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

A comprehensive overview of PPM1B: From biological functions to diseases

Reversible phosphorylation of proteins is an important mechanism that regulates cellular processes, which are precisely regulated by protein kinases and phosphatases. PPM1B is a metal ion-dependent serine/threonine protein phosphatase, which regulates multiple biological functions by targeting substrate dephosphorylation, such as cell cycle, energy metabolism, inflammatory responses. In this review, we summarized the occurrent understandings of PPM1B focused on its regulation of signaling pathways, related diseases, and small-molecular inhibitors, which may provide new insights for the identification of PPM1B inhibitors and the treatment of PPM1B-related diseases.

The role of PP5 and PP2C in cardiac health and disease

Protein phosphatases are important, for example, as functional antagonists of β-adrenergic stimulation of the mammalian heart. While β-adrenergic stimulations increase the phosphorylation state of regulatory proteins and therefore force of contraction in the heart, these phosphorylations are reversed and thus force is reduced by the activity of protein phosphatases. In this context the role of PP5 and PP2C is starting to unravel. They do not belong to the same family of phosphatases with regard to sequence homology, many similarities with regard to location, activation by lipids and putative substrates have been worked out over the years. We also suggest which pathways for regulation of PP5 and/or PP2C described in other tissues and not yet in the heart might be useful to look for in cardiac tissue. Both phosphatases might play a role in signal transduction of sarcolemmal receptors in the heart. Expression of PP5 and PP2C can be increased by extracellular stimuli in the heart. Because PP5 is overexpressed in failing animal and human hearts, and because overexpression of PP5 or PP2C leads to cardiac hypertrophy and KO of PP5 leads to cardiac hypotrophy, one might argue for a role of PP5 and PP2C in heart failure. Because PP5 and PP2C can reduce, at least in vitro, the phosphorylation state of proteins thought to be relevant for cardiac arrhythmias, a role of these phosphatases for cardiac arrhythmias is also probable. Thus, PP5 and PP2C might be druggable targets to treat important cardiac diseases like heart failure, cardiac hypertrophy and cardiac arrhythmias.

PRKAA/AMPKα phosphorylation switches the role of RASAL2 from a suppressor to an activator of autophagy

RASAL2 (RAS protein activator like 2), a RASGTPase activating protein, can catalyze the hydrolysis of RAS-GTP into RAS-GDP to inactivate the RAS pathway in various types of cancer cells. However, the cellular function of RASAL2 remains elusive. Here we showed that RASAL2 can attenuate PRKAA/AMPKα phosphorylation by recruiting phosphatase PPM1B/pp2cβ, thus inhibiting the initiation of basal autophagy under normal conditions. In addition, we found that glucose starvation could induce dissociation of PPM1B from RASAL2 and then RASAL2 at S351 be phosphorylated by PRKAA, followed by the binding of phosphorylated-RASAL2 with to PIK3C3/VPS34-ATG14-BECN1/Beclin1 complex to increase PIK3C3 activity and autophagy. Furthermore, RASAL2 S351 phosphorylation facilitated breast tumor growth and correlated to poor clinical outcomes in breast cancer patients. Our study demonstrated that the phosphorylation status of RASAL2 S351 can function as a molecular switch to either suppress or promote AMPK-mediated autophagy. Inhibition of RASAL2 S351 phosphorylation might be a potential therapeutic strategy to overcome the resistance of AMPK-activation agents.

Infection Reveals a Modification of SIRT2 Critical for Chromatin Association

Sirtuin 2 is a nicotinamide-adenine-dinucleotide-dependent deacetylase that regulates cell processes such as carcinogenesis, cell cycle, DNA damage, and infection. Subcellular localization of SIRT2 is crucial for its function but is poorly understood. Infection with the bacterial pathogen Listeria monocytogenes, which relocalizes SIRT2 from the cytoplasm to the chromatin, provides an ideal stimulus for the molecular study of this process. In this report, we provide a map of SIRT2 post-translational modification sites and focus on serine 25 phosphorylation. We show that infection specifically induces dephosphorylation of S25, an event essential for SIRT2 chromatin association. Furthermore, we identify a nuclear complex formed by the phosphatases PPM1A and PPM1B, with SIRT2 essential for controlling H3K18 deacetylation and SIRT2-mediated gene repression during infection and necessary for a productive Listeria infection. This study reveals a molecular mechanism regulating SIRT2 function and localization, paving the way for understanding other SIRT2-regulated cellular processes.

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Infection induces modification of the host deacetylase SIRT2 to target it to chromatin

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Dephosphorylation of serine 25 on SIRT2 controls its subcellular localization

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PPM1A and PPM1B form a nuclear complex to dephosphorylate serine 25

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The phosphorylation state of serine 25 is crucial for Listeria infection

NV Proteins of Fish Novirhabdovirus Recruit Cellular PPM1Bb Protein Phosphatase and Antagonize RIG-I-Mediated IFN Induction

Non virion (NV) protein expression is critical for fish Novirhabdovirus, viral hemorrhagic septicemia virus (VHSV) and infectious hematopoietic necrosis virus (IHNV), in vivo pathogenesis. However, the mechanism by which NV promotes the viral replication is still unclear. We developed an approach based on reverse genetics and interactomic and identified several NV-associated cellular partners underlying cellular pathways as potential viral targets. Among these cell partners, we showed that NV proteins specifically interact with a protein phosphatase, Mg²⁺/Mn²⁺-dependent, 1Bb (PPM1Bb) and recruit it in the close vicinity of mitochondria, a subcellular compartment important for retinoic acid-inducible gene-I- (RIG-I)-mediated interferon induction pathway. PPM1B proteins belong to the PP2C family of serine/threonine (Ser/Thr) protein phosphatase and have recently been shown to negatively regulate the host antiviral response via dephosphorylating Traf family member-associated NF-κB activator (TANK)-binding kinase 1 (TBK1). We demonstrated that NV proteins and PPM1Bb counteract RIG-I- and TBK1-dependent interferon (IFN) and IFN-stimulated gene promoter induction in fish cells and, hence, the establishment of an antiviral state. Furthermore, the expression of VHSV NV strongly reduced TBK1 phosphorylation and thus its activation. Our findings provide evidence for a previously undescribed mechanism by which a viral protein recruits PPM1Bb protein phosphatase to subvert innate immune recognition.

Ppm1b negatively regulates necroptosis through dephosphorylating Rip3

The auto-phosphorylation of murine receptor-interacting protein 3 (Rip3) on Thr 231 and Ser 232 in the necrosome is required to trigger necroptosis. However, how Rip3 phosphorylation is regulated is still largely unknown. Here we identified protein phosphatase 1B (Ppm1b) as a Rip3 phosphatase and found that Ppm1b restricts necroptosis in two settings: spontaneous necroptosis caused by Rip3 auto-phosphorylation in resting cells, and tumour necrosis factor-α (TNF)-induced necroptosis in cultured cells. We revealed that Ppm1b selectively suppresses necroptosis through the dephosphorylation of Rip3, which then prevents the recruitment of mixed lineage kinase domain-like protein (Mlkl) to the necrosome. We further showed that Ppm1b deficiency (Ppm1b(d/d)) in mice enhanced TNF-induced death in a Rip3-dependent manner, and the role of Ppm1b in inhibiting necroptosis was evidenced by elevated Rip3 phosphorylation and tissue damage in the caecum of TNF-treated Ppm1b(d/d) mice. These data indicate that Ppm1b negatively regulates necroptosis through dephosphorylating Rip3 in vitro and in vivo.

Biochemical characterization of the phosphatase domain of the tumor suppressor PH domain leucine-rich repeat protein phosphatase

PH domain leucine-rich repeat protein phosphatase (PHLPP) directly dephosphorylates and inactivates Akt and protein kinase C and is therefore a prime target for pharmacological intervention of two key signaling pathways, the phosphatidylinositol 3-kinase and diacylglycerol signaling pathways. Here we report on the first biochemical characterization of the phosphatase domain of a PHLPP family member. The human PHLPP1 and PHLPP2 phosphatase domains were expressed and purified from bacteria or insect cells and their activities compared to that of full-length proteins immunoprecipitated from mammalian cells. Biochemical analyses reveal that the PHLPP phosphatase domain effectively dephosphorylates synthetic and peptidic substrates, that its activity is modulated by metals and lipophilic compounds, and that it has relatively high thermal stability. Mutational analysis of PHLPP2 reveals an unusual active site architecture compared to the canonical architecture of PP2C phosphatases and identifies key acidic residues (Asp 806, Glu 989, and Asp 1024) and bulky aromatic residues (Phe 783 and Phe 808) whose mutation impairs activity. Consistent with a unique active site architecture, we identify inhibitors that discriminate between PHLPP2 and PP2Cα. These data establish PHLPP as a member of the PP2C family of phosphatases with a unique active site architecture.

The serine/threonine phosphatase PPM1B (PP2Cβ) selectively modulates PPARγ activity

Reversible phosphorylation is a widespread molecular mechanism to regulate the function of cellular proteins, including transcription factors. Phosphorylation of the nuclear receptor PPARγ (peroxisome-proliferator-activated receptor γ) at two conserved serine residue (Ser(112) and Ser(273)) results in an altered transcriptional activity of this transcription factor. So far, only a very limited number of cellular enzymatic activities has been described which can dephosphorylate nuclear receptors. In the present study we used immunoprecipitation assays coupled to tandem MS analysis to identify novel PPARγ-regulating proteins. We identified the serine/threonine phosphatase PPM1B [PP (protein phosphatase), Mg(2+)/Mn(2+) dependent, 1B; also known as PP2Cβ] as a novel PPARγ-interacting protein. Endogenous PPM1B protein is localized in the nucleus of mature 3T3-L1 adipocytes where it can bind to PPARγ. Furthermore we show that PPM1B can directly dephosphorylate PPARγ, both in intact cells and in vitro. In addition PPM1B increases PPARγ-mediated transcription via dephosphorylation of Ser(112). Finally, we show that knockdown of PPM1B in 3T3-L1 adipocytes blunts the expression of some PPARγ target genes while leaving others unaltered. These findings qualify the phosphatase PPM1B as a novel selective modulator of PPARγ activity.

Function analysis of conserved amino acid residues in a Mn(2+)-dependent protein phosphatase, Pph3, from Myxococcus xanthus

The Myxococcus xanthus protein phosphatase Pph3 belongs to the Mg(2+)- or Mn(2+)-dependent protein phosphatase (PPM) family. Bacterial PPMs contain three divalent metal ions and a flap subdomain. Putative metal- or phosphate-ion binding site-specific mutations drastically reduced enzymatic activity. Pph3 contains a cyclic nucleotide monophosphate (cNMP)-binding domain in the C-terminal region, and it requires 2-mercaptoethanol for phosphatase activity; however, the C-terminal deletion mutant showed high activity in the absence of 2-mercaptoethanol. The phosphatase activity of the wild-type enzyme was higher in the presence of cAMP than in the absence of cAMP, whereas a triple mutant of the cNMP-binding domain showed slightly lower activities than those of wild-type, without addition of cAMP. In addition, mutational disruption of a disulphide bond in the wild-type enzyme increased the phosphatase activity in the absence of 2-mercaptoethanol, but not in the C-terminal deletion mutant. These results suggested that the presence of the C-terminal region may lead to the formation of the disulphide bond in the catalytic domain, and that disulphide bond cleavage of Pph3 by 2-mercaptoethanol may occur more easily with cAMP bound than with no cAMP bound.

PPM1H is a p27 phosphatase implicated in trastuzumab resistance

The HER2 oncogene is overexpressed or amplified in 20% of breast cancers. HER2-positive cancer historically portends a poor prognosis, but the HER2-targeted therapy trastuzumab mitigates this otherwise ominous distinction. Nevertheless, some patients suffer disease recurrence despite trastuzumab, and metastatic disease remains largely incurable due to innate and acquired resistance. Thus, understanding trastuzumab resistance remains an unmet medical need. Through RNA interference screening, we discovered that knockdown of the serine/threonine phosphatase PPM1H confers trastuzumab resistance via reduction in protein levels of the tumor suppressor p27. PPM1H dephosphorylates p27 at threonine 187, thus removing a signal for proteasomal degradation. We further determined that patients whose tumors express low levels of PPM1H trend towards worse clinical outcome on trastuzumab. Identifying PPM1H as a novel p27 phosphatase reveals new insight into how cancer cells destabilize a well-recognized tumor suppressor. Furthermore, low PPM1H expression may identify a subset of HER2-positive tumors that are harder to treat.

Developmental regulation and extracellular release of a VSG expression-site-associated gene product from Trypanosoma brucei bloodstream forms

Trypanosomes evade host immunity by exchanging variant surface glycoprotein (VSG) coats. VSG genes are transcribed from telomeric expression sites, which contain a diverse family of expression-site-associated genes (ESAGs). We have discovered that the mRNAs for one ESAG family, ESAG9, are strongly developmentally regulated, being enriched in stumpy forms, a life-cycle stage in the mammalian bloodstream that is important for the maintenance of chronic parasite infections and for tsetse transmission. ESAG9 gene sequences are highly diverse in the genome and encode proteins with weak similarity to the massively diverse MASP proteins in Trypanosoma cruzi. We demonstrate that ESAG9 proteins are modified by N-glycosylation and can be shed to the external milieu, this being dependent upon coexpression with at least one other family member. The expression profile and extracellular release of ESAG9 proteins represents a novel and unexpected aspect of the transmission biology of trypanosomes in their mammalian host. We suggest that these molecules might interact with the external environment, with possible implications for infection chronicity or parasite transmission.

Pisiferdiol and pisiferic acid isolated from Chamaecyparis pisifera activate protein phosphatase 2C in vitro and induce caspase-3/7-dependent apoptosis via dephosphorylation of Bad in HL60 cells

Protein phosphatase 2C (PP2C) dephosphorylates a broad range of substrates and regulates apoptosis, stress response and growth-related pathways. In the course of screening for PP2C activators from natural sources, we isolated abietane-type diterpenes, pisiferdiol and pisiferic acid from Chamaecyparis pisifera. Pisiferdiol having a unique seven-membered ring showed more specific PP2C activation activity (1.3-fold at 100 microM) than pisiferic acid having a normal six-membered ring and oleic acid, which is known to activate PP2C. Pisiferdiol and pisiferic acid showed mixed-type activation with respect to alpha-casein, and this differed from the non-competitive activation of oleic acid in vitro. In vivo, the cytotoxicity of pisiferdiol toward human promyelocytic leukemia cell line HL60 with an IC(50) value of 18.3 microM was 2-fold and 7-fold stronger than those of pisiferic acid and oleic acid, and pisiferdiol induced apoptosis through a caspase 3/7-dependent mechanism involving the dephosphorylation of Bad(1), which is a PP2C substrate. We thus conclude that pisiferdiol and pisiferic acid are novel PP2C activators, and the more specific activator, pisiferdiol, may be a useful chemical probe to study PP2C-mediated signaling pathways, and a lead compound for pharmaceutical agents.

A Toxoplasma type 2C serine-threonine phosphatase is involved in parasite growth in the mammalian host cell

Toxoplasma gondii is a human protozoan parasite that belongs to the phylum of Apicomplexa and causes toxoplasmosis. As the other members of this phylum, T. gondii obligatory multiplies within a host cell by a peculiar type of mitosis that leads to daughter cell assembly within a mother cell. Although parasite growth and virulence have been linked for years, few molecules controlling mitosis have been yet identified and they include a couple of kinases but not the counteracting phosphatases. Here, we report that in contrast to other animal cells, type 2C is by far the major type of serine threonine phosphatase activity both in extracellular and in intracellular dividing parasites. Using wild type and transgenic parasites, we characterized the 37kDa TgPP2C molecule as an abundant cytoplasmic and nuclear enzyme with activity being under tight regulation. In addition, we showed that the increase in TgPP2C activity significantly affected parasite growth by impairing cytokinesis while nuclear division still occurred. This study supports for the first time that type 2C protein phosphatase is an important regulator of cell growth in T. gondii.

A serine/threonine protein phosphatase-like protein, CaPTC8, from Candida albicans defines a new PPM subfamily

Protein phosphatase M family (PPM; Mg(2+)-dependent protein phosphatases), which specifically dephosphorylates serine/threonine residues, consists of pyruvate dehydrogenase phosphatases, SpoIIE, adenylate cyclase and protein phosphatase type 2Cs (PP2Cs). To identify Candida albicans PP2Cs, the archetype of the PPM Ser/Thr phosphatases, we thoroughly searched the public C. albicans genome database and identified seven PP2C members. One of the PP2Cs in C. albicans, designated as CaPTC8 gene, represents a new member of PP2C genes. Northern blot analysis showed that the expression of CaPTC8 was positively responsive to high osmolarity, temperature or serum-stimulated filamentous growth. Gene disruption further demonstrated that deletion of CaPTC8 gene caused the defect of hyphal formation. Sequence analysis revealed that two conserved amino acids His and Asn in the prototypical members of the PPM family were substituted by Val and Asp in the PTC8p-like proteins. In addition, posterior analysis for site-specific profile showed that seven more sites are under the selection of functional divergence between these two groups of proteins. Three-dimensional homology modeling illustrated the signatures of the two groups in the conserved catalytic region of the protein phosphatases. Hence, CaPTC8 plays a role in stress responses and is required for the yeast-hyphal transition, and the CaPTC8-related genes are evolutionarily conserved. The phylogenetic relationships of all members of the PPM family strongly support the existence of a distinct and new subfamily of the PP2C-related proteins, PP2CR.

PPM1D430, a novel alternative splicing variant of the human PPM1D, can dephosphorylate p53 and exhibits specific tissue expression

PPM1D is a PPM1 type protein phosphatase and is induced in response to DNA damage. PPM1D-deficient mice show defects in spermatogenesis and lymphoid cell functions but the mechanisms underlying these phenotypes remain unknown. In our current study, we identify and characterize an alternative splicing variant (denoted PPM1D430) of human PPM1D at both the mRNA and protein level. PPM1D430 comprises the common 420 residues of the known PPM1D protein (PPM1D605) and contains a stretch of PPM1D430-specific 10 amino acids. Semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR) analysis revealed that PPM1D430 mRNA is also induced in response to the genotoxic stress in a p53-dependent manner. In vitro phosphatase analysis and PPM1D430-specific RNA interference analysis further indicated that PPM1D430 can dephosphorylate Ser15 of human p53 both in vitro and in vivo. On the other hand, expression profiling of this gene by RT-PCR analysis of a human tissue cDNA panel revealed that PPM1D430 is expressed exclusively in testes and in leucocytes whereas PPM1D605 is ubiquitous. In addition, PPM1D430 shows a different subcellular localization pattern and protein stability when compared with PPM1D605 under some conditions. Our current findings thus suggest that PPM1D430 may exert specific functions in immune response and/or spermatogenesis.

Structural insights into mechanism and specificity of O-GlcNAc transferase

Post-translational modification of protein serines/threonines with N-acetylglucosamine (O-GlcNAc) is dynamic, inducible and abundant, regulating many cellular processes by interfering with protein phosphorylation. O-GlcNAcylation is regulated by O-GlcNAc transferase (OGT) and O-GlcNAcase, both encoded by single, essential, genes in metazoan genomes. It is not understood how OGT recognises its sugar nucleotide donor and performs O-GlcNAc transfer onto proteins/peptides, and how the enzyme recognises specific cellular protein substrates. Here, we show, by X-ray crystallography and mutagenesis, that OGT adopts the (metal-independent) GT-B fold and binds a UDP-GlcNAc analogue at the bottom of a highly conserved putative peptide-binding groove, covered by a mobile loop. Strikingly, the tetratricopeptide repeats (TPRs) tightly interact with the active site to form a continuous 120 Å putative interaction surface, whereas the previously predicted phosphatidylinositide-binding site locates to the opposite end of the catalytic domain. On the basis of the structure, we identify truncation/point mutants of the TPRs that have differential effects on activity towards proteins/peptides, giving first insights into how OGT may recognise its substrates.

Role of type 2C protein phosphatases in growth regulation and in cellular stress signaling

A number of interesting features, phenotypes, and potential clinical applications have recently been ascribed to the type 2C family of protein phosphatases. Thus far, 16 different PP2C genes have been identified in the human genome, encoding (by means of alternative splicing) for at least 22 different isozymes. Virtually ever since their discovery, type 2C phosphatases have been predominantly linked to cell growth and to cellular stress signaling. Here, we provide an overview of the involvement of type 2C phosphatases in these two processes, and we show that four of them (PP2Calpha, PP2Cbeta, ILKAP, and PHLPP) can be expected to function as tumor suppressor proteins, and one as an oncoprotein (PP2Cdelta /Wip1). In addition, we demonstrate that in virtually all cases in which they have been linked to the stress response, PP2Cs act as inhibitors of cellular stress signaling. Based on the vast amount of experimental evidence obtained thus far, it therefore seems justified to conclude that type 2C protein phosphatases are important physiological regulators of cell growth and of cellular stress signaling.

Mutational analysis of Ca2+/calmodulin-dependent protein kinase phosphatase (CaMKP)

Ca(2+)/calmodulin-dependent protein kinase phosphatase (CaMKP) is a member of the serine/threonine protein phosphatases and shares 29% sequence identity with protein phosphatase 2Calpha (PP2Calpha) in its catalytic domain. To investigate the functional domains of CaMKP, mutational analysis was carried out using various recombinant CaMKPs expressed in Escherichia coli. Analysis of N-terminal deletion mutants showed that the N-terminal region of CaMKP played important roles in the formation of the catalytically active structure of the enzyme, and a critical role in polycation stimulation. A chimera mutant, a fusion of the N-terminal domain of CaMKP and the catalytic domain of PP2Calpha, exhibited similar substrate specificity to CaMKP but not to PP2Calpha, suggesting that the N-terminal region of CaMKP is crucial for its unique substrate specificity. Point mutations at Arg-162, Asp-194, His-196, and Asp-400, highly conserved amino acid residues in the catalytic domain of PP2C family, resulted in a significant loss of phosphatase activity, indicating that these amino acid residues may play important roles in the catalytic activity of CaMKP. Although CaMKP(1-412), a C-terminal truncation mutant, retained phosphatase activity, it was found to be much less stable upon incubation at 37 degrees C than wild type CaMKP, indicating that the C-terminal region of CaMKP is important for the maintenance of the catalytically active conformation. The results suggested that the N- and C-terminal sequences of CaMKP are essential for the regulation and stability of CaMKP.

Identification of phosphatases for Smad in the BMP/DPP pathway

Phosphorylation of the SSXS motif of Smads is critical in activating the transforming growth factor beta (TGF-beta) and bone morphogenetic protein (BMP) pathways. However, the phosphatase(s) involved in dephosphorylating and hence inactivating Smads remained elusive. Through RNA interference (RNAi)-based screening of serine/threonine phosphatases in Drosophila S2 cells, we identified pyruvate dehydrogenase phosphatase (PDP) to be required for dephosphorylation of Mothers against Decapentaplegic (MAD), a Drosophila Smad. Biochemical and genetic evidence suggest that PDP directly dephosphorylates MAD and inhibits signal transduction of Decapentaplegic (DPP). We show that the mammalian PDPs are important in dephosphorylation of BMP-activated Smad1 but not TGF-beta-activated Smad2 or Smad3. Thus, PDPs specifically inactivate Smads in the BMP/DPP pathway.

p90 ribosomal S6 kinase 2 is associated with and dephosphorylated by protein phosphatase 2Cdelta

RSK2 (p90 ribosomal S6 kinase 2) is activated via the ERK (extracellular-signal-regulated kinase) pathway by phosphorylation on four sites: Ser227 in the activation loop of the N-terminal kinase domain, Ser369 in the linker, Ser386 in the hydrophobic motif and Thr577 in the C-terminal kinase domain of RSK2. In the present study, we demonstrate that RSK2 is associated in vivo with PP2Cdelta (protein phosphatase 2Cdelta). In epidermal growth factorstimulated cells, RSK2 is partially dephosphorylated on all four sites in an Mn2+-dependent manner, leading to reduced protein kinase activity. Furthermore, PP2Cd is phosphorylated by ERK on Thr315 and Thr333 in the catalytic domain. Mutation of Thr315 and Thr333 to alanine in a catalytically inactive mutant PP2Cdelta (H154D) (His154-->Asp) increases the association with RSK2 significantly, whereas mutation to glutamate, mimicking phosphorylation, reduces the binding of RSK2. The domains of interaction are mapped to the N-terminal extension comprising residues 1-71 of PP2Cd and the N-terminal kinase domain of RSK2. The interaction is specific, since PP2Cd associates with RSK1-RSK4, MSK1 (mitogen- and stress-activated kinase 1) and MSK2, but not with p70 S6 kinase or phosphoinositide-dependent kinase 1. We conclude that RSK2 is associated with PP2Cd in vivo and is partially dephosphorylated by it, leading to reduced kinase activity.

Probing a putative active site of the catalytic subunit of pyruvate dehydrogenase phosphatase 1 (PDP1c) by site-directed mutagenesis

The catalytic subunit of pyruvate dehydrogenase phosphatase 1 (PDP1c) is a magnesium-dependent protein phosphatase that regulates the activity of mammalian pyruvate dehydrogenase complex. Based on the sequence analysis, it was hypothesized that PDP1c is related to the mammalian magnesium-dependent protein phosphatase type 1, with Asp54, Asp347, and Asp445 contributing to the binuclear metal-binding center, and Asn49 contributing to the phosphate-binding sites. In this study, we analyzed the functional significance of these amino acid residues using a site-directed mutagenesis. It was found that substitution of each of these residues had a significant impact on PDP1c activity toward the protein substrate. The activities of Asp54, Asp347, and Asp445 mutants were decreased more than 1000-fold. The activity of Asn49 mutant was 2.5-fold lower than the activity of wild-type PDP1c. The decrease in activity of Asp54 and Asp347 came about, most likely, as a result of impaired magnesium binding. Unexpectedly, it was found that the Asp445 mutant bound Mg(2+) ions similarly to the wild-type enzyme. Accordingly, the Asp445 mutant was found to be active with the artificial substrate p-nitrophenyl phosphate (pNPP). Asp54 and Asp347 mutants did not demonstrate any appreciable activity with pNPP. Together, these observations strongly suggest that Asn49, Asp54, and Asp347 are important for the catalysis of the phosphatase reaction, contributing to the phosphate- and metal-binding centers of PDP1c. In contrast, Asp445 is not required for catalysis. The exact role of Asp445 remains to be established, but indirect evidence suggests that it might be involved in the control of interactions between PDP1c and the protein substrate pyruvate dehydrogenase.

Characterization of a novel plant PP2C-like protein Ser/Thr phosphatase as a calmodulin-binding protein

Protein phosphatases regulated by calmodulin (CaM) mediate the action of intracellular Ca2+ and modulate functions of various target proteins by dephosphorylation. In plants, however, the role of Ca2+ in the regulation of protein dephosphorylation is not well understood due to a lack of information on characteristics of CaM-regulated protein phosphatases. Screening of a cDNA library of the moss Physcomitrella patens by using 35S-labeled calmodulin as a ligand resulted in identification of a gene, PCaMPP, that encodes a protein serine/threonine phosphatase with 373 amino acids. PCaMPP had a catalytic domain with sequence similarity to type 2C protein phosphatases (PP2Cs) with six conserved metal-associating amino acid residues and also had an extra C-terminal domain. Recombinant GST fusion proteins of PCaMPP exhibited Mn2+-dependent phosphatase activity, and the activity was inhibited by pyrophosphate and 1 mm Ca2+ but not by okadaic acid, orthovanadate, or beta-glycerophosphate. Furthermore, the PCaMPP activity was increased 1.7-fold by addition of CaM at nanomolar concentrations. CaM binding assays using deletion proteins and a synthetic peptide revealed that the CaM-binding region resides within the basic amphiphilic amino acid region 324-346 in the C-terminal domain. The CaM-binding region had sequence similarity to amino acids in one of three alpha-helices in the C-terminal domain of human PP2Calpha, suggesting a novel role of the C-terminal domains for the phosphatase activity. These results provide the first evidence showing possible regulation of PP2C-related phosphatases by Ca2+/CaM in plants. Genes similar to PCaMPP were found in genomes of various higher plant species, suggesting that PCaMPP-type protein phosphatases are conserved in land plants.

Actin dynamics is controlled by a casein kinase II and phosphatase 2C interplay on Toxoplasma gondii Toxofilin

Actin polymerization in Apicomplexa protozoa is central to parasite motility and host cell invasion. Toxofilin has been characterized as a protein that sequesters actin monomers and caps actin filaments in Toxoplasma gondii. Herein, we show that Toxofilin properties in vivo as in vitro depend on its phosphorylation. We identify a novel parasitic type 2C phosphatase that binds the Toxofilin/G-actin complex and a casein kinase II-like activity in the cytosol, both of which modulate the phosphorylation status of Toxofilin serine53. The interplay of these two molecules controls Toxofilin binding of G-actin as well as actin dynamics in vivo. Such functional interactions should play a major role in actin sequestration, a central feature of actin dynamics in Apicomplexa that underlies the spectacular speed and nature of parasite gliding motility.

Regulation of the interleukin-1-induced signaling pathways by a novel member of the protein phosphatase 2C family (PP2Cepsilon)

Although TAK1 signaling plays essential roles in eliciting cellular responses to interleukin-1 (IL-1), a proinflammatory cytokine, how the IL-1-TAK1 signaling pathway is positively and negatively regulated remains poorly understood. In this study, we investigated the possible role of a novel protein phosphatase 2C (PP2C) family member, PP2Cepsilon, in the regulation of the IL-1-TAK1 signaling pathway. PP2Cepsilon was composed of 303 amino acids, and the overall similarity of amino acid sequence between PP2Cepsilon and PP2Calpha was found to be 26%. Ectopic expression of PP2Cepsilon inhibited the IL-1- and TAK1-induced activation of mitogen-activated protein kinase kinase 4 (MKK4)-c-Jun N-terminal kinase or MKK3-p38 signaling pathway. PP2Cepsilon dephosphorylated TAK1 in vitro. Co-immunoprecipitation experiments indicated that PP2Cepsilon associates stably with TAK1 and attenuates the binding of TAK1 to MKK4 or MKK6. Ectopic expression of a phosphatase-negative mutant of PP2Cepsilon, PP2Cepsilon(D/A), which acted as a dominant negative form, enhanced both the association between TAK1 and MKK4 or MKK6 and the TAK1-induced activation of an AP-1 reporter gene. The association between PP2Cepsilon and TAK1 was transiently suppressed by IL-1 treatment of the cells. Taken together, these results suggest that, in the absence of IL-1-induced signal, PP2Cepsilon contributes to keeping the TAK1 signaling pathway in an inactive state by associating with and dephosphorylating TAK1.

POLTERGEIST encodes a protein phosphatase 2C that regulates CLAVATA pathways controlling stem cell identity at Arabidopsis shoot and flower meristems

BACKGROUND

Receptor kinases are a large gene family in plants and have more than 600 members in Arabidopsis. Receptor kinases in plants regulate a broad range of developmental processes, including steroid hormone perception, organ elongation, self-incompatibility, and abscission. Intracellular signaling components for receptor kinases in plants are largely unknown. The CLAVATA 1 (CLV1) receptor kinase in Arabidopsis regulates stem cell identity and differentiation through its repression of WUSCHEL (WUS) expression. Mutations at the POLTERGEIST (POL) gene were previously described as phenotypic suppressors of mutations within the CLV1 gene. Genetic evidence placed POL as a downstream regulator of CLAVATA1 signaling.

RESULTS

We provide evidence that POL functions in both the CLV1-WUS pathway and a novel WUS-independent CLV1 pathway regulating stem cell identity. We demonstrate that POL encodes a protein phosphatase 2C (PP2C) with a predicted nuclear localization sequence, indicating that it has a role in signal transduction downstream of the CLV1 receptor. The N terminus of POL has a possible regulatory function, and the C terminus has PP2C-like phosphatase catalytic activity. Although the POL catalytic domain is conserved in other PP2Cs, the POL protein represents a unique subclass of plant PP2Cs. POL is broadly expressed throughout the plant.

CONCLUSIONS

POL represents a novel component of the CLV1 receptor kinase signaling pathway. The ubiquitous expression of POL and pol phenotypes outside the meristem suggest that POL may be a common regulator of many signaling pathways.

Uniquely conserved non-translated regions are involved in generation of the two major transcripts of protein phosphatase 2Cbeta

Partial cDNAs of different isoforms of protein phosphatase 2Cbeta (PP2Cbeta or PPM1B) have been characterized in mammals. We disclose here the full cDNAs of two major PP2Cbeta isoforms from human, rat and mouse. These cDNAs (2.6 and 3.3 kb) are able to encode 53 kDa (PP2Cbetal) and 43 kDa (PP2Cbetas) polypeptides, respectively. The isoforms are co-expressed ubiquitously with the highest level in skeletal muscle, as assessed by Northern-blot analysis. Western and in situ analyses using monoclonal antibodies against PP2Cbeta confirmed the existence of two isoforms in the cytoplasm. Comparative sequence analysis revealed that both cDNAs consist of six exons with an alternate usage of the 3' exons that underlies the differences between them. The genomic structure of PP2Cbeta is similar to that of other PP2C paralogs and includes a non-coding first exon followed by a large intron and a large second exon that encoded most of the catalytic domain. Both variants of the ending exon include large non-coding regions. All non-translated regions (NTRs) are highly conserved between the orthologous genes, indicating their regulatory function. The 5'-NTR is long (379 bp), includes upstream start codons and is predicted to contain stable secondary structures. Such features inhibit translation initiation by the scanning mechanism. Introduction of this NTR element into a bi-luciferase expression-cassette enabled expression of the second cistron, suggesting that it might serve as an internal ribosome entry site, or it contains a cryptic promoter. Overexpression of PP2Cbeta under CMV-promoter in 293 cells led to cell-growth arrest or cell death.

Modulation of integrin signal transduction by ILKAP, a protein phosphatase 2C associating with the integrin-linked kinase, ILK1

ILKAP, a protein serine/threonine (S/T) phosphatase of the PP2C family, was isolated in a yeast two-hybrid screen baited with integrin-linked kinase, ILK1. Association of ILK1 and ILKAP was independent of the catalytic activity of either partner, as assayed in co-precipitation and two-hybrid experiments. Condi tional expression of ILKAP in HEK 293 cells resulted in selective inhibition of ECM- and growth factor-stimulated ILK1 activity, but did not inhibit Raf-1 kinase activity. A catalytic mutant of ILKAP, H154D, did not inhibit ILK1 kinase activity. Two cellular targets of ILK1, glycogen synthase kinase 3 beta (GSK3beta) and protein kinase B (PKB)/AKT, were differentially affected by ILKAP-mediated inhibition of ILK1. Catalytically active, but not mutant ILKAP, strongly inhibited insulin-like growth factor-1-stimulated GSK3beta phosphorylation on Ser9, but did not affect phosphorylation of PKB on Ser473, suggesting that ILKAP selectively affects ILK-mediated GSK3beta signalling. Consistent with this, active, but not H154D mutant or the related PP2Calpha, selectively inhibited transactivation of a Tcf/Lef reporter gene, TOPFlash, in 293 cells. We propose that ILKAP regulates ILK1 activity, targeting ILK1 signalling of Wnt pathway components via modulation of GSK3beta phosphorylation.

Regulation of the TAK1 signaling pathway by protein phosphatase 2C

Protein phosphatase 2C (PP2C) is implicated in the negative regulation of stress-activated protein kinase cascades in yeast and mammalian cells. In this study, we determined the role of PP2Cbeta-1, a major isoform of mammalian PP2C, in the TAK1 signaling pathway, a stress-activated protein kinase cascade that is activated by interleukin-1, transforming growth factor-beta, or stress. Ectopic expression of PP2Cbeta-1 inhibited the TAK1-mediated mitogen-activated protein kinase kinase 4-c-Jun amino-terminal kinase and mitogen-activated protein kinase kinase 6-p38 signaling pathways. In vitro, PP2Cbeta-1 dephosphorylated and inactivated TAK1. Coimmunoprecipitation experiments indicated that PP2Cbeta-1 associates with the central region of TAK1. A phosphatase-negative mutant of PP2Cbeta-1, PP2Cbeta-1 (R/G), acted as a dominant negative mutant, inhibiting dephosphorylation of TAK1 by wild-type PP2Cbeta-1 in vitro. In addition, ectopic expression of PP2Cbeta-1(R/G) enhanced interleukin-1-induced activation of an AP-1 reporter gene. Collectively, these results indicate that PP2Cbeta negatively regulates the TAK1 signaling pathway by direct dephosphorylation of TAK1.

Dephosphorylation of human cyclin-dependent kinases by protein phosphatase type 2C alpha and beta 2 isoforms

We previously reported that the activating phosphorylation on cyclin-dependent kinases in yeast (Cdc28p) and in humans (Cdk2) is removed by type 2C protein phosphatases. In this study, we characterize this PP2C-like activity in HeLa cell extract and determine that it is due to PP2C beta 2, a novel PP2C beta isoform, and to PP2C alpha. PP2C alpha and PP2C beta 2 co-purified with Mg(2+)-dependent Cdk2/Cdk6 phosphatase activity in DEAE-Sepharose, Superdex-200, and Mono Q chromatographies. Moreover, purified recombinant PP2C alpha and PP2C beta 2 proteins efficiently dephosphorylated monomeric Cdk2/Cdk6 in vitro. The dephosphorylation of Cdk2 and Cdk6 by PP2C isoforms was inhibited by the binding of cyclins. We found that the PP2C-like activity in HeLa cell extract, partially purified HeLa PP2C alpha and PP2C beta 2 isoforms, and the recombinant PP2Cs exhibited a comparable substrate preference for a phosphothreonine containing substrate, consistent with the conservation of threonine residues at the site of activating phosphorylation in CDKs.

The Bacillus subtilis regulator protein SpoIIE shares functional and structural similarities with eukaryotic protein phosphatases 2C

Dephosphorylation of SpoIIAA-P by SpoIIE is strictly dependent on the presence of the bivalent metal ions Mn2+ or Mg2+. Replacement by Ala of one of the four Asp residues, invariant in all representatives of protein phosphatase 2C, completely abolished the SpoIIE phosphatase activity in vitro, whilst replacement of the Asp residues by another acidic amino acid, Glu, had varying effects on the activities of the resulting mutated proteins. D610E and D795E exhibited some residual activity while D628E and D745E were without enzymatic activity. The results suggest that the functional model in which metal-associated water molecules are involved in the dephosphorylation reaction catalyzed by human protein phosphatase 2C alpha can also be applied to the bacterial protein phosphatase 2C-like protein.

The type 2C Ser/Thr phosphatase PP2Cgamma is a pre-mRNA splicing factor

To identify activities involved in human pre-mRNA splicing, we have developed a procedure to separate HeLa cell nuclear extract into five complementing fractions. An activity called SCF1 was purified from one of these fractions by assaying for reconstitution of splicing in the presence of the remaining four fractions. A component of SCF1 is shown to be PP2Cgamma, a type 2C Ser/Thr phosphatase of previously unknown function. Previous work suggested that dephosphorylation of splicing factors may be important for catalysis after spliceosome assembly, although the identities of the specific phosphatases involved remain unclear. Here we show that human PP2Cgamma is physically associated with the spliceosome in vitro throughout the splicing reaction, but is first required during the early stages of spliceosome assembly for efficient formation of the A complex. The phosphatase activity is required for the splicing function of PP2Cgamma, as an active site mutant does not support spliceosome assembly. The requirement for PP2Cgamma is highly specific, as the closely related phosphatase PP2Calpha cannot substitute for PP2Cgamma. Consistent with a role in splicing, PP2Cgamma localizes to the nucleus in vivo. We conclude that at least one specific dephosphorylation event catalyzed by PP2Cgamma is required for formation of the spliceosome.

Selective suppression of stress-activated protein kinase pathway by protein phosphatase 2C in mammalian cells

Protein phosphatase 2Calpha (PP2Calpha) or PP2Cbeta-1 expressed in COS7 cells suppressed anisomycin- and NaCl-enhanced phosphorylations of p38 co-expressed in the cells. PP2Calpha or PP2Cbeta-1 expression also suppressed both basal and stress-enhanced phosphorylations of MKK3b and MKK6b, which are upstream protein kinases of p38, and of MKK4, which is one of the major upstream protein kinases of JNK. Basal activity of MKK7, another upstream protein kinase of JNK, was also suppressed by PP2Calpha or PP2Cbeta-1 expression. However, basal as well as serum-activated phosphorylation of MKK1alpha, an upstream protein kinase of ERKs, was not affected by PP2Cbeta or PP2Cbeta-1. A catalytically inactive mutant of PP2Cbeta-1 further enhanced the NaCl-stimulated phosphorylations of MMK3b, MKK4 and MKK6b, suggesting that this mutant PP2Cbeta-1 works as a dominant negative form. These results suggest that PP2C selectively inhibits the SAPK pathways through suppression of MKK3b, MKK4, MKK6b and MKK7 activities in mammalian cells.