Negative feedback predominates over cross-regulation to control ERK MAPK activity in response to FGF signalling in embryos

Expression of the gene encoding the MKP-3/Pyst1 protein phosphatase, which inactivates ERK MAPK, is induced by FGF. However, which intracellular signalling pathway mediates this expression is unclear, with essential roles proposed for both ERK and PI(3)K in chick embryonic limb. Here, we report that MKP-3/Pyst1 expression is sensitive to inhibition of ERK or MAPKK, that endogenous MKP-3/Pyst1 co-localizes with activated ERK, and expression of MKP-3/Pyst1 in mice lacking PDK1, an essential mediator of PI(3)K signalling. We conclude that MKP-3/Pyst1 expression is mediated by ERK activation and that negative feedback control predominates in limiting the extent of FGF-induced ERK activity.

The dual-specificity protein phosphatase DUSP9/MKP-4 is essential for placental function but is not required for normal embryonic development

To elucidate the physiological role(s) of DUSP9 (dual-specificity phosphatase 9), also known as MKP-4 (mitogen-activated protein kinase [MAPK] phosphatase 4), the gene was deleted in mice. Crossing male chimeras with wild-type females resulted in heterozygous (DUSP9(+/-)) females. However, when these animals were crossed with wild-type (DUSP9(+/y)) males none of the progeny carried the targeted DUSP9 allele, indicating that both female heterozygous and male null (DUSP9(-/y)) animals die in utero. The DUSP9 gene is on the X chromosome, and this pattern of embryonic lethality is consistent with the selective inactivation of the paternal X chromosome in the extraembryonic tissues of the mouse, suggesting that DUSP9/MKP4 performs an essential function during placental development. Examination of embryos between 8 and 10.5 days postcoitum confirmed that lethality was due to a failure of labyrinth development, and this correlates exactly with the normal expression pattern of DUSP9/MKP-4 in the trophoblast giant cells and labyrinth of the placenta. Finally, when the placental defect was rescued, male null (DUSP9(-/y)) embryos developed to term, appeared normal, and were fertile. Our results indicate that DUSP9/MKP-4 is essential for placental organogenesis but is otherwise dispensable for mammalian embryonic development and highlights the critical role of dual-specificity MAPK phosphatases in the regulation of developmental outcomes in vertebrates.

Feedback interactions between MKP3 and ERK MAP kinase control scleraxis expression and the specification of rib progenitors in the developing chick somite

Cells in the early vertebrate somite receive cues from surrounding tissues,which are important for their specification. A number of signalling pathways involved in somite patterning have been described extensively. By contrast,the interactions between cells from different regions within the somite are less well characterised. Here, we demonstrate that myotomally derived FGFs act through the MAPK signal transduction cascade and in particular, ERK1/2 to activate scleraxis expression in a population of mesenchymal progenitor cells in the dorsal sclerotome. We show that the levels of active,phosphorylated ERK protein in the developing somite are crucial for the expression of scleraxis and Mkp3. MKP3 is a dual specificity phosphatase and a specific antagonist of ERK MAP kinases and we demonstrate that in somites Mkp3 transcription depends on the presence of active ERK. Therefore, MKP3 and ERK MAP kinase constitute a negative feedback loop activated by FGF in sclerotomal progenitor cells. We propose that tight control of ERK signalling strength by MKP3 is important for the appropriate regulation of downstream cellular responses including the activation of scleraxis. We show that increased or decreased levels of phosphorylated ERK result in the loss of scleraxis transcripts and the loss of distal rib development, highlighting the importance of the MKP3-ERK-MAP kinase mediated feedback loop for cell specification and differentiation.

Specific inactivation and nuclear anchoring of extracellular signal-regulated kinase 2 by the inducible dual-specificity protein phosphatase DUSP5

The mechanisms which determine the nuclear accumulation and inactivation of the extracellular signal-regulated kinase 1 (ERK1) or ERK2 mitogen-activated protein (MAP) kinases are poorly understood. Here we demonstrate that DUSP5, an inducible nuclear phosphatase, interacts specifically with ERK2 via a kinase interaction motif (KIM) within its amino-terminal noncatalytic domain. This binding determines the substrate specificity of DUSP5 in vivo, as it inactivates ERK2 but not Jun N-terminal protein kinase or p38 MAP kinase. Using green fluorescent protein fusions, we identify within this same domain of DUSP5 a functional nuclear localization signal (NLS) which functions independently of the KIM. Moreover, we demonstrate that the expression of DUSP5 causes both nuclear translocation and sequestration of inactive ERK2. Nuclear anchoring is ERK2 specific and requires both interactions between the DUSP5 KIM and the common docking site of ERK2 and a functional NLS within DUSP5. Finally, the expression of a catalytically inactive mutant of DUSP5 also tethers ERK2 within the nucleus. Furthermore, this nuclear ERK2 is phosphorylated by MAP kinase kinase in response to growth factors and also activates transcription factor Elk-1. We conclude that DUSP5 is an inducible nuclear ERK-specific MAP kinase phosphatase that functions as both an inactivator of and a nuclear anchor for ERK2 in mammalian cells. In addition, our data indicate that the cytoplasm may not be an exclusive site of MAP kinase activation.

Activation of MK5/PRAK by the atypical MAP kinase ERK3 defines a novel signal transduction pathway

Extracellular signal-regulated kinase 3 (ERK3) is an atypical mitogen-activated protein kinase (MAPK), which is regulated by protein stability. However, its function is unknown and no physiological substrates for ERK3 have yet been identified. Here we demonstrate a specific interaction between ERK3 and MAPK-activated protein kinase-5 (MK5). Binding results in nuclear exclusion of both ERK3 and MK5 and is accompanied by ERK3-dependent phosphorylation and activation of MK5 in vitro and in vivo. Endogenous MK5 activity is significantly reduced by siRNA-mediated knockdown of ERK3 and also in fibroblasts derived from ERK3-/- mice. Furthermore, increased levels of ERK3 protein detected during nerve growth factor-induced differentiation of PC12 cells are accompanied by an increase in MK5 activity. Conversely, MK5 depletion causes a dramatic reduction in endogenous ERK3 levels. Our data identify the first physiological protein substrate for ERK3 and suggest a functional link between these kinases in which MK5 is a downstream target of ERK3, while MK5 acts as a chaperone for ERK3. Our findings provide valuable tools to further dissect the regulation and biological roles of both ERK3 and MK5.

Both nuclear-cytoplasmic shuttling of the dual specificity phosphatase MKP-3 and its ability to anchor MAP kinase in the cytoplasm are mediated by a conserved nuclear export signal

MAP kinase phosphatase (MKP)-3 is a cytoplasmic dual specificity protein phosphatase that specifically binds to and inactivates the ERK1/2 MAP kinases in mammalian cells. However, the molecular basis of the cytoplasmic localization of MKP-3 or its physiological significance is unknown. We have used MKP-3-green fluorescent protein fusions in conjunction with leptomycin B to show that the cytoplasmic localization of MKP-3 is mediated by a chromosome region maintenance-1 (CRM1)-dependent nuclear export pathway. Furthermore, the nuclear translocation of MKP-3 seen in the presence of leptomycin B is mediated by an active process, indicating that MKP-3 shuttles between the nucleus and cytoplasm. The amino-terminal noncatalytic domain of MKP-3 is both necessary and sufficient for nuclear export of the phosphatase and contains a single functional leucine-rich nuclear export signal (NES). Even though this domain of the protein also mediates the binding of MKP-3 to MAP kinase, we show that mutations of the kinase interaction motif which abrogate ERK2 binding do not affect MKP-3 localization. Conversely, mutation of the NES does not affect either the binding or phosphatase activity of MKP-3 toward ERK2, indicating that the kinase interaction motif and NES function independently. Finally, we demonstrate that the ability of MKP-3 to cause the cytoplasmic retention of ERK2 requires both a functional kinase interaction motif and NES. We conclude that in addition to its established function in the regulated dephosphorylation and inactivation of MAP kinase, MKP-3 may also play a role in determining the subcellular localization of its substrate. Our results reinforce the idea that regulatory proteins such as MKP-3 may play a key role in the spatio-temporal regulation of MAP kinase activity.

Negative feedback regulation of FGF signaling levels by Pyst1/MKP3 in chick embryos

BACKGROUND

The importance of endogenous antagonists in intracellular signal transduction pathways is becoming increasingly recognized. There is evidence in cultured mammalian cells that Pyst1/MKP3, a dual specificity protein phosphatase, specifically binds to and inactivates ERK1/2 mitogen-activated protein kinases (MAPKs). High-level Pyst1/Mkp3 expression has recently been found at many sites of known FGF signaling in mouse embryos, but the significance of this association and its function are not known.

RESULTS

We have cloned chicken Pyst1/Mkp3 and show that high-level expression in neural plate correlates with active MAPK. We show that FGF signaling regulates Pyst1 expression in developing neural plate and limb bud by ablating and/or transplanting tissue sources of FGFs and by applying FGF protein or a specific FGFR inhibitor (SU5402). We further show by applying a specific MAP kinase kinase inhibitor (PD184352) that Pyst1 expression is regulated via the MAPK cascade. Overexpression of Pyst1 in chick embryos reduces levels of activated MAPK in neural plate and alters its morphology and retards limb bud outgrowth.

CONCLUSIONS

Pyst1 is an inducible antagonist of FGF signaling in embryos and acts in a negative feedback loop to regulate the activity of MAPK. Our results demonstrate both the importance of MAPK signaling in neural induction and limb bud outgrowth and the critical role played by dual specificity MAP kinase phosphatases in regulating developmental outcomes in vertebrates.

Both binding and activation of p38 mitogen-activated protein kinase (MAPK) play essential roles in regulation of the nucleocytoplasmic distribution of MAPK-activated protein kinase 5 by cellular stress

The p38 mitogen-activated protein kinase (MAPK) pathway is an important mediator of cellular responses to environmental stress. Targets of p38 include transcription factors, components of the translational machinery, and downstream serine/threonine kinases, including MAPK-activated protein kinase 5 (MK5). Here we have used enhanced green fluorescent protein fusion proteins to analyze the subcellular localization of MK5. Although this protein is predominantly nuclear in unstimulated cells, MK5 shuttles between the nucleus and the cytoplasm. Furthermore, we have shown that the C-terminal domain of MK5 contains both a functional nuclear localization signal (NLS) and a leucine-rich nuclear export signal (NES), indicating that the subcellular distribution of this kinase reflects the relative activities of these two signals. In support of this, we have shown that stress-induced activation of the p38 MAPK stimulates the chromosomal region maintenance 1 protein-dependent nuclear export of MK5. This is regulated by both binding of p38 MAPK to MK5, which masks the functional NLS, and stress-induced phosphorylation of MK5 by p38 MAPK, which either activates or unmasks the NES. These properties may define the ability of MK5 to differentially phosphorylate both nuclear and cytoplasmic targets or alternatively reflect a mechanism whereby signals initiated by activation of MK5 in the nucleus may be transmitted to the cytoplasm.

YIL113w encodes a functional dual-specificity protein phosphatase which specifically interacts with and inactivates the Slt2/Mpk1p MAP kinase in S. cerevisiae

We show here that the YIL113w gene of Saccharomyces cerevisiae encodes a functional protein phosphatase. Yil113p shows no activity in vitro towards either phosphorylated casein or myelin basic protein. However, Yil113p dephosphorylates activated extracellular signal-regulated kinase 2 MAP kinase indicating that it is a dual-specificity MAP kinase phosphatase. In support of this we find that Yil113p specifically interacts with the stress-activated Slt2/Mpk1p MAP kinase of S. cerevisiae. Furthermore, expression of Yil113p causes the dephosphorylation of Slt2/Mpk1p in vivo, while expression of an inactive mutant of Yil113p causes the accumulation of phosphorylated Slt2/Mpk1p. We conclude that the physiological target of YIL113p is Slt2/Mpk1p.

Characterization of a murine gene encoding a developmentally regulated cytoplasmic dual-specificity mitogen-activated protein kinase phosphatase

Mitogen-activated protein kinases (MAPKs) play a vital role in cellular growth control, but far less is known about these signalling pathways in the context of embryonic development. Duration and magnitude of MAPK activation are crucial factors in cell fate decisions, and reflect a balance between the activities of upstream activators and specific MAPK phosphatases (MKPs). Here, we report the isolation and characterization of the murine Pyst3 gene, which encodes a cytosolic dual-specificity MKP. This enzyme selectively interacts with, and is catalytically activated by, the 'classical' extracellular signal-regulated kinases (ERKs) 1 and 2 and, to a lesser extent, the stress-activated MAPK p38alpha. These properties define the ability of this enzyme to dephosphorylate and inactivate ERK1/2 and p38alpha, but not JNK (c-Jun N-terminal kinase) in vivo. When expressed in mammalian cells, the Pyst3 protein is predominantly cytoplasmic. Furthermore, leptomycin B causes a complete redistribution of the protein to the nucleus, implicating a CRM (chromosomal region maintenance)1/exportin 1-dependent nuclear export signal in determining the subcellular localization of this enzyme. Finally, whole-mount in situ hybridization studies in mouse embryos reveal that the Pyst3 gene is expressed specifically in the placenta, developing liver and in migratory muscle cells. Our results suggest that this enzyme may have a critical role in regulating the activity of MAPK signalling during early development and organogenesis.

Expression of the ERK-specific MAP kinase phosphatase PYST1/MKP3 in mouse embryos during morphogenesis and early organogenesis

Mitogen-activated-protein kinase (MAP kinase) cascades are effector mechanisms for many growth factor signals implicated in developmental processes, including appendage outgrowth and organogenesis. The cascade culminates in extracellular-signal-regulated MAP kinase (ERK), which enters the nucleus. ERK activity reflects the competing actions of upstream activator kinases and inhibitory MAP kinase phosphatases. We have studied embryonic expression of the dual-specificity MAP kinase phosphatase PYST1/MKP3, which is a specific and potent regulator of the ERK class of MAP kinases. We found dynamic patterns of mPyst1 messenger RNA in important signalling centres associated with cell proliferation and patterning in developing mouse embryos, including presegmental paraxial mesoderm, limb bud and branchial arch mesenchyme, midbrain/hindbrain isthmus, and nasal, dental, hair, and mammary placodes. Most of these have been characterised as sites of FGF/FGFR signalling.

Transcriptional induction of MKP-1 in response to stress is associated with histone H3 phosphorylation-acetylation

Mitogen-activated protein (MAP) kinase phosphatase 1 (MKP-1) has been shown to play a critical role in mediating the feedback control of MAP kinase cascades in a variety of cellular processes, including proliferation and stress responsiveness. Although MKP-1 expression is induced by a broad array of extracellular stimuli, the mechanisms mediating its induction remain poorly understood. Here we show that MKP-1 mRNA was potently induced by arsenite and ultraviolet light and modestly increased by heat shock and hydrogen peroxide. Interestingly, arsenite also dramatically induces phosphorylation-acetylation of histone H3 at a global level which precedes the induction of MKP-1 mRNA. The transcriptional induction of MKP-1, histone H3 modification, and elevation in MKP-1 mRNA in response to arsenite are all partially prevented by the p38 MAP kinase inhibitor SB203580, suggesting that the p38 pathway is involved in these processes. Finally, analysis of the DNA brought down by chromatin immunoprecipitation (ChIP) reveals that arsenite induces phosphorylation-acetylation of histone H3 associated with the MKP-1 gene and enhances binding of RNA polymerase II to MKP-1 chromatin. ChIP assays following exposure to other stress agents reveal various degrees of histone H3 modification at the MKP-1 chromatin. The differential contribution of p38 and ERK MAP kinases in mediating MKP-1 induction by different stress agents further illustrates the complexity and versatility of stress-induced MKP-1 expression. Our results strongly suggest that chromatin remodeling after stress contributes to the transcriptional induction of MKP-1.

Membrane proximal ERK signaling is required for M-calpain activation downstream of epidermal growth factor receptor signaling

Localization of signaling is critical in directing cellular outcomes, especially in pleiotropic signaling pathways. The extracellular signal-regulated kinase (ERK)/microtubule-associated protein kinase, which promotes cell migration, proliferation, and differentiation is found in the nucleus and throughout the cytoplasm. Recently, it has been shown that nuclear translocation of ERK is required for transcriptional changes and cell proliferation. However, the cellular consequences, of cytoplasmic signaling have not been defined. We explored whether cytoplasmic, specifically membrane-proximal, ERK signaling is involved in growth factor-induced cell motility. We previously have demonstrated that increased M-calpain activity downstream of epidermal growth factor receptor (EGFR)-mediated ERK activation is necessary for epidermal growth factor (EGF)-induced motility. Calpain isoforms also have been found in nuclear, cytosolic, and plasma membrane-associated compartments in a variety of cell types. We now employ cell engineering approaches to control localization of the upstream EGFR and ERK activities to examine the spatial effect of upstream signal locale on downstream calpain activity. With differential ligand-induced internalization and trafficking-restricted receptor variants, we find that calpain activity is triggered only by plasma membrane-restricted activated EGFR, not by internalized (although still active) EGFR. Cells transfected with membrane-targeted ERK1 and ERK2, which sequester endogenous ERKs, exhibited normal EGF-induced calpain activity. Transfection of an inactive ERK phosphatase (MKP-3/Pyst1) that sequesters ERK in the cytoplasm prevented calpain activation as well as de-adhesion. These data strongly suggest that EGF-induced calpain activity can be enhanced near sites of membrane-proximal EGFR-mediated ERK signaling, providing insights about how calpain activity might be regulated and targeted to enhance its effects on adhesion-related substrates.

Distinct binding determinants for ERK2/p38alpha and JNK map kinases mediate catalytic activation and substrate selectivity of map kinase phosphatase-1

Mitogen-activated protein (MAP) kinase phosphatase 1 (MKP-1/CL100) is an inducible nuclear dual specificity protein phosphatase that can dephosphorylate and inactivate both mitogen- and stress-activated protein kinases in vitro and in vivo. However, the molecular mechanism responsible for the substrate selectivity of MKP-1 is unknown. In addition, it has been suggested that the signal transducers and activators of transcription 1 (STAT1) transcription factor is a physiological non-MAP kinase substrate for MKP-1. We have used the yeast two-hybrid assay to demonstrate that MKP-1 is able to interact selectively with the extracellular signal-regulated kinase 1/2 (ERK1/2), p38alpha, and c-Jun NH(2)-terminal kinase (JNK) MAP kinase isoforms. Furthermore, this binding is accompanied by catalytic activation of recombinant MKP-1 protein in vitro, and these end points show an absolute correlation with MKP-1 substrate selectivity in vivo. In contrast, MKP-1 does not interact with STAT1. Recombinant STAT1 does not cause catalytic activation of MKP-1; nor does MKP-1 block tyrosine phosphorylation of STAT1 in vivo. Both binding and catalytic activation of MKP-1 are abrogated by mutation of a conserved docking site in ERK2, p38alpha, and JNK1 MAP kinases. Within MKP-1, MAP kinase binding is mediated by the amino-terminal noncatalytic domain of the protein. However, mutation of a conserved cluster of positively charged residues within this domain abolishes the binding and activation of MKP-1 by ERK2 and p38alpha but not JNK1, indicating that there are distinct binding determinants for these MAP kinase isoforms. We conclude that the substrate selectivity of MKP-1 is determined by specific protein-protein interactions coupled with catalytic activation of the phosphatase and that these interactions are restricted to members of the MAP kinase family of enzymes.

CL100/MKP-1 modulates JNK activation and apoptosis in response to cisplatin

Treatment of cells with cisplatin induces a sustained activation of the stress activated protein kinase SAPK/JNK and the mitogen-activated protein kinase p38. Activation of JNK by cisplatin is necessary for the induction of apoptosis. Expression of the MAPK phosphatases CL100/MKP-1 and hVH-5 selectively prevents JNK/SAPK activation by cisplatin in a dose dependent fashion and results in protection against cisplatin-induced apoptosis. In contrast, expression of the ERK-specific phosphatase Pyst1 inhibits JNK/SAPK activity only when expressed at very high levels and does not confer protection against cisplatin. Furthermore, expression of a catalytically inactive mutant of CL100 in 293 cells decreases the IC50 for cisplatin and increases the toxicity of transplatin. This effect seems to be mediated by an increase in JNK activity since p38 activity is unaffected. These results suggest that dual-specificity MAPK phosphatases may be candidate drug targets in order to optimize cisplatin based therapeutic protocols.

Synergistic activation of the mkp-1 gene by protein kinase A signaling and USF, but not c-Myc

Mitogen-activated protein kinase phosphatase 1 (MKP-1) is negatively regulating mitogen-activated protein kinases and is therefore involved in early signaling processes. The expression of the mkp-1 gene is induced by growth factors and stress. The promoters of the human and murine mkp-1 genes contain several conserved DNA binding elements, including two cAMP response elements and an E box. We observed that the upstream stimulatory factor (USF), but not c-Myc, activated mkp-1. USF synergized with protein kinase A, thus providing evidence for a role of the E box, during signal-regulated stimulation of mkp-1.

Protein phosphatases and the regulation of mitogen-activated protein kinase signalling

The magnitude and duration of signalling through mitogen- and stress-activated kinases are critical determinants of biological effect. This reflects a balance between the activities of upstream activators and a complex regulatory network of protein phosphatases. These mitogen-activated protein kinase phosphatases include both dual-specificity (threonine/tyrosine) and tyrosine-specific enzymes, and recent evidence suggests that a single mitogen-activated protein kinase isoform may be acted upon by both classes of protein phosphatase. In both cases, substrate selectivity is determined by specific protein-protein interactions mediated through noncatalytic amino-terminal mitogen-activated protein kinase binding domains. Future challenges include the determination of exactly how this network of protein phosphatases interacts selectively with mitogen-activated protein kinase signalling complexes to achieve precise regulation of these key pathways in mammalian cells.

The role of protein phosphatases in the regulation of mitogen and stress-activated protein kinases

It is now established that a family of dual-specificity protein phosphatases are able to interact with mitogen and stress-activated protein kinases in a highly specific manner to differentially regulate these enzymes in mammalian cells. A role for these proteins in negative feedback regulation of MAP kinase activity is also supported by genetic and biochemical studies in yeasts and Drosophila. More recently it has become clear that other classes of protein phosphatase also play key roles in the regulated dephosphorylation of MAP kinases, including tyrosine-specific protein phosphatases and serine/threonine protein phosphatases. It is likely that a complex balance between upstream activators and these different classes of MAP kinase specific phosphatase are responsible for determining, at least in part, the magnitude and duration of MAP kinase activation and hence the physiological outcome of signalling.

Crystal structure of the MAPK phosphatase Pyst1 catalytic domain and implications for regulated activation

The crystal structure of the catalytic domain from the MAPK phosphatase Pyst1 (Pyst1-CD) has been determined at 2.35 A. The structure adopts a protein tyrosine phosphatase (PTPase) fold with a shallow active site that displays a distorted geometry in the absence of its substrate with some similarity to the dual-specificity phosphatase cdc25. Functional characterization of Pyst1-CD indicates it is sufficient to dephosphorylate activated ERK2 in vitro. Kinetic analysis of Pyst1 and Pyst1-CD using the substrate p-nitrophenyl phosphate (pNPP) reveals that both molecules undergo catalytic activation in the presence of recombinant inactive ERK2, switching from a low- to high-activity form. Mutation of Asp 262, located 5.5 A distal to the active site, demonstrates it is essential for catalysis in the high-activity ERK2-dependent conformation of Pyst1 but not for the low-activity ERK2-independent form, suggesting that ERK2 induces closure of the Asp 262 loop over the active site, thereby enhancing Pyst1 catalytic efficiency.

Isolation of the human genes encoding the pyst1 and Pyst2 phosphatases: characterisation of Pyst2 as a cytosolic dual-specificity MAP kinase phosphatase and its catalytic activation by both MAP and SAP kinases

We have isolated the human genes encoding the Pyst1 (MKP-3) and Pyst2 (MKP-X) MAP kinase phosphatases. Both genes consist of three exons interrupted by two introns and lack an intron which is conserved in all the other members of this gene family characterised to date. This reinforces the conclusion that Pyst1 and Pyst2 are members of a distinct and structurally homologous subfamily of dual-specificity (Thr/Tyr) MAP kinase phosphatases. We find that Pyst2 mRNA is constitutively expressed in a wide variety of human cell lines including those derived from ovarian, bladder and breast cancers. While there is no evidence for inducible expression of Pyst2 mRNA in human skin fibroblasts in response to cellular stress, Pyst2 mRNA levels are moderately increased in response to serum stimulation. Pyst2 protein is predominantly cytosolic when expressed in COS-1 cells. In common with Pyst1, Pyst2 shows substrate selectivity for the classical p42 (ERK2) isoform of MAP kinase both in vitro and in vivo, displaying much reduced activity towards stress activated MAP kinase isoforms such as JNK-1 and p38/RK. Pyst2 binds p42 MAP kinase in vivo and both MAP kinase binding and substrate selectivity correlate with the ability of different recombinant MAP and SAP kinases to cause catalytic activation of the Pyst2 phosphatase in vitro.

Protein phosphatases and the regulation of MAP kinase activity

A family of dual specificity (Thr/Tyr) MAP kinase phosphatases (MKPs) have been identified in mammalian cells. These enzymes are implicated in negative feedback control of MAP kinase activity. This idea is supported by genetic and biochemical evidence which implicates homologous enzymes in the regulation of MAP kinases in yeasts and Drosophila. However, recent work in yeasts has shown that, in addition to these dual specificity MKPs, 'classical' tyrosine-specific phosphatases are also involved in the regulated dephosphorylation of MAP kinases. A picture is emerging in which a complex interplay between upstream activators and multiple protein phosphatases is responsible for the regulation of MAP kinase activity. The activities, substrate specificities and subcellular localisation of these protein phosphatases are likely to be key determinants of the biological outcome of signalling through different MAP kinase pathways in mammalian cells and tissues.

Differential regulation of the MAP, SAP and RK/p38 kinases by Pyst1, a novel cytosolic dual-specificity phosphatase

The Pyst1 and Pyst2 mRNAs encode closely related proteins, which are novel members of a family of dual-specificity MAP kinase phosphatases typified by CL100/MKP-1. Pyst1 is expressed constitutively in human skin fibroblasts and, in contrast to other members of this family of enzymes, its mRNA is not inducible by either stress or mitogens. Furthermore, unlike the nuclear CL100 protein, Pyst1 is localized in the cytoplasm of transfected Cos-1 cells. Like CL100/ MKP-1, Pyst1 dephosphorylates and inactivates MAP kinase in vitro and in vivo. In addition, Pyst1 is able to form a physical complex with endogenous MAP kinase in Cos-1 cells. However, unlike CL100, Pyst1 displays very low activity towards the stress-activated protein kinases (SAPKs) or RK/p38 in vitro, indicating that these kinases are not physiological substrates for Pyst1. This specificity is underlined by the inability of Pyst1 to block either the stress-mediated activation of the JNK-1 SAP kinase or RK/p38 in vivo, or to inhibit nuclear signalling events mediated by the SAP kinases in response to UV radiation. Our results provide the first evidence that the members of the MAP kinase family of enzymes are differentially regulated by dual-specificity phosphatases and also indicate that the MAP kinases may be regulated by different members of this family of enzymes depending on their subcellular location.