The protein kinases ERK1/2 and their roles in pancreatic beta cells

Extracellular signal-regulated protein kinases 1 and 2 (ERK1/2) activities are modulated in a manner that reflects the secretory demand on beta cells to integrate long- and short-term nutrient sensing information. Our studies have focused on the mechanisms of ERK1/2 activation in beta cells and on the actions of ERK1/2 that regulate beta cell function. Insulin and growth factors regulate ERK1/2 in beta cells in a largely calcium-independent manner. Nutrients and anticipatory hormones, in contrast, activate ERK1/2 in a calcium-dependent manner in these cells. We are exploring the key intermediates in these distinct activation pathways and find that calcineurin is essential for the nutrient pathway but is not essential for the growth factor pathway. Using reporter assays, heterologous reconstitution, electrophoretic mobility shift assays, Northern analysis, Q-PCR and chromatin immunoprecipitation, we have examined several genes that are regulated by ERK1/2, primarily the insulin gene and the apoptotic factor C/EBP-homologous protein (CHOP)-10 (GADD153/DDIT-3), a bZIP protein. ERK1/2-sensitive transcriptional regulators common to these two genes are C/EBP-beta and MafA. The insulin promoter is both positively and negatively regulated by glucose and other nutrients. Exposure to glucose for minutes to hours causes an increase in the rate of insulin gene transcription. In contrast, exposure to elevated glucose for 48 h or more results in inhibition of the insulin gene promoter. Both of these processes depend on ERK1/2 activity. Expression of CHOP is induced by stresses including nutrient deprivation and endoplasmic reticulum stress. CHOP gene expression, especially that regulated by nutrients, is also ERK1/2-dependent in beta cells, These studies support the hypothesis that the genes regulated by ERK1/2 and the mechanisms employed are key to maintaining normal beta cell function.

Differential regulation and properties of MAPKs

Mitogen-activated protein kinases (MAPKs) regulate diverse cellular programs including embryogenesis, proliferation, differentiation and apoptosis based on cues derived from the cell surface and the metabolic state and environment of the cell. In mammals, there are more than a dozen MAPK genes. The best known are the extracellular signal-regulated kinases 1 and 2 (ERK1/2), c-Jun N-terminal kinase (JNK(1-3)) and p38(alpha, beta, gamma and delta) families. ERK3, ERK5 and ERK7 are other MAPKs that have distinct regulation and functions. MAPK cascades consist of a core of three protein kinases. Despite the apparently simple architecture of this pathway, these enzymes are capable of responding to a bewildering number of stimuli to produce exquisitely specific cellular outcomes. These responses depend on the kinetics of their activation and inactivation, the subcellular localization of the kinases, the complexes in which they act, and the availability of substrates. Fine-tuning of cascade activity can occur through modulatory inputs to cascade component from the primary kinases to the scaffolding accessory proteins. Here, we describe some of the properties of the three major MAPK pathways and discuss how these properties govern pathway regulation and activity.

ERKs weigh in on ribosome mass

In this issue of Molecular Cell, Stefanovsky et al. demonstrate that the activation of the ERK1/2 MAP kinases by growth factors leads to induction of ribosomal gene transcription, through a mechanism dependent on phosphorylation of the upstream binding factor (UBF). This provides a connection between growth factor signaling and increased translation.

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.

Regulation of stress-responsive mitogen-activated protein (MAP) kinase pathways by TAO2

Previous studies demonstrated that in vitro the protein kinase TAO2 activates MAP/ERK kinases (MEKs) 3, 4, and 6 toward their substrates p38 MAP kinase and c-Jun N-terminal kinase/stress-activated protein kinase (JNK/SAPK). In this study, we examined the ability of TAO2 to activate stress-sensitive MAP kinase pathways in cells and the relationship between activation of TAO2 and potential downstream pathways. Over-expression of TAO2 activated endogenous JNK/SAPK and p38 but not ERK1/2. Cotransfection experiments suggested that TAO2 selectively activates MEK3 and MEK6 but not MEKs 1, 4, or 7. Coimmunoprecipitation demonstrated that endogenous TAO2 specifically associates with MEK3 and MEK6 providing one mechanism for preferential recognition of MEKs upstream of p38. Sorbitol, and to a lesser extent, sodium chloride, Taxol, and nocodazole increased TAO2 activity toward itself and kinase-dead MEKs 3 and 6. Activation of endogenous TAO2 during differentiation of C2C12 myoblasts paralleled activation of p38 but not JNK/SAPK, consistent with the idea that TAO2 is a physiological regulator of p38 under certain circumstances.

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.

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.

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.

Uncoupling Raf1 from MEK1/2 impairs only a subset of cellular responses to Raf activation

The Raf family of serine/threonine protein kinases is intimately involved in the transmission of cell regulatory signals controlling proliferation and differentiation. The best characterized Raf substrates are MEK1 and MEK2. The activation of MEK1/2 by Raf is required to mediate many of the cellular responses to Raf activation, suggesting that MEK1/2 are the dominant Raf effector proteins. However, accumulating evidence suggests that there are additional Raf substrates and that subsets of Raf-induced regulatory events are mediated independently of Raf activation of MEK1/2. To examine the possibility that there is bifurcation at the level of Raf in activation of MEK1/2-dependent and MEK1/2-independent cell regulatory events, we engineered a kinase-active Raf1 variant (RafBXB(T481A)) with an amino acid substitution that disrupts MEK1 binding. We find that disruption of MEK1/2 association uncouples Raf from activation of ERK1/2, induction of serum-response element-dependent gene expression, and induction of growth and morphological transformation. However, activation of NF-kappaB-dependent gene expression and induction of neurite differentiation were unimpaired. In addition, Raf-dependent activation of p90 ribosomal S6 kinase was only slightly impaired. These results support the hypothesis that Raf kinases utilize multiple downstream effectors to regulate distinct cellular activities.

Dual MAP kinase pathways mediate opposing forms of long-term plasticity at CA3-CA1 synapses

Although the function of the p42/p44 mitogen-activated protein (MAP) kinase pathway in long-term potentiation at hippocampal CA3-CA1 synapses has been well described, relatively little is known about the importance of the p38 MAP kinase pathway in synaptic plasticity. Here we show that the p38 MAP kinase pathway, a parallel signaling cascade activated by distinct upstream kinases, mediates the induction of metabotropic glutamate receptor-dependent long-term depression at CA3-CA1 synapses. Thus, two parallel MAP kinase pathways contribute to opposing forms of long-term plasticity at a central synapse.

Stress pathway activation induces phosphorylation of retinoid X receptor

Cellular stresses inhibit retinoid signaling, but the molecular basis for this phenomenon has not been revealed. Here, we present evidence that retinoid X receptor (RXR) is a substrate for both mitogen-activated protein kinase kinase-4 (MKK4/SEK1) and its downstream mediator c-Jun N-terminal kinase (JNK). MKK4/SEK1 and JNK recognized distinct features on RXR in the DE and AB regions, respectively. Phosphorylation by MKK4/SEK1 had profound effects on the biochemical properties of RXR, inhibiting the expression of genes activated by RXR-retinoic acid receptor complexes. Tyr-249 in the RXR DE region was required for the inhibitory effect of MKK4/SEK1. These effects were significantly reduced in MKK4/SEK1-null cells, indicating that MKK4/SEK1 is required for the suppression of retinoid signaling by stress. Findings presented here demonstrate that MKK4/SEK1 can directly modulate transcription by phosphorylating RXR, a novel MKK4/SEK1 substrate.

Stimulation of NFkappa B activity by multiple signaling pathways requires PAK1

The p21-activated kinase (PAK1) is a serine-threonine protein kinase that is activated by binding to the Rho family small G proteins Rac and Cdc42hs. Both Rac and Cdc42hs have been shown to regulate the activity of the transcription factor NFkappaB. Here we show that expression of active Ras, Raf-1, or Rac1 in fibroblasts stimulates NFkappaB in a PAK1-dependent manner and that expression of active PAK1 can stimulate NFkappaB on its own. Similarly, in macrophages activation of NFkappaB as well as transcription from the tumor necrosis factor alpha promoter depends on PAK1. In these cells lipopolysaccharide is a potent activator of PAK1 kinase activity. We also demonstrate that expression of active PAK1 stimulates the nuclear translocation of the p65 subunit of NFkappaB but does not activate the inhibitor of kappaB kinases alpha or beta. These data demonstrate that PAK1 is a crucial signaling molecule involved in NFkappaB activation by multiple stimuli.

MEKK1 interacts with alpha-actinin and localizes to stress fibers and focal adhesions

Mitogen-activated protein (MAP) kinases orchestrate the effects of many extracellular stimuli on cells. The serine/threonine protein kinase MEKK1 is an upstream activator of the MAP kinases c-Jun N-terminal kinase/stress-activated protein kinase (JNK/SAPK), extracellular signal-regulated kinase (ERK), and p38 as well as NF-kappa B. In a yeast two-hybrid interaction screen to identify proteins that bind to an N-terminal fragment of MEKK1 (amino acids 1-719), the actin-crosslinking protein alpha-actinin was identified as a MEKK1-binding protein. Over-expressed MEKK1 co-immunoprecipitated with alpha-actinin in cell lysates. Both endogenous and over-expressed MEKK1 colocalized with alpha-actinin along actin stress fibers and at focal adhesions. Residues 221-559 of MEKK1 bound to purified alpha-actinin in vitro, indicating that the interaction is direct, and this fragment localized to actin filaments in cells. MEKK1 kinase activity was not required for association with actin filaments, because a catalytically inactive mutant of MEKK1 (MEKK1 D1369A) localized to stress fibers. These results provide strong evidence for the interaction between MEKK1 and alpha-actinin. Thus, restriction of the kinase to the actin cytoskeleton may serve to regulate its specificity towards downstream targets.

WNK1, a novel mammalian serine/threonine protein kinase lacking the catalytic lysine in subdomain II

We have cloned and characterized a novel mammalian serine/threonine protein kinase WNK1 (with no lysine (K)) from a rat brain cDNA library. WNK1 has 2126 amino acids and can be detected as a protein of approximately 230 kDa in various cell lines and rat tissues. WNK1 contains a small N-terminal domain followed by the kinase domain and a long C-terminal tail. The WNK1 kinase domain has the greatest similarity to the MEKK protein kinase family. However, overexpression of WNK1 in HEK293 cells exerts no detectable effect on the activity of known, co-transfected mitogen-activated protein kinases, suggesting that it belongs to a distinct pathway. WNK1 phosphorylates the exogenous substrate myelin basic protein as well as itself mostly on serine residues, confirming that it is a serine/threonine protein kinase. The demonstration of activity was striking because WNK1, and its homologs in other organisms lack the invariant catalytic lysine in subdomain II of protein kinases that is crucial for binding to ATP. A model of WNK1 using the structure of cAMP-dependent protein kinase suggests that lysine 233 in kinase subdomain I may provide this function. Mutation of this lysine residue to methionine eliminates WNK1 activity, consistent with the conclusion that it is required for catalysis. This distinct organization of catalytic residues indicates that WNK1 belongs to a novel family of serine/threonine protein kinases.

Catalytic reaction pathway for the mitogen-activated protein kinase ERK2

The structural, functional, and regulatory properties of the mitogen-activated protein kinases (MAP kinases) have long attracted considerable attention owing to the critical role that these enzymes play in signal transduction. While several MAP kinase X-ray crystal structures currently exist, there is by comparison little mechanistic information available to correlate the structural data with the known biochemical properties of these molecules. We have employed steady-state kinetic and solvent viscosometric techniques to characterize the catalytic reaction pathway of the MAP kinase ERK2 with respect to the phosphorylation of a protein substrate, myelin basic protein (MBP), and a synthetic peptide substrate, ERKtide. A minor viscosity effect on k(cat) with respect to the phosphorylation of MBP was observed (k(cat) = 10 +/- 2 s(-1), k(cat)(eta) = 0.18 +/- 0.05), indicating that substrate processing occurs via slow phosphoryl group transfer (12 +/- 4 s(-1)) followed by the faster release of products (56 +/- 4 s(-1)). At an MBP concentration extrapolated to infinity, no significant viscosity effect on k(cat)/K(m(ATP)) was observed (k(cat)/K(m(ATP)) = 0.2 +/- 0.1 microM(-1) s(-1), k(cat)/K(m(ATP))(eta) = -0.08 +/- 0.04), consistent with rapid-equilibrium binding of the nucleotide. In contrast, at saturating ATP, a full viscosity effect on k(cat)/K(m) for MBP was apparent (k(cat)/K(m(MBP)) = 2.4 +/- 1 microM(-1) s(-1), k(cat)/K(m(MBP))(eta) = 1.0 +/- 0.1), while no viscosity effect was observed on k(cat)/K(m) for the phosphorylation of ERKtide (k(cat)/K(m(ERKtide)) = (4 +/- 2) x 10(-3) microM(-1) s(-1), k(cat)/K(m(ERKtide))(eta) = -0.02 +/- 0.02). This is consistent with the diffusion-limited binding of MBP, in contrast to the rapid-equilibrium binding of ERKtide, to form the ternary Michaelis complex. Calculated values for binding constants show that the estimated value for K(d(MBP)) (</=0.5 microM) is significantly lower than that of the measured K(m(MBP)) (4.2 +/- 0.8 microM). Furthermore, MBP binds to the ERK2 x ATP complex at least 1500-fold more tightly than does ERKtide (K(d(ERKtide)) >/= 1.5 mM). The dramatically higher catalytic efficiency of MBP in comparison to that of ERKtide ( approximately 600-fold difference) is largely attributable to the slow dissociation rate of MBP (</=1.2 s(-1)) versus that of the synthetic peptide (>/=56 s(-1)), from the ERK2 active site.

Phosphatidylinositol 3-kinase regulates Raf1 through Pak phosphorylation of serine 338

We have previously shown that inhibition of phosphatidylinositol (PI) 3-kinase severely attenuates the activation of extracellular signal-regulated kinase (Erk) following engagement of integrin/fibronectin receptors and that Raf is the critical target of PI 3-kinase regulation [1]. To investigate how PI 3-kinase regulates Raf, we examined sites on Raf1 required for regulation by PI 3-kinase and explored the mechanisms involved in this regulation. Serine 338 (Ser338), which was critical for fibronectin stimulation of Raf1, was phosphorylated in a PI 3-kinase-dependent manner following engagement of fibronectin receptors. In addition, fibronectin activation of a Raf1 mutant containing a phospho-mimic mutation (S338D) was independent of PI 3-kinase. Furthermore, integrin-induced activation of the serine/threonine kinase Pak-1, which has been shown to phosphorylate Raf1 Ser338, was also dependent on PI 3-kinase activity and expression of a kinase-inactive Pak-1 mutant blocked phosphorylation of Raf1 Ser338. These results indicate that PI 3-kinase regulates phosphorylation of Raf1 Ser338 through the serine/threonine kinase Pak. Thus, phosphorylation of Raf1 Ser338 through PI 3-kinase and Pak provides a co-stimulatory signal which together with Ras leads to strong activation of Raf1 kinase activity by integrins.

IL-1 receptor-associated kinase modulates host responsiveness to endotoxin

Endotoxin triggers many of the inflammatory, hemodynamic, and hematological derangements of Gram-negative septic shock. Recent genetic studies in mice have identified the Toll-like receptor 4 as the transmembrane endotoxin signal transducer. The IL-1 intracellular signaling pathway has been implicated in Toll-like receptor signal transduction. LPS-induced activation of the IL-1 receptor-associated kinase (IRAK), and the influence of IRAK on intracellular signaling and cellular responses to endotoxin has not been explored in relevant innate immune cells. We demonstrate that LPS activates IRAK in murine macrophages. IRAK-deficient macrophages, in contrast, are resistant to LPS. Deletion of IRAK disrupts several endotoxin-triggered signaling cascades. Furthermore, macrophages lacking IRAK exhibit impaired LPS-stimulated TNF-alpha production, and IRAK-deficient mice withstand the lethal effects of LPS. These findings, coupled with the critical role for IRAK in IL-1 and IL-18 signal transduction, demonstrate the importance of this kinase and the IL-1/Toll signaling cassette in sensing and responding to Gram-negative infection.

HPK1 is activated by lymphocyte antigen receptors and negatively regulates AP-1

The serine/threonine kinase HPK1 is a member of the germinal center kinase (GCK) family that has been implicated in the regulation of MAP kinase pathways. Here, we demonstrate the involvement of HPK1 in antigen receptor signaling. Engagement of the TCR or the BCR resulted in a marked induction of HPK1 catalytic activity. Subsequent analysis revealed that Src and Syk/ZAP-70 tyrosine kinases and the adaptor proteins LAT, SLP-76, BLNK, Grb2, and Grap are involved in HPK1 activation. Overexpression of HPK1 inhibited TCR activation of AP-1 and ERK2, whereas the kinase-inactive mutant of HPK1 potentiated these responses. Neither form of HPK1 affected PMA or v-Ras activation of AP-1 and ERK2. Thus, HPK1 is a negative regulator of the TCR-induced AP-1 response pathway.

Dimerization in MAP-kinase signaling

The stimulus-dependent nuclear localization of the extracellular-signal- regulated kinases ERK1 and ERK2 is required for many of their actions, including induction of neurites in PC12 cells and transformation of fibroblasts. Phosphorylation of ERK2 causes it to form dimers, and the most flexible portions of the ERK2 molecule provide the surfaces for dimerization. It is thought that dimerization promotes nuclear localization of ERK2 by its effects on import, export or retention in cytoplasmic and nuclear compartments. Dimerization might also influence substrate interactions.

The N-terminal ERK-binding site of MEK1 is required for efficient feedback phosphorylation by ERK2 in vitro and ERK activation in vivo

An ERK2-binding site at the N terminus of MEK1 was reported to mediate their stable association. We examined the importance of this binding site in the feedback phosphorylation of MEK1 on Thr(292) and Thr(386) by ERK2, the phosphorylation and activation of ERK2 by MEK1, and the interaction of MEK1 with ERK2 and Raf-1. Deletion of the binding site from MEK1 reduced its phosphorylation by ERK2, but had no effect on its phosphorylation by p21-activated protein kinase-1 (PAK1). A MEK1 N-terminal peptide containing the binding site inhibited MEK1 phosphorylation by ERK2. However, it did not affect MEK1 phosphorylation by p21-activated protein kinase or myelin basic protein phosphorylation by ERK2. Deletion of the N-terminal ERK-binding domain of MEK1 also reduced its ability to phosphorylate ERK2 in vitro, to co-immunoprecipitate with ERK2, and to stimulate ERK2 activation in transfected cells, but it did not alter the association with endogenous Raf-1. Using ERK2-p38 chimeras and an ERK2 deletion mutant, a MEK1-binding site of ERK2 was localized to its N terminus.

Scaffolding and protein interactions in MAP kinase modules

MAP kinases are a family of protein kinases that are ubiquitously expressed and play roles in most signal transduction pathways. They are activated within protein kinase cascades consisting of at least three kinases acting in series. In many, if not all cases, the three-kinase cascade, conveniently referred to as a MAP kinase module, is organized on scaffolds with a variety of forms and functions. This review discusses similarities and differences in scaffolding proteins and mechanisms in yeast, flies, worms and mammals.

Contribution of the ERK5/MEK5 pathway to Ras/Raf signaling and growth control

The activity of the catalytic domain of the orphan MAP kinase ERK5 is increased by Ras but not Raf-1 in cells, which suggests that ERK5 might mediate Raf-independent signaling by Ras. We found that Raf-1 does contribute to Ras activation of ERK5 but in a manner that does not correlate with Raf-1 catalytic activity. A clue to the mechanism of action of Raf-1 on ERK5 comes from the observation that endogenous Raf-1 binds to endogenous ERK5, suggesting the involvement of regulatory protein-protein interactions. This interaction is specific because Raf-1 binds only to ERK5 and not ERK2 or SAPK. Finally, we demonstrate the ERK5/MEK5 pathway is required for Raf-dependent cellular transformation and that a constitutively active form of MEK5, MEK5DD, synergizes with Raf to transform NIH 3T3 cells. These observations suggest that ERK5 plays a large role in Raf-1-mediated signal transduction.

Reciprocal signaling between heterotrimeric G proteins and the p21-stimulated protein kinase

p21-activated protein kinase (PAK)-1 phosphorylated Galpha(z), a member of the Galpha(i) family that is found in the brain, platelets, and adrenal medulla. Phosphorylation approached 1 mol of phosphate/mol of Galpha(z) in vitro. In transfected cells, Galpha(z) was phosphorylated both by wild-type PAK1 when stimulated by the GTP-binding protein Rac1 and by constitutively active PAK1 mutants. In vitro, phosphorylation occurred only at Ser(16), one of two Ser residues that are the major substrate sites for protein kinase C (PKC). PAK1 did not phosphorylate other Galpha subunits (i1, i2, i3, o, s, or q). PAK1-phosphorylated Galpha(z) was resistant both to RGSZ1, a G(z)-selective GTPase-activating protein (GAP), and to RGS4, a relatively nonselective GAP for the G(i) and G(q) families of G proteins. Phosphorylation of Ser(27) by PKC did not alter sensitivity to either GAP. The previously described inhibition of G(z) GAPs by PKC is therefore mediated by phosphorylation of Ser(16). Phosphorylation of either Ser(16) by PAK1 or Ser(27) by PKC decreased the affinity of Galpha(z) for Gbetagamma; phosphorylation of both residues by PKC caused no further effect. PAK1 thus regulates Galpha(z) function by attenuating the inhibitory effects of both GAPs and Gbetagamma. In this context, the kinase activity of PAK1 toward several protein substrates was directly inhibited by Gbetagamma, suggesting that PAK1 acts as a Gbetagamma-regulated effector protein. This inhibition of mammalian PAK1 by Gbetagamma contrasts with the stimulation of the PAK homolog Ste20p in Saccharomyces cerevisiae by the Gbetagamma homolog Ste4p/Ste18p.

Isolation of the protein kinase TAO2 and identification of its mitogen-activated protein kinase/extracellular signal-regulated kinase kinase binding domain

We previously reported the cloning of the thousand and one-amino acid protein kinase 1 (TAO1), a rat homolog of the Saccharomyces cerevisiae protein kinase sterile 20 protein. Here we report the complete sequence and properties of a related rat protein kinase TAO2. Like TAO1, recombinant TAO2 selectively activated mitogen-activated protein/extracellular signal-regulated kinase kinases (MEKs) 3, 4, and 6 of the stress-responsive mitogen-activated protein kinase pathways in vitro and copurified with MEK3 endogenous to Sf9 cells. To examine TAO2 interactions with MEKs, the MEK binding domain of TAO2 was localized to an approximately 135-residue sequence just C-terminal to the TAO2 catalytic domain. In vitro this MEK binding domain associated with MEKs 3 and 6 but not MEKs 1, 2, or 4. Using chimeric MEK proteins, we found that the MEK N terminus was sufficient for binding to TAO2. Catalytic activity of full-length TAO2 enhanced its binding to MEKs. However, neither the autophosphorylation of the MEK binding domain of TAO2 nor the activity of MEK itself was required for MEK binding. These results suggest that TAO proteins lie in stress-sensitive kinase cascades and define a mechanism by which these kinases may organize downstream targets.

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.

MAP kinase pathways

MAP kinases help to mediate diverse processes ranging from transcription of protooncogenes to programmed cell death. More than a dozen mammalian MAP kinase family members have been discovered and include, among others, the well studied ERKs and several stress-sensitive enzymes. MAP kinases lie within protein kinase cascades. Each cascade consists of no fewer than three enzymes that are activated in series. Cascades convey information to effectors, coordinates incoming information from other signaling pathways, amplify signals, and allow for a variety of response patterns. Subcellular localization of enzymes in the cascades is an important aspect of their mechanisms of action and contributes to cell-type and ligand-specific responses. Recent findings on these properties of MAP kinase cascades are the major focus of this review.

Lipopolysaccharide-induced tumor necrosis factor-alpha promoter activity is inhibitor of nuclear factor-kappaB kinase-dependent

The adverse effects of lipopolysaccharide (LPS) are primarily mediated by tumor necrosis factor-alpha (TNF-alpha). TNF-alpha production by LPS-stimulated macrophages is regulated both transcriptionally and post-transcriptionally. Transcriptional regulation of the TNF-alpha gene is dependent on nuclear factor-kappaB (NF-kappaB). We examined the signaling pathways involved in the regulation of NF-kappaB that lead to TNF-alpha promoter activity. We determined a role for one or both of the recently identified inhibitor of NF-kappaB kinases, IkappaB kinase-1 and IkappaB kinase-2, in LPS induction of an NF-kappaB reporter and of TNF-alpha promoter activity. IkappaB kinase activation is one of the earliest signaling events known to be induced by LPS. Furthermore, our results suggest roles for the IkappaB kinases NF-kappaB-inducing kinase and mitogen-activated protein kinase/extracellular signal-regulated kinase kinase kinase 1 in the regulation of IkappaB kinase-2, as well as in LPS-induced TNF-alpha transcription.

Isolation of TAO1, a protein kinase that activates MEKs in stress-activated protein kinase cascades

Several components of the budding yeast pheromone-response pathway are conserved in mammalian mitogen-activated protein (MAP) kinase pathways. Thus, we used degenerate oligonucleotides derived from the sequence of the Saccharomyces cerevisiae protein kinase Ste20p to amplify related sequences from the rat. One of these sequences was used to clone a rat Ste20p homolog, which we called TAO1 for its one thousand and one amino acids. Northern analysis shows TAO1 is highly expressed in brain, as is a homolog TAO2. Recombinant TAO1 was expressed and purified from Sf9 cells. In vitro, it activated MAP/extracellular signal-regulated protein kinase (ERK) kinases (MEKs) 3, 4, and 6 of the stress-responsive MAP kinase pathways, but not MEK1 or 2 of the classical MAP kinase pathway. TAO1 activated MEK3 but not MEK4 or MEK6 in transfected cells. MEK3 coimmunoprecipitated with TAO1 when they were expressed in 293 cells. In addition, immunoreactive MEK3 endogenous to Sf9 cells copurified with TAO1 produced from a recombinant baculovirus. The activation of and binding to MEK3 by TAO1 implicates TAO1 in the regulation of the p38-containing stress-responsive MAP kinase pathway.

Differential effects of PAK1-activating mutations reveal activity-dependent and -independent effects on cytoskeletal regulation

PAKs are serine/threonine protein kinases that are activated by binding to Rac or Cdc42hs. Different forms of activated PAK1 have been reported to either promote membrane ruffling and focal adhesion assembly or cause focal adhesion disassembly and stress fiber dissolution. To understand the basis for these distinct morphological effects, we have examined the mechanism of mutational activation of PAK1, and characterized the effects of different active PAK1 proteins on cytoskeletal structure in vivo. We find that PAK1 contains an autoinhibitory domain that overlaps with its small G protein binding domain and that two separate activating mutations within this regulatory region each decrease autoinhibitory activity. Because only one of these mutations affects Cdc42hs binding activity, this indicates that activation of PAK1 by these mutations results from interference with the function of the autoinhibitory domain and not with small G protein binding activity. When we examined the morphological effects of these different forms of PAK1 in vivo, we found that PAK1 kinase activity was associated with disassembly of focal adhesions and actin stress fibers and that this may require interaction with potential SH3 domain-containing proteins. Lamellipodia formation and membrane ruffling caused by active PAK1 expression, however, was independent of PAK1 catalytic activity and likely requires interaction among multiple proteins binding to the PAK1 regulatory domain.

A constitutively active and nuclear form of the MAP kinase ERK2 is sufficient for neurite outgrowth and cell transformation

BACKGROUND

Mitogen-activated protein (MAP) kinases are ubiquitous components of many signal transduction pathways. Constitutively active variants have been isolated for every component of the extracellular-signal-regulated kinase 1 (ERK1) and ERK2 MAP kinase pathway except for the ERK itself.

RESULTS

To create an activated ERK2 variant, we fused ERK2 to the low activity form of its upstream regulator, the MAP kinase kinase MEK1. The ERK2 in this fusion protein was active in the absence of extracellular signals. Expression of the fusion protein in mammalian cells did not activate endogenous ERK1 or ERK2. It was sufficient, however, to induce activation of the transcription factors Elk-1 and AP-1, neurite extension in PC12 cells in the absence of nerve growth factor, and foci of morphologically and growth-transformed NIH3T3 cells, if the fusion protein was localized to the nucleus. A cytoplasmic fusion protein was without effect.

CONCLUSIONS

Activation of ERK2 is sufficient to cause several transcriptional and phenotypic responses in mammalian cells. Nuclear localization of activated ERK2 is required to induce these events.

Structural basis of inhibitor selectivity in MAP kinases

BACKGROUND

The mitogen-activated protein (MAP) kinases are important signaling molecules that participate in diverse cellular events and are potential targets for intervention in inflammation, cancer, and other diseases. The MAP kinase p38 is responsive to environmental stresses and is involved in the production of cytokines during inflammation. In contrast, the activation of the MAP kinase ERK2 (extracellular-signal-regulated kinase 2) leads to cellular differentiation or proliferation. The anti-inflammatory agent pyridinylimidazole and its analogs (SB [SmithKline Beecham] compounds) are highly potent and selective inhibitors of p38, but not of the closely-related ERK2, or other serine/threonine kinases. Although these compounds are known to bind to the ATP-binding site, the origin of the inhibitory specificity toward p38 is not clear.

RESULTS

We report the structural basis for the exceptional selectivity of these SB compounds for p38 over ERK2, as determined by comparative crystallography. In addition, structural data on the origin of olomoucine (a better inhibitor of ERK2) selectivity are presented. The crystal structures of four SB compounds in complex with p38 and of one SB compound and olomoucine in complex with ERK2 are presented here. The SB inhibitors bind in an extended pocket in the active site and are complementary to the open domain structure of the low-activity form of p38. The relatively closed domain structure of ERK2 is able to accommodate the smaller olomoucine.

CONCLUSIONS

The unique kinase-inhibitor interactions observed in these complexes originate from amino-acid replacements in the active site and replacements distant from the active site that affect the size of the domain interface. This structural information should facilitate the design of better MAP-kinase inhibitors for the treatment of inflammation and other diseases.

The MEK1 proline-rich insert is required for efficient activation of the mitogen-activated protein kinases ERK1 and ERK2 in mammalian cells

MEK1 and MEK2 contain a proline-rich insert not present in any other known MEK (MAP (mitogen-activated protein)/ERK (extracellular signal-regulated kinase) kinase) family members. We examined the effect of removing the MEK1 polyproline insert on MEK activity, its binding to Raf, and its ability to activate ERKs in cells. Deletion of the insert had no effect on either the activity of MEK1 or on its ability to bind to Raf-1. Both wild type and constitutively active MEK1 coimmunoprecipitated with Raf-1 whether or not the insert was present. Deletion of the insert did not reduce activation of MEK1 by EGF or activated Raf in cells. The proline-rich insert enhanced the ability of an otherwise equally active MEK1 protein to regulate endogenous ERKs in mammalian cells. Overexpression of either constitutively active MEK1 lacking the insert or ERK2 compensates for the weaker in vivo activity of the MEK1 deletion mutant. Expression of the insert in cells reduced activation of ERKs by EGF. We conclude that the proline-rich insert is not the site of the MEK-Raf interaction and that the polyproline insert is required for its efficient activation of downstream ERKs in cells.

HTLV-I Tax protein binds to MEKK1 to stimulate IkappaB kinase activity and NF-kappaB activation

NF-kappaB, a key regulator of the cellular inflammatory and immune response, is activated by the HTLV-I transforming and transactivating protein Tax. We show that Tax binds to the amino terminus of the protein kinase MEKK1, a component of an IkappaB kinase complex, and stimulates MEKK1 kinase activity. Tax expression increases the activity of IkappaB kinase beta (IKKbeta) to enhance phosphorylation of serine residues in IkappaB alpha that lead to its degradation. Dominant negative mutants of both IKKbeta and MEKK1 prevent Tax activation of the NF-kappaB pathway. Furthermore, recombinant MEKK1 stimulates IKKbeta phosphorylation of IkappaB alpha. Thus, Tax-mediated increases in NF-kappaB nuclear translocation result from direct interactions of Tax and MEKK1 leading to enhanced IKKbeta phosphorylation of IkappaB alpha.

Phosphorylation of the MAP kinase ERK2 promotes its homodimerization and nuclear translocation

The MAP kinase ERK2 is widely involved in eukaryotic signal transduction. Upon activation it translocates to the nucleus of the stimulated cell, where it phosphorylates nuclear targets. We find that nuclear accumulation of microinjected ERK2 depends on its phosphorylation state rather than on its activity or on upstream components of its signaling pathway. Phosphorylated ERK2 forms dimers with phosphorylated and unphosphorylated ERK2 partners. Disruption of dimerization by mutagenesis of ERK2 reduces its ability to accumulate in the nucleus, suggesting that dimerization is essential for its normal ligand-dependent relocalization. The crystal structure of phosphorylated ERK2 reveals the basis for dimerization. Other MAP kinase family members also form dimers. The generality of this behavior suggests that dimerization is part of the mechanism of action of the MAP kinase family.

Identification of substrates and regulators of the mitogen-activated protein kinase ERK5 using chimeric protein kinases

Extracellular signal-regulated protein kinase 5 (ERK5) is a recently discovered orphan mitogen-activated protein kinase for which no substrates or strong activators have been described. Two ERK5 chimeras were created as a novel approach to discover its substrates and upstream regulators. One chimeric protein contained the N-terminal domain of the ERK5 catalytic core (subdomains I-IV) and the C-terminal domain of the ERK2 catalytic core (subdomains V-XI). This chimera was highly responsive to stimuli that regulate ERK2 in vitro and in cells. A second chimeric protein consisted of the N-terminal domain of ERK2 (subdomains I-IV) and the C-terminal domain of the ERK5 catalytic core (subdomains V-XI). This chimera was activated in bacteria by coexpression with a constitutively active mutant of MEK1. Using the activated chimera, we identified three in vitro substrates of ERK5. Assaying ERK5 activity in immune complexes with one of these substrates, c-Myc, we determined that the ERK5 catalytic domain is activated by V12 H-Ras and to a lesser extent by phorbol ester but not by constitutively active mutants of Raf-1. Thus, ERK5 is a target of a novel Ras effector pathway that may communicate with c-Myc.

The mitogen-activated protein kinase p38-2 is necessary for the inhibition of N-type calcium current by bradykinin

Calcium influx via voltage-dependent calcium channels (ICa,V) links depolarization of excitable cells to critical cellular processes, such as secretion, contraction, and gene transcription. Fast regulation of ICa,V (<1 sec) by G-protein-coupled receptors is a relatively well-defined mechanism, whereas slow (30-60 sec) actions of transmitters and hormones on the same current remain poorly understood. In NG108-15 cells, the kinetically slow inhibition of N-type ICa,V by bradykinin (BK) requires the sequential activation of two G-proteins, heterotrimeric G13 and monomeric Rac1/Cdc42. We have now defined a role in this pathway for the relatively fast-acting p38 mitogen-activated protein kinase (MAPK). The slow inhibition of ICa,V by BK was suppressed specifically by SB203580, a compound that inhibits the p38 family of MAPKs. BK potently and selectively activated a newly discovered p38 family member, p38-2. These data provide the first evidence that a MAPK is involved in the regulation of ICa,V by a receptor-mediated process.

MEKK1 binds directly to the c-Jun N-terminal kinases/stress-activated protein kinases

Mitogen-activated protein (MAP) kinases mediate responses to a wide array of cellular stimuli. These cascades consist of a MAP kinase or extracellular signal-regulated kinase (ERK), activated by a MAP/ERK kinase (MEK), in turn activated by a MEK kinase (MEKK). MEKK1 has been shown to be a strong activator of the c-Jun N-terminal kinase/stress-actived protein kinase (JNK/SAPK) pathway. We report here that JNK/SAPK binds directly to the N-terminal, noncatalytic domain of MEKK1 in vitro and in transfected cells. Immobilized MEKK1-derived peptides extract JNK/SAPK selectively from cell lysates. MEKK1 coimmunoprecipitates with multiple JNK/SAPK isoforms in transfected cells. Expression of the N terminus of MEKK1 lacking the kinase domain increases activation of endogenous JNK/SAPK by MEKK1. The data are consistent with a model in which MEKK1-JNK/SAPK binding facilitates the receipt of signals from upstream inputs and localizes JNK/SAPK to intracellular targets of the pathway.

Cross-cascade activation of ERKs and ternary complex factors by Rho family proteins

Mitogens promote cell growth through integrated signal transduction networks that alter cellular metabolism, gene expression and cytoskeletal organization. Many such signals are propagated through activation of MAP kinase cascades partly regulated by upstream small GTP-binding proteins. Interactions among cascades are suspected but not defined. Here we show that Rho family small G proteins such as Rac1 and Cdc42hs, which activate the JNK/SAPK pathway, cooperate with Raf-1 to activate the ERK pathway. This causes activation of ternary complex factors (TCFs), which regulate c-fos gene expression through the serum response element. Examination of ERK pathway kinases shows that neither MEK1 nor Ras will synergize with Rho-type proteins, and that only MEK1 is fully activated, indicating that MEKs are a focal point for cross-cascade regulation. Rho family proteins utilize PAKs for this effect, as expression of an active PAK1 mutant can substitute for Rho family small G proteins, and expression of an interfering PAK1 mutant blocks Rho-type protein stimulation of ERKs. PAK1 phosphorylates MEK1 on Ser298, a site important for binding of Raf-1 to MEK1 in vivo. Expression of interfering PAK1 also reduces stimulation of TCF function by serum growth factors, while expression of active PAK1 enhances EGF-stimulated MEK1 activity. This demonstrates interaction among MAP kinase pathway elements not previously recognized and suggests an explanation for the cooperative effect of Raf-1 and Rho family proteins on cellular transformation.

Jun N-terminal kinase/stress-activated protein kinase (JNK/SAPK) is required for lipopolysaccharide stimulation of tumor necrosis factor alpha (TNF-alpha) translation: glucocorticoids inhibit TNF-alpha translation by blocking JNK/SAPK

The adverse effects of lipopolysaccharide (LPS) are mediated primarily by tumor necrosis factor alpha (TNF-alpha). TNF-alpha production by LPS-stimulated macrophages is regulated at the levels of both transcription and translation. It has previously been shown that several mitogen-activated protein kinases (MAPKs) are activated in response to LPS. We set out to determine which MAPK signaling pathways are activated in our system and which MAPK pathways are required for TNF-alpha gene transcription or TNF-alpha mRNA translation. We confirm activation of the MAPK family members extracellular-signal-regulated kinases 1 and 2 (ERK1 and ERK2), p38, and Jun N-terminal kinase/stress-activated protein kinase (JNK/SAPK), as well as activation of the immediate upstream MAPK activators MAPK/ERK kinases 1 and 4 (MEK1 and MEK4). We demonstrate that LPS also activates MEK2, MEK3, and MEK6. Furthermore, we demonstrate that dexamethasone, which inhibits the production of cytokines, including TNF-alpha, significantly inhibits LPS induction of JNK/SAPK activity but not that of p38, ERK1 and ERK2, or MEK3, MEK4, or MEK6. Dexamethasone also blocks the sorbitol but not anisomycin stimulation of JNK/SAPK activity. A kinase-defective mutant of SAPKbeta, SAPKbeta K-A, blocked translation of TNF-alpha, as determined by using a TNF-alpha translational reporting system. Finally, overexpression of wild-type SAPKbeta was able to overcome the dexamethasone-induced block of TNF-alpha translation. These data confirm that three MAPK family members and their upstream activators are stimulated by LPS and demonstrate that JNK/SAPK is required for LPS-induced translation of TNF-alpha mRNA. A novel mechanism by which dexamethasone inhibits translation of TNF-alpha is also revealed.

Differential regulation of mitogen-activated protein/ERK kinase (MEK)1 and MEK2 and activation by a Ras-independent mechanism

Mitogen-activated protein (MAP)/ERK kinase (MEK)1 and MEK2 are the upstream activators of the MAP kinases, ERK1 and ERK2. MEK1 and MEK2 are approximately 85% identical in sequence but have unique inserts in their C-terminal domains. MEK isoform-specific antibodies were used to examine expression and regulation of each enzyme. MEK1 and MEK2 were expressed in approximately equal amounts in several cell lines; in some, MEK1 was present in slight excess. Activation of tyrosine kinase-containing receptors, heterotrimeric G proteins, and protein kinase C enhanced the activities of both MEK isoforms in 293 and PC12 cells. AIF4-stimulated both MEK1 and MEK2 in PC12 cells expressing a dominant interfering Ras mutant that prevents nerve growth factor-dependent activation of the cascade. Carbachol also stimulated the pathway in these cells. Thus, in addition to their ability to activate Ras/Raf and the downstream ERK pathway, heterotrimeric G proteins also appear to trigger a Ras-independent mechanism to regulate this kinase cascade. In U373, Chinese hamster ovary (CHO), and INS-1 cells, MEK1 was activated by regulators of ERKs, while MEK2 was not. These data suggest that, like the MAP kinases ERK1 and ERK2, in some cell settings the two similar MEK isoforms are differentially regulated.

Activation mechanism of the MAP kinase ERK2 by dual phosphorylation

The structure of the active form of the MAP kinase ERK2 has been solved, phosphorylated on a threonine and a tyrosine residue within the phosphorylation lip. The lip is refolded, bringing the phosphothreonine and phosphotyrosine into alignment with surface arginine-rich binding sites. Conformational changes occur in the lip and neighboring structures, including the P+1 site, the MAP kinase insertion, the C-terminal extension, and helix C. Domain rotation and remodeling of the proline-directed P+1 specificity pocket account for the activation. The conformation of the P+1 pocket is similar to a second proline-directed kinase, CDK2-CyclinA, thus permitting the origin of this specificity to be defined. Conformational changes outside the lip provide loci at which the state of phosphorylation can be felt by other cellular components.

Activation of mitogen-activating protein kinase by glucose is not required for insulin secretion

In the insulinoma cell line INS-1, a model system for glucose-regulated insulin secretion, the mitogen-activating protein (MAP) kinases/extracellular signal-regulated protein kinases, ERK1 and ERK2 are activated up to 15-fold by physiological concentrations of glucose, in the range of 3-12 mM. The related MAP kinase family members, the c-Jun-N-terminal kinases/stress-activated protein kinases are insensitive to glucose, while the p38 MAP kinase is slightly glucose responsive (1.5-fold). ERK activation is dependent on glucose metabolism and the subsequent increase in calcium influx. Inhibiting activation of ERK1 and ERK2 with the MEK1/2 inhibitor PD98059 has no effect on insulin secretion, indicating that ERK activity is not necessary for secretion under these conditions. Glucose activates ERK1 and ERK2 in cytosolic and purified nuclear fractions of INS-1 cells and more of each is found in nuclei from glucose-treated cells. These findings suggest that some of the glucose-dependent actions of ERKs will be exerted in the nucleus.

Reconstitution of mitogen-activated protein kinase phosphorylation cascades in bacteria. Efficient synthesis of active protein kinases

Mitogen-activated protein (MAP) kinase pathways include a three-kinase cascade terminating in a MAP kinase family member. The middle kinase in the cascade is a MAP/extracellular signal-regulated kinase (ERK) kinase or MEK family member and is highly specific for its MAP kinase target. The first kinase in the cascade, a MEK kinase (MEKK), is characterized by its ability to activate one or more MEK family members. A two-plasmid bacterial expression system was employed to express active forms of the following MEK and MAP kinase family members: ERK1, ERK2, alpha-SAPK, and p38 and their upstream activators, MEK1, -2, -3, and -4. In each kinase module, the upstream activator, a constitutively active mutant of MEK1 or MEKK1, was expressed from a low copy plasmid, while one or two downstream effector kinases were expressed from a high copy plasmid with different antibiotic resistance genes and origins of replication. Consistent with their high activity, ERK1 and ERK2 were doubly phosphorylated on Tyr and Thr, were recognized by an antibody specific to the doubly phosphorylated forms, and were inactivated by either phosphoprotein phosphatase 2A or phosphotyrosine phosphatase type 1. Likewise, activated p38 and alpha-stress-activated protein kinase could also be inactivated by either phosphatase, and alpha-stress-activated protein kinase was recognized by an antibody specific to the doubly phosphorylated forms. These three purified, active MAP kinases have specific activities in the range of 0.6-2.3 micromol/min/mg. Coexpression of protein kinases with their substrates in bacteria is of great value in the preparation of numerous phosphoproteins, heretofore not possible in procaryotic expression systems.

Mitogen-activated protein kinase pathways

Nearly all cell surface receptors utilize one or more of the mitogen-activated protein kinase cascades in their repertoire of signal transduction mechanisms. Recent advances in the study of such cascades include the cloning of genes encoding novel members of the cascades, further definition of the roles of the cascades in responses to extracellular signals, and examination of cross-talk between different cascades.

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

The structure of mitogen-activated protein (MAP) kinase p38 has been solved at 2.1-A to an R factor of 21.0%, making p38 the second low activity MAP kinase solved to date. Although p38 is topologically similar to the MAP kinase ERK2, the phosphorylation Lip (a regulatory loop near the active site) adopts a different fold in p38. The peptide substrate binding site and the ATP binding site are also different from those of ERK2. The results explain why MAP kinases are specific for different activating enzymes, substrates, and inhibitors. A model presented for substrate and activator interactions has implications for the evolution of protein kinase cascades.

The highly conserved skb1 gene encodes a protein that interacts with Shk1, a fission yeast Ste20/PAK homolog

The Shk1 protein kinase, a homolog of Saccharomyces cerevisiae Ste20 and mammalian p21Cdc42/Rac-activated kinases, is an essential component of a Ras- and Cdc42-dependent signaling cascade required for cell viability, normal morphology, and mitogen-activated protein kinase-mediated sexual responses in the fission yeast, Schizosaccharomyces pombe. To identify S. pombe proteins that modulate or mediate Shk1 functions, we conducted a two-hybrid screen for Shk1-interacting proteins. One of the genes identified as a result of this screen was skb1. We show that Skb1 interacts with a region of the N-terminal regulatory domain of Shk1 distinct from that to which Cdc42 binds, and that Shk1, Cdc42, and Skb1 are able to form a ternary complex in vivo. S.pombe cells carrying an skb1 null mutation are less elongate in morphology than wild-type cells and exhibit a moderate growth defect. The morphology defect of the skb1 deletion mutant is suppressed by overexpression of Shk1. Overexpression of Skb1 causes wild-type S. pombe cells to become hyperelongated. Additional genetic analyses described herein suggest that Skb1 is a component of the morphology control branch of the Ras signaling cascade in S. pombe and that it positively modulates Shk1 function. Homologs of Skb1 are encoded by open reading frames in the genomes of S. cerevisiae and Caenorhabditis elegans and by an uncharacterized human cDNA sequence. Thus, skb1 may be the first well-characterized member of a highly conserved family of genes encoding potential p21Cdc42/Rac-activated kinase regulators.

Contributions of the mitogen-activated protein (MAP) kinase backbone and phosphorylation loop to MEK specificity

To examine the specificity of MEKs for MAP kinase family members, we determined the abilities of several MEK isoforms to phosphorylate mutants of the MAP kinase ERK2 and the related kinase ERK3 which are modified in the phosphorylation loop. The ERK2 mutants included mutations of the two phosphorylation sites, mutations of the acidic residue between these two sites, and mutations that shorten the length of this loop. All mutants were tested for phosphorylation by six mammalian MEKs and compared with several wild type MAP kinases. MEK1 and MEK2 phosphorylate a majority of the ERK2 mutants. MEK2 but not MEK1 will phosphorylate ERK3. Alteration of the residue between the two phosphorylation sites neither dramatically affected the activity of MEK1 and MEK2 toward ERK2 nor conferred recognition by other MEKs. Likewise, reduction of the length of the phosphorylation loop only partially reduces recognition by MEK1 and MEK2 but does not promote recognition by other MEKs. Thus other yet to be identified factors must contribute to the specificity of MEK recognition of MAP kinases.