The structure of mitogen-activated protein kinase p38 at 2.1-A resolution

Cell-cycle regulation and cell-type specification in the developing Drosophila compound eye

During nervous system development stem cell daughters must exit the proliferative cycle to adopt specific neural and glial fates and they must do so in the correct positions. Cell proliferation in the central nervous system occurs in neuroepithelia such as the neural retina and the ventricular zones. As cells are assigned specific fates they migrate out of the plane of the epithelium to form higher layers. Recent evidence from the Drosophila compound eye suggests that a novel mode of Ras pathway regulation may be crucial in both cell-cycle exit and neural patterning: "MAP Kinase cytoplasmic hold".

A Biacore biosensor method for detailed kinetic binding analysis of small molecule inhibitors of p38α mitogen-activated protein kinase

Protein kinases are emerging as one of the most intensely studied classes of enzymes as their central roles in physiologically and clinically important cellular signaling events become more clearly understood. We report here the development of a real-time, label-free method to study protein kinase inhibitor binding kinetics using surface plasmon resonance-based biomolecular interaction analysis (Biacore). Utilizing p38alpha mitogen-activated protein kinase as a model system, we studied the binding properties of two known small molecule p38alpha inhibitors (SB-203580 and SKF-86002). Direct coupling of p38alpha to the biosensor surface in the presence of a reversible structure-stabilizing ligand (SB-203580) consistently produced greater than 90% active protein on the biosensor surface. The dissociation and kinetic constants derived using this Biacore method are in excellent agreement with values determined by other methods. Additionally, we extend the method to study the thermodynamics of small molecule binding to p38alpha and derive a detailed thermodynamic reaction pathway for SB-203580. The Biacore method reported here provides an efficient way to directly and reproducibly examine dissociation constants, kinetics, and thermodynamics for small molecules binding to p38alpha and possibly other protein kinases. Immobilization in the presence of a stabilizing ligand may further represent a broadly applicable paradigm for creation of highly active biosensor surfaces.

Lattice stabilization and enhanced diffraction in human p38α crystals by protein engineering

Mitogen-activated protein (MAP) kinase p38 alpha is activated in response to environmental stress and cytokines, and plays a significant role in inflammatory responses. For these reasons, it is an important target for the treatment of a wide range of inflammatory and autoimmune diseases. The crystals of p38 alpha that we obtained by published procedures were usually small, quite mosaic, and difficult to reproduce and thus posed a difficulty for the intensive high-resolution studies required for a structure-guided drug discovery approach. Based on crystallographic and biochemical evidences, we prepared a single point mutation of a surface cysteine (C162S) and found that it prevents aggregation and improves the homogeneity and stability of the enzyme. This mutation also facilitates the crystallization process and increases the diffracting power of p38 alpha crystals. Surprisingly, we found that the mutation induces a change in the conformation of a nearby surface loop resulting in stronger lattice interactions, consistent with the improved crystal quality. The mutant protein, because of its improved stability and strengthened lattice interactions, thus provides a significantly improved reagent for use in structure-based drug design for this important disease target.

Structural basis for UCN-01 (7-hydroxystaurosporine) specificity and PDK1 (3-phosphoinositide-dependent protein kinase-1) inhibition

PDK1 (3-phosphoinositide-dependent protein kinase-1) is a member of the AGC (cAMP-dependent, cGMP-dependent, protein kinase C) family of protein kinases, and has a key role in insulin and growth-factor signalling through phosphorylation and subsequent activation of a number of other AGC kinase family members, such as protein kinase B. The staurosporine derivative UCN-01 (7-hydroxystaurosporine) has been reported to be a potent inhibitor for PDK1, and is currently undergoing clinical trials for the treatment of cancer. Here, we report the crystal structures of staurosporine and UCN-01 in complex with the kinase domain of PDK1. We show that, although staurosporine and UCN-01 interact with the PDK1 active site in an overall similar manner, the UCN-01 7-hydroxy group, which is not present in staurosporine, generates direct and water-mediated hydrogen bonds with active-site residues. Inhibition data from UCN-01 tested against a panel of 29 different kinases show a different pattern of inhibition compared with staurosporine. We discuss how these differences in inhibition could be attributed to specific interactions with the additional 7-hydroxy group, as well as the size of the 7-hydroxy-group-binding pocket. This information could lead to opportunities for structure-based optimization of PDK1 inhibitors.

PEA-15 binding to ERK1/2 MAPKs is required for its modulation of integrin activation

Activation of Raf-1 suppresses integrin activation, potentially through the activation of extracellular signal-regulated kinases 1 and 2 (ERK1/2). However, bulk ERK1/2 activation does not correlate with suppression. PEA-15 reverses suppression of integrin activation and binds ERK1/2. Here we report that PEA-15 reversal of integrin suppression depends on its capacity to bind ERK1/2, indicating that ERK1/2 function is indeed required for suppression. Mutations in either the death effector domain or C-terminal tail of PEA-15 that block ERK1/2 binding abrogated the reversal of integrin suppression. Furthermore, we used ERK/p38 chimeras and site-directed mutagenesis to identify ERK1/2 residues required for binding PEA-15. Mutations of residues that precede the alphaG helix and within the mitogen-activated protein kinase insert blocked ERK2 binding to PEA-15, but not activation of ERK2. These ERK2 mutants blocked the ability of PEA-15 to reverse suppression of integrin activation. Thus, PEA-15 regulation of integrin activation depends on its binding to ERK1/2. To directly test the role of ERK1/2 localization in suppression, we enforced membrane association of ERK1 and 2 by joining a membrane-targeting CAAX box sequence to them. Both ERK1-CAAX and ERK2-CAAX were membrane-localized and suppressed integrin activation. In contrast to suppression by membrane-targeted Raf-CAAX, suppression by ERK1/2-CAAX was not reversed by PEA-15. Thus, ERK1/2 are the Raf effectors for suppression of integrin activation, and PEA-15 reverses suppression by binding ERK1/2.

p38 MAP kinases: key signalling molecules as therapeutic targets for inflammatory diseases

The p38 MAP kinases are a family of serine/threonine protein kinases that play important roles in cellular responses to external stress signals. Since their identification about 10 years ago, much has been learned of the activation and regulation of the p38 MAP kinase pathways. Inhibitors of two members of the p38 family have been shown to have anti-inflammatory effects in preclinical disease models, primarily through the inhibition of the expression of inflammatory mediators. Several promising compounds have also progressed to clinical trials. In this review, we provide an overview of the role of p38 MAP kinases in stress-activated pathways and the progress towards clinical development of p38 MAP kinase inhibitors in the treatment of inflammatory diseases.

A bipartite mechanism for ERK2 recognition by its cognate regulators and substrates

Mitogen-activated protein (MAP) kinases control gene expression in response to extracellular stimuli and exhibit exquisite specificity for their cognate regulators and substrates. We performed a structure-based mutational analysis of ERK2 to identify surface areas that are important for recognition of its interacting proteins. We show that binding and activation of MKP3 by ERK2 involve two distinct protein-protein interaction sites in ERK2. Thus, the common docking (CD) site composed of Glu-79, Tyr-126, Arg-133, Asp-160, Tyr-314, Asp-316, and Asp-319 are important for high affinity MKP3 binding but not essential for ERK2-induced MKP3 activation. MKP3 activation requires residues Tyr-111, Thr-116, Leu-119, Lys-149, Arg-189, Trp-190, Glu-218, Arg-223, Lys-229, and His-230 in the ERK2 substrate-binding region, located distal to the common docking site. Interestingly, many of the residues important for MKP3 recognition are also used for Elk1 binding and phosphorylation. In addition to the shared residues, there are also residues that are unique to each target recognition. There is evidence indicating that the CD site and the substrate-binding region defined here are also utilized for MEK1 recognition, and indeed, we demonstrate that the binding of MKP3, Elk1, and MEK1 to ERK2 is mutually exclusive. Taken together, our data suggest that the efficiency and fidelity of ERK2 signaling is achieved by a bipartite recognition process. In this model, one part of the ERK2-binding proteins (e.g. the kinase interaction motif sequence) docks to the CD site located on the back side of the ERK2 catalytic pocket for high affinity association, whereas the interaction of the substrate-binding region with another structural element (e.g. the FXFP motif in MKP3 and Elk1) may not only stabilize binding but also provide contacts crucial for modulating the activity and/or specificity of ERK2 target molecules.

Nuclear translocation of activated MAP kinase is developmentally regulated in the developing Drosophila eye

In proneural groups of cells in the morphogenetic furrow of the developing Drosophila eye phosphorylated mitogen activated protein kinase (MAPK) antigen is held in the cytoplasm for hours. We have developed a reagent to detect nuclear MAPK non-antigenically and report our use of this reagent to confirm that MAPK nuclear translocation is regulated by a second mechanism in addition to phosphorylation. This "cytoplasmic hold" of activated MAPK has not been observed in cell culture systems. We also show that MAPK cytoplasmic hold has an essential function in vivo: if it is overcome, developmental patterning in the furrow is disrupted.

Combination of two activating mutations in one HOG1 gene forms hyperactive enzymes that induce growth arrest

Mitogen-activated protein kinases (MAPKs) play key roles in differentiation, growth, proliferation, and apoptosis. Although MAPKs have been extensively studied, the precise function, specific substrates, and target genes of each MAPK are not known. These issues could be addressed by sole activation of a given MAPK, e.g., through the use of constitutively active MAPK enzymes. We have recently reported the isolation of eight hyperactive mutants of the Saccharomyces cerevisiae MAPK Hog1, each of which bears a distinct single point mutation. These mutants acquired high intrinsic catalytic activity but did not impose the full biological potential of the Hog1 pathway. Here we describe our attempt to obtain a MAPK that is more active than the previous mutants both catalytically and biologically. We combined two different activating point mutations in the same gene and found that two of the resulting double mutants acquired unusual properties. These alleles, HOG1(D170A,F318L) and HOG1(D170A,F318S), induced a severe growth inhibition and had to be studied through an inducible expression system. This growth inhibition correlated with very high spontaneous (in the absence of any stimulation) catalytic activity and strong induction of Hog1 target genes. Furthermore, analysis of the phosphorylation status of these active alleles shows that their acquired intrinsic activity is independent of either phospho-Thr174 or phospho-Tyr176. Through fluorescence-activated cell sorting analysis, we show that the effect on cell growth inhibition is not a result of cell death. This study provides the first example of a MAPK that is intrinsically activated by mutations and induces a strong biological effect.

Molecular recognitions in the MAP kinase cascades

The mitogen-activated protein kinase (MAPK) cascades play a pivotal role in many aspects of cellular functions, and are evolutionarily conserved from yeast to mammals. In mammals, there are four subfamily members in the MAPKs. Each MAPK has its own activators, substrates and inactivators. In order to achieve normal cellular functions, the MAPK cascades should transduce signals with high efficiency and fidelity. However, the molecular basis for the mechanism underlying the specific reactions in the MAPK cascades has not been fully understood. The MAPKs form a globular structure without a distinct domain specific for protein-protein interactions. Recent studies revealed two mechanisms regulating the signalling, the docking interaction and the scaffolding. The docking interaction is achieved through the common docking domain (the CD domain) on MAPKs, and is different from a transient enzyme-substrate interaction through the active centre of the enzymes. Almost all the MAPK-interacting molecules have a conserved motif interacting with the CD domain. The scaffolding usually utilizes a third molecule to tether several components of the MAPK cascades. Both of them are thought to regulate the enzymatic specificity and efficiency.

TAB1beta (transforming growth factor-beta-activated protein kinase 1-binding protein 1beta ), a novel splicing variant of TAB1 that interacts with p38alpha but not TAK1

The mitogen-activated protein kinases (MAPKs) play an important role in a variety of biological processes. Activation of MAPKs is mediated by phosphorylation on specific regulatory tyrosine and threonine sites. We have recently found that activation of p38alpha MAPK can be carried out not only by its upstream MAPK kinases (MKKs) but also by p38alpha autophosphorylation. p38alpha autoactivation requires an interaction of p38alpha with TAB1 (transforming growth factor-beta-activated protein kinase 1-binding protein 1). The autoactivation mechanism of p38alpha has been found to be important in cellular responses to a number of physiologically relevant stimuli. Here, we report the characterization of a splicing variant of TAB1, TAB1beta. TAB1 and TAB1beta share the first 10 exons. The 11th and 12th exons of TAB1 were spliced out in TAB1beta, and an extra exon, termed exon beta, downstream of exons 11 and 12 in the genome was used as the last exon in TAB1beta. The mRNA of TAB1beta was expressed in all cell lines examined. The TAB1beta mRNA encodes a protein with an identical sequence to TAB1 except the C-terminal 69 amino acids were replaced with an unrelated 27-amino acid sequence. Similar to TAB1, TAB1beta interacts with p38alpha but not other MAPKs and stimulates p38alpha autoactivation. Different from TAB1, TAB1beta does not bind or activate TAK1. Inhibition of TAB1beta expression with RNA interference in MDA231 breast cancer cells resulted in the reduction of basal activity of p38alpha and invasiveness of MDA231 cells, suggesting that TauAlphaBeta1beta is involved in regulating p38alpha activity in physiological conditions.

Modular structure of a docking surface on MAPK phosphatases

Mitogen-activated protein kinases (MAPKs) must be precisely inactivated to achieve proper functions in the cells. Ten members of dual specificity phosphatases specifically acting on MAPKs, termed MAPK phosphatases (MKPs), have been reported. Each member has its own substrate specificity that should be tightly regulated. However, the molecular mechanism underlying the regulation of the specificity is largely unknown. In the MAPK signaling pathways, docking interactions, which are different from transient enzyme-substrate interaction, are known to regulate the enzymatic specificity. Here we have identified and characterized a docking surface of MKPs. Our results show that a docking surface is composed of a tandem alignment of three subregions (modules): a cluster of positively charged amino acids, a cluster of hydrophobic amino acids, and a cluster of positively charged amino acids (positive-hydrophobic-positive). This modular structure well fits the docking groove on MAPKs that we have previously identified and may contribute to regulating the docking specificity of the MKP family. The position, number, and species of charged amino acids in each module including the central hydrophobic subregion are important factors in regulation of docking to specific MAPKs. This modular structure in the docking interaction may define a novel model of protein-protein interaction that would also regulate other systems.

Crystal structures of MAP kinase p38 complexed to the docking sites on its nuclear substrate MEF2A and activator MKK3b

The structures of the MAP kinase p38 in complex with docking site peptides containing a phi(A)-X-phi(B) motif, derived from substrate MEF2A and activating enzyme MKK3b, have been solved. The peptides bind to the same site in the C-terminal domain of the kinase, which is both outside the active site and distinct from the "CD" domain previously implicated in docking site interactions. Mutational analysis on the interaction of p38 with the docking sites supports the crystallographic models and has uncovered two novel residues on the docking groove that are critical for binding. The two peptides induce similar large conformational changes local to the peptide binding groove. The peptides also induce unexpected and different conformational changes in the active site, as well as structural disorder in the phosphorylation lip.

Osmoregulation of natriuretic peptide receptor signaling in inner medullary collecting duct. A requirement for p38 MAPK

In the inner medullary collecting duct of the terminal nephron, the type A natriuretic peptide receptor (NPR-A) plays a major role in determining urinary sodium content. This nephron segment, by virtue of its medullary location, is subject to very high levels of extracellular tonicity. We have examined the ability of medium tonicity to regulate the activity and expression of this receptor in cultured rat inner medullary collecting duct cells. We found that NaCl (75 mm) and sucrose (150 mm), but not urea (150 mm), increased natriuretic peptide receptor activity, gene expression, and promoter activity. The osmotic stimulus also activated extracellular signal-regulated kinase (ERK), c-Jun NH(2)-terminal kinase (JNK), and p38 mitogen-activated protein kinase (p38 MAPK). In the latter instance the beta isoform was selectively activated. Inhibition of p38 MAPK with SB203580 blocked the osmotic induction of receptor activity and expression, as well as receptor gene promoter activity, whereas inhibition of ERK with PD98059 had no effect. Cotransfection of p38 beta MAPK together with the receptor gene promoter resulted in amplification of the osmotic stimulation of the latter, whereas cotransfection of dominant negative MKK6, but not dominant-negative MEK, completely blocked the osmotic induction of receptor promoter activity. Collectively, the data indicate that extracellular osmolality stimulates receptor activity and receptor gene expression through a specific p38 beta-dependent mechanism, raising the possibility that changes in medullary tonicity could play an important role in the regulation of renal sodium handling in the terminal nephron.

Docking interactions in the mitogen-activated protein kinase cascades

Regulation of cellular functions and responses utilizes a number of the signal transduction pathways. Each pathway should transduce signals with high efficiency and fidelity to avoid unnecessary crosstalks. The mitogen-activated protein kinase (MAPK) cascades regulate a wide variety of cellular functions, including cell proliferation, differentiation, and stress responses. MAPK is activated by MAPK kinase; phosphorylates various targets, including transcription factors and MAPK-activated protein kinases; and is inactivated by several phosphatases. Recent studies have provided a cue to understand the molecular mechanism underlying the signal transduction through the MAPK cascades. In the MAPK cascades, docking interactions, which are achieved through a site outside the catalytic domain of MAPKs, regulate the efficiency and specificity of the enzymatic reactions. The docking interaction is different from a transient enzyme-substrate interaction through the active center. It has been shown that activators, substrates, and inactivators of MAPKs utilize a common site on MAPKs in the docking interaction. Then, the docking interaction may regulate not only the efficiency and specificity of the cascades, but also the ordered and integrated signaling.

Pharmacological inhibitors of MAPK pathways

Mitogen-activated protein kinases [MAPKs, also called extracellular signal-regulated kinases (ERKs)] are constituents of numerous signal transduction pathways, and are activated by protein kinase cascades. Intense efforts are under way to develop and evaluate compounds that target components of MAPK pathways. In this article, the current status of inhibitors of MAPK pathways will be presented with a focus on the properties of small-molecule inhibitors of p38, MEK1 and MEK2 protein kinases. Several of these inhibitors are effective in animal models of disease and have advanced to clinical trials for the treatment of inflammatory diseases and cancer. The clinical utility of specifically targeting a subset of cellular signaling cascades and signaling cascades that regulate pleiotropic cellular processes are being evaluated. The results of these efforts have broad implications for the treatment of many diseases.

Recent advances in plant MAP kinase signalling

Mitogen activated protein kinases (MAPK) are important mediators in signal transmission, connecting the perception of external stimuli to cellular responses. MAPK cascades are involved in signalling various biotic and abiotic stresses, like wounding and pathogen infection, temperature stress or drought, but are also involved in mediating the action of some plant hormones, such as ethylene and auxin. Moreover, MAPKs have been implicated in cell cycle and developmental processes. In Arabidopsis mutant screens and in vivo assays several components of plant MAPK cascades have been identified. This review gives an update of recent advances in plant MAPK signalling and discusses the emerging mechanisms of some selected MAPK pathways.

Isolation of hyperactive mutants of the MAPK p38/Hog1 that are independent of MAPK kinase activation

Mitogen-activated protein kinases (MAPKs) play pivotal roles in growth, development, differentiation, and apoptosis. The exact role of a given MAPK in these processes is not fully understood. This question could be addressed using active forms of these enzymes that are independent of external stimulation and upstream regulation. Yet, such molecules are not available. MAPK activation requires dual phosphorylation, on neighboring Tyr and Thr residues, catalyzed by MAPK kinases (MAPKKs). It is not known how to force MAPK activation independent of MAPKK phosphorylation. Here we describe a series of nine hyperactive (catalytically and biologically), MAPKK-independent variants of the MAPK Hog1. Each of the active molecules contains just a single point mutation. Six mutations are in the conserved L16 domain of the protein. The active Hog1 mutants were obtained through a novel genetic screen that could be applied for isolation of active MAPKs of other families. Equivalent mutations, introduced to the human p38alpha, rendered the enzyme active even when produced in Escherichia coli, showing that the mutations increased the intrinsic catalytic activity of p38. It implies that the activating mutations could be directly used for production of active forms of MAPKs from yeasts to humans and could open the way to revealing their biological functions.

Hydrophobic as Well as Charged Residues in Both MEK1 and ERK2 Are Important for Their Proper Docking

Docking between MEK1 and ERK2 is required for their stable interaction and efficient signal transmission. The MEK1 N terminus contains the ERK docking or D domain that consists of conserved hydrophobic and basic residues. We mutated the hydrophobic and basic residues individually and found that loss of either type reduced MEK1 phosphorylation of ERK2 in vitro and its ability to bind to ERK2 in vivo. Moreover, ERK2 was localized in both the cytoplasm and the nucleus when co-expressed with MEK1 that had mutations in either the hydrophobic or the basic residues. We then identified two conserved hydrophobic residues on ERK2 that play roles in docking with MEK1. Mutating these residues to alanine reduced the interaction of ERK2 with MEK1 in cells. These mutations also reduced the phosphorylation of MEK1 by ERK2 but had little effect on phosphorylation of MBP by ERK2. Finally, we generated docking site mutants in ERK2-MEK1 fusion proteins. Although the mutation of the MEK1 D domain significantly reduced ERK2-MEK1 activity, mutations of the putatively complementary acidic residues and hydrophobic residues on ERK2 did not change its activity. However, both types of mutations decreased the phosphorylation of Elk-1 caused by ERK2-MEK1 fusion proteins. These findings suggest complex interactions of MEK1 D domains with ERK2 that influence its activation and its effects on substrates.

Crystal structure of glycogen synthase kinase 3 beta: structural basis for phosphate-primed substrate specificity and autoinhibition

Glycogen synthase kinase 3 beta (GSK3 beta) plays a key role in insulin and Wnt signaling, phosphorylating downstream targets by default, and becoming inhibited following the extracellular signaling event. The crystal structure of human GSK3 beta shows a catalytically active conformation in the absence of activation-segment phosphorylation, with the sulphonate of a buffer molecule bridging the activation-segment and N-terminal domain in the same way as the phosphate group of the activation-segment phospho-Ser/Thr in other kinases. The location of this oxyanion binding site in the substrate binding cleft indicates direct coupling of P+4 phosphate-primed substrate binding and catalytic activation, explains the ability of GSK3 beta to processively hyperphosphorylate substrates with Ser/Thr pentad-repeats, and suggests a mechanism for autoinhibition in which the phosphorylated N terminus binds as a competitive pseudosubstrate with phospho-Ser 9 occupying the P+4 site.

Mitogen-activated protein (MAP) kinase pathways: regulation and physiological functions

Mitogen-activated protein (MAP) kinases comprise a family of ubiquitous proline-directed, protein-serine/threonine kinases, which participate in signal transduction pathways that control intracellular events including acute responses to hormones and major developmental changes in organisms. MAP kinases lie in protein kinase cascades. This review discusses the regulation and functions of mammalian MAP kinases. Nonenzymatic mechanisms that impact MAP kinase functions and findings from gene disruption studies are highlighted. Particular emphasis is on ERK1/2.

The kinetic mechanism of the dual phosphorylation of the ATF2 transcription factor by p38 mitogen-activated protein (MAP) kinase alpha. Implications for signal/response profiles of MAP kinase pathways

The mitogen-activated protein kinases (MAPKs) are a family of enzymes conserved among eukaryotes that regulate cellular activities in response to numerous external signals. They are the terminal component of a three-kinase cascade that is evolutionarily conserved and whose arrangement appears to offer considerable flexibility in encompassing the diverse biological situations for which they are employed. Although multistep protein phosphorylation within mitogen-activated protein kinase (MAPK) cascades can dramatically influence the sensitivity of signal propagation, an investigation of the mechanism of multisite phosphorylation by a MAPK has not been reported. Here we report a kinetic examination of the phosphorylation of Thr-69 and Thr-71 of the glutathione S-transferase fusion protein of the trans-activation domain of activating transcription factor-2 (GST-ATF2-(1-115)) by p38 MAPKalpha (p38alpha) as a model system for the phosphorylation of ATF2 by p38alpha. Our experiments demonstrated that GST-ATF2-(1-115) is phosphorylated in a two-step distributive mechanism, where p38alpha dissociates from GST-ATF2-(1-115) after the initial phosphorylation of either Thr-69 or Thr-71. Whereas p38alpha showed similar specificity for Thr-71 and Thr-69 in the unphosphorylated protein, it displayed a marked difference in specificity toward the mono-phosphoisomers. Phosphorylation of Thr-71 had no significant effect on the rate of Thr-69 phosphorylation, but Thr-69 phosphorylation reduced the specificity, k(cat)/K(M), of p38alpha for Thr-71 by approximately 40-fold. Computer simulation of the mechanism suggests that the activation of ATF2 by p38alpha in vivo is essentially Michaelian and provides insight into how the kinetics of a two-step distributive mechanism can be adapted to modulate effectively the sensitivity of a signal transduction pathway. This work also suggests that whereas MAPKs utilize docking interactions to bind substrates, they can be weak and transient in nature, providing just enough binding energy to promote the phosphorylation of a specific substrate.

Identification of a docking groove on ERK and p38 MAP kinases that regulates the specificity of docking interactions

MAP kinases (MAPKs) form a complex with MAPK kinases (MAPKKs), MAPK-specific phosphatases (MKPs) and various targets including MAPKAPKs. These docking interactions contribute to regulation of the specificity and efficiency of the enzymatic reactions. We have previously identified a docking site on MAPKs, termed the CD (common docking) domain, which is utilized commonly for docking interactions with MAPKKs, MKPs and MAPKAPKs. However, the CD domain alone does not determine the docking specificity. Here we have identified a novel site on p38 and ERK2 MAPKs that regulates the docking specificity towards MAPKAPKs. Remarkably, exchange of two amino acids in this site of ERK2 for corresponding residues of p38 converted the docking specificity for MAPKAPK-3/3pk, which is a dominant target of p38, from the ERK2 type to the p38 type, and vice versa. Furthermore, our detailed analyses with a number of MAPKAPKs and MKPs suggest that a groove in the steric structure of MAPKs, which comprises the CD domain and the site identified here, serves as a common docking region for various MAPK-interacting molecules.

Substrate recognition domains within extracellular signal-regulated kinase mediate binding and catalytic activation of mitogen-activated protein kinase phosphatase-3

Mitogen-activated protein (MAP) kinase phosphatase-3 (MKP-3) is a dual specificity phosphatase that inactivates extracellular signal-regulated kinase (ERK) MAP kinases. This reflects tight and specific binding between ERK and the MKP-3 amino terminus with consequent phosphatase activation and dephosphorylation of the bound MAP kinase. We have used a series of p38/ERK chimeric molecules to identify domains within ERK necessary for binding and catalytic activation of MKP-3. These studies demonstrate that ERK kinase subdomains V-XI are necessary and sufficient for binding and catalytic activation of MKP-3. These domains constitute the major COOH-terminal structural lobe of ERK. p38/ERK chimeras possessing these regions display increased sensitivity to inactivation by MKP-3. These data also reveal an overlap between ERK domains interacting with MKP-3 and those known to confer substrate specificity on the ERK MAP kinase. Consistent with this, we show that peptides representing docking sites within the target substrates Elk-1 and p90(rsk) inhibit ERK-dependent activation of MKP-3. In addition, abolition of ERK-dependent phosphatase activation following mutation of a putative kinase interaction motif (KIM) within the MKP-3 NH(2) terminus suggests that key sites of contact for the ERK COOH-terminal structural lobe include residues localized between the Cdc25 homology domains (CH2) found conserved between members of the DSP gene family.

p38 MAPK signalling cascades: ancient roles and new functions

p38 MAPKs are a conserved subfamily of MAPKs involved in the response to stress found in eukaryotic cells from yeast to mammals. The recent isolation of genes coding for members of this signalling cascade in Drosophila has provided us with the genetic tools to study their various biological roles and their regulatory interactions with other signalling pathways. This cascade participates in the immune response, a function that is remarkably conserved between flies and humans. Additionally, it appears to exert other fundamental roles during development, in cell fate specification in imaginal discs, and in cell polarity during oogenesis. These functions involve genetic and biochemical interactions with other signalling cascades, the decapentaplegic/TGFbeta, the wingless/Wnt and the torpedo/Ras-ERK pathways. In the near future, we can expect a flurry of information that will allow us to draw a comprehensive picture of the roles of signalling networks mediated by p38s during development.

Development of a p38 Kinase Binding Assay for High Throughput Screening

p38 is a member of the mitogen-activated protein kinase (MAPK) family of serine/threonine kinases, which is activated by cellular stressors and has been shown to be a critical enzyme in the synthesis and action of proinflammatory cytokines, tumor necrosis factor-a (TNF-alpha) and interleukin-1beta (IL-1beta). A group of pyridinyl imidazole compounds such as SB202190 have been identified as selective inhibitors of p38 that bind directly to the ATP pocket of the enzyme. These compounds inhibit the p38 kinase activity, block TNF-alpha and IL-1beta secretion both in vivo and in vitro and are found to be effective in animal models of arthritis, bone resorption, and endotoxin shock. We postulated that other classes of compounds capable of competing the binding of pyridinyl imidazole with p38 enzyme could have efficacy in the treatment of inflammatory diseases. Therefore, a simple and robust assay was developed to measure the ability of small molecules to inhibit the binding of tritium-labeled pyridinyl imidazole, SB202190, to recombinant p38 kinase. For assay development, the human p38 gene was cloned in baculovirus and then expressed in insect cells. Tritiated SB202190 was synthesized and used as the p38 ligand for a competitive filter binding assay. This assay has been used successfully to screen both synthetic and combinatorial chemical libraries for other classes of p38 kinase inhibitors.

Amino-terminal-derived JNK fragment alters expression and activity of c-Jun, ATF2, and p53 and increases H2O2-induced cell death

The stress-activated protein kinase JNK plays an important role in the stability and activities of key regulatory proteins, including c-Jun, ATF2, and p53. To better understand mechanisms underlying the regulation of JNK activities, we studied the effect of expression of the amino-terminal JNK fragment (N-JNK; amino acids 1-206) on the stability and activities of JNK substrates under nonstressed growth conditions, as well as after exposure to hydrogen peroxide. Mouse fibroblasts that express N-JNK under tetracycline-off (tet-off) inducible promoter exhibited elevated expression of c-Jun, ATF2, and p53 upon tetracycline removal. This increased coincided with elevated transcriptional activities of p53, but not of c-Jun or ATF2, as reflected in luciferase activities of p21(Waf1/Cip1)-Luc, AP1-Luc, and Jun2-Luc, respectively. Expression of N-JNK in cells that were treated with H(2)O(2) impaired transcriptional output as reflected in a delayed and lower level of c-Jun-, limited ATF2-, and reduced p53-transcriptional activities. N-JNK elicited an increase in H(2)O(2)-induced cell death, which is p53-dependent, because it was not seen in p53 null cells yet could be observed upon coexpression of p53 and N-JNK. The ability to alter the activity of ATF2, c-Jun, and p53 and the degree of stress-induced cell death by a JNK-derived fragment identifies new means to elucidate the nature of JNK regulation and to alter the cellular response to stress.

Mkp1 of Pneumocystis carinii associates with the yeast transcription factor Rlm1 via a mechanism independent of the activation state

The mitogen-activated protein (MAP) kinase Mkp1 of the fungal pathogen Pneumocystis carinii is a functional MAP kinase that complements the loss of Slt2p, the MAP kinase component of the cell integrity pathway of Saccharomyces cerevisiae, and is activated within P. carinii in response to oxidative stress. Mkp1 displays an unusual feature in that it contains a phosphorylation motif repeat (TEYMTEY) within the activation loop not present in any other fungal MAPK identified to date. Mutagenesis of the T186,Y188 phosphorylation motif within the activation domain of Mkp1 results in the loss of detectable kinase activity but still retains partial complementation function. In addition to the ability of Mkp1 to restore partial activity to the cell integrity pathway in the absence of phosphorylatable residues within the activation loop, the association of Mkp1 with a substrate of Slt2p, the transcription factor Rlm1p, can also occur in the absence of MAP kinase activation. The results of this study suggest that the presence of phosphorylatable residues within the activation loop of Mkp1 is not absolutely required for functional (complementation) activity or for the association of Mkp1 with the transcription factor Rlm1p. In contrast, the catalytic lysine of the ATP-binding domain of Mkp1 is necessary for both complementation function and interaction with Rlm1p.

Inhibition of p38 MAP kinase as a therapeutic strategy

Since the discovery of p38 MAP kinase in 1994, our understanding of its biology has progressed dramatically. The key advances include (1) identification of p38 MAP kinase homologs and protein kinases that act upstream and downstream from p38 MAP kinase, (2) identification of interesting and potentially important substrates, (3) elucidation of the role of p38 MAP kinase in cellular processes and (4) the establishment of the mechanism by which the pyridinylimidazole p38 MAP kinase inhibitors inhibit enzyme activity. It is now known that there are four members of the p38 MAP kinase family. They differ in their tissue distribution, regulation of kinase activation and subsequent phosphorylation of downstream substrates. They also differ in terms of their sensitivities toward the p38 MAP kinase inhibitors. The best-studied isoform is p38 alpha, whose activation has been observed in many hematopoietic and non-hematopoietic cell types upon treatment with appropriate stimuli. The pyridinylimidazole compounds, exemplified by SB 203580, were originally prepared as inflammatory cytokine synthesis inhibitors that subsequently were found to be selective inhibitors of p38 MAP kinase. SB 203580 inhibits the catalytic activity of p38 MAP kinase by competitive binding in the ATP pocket. X-ray crystallographic studies of the target enzyme complexed with inhibitor reinforce the observations made from site-directed mutagenesis studies, thereby providing a molecular basis for understanding the kinase selectivity of these inhibitors. The p38 MAP kinase inhibitors are efficacious in several disease models, including inflammation, arthritis and other joint diseases, septic shock, and myocardial injury. In all cases, p38 activation in key cell types correlated with disease initiation and progression. Treatment with p38 MAP kinase inhibitors attenuated both p38 activation and disease severity. Structurally diverse p38 MAP kinase inhibitors have been tested extensively in preclinical studies.

Structure and polymorphism of two stress-activated protein kinase genes centromeric of the MHC: SAPK2a and SAPK4

As MHC genes are potent determinants of susceptibility to immunopathological diseases, the mapping of SAPK2a (CSBP) and SAPK4 to chromosome 6p 21.2-21.3 suggested that these genes may mediate the effects of the MHC on disease. Here we describe the genomic structure and localisation of both genes approximately 2.3Mb centromeric of HLA-DP. Examination of the complete coding region and selected intronic regions of SAPK2a and SAPK4 from 22 human EBV-transformed B-cell lines of different MHC haplotypes and racial background revealed complete sequence conservation. There were no notable differences in levels of expression of SAPK2a and SAPK4 mRNA in cell lines of different MHC haplotypes or racial origin. Examination of the SAPK2a and SAPK4 sequences from two chimpanzees revealed 3 nucleotide differences between human and chimpanzee in each gene resulting in only one amino acid change in SAPK4, and 6 nucleotide substitutions plus 2 deletions in 600bp of intronic sequence from SAPK4. This highlights the selective pressure placed on these genes to maintain their protein sequence, but does not favour a role in genetic regulation of disease or provide evidence of linkage disequilibrium with the MHC.

MAP kinase pathways activated by stress: the p38 MAPK pathway

A stress-activated serine/threonine protein kinase, p38 mitogen-activated protein kinase (p38 MAPK), belongs to the MAP kinase superfamily. Diverse extracellular stimuli, including ultraviolet light, irradiation, heat shock, high osmotic stress, proinflammatory cytokines and certain mitogens, trigger a stress-regulated protein kinase cascade culminating in activation of p38 MAPK through phosphorylation on a TGY motif within the kinase activation loop. p38 MAPK appears to play a major role in apoptosis, cytokine production, transcriptional regulation, and cytoskeletal reorganization, and has been causally implicated in sepsis, ischemic heart disease, arthritis, human immunodeficiency virus infection, and Alzheimer's disease. The availability of specific inhibitors helps to clarify the role that p38 MAPK plays in these processes, and may ultimately offer therapeutic benefit for certain critically ill patients.

A method to predict residues conferring functional differences between related proteins: application to MAP kinase pathways

Physicochemical properties are potentially useful in predicting functional differences between aligned protein subfamilies. We present a method that considers physicochemical properties from ancestral sequences predicted to have given rise to the subfamilies of interest by gene duplication. Comparison between two map kinases subfamilies, p38 and ERK, revealed a region that had an excess of change in properties after gene duplication followed by conservation within the two subfamilies. This region corresponded to that experimentally defined as important for substrate and pathway specificity. The derived scores for the region of interest were found to differ significantly in their distribution compared to the rest of the protein when the Kolmogorov-Smirnov test was applied (p = 0.005). Thus, the incorporation of ancestral physicochemical properties is useful in predicting functional differences between protein subfamilies. In addition, the method was applied to the MKK and MAPK components of the p38 and JNK pathways. These proteins showed a similar pattern in their evolution and regions predicted to confer functional differences are discussed.

Mxi2, a splice variant of p38 stress-activated kinase, is a distal nephron protein regulated with kidney ischemia

Mxi2 is one of three known alternative spliced forms of the stress-activated mitogen-activated protein kinase p38 (CSBP). Mxi2 was originally identified as a Max-interacting protein and is the smallest member of the family of stress-activated kinases isolated to date. Mxi2 lacks most of the XI domain found in p38 and instead has a distinct COOH-terminal sequence of 17 amino acids. Here we present the genomic structure of the Mxi2/p38 locus on human chromosome 6q21.2/21.3 and establish the origin of the three spliced forms of p38. Using Mxi2-specific antibodies in mouse organs, we found the Mxi2 protein to be present exclusively in the kidney. Mxi2 is present predominantly in the distal tubule of the nephron and the level of the protein decreased during kidney ischemia-reperfusion. Stress signals or other known activators of the p38 pathway including MAP kinase-kinase 3 and MAP kinase-kinase 6 did not induce the kinase activity of Mxi2 using ATF-2 as a substrate. With the use of hybrid proteins encoding different portions of Mxi2 and p38 polypeptides, the different properties of Mxi2 can be assigned to its unique COOH terminus.

Kinetic mechanism of the p38-alpha MAP kinase: phosphoryl transfer to synthetic peptides

p38 is a member of the mitogen-activated protein (MAP) kinase family. Activation (phosphorylation) of p38 acts as a switch for the transcriptional and translational regulation of a number of proteins, including the proinflammatory cytokines. Investigation of a set of small peptides revealed that, as with protein substrates, p38-alpha behaves as a proline-directed Ser/Thr MAP kinase for a peptide substrate, peptide 4 (IPTSPITTTYFFFKKK). We investigated the steady-state kinetic mechanism of the p38-alpha-catalyzed kinase reaction with EGF receptor peptide, peptide 1, as a substrate. Lineweaver-Burk analysis of the substrate kinetics yielded a family of lines intersecting to the left of the ordinate, with either ATP or peptide 1 as the varied substrate. Kinetic analysis in the presence of ADP yielded a competitive inhibition pattern when ATP was the varied substrate and a noncompetitive pattern if peptide 1 was the varied substrate. At saturating peptide substrate concentrations, inhibition by phosphopeptide product yielded an uncompetitive pattern when ATP was the varied substrate. These data are consistent with ordered binding with ATP as the initial substrate. We provide further evidence of the existence of a productive p38.ATP binary complex in that (a) activated p38-alpha has intrinsic ATPase activity, (b) ATPase and kinase activities are coupled, and (c) inhibitors of ATPase activity also inhibit the kinase activity with a similar inhibition constant. The k(cat) for the kinase reaction was lowered by 1.8-fold when ATP-gamma-S was used. Microviscosity linearly affected the k(cat) values of both the ATP and ATP-gamma-S reactions with a slope of about 0.8. These observations were interpreted to mean that the phosphoryl transfer step is not rate-limiting and that the release of product and/or enzyme isomerization is a possible rate-limiting step(s).

A conserved docking motif in MAP kinases common to substrates, activators and regulators

Mitogen-activated protein kinases (MAPKs) are specifically phosphorylated and activated by the MAPK kinases, phosphorylate various targets such as MAPK-activated protein kinases and transcription factors, and are inactivated by specific phosphatases. Recently, docking interactions via the non-catalytic regions of MAPKs have been suggested to be important in regulating these reactions. Here we identify docking sites in MAPKs and in MAPK-interacting enzymes. A docking domain in extracellular-signal-regulated kinase (ERK), a MAPK, serves as a common site for binding to the MAPK kinase MEK1, the MAPK-activated protein kinase MNK1 and the MAPK phosphatase MKP3. Two aspartic acids in this domain are essential for docking, one of which is mutated in the sevenmaker mutant of Drosophila ERK/Rolled. A corresponding domain in the MAPKs p38 and JNK/SAPK also serves as a common docking site for their MEKs, MAPK-activated protein kinases and MKPs. These docking interactions increase the efficiency of the enzymatic reactions. These findings reveal a hitherto unidentified docking motif in MAPKs that is used in common for recognition of their activators, substrates and regulators.

The p38 signal transduction pathway: activation and function

The p38 signalling transduction pathway, a Mitogen-activated protein (MAP) kinase pathway, plays an essential role in regulating many cellular processes including inflammation, cell differentiation, cell growth and death. Activation of p38 often through extracellular stimuli such as bacterial pathogens and cytokines, mediates signal transduction into the nucleus to turn on the responsive genes. p38 also transduces signals to other cellular components to execute different cellular responses. In this review, we summarize the characteristics of the major components of the p38 signalling transduction pathway and highlight the targets of this pathway and the physiological function of the p38 activation.

Identification of two distinct regions of p38 MAPK required for substrate binding and phosphorylation

The mechanism by which different mitogen activated protein kinases (MAPKs) distinguish between different substrates is poorly understood. For example, p38 and SAPK4 are two closely related p38 MAPKs that both phosphorylate ATF2 and MBP. However, p38 phosphorylates MAPKAPK-2 and -3, whereas SAPK4 does not. In this study, we have used mutagenesis to determine the regions of p38 required for substrate selection. Alanine scanning mutagenesis identified one region of p38 that was required for its ability to phosphorylate MAPKAPK-2 and -3, but that did not significantly affect its binding to these substrates. Chimeras of p38 and SAPK4 identified a second region of p38 that affected the ability of p38 to both bind and phosphorylate MAPKAPK-2 and -3. Hence, we show for the first time that MAPKs contain two distinct regions for recognizing and phosphorylating protein substrates.

Kinetic mechanism and ATP-binding site reactivity of p38gamma MAP kinase

Activated p38gamma MAP kinase exhibited significant basal ATPase activity in the absence of a kinase substrate, and addition of a phosphoacceptor substrate increased k(cat)/K(m)20-fold. AMP-PCP was competitive with ATP binding and non-competitive with phosphoacceptor substrate binding. The nucleotide binding site affinity label 5'-(p-fluorosulfonylbenzoyl)adenosine (FSBA) bound stoichiometrically at Lys-56 in the ATP site of both unphosphorylated and activated p38gamma. AMP-PCP only protected the activated enzyme from FSBA inactivation, implying that AMP-PCP does not bind unphosphorylated p38gamma. Basal ATPase activities were also observed for activated p38alpha, ERK2 and JNK3 suggesting that the enzymatic mechanism may be similar for all classes of MAP kinases.

MAP kinases in plant signal transduction: how many, and what for?

Mitogen-activated protein kinase (MAPK) pathways are protein kinase cascades that have a function in the transduction of extracellular signals to intracellular targets in all eukaryotes. Distinct MAPK pathways are regulated by different signals and have a role in a wide variety of physiological processes. In plants there is evidence for a role of MAPKs in the signaling of pathogens, abiotic stresses, plant hormones, and cell cycle cues. A large number of distinct MAPKs in plants have been identified that are all most similar to the animal ERK MAPKs. By sequence alignment all available full length plant MAPKs can be grouped into five subfamilies. Functional data exist for members of four subfamilies and show that different subfamilies encode MAPKs for specific functions. Analysis of partial MAPK sequences from full length, truncated cDNAs and expressed sequence tags (ESTs) revealed the presence of two new subfamilies in the plant MAPK superfamily. Signature sequences valid for the superfamily of plant MAPKs and each subfamily were derived and should help in future classification of novel MAPKs. The future challenge is to unambiguously assign functions to each MAPK and decipher the other partners of their signaling pathways.

Pyridinylimidazole compound SB 203580 inhibits the activity but not the activation of p38 mitogen-activated protein kinase

p38 MAPK is a Ser/Thr protein kinase activated by various inflammatory cytokines and a variety of stress stimuli. It is involved in many physiological processes, including the production of inflammatory cytokines. We have previously reported the design and synthesis of a series of pyridinylimidazole compounds that are selective inhibitors of p38 MAPK. These compounds, exemplified by SB 203580, are exceptionally effective in cell-based assays, including the inhibition of inflammatory cytokine production. SB 203580 is widely used as a tool to dissect the role of p38 MAPK in various physiological processes. It has previously been established that SB 203580 acts primarily to block the catalytic activity of p38 MAPK. However, it has been suggested that in cells, the compounds could also inhibit p38 MAPK activation by virtue of their ability to bind to the inactive enzyme. We undertook careful studies to definitively demonstrate that treatment with SB 203580 had no effect on Thr(180) and Tyr(182) phosphorylation, and hence activation of p38 in vivo. SB 203580, however, potently inhibited the activity of p38 MAPK as demonstrated by the inhibition of the activation of MAPKAP K2, a specific physiological substrate of p38 MAPK. This was observed regardless of stimuli or cell type. Identical results were obtained when the p38 MAPK cascade was partially reconstituted in vitro. Thus, our data clearly indicate that SB 203580 specifically inhibits the activity of p38 MAPK but not its activation by upstream MAPKK.

Opposing effects of sodium salicylate and haematopoietic cytokines IL-3, IL-5 and GM-CSF on mitogen-activated protein kinases and apoptosis of EoL-1 cells

Haematopoietic cytokines such as IL-3, IL-5 and GM-CSF not only activate eosinophils but also prolong their life span by inhibiting their apoptotic cell death. We have studied the effects of IL-3, IL-5 and GM-CSF on apoptosis and mitogen-activated protein kinases (MAPKs) in a human eosinophilic leukaemic cell line (EoL-1). Results demonstrated that all three cytokines could trigger the receptor-mediated activation of extracellular signal-regulated kinase (ERK) within one hour but not p38 MAPK activity in EoL-1 cells. In contrast, sodium salicylate (NaSal), a nonsteroidal anti-inflammatory drug (NSAID), could activate p38 MAPK but not ERK within one hour. Both cytokines and specific p38 MAPK inhibitor SB 203580 could partly block the NaSal-induced apoptosis in EoL-1 cells. A specific MAPK/ERK kinase (MEK) inhibitor, PD 098059, could induce apoptosis and eliminate the protective effect of IL-3, IL-5 and GM-CSF against NaSal-induced apoptosis in EoL-1 cells. Taken together, cytokines IL-3, IL-5 and GM-CSF could prolong EoL-1 cells survival through the transient activation of ERK. On the other hand, activation of p38 MAPK in EoL-1 cells by NaSal could lead to apoptosis. Activation of p38 MAPK and the resulting induction of apoptosis in EoL-1 cells may be important to explain the anti-inflammatory action of NSAID in allergic inflammation.

Activation and expression of ERK, JNK, and p38 MAP-kinases in isolated islets of Langerhans: implications for cultured islet survival

Isolation and purification of islet cells exposes them to ischemic, osmotic and mechanical stresses. The objective of this study was to determine the roles of the MAP-kinases in islets immediately following isolation. During the first 48 h, activity of JNK1 and JNK2 declined markedly. Activity of p38 increased steadily with time in culture while extracellular signal regulated kinase (ERK) activity declined dramatically within 24 h post-isolation. High p38 activation relative to ERK activation immediately following isolation correlated with a decrease in islet survival after 36 h in culture. Absence and/or transiency of ERK signaling in conjunction with sustained activation of p38 pathway could be an important regulator of cell death in islets during and following their isolation by commonly employed procedures.

Phosphorylation of MAP kinases by MAP/ERK involves multiple regions of MAP kinases

Mitogen-activated protein (MAP) kinases are activated with great specificity by MAP/ERK kinases (MEKs). The basis for the specific activation is not understood. In this study chimeras composed of two MAP kinases, extracellular signal-regulated protein kinase 2 and p38, were assayed in vitro for phosphorylation and activation by different MEK isoforms to probe the requirements for productive interaction of MAP kinases with MEKs. Experimental results and modeling support the conclusion that the specificity of MEK/MAP kinase phosphorylation results from multiple contacts, including surfaces in both the N- and C-terminal domains.