Identification and characterization of a general nuclear translocation signal in signaling proteins

Mechanisms of GnRH-induced extracellular signal-regulated kinase nuclear localization

Gonadotropin-releasing hormone receptors (GnRHR) mediate activation and nuclear translocation of the extracellular signal regulated kinases 1 and 2 (ERK) by phosphorylation on the TEY motif. This is necessary for GnRH to initiate transcriptional programmes controlling fertility, but mechanisms that govern ERK targeting are unclear. Using automated microscopy to explore ERK regulation in single cells, we find that GnRHR activation induces marked redistribution of ERK to the nucleus and that this effect can be uncoupled from the level of TEY phosphorylation of ERK. Thus, 5 min stimulation with 100 nM GnRH increased phospho-ERK levels (from 89 ± 34 to 555 ± 45 arbitrary fluorescence units) and increased the nuclear:cytoplasmic (N:C) ERK ratio (from 1.36 ± 0.06 to 2.16 ± 0.05) in the whole cell population, but it also significantly increased N:C ERK in cells binned according to phospho-ERK levels. This phosphorylation unattributable component of the ERK translocation response occurs at a broad range of GnRHR expression levels, in the presence of tyrosine phosphatase and protein synthesis inhibitors, and in ERK mutants unable to undergo catalytic activation. It also occurred in mutants incapable of binding the DEF (docking site for ERK, F/Y-X-F/Y-P) domains found in many ERK binding partners. It was however, reduced by MEK or PKC inhibition and by mutations preventing TEY phosphorylation or that abrogate ERK binding to D (docking) domain partners. We therefore show that TEY phosphorylation of ERK is necessary, but not sufficient for the full nuclear localization response. We further show that this "phosphorylation unattributable" component of GnRH-mediated ERK nuclear translocation requires both PKC activity and association with partner proteins via the D-domain.

Biological significance of nuclear localization of mitogen-activated protein kinase Pmk1 in fission yeast

Mitogen-activated protein kinase (MAPK) signaling pathways play a fundamental role in the response of eukaryotic cells to environmental changes. Also, much evidence shows that the stimulus-dependent nuclear targeting of this class of regulatory kinases is crucial for adequate regulation of distinct cellular events. In the fission yeast Schizosaccharomyces pombe, the cell integrity MAPK pathway, whose central element is the MAPK Pmk1, regulates multiple processes such as cell wall integrity, vacuole fusion, cytokinesis, and ionic homeostasis. In non-stressed cells Pmk1 is constitutively localized in both cytoplasm and nucleus, and its localization pattern appears unaffected by its activation status or in response to stress, thus questioning the biological significance of the presence of this MAPK into the nucleus. We have addressed this issue by characterizing mutants expressing Pmk1 versions excluded from the cell nucleus and anchored to the plasma membrane in different genetic backgrounds. Although nuclear Pmk1 partially regulates cell wall integrity at a transcriptional level, membrane-tethered Pmk1 performs many of the biological functions assigned to wild type MAPK like regulation of chloride homeostasis, vacuole fusion, and cellular separation. However, we found that down-regulation of nuclear Pmk1 by MAPK phosphatases induced by the stress activated protein kinase pathway is important for the fine modulation of extranuclear Pmk1 activity. These results highlight the importance of the control of MAPK activity at subcellular level.

Mathematical modeling reveals the functional implications of the different nuclear shuttling rates of Erk1 and Erk2

The mitogen-activated protein kinase (MAPK) family of proteins is involved in regulating cellular fates such as proliferation, differentiation and apoptosis. In particular, the dynamics of the Erk/Mek system, which has become the canonical example for MAPK signaling systems, have attracted considerable attention. Erk is encoded by two genes, Erk1 and Erk2, that until recently had been considered equivalent as they differ only subtly at the sequence level. However, these proteins exhibit radically different trafficking between cytoplasm and nucleus and this fact may have functional implications. Here we use spatially resolved data on Erk1/2 to develop and analyze spatio-temporal models of these cascades, and we discuss how sensitivity analysis can be used to discriminate between mechanisms. Our models elucidate some of the factors governing the interplay between signaling processes and the Erk1/2 localization in different cellular compartments, including competition between Erk1 and Erk2. Our approach is applicable to a wide range of signaling systems, such as activation cascades, where translocation of molecules occurs. Our study provides a first model of Erk1 and Erk2 activation and their nuclear shuttling dynamics, revealing a role in the regulation of the efficiency of nuclear signaling.

Epstein-Barr virus protein kinase BGLF4 targets the nucleus through interaction with nucleoporins

BGLF4 of Epstein-Barr virus (EBV) encodes a serine/threonine protein kinase that phosphorylates multiple viral and cellular substrates to optimize the cellular environment for viral DNA replication and the nuclear egress of viral nucleocapsids. BGLF4 is expressed predominantly in the nucleus at early and late stages of virus replication, while a small portion of BGLF4 is distributed in the cytoplasm at the late stage of virus replication and packaged into the virion. Here, we analyzed systematically the functional domains crucial for nuclear localization of BGLF4 and found that both the N and C termini play important modulating roles. Analysis of amino acid substitution mutants revealed that the C terminus of BGLF4 does not contain a conventional nuclear localization signal (NLS). Additionally, deletion of the C-terminal putative helical regions at amino acids 386 to 393 and 410 to 419 diminished the nuclear translocation of BGLF4, indicating that the secondary structure of the C terminus is important for the localization of BGLF4. The green fluorescent protein-fused wild-type or C-terminal helical regions of BGLF4 associate with phenylalanine/glycine repeat-containing nucleoporins (Nups) in nuclear envelope fractionation. Both coimmunoprecipitation and in vitro pull-down assays further demonstrated that BGLF4 binds to Nup62 and Nup153. Remarkably, nuclear import assay with permeabilized HeLa cells demonstrated that BGLF4 translocated into nucleus independent of cytosolic factors. Data presented here suggest that BGLF4 employs a novel mechanism through direct interactions with nucleoporins for its nuclear targeting.

ERK1/2 MAP kinases: structure, function, and regulation

ERK1 and ERK2 are related protein-serine/threonine kinases that participate in the Ras-Raf-MEK-ERK signal transduction cascade. This cascade participates in the regulation of a large variety of processes including cell adhesion, cell cycle progression, cell migration, cell survival, differentiation, metabolism, proliferation, and transcription. MEK1/2 catalyze the phosphorylation of human ERK1/2 at Tyr204/187 and then Thr202/185. The phosphorylation of both tyrosine and threonine is required for enzyme activation. Whereas the Raf kinase and MEK families have narrow substrate specificity, ERK1/2 catalyze the phosphorylation of hundreds of cytoplasmic and nuclear substrates including regulatory molecules and transcription factors. ERK1/2 are proline-directed kinases that preferentially catalyze the phosphorylation of substrates containing a Pro-Xxx-Ser/Thr-Pro sequence. Besides this primary structure requirement, many ERK1/2 substrates possess a D-docking site, an F-docking site, or both. A variety of scaffold proteins including KSR1/2, IQGAP1, MP1, β-Arrestin1/2 participate in the regulation of the ERK1/2 MAP kinase cascade. The regulatory dephosphorylation of ERK1/2 is mediated by protein-tyrosine specific phosphatases, protein-serine/threonine phosphatases, and dual specificity phosphatases. The combination of kinases and phosphatases make the overall process reversible. The ERK1/2 catalyzed phosphorylation of nuclear transcription factors including those of Ets, Elk, and c-Fos represents an important function and requires the translocation of ERK1/2 into the nucleus by active and passive processes involving the nuclear pore. These transcription factors participate in the immediate early gene response. The activity of the Ras-Raf-MEK-ERK cascade is increased in about one-third of all human cancers, and inhibition of components of this cascade by targeted inhibitors represents an important anti-tumor strategy. Thus far, however, only inhibition of mutant B-Raf (Val600Glu) has been found to be therapeutically efficacious.

Mouse tissues that undergo neoplastic progression after K-Ras activation are distinguished by nuclear translocation of phospho-Erk1/2 and robust tumor suppressor responses

Mutation of K-Ras is a frequent oncogenic event in human cancers, particularly cancers of lungs, pancreas, and colon. It remains unclear why some tissues are more susceptible to Ras-induced transformation than others. Here, we globally activated a mutant oncogenic K-Ras allele (K-Ras(G12D)) in mice and examined the tissue-specific effects of this activation on cancer pathobiology, Ras signaling, tumor suppressor, DNA damage, and inflammatory responses. Within 5 to 6 weeks of oncogenic Ras activation, mice develop oral and gastric papillomas, lung adenomas, and hematopoietic hyperproliferation and turn moribund. The oral, gastric, and lung premalignant lesions display activated extracellular signal-regulated kinases (Erk)1/2 and NF-κB signaling as well as activated tumor suppressor and DNA damage responses. Other organs such as pancreas, liver, and small intestine do not exhibit neoplastic progression within 6 weeks following K-Ras(G12D) activation and do not show a potent tumor suppressor response. Even though robust Erk1/2 signaling is activated in all the tissues examined, the pErk1/2 distribution remains largely cytoplasmic in K-Ras(G12D)-refractory tissues (pancreas, liver, and intestines) as opposed to a predominantly nuclear localization in K-Ras(G12D)-induced neoplasms of lung, oral, and gastric mucosa. The downstream targets of Ras signaling, pElk-1 and c-Myc, are elevated in K-Ras(G12D)-induced neoplastic lesions but not in K-Ras(G12D)-refractory tissues. We propose that oncogenic K-Ras-refractory tissues delay oncogenic progression by spatially limiting the efficacy of Ras/Raf/Erk1/2 signaling, whereas K-Ras-responsive tissues exhibit activated Ras/Raf/Erk1/2 signaling, rapidly form premalignant tumors, and activate potent antitumor responses that effectively prevent further malignant progression.

Sef downregulation by Ras causes MEK1/2 to become aberrantly nuclear localized leading to polyploidy and neoplastic transformation

Subcellular trafficking of key oncogenic signal pathway components is likely to be crucial for neoplastic transformation, but little is known about how such trafficking processes are spatially controlled. In this study, we show how Ras activation causes aberrant nuclear localization of phosphorylated mitogen-activated protein (MAP)/extracellular signal-regulated kinase (ERK; MEK) MEK1/2 to drive neoplastic transformation. Phosphorylated MEK1/2 was aberrantly located within the nucleus of primary colorectal tumors and human colon cancer cells, and oncogenic activation of Ras was sufficient to induce nuclear accumulation of phosphorylated MEK1/2 and ERK1/2 in intestinal epithelial cells. Enforced nuclear localization of MEK1 in epithelial cells or fibroblasts was sufficient for hyperactivation of ERK1/2, thereby driving cell proliferation, chromosomal polyploidy, and tumorigenesis. Notably, Ras-induced nuclear accumulation of activated MEK1/2 was reliant on downregulation of the spatial regulator Sef, the reexpression of which was sufficient to restore normal MEK1/2 localization and a reversal of Ras-induced proliferation and tumorigenesis. Taken together, our findings indicate that Ras-induced downregulation of Sef is an early oncogenic event that contributes to genetic instability and tumor progression by sustaining nuclear ERK1/2 signaling.

ERK phosphorylation and nuclear accumulation: insights from single-cell imaging

Many stimuli mediate activation and nuclear translocation of ERK (extracellular-signal-regulated kinase) by phosphorylation on the TEY (Thr-Glu-Tyr) motif. This is necessary to initiate transcriptional programmes controlling cellular responses, but the mechanisms that govern ERK nuclear targeting are unclear. Single-cell imaging approaches have done much to increase our understanding of input–output relationships in the ERK cascade, but few studies have addressed how the range of ERK phosphorylation responses observed in cell populations influences subcellular localization. Using automated microscopy to explore ERK regulation in single adherent cells, we find that nuclear localization responses increase in proportion to stimulus level, but not the level of TEY phosphorylation. This phosphorylation-unattributable nuclear localization response occurs in the presence of tyrosine phosphatase and protein synthesis inhibitors. It is also seen with a catalytically inactive ERK2–GFP (green fluorescent protein) mutant, and with a mutant incapable of binding the DEF (docking site for ERK, F/Y-X-F/Y-P) domains found in many ERK-binding partners. It is, however, reduced by MEK (mitogen-activated protein kinase/ERK kinase) inhibition and by mutations preventing TEY phosphorylation or in the ERK common docking region. We therefore show that TEY phosphorylation of ERK is necessary, but not sufficient, for the full nuclear accumulation response and that this ‘phosphorylation-unattributable’ component of stimulus-mediated ERK nuclear localization requires association with partner proteins via the common docking motif.

Measles virus C protein interferes with Beta interferon transcription in the nucleus

Transcriptional induction of beta interferon (IFN-β) through pattern recognition receptors is a key event in the host defense against invading viruses. Infection of cells by paramyxoviruses, like measles virus (MV) (genus Morbillivirus), is sensed predominantly by the ubiquitous cytoplasmic helicase RIG-I, recognizing viral 5'-triphosphate RNAs, and to some degree by MDA5. While MDA5 activation is effectively prevented by the MV V protein, the viral mechanisms for inhibition of MDA5-independent induction of IFN-β remained obscure. Here, we identify the 186-amino-acid MV C protein, which shuttles between the nucleus and the cytoplasm, as a major viral inhibitor of IFN-β transcription in human cells. Activation of the transcription factor IRF3 by upstream kinases and nuclear import of activated IRF3 were not affected in the presence of C protein, suggesting a nuclear target. Notably, C proteins of wild-type MV isolates, which are poor IFN-β inducers, were found to comprise a canonical nuclear localization signal (NLS), whereas the NLSs of all vaccine strains, irrespective of their origins, were mutated. Site-directed mutagenesis of the C proteins from an MV wild-type isolate and from the vaccine virus strain Schwarz confirmed a correlation of nuclear localization and inhibition of IFN-β transcription. A functional NLS and efficient nuclear accumulation are therefore critical for MV C to retain its potential to downregulate IFN-β induction. We suggest that a defect in efficient nuclear import of C protein contributes to attenuation of MV vaccine strains.

Long-distance signaling from synapse to nucleus via protein messengers

The communication between synapses and the cell nucleus has attracted considerable interest for many years. This interest is largely fueled by the idea that synapse-to-nucleus signaling might specifically induce the expression of genes that make long-term memory "stick." However, despite many years of research, it is still essentially unclear how synaptic signals are conveyed to the nucleus, and it remains to a large degree enigmatic how activity-induced gene expression feeds back to synaptic function. In this chapter, we will focus on the activity-dependent synapto-nuclear trafficking of protein messengers and discuss the underlying mechanisms of their retrograde transport and their supposed functional role in neuronal plasticity.

Mechanism and regulation of nucleocytoplasmic trafficking of smad

Smad proteins are the intracellular mediators of transforming growth factor β (TGF-β) signaling. Smads function as transcription factors and their activities require carboxyl-terminal phosphorylation by TGF-β receptor kinases which are embedded in the cell membrane. Therefore, the translocation of activated Smads from the cytoplasm into the nucleus is a rate-limiting step in TGF-β signal transduction into the nucleus. On the other hand, the export of Smads out of the nucleus turns off TGF-β effect. Such spatial control of Smad ensures a tight regulation of TGF-β target genes. Several cross-talk pathways have been shown to affect TGF-β signaling by impairing nuclear translocation of Smad, exemplifying the biological importance of the nuclear transport process. Many laboratories have investigated the underlying molecular mechanism of Smad nucleocytoplasmic translocation, combining genetics, biochemistry and sophisticated live cell imaging approaches. The last few years have witnessed the elucidation of several key players in Smad nuclear transport, most importantly the karyopherins that carry Smads across the nuclear envelope and nuclear pore proteins that facilitate the trans-nuclear envelope movement. The foundation is now set to further elucidate how the nuclear transport process is regulated and exploit such knowledge to manipulate TGF-β signaling. In this review we will discuss the current understanding of the molecular machinery responsible for nuclear import and export of Smads.

Identification and functional characterization of novel phosphorylation sites in TAK1-binding protein (TAB) 1

TAB1 was defined as a regulatory subunit of the protein kinase TAK1, which functions upstream in the pathways activated by interleukin (IL)-1, tumor necrosis factor (TNF), toll-like receptors (TLRs) and stressors. However, TAB1 also functions in the p38 MAPK pathway downstream of TAK1. We identified amino acids (aa) 452/453 and 456/457 of TAB1 as novel sites phosphorylated by TAK1 as well as by p38 MAPK in intact cells as well as in vitro. Serines 452/453 and 456/457 were phosphorylated upon phosphatase blockade by calyculin A, or in response to IL-1 or translational stressors such as anisomycin and sorbitol. Deletion or phospho-mimetic mutations of aa 452–457 of TAB1 retain TAB1 and p38 MAPK in the cytoplasm. The TAB1 mutant lacking aa 452–457 decreases TAB1-dependent phosphorylation of p38 MAPK. It also enhances TAB1-dependent CCL5 secretion in response to IL-1 and increases activity of a post-transcriptional reporter gene, which contains the CCL5 3′ untranslated region. These data suggest a complex role of aa 452–457 of TAB1 in controlling p38 MAPK activity and subcellular localization and implicate these residues in TAK1- or p38 MAPK-dependent post-transcriptional control of gene expression.

The ERK Cascade: Distinct Functions within Various Subcellular Organelles

The extracellular signal-regulated kinase 1/2 (ERK1/2) cascade is a central signaling pathway that regulates a wide variety of stimulated cellular processes, including mainly proliferation, differentiation, and survival, but apoptosis and stress response as well. The ability of this linear cascade to induce so many distinct and even opposing effects after various stimulations raises the question as to how the signaling specificity of the cascade is regulated. Over the past years, several specificity-mediating mechanisms have been elucidated, including temporal regulation, scaffolding interactions, crosstalks with other signaling components, substrate competition, and multiple components in each tier of the cascade. In addition, spatial regulation of various components of the cascade is probably one of the main ways by which signals can be directed to some downstream targets and not to others. In this review, we describe first the components of the ERK1/2 cascade and their mode of regulation by kinases, phosphatases, and scaffold proteins. In the second part, we focus on the role of MEK1/2 and ERK1/2 compartmentalization in the nucleus, mitochondria, endosomes, plasma membrane, cytoskeleton, and Golgi apparatus. We explain that this spatial distribution may direct ERK1/2 signals to regulate the organelles' activities. However, it can also direct the activity of the cascade's components to the outer surface of the organelles in order to bring them to close proximity to specific cytoplasmic targets. We conclude that the dynamic localization of the ERK1/2 cascade components is an important regulatory mechanism in determining the signaling specificity of the cascade, and its understanding should shed a new light on the understanding of many stimulus-dependent processes.

Calcium signaling in synapse-to-nucleus communication

Changes in the intracellular concentration of calcium ions in neurons are involved in neurite growth, development, and remodeling, regulation of neuronal excitability, increases and decreases in the strength of synaptic connections, and the activation of survival and programmed cell death pathways. An important aspect of the signals that trigger these processes is that they are frequently initiated in the form of glutamatergic neurotransmission within dendritic trees, while their completion involves specific changes in the patterns of genes expressed within neuronal nuclei. Accordingly, two prominent aims of research concerned with calcium signaling in neurons are determination of the mechanisms governing information conveyance between synapse and nucleus, and discovery of the rules dictating translation of specific patterns of inputs into appropriate and specific transcriptional responses. In this article, we present an overview of the avenues by which glutamatergic excitation of dendrites may be communicated to the neuronal nucleus and the primary calcium-dependent signaling pathways by which synaptic activity can invoke changes in neuronal gene expression programs.

Rasd1 modulates the coactivator function of NonO in the cyclic AMP pathway

All living organisms exhibit autonomous daily physiological and behavioural rhythms to help them synchronize with the environment. Entrainment of circadian rhythm is achieved via activation of cyclic AMP (cAMP) and mitogen-activated protein kinase signaling pathways. NonO (p54nrb) is a multifunctional protein involved in transcriptional activation of the cAMP pathway and is involved in circadian rhythm control. Rasd1 is a monomeric G protein implicated to play a pivotal role in potentiating both photic and nonphotic responses of the circadian rhythm. In this study, we have identified and validated NonO as an interacting partner of Rasd1 via affinity pulldown, co-immunoprecipitation and indirect immunofluorescence studies. The GTP-hydrolysis activity of Rasd1 is required for the functional interaction. Functional interaction of Rasd1-NonO in the cAMP pathway was investigated via reporter gene assays, chromatin immunoprecipitation and gene knockdown. We showed that Rasd1 and NonO interact at the CRE-site of specific target genes. These findings reveal a novel mechanism by which the coregulator activity of NonO can be modulated.

Signaling from the cytoplasm to the nucleus in striatal medium-sized spiny neurons

Striatal medium-sized spiny neurons (MSNs) receive massive glutamate inputs from the cerebral cortex and thalamus and are a major target of dopamine projections. Interaction between glutamate and dopamine signaling is crucial for the control of movement and reward-driven learning, and its alterations are implicated in several neuropsychiatric disorders including Parkinson's disease and drug addiction. Long-lasting forms of synaptic plasticity are thought to depend on transcription of gene products that alter the structure and/or function of neurons. Although multiple signal transduction pathways regulate transcription, little is known about signal transmission between the cytoplasm and the nucleus of striatal neurons and its regulation. Here we review the current knowledge of the signaling cascades that target the nucleus of MSNs, most of which are activated by cAMP and/or Ca(2+). We outline the mechanisms by which signals originating at the plasma membrane and amplified in the cytoplasm are relayed to the nucleus, through the regulation of several protein kinases and phosphatases and transport through the nuclear pore. We also summarize the identified mechanisms of transcription regulation and chromatin remodeling in MSNs that appear to be important for behavioral adaptations, and discuss their relationships with epigenetic regulation.

Nuclear extracellular signal-regulated kinase 1 and 2 translocation is mediated by casein kinase 2 and accelerated by autophosphorylation

The extracellular signal-regulated kinases (ERK) 1 and 2 (ERK1/2) are members of the mitogen-activated protein kinase [MAPK] family. Upon stimulation, these kinases translocate from the cytoplasm to the nucleus, where they induce physiological processes such as proliferation and differentiation. The mechanism of translocation of this kinase involves phosphorylation of two Ser residues within a nuclear translocation signal (NTS), which allows binding to importin7 and a subsequent penetration via nuclear pores. Here we show that the phosphorylation of both Ser residues is mediated mainly by casein kinase 2 (CK2) and that active ERK may assist in the phosphorylation of the N-terminal Ser. We also demonstrate that the phosphorylation is dependent on the release of ERK from cytoplasmic anchoring proteins. Crystal structure of the phosphomimetic ERK revealed that the NTS phosphorylation creates an acidic patch in ERK. Our model is that in resting cells ERK is bound to cytoplasmic anchors, which prevent its NTS phosphorylation. Upon stimulation, phosphorylation of the ERK TEY domain releases ERK and allows phosphorylation of its NTS by CK2 and active ERK to generate a negatively charged patch in ERK, binding to importin 7 and nuclear translocation. These results provide an important role of CK2 in regulating nuclear ERK activities.

The Ras inhibitors caveolin-1 and docking protein 1 activate peroxisome proliferator-activated receptor γ through spatial relocalization at helix 7 of its ligand-binding domain

Peroxisome proliferator-activated receptor γ (PPARγ) is a transcription factor that promotes differentiation and cell survival in the stomach. PPARγ upregulates and interacts with caveolin-1 (Cav1), a scaffold protein of Ras/mitogen-activated protein kinases (MAPKs). The cytoplasmic-to-nuclear localization of PPARγ is altered in gastric cancer (GC) patients, suggesting a so-far-unknown role for Cav1 in spatial regulation of PPARγ signaling. We show here that loss of Cav1 accelerated proliferation of normal stomach and GC cells in vitro and in vivo. Downregulation of Cav1 increased Ras/MAPK-dependent phosphorylation of serine 84 in PPARγ and enhanced nuclear translocation and ligand-independent transcription of PPARγ target genes. In contrast, Cav1 overexpression sequestered PPARγ in the cytosol through interaction of the Cav1 scaffolding domain (CSD) with a conserved hydrophobic motif in helix 7 of PPARγ's ligand-binding domain. Cav1 cooperated with the endogenous Ras/MAPK inhibitor docking protein 1 (Dok1) to promote the ligand-dependent transcriptional activity of PPARγ and to inhibit cell proliferation. Ligand-activated PPARγ also reduced tumor growth and upregulated the Ras/MAPK inhibitors Cav1 and Dok1 in a murine model of GC. These results suggest a novel mechanism of PPARγ regulation by which Ras/MAPK inhibitors act as scaffold proteins that sequester and sensitize PPARγ to ligands, limiting proliferation of gastric epithelial cells.

Importin beta plays an essential role in the regulation of the LysRS-Ap(4)A pathway in immunologically activated mast cells

We recently reported that diadenosine tetraphosphate hydrolase (Ap(4)A hydrolase) plays a critical role in gene expression via regulation of intracellular Ap(4)A levels. This enzyme serves as a component of our newly described lysyl tRNA synthetase (LysRS)-Ap(4)A biochemical pathway that is triggered upon immunological challenge. Here we explored the mechanism of this enzyme's translocation into the nucleus and found its immunologically dependent association with importin beta. Silencing of importin beta prevented Ap(4)A hydrolase nuclear translocation and affected the local concentration of Ap(4)A, which led to an increase in microphthalmia transcription factor (MITF) transcriptional activity. Furthermore, immunological activation of mast cells resulted in dephosphorylation of Ap(4)A hydrolase, which changed the hydrolytic activity of the enzyme.

Phosphorylation meets nuclear import: a review

Phosphorylation is the most common and pleiotropic modification in biology, which plays a vital role in regulating and finely tuning a multitude of biological pathways. Transport across the nuclear envelope is also an essential cellular function and is intimately linked to many degeneration processes that lead to disease. It is therefore not surprising that phosphorylation of cargos trafficking between the cytoplasm and nucleus is emerging as an important step to regulate nuclear availability, which directly affects gene expression, cell growth and proliferation. However, the literature on phosphorylation of nucleocytoplasmic trafficking cargos is often confusing. Phosphorylation, and its mirror process dephosphorylation, has been shown to have opposite and often contradictory effects on the ability of cargos to be transported across the nuclear envelope. Without a clear connection between attachment of a phosphate moiety and biological response, it is difficult to fully understand and predict how phosphorylation regulates nucleocytoplasmic trafficking. In this review, we will recapitulate clue findings in the field and provide some general rules on how reversible phosphorylation can affect the nuclear-cytoplasmic localization of substrates. This is only now beginning to emerge as a key regulatory step in biology.

The MAPK cascades: signaling components, nuclear roles and mechanisms of nuclear translocation

The MAPK cascades are central signaling pathways that regulate a wide variety of stimulated cellular processes, including proliferation, differentiation, apoptosis and stress response. Therefore, dysregulation, or improper functioning of these cascades, is involved in the induction and progression of diseases such as cancer, diabetes, autoimmune diseases, and developmental abnormalities. Many of these physiological, and pathological functions are mediated by MAPK-dependent transcription of various regulatory genes. In order to induce transcription and the consequent functions, the signals transmitted via the cascades need to enter the nucleus, where they may modulate the activity of transcription factors and chromatin remodeling enzymes. In this review, we briefly cover the composition of the MAPK cascades, as well as their physiological and pathological functions. We describe, in more detail, many of the important nuclear activities of the MAPK cascades, and we elaborate on the mechanisms of ERK1/2 translocation into the nucleus, including the identification of their nuclear translocation sequence (NTS) binding to the shuttling protein importin7. Overall, the nuclear translocation of signaling components may emerge as an important regulatory layer in the induction of cellular processes, and therefore, may serve as targets for therapeutic intervention in signaling-related diseases such as cancer and diabetes. This article is part of a Special Issue entitled: Regulation of Signaling and Cellular Fate through Modulation of Nuclear Protein Import.

Activity-dependent calcium signaling and ERK-MAP kinases in neurons: a link to structural plasticity of the nucleus and gene transcription regulation

Activity-dependent gene expression is important for the formation and maturation of neuronal networks, neuronal survival and for plastic modifications within mature networks. At the level of individual neurons, expression of new protein is required for dendritic branching, synapse formation and elimination. Experience-driven synaptic activity induces membrane depolarization, which in turn evokes intracellular calcium transients that are decoded according to their source and strength by intracellular calcium sensing proteins. In order to activate the gene transcription machinery of the cell, calcium signals have to be conveyed from the site of their generation in the cytoplasm to the cell nucleus. This can occur via a variety of mechanisms and with different kinetics depending on the source and amplitude of calcium influx. One mechanism involves the propagation of calcium itself, leading to nuclear calcium transients that subsequently activate transcription. The mitogen-activated protein kinase (MAPK) cascade represents a second central signaling module that transduces information from the site of calcium signal generation at the plasma membrane to the nucleus. Nuclear signaling of the MAPK cascades catalyzes the phosphorylation of transcription factors but also regulates gene transcription more globally at the level of chromatin remodeling as well as through its recently identified role in the modulation of nuclear shape. Here we discuss the possible mechanisms by which the MAPKs ERK1 and ERK2, activated by synaptically evoked calcium influx, can signal to the nucleus and regulate gene transcription. Moreover, we describe how MAPK-dependent structural plasticity of the nuclear envelope enhances nuclear calcium signaling and suggest possible implications for the regulation of gene transcription in the context of nuclear geometry.

Stimulus-induced uncoupling of extracellular signal-regulated kinase phosphorylation from nuclear localization is dependent on docking domain interactions

Many stimuli activate the extracellular signal-regulated kinase (ERK) by phosphorylation on the TEY motif. Activated ERK characteristically accumulates in the nucleus, but the underlying mechanisms involved are unclear. Using automated microscopy to explore ERK regulation in single intact cells, we find that, when protein kinase C or epidermal growth factor receptors are activated, a substantial fraction of the ERK nuclear localization response is uncoupled from TEY phosphorylation. This phosphorylation-unattributable nuclear localization response occurs in the presence of inhibitors of tyrosine phosphatases and protein synthesis. It was also evident with a catalytically inactive ERK2-GFP mutant, and with a mutant incapable of binding the DEF (docking site for ERK, F/Y-X-F/Y-P) domains found in many ERK binding partners. It was, however, reduced by MEK inhibition and by mutations preventing either TEY phosphorylation or D (docking)-domain-dependent ERK binding (D319N). Thus, we show that MEK-catalysed ERK phosphorylation is necessary but not sufficient for the full nuclear localization response: there is an additional phosphorylation-unattributable component of the response that does not reflect induced expression of nuclear anchors and is independent of ERK catalytic activity or DEF-domain binding. It is, however, dependent upon D-domain binding, highlighting distinct roles of ERK motifs during nuclear targeting.

The MAP kinase signaling cascades: a system of hundreds of components regulates a diverse array of physiological functions

Sequential activation of kinases within the mitogen-activated protein (MAP) kinase (MAPK) cascades is a common, and evolutionary-conserved mechanism of signal transduction. Four MAPK cascades have been identified in the last 20 years and those are usually named according to the MAPK components that are the central building blocks of each of the cascades. These are the extracellular signal-regulated kinase 1/2 (ERK1/2), c-Jun N-Terminal kinase (JNK), p38, and ERK5 cascades. Each of these cascades consists of a core module of three tiers of protein kinases termed MAPK, MAPKK, and MAP3K, and often two additional tiers, the upstream MAP4K and the downstream MAPKAPK, which can complete five tiers of each cascade in certain cell lines or stimulations. The transmission of the signal via each cascade is mediated by sequential phosphorylation and activation of the components in the sequential tiers. These cascades cooperate in transmitting various extracellular signals and thus control a large number of distinct and even opposing cellular processes such as proliferation, differentiation, survival, development, stress response, and apoptosis. One way by which the specificity of each cascade is regulated is through the existence of several distinct components in each tier of the different cascades. About 70 genes, which are each translated to several alternatively spliced isoforms, encode the entire MAPK system, and allow the wide array of cascade's functions. These components, their regulation, as well as their involvement together with other mechanisms in the determination of signaling specificity by the MAPK cascade is described in this review. Mis-regulation of the MAPKs signals usually leads to diseases such as cancer and diabetes; therefore, studying the mechanisms of specificity-determination may lead to better understanding of these signaling-related diseases.

Defective anchoring of JNK1 in the cytoplasm by MKK7 in Jurkat cells is associated with resistance to Fas-mediated apoptosis

The c-Jun N-terminal protein kinase (JNK) plays a context-dependent role in tumorigenesis. Stress-induced redistribution of JNK from the cytoplasm to the nucleus has been demonstrated as essential for stress-induced cell death. However, accumulation of basal JNK activity in the nucleus has frequently been seen in tumor cells. Our previous report revealed aberrant nuclear entry of JNK protein in Jurkat human leukemic T-cells even without JNK hyperactivation. Because inhibition of JNK activity, especially JNK1 activity, in Jurkat cells results in augmented Fas-mediated apoptosis, it is possible that aberrant subcellular localization of JNK, especially the JNK1 isoform, contributes to the resistance to Fas-mediated apoptosis. Here we report that MKK7 works as a cytoplasmic anchoring protein for JNK1 in various types of cells, including human peripheral blood mononuclear cell (PBMC) T-cells, but exhibits aberrant nuclear entry in Jurkat cells. Ectopic expression of a JNK1 mutant defective of nuclear entry or a nuclear JNK inhibitor leads to impaired UV-induced apoptosis in both PBMC T- and Jurkat cells. The same treatment shows no effect on Fas-mediated apoptosis of PBMC T-cells but sensitizes Jurkat cells to Fas-mediated apoptosis. Taken together, our work suggests that aberrant subcellular organization of the JNK pathway might render certain tumor cells resistant to Fas-mediated apoptosis.

Nucleocytoplasmic distribution is required for activation of resistance by the potato NB-LRR receptor Rx1 and is balanced by its functional domains

The Rx1 protein, as many resistance proteins of the nucleotide binding-leucine-rich repeat (NB-LRR) class, is predicted to be cytoplasmic because it lacks discernable nuclear targeting signals. Here, we demonstrate that Rx1, which confers extreme resistance to Potato virus X, is located both in the nucleus and cytoplasm. Manipulating the nucleocytoplasmic distribution of Rx1 or its elicitor revealed that Rx1 is activated in the cytoplasm and cannot be activated in the nucleus. The coiled coil (CC) domain was found to be required for accumulation of Rx1 in the nucleus, whereas the LRR domain promoted the localization in the cytoplasm. Analyses of structural subdomains of the CC domain revealed no autonomous signals responsible for active nuclear import. Fluorescence recovery after photobleaching and nuclear fractionation indicated that the CC domain binds transiently to large complexes in the nucleus. Disruption of the Rx1 resistance function and protein conformation by mutating the ATP binding phosphate binding loop in the NB domain, or by silencing the cochaperone SGT1, impaired the accumulation of Rx1 protein in the nucleus, while Rx1 versions lacking the LRR domain were not affected in this respect. Our results support a model in which interdomain interactions and folding states determine the nucleocytoplasmic distribution of Rx1.

A novel domain mediates insulin-induced proteasomal degradation of insulin receptor substrate 1 (IRS-1)

Insulin receptor substrate-1 (IRS-1) plays a pivotal role in insulin signaling, therefore its degradation is exquisitely regulated. Here, we show that insulin-stimulated degradation of IRS-1 requires the presence of a highly conserved Ser/Thr-rich domain that we named domain involved in degradation of IRS-1 (DIDI). DIDI (amino acids 386-430 of IRS-1) was identified by comparing the intracellular degradation rate of several truncated forms of IRS-1 transfected into CHO cells. The isolated DIDI domain underwent insulin-stimulated Ser/Thr phosphorylation, suggesting that it serves as a target for IRS-1 kinases. The effects of deletion of DIDI were studied in Fao rat hepatoma and in CHO cells expressing Myc-IRS-1(WT) or Myc-IRS-1(Δ386-430). Deletion of DIDI maintained the ability of IRS-1(Δ386-434) to undergo ubiquitination while rendering it insensitive to insulin-induced proteasomal degradation, which affected IRS-1(WT) (80% at 8 h). Consequently, IRS-1(Δ386-434) mediated insulin signaling (activation of Akt and glycogen synthesis) better than IRS-1(WT). IRS-1(Δ386-434) exhibited a significant greater preference for nuclear localization, compared with IRS-1(WT). Higher nuclear localization was also observed when cells expressing IRS-1(WT) were incubated with the proteasome inhibitor MG-132. The sequence of DIDI is conserved more than 93% across species, from fish to mammals, as opposed to approximately 40% homology of the entire IRS-1. These findings implicate DIDI as a novel, highly conserved domain of IRS-1, which mediates its cellular localization, rate of degradation, and biological activity, with a direct impact on insulin signal transduction.

Mechanisms regulating the nuclear translocation of p38 MAP kinase

p38 mitogen-activated protein kinase (MAPK) is of fundamental importance in a cell's response to environmental stresses, cytokines and DNA damage. p38 resides in the cytoplasm of resting cells, and translocates into the nucleus upon activation, yet the exact mechanisms remain largely unclear. We show here that the phosphorylation-dependent nuclear translocation of p38 is a common phenomenon when cells are stimulated with various stresses. On the other hand, the nuclear export of p38 requires its dephosphorylation, and it is exported both in a MK2-dependent and a nuclear export signal (NES)-independent manner. Although different p38-regulated/activated protein kinase (PRAK) mutants all dictate the intracellular localization of p38, results from a PRAK-deficient cell line indicate that it plays no role in this process. Microtubule depolymerizing reagent nocodazole and dynein inhibitor EHNA both block the nuclear translocation of p38, demonstrating roles for microtubules and dynein in p38 transport. Taken together, stress-induced nuclear accumulation of p38 is a phosphorylation-dependent, microtubule- and dynein-associated process.

Nuclear import by karyopherin-βs: recognition and inhibition

Proteins in the karyopherin-β family mediate the majority of macromolecular transport between the nucleus and the cytoplasm. Eleven of the 19 known human karyopherin-βs and 10 of the 14S. cerevisiae karyopherin-βs mediate nuclear import through recognition of nuclear localization signals or NLSs in their cargos. This receptor-mediated process is essential to cellular viability as proteins are translated in the cytoplasm but many have functional roles in the nucleus. Many known karyopherin-β-cargo interactions were discovered through studies of the individual cargos rather than the karyopherins, and this information is thus widely scattered in the literature. We consolidate information about cargos that are directly recognized by import-karyopherin-βs and review common characteristics or lack thereof among cargos of different import pathways. Knowledge of karyopherin-β-cargo interactions is also critical for the development of nuclear import inhibitors and the understanding of their mechanisms of inhibition. This article is part of a Special Issue entitled: Regulation of Signaling and Cellular Fate through Modulation of Nuclear Protein Import.

Identification of a nuclear export signal in the catalytic subunit of AMP-activated protein kinase

In this study, we utilized genetic and cell biological approaches to evaluate potential functions for the AMPKα C-terminus. We identify a critical new function for the carboxy-terminal amino acids of AMPKα in vivo, which affects AMPKα subcellular localization, phosphorylation, and ultimately organismal viability.

The metabolic regulator AMP-activated protein kinase (AMPK) maintains cellular homeostasis through regulation of proteins involved in energy-producing and -consuming pathways. Although AMPK phosphorylation targets include cytoplasmic and nuclear proteins, the precise mechanisms that regulate AMPK localization, and thus its access to these substrates, are unclear. We identify highly conserved carboxy-terminal hydrophobic amino acids that function as a leptomycin B–sensitive, CRM1-dependent nuclear export sequence (NES) in the AMPK catalytic subunit (AMPKα). When this sequence is modified AMPKα shows increased nuclear localization via a Ran-dependent import pathway. Cytoplasmic localization can be restored by substituting well-defined snurportin-1 or protein kinase A inhibitor (PKIA) CRM1-binding NESs into AMPKα. We demonstrate a functional requirement in vivo for the AMPKα carboxy-terminal NES, as transgenic Drosophila expressing AMPKα lacking this NES fail to rescue lethality of AMPKα null mutant flies and show decreased activation loop phosphorylation under heat-shock stress. Sequestered to the nucleus, this truncated protein shows highly reduced phosphorylation at the key Thr172 activation residue, suggesting that AMPK activation predominantly occurs in the cytoplasm under unstressed conditions. Thus, modulation of CRM1-mediated export of AMPKα via its C-terminal NES provides an additional mechanism for cells to use in the regulation of AMPK activity and localization.

The Ras-ERK pathway: understanding site-specific signaling provides hope of new anti-tumor therapies

Recent discoveries have suggested the concept that intracellular signals are the sum of multiple, site-specified subsignals, rather than single, homogeneous entities. In the context of cancer, searching for compounds that selectively block subsignals essential for tumor progression, but not those regulating "house-keeping" functions, could help in producing drugs with reduced side effects compared to compounds that block signaling completely. The Ras-ERK pathway has become a paradigm of how space can differentially shape signaling. Today, we know that Ras proteins are found in different plasma membrane microdomains and endomembranes. At these localizations, Ras is subject to site-specific regulatory mechanisms, distinctively engaging effector pathways and switching-on diverse genetic programs to generate different biological responses. The Ras effector pathway leading to ERKs activation is also under strict, space-related regulatory processes. These findings may open a gate for aiming at the Ras-ERK pathway in a spatially restricted fashion, in our quest for new anti-tumor therapies.

Ligand interaction scan (LIScan) in the study of ERK8

ERK8 is the most recent addition for the MAPK family, and its mechanism of activation and function are not yet known, mainly due to the lack of any known physiological stimulator. In this report, we describe the preparation of reagents for the use of a novel method, the ligand interaction scan (LIScan), to study the function of this protein kinase. We generated a set of mutants of ERK8, and identified inhibited as well as stimulated forms. By specifically inhibiting or stimulating the mutants of ERK8, we show that the ERK8-induced inhibition of proliferation is altered. Moreover, we used the developed mutants to show for the first time that ERK8 translocates to the nucleus upon activation. The use of methods such as the ligand interaction scan may thus promote the analyses of the functions of uncharacterized proteins such as ERK8, and possibly help in controlling the activity of target proteins in various experimental systems and applications.

Activation of the cell wall integrity pathway promotes escape from G2 in the fungus Ustilago maydis

It is widely accepted that MAPK activation in budding and fission yeasts is often associated with negative effects on cell cycle progression, resulting in delay or arrest at a specific stage in the cell cycle, thereby enabling cells to adapt to changing environmental conditions. For instance, activation of the Cell Wall Integrity (CWI) pathway in the budding yeast Saccharomyces cerevisiae signals an increase in CDK inhibitory phosphorylation, which leads cells to remain in the G2 phase. Here we characterized the CWI pathway of Ustilago maydis, a fungus evolutionarily distant from budding and fission yeasts, and show that activation of the CWI pathway forces cells to escape from G2 phase. In spite of these disparate cell cycle responses in S. cerevisiae and U. maydis, the CWI pathway in both organisms appears to respond to the same class cell wall stressors. To understand the basis of such a difference, we studied the mechanism behind the U. maydis response. We found that activation of CWI pathway in U. maydis results in a decrease in CDK inhibitory phosphorylation, which depends on the mitotic phosphatase Cdc25. Moreover, in response to activation of the CWI pathway, Cdc25 accumulates in the nucleus, providing a likely explanation for the increase in the unphosphorylated form of CDK. We also found that the extended N-terminal domain of Cdc25, which is dispensable under normal growth conditions, is required for this G2 escape as well as for resistance to cell wall stressors. We propose that the process of cell cycle adaptation to cell stress evolved differently in these two divergent organisms so that each can move towards a cell cycle phase most appropriate for responding to the environmental signals encountered.

Phosphorylation of the RNase III enzyme Drosha at Serine300 or Serine302 is required for its nuclear localization

The RNaseIII enzyme Drosha plays a pivotal role in microRNA (miRNA) biogenesis by cleaving primary miRNA transcripts to generate precursor miRNA in the nucleus. The RNA binding and enzymatic domains of Drosha have been characterized and are on its C-terminus. Its N-terminus harbors a nuclear localization signal. Using a series of truncated Drosha constructs, we narrowed down the segment responsible for nuclear translocation to a domain between aa 270 and aa 390. We further identified two phosphorylation sites at Serine300 (S300) and Serine302 (S302) by mass spectrometric analysis. Double mutations of S→A at S300 and S302 completely disrupted nuclear localization. Single mutation of S→A at S300 or S302, however, had no effect on nuclear localization indicating that phosphorylation at either site is sufficient to locate Drosha to the nucleus. Furthermore, mimicking phosphorylation status by mutating S→E at S300 and/or S→D at S302 restored nuclear localization. Our findings add a further layer of complexity to the molecular anatomy of Drosha as it relates to miRNA biogenesis.

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.

Characterization of ERK activation in human mast cells stimulated by contact with T cells

Close physical proximity between mast cells and T cells has been demonstrated in several human conditions. We have identified and characterized a novel mast cell activation pathway initiated by contact with T cells, and showed that this pathway is associated with cytokine release. It has been shown recently that Ras is activated in this pathway. Thus, in the present study we further explore the downstream events associated with Ras activation and cytokine release in human mast cells stimulated by contact with T cells. ERK activation in human mast cells stimulated by either contact with T cells or by crosslinking the FC epsilon receptor was studied. Photobleaching experiments were used to study ERK localization. Enzyme linked immunosorbent assay was used to study the cytokine release by human mast cells. We show that stimulation of human mast cells by contact with activated T cells results is sustained ERK activation. Furthermore, sustained ERK activation in these cells is associated with increased dwell time at the nucleus and with IL-8 release. Interestingly, when mast cells were stimulated by crosslinking the FC epsilon receptor I, ERK activation was transient. ERK activation was associated with a shorter dwell time at the nucleus and with TNF-alpha release. Thus, retaining ERK in the nucleus might be a mechanism utilized by human mast cells to generate different cytokines from a single signaling cascade.

Phosphorylation of nucleoporins: signal transduction-mediated regulation of their interaction with nuclear transport receptors

The nuclear pore complex (NPC) is composed of ∼30 unique proteins, collectively referred to as nucleoporins or Nups. While metazoan Nups are known to be phosphorylated during mitosis to cause disassembly of the NPC, what is less clear is whether Nups are phosphorylated and regulated by extracellular stimuli in interphase cells. Our multi-step phosphoproteomic approach revealed a number of physiologically relevant extracellular signal-regulated kinase (ERK) targets, including Nups containing FG repeats (FG Nups) that provide binding sites for nuclear transport receptors (NTRs) during the NPC passage. The phosphorylation of FG Nups by ERK does not affect the overall architecture of the NPC but directly inhibits their interactions with NTRs and regulates the permeability barrier properties of the NPC. Such regulation at the levels of transport machinery is expected to have a broad impact on cellular physiology through the spatiotemporal control of signaling events. Until recently, many studies have focused on cellular signaling-mediated phosphorylation of individual cargo proteins, such as transcription factors. An understanding of the effects of signaling pathways on nucleocytoplasmic transport machinery is only beginning to emerge.

Upregulated claudin-1 expression confers resistance to cell death of nasopharyngeal carcinoma cells

Accumulating evidence reveals that aberrant expression of claudins manifests in various tumors; however, their biological functions are poorly understood. Here, we report on the elevated expression of claudin-1 in nasopharyngeal carcinoma (NPC) cell lines under serum deprivation or fluorouracil (5-FU) treatment. Interestingly, an increase in expression of claudin-1 considerably reduced apoptosis rather than enhancing cell proliferation. However, claudin-1 expression and activity were unaffected by external stimuli or Akt and NF-kappaB activation. Notably, predominant cytoplasmic and nuclear localization of claudin-1 in NPC cells reflected the aforementioned feature. On the other hand, loss of epithelial morphology and E-cadherin expression was associated with serum withdrawal in NPC cells. Interestingly, restoration of E-cadherin inhibited the protein elevation and antiapoptotic activity of claudin-1. In conclusion, our data demonstrate the regulation and novel biological function of claudin-1 and indicate the important role of claudin-1 in NPC tumorigenesis.

Nuclear entry of activated MAPK is restricted in primary ovarian and mammary epithelial cells

Background

The MAPK/ERK1/2 serine kinases are primary mediators of the Ras mitogenic signaling pathway. Phosphorylation by MEK activates MAPK/ERK in the cytoplasm, and phospho-ERK is thought to enter the nucleus readily to modulate transcription.

Principal Findings

Here, however, we observe that in primary cultures of breast and ovarian epithelial cells, phosphorylation and activation of ERK1/2 are disassociated from nuclear translocalization and transcription of downstream targets, such as c-Fos, suggesting that nuclear translocation is limited in primary cells. Accordingly, in import assays in vitro, primary cells showed a lower import activity for ERK1/2 than cancer cells, in which activated MAPK readily translocated into the nucleus and activated c-Fos expression. Primary cells express lower levels of nuclear pore complex proteins and the nuclear transport factors, importin B1 and importin 7, which may explain the limiting ERK1/2 import found in primary cells. Additionally, reduction in expression of nucleoporin 153 by siRNA targeting reduced ERK1/2 nuclear activity in cancer cells.

Conclusion

ERK1/2 activation is dissociated from nuclear entry, which is a rate limiting step in primary cells and in vivo, and the restriction of nuclear entry is disrupted in transformed cells by the increased expression of nuclear pores and/or nuclear transport factors.

The subcellular localization of MEK and ERK--a novel nuclear translocation signal (NTS) paves a way to the nucleus

The ERK cascade is a central signaling pathway that regulates a large number of intracellular processes including proliferation, differentiation, development and also survival or apoptosis. The induction of so many distinct and even opposing cellular processes raises the question as to how the signaling specificity of the cascade is regulated. In the past few years, subcellular localization of components of the ERK cascade was shown to play an important role in specificity determination. Here we describe the dynamic subcellular localization of Raf kinases, MEKs, and particularly ERKs, which translocate into the nucleus during many cellular processes to induce transcription. We also describe in details the recent identification of a novel nuclear translocation mechanism for ERKs, which is based on a nuclear translocation sequence (NTS) within their kinase insert domain (KID). Phosphorylation of this domain, mainly upon stimulation, allows ERKs to interact with the nuclear importing protein - importin7, which mediates the penetration of the interacting ERKs into the nucleus via nuclear pores. Interestingly, the NTS is not specific to ERKs, and seems to be a general signal for regulating nuclear accumulation of various proteins, including MEKs, upon their stimulation. Better understanding of this mechanism may clarify the role of the massive nuclear translocation of many regulatory proteins shortly after their stimulation.

Using high-content microscopy to study gonadotrophin-releasing hormone regulation of ERK

Gonadotrophin-releasing hormone (GnRH) is a hypothalamic peptide that acts via G(q/11)-coupled 7TM receptors on pituitary gonadotrophs and mediates the central control of reproduction. Recent evidence also indicates that GnRH can affect numerous tissues, but the molecular mechanisms of GnRH receptor stimulation are cell type-specific. Extracellular signal-regulated kinase (ERK) 1 and 2 are key regulators of GnRH function in several cell types, but they also integrate signals from a wide variety of other stimuli. This leads to the obvious question of how specific cellular responses to ERK activation occur, and it is now clear that this is, in part, achieved through strict spatiotemporal control of ERK activity. This means that, in order to infer the function of ERK regulation accurately, multiple readouts for ERK activity, localisation and downstream consequences (e.g. transcriptional activation or cell growth) must be compared simultaneously. Here, we describe some of our findings in the investigation of GnRH signalling to ERK, with particular emphasis on novel, high-content microscopy methods for studying ERK regulation.

Reconstitution of the nuclear transport of the MAP kinase ERK2

The nuclear-cytoplasmic distribution of ERK2 is regulated in response to various stimuli and changes in cell context. Furthermore, the nuclear flux of ERK2 occurs by several energy- and carrier-dependent and -independent mechanisms. ERK2 has been shown to translocate into and out of the nucleus by facilitated diffusion through the nuclear pore, interacting directly with proteins within the nuclear pore complex, as well as by karyopherin-mediated transport. Nuclear export has been suggested to be CRM1- and MEK1/2-dependent. Here, we describe a general nuclear import assay of wild-type ERK2 that can be employed to identify different mechanisms governing nuclear entry of the protein kinase, adapted to evaluate ERK2 mutants that impair nuclear entry to dissect energy- and carrier-dependent and -independent mechanisms, and extended to characterize export mechanisms.

Determination of ERK activity: anti-phospho-ERK antibodies and in vitro phosphorylation

The ERK signaling cascade is composed of several protein kinases that sequentially activate each other by phosphorylation. This pathway is a central component of a complex signaling network that regulates important cellular processes including proliferation, differentiation, and survival. In most of these cases, the ERK cascade is activated downstream of the small GTPase Ras that, upon activation, recruits and activates the first tier in the cascade, which contains the Raf kinases. Afterward the signal is further transmitted by MEKs, ERKs, and often RSKs in the MAPKK, MAPK, and MAPKAPKs tiers of the cascade, respectively. ERKs and RSKs can further disseminate the signal by phosphorylating and modulating the activity of a large number of regulatory proteins including transcription factors and chromatin modifying enzymes. Understanding the mechanisms of activation and the regulation of the various components of this cascade will enhance our insight into the regulation of the ERK-dependent cellular processes in normal cells or of their malfunctioning in various diseases, including cancer. In this chapter, we describe methods used to determine the activity of ERKs, which upon slight modifications can also be used for the study of other signaling kinases, either within the cascade or in other pathways. These methods have been successfully applied to study the ERK signaling cascades in a variety of tissue-cultured cell lines, homo-genized animal organs, and lower organisms. As such, the use of these methods should expand our knowledge on the regulation of many distinct systems and upon induction of various stimulations.