatase (PP2C) enzymes in Saccharomyces cerevisiae, where seven PP2C-encoding genes (PTC1 to -7) with diverse functions can be found. More recently, data on several Candida albicans PP2C proteins became available, suggesting that some of them can be involved in virulence. In this work we review the available literature on fungal PP2Cs and explore sequence databases to provide a comprehensive overview of these enzymes in fungi.

Dealing with misfolded proteins: examining the neuroprotective role of molecular chaperones in neurodegeneration

Human neurodegenerative diseases arise from a wide array of genetic and environmental factors. Despite the diversity in etiology, many of these diseases are considered "conformational" in nature, characterized by the accumulation of pathological, misfolded proteins. These misfolded proteins can induce cellular stress by overloading the proteolytic machinery, ultimately resulting in the accumulation and deposition of aggregated protein species that are cytotoxic. Misfolded proteins may also form aberrant, non-physiological protein-protein interactions leading to the sequestration of other normal proteins essential for cellular functions. The progression of such disease may therefore be viewed as a failure of normal protein homeostasis, a process that involves a network of molecules regulating the synthesis, folding, translocation and clearance of proteins. Molecular chaperones are highly conserved proteins involved in the folding of nascent proteins, and the repair of proteins that have lost their typical conformations. These functions have therefore made molecular chaperones an active area of investigation within the field of conformational diseases. This review will discuss the role of molecular chaperones in neurodegenerative diseases, highlighting their functional classification, regulation, and therapeutic potential for such diseases.

Atf1 is a target of the mitogen-activated protein kinase Pmk1 and regulates cell integrity in fission yeast

In fission yeast, knockout of the calcineurin gene resulted in hypersensitivity to Cl(-), and the overexpression of pmp1(+) encoding a dual-specificity phosphatase for Pmk1 mitogen-activated protein kinase (MAPK) or the knockout of the components of the Pmk1 pathway complemented the Cl(-) hypersensitivity of calcineurin deletion. Here, we showed that the overexpression of ptc1(+) and ptc3(+), both encoding type 2C protein phosphatase (PP2C), previously known to inactivate the Wis1-Spc1-Atf1 stress-activated MAPK signaling pathway, suppressed the Cl(-) hypersensitivity of calcineurin deletion. We also demonstrated that the mRNA levels of these two PP2Cs and pyp2(+), another negative regulator of Spc1, are dependent on Pmk1. Notably, the deletion of Atf1, but not that of Spc1, displayed hypersensitivity to the cell wall-damaging agents and also suppressed the Cl(-) hypersensitivity of calcineurin deletion, both of which are characteristic phenotypes shared by the mutation of the components of the Pmk1 MAPK pathway. Moreover, micafungin treatment induced Pmk1 hyperactivation that resulted in Atf1 hyperphosphorylation. Together, our results suggest that PP2C is involved in a negative feedback loop of the Pmk1 signaling, and results also demonstrate that Atf1 is a key component of the cell integrity signaling downstream of Pmk1 MAPK.

Expression of plant protein phosphatase 2C interferes with nuclear import of the Agrobacterium T-complex protein VirD2

Agrobacterium tumefaciens transfers DNA to plant cells as a single-stranded DNA molecule (the T-strand) covalently linked to VirD2 protein. VirD2 contains nuclear localization signal sequences that presumably help direct the T-strand to the plant nucleus. We identified a tomato cDNA clone, DIG3, that encodes a protein that interacts with the C-terminal region of VirD2. DIG3 encodes an enzymatically active type 2C serine/threonine protein phosphatase. Overexpression of DIG3 in tobacco BY-2 protoplasts inhibited nuclear import of a beta-glucuronidase-VirD2 nuclear localization signal fusion protein. Thus, DIG3 may be involved in nuclear import of the VirD2 protein and, consequently, the VirD2/transferred DNA complex.

Cytoplasmic localization of Wis1 MAPKK by nuclear export signal is important for nuclear targeting of Spc1/Sty1 MAPK in fission yeast

Mitogen-activated protein kinase (MAPK) cascade is a ubiquitous signaling module that transmits extracellular stimuli through the cytoplasm to the nucleus; in response to activating stimuli, MAPKs translocate into the nucleus. Mammalian MEK MAPK kinases (MAPKKs) have in their N termini an MAPK-docking site and a nuclear export signal (NES) sequence, which are known to play critical roles in maintaining ERK MAPKs in the cytoplasm of unstimulated cells. Herein, we show that the Wis1 MAPKK of the stress-activated Spc1 MAPK cascade in fission yeast also has a MAPK-docking site and an NES sequence in its N-terminal domain. Unexpectedly, an inactivating mutation to the NES of chromosomal wis1(+) does not affect the subcellular localization of Spc1 MAPK, whereas this NES mutation disturbs the cytoplasmic localization of Wis1. However, when Wis1 is targeted to the nucleus by fusing to a nuclear localization signal sequence, stress-induced nuclear translocation of Spc1 is abrogated, indicating that cytoplasmic Wis1 is required for nuclear transport of Spc1 upon stress. Moreover, we have observed that a fraction of Wis1 translocates into the nucleus in response to stress. These results suggest that cytoplasmic localization of Wis1 MAPKK by its NES is important for stress signaling to the nucleus.

Temporal and mechanistic effects of heat shock on LPS-mediated degradation of IkappaBalpha in macrophages

Previous studies demonstrated important interactions between the heat shock response and the IkappaBalpha/NF-kappaB pathway when these two pathways are induced sequentially. One such interaction involves the ability of heat shock to inhibit subsequent degradation of IkappaBalpha in response to a proinflammatory signal. Herein we investigated the temporal relationship between recovery from heat shock and inhibition of IkappaBalpha degradation, and the proximal mechanisms by which heat shock inhibits degradation of IkappaBalpha in macrophages. In RAW 264.7 murine macrophages, prior heat shock inhibited LPS-mediated IkappaBalpha degradation up to 4 h after recovery from heat shock, and this effect correlated with inhibition of LPS-mediated activation of NF-kappaB. Beyond these recovery periods, heat shock did not inhibit IkappaBalpha degradation. IkappaB kinase (IKK) assays demonstrated that heat shock inhibited LPS-mediated activation of IKK up to 1 h after recovery from heat shock. Heat shock also increased intracellular phosphatase activity, and inhibition of intracellular phosphatase activity partially reversed the ability of heat shock to inhibit both LPS-mediated degradation of IkappaBalpha and LPS-mediated activation of IKK. These data demonstrate that the ability of heat shock to inhibit degradation of IkappaBalpha is dependent on the recovery period between the heat shock stimulus and the proinflammatory stimulus. The mechanism by which heat shock inhibits degradation of IkappaBalpha involves dual modulation of IKK and intracellular phosphatase activity.

Dephosphorylation of cyclin-dependent kinases by type 2C protein phosphatases

Activating phosphorylation of cyclin-dependent protein kinases (CDKs) is necessary for their kinase activity and cell cycle progression. This phosphorylation is carried out by the Cdk-activating kinase (CAK); in contrast, little is known about the corresponding protein phosphatase. We show that type 2C protein phosphatases (PP2Cs) are responsible for this dephosphorylation of Cdc28p, the major budding yeast CDK. Two yeast PP2Cs, Ptc2p and Ptc3p, display Cdc28p phosphatase activity in vitro and in vivo, and account for approximately 90% of Cdc28p phosphatase activity in yeast extracts. Overexpression of PTC2 or PTC3 results in synthetic lethality in strains temperature-sensitive for yeast CAK1, and disruptions of PTC2 and PTC3 suppress the growth defect of a cak1 mutant. Furthermore, PP2C-like enzymes are the predominant phosphatases toward human Cdk2 in HeLa cell extracts, indicating that the substrate specificity of PP2Cs toward CDKs is evolutionarily conserved.

Vacuole fusion regulated by protein phosphatase 2C in fission yeast

The gene ptc4+ encodes one of four type 2C protein phosphatases (PP2C) in the fission yeast Schizosaccharomyces pombe. Deletion of ptc4+ is not lethal; however, Deltaptc4 cells grow slowly in defined minimal medium and undergo premature growth arrest in response to nitrogen starvation. Interestingly, Deltaptc4 cells are unable to fuse vacuoles in response to hypotonic stress or nutrient starvation. Conversely, Ptc4 overexpression appears to induce vacuole fusion. These findings reveal a hitherto unrecognized function of type 2C protein phosphatases: regulation of vacuole fusion. Ptc4 localizes in vacuole membranes, which suggests that Ptc4 regulates vacuole fusion by dephosphorylation of one or more proteins in the vacuole membrane. Vacuole function is required for the process of autophagy that is induced by nutrient starvation; thus, the vacuole defect of Deltaptc4 cells might explain why these cells undergo premature growth arrest in response to nitrogen starvation.

Heat-shock-induced activation of stress MAP kinase is regulated by threonine- and tyrosine-specific phosphatases

In eukaryotic species from yeast to human, stress-activated protein kinases (SAPKs), members of a MAP kinase (MAPK) subfamily, regulate the transcriptional response to various environmental stress. It is poorly understood how diverse forms of stress are sensed and transmitted to SAPKs. Here, we report the heat shock regulation of the fission yeast Spc1 SAPK, a homolog of human p38 and budding yeast Hog1p. Although osmostress and oxidative stress induce strong activation of the Wis1 MAPK kinase (MEK), which activates Spc1 through Thr-171/Tyr-173 phosphorylation, activation of Wis1 upon heat shock is relatively weak and transient. However, in heat-shocked cells, Pyp1, the major tyrosine phosphatase that dephosphorylates and inactivates Spc1, is inhibited for its interaction with Spc1, which leads to strong activation of Spc1. Subsequently, Spc1 activity is rapidly attenuated by Thr-171 dephosphorylation, whereas Tyr-173 remains phosphorylated. Thr-171 dephosphorylation is compromised in a strain lacking functional type 2C serine/threonine phosphatases (PP2C), Ptc1 and Ptc3. Moreover, Ptc1 and Ptc3 can dephosphorylate Thr-171 of Spc1 both in vivo and in vitro. These observations strongly suggest that PP2C enzymes play an important role in the attenuation of Spc1 activity in heat-shocked cells. Thus, transient activation of Spc1 upon heat shock is ensured by differential regulation of threonine and tyrosine phosphorylation.

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.

Heat stress activates fission yeast Spc1/StyI MAPK by a MEKK-independent mechanism

Fission yeast Spc1/StyI MAPK is activated by many environmental insults including high osmolarity, oxidative stress, and heat shock. Spc1/StyI is activated by Wis1, a MAPK kinase (MEK), which is itself activated by Wik1/Wak1/Wis4, a MEK kinase (MEKK). Spc1/StyI is inactivated by the tyrosine phosphatases Pyp1 and Pyp2. Inhibition of Pyp1 was recently reported to play a crucial role in the oxidative stress and heat shock responses. These conclusions were based on three findings: 1) osmotic, oxidative, and heat stresses activate Spc1/StyI in wis4 cells; 2) oxidative stress and heat shock activate Spc1/StyI in cells that express Wis1AA, in which MEKK consensus phosphorylation sites were replaced with alanine; and 3) Spc1/StyI is maximally activated in Deltapyp1 cells. Contrary to these findings, we report: 1) Spc1/StyI activation by osmotic stress is greatly reduced in wis4 cells; 2) wis1-AA and Deltawis1 cells have identical phenotypes; and 3) all forms of stress activate Spc1/StyI in Deltapyp1 cells. We also report that heat shock, but not osmotic or oxidative stress, activate Spc1 in wis1-DD cells, which express Wis1 protein that has the MEKK consensus phosphorylation sites replaced with aspartic acid. Thus osmotic and oxidative stress activate Spc1/StyI by a MEKK-dependent process, whereas heat shock activates Spc1/StyI by a novel mechanism that does not require MEKK activation or Pyp1 inhibition.

Mutational analysis of the domain structure of mouse protein phosphatase 2Cbeta

The structures of five distinct isoforms of mammalian protein phosphatase 2Cbeta (PP2Cbeta-1, -2, -3, -4 and -5) have previously been found to differ only at their C-terminal regions. In the present study, we performed mutational analysis of recombinant mouse PP2Cbeta-1 to determine the functional domains of the molecule and elucidate the biochemical significance of the structural differences in the isoforms. Differences in affinity for [32P]phosphohistone but not for [32P]phosphocasein were observed among the five PP2Cbeta isoforms. Deletion of 12 amino acids from the C-terminal end, which form a unique sequence for PP2Cbeta-1, caused a 35% loss of activity against [32P]phosphohistone but no loss of activity against [32P]phosphocasein. Deletion of up to 78 amino acids from this end did not cause any further alteration in activity, whereas deletion of 100 amino acids totally eliminated the activity against both [32P]phosphohistone and [32P]phosphocasein. On the other hand, deletion of 11 amino acids from the N-terminal end caused a 97% loss of enzyme activity, and further deletions caused a total loss of activity. Substitution of any of the six specific amino acids among 16 tested in this study, which were located among the 250 N-terminal residues, caused 98-100% loss of enzyme activity. Among these amino acids, three (Glu-38, -60 and -243) have recently been reported to be essential for the binding of metal ions in the catalytic site of the PP2C molecule [Das, Helps, Cohen and Barford (1996) EMBO J. 15, 6798-6809]. These observations indicate that PP2Cbeta is composed of at least two distinct functional domains, an N-terminal catalytic domain of about 310 amino acids and the remaining C-terminal domain, which is involved in determination of substrate specificity.

Protein serine/threonine phosphatase Ptc2p negatively regulates the unfolded-protein response by dephosphorylating Ire1p kinase

Cells respond to the accumulation of unfolded proteins in the endoplasmic reticulum (ER) by increasing the transcription of the genes encoding ER-resident chaperone proteins. Ire1p is a transmembrane protein kinase that transmits the signal from unfolded proteins in the lumen of the ER by a mechanism that requires oligomerization and trans-autophosphorylation of its cytoplasmic-nucleoplasmic kinase domain. Activation of Ire1p induces a novel spliced form of HAC1 mRNA that produces Hac1p, a transcription factor that is required for activation of the transcription of genes under the control of the unfolded-protein response (UPR) element. Searching for proteins that interact with Ire1p in Saccharomyces cerevisiae, we isolated PTC2, which encodes a serine/threonine phosphatase of type 2C. The Ptc2p interaction with Ire1p is specific, direct, dependent on Ire1p phosphorylation, and mediated through a kinase interaction domain within Ptc2p. Ptc2p dephosphorylates Ire1p efficiently in an Mg2+-dependent manner in vitro. PTC2 is nonessential for growth and negatively regulates the UPR pathway. Strains carrying null alleles of PTC2 have a three- to fourfold-increased UPR and increased levels of spliced HAC1 mRNA. Overexpression of wild-type Ptc2p but not catalytically inactive Ptc2p reduces levels of spliced HAC1 mRNA and attenuates the UPR, demonstrating that the phosphatase activity of Ptc2p is required for regulation of the UPR. These results demonstrate that Ptc2p downregulates the UPR by dephosphorylating Ire1p and reveal a novel mechanism of regulation in the UPR pathway upstream of the HAC1 mRNA splicing event.

MP2C, a plant protein phosphatase 2C, functions as a negative regulator of mitogen-activated protein kinase pathways in yeast and plants

By interference of the yeast pheromone mitogen-activated protein kinase (MAPK) pathway with an alfalfa cDNA expression library, we have isolated the MP2C gene encoding a functional protein phosphatase type 2C. Epistasis analysis in yeast indicated that the molecular target of the MP2C phosphatase is Ste11, a MAPK kinase kinase that is a central regulator of the pheromone and osmosensing pathways. In plants, MP2C functions as a negative regulator of the stress-activated MAPK (SAMK) pathway that is activated by cold, drought, touch, and wounding. Although activation of the SAMK pathway occurs by a posttranslational mechanism, de novo transcription and translation of protein factor(s) are necessary for its inactivation. MP2C is likely to be this or one of these factors, because wound-induced activation of SAMK is followed by MP2C gene expression and recombinant glutathione S-transferase-MP2C is able to inactivate extracts containing wound-induced SAMK. Wound-induced MP2C expression is a transient event and correlates with the refractory period, i.e., the time when restimulation of the SAMK pathway is not possible by a second stimulation. These data suggest that MP2C is part of a negative feedback mechanism that is responsible for resetting the SAMK cascade in plants.

Biochemical characterization and deletion analysis of recombinant human protein phosphatase 2C alpha

The use of protein phosphatase inhibitors has been instrumental in defining the intracellular roles of protein phosphatase 1 (PP1), PP2A and PP2B. Identification of the role of PP2C in vivo has been hampered, in part, by the unavailability of specific inhibitors. In order to facilitate the identification of novel and specific inhibitors of PP2C by random screening of compounds, and to further characterize this enzyme at the molecular level by site-directed mutagenesis and X-ray crystallography, we have expressed active recombinant human PP2C alpha (rPP2C alpha) in Escherichia coli. Biochemical characterization of rPP2C alpha showed that it could hydrolyse p-nitrophenyl phosphate (pNPP) although, in contrast with native PP2C, this was not stimulated by Mg2+. As with native PP2C, okadaic acid failed to inhibit rPP2C alpha, whereas 50 mM NaF dramatically inhibited its activity. An alignment of the amino acid sequence of AMP-activated protein kinase (AMPK) with those of other serine/threonine protein kinases around the regulatory phosphorylation site (subdomains VII-VIII) revealed a high degree of conservation. Phosphopeptides derived from this region of AMPK and containing the almost invariant threonine (Thr172 in AMPK) were found to be good substrates for rPP2C alpha. We also showed that rPP2C alpha can inactivate AMPK, but only in the presence of Mg2+. To define the regions of PP2C alpha important for catalytic activity, we expressed a number of truncated proteins based on the sequence and proposed domain structure of the PP2C alpha homologue from Paramecium tetraurelia. Deletion of 75 residues (9 kDa) from the C-terminus appeared to have little effect on the catalytic activity using pNPP, phosphopeptides or AMPK as substrates. This suggests that the residues important in catalysis lie elsewhere in the protein. A further deletion of the C-terminus led to a completely inactive and very poorly soluble protein.

Schizosaccharomyces pombe skp1+ encodes a protein kinase related to mammalian glycogen synthase kinase 3 and complements a cdc14 cytokinesis mutant

We report the cloning of the skp1+ gene, a Schizosaccharomyces pombe homolog of the glycogen synthase kinase 3 (GSK-3) family whose members in higher eukaryotes are involved in cell fate determination, nuclear signalling, and hormonal regulation. skp1 is 67% identical to mammalian GSK-3 beta and displays similar biochemical properties in vitro. Like GSK-3 beta, skp1 is phosphorylated on a conserved tyrosine residue, and this phosphorylation is required for efficient activity. skp1 is also phosphorylated at a serine which has been identified as S-335. Phosphorylation at this site is likely to inhibit its function. Unlike the mammalian enzyme, skp1 both tyrosine autophosphorylates in yeast cells and can phosphorylate other proteins on tyrosine in bacteria. The skp1+ gene is not essential. However, cells with deletions in skp1+ are sensitive to heat shock and exhibit defects in sporulation. Overexpression of wild-type skp1+ specifically complements cdc14-118, one of several mutations causing a defect in cytokinesis. In addition, certain phosphorylation site mutants induce a delay or block in cytokinesis when overexpressed. Together, these data identify novel interactions of a fission yeast GSK-3 homolog with elements of the cytokinesis machinery.

Sensitivity to abscisic acid of guard-cell K+ channels is suppressed by abi1-1, a mutant Arabidopsis gene encoding a putative protein phosphatase

Abscisic acid (ABA) modulates the activities of three major classes of ion channels--inward- and outward-rectifying K+ channels (IK,in and IK,out, respectively) and anion channels--at the guard-cell plasma membrane to achieve a net efflux of osmotica and stomatal closure. Disruption of ABA sensitivity in wilty abi1-1 mutants of Arabidopsis and evidence that this gene encodes a protein phosphatase suggest that protein (de)-phosphorylation contributes to guard-cell transport control by ABA. To pinpoint the role of ABI1, the abi1-1 dominant mutant allele was stably transformed into Nicotiana benthamiana and its influence on IK,in, IK,out, and the anion channels was monitored in guard cells under voltage clamp. Compared with guard cells from wild-type and vector-transformed control plants, expression of the abi1-1 gene was associated with 2- to 6-fold reductions in IK,out and an insensitivity of both IK,in and IK,out to 20 microM ABA. In contrast, no differences between control and abi1-1 transgenic plants were observed in the anion current or its response to ABA. Parallel measurements of intracellular pH (pHi) using the fluorescent dye 2',7'-bis(2-carboxyethyl)-5-(and -6)-carboxyfluorescein (BCECF) in every case showed a 0.15- to 0.2-pH-unit alkalinization in ABA, demonstrating that the transgene was without effect on the pHi signal that mediates in ABA-evoked K+ channel control. In guard cells from the abi1-1 transformants, normal sensitivity of both K+ channels to and stomatal closure in ABA was recovered in the presence of 100 microM H7 and 0.5 microM staurosporine, both broad-range protein kinase antagonists. These results demonstrate an aberrant K+ channel behavior--including channel insensitivity to ABA-dependent alkalinization of pHi--as a major consequence of abi1-1 action and implicate AB11 as part of a phosphatase/kinase pathway that modulates the sensitivity of guard-cell K+ channels to ABA-evoked signal cascades.

Serine/threonine protein phosphatases

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Cloning of cDNAs from Arabidopsis thaliana that encode putative protein phosphatase 2C and a human Dr1-like protein by transformation of a fission yeast mutant

We characterized three Arabidopsis thaliana cDNA clones that could rescue the sterile phenotype of the Schizosaccharomyces pombe pde1 mutant, which is defective in cAMP phosphodiesterase. The first clone had a coding capacity of 399 amino acids that is 35% identical with rat protein phosphatase 2C (PP2C). The second had a coding capacity of 159 amino acids that is 41% identical with human Dr1. Dr1 has been shown to interact with TATA-binding protein (TBP) and block its ability to activate transcription. The third encoded Arabidopsis TBP itself. Saccharomyces cerevisiae TBP also could suppress the sterile phenotype if expressed in S.pombe pde1 cells, but overexpression of S.pombe TBP could do so very poorly. These observations suggest preliminarily that PP2C may counteract cAMP-dependent protein kinase in fission yeast cells, and that the heterologous TBPs and Dr1 may interfere with the general transcription factors of S.pombe so that the gene expression in the host cell becomes affirmative of sexual development. Furthermore, the identification of a Dr1-like protein in A.thaliana strongly argues for the ubiquity of this protein among eukaryotic genera and for a conserved mechanism to regulate transcription initiation that involves Dr1.