A novel nuclear export signal sensitive to oxidative stress in the fission yeast transcription factor Pap1

Quantifying redox transcription factor dynamics as a tool to investigate redox signalling

A critical feature of the cellular antioxidant response is the induction of gene expression by redox-sensitive transcription factors. In many cells, activating these transcription factors is a dynamic process involving multiple redox steps, but it is unclear how these dynamics should be measured. Here, we show how the dynamic profile of the Schizosaccharomyces pombe Pap1 transcription factor is quantifiable by three parameters: signal amplitude, signal time and signal duration. In response to increasing hydrogen peroxide concentrations, the Pap1 amplitude decreased while the signal time and duration showed saturable increases. In co-response plots, these parameters showed a complex, non-linear relationship to the mRNA levels of four Pap1-regulated genes. We also demonstrate that hydrogen peroxide and tert-butyl hydroperoxide trigger quantifiably distinct Pap1 activation profiles and transcriptional responses. Based on these findings, we propose that different oxidants and oxidant concentrations modulate the Pap1 dynamic profile, leading to specific transcriptional responses. We further show how the effect of combination and pre-exposure stresses on Pap1 activation dynamics can be quantified using this approach. This method is therefore a valuable addition to the redox signalling toolbox that may illuminate the role of dynamics in determining appropriate responses to oxidative stress.

ACA-28, an anticancer compound, induces Pap1 nuclear accumulation via ROS-dependent and -independent mechanisms in fission yeast

The nucleocytoplasmic transport of proteins is an important mechanism to control cell fate. Pap1 is a fission yeast nucleocytoplasmic shuttling transcription factor of which localization is redox regulated. The nuclear export factor Crm1/exportin negatively regulates Pap1 by exporting it from the nucleus to the cytoplasm. Here, we describe the effect of an anti-cancer compound ACA-28, an improved derivative of 1'-acetoxychavicol acetate (ACA), on the subcellular distribution of Pap1. ACA-28 induced nuclear accumulation of Pap1 more strongly than did ACA. ROS inhibitor N-acetyl-L-cysteine (NAC) partly antagonized the Pap1 nuclear accumulation induced by ACA-28. NAC almost abolished Pap1 nuclear localization upon H 2 O 2 , whereas leptomycin B (LMB)-mediated inhibition of Pap1 nuclear export was resistant to NAC. Collectively, ACA-28-mediated apoptosis in cancer cells may involve ROS-dependent and -independent mechanisms.

Perturbed fatty-acid metabolism is linked to localized chromatin hyperacetylation, increased stress-response gene expression and resistance to oxidative stress

Oxidative stress is associated with cardiovascular and neurodegenerative diseases, diabetes, cancer, psychiatric disorders and aging. In order to counteract, eliminate and/or adapt to the sources of stress, cells possess elaborate stress-response mechanisms, which also operate at the level of regulating transcription. Interestingly, it is becoming apparent that the metabolic state of the cell and certain metabolites can directly control the epigenetic information and gene expression. In the fission yeast Schizosaccharomyces pombe, the conserved Sty1 stress-activated protein kinase cascade is the main pathway responding to most types of stresses, and regulates the transcription of hundreds of genes via the Atf1 transcription factor. Here we report that fission yeast cells defective in fatty acid synthesis (cbf11, mga2 and ACC/cut6 mutants; FAS inhibition) show increased expression of a subset of stress-response genes. This altered gene expression depends on Sty1-Atf1, the Pap1 transcription factor, and the Gcn5 and Mst1 histone acetyltransferases, is associated with increased acetylation of histone H3 at lysine 9 in the corresponding gene promoters, and results in increased cellular resistance to oxidative stress. We propose that changes in lipid metabolism can regulate the chromatin and transcription of specific stress-response genes, which in turn might help cells to maintain redox homeostasis.

Domesticated LTR-Retrotransposon gag-Related Gene (Gagr) as a Member of the Stress Response Network in Drosophila

The most important sources of new components of genomes are transposable elements, which can occupy more than half of the nucleotide sequence of the genome in higher eukaryotes. Among the mobile components of a genome, a special place is occupied by retroelements, which are similar to retroviruses in terms of their mechanisms of integration into a host genome. The process of positive selection of certain sequences of transposable elements and retroviruses in a host genome is commonly called molecular domestication. There are many examples of evolutionary adaptations of gag (retroviral capsid) sequences as new regulatory sequences of different genes in mammals, where domesticated gag genes take part in placenta functioning and embryogenesis, regulation of apoptosis, hematopoiesis, and metabolism. The only gag-related gene has been found in the Drosophila genome—Gagr. According to the large-scale transcriptomic and proteomic analysis data, the Gagr gene in D. melanogaster is a component of the protein complex involved in the stress response. In this work, we consider the evolutionary processes that led to the formation of a new function of the domesticated gag gene and its adaptation to participation in the stress response. We discuss the possible functional role of the Gagr as part of the complex with its partners in Drosophila, and the pathway of evolution of proteins of the complex in eukaryotes to determine the benefit of the domesticated retroelement gag gene.

Overproduction of plant nuclear export signals enhances diamide tolerance in Schizosaccharomyces pombe

The nuclear export signal (NES) endows a protein nuclear export ability. Surprisingly, our previous study shows that just the NES peptide of Schizosaccharomyces pombe Oxs1 (SpOxs1^NES) can confer diamide tolerance by competing with transcription factor Pap1 for nuclear transport. This finding intrigued us to test the function of NESs from heterologous organisms. The Arabidopsis thaliana zinc finger transcription factor OXIDATIVE STRESS 2 (AtOXS2) is a nucleocytoplasmic shuttling protein and nearly all OXS2 members from maize and rice contain an NES. In this study, we find that the plant OXS2 members and their C-terminus (AT3 peptide) can confer diamide tolerance due to their NESs, and amino acids in non-conserved as well as conserved positions are necessary for the diamide tolerance. As in SpOxs1^NES, the enhanced tolerance to diamide in fission yeast depends on Pap1. Like SpOxs1^NES, OXS2 family NESs appear to compete for nuclear transport of the Pap1-like Arabidopsis protein bZIP10, as when overproduced in Arabidopsis protoplasts, bZIP10 is retained in the nucleus.

Epigenetic gene silencing by heterochromatin primes fungal resistance

Heterochromatin that depends on histone H3 lysine 9 methylation (H3K9me) renders embedded genes transcriptionally silent^1-3. In the fission yeast Schizosaccharomyces pombe, H3K9me heterochromatin can be transmitted through cell division provided the counteracting demethylase Epe1 is absent^4,5. Heterochromatin heritability might allow wild-type cells under certain conditions to acquire epimutations, which could influence phenotype through unstable gene silencing rather than DNA change^6,7. Here we show that heterochromatin-dependent epimutants resistant to caffeine arise in fission yeast grown with threshold levels of caffeine. Isolates with unstable resistance have distinct heterochromatin islands with reduced expression of embedded genes, including some whose mutation confers caffeine resistance. Forced heterochromatin formation at implicated loci confirms that resistance results from heterochromatin-mediated silencing. Our analyses reveal that epigenetic processes promote phenotypic plasticity, letting wild-type cells adapt to unfavourable environments without genetic alteration. In some isolates, subsequent or coincident gene-amplification events augment resistance. Caffeine affects two anti-silencing factors: Epe1 is downregulated, reducing its chromatin association, and a shortened isoform of Mst2 histone acetyltransferase is expressed. Thus, heterochromatin-dependent epimutation provides a bet-hedging strategy allowing cells to adapt transiently to insults while remaining genetically wild type. Isolates with unstable caffeine resistance show cross-resistance to antifungal agents, suggesting that related heterochromatin-dependent processes may contribute to resistance of plant and human fungal pathogens to such agents.

Identification of ubiquitin-proteasome system components affecting the degradation of the transcription factor Pap1

Signaling cascades respond to specific inputs, but also require active interventions to be maintained in their basal/inactive levels in the absence of the activating signal(s). In a screen to search for protein quality control components required for wild-type tolerance to oxidative stress in fission yeast, we have isolated eight gene deletions conferring resistance not only to H₂O₂ but also to caffeine. We show that dual resistance acquisition is totally or partially dependent on the transcription factor Pap1. Some gene products, such as the ribosomal-ubiquitin fusion protein Ubi1, the E2 conjugating enzyme Ubc2 or the E3 ligase Ubr1, participate in basal ubiquitin labeling of Pap1, and others, such as Rpt4, are non-essential constituents of the proteasome. We demonstrate here that basal nucleo-cytoplasmic shuttling of Pap1, occurring even in the absence of stress, is sufficient for the interaction of the transcription factor with nuclear Ubr1, and we identify a 30 amino acids peptide in Pap1 as the degron for this important E3 ligase. The isolated gene deletions increase only moderately the concentration of the transcription factor, but it is sufficient to enhance basal tolerance to stress, probably by disturbing the inactive stage of this signaling cascade.

Protection from Disulfide Stress by Inhibition of Pap1 Nuclear Export in Schizosaccharomyces pombe

Appropriate subcellular localization of regulatory factors is critical for cellular function. Pap1, a nucleocytoplasmic shuttling transcription factor of Schizosaccharomyces pombe, is redox regulated for localization and antistress function. In this study, we find that overproduction of a peptide conjugate containing the nuclear export signal of Oxs1, a conserved eukaryotic protein that, along with Pap1, regulates certain diamide responsive genes, can retain Pap1 in the nucleus before stress by competing for nuclear export. The nuclear retention of Pap1 upregulates several drug resistance genes to prime the cells for higher tolerance to disulfide stress. Overproduction of Oxs1 also upregulates these same genes, not by competing for export but by binding directly to the drug resistance gene promoters for Pap1-mediated activation. Of medical relevance is that this may suggest a gene therapy approach of using nuclear export signal conjugates to suppress the nuclear export of biomolecules.

Tschimganine and its derivatives extend the chronological life span of yeast via activation of the Sty1 pathway

Most antiaging factors or life span extenders are associated with calorie restriction (CR). Very few of these factors function independently of, or additively with, CR. In this study, we focused on tschimganine, a compound that was reported to extend chronological life span (CLS). Although tschimganine led to the extension of CLS, it also inhibited yeast cell growth. We acquired a Schizosaccharomyces pombe mutant with a tolerance for tschimganine due to the gene crm1. The resulting Crm1 protein appears to export the stress-activated protein kinase Sty1 from the nucleus to the cytosol even under stressful conditions. Furthermore, we synthesized two derivative compounds of tschimganine, α-hibitakanine and β-hibitakanine; these derivatives did not inhibit cell growth, as seen with tschimganine. α-hibitakanine extended the CLS, not only in S. pombe but also in Saccharomyces cerevisiae, indicating the possibility that life span regulation by tschimganine derivative may be conserved across various yeast species. We found that the longevity induced by tschimganine was dependent on the Sty1 pathway. Based on our results, we propose that tschimganine and its derivatives extend CLS by activating the Sty1 pathway in fission yeast, and CR extends CLS via two distinct pathways, one Sty1-dependent and the other Sty1-independent. These findings provide the potential for creating an additive life span extension effect when combined with CR, as well as a better understanding of the mechanism of CLS.

MoYcp4 is required for growth, conidiogenesis and pathogenicity in Magnaporthe oryzae

The transcription factor MoAP1 has been shown previously to be required for pathogenicity in Magnaporthe oryzae via mediation of the oxidative stress response. In the serial analysis gene expression database, it was found that expression of MoYcp4, a homologue of the Saccharomyces cerevisiae flavodoxin-like protein ScYcp4, was affected by MoAP1. Transcriptional analysis demonstrated that MoYCP4 was significantly up-regulated during conidiation, appressorium formation and infection. The growth rate of a ΔMoycp4 mutant was reduced slightly, but conidial production was increased significantly (more than 10-fold), compared with the wild-type strain. Although the rate of appressorium formation was unaffected, the appressorial turgor was abnormal and the ability to infect rice and barley was reduced, resulting in decreased pathogenicity. In summary, MoYcp4, a target of MoAP1, is involved in the growth, conidiogenesis and pathogenicity of M. oryzae. Our studies provide a comprehensive analysis of flavodoxin-like proteins and will aid in the study of pathogen-related molecular mechanisms.

Spatial regulation of the KH domain RNA-binding protein Rnc1 mediated by a Crm1-independent nuclear export system in Schizosaccharomyces pombe

RNA-binding proteins (RBPs) play important roles in the posttranscriptional regulation of gene expression, including mRNA stability, transport and translation. Fission yeast rnc1+ encodes a K Homology (KH)-type RBP, which binds and stabilizes the Pmp1 MAPK phosphatase mRNA thereby suppressing the Cl- hypersensitivity of calcineurin deletion and MAPK signaling mutants. Here, we analyzed the spatial regulation of Rnc1 and discovered a putative nuclear export signal (NES)Rnc1 , which dictates the cytoplasmic localization of Rnc1 in a Crm1-independent manner. Notably, mutations in the NESRnc1 altered nucleocytoplasmic distribution of Rnc1 and abolished its function to suppress calcineurin deletion, although the Rnc1 NES mutant maintains the ability to bind Pmp1 mRNA. Intriguingly, the Rnc1 NES mutant destabilized Pmp1 mRNA, suggesting the functional importance of the Rnc1 cytoplasmic localization. Mutation in Rae1, but not Mex67 deletion or overproduction, induced Rnc1 accumulation in the nucleus, suggesting that Rnc1 is exported from the nucleus to the cytoplasm via the mRNA export pathway involving Rae1. Importantly, mutations in the Rnc1 KH-domains abolished the mRNA-binding ability and induced nuclear localization, suggesting that Rnc1 may be exported from the nucleus together with its target mRNAs. Collectively, the functional Rae1-dependent mRNA export system may influence the cytoplasmic localization and function of Rnc1.

Latrunculin A-Induced Perturbation of the Actin Cytoskeleton Mediates Pap1p-Dependent Induction of the Caf5p Efflux Pump in Schizosaccharomyces pombe

As part of an earlier study aimed at uncovering gene products with roles in defending against latrunculin A (LatA)-induced cytoskeletal perturbations, we identified three members of the oxidative stress response pathway: the Pap1p AP-1-like transcription factor, the Imp1p α-importin, and the Caf5p efflux pump. In this report, we characterize the pathway further and show that Pap1p translocates from the cytoplasm to the nucleus in an Imp1p-dependent manner upon LatA treatment. Moreover, preventing this translocation, through the addition of a nuclear export signal (NES), confers the same characteristic LatA-sensitive phenotype exhibited by pap1Δ cells. Lastly, we show that the caf5 gene is induced upon exposure to LatA and that Pap1p is required for this transcriptional upregulation. Importantly, the expression of trr1, a Pap1p target specifically induced in response to oxidative stress, is not significantly altered by LatA treatment. Taken together, these results suggest a model in which LatA-mediated cytoskeletal perturbations are sensed, triggering the Imp1p-dependent translocation of Pap1p to the nucleus and the induction of the caf5 gene (independently of oxidative stress).

Oxidative stress response pathways: Fission yeast as archetype

Schizosaccharomyces pombe is a popular model eukaryotic organism to study diverse aspects of mammalian biology, including responses to cellular stress triggered by redox imbalances within its compartments. The review considers the current knowledge on the signaling pathways that govern the transcriptional response of fission yeast cells to elevated levels of hydrogen peroxide. Particular attention is paid to the mechanisms that yeast cells employ to promote cell survival in conditions of intermediate and acute oxidative stress. The role of the Sty1/Spc1/Phh1 mitogen-activated protein kinase in regulating gene expression at multiple levels is discussed in detail.

A Pap1-Oxs1 signaling pathway for disulfide stress in Schizosaccharomyces pombe

We describe a Pap1–Oxs1 pathway for diamide-induced disulfide stress in Schizosaccharomyces pombe, where the nucleocytoplasmic HMG protein Oxs1 acts cooperatively with Pap1 to regulate transcription. Oxs1 and Pap1 form a complex when cells are exposed to diamide or Cd that causes disulfide stress. When examined for promoters up-regulated by diamide, effective Pap1 binding to these targets requires Oxs1, and vice versa. With some genes, each protein alone enhances transcription, but the presence of both exerts an additive positive effect. In other genes, although transcription is induced by diamide, Oxs1 or Pap1 plays a negative role with full de-repression requiring loss of both proteins. In a third class of genes, Oxs1 positively regulates expression, but in its absence, Pap1 plays a negative role. The Oxs1–Pap1 regulatory interaction appears evolutionarily conserved, as heterologous (human, mouse and Arabidopsis) Oxs1 and Pap1-homologues can bind interchangeably with each other in vitro, and at least in the fission yeast, heterologous Oxs1 and Pap1-homologues can substitute for S. pombe Oxs1 and Pap1 to enhance stress tolerance.

Inside out: the role of nucleocytoplasmic transport in ALS and FTLD

Neurodegenerative diseases are characterized by the presence of protein inclusions with a different protein content depending on the type of disease. Amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD) are no exceptions to this common theme. In most ALS and FTLD cases, the predominant pathological species are RNA-binding proteins. Interestingly, these proteins are both depleted from their normal nuclear localization and aggregated in the cytoplasm. This key pathological feature has suggested a potential dual mechanism with both nuclear loss of function and cytoplasmic gain of function being at play. Yet, why and how this pathological cascade is initiated in most patients, and especially sporadic cases, is currently unresolved. Recent breakthroughs in C9orf72 ALS/FTLD disease models point at a pivotal role for the nuclear transport system in toxicity. To address whether defects in nuclear transport are indeed implicated in the disease, we reviewed two decades of ALS/FTLD literature and combined this with bioinformatic analyses. We find that both RNA-binding proteins and nuclear transport factors are key players in ALS/FTLD pathology. Moreover, our analyses suggest that disturbances in nucleocytoplasmic transport play a crucial initiating role in the disease, by bridging both nuclear loss and cytoplasmic gain of functions. These findings highlight this process as a novel and promising therapeutic target for ALS and FTLD.

Tip-to-nucleus migration dynamics of the asexual development regulator FlbB in vegetative cells

In Aspergillus nidulans, asexual differentiation requires the presence of the transcription factor FlbB at the cell tip and apical nuclei. Understanding the relationship between these two pools is crucial for elucidating the biochemical processes mediating conidia production. Tip-to-nucleus communication was demonstrated by photo-convertible FlbB::Dendra2 visualization. Tip localization of FlbB depends on Cys382 in the C-terminus and the bZIP DNA-binding domain in the N-terminus. FlbE, a critical FlbB interactor, binds the bZIP domain. Furthermore, the absence of FlbE results in loss of tip localization but not nuclear accumulation. flbE deletion also abrogates transcriptional activity indicating that FlbB gains transcriptional competence from interactions with FlbE at the tip. Finally, a bipartite nuclear localization signal is required for nuclear localization of FlbB. Those motifs of FlbB may play various roles in the sequence of events necessary for the distribution and activation of this transcriptionally active developmental factor. The tip accumulation, FlbE-dependent activation, transport and nuclear import sketch out a process of relaying an environmentally triggered signal from the tip to the nuclei. As the first known instance of transcription factor-mediated tip-to-nucleus communication in filamentous fungi, this provides a general framework for analyses focused on elucidating the set of molecular mechanisms coupling apical signals to transcriptional events.

Characterization of the nuclear import mechanism of the CCAAT-regulatory subunit Php4

Php4 is a nucleo-cytoplasmic shuttling protein that accumulates in the nucleus during iron deficiency. When present in the nucleus, Php4 associates with the CCAAT-binding protein complex and represses genes encoding iron-using proteins. Here, we show that nuclear import of Php4 is independent of the other subunits of the CCAAT-binding complex. Php4 nuclear import relies on two functionally independent nuclear localization sequences (NLSs) that are located between amino acid residues 171 to 174 (KRIR) and 234 to 240 (KSVKRVR). Specific substitutions of basic amino acid residues to alanines within these sequences are sufficient to abrogate nuclear targeting of Php4. The two NLSs are biologically redundant and are sufficient to target a heterologous reporter protein to the nucleus. Under low-iron conditions, a functional GFP-Php4 protein is only partly targeted to the nucleus in imp1Δ and sal3Δ mutant cells. We further found that cells expressing a temperature-sensitive mutation in cut15 exhibit increased cytosolic accumulation of Php4 at the nonpermissive temperature. Further analysis by pull-down experiments revealed that Php4 is a cargo of the karyopherins Imp1, Cut15 and Sal3. Collectively, these results indicate that Php4 can be bound by distinct karyopherins, connecting it into more than one nuclear import pathway.

The oxidative stress responsive transcription factor Pap1 confers DNA damage resistance on checkpoint-deficient fission yeast cells

Eukaryotic cells invoke mechanisms to promote survival when confronted with cellular stress or damage to the genome. The protein kinase Chk1 is an integral and conserved component of the DNA damage response pathway. Mutation or inhibition of Chk1 results in mitotic death when cells are exposed to DNA damage. Oxidative stress activates a pathway that results in nuclear accumulation of the bZIP transcription factor Pap1. We report the novel finding that fission yeast Pap1 confers resistance to drug- and non-drug-induced DNA damage even when the DNA damage checkpoint is compromised. Multi-copy expression of Pap1 restores growth to chk1-deficient cells exposed to camptothecin or hydroxyurea. Unexpectedly, increased Pap1 expression also promotes survival of chk1-deficient cells with mutations in genes encoding DNA ligase (cdc17) or DNA polymerase δ (cdc6), but not DNA replication initiation mutants. The ability of Pap1 to confer resistance to DNA damage was not specific to chk1 mutants, as it also improved survival of rad1- and rad9-deficient cells in the presence of CPT. To confer resistance to DNA damage Pap1 must localize to the nucleus and be transcriptionally active.

Redox regulation of SIRT1 in inflammation and cellular senescence

Sirtuin 1 (SIRT1) regulates inflammation, aging (life span and health span), calorie restriction/energetics, mitochondrial biogenesis, stress resistance, cellular senescence, endothelial functions, apoptosis/autophagy, and circadian rhythms through deacetylation of transcription factors and histones. SIRT1 level and activity are decreased in chronic inflammatory conditions and aging, in which oxidative stress occurs. SIRT1 is regulated by a NAD(+)-dependent DNA repair enzyme, poly(ADP-ribose) polymerase-1 (PARP1), and subsequent NAD(+) depletion by oxidative stress may have consequent effects on inflammatory and stress responses as well as cellular senescence. SIRT1 has been shown to undergo covalent oxidative modifications by cigarette smoke-derived oxidants/aldehydes, leading to posttranslational modifications, inactivation, and protein degradation. Furthermore, oxidant/carbonyl stress-mediated reduction of SIRT1 leads to the loss of its control on acetylation of target proteins including p53, RelA/p65, and FOXO3, thereby enhancing the inflammatory, prosenescent, and apoptotic responses, as well as endothelial dysfunction. In this review, the mechanisms of cigarette smoke/oxidant-mediated redox posttranslational modifications of SIRT1 and its roles in PARP1 and NF-κB activation, and FOXO3 and eNOS regulation, as well as chromatin remodeling/histone modifications during inflammaging, are discussed. Furthermore, we have also discussed various novel ways to activate SIRT1 either directly or indirectly, which may have therapeutic potential in attenuating inflammation and premature senescence involved in chronic lung diseases.

Reversible thiol oxidation in the H2O2-dependent activation of the transcription factor Pap1

Reversible thiol oxidation is both a mark of hydrogen peroxide (H2O2) toxicity and an initiator of signalling events. H2O2 sensors contain exposed and reactive cysteine residues, which become transiently oxidized as an activation mechanism. In fission yeast, the Pap1 (pombe AP-1) transcription factor is normally cytosolic, and upon H2O2 stress it undergoes post-translational modifications impairing its nuclear export; genetic evidences suggested the formation of a disulphide bond in Pap1 as a triggering activation event. Nuclear Pap1 is then recruited to about 50-80 promoters and induces an adaptation response. We have now dissected the role of all seven cysteine residues in Pap1 using genetic and proteomic techniques, and we show that four of them are required for Pap1 to be activated by H2O2 stress. Thus, mutants lacking each one of these cysteine residues display sensitivity to peroxides. Furthermore, these mutant proteins do not become oxidized by H2O2 and cannot bind to promoters or trigger the Pap1-dependent gene expression program. We also demonstrate, by proteomic analysis of reduced and oxidized Pap1, that these four cysteine residues are reversibly oxidized upon H2O2 stress. Our study suggests that not just one but probably two disulphide bonds are required to promote the important conformational changes that trigger Pap1 activation and nuclear accumulation.

Selective inhibitors of nuclear export block pancreatic cancer cell proliferation and reduce tumor growth in mice

BACKGROUND & AIMS

Tumor-suppressor proteins are inactivated by many different mechanisms, including nuclear exclusion by chromosome region maintenance (CRM)-1. Increased tumor levels of CRM-1 have been correlated with poor prognosis of patients with pancreatic cancer, making it a therapeutic target. Selective inhibitors of nuclear export (SINEs) bind to CRM-1 to irreversibly inhibit its ability to export proteins; we investigated a new class of SINEs in pancreatic cancer cells.

METHODS

We studied the effects of SINE analogs in a panel of pancreatic cancer cell lines and nontransformed human pancreatic ductal epithelial cells using proliferation, apoptosis, immunoblot, co-immunoprecipitation, small inhibitor RNA, and fluorescence microscopy analyses. The effects of the SINEs also were investigated in mice with subcutaneous and orthotopic tumors.

RESULTS

SINEs (KPT-185, KPT-127, KPT-205, and KPT-227) inhibited proliferation and promoted apoptosis of pancreatic cancer cells, but did not affect human pancreatic ductal epithelial cells. The nuclei of cells incubated with KPT-185 accumulated tumor-suppressor proteins (p27, FOXO, p73, and prostate apoptosis response-4 [PAR-4]) and inhibited interactions between CRM-1 and these proteins. Mutations in the region of CRM-1 that bind to SINEs (Cys-528), or small inhibitor RNA knockdown of PAR-4, prevented the ability of KPT-185 to block proliferation and induce apoptosis of pancreatic cancer cells. Oral administration of KPT-330 to mice reduced growth of subcutaneous and orthotopic xenograft tumors without major toxicity. Analysis of tumor remnants showed that KPT-330 disrupted the interaction between CRM-1 and PAR-4, activated PAR-4 signaling, and reduced proliferation of tumor cells.

CONCLUSIONS

We identified SINEs that inhibit CRM-1 and promote nuclear accumulation of tumor-suppressor proteins in pancreatic cancer cells. Oral administration of the drug to mice reduces growth of xenograft tumors.

ValidNESs: a database of validated leucine-rich nuclear export signals

ValidNESs (http://validness.ym.edu.tw/) is a new database for experimentally validated leucine-rich nuclear export signal (NES)-containing proteins. The therapeutic potential of the chromosomal region maintenance 1 (CRM1)-mediated nuclear export pathway and disease relevance of its cargo proteins has gained recognition in recent years. Unfortunately, only about one-third of known CRM1 cargo proteins are accessible in a single database since the last compilation in 2003. CRM1 cargo proteins are often recognized by a classical NES (leucine-rich NES), but this signal is notoriously difficult to predict from sequence alone. Fortunately, a recently developed prediction method, NESsential, is able to identify good candidates in some cases, enabling valuable hints to be gained by in silico prediction, but until now it has not been available through a web interface. We present ValidNESs, an integrated, up-to-date database holding 221 NES-containing proteins, combined with a web interface to prediction by NESsential.

Oxidative stress and programmed cell death in yeast

Yeasts, such as Saccharomyces cerevisiae, have long served as useful models for the study of oxidative stress, an event associated with cell death and severe human pathologies. This review will discuss oxidative stress in yeast, in terms of sources of reactive oxygen species (ROS), their molecular targets, and the metabolic responses elicited by cellular ROS accumulation. Responses of yeast to accumulated ROS include upregulation of antioxidants mediated by complex transcriptional changes, activation of pro-survival pathways such as mitophagy, and programmed cell death (PCD) which, apart from apoptosis, includes pathways such as autophagy and necrosis, a form of cell death long considered accidental and uncoordinated. The role of ROS in yeast aging will also be discussed.

A CRM1-dependent nuclear export signal controls nucleocytoplasmic translocation of HSCARG, which regulates NF-κB activity

HSCARG is a newly identified nuclear factor-κB (NF-κB) inhibitor that plays important roles in cell growth. Our previous study found that HSCARG could shuttle between the nucleus and cytoplasm by sensing the change in cellular redox states. To further investigate the mechanism of HSCARG translocation and its effect on the regulation of NF-κB activity, we identified a previously uncharacterized nuclear export signal (NES) at residues 272-278 of HSCARG that is required for its cytoplasmic translocation. This leucine-rich NES was found to be mediated by chromosome region maintenance 1. More importantly, accumulation of HSCARG in the nucleus occurred following a mutation in the NES or oxidative stress, which attenuated the inhibition of NF-κB by HSCARG. These results indicate that nucleocytoplasmic translocation of HSCARG plays an important role in fine-tuning NF-κB signaling.

The transcription factors Pap1 and Prr1 collaborate to activate antioxidant, but not drug tolerance, genes in response to H2O2

In response to hydrogen peroxide (H₂O₂), the transcription factor Pap1 from Schizosaccharomyces pombe regulates transcription of genes required for adaptation to oxidative stress and for tolerance to toxic drugs. H₂O₂ induces oxidation of Pap1, its nuclear accumulation and expression of more than fifty Pap1-dependent genes. Oxidation and nuclear accumulation of Pap1 can also be accomplished by genetic inhibition of thioredoxin reductase. Furthermore, genetic alteration of the nuclear export pathway, or mutations in Pap1 nuclear export signal trigger nuclear accumulation of reduced Pap1. We show here that a subset of Pap1-dependent genes, such as those coding for the efflux pump Caf5, the ubiquitin-like protein Obr1 or the dehydrogenase SPCC663.08c, only require nuclear Pap1 for activation, whereas another subset of genes, those coding for the antioxidants catalase, sulfiredoxin or thioredoxin reductase, do need oxidized Pap1 to form a heterodimer with the constitutively nuclear transcription factor Prr1. The ability of Pap1 to bind and activate drug tolerance promoters is independent on Prr1, whereas its affinity for the antioxidant promoters is significantly enhanced upon association with Prr1. This finding suggests that the activation of both antioxidant and drug resistance genes in response to oxidative stress share a common inducer, H₂O₂, but alternative effectors.

Characterization of the ptr5+ gene involved in nuclear mRNA export in fission yeast

To analyze the mechanisms of mRNA export from the nucleus to the cytoplasm, we have isolated eleven mutants, ptr [poly(A)(+) RNA transport] 1 to 11, which accumulate poly(A)(+) RNA in the nucleus at a nonpermissive temperature in Schizosaccharomyces pombe. Of those, the ptr5-1 mutant shows dots- or a ring-like accumulation of poly(A)(+) RNA at the nuclear periphery after shifting to the nonpermissive temperature. We cloned the ptr5(+) gene and found that it encodes a component of the nuclear pore complex (NPC), nucleoporin 85 (Nup85). The ptr5-1 mutant shows no defects in protein transport, suggesting the specific involvement of Ptr5p/Nup85p in nuclear mRNA export in S. pombe. We identified Seh1p, a nucleoporin interacting with Nup85p, an mRNA-binding protein Mlo3p, and Sac3p, a component of the TREX-2 complex involved in coupling of nuclear mRNA export with transcription, as multi-copy suppressors for the ptr5-1 mutation. In addition, we found that the ptr5-1 mutation is synthetically lethal with a mutation of the mRNA export factor Rae1p, and that the double mutant exaggerates defective nuclear mRNA export, suggesting that Ptr5p/Nup85p is involved in nuclear mRNA export through Rae1p. Interestingly, the ptr5-1 mutation also showed synthetic effects with several prp pre-mRNA splicing mutations, suggesting a functional linkage between the NPCs and the splicing apparatus in the yeast nucleus.

Prediction of leucine-rich nuclear export signal containing proteins with NESsential

The classical nuclear export signal (NES), also known as the leucine-rich NES, is a protein localization signal often involved in important processes such as signal transduction and cell cycle regulation. Although 15 years has passed since its discovery, limited structural information and high sequence diversity have hampered understanding of the NES. Several consensus sequences have been proposed to describe it, but they suffer from poor predictive power. On the other hand, the NetNES server provides the only computational method currently available. Although these two methods have been widely used to attempt to find the correct NES position within potential NES-containing proteins, their performance has not yet been evaluated on the basic task of identifying NES-containing proteins. We propose a new predictor, NESsential, which uses sequence derived meta-features, such as predicted disorder and solvent accessibility, in addition to primary sequence. We demonstrate that it can identify promising NES-containing candidate proteins (albeit at low coverage), but other methods cannot. We also quantitatively demonstrate that predicted disorder is a useful feature for prediction and investigate the different features of (predicted) ordered versus disordered NES's. Finally, we list 70 recently discovered NES-containing proteins, doubling the number available to the community.

Fission yeast Ubr1 ubiquitin ligase influences the oxidative stress response via degradation of active Pap1 bZIP transcription factor in the nucleus

Cells adapt to oxidative stress by transcriptional activation of genes encoding antioxidants and proteins of other protective roles. A bZIP transcription factor, Pap1, plays a critical role in this process and overexpression of Pap1 confers resistance to various oxidants and drugs in fission yeast. Pap1 temporarily enters the nucleus upon oxidative stress but returns to the cytoplasm once cells adapt to the stress, suggesting that cellular localization regulates Pap1 function. We report here an additional regulatory mechanism that Ubr1 ubiquitin ligase-dependent degradation lowered the Pap1 protein levels. ubr1 cells were causally resistant to hydrogen peroxide because of the increment of Pap1 levels. Pap1 was preferentially degraded in the nucleus where Ubr1 was consistently enriched. Proteolysis was critical to downregulate Pap1 especially when its activation persisted, as constitutively nuclear Pap1 severely inhibited growth in ubr1 mutants. Inactive mutations in the bZIP DNA binding domain stabilized Pap1 but rescued the lethality caused by constitutively active Pap1 in ubr1 mutants. These findings indicate that either nuclear export or Ubr1-mediated proteolysis must be operative to prevent uncontrolled Pap1 function. Coincidental dysfunction in both inhibitory pathways causes lethality because of prolonged activation of Pap1. Ubr1 is a critical regulator for the homeostasis of oxidative stress response.

Elucidation of functional markers from Aspergillus nidulans developmental regulator FlbB and their phylogenetic distribution

Aspergillus nidulans is a filamentous fungus widely used as a model for biotechnological and clinical research. It is also used as a platform for the study of basic eukaryotic developmental processes. Previous studies identified and partially characterized a set of proteins controlling cellular transformations in this ascomycete. Among these proteins, the bZip type transcription factor FlbB is a key regulator of reproduction, stress responses and cell-death. Our aim here was the prediction, through various bioinformatic methods, of key functional residues and motifs within FlbB in order to inform the design of future laboratory experiments and further the understanding of the molecular mechanisms that control fungal development. A dataset of FlbB orthologs and those of its key interaction partner FlbE was assembled from 40 members of the Pezizomycotina. Unique features were identified in each of the three structural domains of FlbB. The N-terminal region encoded a bZip transcription factor domain with a novel histidine-containing DNA binding motif while the dimerization determinants exhibited two distinct profiles that segregated by class. The C-terminal region of FlbB showed high similarity with the AP-1 family of stress response regulators but with variable patterns of conserved cysteines that segregated by class and order. Motif conservation analysis revealed that nine FlbB orthologs belonging to the Eurotiales order contained a motif in the central region that could mediate interaction with FlbE. The key residues and motifs identified here provide a basis for the design of follow-up experimental investigations. Additionally, the presence or absence of these residues and motifs among the FlbB orthologs could help explain the differences in the developmental programs among fungal species as well as define putative complementation groups that could serve to extend known functional characterizations to other species.

Adaptive response to oxidative stress: Bacteria, fungi, plants and animals

Reactive oxygen species (ROS) are continuously produced and eliminated by living organisms normally maintaining ROS at certain steady-state levels. Under some circumstances, the balance between ROS generation and elimination is disturbed leading to enhanced ROS level called "oxidative stress". The primary goal of this review is to characterize two principal mechanisms of protection against oxidative stress - regulation of membrane permeability and antioxidant potential. The ancillary goals of this work are to describe up to date knowledge on the regulation of the previously mentioned mechanisms and to identify areas of prospective research and emerging directions in investigation of adaptation to oxidative stress. The ubiquity for challenges leading to oxidative stress development calls for identification of common mechanisms. They are cysteine residues and [Fe,S]-clusters of specific regulatory proteins. The latter mechanism is realized via SoxR bacterial protein, whereas the former mechanism is involved in operation of bacterial OxyR regulon, yeast H(2)O(2)-stimulon, plant NPR1/TGA and Rap2.4a systems, and animal Keap1/Nrf2, NF-κB and AP-1, and others. Although hundreds of studies have been carried out in the field with different taxa, the comparative analysis of adaptive response is quite incomplete and therefore, this work aims to cover a plethora of phylogenetic groups to delineate common mechanisms. In addition, this article raises some questions to be elucidated and points out future directions of this research. The comparative approach is used to shed light on fundamental principles and mechanisms of regulation of antioxidant systems. The idea is to provide starting points from which we can develop novel tools and hypothesis to facilitate meaningful investigations in the physiology and biochemistry of organismic response to oxidative stress.

Redox control systems in the nucleus: mechanisms and functions

Proteins with oxidizable thiols are essential to many functions of cell nuclei, including transcription, chromatin stability, nuclear protein import and export, and DNA replication and repair. Control of the nuclear thiol-disulfide redox states involves both the elimination of oxidants to prevent oxidation and the reduction of oxidized thiols to restore function. These processes depend on the common thiol reductants, glutathione (GSH) and thioredoxin-1 (Trx1). Recent evidence shows that these systems are controlled independent of the cytoplasmic counterparts. In addition, the GSH and Trx1 couples are not in redox equilibrium, indicating that these reductants have nonredundant functions in their support of proteins involved in transcriptional regulation, nuclear protein trafficking, and DNA repair. Specific isoforms of glutathione peroxidases, glutathione S-transferases, and peroxiredoxins are enriched in nuclei, further supporting the interpretation that functions of the thiol-dependent systems in nuclei are at least quantitatively distinct, and probably also qualitatively distinct, from similar processes in the cytoplasm. Elucidation of the distinct nuclear functions and regulation of the thiol redox pathways in nuclei can be expected to improve understanding of nuclear processes and also to provide the basis for novel approaches to treat aging and disease processes associated with oxidative stress in the nuclei.

Oxidative stress in yeast

The mechanisms of production and elimination of reactive oxygen species in the cells of the budding yeast Saccharomyces cerevisiae are analyzed. Coordinative role of special regulatory proteins including Yap1p, Msn2/4p, and Skn7p (Pos9p) in regulation of defense mechanisms in S. cerevisiae is described. A special section is devoted to two other well-studied species from the point of view of oxidative stress -- Schizosaccharomyces pombe and Candida albicans. Some examples demonstrating the use of yeast for investigation of apoptosis, aging, and some human diseases are given in the conclusion part.

Both Php4 function and subcellular localization are regulated by iron via a multistep mechanism involving the glutaredoxin Grx4 and the exportin Crm1

In Schizosaccharomyces pombe, the CCAAT-binding factor is a multisubunit complex that contains the proteins Php2, Php3, Php4, and Php5. Under low iron conditions, Php4 acts as a negative regulatory subunit of the CCAAT-binding factor and fosters repression of genes encoding iron-using proteins. Under conditions of iron excess, Php4 expression is turned off by the iron-dependent transcriptional repressor Fep1. In this study, we developed a biological system that allows us to unlink iron-dependent behavior of Php4 protein from its transcriptional regulation by Fep1. Microscopic analyses revealed that a functional GFP-Php4 protein accumulates in the nucleus under conditions of iron starvation. Conversely, in cells undergoing a transition from low to high iron, GFP-Php4 is exported from the nucleus to the cytoplasm. We mapped a leucine-rich nuclear export signal that is necessary for nuclear exclusion of Php4. This latter process was blocked by leptomycin B. By using coimmunoprecipitation analysis, we showed that Php4 and Crm1 physically interact with each other. Although we determined that nuclear retention of Php4 per se is not sufficient to cause a constitutive repression of iron-using genes, we found that deletion of the grx4(+)-encoded glutaredoxin-4 renders Php4 constitutively active and invariably localized in the nucleus. Further analysis by bimolecular fluorescence complementation assay and by two-hybrid assays showed that Php4 and Grx4 are physically associated in vivo. Taken together, our findings indicate that Grx4 and Crm1 are novel components involved in the mechanism by which Php4 is inactivated by iron in a Fep1-independent manner.

Activation of the oxidative stress regulator PpYap1 through conserved cysteine residues during methanol metabolism in the yeast Pichia pastoris

The methylotrophic yeast Pichia pastoris can grow on methanol as sole source of carbon and energy. The first reaction in yeast methanol metabolism, catalyzed by an abundant peroxisomal enzyme, alcohol oxidase, generates high levels of H(2)O(2), but the oxidative stress response during methanol metabolism has not been elucidated. In this study, we isolated the Yap1 homolog of P. pastoris (PpYap1) and analyzed the properties of a PpYAP1-disruption strain. The PpYap1 transcription factor is activated after exposure to various reactive agents, and therefore functions as a regulator of the redox system in P. pastoris. We have also identified PpGPX1, the unique glutathione peroxidase-encoding gene in P. pastoris whose expression is induced by PpYap1. PpGpx1, but not the ScTsa1 or SpTpx1 homolog PpTsa1, functions as a H(2)O(2) sensor and activates PpYap1. This study is the first demonstration of a yeast Yap1 family protein activated during conventional metabolism.

The role and regulation of Trxl, a cytosolic thioredoxin in Schizosaccharomyces pombe

The genome of fission yeast Schizosaccharomyces pombe harbors two genes for thioredoxins, trx1(+) and trx2(+), which encode cytosolic and mitochondrial thioredoxins, respectively. The Deltatrx1 mutant was found sensitive to diverse external stressors such as various oxidants, heat, and salt, whereas Deltatrx2 mutant was not sensitive except to paraquat, a superoxide generator. Both Deltatrx1 and Deltatrx2 mutants were more resistant to diamide, a thiol-specific oxidant, than the wild type. The trx1(+) gene expression was induced by H(2)O(2) and menadione, being mediated through a stress-responsive transcription factor Papl. In Deltatrx1 cells, the basal expression of Pap1-regulated genes were elevated, suggesting a role for Trxl as a reducer for oxidized (activated) Papl. The Deltatrx1 mutant exhibited cysteine auxotrophy, which can be overcome by adding sulfite. This suggests that Trxl serves as a primary electron donor for 3'-phosphoadenosine-5'-phosphosulfate (PAPS) reductase and thus is an essential protein for sulfur assimilation in S. pombe. These results suggest that, in contrast to Trx2 whose role is more confined to mitochondrial functions, Trxl plays a major role in protecting S. pombe against various stressful conditions and enables proper sulfur metabolism.

Nucleocytoplasmic transport of proteins

In eukaryotic cells, the movement of macromolecules between the nucleus and cytoplasm occurs through the nuclear pore complex (NPC)--a large protein complex spanning the nuclear envelope. The nuclear transport of proteins is usually mediated by a family of transport receptors known as karyopherins. Karyopherins bind to their cargoes via recognition of nuclear localization signal (NLS) for nuclear import or nuclear export signal (NES) for export to form a transport complex. Its transport through NPC is facilitated by transient interactions between the karyopherins and NPC components. The interactions of karyopherins with their cargoes are regulated by GTPase Ran. In the current review, we describe the NPC structure, NLS, and NES, as well as the model of classic Ran-dependent transport, with special emphasis on existing alternative mechanisms; we also propose a classification of the basic mechanisms of protein transport regulation.

An evolutionarily conserved nuclear export signal facilitates cytoplasmic localization of the Tbx5 transcription factor

During cardiac development, the T-box transcription factor Tbx5 displays dynamic changes in localization from strictly nuclear to both nuclear and cytoplasmic to exclusively cytoplasmic along the actin cytoskeleton in cells coexpressing its binding protein LMP4. Although nuclear localization signals (NLSs) have been described, the mechanism by which Tbx5 exits the nucleus remained elusive. Here, we describe for Tbx5 a nuclear export signal (NES) that is recognized by the CRM1 export protein. Site-directed mutagenesis of a critical amino acid(s) within this sequence determined the functionality of this NES. Confocal localization studies and luciferase transcriptional reporter assays with NES mutant Tbx5 forms demonstrated retention in the nucleus, regardless of the presence of LMP4. Coimmunoprecipitation and pharmacological interference studies demonstrated a direct interaction between Tbx5 and CRM1, revealing that Tbx5 is using the CRM1 pathway for nuclear export. In addition to Tbx5, we identified NESs in all T-box proteins and demonstrated interaction of the family members Tbx3 and Brachyury with the CRM1 exporter, suggesting general significance. This first demonstration of evolutionarily conserved NESs in all T-box proteins in conjunction with NLSs indicates a primordial function of T-box proteins to dynamically shuttle between nuclear and cytoplasmic compartments of the cell.

Multiple pathways differentially regulate global oxidative stress responses in fission yeast

Cellular protection against oxidative damage is relevant to ageing and numerous diseases. We analyzed the diversity of genome-wide gene expression programs and their regulation in response to various types and doses of oxidants in Schizosaccharomyces pombe. A small core gene set, regulated by the AP-1-like factor Pap1p and the two-component regulator Prr1p, was universally induced irrespective of oxidant and dose. Strong oxidative stresses led to a much larger transcriptional response. The mitogen-activated protein kinase (MAPK) Sty1p and the bZIP factor Atf1p were critical for the response to hydrogen peroxide. A newly identified zinc-finger protein, Hsr1p, is uniquely regulated by all three major regulatory systems (Sty1p-Atf1p, Pap1p, and Prr1p) and in turn globally supports gene expression in response to hydrogen peroxide. Although the overall transcriptional responses to hydrogen peroxide and t-butylhydroperoxide were similar, to our surprise, Sty1p and Atf1p were less critical for the response to the latter. Instead, another MAPK, Pmk1p, was involved in surviving this stress, although Pmk1p played only a minor role in regulating the transcriptional response. These data reveal a considerable plasticity and differential control of regulatory pathways in distinct oxidative stress conditions, providing both specificity and backup for protection from oxidative damage.

Characterization of a ribonuclease III-like protein required for cleavage of the pre-rRNA in the 3'ETS in Arabidopsis

Ribonuclease III (RNaseIII) is responsible for processing and maturation of RNA precursors into functional rRNA, mRNA and other small RNA. In contrast to bacterial and yeast cells, higher eukaryotes contain at least three classes of RNaseIII, including class IV or dicer-like proteins. Here, we describe the functional characterization of AtRTL2, an Arabidopsis thaliana RNaseIII-like protein that belongs to a small family of genes distinct from the dicer family. We demonstrate that AtRTL2 is required for 3′external transcribed spacer (ETS) cleavage of the pre-rRNA in vivo. AtRTL2 localizes in the nucleus and cytoplasm, a nuclear export signal (NES) in the N-terminal sequence probably controlling AtRTL2 cellular localization. The modeled 3D structure of the RNaseIII domain of AtRTL2 is similar to the bacterial RNaseIII domain, suggesting a comparable catalytic mechanism. However, unlike bacterial RNaseIII, the AtRTL2 protein forms a highly salt-resistant homodimer that is only disrupted on treatment with DTT. These data indicate that AtRTL2 may use a dimeric mechanism to cleave double-stranded RNA, but unlike bacterial or yeast RNase III proteins, AtRTL2 forms homodimers through formation of disulfide bonds, suggesting that redox conditions may operate to regulate the activity of RNaseIII.

Crm1-mediated nuclear export of the Schizosaccharomyces pombe transcription factor Cuf1 during a shift from low to high copper concentrations

In this study, we examine the fate of the nuclear pool of the Schizosaccharomyces pombe transcription factor Cuf1 in response to variations in copper levels. A nuclear pool of Cuf1-green fluorescent protein (GFP) was generated by expressing a functional cuf1(+)-GFP allele in the presence of a copper chelator. We then extinguished cuf1(+)-GFP expression and tracked the changes in the localization of the nuclear pool of Cuf1-GFP in the presence of low or high copper concentrations. Treating cells with copper as well as silver ions resulted in the nuclear export of Cuf1. We identified a leucine-rich nuclear export signal (NES), (349)LAALNHISAL(358), within the C-terminal region of Cuf1. Mutations in this sequence abrogated Cuf1 export from the nucleus. Furthermore, amino acid substitutions that impair Cuf1 NES function resulted in increased target gene expression and a concomitant cellular hypersensitivity to copper. Export of the wild-type Cuf1 protein was inhibited by leptomycin B (LMB), a specific inhibitor of the nuclear export protein Crm1. We further show that cells expressing a temperature-sensitive mutation in crm1(+) exhibit increased nuclear accumulation of Cuf1 at the nonpermissive temperature. Although wild-type Cuf1 is localized in the nucleus in both conditions, we observed that the protein can still be inactivated by copper, resulting in the repression of ctr4(+) gene expression in the presence of exogenous copper. These results demonstrate that nuclear accumulation of Cuf1 per se is not sufficient to cause the unregulated expression of the copper transport genes like ctr4(+). In addition to nuclear localization, a functional Cys-rich domain or NES element in Cuf1 is required to appropriately regulate copper transport gene expression in response to changes in intracellular copper concentration.

Participation of XPB/Ptr8p, a component of TFIIH, in nucleocytoplasmic transport of mRNA in fission yeast

To identify novel factors involved in nuclear mRNA export in Schizosaccharomyces pombe, we isolated and characterized the ptr8(+) gene, mutation of which causes nuclear accumulation of poly (A)(+) RNA. The ptr8(+) gene encodes an S. pombe homologue of human XPB, a component of TFIIH involved in nucleotide excision repair (NER) and transcription. A temperature-sensitive mutant of ptr8(+) (ptr8-1) was highly sensitive to UV irradiation, as are human XPB cells. Northern blot analysis demonstrated that the amount of total poly (A)(+) mRNAs does not decrease significantly at the nonpermissive temperature in ptr8-1 cells, whereas a pulse-labeling assay using (35)S-methionine showed that protein synthesis decreases rapidly after incubation of cells at the nonpermissive temperature, suggesting that ptr8-1 cells have a defect in nuclear mRNA export. In Saccharomyces cerevisiae, a mutation in the SSL2 gene encoding a homologue of Ptr8p also causes a block of mRNA export at the nonpermissive temperature. In addition, expression of human XPB in ptr8-1 cells rescued the ts phenotype and the mRNA export defects, suggesting that human XPB may also play a role in mRNA export. Furthermore, we revealed a functional interaction between Ptr8p and Tho2p, a component of the TREX complex involved in mRNA export. These results suggest that XPB/Ptr8p plays roles not only in NER and transcription, but also plays a conserved role in mRNA export.

Dynamic regulation of ERK2 nuclear translocation and mobility in living cells

The extracellular signal-regulated protein kinase ERK1/2 is a crucial effector linking extracellular stimuli to cellular responses: upon phosphorylation ERK [also known as mitogen-activated protein kinase P42/P44 (MAPK)] concentrates in the nucleus where it activates specific programs of gene expression. Notwithstanding the importance of this process, little is known about the modalities, time course and regulation of ERK exchange between nucleus and cytoplasm in living cells. We visualized the dynamic of nuclear translocation by expressing low levels (<150 nM) of fluorescently tagged ERK2 in living fibroblasts. Time-lapse imaging demonstrated that nuclear concentration can change bidirectionally with a time constant of a few minutes. The increase of nuclear concentration requires continuous MEK (also known as MAPK kinase) activity upstream of ERK and is rapidly reduced by the operation of phosphatases. We measured quantitatively the speed of ERK2 shuttling between nucleus and cytoplasm and determined that shuttling accelerated after ERK activation, becoming fast enough not to be rate-limiting for translocation. Finally, we demonstrated that ERK2 did not diffuse freely in the nucleus and that diffusion was further impeded after phosphorylation, suggesting the formation of complexes of low mobility. These results show that nucleocytoplasmic trafficking of ERK2 and its mobility are dynamically regulated in living cells.

Green tea polyphenols function as prooxidants to activate oxidative-stress-responsive transcription factors in yeasts

Epigallocatechin gallate (EGCG) is the most abundant polyphenolic flavonoid in green tea. Catechin and its derivatives, including EGCG, are widely believed to function as antioxidants. Here we demonstrate that both EGCG and green tea extract (GTE) cause oxidative stress-related responses in the budding yeast Saccharomyces cerevisiae and the fission yeast Schizosaccharomyces pombe under weak alkaline conditions in terms of the activation of oxidative-stress-responsive transcription factors. GTE as well as EGCG induced the nuclear localization of Yap1 in S. cerevisiae, which was repressed by the addition of catalase but not by the addition of superoxide dismutase. The same phenomena were observed for the nucleocytoplasmic localization of Msn2 in S. cerevisiae and Pap1, a Yap1 homologue, in S. pombe. The formation of intramolecular disulfide bonds has been proposed to be crucial for the H(2)O(2)-induced nuclear localization of Yap1, and we verified the importance of cysteine residues of Yap1 in response to EGCG and GTE. Additionally, we show that EGCG and GTE produce H(2)O(2) in a weak alkaline medium. Finally, we conclude that tea polyphenols are able to act as prooxidants to cause a response to oxidative stress in yeasts under certain conditions.

Oxidative stress in Schizosaccharomyces pombe: different H2O2 levels, different response pathways

Schizosaccharomyces pombe triggers different signalling pathways depending on the severity of the oxidative stress exerted, the main ones being the Pap1 and the Sty1 pathways. The Pap1 transcription factor is more sensitive to hydrogen peroxide (H(2)O(2)) than the MAP kinase Sty1 pathway, and is designed to induce adaptation, rather than survival, responses. The peroxiredoxin Tpx1 acts as a H(2)O(2) sensor and the upstream activator of the Pap1 pathway. Therefore, sensitivity to H(2)O(2) depends on this thioredoxin peroxidase. In order to achieve maximal activation of the MAP kinase pathway, the concentration of H(2)O(2) needs to be at least fivefold higher than that to fully activate Pap1. Tpx1 is a H(2)O(2) scavenger, thus its peroxidase activity is essential for aerobic growth. As described for other eukaryotic peroxiredoxins, high doses of H(2)O(2) temporarily inactivate Tpx1 and delay Pap1 activation, whereas the Sty1 pathway remains fully functional under these conditions. As part of the Sty1-dependent transcriptional response, the expression of Srx1 is induced and this reductase re-activates the over-oxidised Tpx1. Therefore, the antioxidant pathways of the fission yeast are perfectly designed so that the transcriptional programs triggered by the different signalling pathways never overlap.

Ltv1 is required for efficient nuclear export of the ribosomal small subunit in Saccharomyces cerevisiae

In eukaryotes, 40S and 60S ribosomal subunits are assembled in the nucleus and exported to the cytoplasm independently of one another. Nuclear export of the 60S requires the adapter protein Nmd3, but no analogous adapter has been identified for the 40S. Ltv1 is a nonessential, nonribosomal protein that is required for 40S subunit biogenesis in yeast. Cells lacking LTV1 grow slowly, are hypersensitive to inhibitors of protein synthesis, and produce about half as many 40S subunits as do wild-type cells. Ltv1 interacts with Crm1, co-sediments in sucrose gradients with 43S/40S subunits, and copurifies with late 43S particles. Here we show that Ltv1 shuttles between nucleus and cytoplasm in a Crm1-dependent manner and that it contains a functional NES that is sufficient to direct the export of an NLS-containing reporter. Small subunit export is reduced in Deltaltv1 mutants, as judged by the altered distribution of the 5'-ITS1 rRNA and the 40S ribosomal protein RpS3. Finally, we show a genetic interaction between LTV1 and YRB2, a gene that encodes a Ran-GTP-, Crm1-binding protein that facilitates the small subunit export. We propose that Ltv1 functions as one of several possible adapter proteins that link the nuclear export machinery to the small subunit.