Biochemical and biological functions of the N-terminal, noncatalytic domain of extracellular signal-regulated kinase 2

ERK2-topoisomerase II regulatory axis is important for gene activation in immediate early genes

The function of the mitogen-activated protein kinase signaling pathway is required for the activation of immediate early genes (IEGs), including EGR1 and FOS, for cell growth and proliferation. Recent studies have identified topoisomerase II (TOP2) as one of the important regulators of the transcriptional activation of IEGs. However, the mechanism underlying transcriptional regulation involving TOP2 in IEG activation has remained unknown. Here, we demonstrate that ERK2, but not ERK1, is important for IEG transcriptional activation and report a critical ELK1 binding sequence for ERK2 function at the EGR1 gene. Our data indicate that both ERK1 and ERK2 extensively phosphorylate the C-terminal domain of TOP2B at mutual and distinctive residues. Although both ERK1 and ERK2 enhance the catalytic rate of TOP2B required to relax positive DNA supercoiling, ERK2 delays TOP2B catalysis of negative DNA supercoiling. In addition, ERK1 may relax DNA supercoiling by itself. ERK2 catalytic inhibition or knock-down interferes with transcription and deregulates TOP2B in IEGs. Furthermore, we present the first cryo-EM structure of the human cell-purified TOP2B and etoposide together with the EGR1 transcriptional start site (-30 to +20) that has the strongest affinity to TOP2B within -423 to +332. The structure shows TOP2B-mediated breakage and dramatic bending of the DNA. Transcription is activated by etoposide, while it is inhibited by ICRF193 at EGR1 and FOS, suggesting that TOP2B-mediated DNA break to favor transcriptional activation. Taken together, this study suggests that activated ERK2 phosphorylates TOP2B to regulate TOP2-DNA interactions and favor transcriptional activation in IEGs. We propose that TOP2B association, catalysis, and dissociation on its substrate DNA are important processes for regulating transcription and that ERK2-mediated TOP2B phosphorylation may be key for the catalysis and dissociation steps.

Navigating the ERK1/2 MAPK Cascade

The RAS-ERK pathway is a fundamental signaling cascade crucial for many biological processes including proliferation, cell cycle control, growth, and survival; common across all cell types. Notably, ERK1/2 are implicated in specific processes in a context-dependent manner as in stem cells and pancreatic β-cells. Alterations in the different components of this cascade result in dysregulation of the effector kinases ERK1/2 which communicate with hundreds of substrates. Aberrant activation of the pathway contributes to a range of disorders, including cancer. This review provides an overview of the structure, activation, regulation, and mutational frequency of the different tiers of the cascade; with a particular focus on ERK1/2. We highlight the importance of scaffold proteins that contribute to kinase localization and coordinate interaction dynamics of the kinases with substrates, activators, and inhibitors. Additionally, we explore innovative therapeutic approaches emphasizing promising avenues in this field.

The MEK/ERK Network as a Therapeutic Target in Human Cancer

The RAS-RAF-MEK-ERK pathway is the most well-studied of the MAPK cascades and is critical for cell proliferation, differentiation, and survival. Abnormalities in regulation resulting from mutations in components of this pathway, particularly in upstream proteins, RAS and RAF, are responsible for a significant fraction of human cancers and nearly all cutaneous melanomas. Activation of receptor tyrosine kinases by growth factors and various extracellular signals leads to the sequential activation of RAS, RAF, MEK, and finally ERK, which activates numerous transcription factors and facilitates oncogenesis in the case of aberrant pathway activation. While extensive studies have worked to elucidate the activation mechanisms and structural components of upstream MAPK components, comparatively less attention has been directed toward the kinases, MEK and ERK, due to the infrequency of oncogenic-activating mutations in these kinases. However, acquired drug resistance has become a major issue in the treatment of RAS- and RAF-mutated cancers. Targeting the terminal kinases in the MAPK cascade has shown promise for overcoming many of these resistance mechanisms and improving treatment options for patients with MAPK-aberrant cancers. Here, we will describe the role of MEK and ERK in MAPK signaling and summarize the current understanding of their interaction and activation mechanisms. We will also discuss existing approaches for targeting MEK and ERK, and the benefits of alternative strategies. Areas requiring further exploration will be highlighted to guide future research endeavors and aid in the development of alternative therapeutic strategies to combat surmounting drug resistance in treating MAPK-mediated cancers. VISUAL OVERVIEW: http://mcr.aacrjournals.org/content/molcanres/19/3/361/F1.large.jpg.

EDD enhances cell survival and cisplatin resistance and is a therapeutic target for epithelial ovarian cancer

The E3 ubiquitin ligase EDD is overexpressed in recurrent, platinum-resistant ovarian cancers, suggesting a role in tumor survival and/or platinum resistance. EDD knockdown by small interfering RNA (siRNA) induced apoptosis in A2780ip2, OVCAR5 and ES-2 ovarian cancer cells, correlating with loss of the prosurvival protein myeloid cell leukemia sequence 1 (Mcl-1) through a glycogen synthase kinase 3 beta-independent mechanism. SiRNA to EDD or Mcl-1 induced comparable levels of apoptosis in A2780ip2 and ES-2 cells. Stable overexpression of Mcl-1 protected cells from apoptosis following EDD knockdown, accompanied by a loss of endogenous, but not exogenous, Mcl-1 protein, suggesting that EDD regulated Mcl-1 synthesis. Indeed, EDD knockdown induced a 1.87-fold decrease in Mcl-1 messenger RNA and EDD transfection enhanced murine Mcl-1 promoter-driven luciferase expression 5-fold. To separate EDD survival and potential cisplatin resistance functions, we generated EDD shRNA stable cell lines that could survive initial EDD knockdown and showed that these cells were 4- to 21-fold more sensitive to cisplatin. Moreover, transient EDD overexpression in COS-7 cells was sufficient to promote cisplatin resistance 2.4-fold, dependent upon its E3 ligase activity. In vivo, mouse intraperitoneal ES-2 and A2780ip2 xenograft experiments showed that mice treated with EDD siRNA by nanoliposomal delivery [1,2-dioleoyl-sn-glycero-3-phophatidylcholine (DOPC)] and cisplatin had significantly less tumor burden than those treated with control siRNA/DOPC alone (ES-2, 77.9% reduction, P = 0.004; A2780ip2, 75.9% reduction, P = 0.042) or control siRNA/DOPC with cisplatin in ES-2 (64.4% reduction, P = 0.035), with a trend in A2780ip2 (60.3% reduction, P = 0.168). These results identify EDD as a dual regulator of cell survival and cisplatin resistance and suggest that EDD is a therapeutic target for ovarian cancer.

Phosphorylation of the alternative mRNA splicing factor 45 (SPF45) by Clk1 regulates its splice site utilization, cell migration and invasion

Alternative mRNA splicing is a mechanism to regulate protein isoform expression and is regulated by alternative splicing factors. The alternative splicing factor 45 (SPF45) is overexpressed in cancer, although few biological effects of SPF45 are known, and few splicing targets have been identified. We previously showed that Extracellular Regulated Kinase 2 (ERK2) phosphorylation of SPF45 regulates cell proliferation and adhesion to fibronectin. In this work, we show that Cdc2-like kinase 1 (Clk1) phosphorylates SPF45 on eight serine residues. Clk1 expression enhanced, whereas Clk1 inhibition reduced, SPF45-induced exon 6 exclusion from Fas mRNA. Mutational analysis of the Clk1 phosphorylation sites on SPF45 showed both positive and negative regulation of splicing, with a net effect of inhibiting SPF45-induced exon 6 exclusion, correlating with reduced Fas mRNA binding. However, Clk1 enhanced SPF45 protein expression, but not mRNA expression, whereas inhibition of Clk1 increased SPF45 degradation through a proteasome-dependent pathway. Overexpression of SPF45 or a phospho-mimetic mutant, but not a phospho-inhibitory mutant, stimulated ovarian cancer cell migration and invasion, correlating with increased fibronectin expression, ERK activation and enhanced splicing and phosphorylation of full-length cortactin. Our results demonstrate for the first time that SPF45 overexpression enhances cell migration and invasion, dependent on biochemical regulation by Clk1.

Mitogen-activated protein kinase phosphorylation of splicing factor 45 (SPF45) regulates SPF45 alternative splicing site utilization, proliferation, and cell adhesion

The regulation of alternative mRNA splicing factors by extracellular cues and signal transduction cascades is poorly understood. Using an engineered extracellular signal-regulated kinase 2 (ERK2) that can utilize ATP analogs, we have identified the alternative mRNA splicing factor 45 (SPF45), which is overexpressed in cancer, as a novel coimmunoprecipitating ERK2 substrate. ERK2 phosphorylated SPF45 on Thr71 and Ser222 in vitro and in cells in response to H-RasV12, B-RAF-V600E, and activated MEK1. Jun N-terminal kinase 1 (JNK1) and p38α also phosphorylated SPF45 in vitro and associated with SPF45 in cells. SPF45 was differentially phosphorylated in cells by all three mitogen-activated protein (MAP) kinases in response to phorbol myristate acid (PMA), H(2)O(2), UV, and anisomycin stimulation. ERK and p38 activation decreased SPF45-dependent exon 6 exclusion from fas mRNA in a minigene assay in cells. Stable overexpression of SPF45 in SKOV-3 cells dramatically inhibited cell proliferation in a phosphorylation-dependent manner through inhibition of ErbB2 expression. SPF45 overexpression also induced EDA inclusion into fibronectin transcripts and fibronectin expression in a phosphorylation-dependent and -independent manner, respectively, specifically affecting cellular adhesion to a fibronectin matrix. These data identify SPF45 as the first splicing factor regulated by multiple MAP kinase pathways and show effects of both SPF45 overexpression and phosphorylation.

Monomeric and dimeric models of ERK2 in conjunction with studies on cellular localization, nuclear translocation, and in vitro analysis

Extracellular signal-regulated protein kinase 2 (ERK2) plays many vital roles in cellular signal regulation. Phosphorylation of ERK2 leads to propagation and execution of various extracellular stimuli, which influence cellular responses to stress. The final response of the ERK2 signaling pathway is determined by localization and duration of active ERK2 at specific target cell compartments through protein-protein interactions of ERK2 with various cytoplasmic and nuclear substrates, scaffold proteins, and anchoring counterparts. In this respect, dimerization of phosphorylated ERK2 has been suggested to be a part of crucial regulating mechanism in various protein-protein interactions. After the report of putative dimeric structure of active ERK2 (Canagarajah et al., 1997), dimeric model was employed to explain many in vivo and in vitro experimental results. But more recently, many reports have been presented questioning the validity of dimer hypothesis of active ERK2. In this review, we summarize the various in vitro and in vivo studies concerning the Monomeric or the dimeric forms of ERK2 and the validity of the dimer hypothesis.

Identification of novel substrates of MAP Kinase cascades using bioengineered kinases that uniquely utilize analogs of ATP to phosphorylate substrates

The Mitogen-Activated Protein Kinase (MAPK) family of signaling molecules regulates a number of cellular processes through the direct phosphorylation and regulation of a plethora of cellular proteins. Identifying the direct substrates of the MAPK pathway proteins is important for determining how the effects of MAPK activation have such profound effects on cell biology. In this chapter, we describe one method for specific labeling and identification of direct MAPK substrates. A single or double point mutation is generated within the ATP binding domain at a particular residue(s) termed the "gatekeeper" that comes into close contact with the N6 position of ATP. Most kinases contain an amino acid larger than alanine at this position. Mutation of the residue(s) to glycine or alanine generates a "pocket" that allows the mutant kinase to bind and uniquely utilize an analog of ATP that contains a chemical substituent at the N6 position. When radiolabeled analog is added to the mutant kinase and a complex mixture of cellular proteins, the only proteins that become radiolabeled are direct substrates of the mutant kinase. To label biologically relevant substrates, we take advantage of the direct binding of MAPKs to their substrates. An epitope tagged mutant kinase is expressed in cells and immunoprecipitated with associated substrates, which are then radiolabeled in an in vitro kinase reaction using (gamma-(32)P) ATP analog. Larger, unlabeled kinase reactions are run in parallel and used to identify the substrates by mass spectrometry.

ERK1 nucleocytoplasmic shuttling rate depends on specific N-terminal aminoacids

Despite ERK1 and ERK2 were considered interchangeable isoforms for a long time, their roles are now emerging as only partially overlapping. We recently reported that the nucleocytoplasmic trafficking of GFP-tagged ERK1 is slower than that of ERK2, this difference being caused by a unique domain of ERK1 located at its N-terminus (ERK1-Nt). In the present report we further investigated this issue by asking which were the specific aminoacids involved in such process. By photobleaching strategy, we demonstrated that ERK1-Nt is a domain capable to slow down the nucleocytoplasmic shuttling rate even of a small cargo protein. ERK1-Nt was then dissected into three regions as follows: 1 (aa 1-9), 2 (aa 10-29) and 3, (aa 30-39) that were deleted or mutated at specific sites. Dynamic imaging assessment of the role played by each region in determining the shuttling rate revealed that: region 1 has no significant role, region 2 and specific aminoacids of region 3 (V31, K33, P36) are critical, but singularly do not totally account for the difference in the shuttling rate between ERK1 and 2. Finally, we demonstrated that the nucleocytoplasmic shuttling rate of a passively diffusing protein (mRED) is inversely related to ERK1-Nt-GFP concentrations inside the cell, thus suggesting that ERK1-Nt-GFP occupies the nuclear pore perhaps because of an important affinity of ERK1-Nt for nucleoporins. In conclusion, ERK1-Nt is a domain able per se to confer a slower shuttling rate to a cargo protein. Specific regions within this domain were identified as responsible for this biophysical property.

GIT1 is a novel MEK1-ERK1/2 scaffold that localizes to focal adhesions

Cell polarity is critical for cell migration and requires localized signal transduction in subcellular domains. Recent evidence demonstrates that activation of ERK1/2 (extracellular-signal-regulated kinase 1/2) in focal adhesions is essential for cell migration. GIT1 (G-protein-coupled receptor kinase-interacting protein 1) has been shown to bind paxillin and regulate focal-adhesion disassembly. We have previously reported that GIT1 binds to MEK1 [MAPK (mitogen-activated protein kinase)/ERK kinase 1] and acts as a scaffold to enhance ERK1/2 activation in response to EGF (epidermal growth factor). In the present study we show that GIT1 associates with ERK1/2 in focal adhesions and this association increases after EGF stimulation. The CC (coiled-coil) domain of ERK1/2 is required for association with GIT1, translocation to focal adhesions, and cell spreading and migration. Immunofluorescent staining showed that, after EGF stimulation, GIT1 co-localized with pERK1/2 (phosphorylated ERK1/2) in focal adhesions. The binding of GIT1 and ERK1/2 was functionally important, since transfecting an ERK2 mutant lacking the CC domain [ERK2(del CC)] significantly decreased pERK1/2 translocation to focal adhesions, cell spreading and migration induced by EGF. In summary, the CC domain of ERK1/2 is necessary for binding to GIT1, for ERK1/2 activation in focal adhesions, and for cell spreading and migration.

The ERK signaling cascade--views from different subcellular compartments

The extracellular signal-regulated kinase cascade is a central signaling pathway that is stimulated by various extracellular stimuli. The signals of these stimuli are then transferred by the cascade's components to a large number of targets at distinct subcellular compartments, which in turn induce and regulate a large number of cellular processes. To achieve these functions, the cascade exhibits versatile and dynamic subcellular distribution that allows proper temporal and spatial modulation of the appropriate processes. In this review, we discuss the intracellular localizations of different components of the ERK cascade, and the impact of these localizations on their activation and specificity.

ERK activation and nuclear signaling induced by the loss of cell/matrix adhesion stimulates anchorage-independent growth of ovarian cancer cells

Ovarian cancer metastasis involves the sloughing of epithelial cells from the ovary into the peritoneal cavity, where the cells can survive and proliferate in peritoneal ascites under anchorage-independent conditions. For normal epithelial cells and fibroblasts, cell adhesion to the extracellular matrix is required to prevent apoptosis and for proper activation and nuclear signaling of the ERK MAP kinase. The mechanisms of ERK regulation by adhesion have been determined by our lab and others. In this report, we elucidate a novel means of ERK regulation by cellular adhesion in ovarian cancer cells. We demonstrate that ERK and its activator MEK are robustly stimulated after cell detachment from a substratum in several ovarian cancer cell lines, but not a benign ovarian cell line, independent of serum and FAK or PAK activity. MEK and ERK activation was sustained for 48 h after detachment, while activation by serum or growth factors in adherent cells was transient. Re-attachment of suspended ovarian cells to fibronectin restored basal levels of MEK and ERK activity. ERK activity in suspended cells was dynamically controlled through an autocrine stimulatory pathway and prevalent phosphatase activity. Suspended cells demonstrated higher levels of ERK nuclear signaling to Elk1 compared to adherent cells. Inhibition of ERK activation with the MEK inhibitor U0126 had minor effects on adherent cell growth, but greatly decreased growth in soft agar. These data demonstrate a unique regulation of ERK by cellular adhesion and suggest a mechanism by which ERK may regulate anchorage-independent growth of metastatic ovarian cancer cells.

Akt as a therapeutic target in cancer

BACKGROUND

The phosphatidylinositol 3-kinase (PI3K)/phosphatase and tensin homolog (PTEN)/v-akt murine thymoma viral oncogene homolog (Akt)/mammalian target of rapamycin (mTOR) pathway is central in the transmission of growth regulatory signals originating from cell surface receptors.

OBJECTIVE

This review discusses how mutations occur that result in elevated expression the PI3K/PTEN/Akt/mTOR pathway and lead to malignant transformation, and how effective targeting of this pathway may result in suppression of abnormal growth of cancer cells.

METHODS

We searched the literature for articles which dealt with altered expression of this pathway in various cancers including: hematopoietic, melanoma, non-small cell lung, pancreatic, endometrial and ovarian, breast, prostate and hepatocellular.

RESULTS/CONCLUSIONS

The PI3K/PTEN/Akt/mTOR pathway is frequently aberrantly regulated in various cancers and targeting this pathway with small molecule inhibitors and may result in novel, more effective anticancer therapies.

Role of Non-phosphorylated Activation Loop Residues in Determining ERK2 Dephosphorylation, Activity, and Subcellular Localization

Extracellular signal-regulated kinases (ERKs) activity is regulated by MAPK/ERK kinases (MEKs), which phosphorylate the regulatory Tyr and Thr residues in ERKs activation loop, and by various phosphatases that remove the incorporated phosphates. Although the role of the phosphorylated residues in the activation loop of ERKs is well studied, much less is known about the role of other residues within this loop. Here we substituted several residues within amino acids 173-177 of ERK2 and studied their role in ERK2 phosphorylation, substrate recognition, and subcellular localization. We found that substitution of residues 173-175 and particularly Pro(174) to alanines reduces the EGF-induced ERK2 phosphorylation, without modifying its in vitro phosphorylation by MEK1. Examining the ability of these mutants to be dephosphorylated revealed that 173-5A mutants are hypersensitive to phosphatases, indicating that these residues are important for setting the phosphorylation/dephosphorylation balance of ERKs. In addition, 173-5A mutants reduced ERK2 activity toward Elk-1, without affecting the activity of ERK2 toward MBP, while substitution of residues 176-8 decreased ERK2 activity toward both substrates. Substitution of Asp(177) to alanine increased nuclear localization of the construct in MEK1-overexpressing cells, suggesting that this residue together with His(176) is involved in the dissociation of ERK2 from MEKs. Combining CRS/CD motif and the activation loop mutations revealed that these two regions cooperate in determining the net phosphorylation of ERK2, but the role of the CRS/CD motif predominates that of the activation loop residues. Thus, we show here that residues 173-177 of ERK2 join other regulatory regions of ERKs in governing ERK activity.

The interaction of endoglin with beta-arrestin2 regulates transforming growth factor-beta-mediated ERK activation and migration in endothelial cells

In endothelial cells, transforming growth factor beta (TGF-beta) signals through two distinct pathways to regulate endothelial cell proliferation and migration, the ALK-1/Smads 1/5/8 pathway and the ALK-5/Smads 2/3 pathway. TGF-beta signaling through these pathways is further regulated in endothelial cells by the endothelial specific TGF-beta superfamily co-receptor, endoglin. The importance of endoglin, ALK-1, and ALK-5 in endothelial biology is underscored by the embryonic lethal phenotypes of knock-outs in mice due to defects in angiogenesis, and by the presence of disease-causing mutations in these genes in human vascular diseases. However, the mechanism of action of endoglin is not well defined. Here we define a novel interaction between endoglin and the scaffolding protein beta-arrestin2. Both co-immunoprecipitation and fluorescence confocal studies demonstrate the specific interaction between endoglin and beta-arrestin2 in endothelial cells, enhanced by ALK-1 and to a lesser extent by the type II TGF-beta receptor. The endoglin/beta-arrestin2 interaction results in endoglin internalization and co-accumulation of endoglin and beta-arrestin2 in endocytic vesicles. Whereas endoglin did not have a direct impact on either Smad 2/3 or Smad 1/5/8 activation, endoglin antagonized TGF-beta-mediated ERK signaling, altered the subcellular distribution of activated ERK, and inhibited endothelial cell migration in a manner dependent on the ability of endoglin to interact with beta-arrestin2. Reciprocally, small interfering RNA-mediated silencing of endogenous beta-arrestin2 expression restored TGF-beta-mediated ERK activation and increased endothelial cell migration in an endoglin-dependent manner. These studies define a novel function for endoglin, and further expand the roles mediated by the ubiquitous scaffolding protein beta-arrestin2.

DNA-independent PARP-1 activation by phosphorylated ERK2 increases Elk1 activity: a link to histone acetylation

PolyADP-ribose polymerases (PARPs) catalyze a posttranslational modification of nuclear proteins by polyADP-ribosylation. The catalytic activity of the abundant nuclear protein PARP-1 is stimulated by DNA strand breaks, and PARP-1 activation is required for initiation of DNA repair. Here we show that PARP-1 also acts within extracellular signal-regulated kinase (ERK) signaling cascade that mediates growth and differentiation. The findings reveal an alternative mode of PARP-1 activation, which does not involve binding to DNA or DNA damage. In a cell-free system, recombinant PARP-1 was intensively activated and thereby polyADP-ribosylated by a direct interaction with phosphorylated ERK2, and the activated PARP-1 dramatically increased ERK2-catalyzed phosphorylation of the transcription factor Elk1. In cortical neurons treated with nerve growth factors and in stimulated cardiomyocytes, PARP-1 activation enhanced ERK-induced Elk1-phosphorylation, core histone acetylation, and transcription of the Elk1-target gene c-fos. These findings constitute evidence for PARP-1 activity within the ERK signal-transduction pathway.

The MEK/ERK cascade: from signaling specificity to diverse functions

The ERK signaling cascade is a central MAPK pathway that plays a role in the regulation of various cellular processes such as proliferation, differentiation, development, learning, survival and, under some conditions, also apoptosis. The ability of this cascade to regulate so many distinct, and even opposing, cellular processes, raises the question of signaling specificity determination by this cascade. Here we describe mechanisms that cooperate to direct MEK-ERK signals to their appropriate downstream destinations. These include duration and strength of the signals, interaction with specific scaffolds, changes in subcellular localization, crosstalk with other signaling pathways, and presence of multiple components with distinct functions in each tier of the cascade. Since many of the mechanisms do not function properly in cancer cells, understanding them may shed light not only on the regulation of normal cell proliferation, but also on mechanisms of oncogenic transformation.

Vimentin binding to phosphorylated Erk sterically hinders enzymatic dephosphorylation of the kinase

Cleavage fragments of de novo synthesized vimentin were recently reported to interact with phosphorylated Erk1 and Erk2 MAP kinases (pErk) in injured sciatic nerve, thus linking pErk to a signaling complex retrogradely transported on importins and dynein. Here we clarify the structural basis for this interaction, which explains how pErk is protected from dephosphorylation while bound to vimentin. Pull-down and ELISA experiments revealed robust calcium-dependent binding of pErk to the second coiled-coil domain of vimentin, with observed affinities of binding increasing from 180 nM at 0.1 microM calcium to 15 nM at 10 microM calcium. In contrast there was little or no binding of non-phosphorylated Erk to vimentin under these conditions. Geometric and electrostatic complementarity docking generated a number of solutions wherein vimentin binding to pErk occludes the lip containing the phosphorylated residues in the kinase. Binding competition experiments with Erk peptides confirmed a solution in which vimentin covers the phosphorylation lip in pErk, interacting with residues above and below the lip. The same peptides inhibited pErk binding to the dynein complex in sciatic nerve axoplasm, and interfered with protection from phosphatases by vimentin. Thus, a soluble intermediate filament fragment interacts with a signaling kinase and protects it from dephosphorylation by calcium-dependent steric hindrance.

The ERK cascade: a prototype of MAPK signaling

Sequential activation of protein kinases within the mitogen-activated protein kinase (MAPK) cascades is a common mechanism of signal transduction in many cellular processes. Four such cascades have been elucidated thus far, and named according to their MAPK tier component as the ERK1/2, JNK, p38MAPK, and ERK5 cascades. These cascades cooperate in transmitting various extracellular signals, and thus control cellular processes such as proliferation, differentiation, development, stress response, and apoptosis. Here we describe the classic ERK1/2 cascade, and concentrate mainly on the properties of MEK1/2 and ERK1/2, including their mode of regulation and their role in various cellular processes and in oncogenesis. This cascade may serve as a prototype of the other MAPK cascades, and the study of this cascade is likely to contribute to the understanding of mitogenic and other processes in many cell lines and tissues.

Protein-protein interactions in the regulation of the extracellular signal-regulated kinase

The extracellular signal-regulated kinase (ERK) cascade is a central intracellular signaling pathway that is activated by a variety of extracellular stimuli, and thereby regulates cellular processes such as proliferation, differentiation, and oncogenic transformation. To execute these functions, the signals of those stimuli are transmitted to the cytosolic and nuclear targets in a rapid and specific manner. In the last few years it has become clear that the specificity and the rapid function of the ERK cascade is largely determined by protein-protein interactions with various signaling components and substrates. This review describes interactions of ERK with its immediate regulators, scaffold proteins, substrates, and localizing proteins, and shows their involvement in the functioning of the ERK cascade. Understanding the full scope of ERK-interactions is important for the development of new drugs for the treatment of cancer and other diseases.

The extracellular signal-regulated kinase: multiple substrates regulate diverse cellular functions

The extracellular signal-regulated kinase (ERK) cascade is a central pathway that transmits signals from many extracellular agents to regulate cellular processes such as proliferation, differentiation and cell cycle progression. The signaling via the ERK cascade is mediated by sequential phosphorylation and activation of protein kinases in the different tiers of the cascade. Although the main core phosphorylation chain of the cascade includes Raf kinases, MEK1/2, ERK1/2 (ERKs) and RSKs, other alternatively spliced forms and distinct components exist in the different tiers, and participate in ERK signaling under specific conditions. These components enhance the complexity of the ERK cascade and thereby, enable the wide variety of functions that are regulated by it. Another factor that is important for the dissemination of ERKs' signals is the multiplicity of the cascade's substrates, which include transcription factors, protein kinases and phosphatases, cytoskeletal elements, regulators of apoptosis, and a variety of other signaling-related molecules. About 160 substrates have already been discovered for ERKs, and the list of these substrates, as well as the function and mechanism of activation of representative substrates, are described in the current review. Many of these substrates are localized in the nucleus, and seem to participate in the regulation of transcription upon stimulation. However, other substrates are found in the cytosol as well as in other cellular organelles, and those are responsible for processes such as translation, mitosis and apoptosis. Understanding of these processes may provide a full picture of the distinct, and even opposing cellular processes that are regulated by the ERK cascade.

Regulation of p90RSK phosphorylation by SARS-CoV infection in Vero E6 cells

The 90 kDa ribosomal S6 kinases (p90RSKs) are a family of broadly expressed serine/threonine kinases with two kinase domains activated by extracellular signal‐regulated protein kinase in response to many growth factors. Our recent study demonstrated that severe acute respiratory syndrome (SARS)‐coronavirus (CoV) infection of monkey kidney Vero E6 cells induces phosphorylation and dephosphorylation of signaling pathways, resulting in apoptosis. In the present study, we investigated the phosphorylation status of p90RSK, which is a well‐known substrate of these signaling pathways, in SARS‐CoV‐infected cells. Vero E6 mainly expressed p90RSK1 and showed weak expression of p90RSK2. In the absence of viral infection, Ser221 in the N‐terminal kinase domain was phosphorylated constitutively, whereas both Thr573 in the C‐terminal kinase domain and Ser380 between the two kinase domains were not phosphorylated in confluent cells. Ser380, which has been reported to be involved in autophosphorylation by activation of the C‐terminal kinase domain, was phosphorylated in confluent SARS‐CoV‐infected cells, and this phosphorylation was inhibited by http://SB203580, which is an inhibitor of p38 mitogen‐activated protein kinases (MAPK). Phosphorylation of Thr573 was not upregulated in SARS‐CoV‐infected cells. Thus, in virus‐infected cells, phosphorylation of Thr573 was not necessary to induce phosphorylation of Ser380. On the other hand, Both Thr573 and Ser380 were phosphorylated by treatment with epidermal growth factor (EGF) in the absence of p38 MAPK activation. Ser220 was constitutively phosphorylated despite infection. These results indicated that phosphorylation status of p90RSK by SARS‐CoV infection is different from that by stimulation of EGF. This is the first detailed report regarding regulation of p90RSK phosphorylation by virus infection.

Trichoderma mitogen-activated protein kinase signaling is involved in induction of plant systemic resistance

The role of a mitogen-activated protein kinase (MAPK) TmkA in inducing systemic resistance in cucumber against the bacterial pathogen Pseudomonas syringae pv. lacrymans was investigated by using tmkA loss-of-function mutants of Trichoderma virens. In an assay where Trichoderma spores were germinated in proximity to cucumber roots, the mutants were able to colonize the plant roots as effectively as the wild-type strain but failed to induce full systemic resistance against the leaf pathogen. Interactions with the plant roots enhanced the level of tmkA transcript in T. virens and its homologue in Trichoderma asperellum. At the protein level, we could detect the activation of two forms reacting to the phospho-p44/42 MAPK antibody. Biocontrol experiments demonstrated that the tmkA mutants retain their biocontrol potential against Rhizoctonia solani in soil but are not effective against Sclerotium rolfsii in reducing disease incidence. Our results show that, unlike in many plant-pathogen interactions, Trichoderma TmkA MAPK is not involved in limited root colonization. Trichoderma, however, needs MAPK signaling in order to induce full systemic resistance in the plant.

Quantifying ERK2–protein interactions by fluorescence anisotropy: PEA-15 inhibits ERK2 by blocking the binding of DEJL domains

While mitogen-activated protein kinase signaling pathways constitute highly regulated networks of protein-protein interactions, little quantitative information for these interactions is available. Here we highlight recent fluorescence anisotropy binding studies that focus on the interactions of ERK1 and ERK2 with PEA-15 (antiapoptotic phosphoprotein enriched in astrocytes-15 kDa), a small protein that sequesters ERK2 in the cytoplasm. The regulation of ERK2 by PEA-15 is appraised in the light of a simple equilibrium-binding model for reversible ERK2 nucleoplasmic-cytoplasmic shuttling, which elaborates on the theory of Burack and Shaw (J. Biol. Chem. 280, 3832-3837; 2005). Also highlighted is the recent observation that the peptide N-QKGKPRDLELPLSPSL-C, derived from the docking site for ERK/JNK and LEL (DEJL) in Elk-1, displaces PEA-15 from ERK2. It is proposed that the C-terminus of PEA-15 ((121)LXLXXXXKK(129)) is a reverse DEJL domain [which has a general consensus of R/K-phi(A)-X(3/4)-phi(B), where phi(A) and phi(B) are hydrophobic residues (Leu, Ile, or Val)], which mediates one arm of a bidentate PEA-15 interaction with ERK2. The notion that PEA-15 is a potent inhibitor of many ERK2-mediated phosphorylations, by virtue of its ability to block ERK2-DEJL domain interactions, is proposed.

Stress kinase signaling regulates androgen receptor phosphorylation, transcription, and localization

Activation of signal transduction kinase cascades is known to alter androgen receptor (AR) activity, but the molecular mechanisms are still poorly defined. Here we show that stress kinase signaling regulates Ser 650 phosphorylation and AR nuclear export. In LNCaP prostate cancer cells, activation of either MAPK kinase (MKK) 4:c-Jun N-terminal kinase (JNK) or MKK6:p38 signaling pathways increased Ser 650 phosphorylation, whereas pharmacologic inhibition of JNK or p38 signaling led to a reduction of AR Ser 650 phosphorylation. Both p38alpha and JNK1 phosphorylated Ser 650 in vitro. Small interfering RNA-mediated knockdown of either MKK4 or MKK6 increased endogenous prostate-specific antigen (PSA) transcript levels, and this increase was blocked by either bicalutamide or AR small interfering RNA. Stress kinase inhibition of PSA transcription is, therefore, dependent on the AR. Similar experiments involving either activation or inhibition of MAPK/ERK kinase:ERK signaling had little effect on Ser 650 phosphorylation or PSA mRNA levels. Ser 650 is proximal to the DNA binding domain that contains a nuclear export signal. Mutation of Ser 650 to alanine reduced nuclear export of the AR, whereas mutation of Ser 650 to the phosphomimetic amino acid aspartate restored AR nuclear export. Pharmacologic inhibition of stress kinase signaling reduced wild-type AR nuclear export equivalent to the S650A mutant without affecting nuclear export of the S650D mutant. Our data suggest that stress kinase signaling and nuclear export regulate AR transcriptional activity.

Exposure of protein kinase motifs that trigger binding of Hsp90 and Cdc37

Hsp90 and its co-chaperone Cdc37 are required for the activity of numerous eukaryotic protein kinases. c-Jun N-terminal kinases (JNKs) appear to be Hsp90-independent kinases, as their activity is unaffected by Hsp90 inhibition. It is currently unknown why some protein kinases are Hsp90- and Cdc37-dependent for their function, while others are not. Therefore, we investigated what structural motifs within JNKs confer or defer Hsp90 and Cdc37 interaction. Both Hsp90 and Cdc37 recognized structural features that were exposed or destabilized upon deletion of JNK1alpha1's N-terminal non-catalytic structural motif, while only Hsp90 bound JNK when its C-terminal non-catalytic structural motif was deleted. Mutations in JNK's activation loop that are known to constitutively activate or inactivate its kinase activity had no effect on JNK's lack of interaction with Hsp90 and Cdc37. Our findings suggest a model in which Hsp90 and Cdc37 each recognize distinct features within the catalytic domains of kinases.

ERK2 shows a restrictive and locally selective mechanism of recognition by its tyrosine phosphatase inactivators not shared by its activator MEK1

The two regulatory residues that control the enzymatic activity of the mitogen-activated protein (MAP) kinase ERK2 are phosphorylated by the unique MAP kinase kinases MEK1/2 and dephosphorylated by several tyrosine-specific and dual specificity protein phosphatases. Selective docking interactions facilitate these phosphorylation and dephosphorylation events, controlling the specificity and duration of the MAP kinase activation-inactivation cycles. We have analyzed the contribution of specific residues of ERK2 in the physical and functional interaction with the ERK2 phosphatase inactivators PTP-SL and MKP-3 and with its activator MEK1. Single mutations in ERK2 that abrogated the dephosphorylation by endogenous tyrosine phosphatases from HEK293 cells still allowed efficient phosphorylation by endogenous MEK1/2. Discrete ERK2 mutations at the ERK2 docking groove differentially affected binding and inactivation by PTP-SL and MKP-3. Remarkably, the cytosolic retention of ERK2 by its activator MEK1 was not affected by any of the analyzed ERK2 single amino acid substitutions. A chimeric MEK1 protein, containing the kinase interaction motif of PTP-SL, bound tightly to ERK2 through its docking groove and behaved as a gain-of-function MAP kinase kinase that hyperactivated ERK2. Our results provide evidence that the ERK2 docking groove is more restrictive and selective for its tyrosine phosphatase inactivators than for MEK1/2 and indicate that distinct ERK2 residues modulate the docking interactions with activating and inactivating effectors.

Mitogen-activated protein kinase (MAPK)-docking sites in MAPK kinases function as tethers that are crucial for MAPK regulation in vivo

Docking sites on targets of mitogen-activated protein kinases (MAPKs) facilitate accurate and efficient substrate phosphorylation. MAPK/ERK kinases (MEKs, or MKKs), the upstream regulators of MAPKs, also contain N-terminal MAPK-docking sites, or 'D-sites'; however, the in vivo functions of MEK D-sites are incompletely understood. Here we found that the ability of constitutively-active human MEK1 and MEK2 to stimulate ERK phosphorylation and to induce the neoplastic transformation of NIH 3T3 cells required the integrity of the D-site. In addition, D-site mutants of otherwise wild-type MEK1/2 were unable to anchor unphosphorylated ERK2 in the cytoplasm. ERK activation, cytoplasmic anchoring and release were completely retained in 'swap' mutants in which MEK2's D-site was replaced with the D-site of MEK1 or yeast Ste7. Furthermore, these abilities were significantly retained when MEK2's D-site was moved to its C-terminus, or replaced by an unrelated MAPK-binding domain taken from the Ets-1 transcription factor. We conclude that the D-sites in MEKs are crucial for the activation of their cognate MAPKs in vivo, and that their primary function is to tether their cognate MAPKs near the MEK's kinase domain. This proximity effect is sufficient to explain the contribution that the D-site interaction makes to several biologically important signaling events.

Identifying specific kinase substrates through engineered kinases and ATP analogs

Intracellular signaling by protein kinases controls many aspects of cellular biochemistry and physiology. Determining the direct substrates of protein kinases is important in understanding how these signaling enzymes exert their effect on cellular functions. One of the recent developments in this area takes advantage of the similarity in the ATP binding domains of protein kinases, where a few conserved amino acids containing large side chains come in close contact with the N-6 position of bound ATP. Mutation of one or more of these residues generates a "pocket" in the ATP binding site that allows the mutant kinase, but not other cellular kinases, to utilize analogs of ATP with bulky substituents synthesized onto the N-6 position. The use of such a mutated kinase and radiolabeled ATP analogs allows for the specific labeling of direct substrates of the kinase within a mixture of cellular proteins. We have recently reported the generation of "pocket" mutants of extracellular regulated kinase 2 (ERK2) and their use in the identification of two novel substrates of ERK2. In this report, we discuss the generation and characterization of ERK2 mutants that utilize analogs of ATP and describe the methodology used to identify ERK2-associated substrates. We also describe the direct labeling of ERK2 substrates in cell lysates. These methodologies can be adapted for use with other protein kinases to increase the understanding of intracellular signal transduction.

Mitogen-activated protein kinase feedback phosphorylation regulates MEK1 complex formation and activation during cellular adhesion

Cell adhesion and spreading depend on activation of mitogen-activated kinase, which in turn is regulated both by growth factor and integrin signaling. Growth factors, such as epidermal growth factor, are capable of activating Ras and Raf, but integrin signaling is required to couple Raf to MEK and MEK to extracellular signal-regulated protein kinase (ERK). It was previously shown that Rac-p21-activated kinase (PAK) signaling regulated the physical association of MEK1 with ERK2 through phosphorylation sites in the proline-rich sequence (PRS) of MEK1. It was also shown that activation of MEK1 and ERK by integrins depends on PAK phosphorylation of S298 in the PRS. Here we report a novel MEK1-specific regulatory feedback mechanism that provides a means by which activated ERK can terminate continued PAK phosphorylation of MEK1. Activated ERK can phosphorylate T292 in the PRS, and this blocks the ability of PAK to phosphorylate S298 and of Rac-PAK signaling to enhance MEK1-ERK complex formation. Preventing ERK feedback phosphorylation on T292 during cellular adhesion prolonged phosphorylation of S298 by PAK and phosphorylation of S218 and S222, the MEK1 activating sites. We propose that activation of ERK during adhesion creates a feedback system in which ERK phosphorylates MEK1 on T292, and this in turn blocks additional S298 phosphorylation in response to integrin signaling.

Signalling specificity of Ser/Thr protein kinases through docking-site-mediated interactions

Signal transduction pathways use protein kinases for the modification of protein function by phosphorylation. A major question in the field is how protein kinases achieve the specificity required to regulate multiple cellular functions. Here we review recent studies that illuminate the mechanisms used by three families of Ser/Thr protein kinases to achieve substrate specificity. These kinases rely on direct docking interactions with substrates, using sites distinct from the phospho-acceptor sequences. Docking interactions also contribute to the specificity and regulation of protein kinase activities. Mitogen-activated protein kinase (MAPK) family members can associate with and phosphorylate specific substrates by virtue of minor variations in their docking sequences. Interestingly, the same MAPK docking pocket that binds substrates also binds docking sequences of positive and negative MAPK regulators. In the case of glycogen synthase kinase 3 (GSK3), the presence of a phosphate-binding site allows docking of previously phosphorylated (primed) substrates; this docking site is also required for the mechanism of GSK3 inhibition by phosphorylation. In contrast, non-primed substrates interact with a different region of GSK3. Phosphoinositide-dependent protein kinase-1 (PDK1) contains a hydrophobic pocket that interacts with a hydrophobic motif present in all known substrates, enabling their efficient phosphorylation. Binding of the substrate hydrophobic motifs to the pocket in the kinase domain activates PDK1 and other members of the AGC family of protein kinases. Finally, the analysis of protein kinase structures indicates that the sites used for docking substrates can also bind N- and C-terminal extensions to the kinase catalytic core and participate in the regulation of its activity.

p38alpha isoform Mxi2 binds to extracellular signal-regulated kinase 1 and 2 mitogen-activated protein kinase and regulates its nuclear activity by sustaining its phosphorylation levels

Mxi2 is a p38alpha splice isoform that is distinctively activated by mitogenic stimuli. Here we show that Mxi2 immunoprecipitates carry a kinase activity that is persistently activated by epidermal growth factor in a fashion regulated by Ras, Raf, and MEK. We demonstrate that this kinase activity can be attributed not to Mxi2 but rather to extracellular signal-regulated kinases 1 and 2 (ERK1/2), which coimmunoprecipitated with Mxi2 both by ectopic expression and in a physiological environment like the kidney. Furthermore, we provide evidence that Mxi2-ERK interaction has profound effects on ERK function, demonstrating that Mxi2 prolongs the duration of the ERK signal by sustaining its phosphorylation levels. Interestingly, we show that the effects of Mxi2 on ERK are restricted to nuclear events. Mxi2 potently up-regulates ERK-mediated activation of the transcription factors Elk1 and HIF1alpha but has no effect on the activity of ERK cytoplasmic substrates RSK2 and cPLA(2), induced by epidermal growth factor or by MEK. Overall, our findings point to Mxi2 as a unique member of the p38 family that may have an unprecedented role in the regulation of the functions of ERK mitogen-activated protein kinases.

Identification of novel ERK2 substrates through use of an engineered kinase and ATP analogs

The mitogen-activated protein kinases are key regulators of cellular organization and function. To understand the mechanisms(s) by which these ubiquitous kinases affect specific cellular changes, it is necessary to identify their diverse and numerous substrates in different cell contexts and compartments. As a first step in achieving this goal, we engineered a mutant ERK2 in which a bulky amino acid residue in the ATP binding site (glutamine 103) is changed to glycine, allowing this mutant to utilize an analog of ATP (cyclopentyl ATP) that cannot be used by wild-type ERK2 or other cellular kinases. The mutation did not inhibit ERK2 kinase activity or substrate specificity in vitro or in vivo. This method allowed us to detect only ERK2-specific phosphorylations within a mixture of proteins. Using this ERK2 mutant/analog pair to phosphorylate ERK2-associated proteins in COS-1 cells, we identified the ubiquitin ligase EDD (E3 identified by differential display) and the nucleoporin Tpr (translocated promoter region) as two novel substrates of ERK2, in addition to the known ERK2 substrate Rsk1. To further validate the method, we present data that confirm that ERK2 phosphorylates EDD in vitro and in vivo. These results not only identify two novel ERK2 substrates but also provide a framework for the future identification of numerous cellular targets of this important signaling cascade.

Docking sites on mitogen-activated protein kinase (MAPK) kinases, MAPK phosphatases and the Elk-1 transcription factor compete for MAPK binding and are crucial for enzymic activity

Mitogen-activated protein kinase (MAPK) cascades control gene expression patterns in response to extracellular stimuli. MAPK/ERK (extracellular-signal-regulated kinase) kinases (MEKs) activate MAPKs by phosphorylating them; activated MAPKs, in turn, phosphorylate target transcription factors, and are deactivated by phosphatases. One mechanism for maintaining signal specificity and efficiency is the interaction of MAPKs with their substrates and regulators through high-affinity docking sites. In the present study, we show that peptides corresponding to the MAPK-docking sites of MEK1, MEK2, Ste7, Elk-1 and MAPK phosphatase (MKP)-2 potently inhibit MEK2 phosphorylation of ERK2, ERK2 phosphorylation of Elk-1, and MKP-1 dephosphorylation of ERK2. Each peptide inhibited multiple reactions; for example, the MEK2 peptide inhibited not only MEK2, but also ERK2 and MKP-1. In addition, these docking-site peptides inhibited MEK2-ERK2 binding. The MAPK-docking site of MEK1 also potently stimulated ERK2-mediated phosphorylation of a target site on the same peptide. Control peptides with mutations of conserved basic and hydrophobic residues of the MAPK-docking site consensus lacked biological activity. We conclude that MEKs, MKPs and the Elk-1 transcription factor compete for binding to the same region of ERK2 via protein-protein interactions that are crucial for kinase/phosphatase activity.

Rac-PAK signaling stimulates extracellular signal-regulated kinase (ERK) activation by regulating formation of MEK1-ERK complexes

Utilizing mutants of extracellular signal-regulated kinase 2 (ERK2) that are defective for intrinsic mitogen-activated protein kinase or ERK kinase (MEK) binding, we have identified a convergent signaling pathway that facilitates regulated MEK-ERK association and ERK activation. ERK2-delta19-25 mutants defective in MEK binding could be phosphorylated in response to mitogens; however, signaling from the Raf-MEK pathway alone was insufficient to stimulate their phosphorylation in COS-1 cells. Phosphorylation of ERK2-delta19-25 but not of wild-type ERK2 in response to Ras V12 was greatly inhibited by dominant-negative Rac. Activated forms of Rac and Cdc42 could enhance the association of wild-type ERK2 with MEK1 but not with MEK2 in serum-starved adherent cells. This effect was p21-activated kinase (PAK) dependent and required the putative PAK phosphorylation sites T292 and S298 of MEK1. In detached cells placed in suspension, ERK2 was complexed with MEK2 but not with MEK1. However, upon replating of cells onto a fibronectin matrix, there was a substantial induction of MEK1-ERK2 association and ERK activation, both of which could be inhibited by dominant-negative PAK1. These data show that Rac facilitates the assembly of a mitogen-activated protein kinase signaling complex required for ERK activation and that this facilitative signaling pathway is active during adhesion to the extracellular matrix. These findings reveal a novel mechanism by which adhesion and growth factor signals are integrated during ERK activation.

Identification of novel point mutations in ERK2 that selectively disrupt binding to MEK1

Extracellular signal-regulated kinases 1 and 2 (ERK1 and ERK2) are essential components of pathways through which signals received at membrane receptors are converted into specific changes in protein function and gene expression. As with other members of the mitogen-activated protein (MAP) kinase family, ERK1 and ERK2 are activated by phosphorylations catalyzed by dual-specificity protein kinases known as MAP/ERK kinases (MEKs). MEKs exhibit stringent specificity for individual MAP kinases. Indeed, MEK1 and MEK2 are the only known activators of ERK1 and ERK2. ERK2 small middle dotMEK1/2 complexes can be detected in vitro and in vivo. The biochemical nature of such complexes and their role in MAP kinase signaling are under investigation. This report describes the use of a yeast two-hybrid screen to identify point mutations in ERK2 that impair its interaction with MEK1/2, yet do not alter its interactions with other proteins. ERK2 residues identified in this screen are on the surface of the C-terminal domain of the kinase, either within or immediately preceding alpha-helix G, or within the MAP kinase insert. Some mutations identified in this manner impaired the two-hybrid interaction of ERK2 with both MEK1 and MEK2, whereas others had a predominant effect on the interaction with either MEK1 or MEK2. Mutant ERK2 proteins displayed reduced activation in HEK293 cells following epidermal growth factor treatment, consistent with their impaired interaction with MEK1/2. However, ERK2 proteins containing MEK-specific mutations retained kinase activity, and were similar to wild type ERK2 in their activation following overexpression of constitutively active MEK1. Unlike wild type ERK2, proteins containing MEK-specific point mutations were constitutively localized in the nucleus, even in the presence of overexpressed MEK1. These data suggest an essential role for the MAP kinase insert and residues within or just preceding alpha-helix G in the interaction of ERK2 with MEK1/2.

Different domains of the mitogen-activated protein kinases ERK3 and ERK2 direct subcellular localization and upstream specificity in vivo

Extracellular signal-regulated kinase 3 (ERK3) is a member of the mitogen-activated protein (MAP) kinase family. ERK3 is most similar in its kinase catalytic domain to ERK2, yet it displays many unique properties. Among these, unlike ERK2, which translocates to the nucleus following activation, ERK3 is constitutively localized to the nucleus, despite the lack of a defined nuclear localization sequence. We created two chimeras between ERK2 and the catalytic domain of ERK3 (ERK3DeltaC), and some mutants of these chimeras, to examine the basis for the different behaviors of these two MAP kinase family members. We find the following: 1) the N-terminal folding domain of ERK3 functions in phosphoryl transfer reactions with the C-terminal folding domain of ERK2; 2) the C-terminal halves of ERK2 and ERK3DeltaC are primarily responsible for their subcellular localization in resting cells; and 3) the N-terminal folding domain of ERK2 is required for its activation in cells, its interaction with MEK1, and its accumulation in the nucleus.

Two clusters of residues at the docking groove of mitogen-activated protein kinases differentially mediate their functional interaction with the tyrosine phosphatases PTP-SL and STEP

Regulated function of mitogen-activated protein (MAP) kinases involves their selective association through docking sites with both activating MAP kinase kinases and inactivating phosphatases, including dual specificity and protein-tyrosine phosphatases (PTP). Site-directed mutagenesis on the mammalian MAP kinases ERK2 and p38alpha identified within their C-terminal docking grooves two clusters of residues important for association with their regulatory PTPs, PTP-SL and STEP. ERK2 and p38alpha mutations that resembled the sevenmaker gain-of-function mutation in the Rolled D. melanogaster ERK2 homologue failed to associate with PTP-SL, were not retained in the cytosol, and were poorly inactivated by this PTP. Additional ERK2 mutations at the docking groove showed deficient association and dephosphorylation by PTP-SL, although their cytosolic retention was unaffected. Other ERK2 mutations, resembling gain-of-function mutations in the FUS3 yeast ERK2 homologue, associated to PTP-SL and were inactivated normally by this PTP. Our results demonstrate that mutations at distinct regions of the docking groove of ERK2 and p38alpha differentially affect their association and regulation by the PTP-SL and STEP PTPs.

Unexpected effects of FERM domain mutations on catalytic activity of Jak3: structural implication for Janus kinases

Janus kinases comprise carboxyterminal kinase, pseudokinase, SH2-like, and N-terminal FERM domains. We identified three patient-derived mutations in the FERM domain of Jak3 and investigated the functional consequences of these mutations. These mutations inhibited receptor binding and also abrogated kinase activity, suggesting interactions between the FERM and kinase domains. In fact, the domains were found to physically associate, and coexpression of the FERM domain enhanced activity of the isolated kinase domain. Conversely, staurosporine, which alters kinase domain structure, disrupted receptor binding, even though the catalytic activity of Jak3 is dispensable for receptor binding. Thus, the Jak FERM domain appears to have two critical functions: receptor interaction and maintenance of kinase integrity.

Involvement of the activation loop of ERK in the detachment from cytosolic anchoring

The Extracellular signal-regulated kinases (ERKs) are translocated into the nucleus in response to mitogenic stimulation. The mechanism of translocation and the residues in ERKs that govern this process are not clear as yet. Here we studied the involvement of residues in the activation loop of ERK2 in determining its subcellular localization. Substitution of residues in the activation loop to alanines indicated that residues 173-181 do not play a significant role in the phosphorylation and activation of ERK2. However, residues 176-181 are responsible for the detachment of ERK2 from MEK1 upon mitogenic stimulation. This dissociation can be mimicked by substitution of residues 176-178 to alanines and is prevented by deletion of these residues or by substitution of residues 179-181 to alanines. On the other hand, residues 176-181, as well as residues essential for ERK2 dimerization, do not play a role in the shuttle of ERK2 through nuclear pores. Thus, phosphorylation-induced conformational rearrangement of residues in the activation loop of ERK2 plays a major role in the control of subcellular localization of this protein.

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.