Structural organization of MAP-kinase signaling modules by scaffold proteins in yeast and mammals

Specificity of MAP kinase signaling in yeast differentiation involves transient versus sustained MAPK activation

Signals transmitted by common components often elicit distinct (yet appropriate) outcomes. In yeast, two developmental options-mating and invasive growth-are both regulated by the same MAP kinase cascade. Specificity has been thought to result from specialized roles for the two MAP kinases, Kss1 and Fus3, and because Fus3 prevents Kss1 from gaining access to the mating pathway. Kss1 has been thought to participate in mating only when Fus3 is absent. Instead, we show that Kss1 is rapidly phosphorylated and potently activated by mating pheromone in wild-type cells, and that this is required for normal pheromone-induced gene expression. Signal identity is apparently maintained because active Fus3 limits the extent of Kss1 activation, thereby preventing inappropriate signal crossover.

Harpin induces activation of the Arabidopsis mitogen-activated protein kinases AtMPK4 and AtMPK6

Mitogen-activated protein kinases (MAPKs) are key enzymes that mediate adaptive responses to various abiotic and biotic stresses, including pathogen challenge. The proteinaceous bacterial elicitor harpin (secreted by Pseudomonas syringae pv syringae) activates two MAPKs in suspension cultures of Arabidopsis var. Landsberg erecta. In this study, we show that harpin and exogenous hydrogen peroxide (H(2)O(2)) activate myelin basic protein kinases in Arabidopsis leaves. Using anti-AtMPK4 and anti-AtMPK6 antibodies, we identify the harpin-activated MAPKs in both leaves and suspension cultures as AtMPK4 and AtMPK6, and show that H(2)O(2), generated by Arabidopsis cells in response to challenge with harpin, activates only AtMPK6. However, treatments with catalase, which removes H(2)O(2), or diphenylene iodonium, which inhibits superoxide and H(2)O(2) production, do not inhibit harpin-induced activation of AtMPK4 or AtMPK6. In addition, activation of AtMPK4 but not AtMPK6 is inhibited by the MAPK kinase inhibitor PD98059. Neither harpin nor H(2)O(2) has any effect on AtMPK4 or AtMPK6 gene expression. In addition, the expression of AtMEKK1, AtMEK1, or AtMKK2, previously shown to be potential functional partners of AtMPK4, were not affected by either harpin or H(2)O(2) treatments. These data suggest that harpin activates several signaling pathways, one leading to stimulation of the oxidative burst and others leading to the activation of AtMPK4 or AtMPK6.

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.

The role of ERK 1/2 and p38 MAP-kinase pathways in taxol-induced apoptosis in human ovarian carcinoma cells

Taxol is an anticancer agent of natural origin with significant activity against a number of human cancers including ovarian and breast carcinomas. Its cytotoxic activity has been attributed to its ability to stabilize microtubules and to promote microtubule assembly. Recently it has become clearer that Taxol has additional activities including effects in cell signaling and gene expression. We have shown previously that Taxol activates ERK 1/2 MAP-kinases and results in the formation of GRB2/SHC complexes in murine macrophage-like RAW 267.4 cells. Here we demonstrate that Taxol activates ERK 1/2 and p38 MAP-kinases in human ovarian carcinoma cells with distinct kinetics. Activation of ERK1/2 has been observed at low concentrations of Taxol (1-100 nM) within 0.5-6 h, whereas longer exposure(24 h) to nanomolar concentrations of Taxol resulted in an abrogation of the ERK1/2 phosphorylation/activation. Higher concentrations (1-10 microM) resulted in a sharp inhibition of ERK1/2 activity. p38 kinase was activated by high concentrations (1-10 microM) of Taxol within 2 h and remained active for more than 24 h. The kinetic studies showed that these effects of Taxol coincided with an inhibition of proliferation, and the onset of apoptosis. The appearance of the fragmented chromatin visualized by DAPI staining, and DNA fragments seen on an agarose gel, coincided with the decrease in ERK1/2 activation and concomitant increase of the level of active p38 MAPK. The inhibitor PD98059 abrogated ERK 1/2 activation and increased the cytotoxic effect of Taxol. An inhibitor of p38 kinase, SB203580, protected the cells partially from Taxol and, unexpectedly, activated ERK 1/2 kinases. We conclude that the alternative use of ERK1/2 and p38 MAP-kinase pathways may be necessary for the transition from proliferation state to Taxol-induced apoptosisin human ovarian carcinoma cells.

MEK6 regulates human involucrin gene expression via a p38alpha - and p38delta -dependent mechanism

A signaling cascade that includes protein kinase C (PKC), Ras, and MEKK1 regulates involucrin (hINV) gene expression in epidermal keratinocytes (Efimova, T., LaCelle, P., Welter, J. F., and Eckert, R. L. (1998) J. Biol. Chem. 273, 24387-24395 and Efimova, T., and Eckert, R. L. (2000) J. Biol. Chem. 275, 1601-1607). Because signal transfer downstream of MEKK1 may involve several MAPK kinases (MEKs), it is important to evaluate the regulatory role of each MEK isoform. In the present study we evaluate the role of MEK6 in transmitting this signal. Constitutively active MEK6 (caMEK6) increases hINV promoter activity and increases endogenous hINV levels. The caMEK6-dependent increase in gene expression is inhibited by the p38 MAPK inhibitor, SB203580, and is associated with a marked increase in p38alpha MAPK activity; JNK and ERK kinases are not activated. In addition, hINV gene expression is inhibited by dominant-negative p38alpha and increased when caMEK6 and p38alpha are co-expressed. caMEK6 also activates p38delta, but p38delta inhibits the caMEK6-dependent activation. These results suggest that MEK6 increases hINV gene expression by regulating the balance between activation of p38alpha, which increases gene expression, and p38delta, which decreases gene expression.

Fibroblast growth factor homologous factors are intracellular signaling proteins

Fibroblast growth factors (FGFs) mediate cell growth, differentiation, migration, and morphogenesis by binding to the extracellular domain of cell surface receptors, triggering receptor tyrosine phosphorylation and signal transduction [1-5]. FGF homologous factors (FHFs) were discovered within vertebrate DNA sequence databases by virtue of their sequence similarity to FGFs [3, 6, 7], but the mechanism of FHF action has not been reported. We show here that FHF-1 is associated with the MAP kinase (MAPK) scaffold protein Islet-Brain-2 (IB2) [8] in the brain and in specific cell lines. FHF/IB2 interaction is highly specific, as FHFs do not bind to the related scaffold protein IB1(JIP-1b) [9, 10], nor can FGF-1 bind to IB2. We further show that FHFs enable IB2 to recruit a specific MAPK in transfected cells, and our data suggest that the scaffolds IB1 and IB2 have different MAPK specificities. Hence, FHFs are intracellular components of a tissue-specific protein kinase signaling module.

Mixed lineage kinase-dependent JNK activation is governed by interactions of scaffold protein JIP with MAPK module components

It has been proposed that JNK-interacting proteins (JIP) facilitate mixed lineage kinase-dependent signal transduction to JNK by aggregating the three components of a JNK module. A new model for the assembly and regulation of these modules is proposed based on several observations. First, artificially induced dimerization of dual leucine zipper-bearing kinase (DLK) confirmed that DLK dimerization is sufficient to induce DLK activation. Secondly, under basal conditions, DLK associated with JIP is held in a monomeric, unphosphorylated and catalytically inactive state. Thirdly, JNK recruitment to JIP coincided with significantly decreased affinity of JIP and DLK. JNK promoted the dimerization, phosphorylation and activation of JIP-associated DLK. Similarly, treatment of cells with okadaic acid inhibited DLK association with JIP and resulted in DLK dimerization in the presence of JIP. In summary, JIP maintains DLK in a monomeric, unphosphorylated, inactive state. Upon stimulation, JNK-JIP binding affinity increases while JIP-DLK interaction affinity is attenuated. Dissociation of DLK from JIP results in subsequent DLK dimerization, autophosphorylation and module activation. Evidence is provided that this model holds for other MLK-dependent JNK modules.

Heterogeneity of signal transduction at the subcellular level: microsphere-based focal EGF receptor activation and stimulation of Shc translocation

Epidermal growth factor receptor (EGFR, erbB1) activation and translocation of the Shc adaptor protein to activated receptors were analyzed at the subcellular level by dual-label immunofluorescence and confocal laser scanning microscopy in conjunction with a new microsphere-based protocol. In the Quantitative Microsphere Recruitment Assay (QMRA) introduced here, epidermal growth factor-coated 1 μm diameter microspheres were distributed over the surface of adherent tissue culture cells expressing the receptor. High-resolution confocal microscopy of a fusion construct of the receptor and the green fluorescent protein expressed in Chinese hamster ovary cells demonstrated that engulfment and internalization of the microspheres occurred rapidly within minutes, and in a receptor activation-dependent manner. In human epidermoid carcinoma A431 cells, receptor activation and Shc translocation persisted over the 20-minute time course of the experiments. However, at the subcellular level the positive correlation of receptor activation and Shc translocation observed at 5-8 minutes dissipated, indicating a time-dependent decoupling of the two events and variation in the kinetics of signal transduction for different subcellular locations.

The MLK family mediates c-Jun N-terminal kinase activation in neuronal apoptosis

Neuronal apoptotic death induced by nerve growth factor (NGF) deprivation is reported to be in part mediated through a pathway that includes Rac1 and Cdc42, mitogen-activated protein kinase kinases 4 and 7 (MKK4 and -7), c-Jun N-terminal kinases (JNKs), and c-Jun. However, additional components of the pathway remain to be defined. We show here that members of the mixed-lineage kinase (MLK) family (including MLK1, MLK2, MLK3, and dual leucine zipper kinase [DLK]) are expressed in neuronal cells and are likely to act between Rac1/Cdc42 and MKK4 and -7 in death signaling. Overexpression of MLKs effectively induces apoptotic death of cultured neuronal PC12 cells and sympathetic neurons, while expression of dominant-negative forms of MLKs suppresses death evoked by NGF deprivation or expression of activated forms of Rac1 and Cdc42. CEP-1347 (KT7515), which blocks neuronal death caused by NGF deprivation and a variety of additional apoptotic stimuli and which selectively inhibits the activities of MLKs, effectively protects neuronal PC12 cells from death induced by overexpression of MLK family members. In addition, NGF deprivation or UV irradiation leads to an increase in both level and phosphorylation of endogenous DLK. These observations support a role for MLKs in the neuronal death mechanism. With respect to ordering the death pathway, dominant-negative forms of MKK4 and -7 and c-Jun are protective against death induced by MLK overexpression, placing MLKs upstream of these kinases. Additional findings place the MLKs upstream of mitochondrial cytochrome c release and caspase activation.

MAP kinase signal transduction pathways in plants

The mitogen-activated protein kinase (MAP kinase) signal transduction cascades are routes through which eukaryotic cells deliver extracellular messages to the cytosol and nucleus. These signalling pathways direct cell division, cellular differentiation, metabolism, and both biotic and abiotic stress responses. In plants, MAP kinases and the upstream components of the cascades are represented by multigene families, organized into different pathways which are stimulated and interact in complex ways. Experimental strategies for the analysis of MAP kinase cascades include the yeast two-hybrid system; using this approach in vitro interactions between specific MAP kinase cascade components have been analysed and putative plant cascades postulated. Transient transformation of protoplasts with epitope-tagged kinases has allowed cascades to be tested in planta. There is clear evidence for the involvement of MAP kinases in plant cell division and in the regulation of auxin signalling. Biotic (pathogens and pathogen-derived elicitors from fungi, bacteria and viruses) and abiotic stresses including wounding, mechanical stimulation, cold, drought and ozone can elicit defence responses in plants through MAP kinase pathways. There are data suggesting that ABA signalling utilizes a MAP kinase pathway, and probably ethylene and perhaps cytokinins do so also. The objective of this paper is to review this rapidly advancing field. Contents Summary 67 I. Introduction 68 II. Background 68 III. MAP kinase targets and targeting specificity 69 IV. Assays and inhibitors 70 V. Two well characterized MAP kinase pathways, Hog1 and Sevenless 71 VI. MAP kinases in plants 73 VII. MAP kinases and cell division 76 VIII. MAP kinases and plant hormones 76 IX. MAP kinase and abiotic stress 78 X. MAP kinase and biotic stress 80 XI. Future perspectives for MAP kinase research in plants 83 Acknowledgements 84 References 84.

MKK7 is an essential component of the JNK signal transduction pathway activated by proinflammatory cytokines

Mitogen-activated protein kinases (MAPK) are activated by phosphorylation on Thr and Tyr by MAPK kinases. Two MAPK kinases (MKK4 and MKK7) can activate the c-Jun NH(2)-terminal kinase (JNK) group of MAPK in vitro. JNK is phosphorylated preferentially on Tyr by MKK4 and on Thr by MKK7. Targeted gene-disruption studies in mice were performed to examine the role of MKK4 and MKK7 in vivo. Simultaneous disruption of the Mkk4 and Mkk7 genes was required to block JNK activation caused by exposure of cells to environmental stress. In contrast, disruption of the Mkk7 gene alone was sufficient to prevent JNK activation caused by proinflammatory cytokines. These data demonstrate that MKK4 and MKK7 serve different functions in the JNK signal transduction pathway.

Expanding roles for beta-arrestins as scaffolds and adapters in GPCR signaling and trafficking

beta-arrestins play previously unsuspected and important roles as adapters and scaffolds that localize signaling proteins to ligand-activated G-protein-coupled receptors. As with the paradigmatic role of the beta-arrestins in uncoupling receptors from G proteins (desensitization), these novel functions involve the interaction of beta-arrestin with phosphorylated heptahelical receptors. beta-arrestins interact with at least two main classes of signaling proteins. First, interaction with molecules such as clathrin, AP-2 and NSF directs the clathrin-mediated internalization of G-protein-coupled receptors. Second, interaction with molecules such as Src, Raf, Erk, ASK1 and JNK3 appears to regulate several pathways that result in the activation of MAP kinases. These recent discoveries indicate that the beta-arrestins play widespread roles as scaffolds and/or adapter molecules that organize a variety of complex signaling pathways emanating from heptahelical receptors. It is likely that additional roles for the beta-arrestins remain to be discovered.

A conserved docking site in MEKs mediates high-affinity binding to MAP kinases and cooperates with a scaffold protein to enhance signal transmission

The recognition of mitogen-activated protein kinases (MAPKs) by their upstream activators, MAPK/ERK kinases (MEKs), is crucial for the effective and accurate transmission of many signals. We demonstrated previously that the yeast MAPKs Kss1 and Fus3 bind with high affinity to the N terminus of the MEK Ste7, and proposed that a conserved motif in Ste7, the MAPK-docking site, mediates this interaction. Here we show that the corresponding sequences in human MEK1 and MEK2 are necessary and sufficient for the direct binding of the MAPKs ERK1 and ERK2. Mutations in MEK1, MEK2, or Ste7 that altered conserved residues in the docking site diminished binding of the cognate MAPKs. Furthermore, short peptides corresponding to the docking sites in these MEKs inhibited MEK1-mediated phosphorylation of ERK2 in vitro. In yeast cells, docking-defective alleles of Ste7 were modestly compromised in their ability to transmit the mating pheromone signal. This deficiency was dramatically enhanced when the ability of the Ste5 scaffold protein to associate with components of the MAPK cascade was also compromised. Thus, both the MEK-MAPK docking interaction and binding to the Ste5 scaffold make mutually reinforcing contributions to the efficiency of signaling by this MAPK cascade in vivo.

Leukocyte-specific adaptor protein Grap2 interacts with hematopoietic progenitor kinase 1 (HPK1) to activate JNK signaling pathway in T lymphocytes

Immune cell-specific adaptor proteins create various combinations of multiprotein complexes and integrate signals from cell surface receptors to the nucleus, modulating the specificity and selectivity of intracellular signal transduction. Grap2 is a newly identified adaptor protein specifically expressed in lymphoid tissues. This protein shares 40--50% sequence homology in the SH3 and the SH2 domain with Grb2 and Grap. However, the Grap2 protein has a unique 120-amino acid glutamine- and proline-rich domain between the SH2 and C-terminal SH3 domains. The expression of Grap2 is highly restricted to lymphoid organs and T lymphocytes. In order to understand the role of this specific adaptor protein in immune cell signaling and activation, we searched for the Grap2 interacting protein in T lymphocytes. We found that Grap2 interacted with the hematopoietic progenitor kinase 1 (HPK1) in vitro and in Jurkat T cells. The interaction was mediated by the carboxyl-terminal SH3 domain of Grap2 with the second proline-rich motif of HPK1. Coexpression of Grap2 with HPK1 not only increased the kinase activity of HPK1 in the cell, but also had an additive effect on HPK1 mediated JNK activation. Furthermore, over expression of Grap2 and HPK1 induced significant transcriptional activation of c-Jun in the JNK signaling pathway and IL-2 gene reporter activity in stimulated Jurkat T cells. Therefore, our data suggest that the hematopoietic specific proteins Grap2 and HPK1 form a signaling complex to mediate the c-Jun NH(2)-terminal kinase (JNK) signaling pathway in T cells.

MEK7-dependent activation of p38 MAP kinase in keratinocytes

Previous studies suggest that a PKC/Ras/MEKK1 cascade regulates involucrin (hINV) gene expression in human epidermal keratinocytes. MEK7, which is expressed in epidermis, has been identified as a member of this cascade (Efimova, T., LaCelle, P., Welter, J. F., and Eckert, R. L. (1998) J. Biol. Chem. 273, 24387-24395 and Efimova, T., and Eckert, R. L. (2000) J. Biol. Chem. 275, 1601-1607). However, the kinase that functions downstream of MEK7 has not been identified. Our present studies show that MEK7 expression in keratinocytes markedly activates p38alpha and modestly activates JNK. Activation of p38 MAPK by MEK7 is a novel finding, as previous reports have assigned MEK7 as a JNK regulator. We also demonstrate that this regulation is physiologically important, as the p38alpha- and JNK-dependent activities regulate hINV promoter activity and expression of the endogenous hINV gene.

Assembly of scaffold-mediated complexes containing Cdc42p, the exchange factor Cdc24p, and the effector Cla4p required for cell cycle-regulated phosphorylation of Cdc24p

In budding yeast cells, the cytoskeletal polarization and depolarization events that shape the bud are triggered at specific times during the cell cycle by the cyclin-dependent kinase Cdc28p. Polarity establishment also requires the small GTPase Cdc42p and its exchange factor, Cdc24p, but the mechanism whereby Cdc28p induces Cdc42p-dependent polarization is unknown. Here we show that Cdc24p becomes phosphorylated in a cell cycle-dependent manner, triggered by Cdc28p. However, the role of Cdc28p is indirect, and the phosphorylation appears to be catalyzed by the p21-activated kinase family member Cla4p and also depends on Cdc42p and the scaffold protein Bem1p. Expression of GTP-Cdc42p, the product of Cdc24p-mediated GDP/GTP exchange, stimulated Cdc24p phosphorylation independent of cell cycle cues, raising the possibility that the phosphorylation is part of a feedback regulatory pathway. Bem1p binds directly to Cdc24p, to Cla4p, and to GTP-bound Cdc42p and can mediate complex formation between these proteins in vitro. We suggest that Bem1p acts to concentrate polarity establishment proteins at a discrete site, facilitating polarization and promoting Cdc24p phosphorylation at specific times during the cell cycle.

Mitogen-activated protein [MAP] kinase pathways in plants: versatile signaling tools

Mitogen-activated protein kinases (MAPKs) are important signaling tools in all eukaryotes, and function in mediating an enormous variety of external signals to appropriate cellular responses. MAPK pathways have been studied extensively in yeast and mammalian cells, and a large body of knowledge on their functioning has accumulated, which is summarized briefly. Plant MAPK pathways have attracted increasing interest, resulting in the isolation of a large number of different components of MAPK cascades. Studies on the functions of these components have revealed that MAPKs play important roles in the response to a broad variety of stresses, as well as in the signaling of most plant hormones and in developmental processes. Finally, the involvement of various plant phosphatases in the inactivation of MAPKs is discussed.

Molecular interaction maps as information organizers and simulation guides

A graphical method for mapping bioregulatory networks is presented that is suited for the representation of multimolecular complexes, protein modifications, as well as actions at cell membranes and between protein domains. The symbol conventions defined for these molecular interaction maps are designed to accommodate multiprotein assemblies and protein modifications that can generate combinatorially large numbers of molecular species. Diagrams can either be "heuristic," meaning that detailed knowledge of all possible reaction paths is not required, or "explicit," meaning that the diagrams are totally unambiguous and suitable for simulation. Interaction maps are linked to annotation lists and indexes that provide ready access to pertinent data and references, and that allow any molecular species to be easily located. Illustrative interaction maps are included on the domain interactions of Src, transcription control of E2F-regulated genes, and signaling from receptor tyrosine kinase through phosphoinositides to Akt/PKB. A simple method of going from an explicit interaction diagram to an input file for a simulation program is outlined, in which the differential equations need not be written out. The role of interaction maps in selecting and defining systems for modeling is discussed. (c) 2001 American Institute of Physics.

Distinct PKC isozymes regulate bufalin-induced differentiation and apoptosis in human monocytic cells

Bufalin, an Na(+)-K(+)-ATPase inhibitor, simultaneously induced cell differentiation and apoptosis in human monocytic leukemia THP-1 cells. In this study, we investigated the regulatory role of protein kinase C (PKC) isozymes in bufalin-induced cell differentiation and apoptosis. A PKC-specific but isozyme-nonselective inhibitor, Ro-31-8220, and a cPKC selective inhibitor, Gö-6976, caused significant attenuation of bufalin-induced interleukin-1beta (IL-1beta) gene expression, a mature monocytic marker, indicating that cPKC participates in the bufalin-induced cell differentiation. On the other hand, cPKCbeta- and nPKCdelta-defective THP-1/TPA cells displayed strong resistance to the bufalin-induced DNA ladder formation. Rottlerin, an nPKCdelta-specific inhibitor, partially attenuated preapoptotic effects of bufalin, such as the limited proteolysis of nPKCdelta and poly(ADP-ribose) polymerase and the cell staining by terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling, suggesting that nPKCdelta is involved, at least in part, in bufalin-induced apoptosis. In contrast, Gö-6976 and rottlerin significantly augmented bufalin-induced apoptosis and differentiation, respectively. The findings suggest that bufalin-induced cell differentiation and apoptosis are interlinked and that distinct PKC isozymes are involved in the fate of the cell.

A novel 14-kilodalton protein interacts with the mitogen-activated protein kinase scaffold mp1 on a late endosomal/lysosomal compartment

We have identified a novel, highly conserved protein of 14 kD copurifying with late endosomes/lysosomes on density gradients. The protein, now termed p14, is peripherally associated with the cytoplasmic face of late endosomes/lysosomes in a variety of different cell types. In a two-hybrid screen with p14 as a bait, we identified the mitogen-activated protein kinase (MAPK) scaffolding protein MAPK/extracellular signal-regulated kinase (ERK) kinase (MEK) partner 1 (MP1) as an interacting protein. We confirmed the specificity of this interaction in vitro by glutathione S-transferase pull-down assays and by coimmunoprecipitation, cosedimentation on glycerol gradients, and colocalization. Moreover, expression of a plasma membrane-targeted p14 causes mislocalization of coexpressed MP1. In addition, we could reconstitute protein complexes containing the p14-MP1 complex associated with ERK and MEK in vitro.The interaction between p14 and MP1 suggests a MAPK scaffolding activity localized to the cytoplasmic surface of late endosomes/lysosomes, thereby combining catalytic scaffolding and subcellular compartmentalization as means to modulate MAPK signaling within a cell.

The MAPK kinase kinase TAK1 plays a central role in coupling the interleukin-1 receptor to both transcriptional and RNA-targeted mechanisms of gene regulation

Mechanisms of fulminant gene induction during an inflammatory response were investigated using expression of the chemoattractant cytokine interleukin-8 (IL-8) as a model. Recently we found that coordinate activation of NF-kappaB and c-Jun N-terminal protein kinase (JNK) is required for strong IL-8 transcription, whereas the p38 MAP kinase (MAPK) pathway stabilizes the IL-8 mRNA. It is unclear how these pathways are coupled to the receptor for IL-1, an important physiological inducer of IL-8. Expression of the MAP kinase kinase kinase (MAPKKK) TAK1 together with its coactivator TAB1 in HeLa cells activated all three pathways and was sufficient to induce IL-8 formation, NF-kappaB + JNK2-mediated transcription from a minimal IL-8 promoter, and p38 MAPK-mediated stabilization of a reporter mRNA containing IL-8-derived regulatory mRNA sequences. Expression of a kinase-inactive mutant of TAK1 largely blocked IL-1-induced transcription and mRNA stabilization, as well as formation of endogenous IL-8. Truncated TAB1, lacking the TAK1 binding domain, or a TAK1-derived peptide containing a TAK1 autoinhibitory domain were also efficient in inhibition. These data indicate that the previously described three-pathway model of IL-8 induction is operative in response to a physiological stimulus, IL-1, and that the MAPKKK TAK1 couples the IL-1 receptor to both transcriptional and RNA-targeted mechanisms mediated by the three pathways.

Specificity of signaling by STAT1 depends on SH2 and C-terminal domains that regulate Ser727 phosphorylation, differentially affecting specific target gene expression

Complete activation of signal transducer and activator of transcription 1 (STAT1) requires phosphorylation at both Y701 and a conserved PMS(727)P sequence. S727 phosphorylation of STAT1 in interferon-gamma (IFN-gamma)-treated mouse fibroblasts occurred without a need for p38 mitogen-activated protein kinase (MAPK), extracellular signal-regulated kinases 1 and 2 or c-Jun kinases, and required both an intact SH2 domain and phosphorylation of Y701. In contrast, UV irradiation-induced STAT1 phosphorylation on S727 required p38MAPK, but no SH2 domain- phosphotyrosine interactions. Mutation of S727 differentially affected IFN-gamma target genes, at the level of both basal and induced expression. Particularly strong effects were noted for the GBP1 and TAP1 genes. The PMS(727)P motif of STAT3 was phosphorylated by stimuli and signaling pathways different from those for STAT1 S727. Transfer of the STAT3 C-terminus to STAT1 changed the stimulus and pathway specificity of STAT1 S727 phosphorylation to that of STAT3. Our data suggest that STAT C-termini contribute to the specificity of cellular responses by linking individual STATs to different serine kinase pathways and through an intrinsically different requirement for serine phosphorylation at different target gene promoters.

Selective targeting of MAPKs to the ETS domain transcription factor SAP-1

MAPK pathways play important roles in regulating the key cellular processes of proliferation, differentiation, and apoptosis. There are multiple MAPK pathways, which are subject to different regulatory cues. It is important that these pathways maintain specificity in signaling to elicit the activation of a specific program of gene expression. MAPK-docking domains in several transcription factors have been shown to play important roles in determining the specificity and efficiency of their phosphorylation by MAPKs. Here we investigate the mechanisms by which MAPKs are targeted to the ETS domain transcription factor SAP-1. We demonstrate that SAP-1 contains two different domains that are required for its efficient phosphorylation in vitro and activation in vivo by ERK2 and a subset of p38 MAPKs. The D-domain is closely related to other MAPK-docking domains, but exhibits a novel specificity and serves to promote selective targeting of ERK2, p38alpha, and p38beta(2) to SAP-1. A second important region, the FXF motif, also plays an important role in directing MAPKs to phosphorylate SAP-1. The FXF motif promotes targeting by ERK2 and, to a lesser extent, p38alpha, but not p38beta(2). Our data therefore demonstrate that a modular system of motifs is responsible for directing specific MAPK subtypes to SAP-1, but also point to important distinctions in the mechanism of action of the D-domain and FXF motif.

Induction of cytosolic phospholipase A2 by oncogenic Ras is mediated through the JNK and ERK pathways in rat epithelial cells

Mutations in ras genes have been detected with high frequency in nonsmall cell lung cancer cells (NSCLC) and contribute to transformed growth of these cells. It has previously been shown that expression of oncogenic forms of Ras in these cells is associated with elevated expression of cytosolic phospholipase A(2) (cPLA(2)) and cyclooxygenase-2 (COX-2), resulting in high constitutive levels of prostaglandin production. To determine whether expression of constitutively active Ras is sufficient to induce expression of these enzymes in nontransformed cells, normal lung epithelial cells were transfected with H-Ras. Stable expression of H-Ras increased expression of cPLA(2) and COX-2 protein. Transient transfection with H-Ras increased promoter activity for both enzymes. H-Ras expression also activated all three families of MAP kinase: ERKs, JNKs, and p38 MAP kinase. Expression of constitutively active Raf did not increase either cPLA(2) or COX-2 promoter activity, but inhibition of the ERK pathway with pharmacological agents or expression of dominant negative ERK partially blocked the H-Ras-mediated induction of cPLA(2) promoter activity. Expression of dominant negative JNK kinases decreased cPLA(2) promoter activity in NSCLC cell lines and inhibited H-Ras-mediated induction in normal epithelial cells, whereas expression of constructs encoding constitutively active JNKs increased promoter activity. Inhibition of p38 MAP kinase or NF-kappaB had no effect on cPLA(2) expression. Truncational analysis revealed that the region of the cPLA(2) promoter from -58 to +12 contained sufficient elements to mediate H-Ras induction. We conclude that expression of oncogenic forms of Ras directly increases cPLA(2) expression in normal epithelial cells through activation of the JNK and ERK pathways.

MEKK1 binds raf-1 and the ERK2 cascade components

Mitogen-activated protein (MAP) kinase cascades are involved in transmitting signals that are generated at the cell surface into the cytosol and nucleus and consist of three sequentially acting enzymes: a MAP kinase, an upstream MAP/extracellular signal-regulated protein kinase (ERK) kinase (MEK), and a MEK kinase (MEKK). Protein-protein interactions within these cascades provide a mechanism to control the localization and function of the proteins. MEKK1 is implicated in activation of the c-Jun N-terminal kinase/stress-activated protein kinase (JNK/SAPK) and ERK1/2 MAP kinase pathways. We showed previously that MEKK1 binds directly to JNK/SAPK. In this study we demonstrate that endogenous MEKK1 binds to endogenous ERK2, MEK1, and another MEKK level kinase, Raf-1, suggesting that it can assemble all three proteins of the ERK2 MAP kinase module.

Akt negatively regulates the cJun N-terminal kinase pathway in PC12 cells

The protein serine/threonine kinase Akt is a target of phosphatidylinositol 3-kinase that mediates many of the trophic actions of growth factors on cells. In PC12 cells, complete removal of serum leads to rapid stimulation of the cJun N-terminal kinase (JNK) pathway. Inclusion of insulin-like growth factor-1, a stimulator of Akt in PC12 cells, inhibits JNK activation in this setting, whereas addition of wortmannin to PC12 cells in the presence of serum stimulates JNK activity, suggesting that growth factor-mediated signaling through the phosphatidylinositol 3-kinase/Akt pathway chronically inhibits the JNK pathway in PC12 cells. To explore the possible role of Akt as a negative regulator of JNK activity in PC12 cells, a myristoylated, gain-of-function Akt polypeptide (Myr-Akt) was expressed by retrovirus-mediated gene transfer. Stimulation of JNK activity by serum withdrawal or UV irradiation in PC12 cell clones stably expressing Myr-Akt was inhibited approximately 95% or 50%, respectively, relative to control transfected PC12 cells. Phosphorylation of both JNKs and a proximal activator, MAP kinase kinase 4 (MKK4), in response to UV irradiation was inhibited in Myr-Akt-expressing PC12 cells. Furthermore, transient expression of Myr-Akt strongly inhibited cJun transactivation mediated by MEKK1 or MKK7-JNK3, a gain-of-function MKK7-JNK fusion protein. Interestingly, inhibited JNK activation in the Myr-Akt-expressing PC12 cells is associated with marked induction of JNK-interacting protein-1 (JIP-1). We propose that negative regulation of the JNK pathway through Akt-dependent induction of specfic JIP proteins contributes to the antiapoptotic actions of Akt in neuronal cell types.

Dissection of Ras-dependent signaling pathways controlling aggressive tumor growth of human fibrosarcoma cells: evidence for a potential novel pathway

Activation of multiple signaling pathways is required to trigger the full spectrum of in vitro and in vivo phenotypic traits associated with neoplastic transformation by oncogenic Ras. To determine which of these pathways are important for N-ras tumorigenesis in human cancer cells and also to investigate the possibility of cross talk among the pathways, we have utilized a human fibrosarcoma cell line (HT1080), which contains an endogenous mutated allele of the N-ras gene, and its derivative (MCH603c8), which lacks the mutant N-ras allele. We have stably transfected MCH603c8 and HT1080 cells with activating or dominant-negative mutant cDNAs, respectively, of various components of the Raf, Rac, and RhoA pathways. In previous studies with these cell lines we showed that loss of mutant Ras function results in dramatic changes in the in vitro phenotypic traits and conversion to a weakly tumorigenic phenotype in vivo. We report here that only overexpression of activated MEK contributed significantly to the conversion of MCH603c8 cells to an aggressive tumorigenic phenotype. Furthermore, we have demonstrated that blocking the constitutive activation of the Raf-MEK, Rac, or RhoA pathway alone is not sufficient to block the aggressive tumorigenic phenotype of HT1080, despite affecting a number of in vitro-transformed phenotypic traits. We have also demonstrated the possibility of bidirectional cross talk between the Raf-MEK-ERK pathway and the Rac-JNK or RhoA pathway. Finally, overexpression of activated MEK in MCH603c8 cells appears to result in the activation of an as-yet-unidentified target(s) that is critical for the aggressive tumorigenic phenotype.

Various abiotic stresses rapidly activate Arabidopsis MAP kinases ATMPK4 and ATMPK6

Mitogen-activated protein kinase (MAP kinase, MAPK) cascades play pivotal roles in signal transduction of extracellular stimuli, such as environmental stresses and growth regulators, in various organisms. Arabidopsis thaliana MAP kinases constitute a gene family, but stimulatory signals for each MAP kinase have not been elucidated. Here we show that environmental stresses such as low temperature, low humidity, hyper-osmolarity, touch and wounding induce rapid and transient activation of the Arabidopsis MAP kinases ATMPK4 and ATMPK6. Activation of ATMPK4 and ATMPK6 was associated with tyrosine phosphorylation but not with the amounts of mRNA or protein. Kinetics during activation differ between these two MAP kinases. These results suggest that ATMPK4 and ATMPK6 are involved in distinct signal transduction pathways responding to these environmental stresses.

The budding yeast, Saccharomyces cerevisiae, as a model for aging research: a critical review

In this review we discuss the yeast as a paradigm for the study of aging. The budding yeast Saccharomyces cerevisiae, which can proliferate in both haploid and diploid states, has been used extensively in aging research. The budding yeast divides asymmetrically to form a 'mother' cell and a bud. Two major approaches, 'budding life span' and 'stationary phase' have been used to determine 'senescence' and 'life span' in yeast. Discrepancies observed in metabolic behavior and longevity between cells studied by these two systems raise questions of how 'life span' in yeast is defined and measured. Added to this variability in experimental approach and results is the variety of yeast strains with different genetic make up used as 'wild type' and experimental organisms. Another problematic genetic point in the published studies on yeast is the use of both diploid and haploid strains. We discuss the inherent, advantageous attributes that make the yeast an attractive choice for modern biological research as well as certain pitfalls in the choice of this model for the study of aging. The significance of the purported roles of the Sir2 gene, histone deacetylases, gene silencing, rDNA circles and stress genes in determination of yeast 'life span' and aging is evaluated. The relationship between cultivation conditions and longevity are assessed. Discrepancies between the yeast and mammalian systems with regard to aging are pointed out. We discuss unresolved problems concerning the suitability of the budding yeast for the study of basic aging phenomena.

Beta-arrestin 2: a receptor-regulated MAPK scaffold for the activation of JNK3

beta-Arrestins, originally discovered in the context of heterotrimeric guanine nucleotide binding protein-coupled receptor (GPCR) desensitization, also function in internalization and signaling of these receptors. We identified c-Jun amino-terminal kinase 3 (JNK3) as a binding partner of beta-arrestin 2 using a yeast two-hybrid screen and by coimmunoprecipitation from mouse brain extracts or cotransfected COS-7 cells. The upstream JNK activators apoptosis signal-regulating kinase 1 (ASK1) and mitogen-activated protein kinase (MAPK) kinase 4 were also found in complex with beta-arrestin 2. Cellular transfection of beta-arrestin 2 caused cytosolic retention of JNK3 and enhanced JNK3 phosphorylation stimulated by ASK1. Moreover, stimulation of the angiotensin II type 1A receptor activated JNK3 and triggered the colocalization of beta-arrestin 2 and active JNK3 to intracellular vesicles. Thus, beta-arrestin 2 acts as a scaffold protein, which brings the spatial distribution and activity of this MAPK module under the control of a GPCR.

Signal transduction. An arresting start for MAPK

Exactly how signaling proteins know where they need to be in the cell is one of the intriguing mysteries of signal transduction biology. In a Perspective, Pouysségur reviews new results that identify b-arrestin 2 as a scaffolding protein that holds together the different components of a MAPK signaling pathway that activates the transcription factor kinase, JNK3.

Regulation of anchorage-dependent signal transduction by protein kinase A and p21-activated kinase

Activation of the canonical mitogen-activated protein kinase (MAPK) cascade by soluble mitogens is blocked in non-adherent cells. It is also blocked in cells in which the cAMP-dependent protein kinase (PKA) is activated. Here we show that inhibition of PKA allows anchorage-independent stimulation of the MAPK cascade by growth factors. This effect is transient, and its duration correlates with sustained tyrosine phosphorylation of paxillin and focal-adhesion kinase (FAK) in non-adherent cells. The effect is sensitive to cytochalasin D, implicating the actin cytoskeleton as an important factor in mediating this anchorage-independent signalling. Interestingly, constitutively active p21-activated kinase (PAK) also allows anchorage-independent MAPK signalling. Furthermore, PKA negatively regulates PAK in vivo, and whereas the induction of anchorage-independent signaling resulting from PKA suppression is blocked by dominant negative PAK, it is markedly prolonged by constitutively active PAK. These observations indicate that PKA and PAK are important regulators of anchorage-dependent signal transduction.

The SMRT corepressor is regulated by a MEK-1 kinase pathway: inhibition of corepressor function is associated with SMRT phosphorylation and nuclear export

The SMRT (silencing mediator of retinoic acid and thyroid hormone receptor) corepressor participates in the repression of target gene expression by a variety of transcription factors, including the nuclear hormone receptors, promyelocytic leukemia zinc finger protein, and B-cell leukemia protein 6. The ability of SMRT to associate with these transcription factors and thereby to mediate repression is strongly inhibited by activation of tyrosine kinase signaling pathways, such as that represented by the epidermal growth factor receptor. We report here that SMRT function is potently inhibited by a mitogen-activated protein kinase (MAPK) kinase kinase (MAPKKK) cascade that operates downstream of this growth factor receptor. Intriguingly, the SMRT protein is a substrate for phosphorylation by protein kinases operating at multiple levels in this MAPKKK pathway, including the MAPKs, MAPK-extracellular signal-regulated kinase 1 (MEK-1), and MEK-1 kinase (MEKK-1). Phosphorylation of SMRT by MEKK-1 and, to a lesser extent, MEK-1 inhibits the ability of SMRT to physically tether to its transcription factor partners. Notably, activation of MEKK-1 or MEK-1 signaling in transfected cells also leads to a redistribution of the SMRT protein from a nuclear compartment to a more perinuclear or cytoplasmic compartment. We suggest that SMRT-mediated repression is regulated by the MAPKKK cascade and that changes both in the affinity of SMRT for its transcription factors and in the subcellular distribution of SMRT contribute to the loss of SMRT function that is observed in response to kinase signal transduction.

Docking domains and substrate-specificity determination for MAP kinases

Signalling specificity in eukaryotic cells is maintained by several mechanisms. One mechanism by which mitogen-activated protein (MAP) kinases ensure their specificity of action is by interacting with their substrates through docking domains. These docking domains recruit the kinases to the correct substrates and enhance their fidelity and efficiency of action. Additional specificity determinants in the substrates serve to enhance the specificity of substrate phosphorylation by MAP kinases further.

Polarized localization of yeast Pbs2 depends on osmostress, the membrane protein Sho1 and Cdc42

In Saccharomyces cerevisiae cells, high external osmolarity leads to the activation of a p38-related mitogen-activated protein (MAP) kinase though Pbs2. Pbs2 tagged with green fluorescent protein (Pbs2-GFP) is evenly distributed in the cytoplasm but excluded from the nucleus before and after exposure to stress. Here we show that a catalytically inactive form of Pbs2 attains a highly polarised localization during osmostress. This phenomenon depends of the osmosensor Sho1 and on a functional Cdc42 GTPase. Cdc42, but not the actin cytoskeleton, influences Sho1-dependent activation of the MAP kinase. Sho1 itself accumulates at sites of polar growth, but independently of stress conditions and Cdc42. These observations allow us to define the sequence of events that occurs during propogation of osmostress signals.

A Stat3-interacting protein (StIP1) regulates cytokine signal transduction

Genetic and biochemical studies have led to the identification of the Stat3-Interacting Protein StIP1. The preferential association of StIP1 with inactive (i.e., unphosphorylated) Stat3 suggests that it may contribute to the regulation of Stat3 activation. Consistent with this possibility, StIP1 also exhibits an affinity for members of the Janus kinase family. Overexpression of the Stat3-binding domain of StIP1 blocks Stat3 activation, nuclear translocation, and Stat3-dependent induction of a reporter gene. These studies indicate that StIP1 regulates the ligand-dependent activation of Stat3, potentially by serving as a scaffold protein that promotes the interaction between Janus kinases and their Stat3 substrate. The ability of StIP1 to associate with several additional members of the signal transducer and activator of transcription family suggests that StIP1 may serve a broader role in cytokine-signaling events.

Transcriptional scaffold: CIITA interacts with NF-Y, RFX, and CREB to cause stereospecific regulation of the class II major histocompatibility complex promoter

Scaffold molecules interact with multiple effectors to elicit specific signal transduction pathways. CIITA, a non-DNA-binding regulator of class II major histocompatibility complex (MHC) gene transcription, may serve as a transcriptional scaffold. Regulation of the class II MHC promoter by CIITA requires strict spatial-helical arrangements of the X and Y promoter elements. The X element binds RFX (RFX5/RFXANK-RFXB/RFXAP) and CREB, while Y binds NF-Y/CBF (NF-YA, NF-YB, and NF-YC). CIITA interacts with all three. In vivo analysis using both N-terminal and C-terminal deletion constructs identified critical domains of CIITA that are required for interaction with NF-YB, NF-YC, RFX5, RFXANK/RFXB, and CREB. We propose that binding of NF-Y/CBF, RFX, and CREB by CIITA results in a macromolecular complex which allows transcription factors to interact with the class II MHC promoter in a spatially and helically constrained fashion.

Induction of smooth muscle alpha-actin in vascular smooth muscle cells by arginine vasopressin is mediated by c-Jun amino-terminal kinases and p38 mitogen-activated protein kinase

Exposure of vascular smooth muscle cells to arginine vasopressin (AVP) increases smooth muscle alpha-actin (SM-alpha-actin) expression through activation of the SM- alpha-actin promoter. The goal of this study was to determine the role of the mitogen-activated protein kinase (MAP kinase) family in regulation of SM-alpha-actin expression. AVP activated all three MAP kinase family members: ERKs, JNKs, and p38 MAP kinase. Inhibition of JNKs or p38 decreased AVP-stimulated SM-alpha-actin promoter activity, whereas inhibition of ERKs had no effect. A 150-base pair region of the promoter containing two CArG boxes was sufficient to mediate regulation by vasoconstrictors. Mutations in either CArG box decreased AVP-stimulated promoter activity. Electrophoretic mobility shift assays using oligonucleotides corresponding to either CArG box resulted in a complex of similar mobility whose intensity was increased by AVP. Antibodies against serum response factor (SRF) completely super-shifted this complex, indicating that SRF binds to both CArG boxes. Overexpression of SRF increased basal promoter activity, but activity was still stimulated by AVP. AVP stimulation rapidly increased SRF phosphorylation. These data indicate that both JNKs and p38 participate in regulation of SM- alpha-actin expression. SRF, which binds to two critical CArG boxes in the promoter, represents a potential target of these kinases.

Human ERK1 induces filamentous growth and cell wall remodeling pathways in Saccharomyces cerevisiae

Expression of an activated extracellular signal-regulated kinase 1 (ERK1) construct in yeast cells was used to examine the conservation of function among mitogen-activated protein (MAP) kinases. Sequence alignment of the human MAP kinase ERK1 with all Saccharomyces cerevisiae kinases reveals a particularly strong kinship with Kss1p (invasive growth promoting MAP kinase), Fus3p (pheromone response MAP/ERK kinase), and Mpk1p (cell wall remodeling MAP kinase). A fusion protein of constitutively active human MAP/ERK kinase 1 (MEK) and human ERK1 was introduced under regulated expression into yeast cells. The fusion protein (MEK/ERK) induced a filamentation response element promoter and led to a growth retardation effect concomitant with a morphological change resulting in elongated cells, bipolar budding, and multicell chains. Induction of filamentous growth was also observed for diploid cells following MEK/ERK expression in liquid culture. Neither haploids nor diploids, however, showed marked penetration of agar medium. These effects could be triggered by either moderate MEK/ERK expression at 37 degrees C or by high level MEK/ERK expression at 30 degrees C. The combination of high level MEK/ERK expression and 37 degrees C resulted in cell death. The deleterious effects of MEK/ERK expression and high temperature were significantly mitigated by 1 m sorbitol, which also enhanced the filamentous phenotype. MEK/ERK was able to constitutively activate a cell wall maintenance reporter gene, suggesting misregulation of this pathway. In contrast, MEK/ERK effectively blocked expression from a pheromone-responsive element promoter and inhibited mating. These results are consistent with MEK/ERK promoting filamentous growth and altering the cell wall through its ability to partially mimic Kss1p and stimulate a pathway normally controlled by Mpk1p, while appearing to inhibit the normal functioning of the structurally related yeast MAP kinase Fus3p.

Interferon-alpha2b reduces phosphorylation and activity of MEK and ERK through a Ras/Raf-independent mechanism

Interferon (IFN)-alpha affects the growth, differentiation and function of various cell types by transducing regulatory signals through the Janus tyrosine kinase/signal transducers of activation and transcription (Jak/STAT) pathway. The signalling pathways employing the mitogen-activated ERK-activating kinase (MEK) and the extracellular-regulated kinase (ERK) are critical in growth factors signalling. Engagement of the receptors, and subsequent stimulation of Ras and Raf, initiates a phosphorylative cascade leading to activation of several proteins among which MEK and ERK play a central role in routing signals critical in controlling cell development, activation and proliferation. We demonstrate here that 24-48 h following treatment of transformed T- and monocytoid cell lines with recombinant human IFN-alpha2b both the phosphorylation and activity of MEK1 and its substrates ERK1/2 were reduced. In contrast, the activities of the upstream molecules Ras and Raf-1 were not affected. No effect on MEK/ERK activity was observed upon short-term exposure (1-30 min) to IFN. The anti-proliferative effect of IFN-alpha was increased by the addition in the culture medium of a specific inhibitor of MEK, namely PD98059. In conclusion, our results indicate that IFN-alpha regulates the activity of the MEK/ERK pathway and consequently modulates cellular proliferation through a Ras/Raf-independent mechanism. Targeting the MEK/ERK pathway may strengthen the IFN-mediated anti-cancer effect.

Response of Saccharomyces cerevisiae to severe osmotic stress: evidence for a novel activation mechanism of the HOG MAP kinase pathway

The HOG/p38 MAP kinase route is an important stress-activated signal transduction pathway that is well conserved among eukaryotes. Here, we describe a novel mechanism of activation of the HOG pathway in budding yeast. This mechanism operates upon severe osmostress conditions (1.4 M NaCl) and is independent of the Sln1p and Sho1p osmosensors. The alternative input feeds into the HOG pathway MAPKK Pbs2p and requires activation of Pbs2p by phosphorylation. We show that, upon severe osmotic shock, Hog1p nuclear accumulation and phosphorylation is delayed compared with mild stress. Moreover, both events lost their transient pattern, presumably because of the absence of negative feedback mediated by Ptp2p tyrosine phosphatase, which we found to be localized in the nucleus. Under severe osmotic stress conditions, the delayed nuclear accumulation correlates with a delay in stress-responsive gene expression. Severe osmoshock leads to a situation in which active and nuclear-localized Hog1p is transiently unable to induce transcription of osmotic stress-responsive genes. It also appeared from our studies that the Sho1p osmosensor is less active under severe osmotic stress conditions, whereas the Sln1p/Ypd1p/Ssk1p sensor and signal transducer functions normally under these circumstances.

c-Jun NH(2)-terminal kinase inhibits targeting of the protein phosphatase calcineurin to NFATc1

The protein phosphatase calcineurin is a critical mediator of calcium signals during T-cell activation. One substrate of calcineurin is the transcription factor NFATc1, which is retained in the cytoplasm of quiescent cells. NFATc1 activation requires the translocation of the transcription factor into the nucleus, a process that is mediated by calcineurin. This interaction with calcineurin requires a targeting domain (PxIxIT motif) located in the NH(2)-terminal region of NFATc1. Here we demonstrate that the calcineurin targeting domain of NFATc1 is phosphorylated and inactivated by the c-Jun NH(2)-terminal kinase (JNK). This disruption of calcineurin targeting inhibits the nuclear accumulation and transcription activity of NFATc1 and accounts for the observation that Jnk1(-/-) T cells exhibit greatly increased NFATc1-dependent nuclear responses.