Regulation of nuclear localization during signaling

Identification of the RGG box motif in Shadoo: RNA-binding and signaling roles?

Using comparative genomics and in-silico analyses, we previously identified a new member of the prion-protein (PrP) family, the gene SPRN, encoding the protein Shadoo (Sho), and suggested its functions might overlap with those of PrP. Extended bioinformatics and conceptual biology studies to elucidate Sho’s functions now reveal Sho has a conserved RGG-box motif, a well-known RNA-binding motif characterized in proteins such as FragileX Mental Retardation Protein. We report a systematic comparative analysis of RGG-box containing proteins which highlights the motif’s functional versatility and supports the suggestion that Sho plays a dual role in cell signaling and RNA binding in brain. These findings provide a further link to PrP, which has well-characterized RNA-binding properties.

Frequency-modulated nuclear localization bursts coordinate gene regulation

In yeast, the transcription factor Crz1 is dephosphorylated and translocates into the nucleus in response to extracellular calcium. Here we show, using time-lapse microscopy, that Crz1 exhibits short bursts of nuclear localization (typically lasting 2 min) that occur stochastically in individual cells and propagate to the expression of downstream genes. Strikingly, calcium concentration controls the frequency, but not the duration, of localization bursts. Using an analytic model, we also show that this frequency modulation of bursts ensures proportional expression of multiple target genes across a wide dynamic range of expression levels, independent of promoter characteristics. We experimentally confirm this theory with natural and synthetic Crz1 target promoters. Another stress-response transcription factor, Msn2, exhibits similar, but largely uncorrelated, localization bursts under calcium stress suggesting that frequency-modulation regulation of localization bursts may be a general control strategy used by the cell to coordinate multi-gene responses to external signals.

A quantitative model of transcription factor-activated gene expression

A challenge facing biology is to develop quantitative, predictive models of gene regulation. Eukaryotic promoters contain transcription factor binding sites of differing affinity and accessibility, but we understand little about how these variables combine to generate a fine-tuned, quantitative transcriptional response. Here we used the PHO5 promoter in budding yeast to quantify the relationship between transcription factor input and gene expression output, termed the gene-regulation function (GRF). A model that captures variable interactions between transcription factors, nucleosomes and the promoter faithfully reproduced the observed quantitative changes in the GRF that occur upon altering the affinity of transcription factor binding sites, and implicates nucleosome-modulated accessibility of transcription factor binding sites in increasing the diversity of gene expression profiles. This work establishes a quantitative framework that can be applied to predict GRFs of other eukaryotic genes.

Subcellular localization of mRNA and factors involved in translation initiation

Both the process and synthesis of factors required for protein synthesis (or translation) account for a large proportion of cellular activity. In eukaryotes, the most complex and highly regulated phase of protein synthesis is that of initiation. For instance, across eukaryotes, at least 12 factors containing 22 or more proteins are involved, and there are several regulated steps. Recently, the localization of mRNA and factors involved in translation has received increased attention. The present review provides a general background to the subcellular localization of mRNA and translation initiation factors, and focuses on the potential functions of localized translation initiation factors. That is, as genuine sites for translation initiation, as repositories for factors and mRNA, and as sites of regulation.

LIM domain protein TES changes its conformational states in different cellular compartments

The human TESTIN (TES) is a putative tumor suppressor and localizes to the cytoplasm as a component of focal adhesions and cell contacts. TES contains a PET domain in the NH(2)-terminus and three tandem LIM domains in the COOH-terminus. It has been hypothesized that interactions between two termini of TES might lead to a "closed" conformational state of the protein. Here, we provide evidence for different conformational states of TES. We confirmed that the NH(2)-terminus of TES can interact with its third LIM domain in the COOH-terminus by GST pull-down assays. In addition, antisera against the full-length or two truncations of TES were prepared to examine the relationship between the conformation and cellular distribution of the protein. We found that these antisera recognize different regions of TES and showed that TES is co-localised with the marker protein B23 in nucleolus, in addition to its localization in endoplasmic reticulum (ER). Furthermore, our co-immunoprecipitation (co-IP) analysis of TES and B23 demonstrated their co-existence in the same complex. Taken together, our results suggest that TES has different conformational states in different cellular compartments, and a "closed" conformational state of TES may be involved in nucleolar localization.

Cyclophilin and the regulation of symbiosis in Aiptasia pallida

The sea anemone Aiptasia pallida, symbiotic with intracellular dinoflagellates, expresses a peptydyl-prolyl cis-trans isomerase (PPIase) belonging to the conserved family of cytosolic cyclophilins (ApCypA). Protein extracts from A. pallida exhibited PPIase activity. Given the high degree of conservation of ApCypA and its known function in the cellular stress response, we hypothesized that it plays a similar role in the cnidarian-dinoflagellate symbiosis. To explore its role, we inhibited the activity of cyclophilin with cyclosporin A (CsA). CsA effectively inhibited the PPIase activity of protein extracts from symbiotic A. pallida. CsA also induced the dose-dependent release of symbiotic algae from host tissues (bleaching). Laser scanning confocal microscopy using superoxide and nitric oxide-sensitive fluorescent dyes on live specimens of A. pallida revealed that CsA strongly induced the production of these known mediators of bleaching. We tested whether the CsA-sensitive isomerase activity is important for maintaining the activity of the antioxidant enzyme superoxide dismutase (SOD). SOD activity of protein extracts was not affected by pre-incubation with CsA in vitro.

Cytokine-induced nuclear translocation of signaling proteins and their analysis using the inducible translocation trap system

Binding of cytokines to their specific receptors induces activation of signal transduction pathways, many of which involve nuclear translocation of signaling proteins. In this review, an overview of cytokine-induced nuclear translocation of signaling proteins is provided. In addition, inducible translocation trap (ITT), a novel reporter-based system to detect nuclear translocation, and its application for identification of nuclear translocating proteins are elaborated. Finally, analysis of "nuclear translocatome", the entire set of proteins that translocate into or out of the nucleus in response to extracellular stimuli, by ITT is discussed.

Tonicity-regulated gene expression

Hypertonicity activates several different transcription factors, including TonEBP/OREBP, that in turn increase transcription of numerous genes. Hypertonicity elevates TonEBP/OREBP transcriptional activity by moving it into the nucleus, where it binds to its cognate DNA element (ORE), and by increasing its transactivational activity. This chapter presents protocols for measuring the transcriptional activity of TonEBP/OREBP and determining its subcellular localization, its binding to OREs, and activity of its transactivation domain.

Cellular Response to Hyperosmotic Stresses

Cells in the renal inner medulla are normally exposed to extraordinarily high levels of NaCl and urea. The osmotic stress causes numerous perturbations because of the hypertonic effect of high NaCl and the direct denaturation of cellular macromolecules by high urea. High NaCl and urea elevate reactive oxygen species, cause cytoskeletal rearrangement, inhibit DNA replication and transcription, inhibit translation, depolarize mitochondria, and damage DNA and proteins. Nevertheless, cells can accommodate by changes that include accumulation of organic osmolytes and increased expression of heat shock proteins. Failure to accommodate results in cell death by apoptosis. Although the adapted cells survive and function, many of the original perturbations persist, and even contribute to signaling the adaptive responses. This review addresses both the perturbing effects of high NaCl and urea and the adaptive responses. We speculate on the sensors of osmolality and document the multiple pathways that signal activation of the transcription factor TonEBP/OREBP, which directs many aspects of adaptation. The facts that numerous cellular functions are altered by hyperosmolality and remain so, even after adaptation, indicate that both the effects of hyperosmolality and adaptation to it involve profound alterations of the state of the cells.

Hyperosmotic stress signaling to the nucleus disrupts the Ran gradient and the production of RanGTP

The RanGTP gradient depends on nucleocytoplasmic shuttling of Ran and its nucleotide exchange in the nucleus. Here we show that hyperosmotic stress signaling induced by sorbitol disrupts the Ran protein gradient and reduces the production of RanGTP. Ran gradient disruption is rapid and is followed by early (10-20 min) and late (30-60 min) phases of recovery. Results from SB203580 and siRNA experiments suggest the stress kinase p38 is important for Ran gradient recovery. NTF2 and Mog1, which are transport factors that regulate the nuclear localization of Ran, showed kinetics of delocalization and recovery similar to Ran. Microinjection of a nuclear localization signal reporter protein revealed that sorbitol stress decreases the rate of nuclear import. Sorbitol stress also slowed RCC1 mobility in the nucleus, which is predicted to reduce RCC1 dissociation from chromatin and RanGTP production. This was tested using a FRET biosensor that registers nuclear RanGTP levels, which were reduced in response to sorbitol stress. Although sorbitol alters nucleotide levels, we show that inverting the GTP/GDP ratio in cells is not sufficient to disrupt the Ran gradient. Thus, the Ran system is a target of hyperosmotic stress signaling, and cells use protein localization-based mechanisms as part of a rapid stress response.

Phosphorylation-dependent regulation of nuclear localization and functions of integrin-linked kinase

Integrin-linked kinase (ILK) is a phosphorylated protein that regulates physiological processes that overlap with those regulated by p21-activated kinase 1 (PAK1). Here we report the possible role of ILK phosphorylation by PAK1 in ILK-mediated signaling and intracellular translocation. We found that PAK1 phosphorylates ILK at threonine-173 and serine-246 in vitro and in vivo. Depletion of PAK1 decreased the levels of endogenous ILK phosphorylation in vivo. Mutation of PAK1 phosphorylation sites on ILK to alanine reduced cell motility and cell proliferation. Biochemical fractionation, confocal microscopy, and chromatin-interaction analyses of human cells revealed that ILK localizes predominantly in the cytoplasm but also resides in the nucleus. Transfection of MCF-7 cells with point mutants ILK-T173A, ILK-S246A, or ILK-T173A; S246A (ILK-DM) altered ILK localization. Selective depletion of PAK1 dramatically increased the nuclear and focal point accumulation of ILK, further demonstrating a role for PAK1 in ILK translocation. We also identified functional nuclear localization sequence and nuclear export sequence motifs in ILK, delineated an apparently integral role for ILK in maintaining normal nuclear integrity, and established that ILK interacts with the regulatory region of the CNKSR3 gene chromatin to negatively modulate its expression. Together, these results suggest that ILK is a PAK1 substrate, undergoes phosphorylation-dependent shuttling between the cell nucleus and cytoplasm, and interacts with gene-regulatory chromatin.

A high-throughput screen of cell-death-inducing factors in Nicotiana benthamiana identifies a novel MAPKK that mediates INF1-induced cell death signaling and non-host resistance to Pseudomonas cichorii

A high-throughput overexpression screen of Nicotiana benthamiana cDNAs identified a gene for a mitogen-activated protein kinase kinase (MAPKK) as a potent inducer of the hypersensitive response (HR)-like cell death. NbMKK1 protein is localized to the nucleus, and the N-terminal putative MAPK docking site of NbMKK1 is required for its function as a cell-death inducer. NbMKK1-mediated leaf-cell death was compromised in leaves where NbSIPK expression was silenced by virus-induced gene silencing. A yeast two-hybrid assay showed that NbMKK1 and NbSIPK physically interact, suggesting that NbSIPK is one of the downstream targets of NbMKK1. Phytophthora infestans INF1 elicitor-mediated HR was delayed in NbMKK1-silenced plants, indicating that NbMKK1 is involved in this HR pathway. Furthermore, the resistance of N. benthamiana to a non-host pathogen Pseudomonas cichorii was compromised in NbMKK1-silenced plants. These results demonstrate that MAPK cascades involving NbMKK1 control non-host resistance including HR cell death.

Classical nuclear localization signals: definition, function, and interaction with importin alpha

The best understood system for the transport of macromolecules between the cytoplasm and the nucleus is the classical nuclear import pathway. In this pathway, a protein containing a classical basic nuclear localization signal (NLS) is imported by a heterodimeric import receptor consisting of the beta-karyopherin importin beta, which mediates interactions with the nuclear pore complex, and the adaptor protein importin alpha, which directly binds the classical NLS. Here we review recent studies that have advanced our understanding of this pathway and also take a bioinformatics approach to analyze the likely prevalence of this system in vivo. Finally, we describe how a predicted NLS within a protein of interest can be confirmed experimentally to be functionally important.

Signal integration during development: mechanisms of EGFR and Notch pathway function and cross-talk

Metazoan development relies on a highly regulated network of interactions between conserved signal transduction pathways to coordinate all aspects of cell fate specification, differentiation, and growth. In this review, we discuss the intricate interplay between the epidermal growth factor receptor (EGFR; Drosophila EGFR/DER) and the Notch signaling pathways as a paradigm for signal integration during development. First, we describe the current state of understanding of the molecular architecture of the EGFR and Notch signaling pathways that has resulted from synergistic studies in vertebrate, invertebrate, and cultured cell model systems. Then, focusing specifically on the Drosophila eye, we discuss how cooperative, sequential, and antagonistic relationships between these pathways mediate the spatially and temporally regulated processes that generate this sensory organ. The common themes underlying the coordination of the EGFR and Notch pathways appear to be broadly conserved and should, therefore, be directly applicable to elucidating mechanisms of information integration and signaling specificity in vertebrate systems.

Inhibition of Nuclear Import of Calcineurin Prevents Myocardial Hypertrophy

The time that transcription factors remain nuclear is a major determinant for transcriptional activity. It has recently been demonstrated that the phosphatase calcineurin is translocated to the nucleus with the transcription factor nuclear factor of activated T cells (NF-AT). This study identifies a nuclear localization sequence (NLS) and a nuclear export signal (NES) in the sequence of calcineurin. Furthermore we identified the nuclear cargo protein importinβ 1 to be responsible for nuclear translocation of calcineurin. Inhibition of the calcineurin/importin interaction by a competitive peptide (KQECKIKYSERV), which mimicked the calcineurin NLS, prevented nuclear entry of calcineurin. A noninhibitory control peptide did not interfere with the calcineurin/importin binding. Using this approach, we were able to prevent the development of myocardial hypertrophy. In angiotensin II-stimulated cardiomyocytes, [ 3 H]-leucine incorporation (159%±9 versus 111%±11; P <0.01) and cell size were suppressed significantly by the NLS peptide compared with a control peptide. The NLS peptide inhibited calcineurin/NF-AT transcriptional activity (227%±11 versus 133%±8; P <0.01), whereas calcineurin phosphatase activity was unaffected (298%±9 versus 270%±11; P =NS). We conclude that calcineurin is not only capable of dephosphorylating NF-AT, thus enabling its nuclear import, but the presence of calcineurin in the nucleus is also important for full NF-AT transcriptional activity.

High-Content Screening Analysis of the p38 Pathway: Profiling of Structurally Related p38α Kinase Inhibitors Using Cell-Based Assays

The complexity of the p38 mitogen-activated protein kinase (MAPK) signaling pathway presents challenges to understanding the efficacy of p38 inhibitors. Biochemical recombinant kinase assays and tumor necrosis factor alpha (TNFalpha) secretion assays are typically used to evaluate p38alpha inhibitors, but they do not provide insight into proximal intracellular events. Stimulation of the pathway evokes a cascade of phosphorylation events, accompanied by movement of molecules to different cellular compartments. Herein, we describe the profiling and potency comparison of a large set of p38alpha inhibitors with a pyrimidinone, imidazopyrimidine, or triazolopyrimidine core against biochemical recombinant p38alpha kinase activity, lipopolysaccharide (LPS)-mediated TNFalpha secretion by THP-1 cells, and a set of cellular imaging assays in SW1353 chondrocytes and baby hamster kidney cells. These pathway assays included p38 phosphorylation, MAPK-activated protein kinase 2 translocation, and heat shock protein (HSP) 27 phosphorylation. We established that HSP27 phosphorylation correlates well with LPS-induced TNFalpha secretion, validating our cellular imaging assays. We also found that the choice of cells and inducer can profoundly affect cellular potency results. High-content analysis may reveal signaling details, enriching our understanding of the mechanism of action of p38alpha inhibitors.

Neural progenitor number is regulated by nuclear factor-kappaB p65 and p50 subunit-dependent proliferation rather than cell survival

The number of cells generated by a proliferating stem or precursor cell can be influenced both by proliferation and by the degree of cell death/survival of the progeny generated. In this study, the extent to which cell survival controls progenitor number was examined by comparing the growth characteristics of neurosphere cultures derived from mice lacking genes for the death-inducing Bcl-2 homologue Hara Kiri (Hrk), apoptosis-associated protein 1 (Apaf1), or the prosurvival nuclear factor-kappaB (NFkappaB) subunits p65, p50, or c-rel. We found no evidence that Hrk or Apaf1, and by inference the mitochondrial cell death pathway, are involved in regulating the number of neurosphere-derived progeny. However, we identified the p65p50 NFkappaB dimer as being required for the normal growth and expansion of neurosphere cultures. Genetic loss of both p65 and p50 NFkappaB subunits resulted in a reduced number of progeny but an increased proportion of neurons. No effect on cell survival was observed. This suggests that the number and fate of neural progenitor cells are more strongly regulated by cell cycle control than survival.

Analysis of the influence of subcellular localization of the HIV Rev protein on Rev-dependent gene expression by multi-fluorescence live-cell imaging

The human immunodeficiency virus Rev protein is a post-transcriptional activator of HIV gene expression. Rev is a nucleocytoplasmic shuttle protein that displays characteristic nuclear/nucleolar subcellular localization in various cell lines. Cytoplasmic localization of Rev occurs under various conditions disrupting Rev function. The goal of this study was to investigate the relationship between localization of Rev and its functional activity in living cells. A triple-fluorescent imaging assay, called AQ-FIND, was established for automatic quantitative evaluation of nucleocytoplasmic distribution of fluorescently tagged proteins. This assay was used to screen 500 rev genes generated by error-prone PCR for Rev mutants with different localization phenotypes. Activities of the Rev mutants were determined with a second quantitative, dual-fluorescent reporter assay. In HeLa cells, the majority of nuclear Rev mutants had activities similar to wild-type Rev. The activities of Rev mutants with abnormal cytoplasmic localization ranged from moderately impaired to nonfunctional. There was no linear correlation between subcellular distribution and levels of Rev activity. In astrocytes, nuclear Rev mutants showed similar impaired activities as the cytoplasmic wild-type Rev. Our data suggest that steady-state subcellular localization is not a primary regulator of Rev activity but may change as a secondary consequence of altered Rev function. The methodologies described here have potential for studying the significance of subcellular localization for functions of other regulatory factors.

Cell growth-dependent subcellular localization of p8

p8 is a stress-induced protein, biochemically related to the architectural factor HMG-I/Y, overexpressed in many cancers and required for tumor expansion. The molecular mechanisms by which p8 may exert its effect in aspects of growth is unknown. Using immunocytochemistry, we found that p8 presents nuclear localization in sub-confluent cells, but it localizes throughout the whole cell in high density grown cells. Cells arrested in Go/G1, either by serum deprivation or by hydroxyurea treatment, show a nucleo-cytoplasmic localization of p8, whether in the rest of the cell cycle stages of actively dividing cells the localization is nuclear. A comparison of p8 sequences from human to fly predicts a conserved bipartite nuclear localization sequence (NLS). The putative NLS has been demonstrated to be functional, since nuclear import is energy dependent (inhibited by sodium azide plus 2-deoxyglucose), and fusion proteins GFP-p8 and GFP-NLSp8 localize to the nucleus, whereas GFP-p8NLSmut in which with Lys 65, 69, 76, and 77 mutated to Ala localized to the whole cell. p8 localization does not involve the CRM1 transporter, since it is insensitive to leptomycin B. Inhibitors of MAPK pathways did not affect p8 subcellular localization. The inhibition of deacetylation with Trichostatin A promotes cytoplasmic accumulation of p8. The results suggest that p8 growth stage-dependent localization is regulated by acetylation, that p8 is not free within the cell but forming part of a complex and that it may exert a role in both subcellular localizations.

The nuclear import of protein kinase D3 requires its catalytic activity

The protein kinase D (PKD) family consists of three serine/threonine protein kinases termed PKD, PKD2, and PKD3, which are similar in overall structure and primary amino acid sequence. However, each isozyme displays a distinctive intracellular localization. Taking advantage of the structural homology and opposite nuclear localization of PKD2 and PKD3, we generated an extensive set of chimeric proteins between both isozymes to determine which PKD3 domain(s) mediates its nuclear localization. We found that the C-terminal region of PKD3, which contains its catalytic domain, is necessary but not sufficient for its nuclear localization. Real time imaging of a photoactivatable green fluorescent protein fused to PKD3 revealed that point mutations that render PKD3 catalytically inactive completely prevented its nuclear import despite its interaction with importin alpha and beta. We also found that activation loop phosphorylation of PKD3 did not require its nuclear localization, and it was not sufficient to promote the nuclear import of PKD3. These results identify a novel function for the kinase activity of PKD3 in promoting its nuclear entry and suggest that the catalytic activity of PKD3 may regulate its nuclear import through autophosphorylation and/or interaction with another protein(s).

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

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

Importin 4 Is Responsible for Ligand-independent Nuclear Translocation of Vitamin D Receptor

Vitamin D receptor (VDR) is localized in nuclei and acts as a ligand-dependent transcription factor. To clarify the molecular mechanisms underlying the nuclear translocation of VDR, we utilized an in vitro nuclear transport assay using digitonin-permeabilized semi-intact cells. In this assay, recombinant whole VDR-(4-427) and a truncated mutant VDR-(4-232) lacking the carboxyl terminus of VDR were imported to nuclei even in the absence of ligand. In contrast, VDR-(91-427) lacking the amino-terminal DNA-binding domain was not imported to nuclei in the absence of ligand, and was efficiently imported in its liganded form. These results suggested that there are two distinct mechanisms underlying the nuclear transport of VDR; ligand-dependent and -independent pathways, and that the different regions of VDR are responsible for these processes. Therefore, we performed the yeast two-hybrid screening using VDR-(4-232) as the bait to explore the molecules responsible for ligand-independent nuclear translocation of VDR, and have identified importin 4 as an interacting protein. In the reconstruction experiments where transport factors were applied as recombinant proteins, recombinant importin 4 facilitated nuclear translocation of VDR regardless of its ligand, whereas importin beta failed in transporting VDR even in the presence of ligand. In conclusion, importin 4, not importin beta, is responsible for the ligand-independent nuclear translocation of VDR.

Regulated cellular partitioning of SR protein-specific kinases in mammalian cells

Reversible phosphorylation of the SR family of splicing factors plays an important role in pre-mRNA processing in the nucleus. Interestingly, the SRPK family of kinases specific for SR proteins is localized in the cytoplasm, which is critical for nuclear import of SR proteins in a phosphorylation-dependent manner. Here, we report molecular dissection of the mechanism involved in partitioning SRPKs in the cytoplasm. Common among all SRPKs, the bipartite kinase catalytic core is separated by a unique spacer sequence. The spacers in mammalian SRPK1 and SRPK2 share little sequence homology, but they function interchangeably in restricting the kinases in the cytoplasm. Removal of the spacer in SRPK1 had little effect on the kinase activity, but it caused a quantitative translocation of the kinase to the nucleus and consequently induced aggregation of splicing factors in the nucleus. Rather than carrying a nuclear export signal as suggested previously, we found multiple redundant signals in the spacer that act together to anchor the kinase in the cytoplasm. Interestingly, a cell cycle signal induced nuclear translocation of the kinase at the G2/M boundary. These findings suggest that SRPKs may play an important role in linking signaling to RNA metabolism in higher eukaryotic cells.

Quantitation of p53 nuclear relocation in response to stress using a yeast functional assay: effects of irradiation and modulation by heavy metal ions

Many regulatory proteins undergo transient nuclear relocation under physical or chemical stress. This phenomenon is, however, difficult to assess due to the lack of sensitive and standardized biological assays. Here, we describe a new quantitative nuclear relocation assay (QNR), based on expression in yeasts of chimeric proteins in which an artificial transcription factor is fused to a target protein acting as driver for relocation. This assay combines the experimental versatility of yeast with quantitation of nuclear relocation at low levels of protein expression. We have assessed the nuclear relocation of yeast Yap1 and human p53, two transcription factors that relocate to the nucleus in response to oxidative-stress and DNA damage, respectively. We show that p53 efficiently drives the relocation of the chimeric reporter in response to irradiation and that this process requires the C-terminal nuclear export signal (NES). Cd2+ and Hg2+, two metal ions inducing DNA damage as well as conformational changes in p53, have opposite effects on p53 relocation in response to DNA damage. Whereas Hg2+ effects are synergistic to DNA damage, Cd2+ inhibits relocation and sequesters p53 into the cytoplasm. These results demonstrate the effectiveness of QNR to investigate the regulation of p53 shuttling in response to stress signals including suspected environmental carcinogens.

Synergistic operation of four cis-acting elements mediate high level DAL5 transcription in Saccharomyces cerevisiae

The Saccharomyces cerevisiae allantoate/ureidosuccinate permease gene (DAL5) is often used as a reporter in studies of the Tor1/2 protein kinases which are specifically inhibited by the clinically important immunosuppressant and anti-neoplastic drug, rapamycin. To date, only a single type of cis-acting element has been shown to be required for DAL5 expression, two copies of the GATAA-containing UAS(NTR) element that mediates nitrogen catabolite repression-sensitive transcription. UAS(NTR) is the binding site for the transcriptional activator, Gln3 whose intracellular localization responds to the nitrogen supply, accumulating in the nuclei of cells provided with poor nitrogen sources and in the cytoplasm when excess nitrogen is available. Recent data raised the possibility that DAL5 might also be regulated by the retrograde system responsible for control of early TCA cycle gene expression, prompting us to investigate the structure of the DAL5 promoter in more detail. Here, we show that clearly one (UAS(B)), and possibly two (UAS(A)), additional cis-acting elements are required for full DAL5 expression. One of these elements (UAS(B)) is in a region that is heavily protected from DNaseI digestion and functions in a highly synergistic manner with the two UAS(NTR) elements. Cis-acting elements UAS(NTR)-UAS(A) and UAS(NTR)-UAS(B) are situated on the same face of the DNA two and one turn apart, respectively. We also found that decreased DAL5 expression in glutamate-grown cells, a characteristic shared with retrograde regulation, likely derives from decreased nuclear Gln3 levels that occur under these growth conditions rather than direct retrograde system control.

Effects of celecoxib on cell proliferation and ERK2 expression of ERK2 in gastric cancer cells

AIM: To explore the role of celecoxib, a selective COX-2 inhibitor, in the proliferation of human gastric carcinoma cells SGC7901, and to investigate the relationship between cell proliferation and the expression of extracellular signal-regulated kinase 2 (ERK2).

METHODS: The cell proliferation of SGC7901 cells was measured by MTT assay, and the apoptoticsis rate of the cells was examined by TUNEL staining method and flow cytometry. The expression of phosphated ERK2 was detected by immunohistochemistry and quantified measured by image analysis systems.

RESULTS: Celecoxib inhibited the proliferation of SGC7901 gastric carcinoma cells and the expression of phosphated ERK2, as well as induced the apoptosis of the SGC-7901 cells in a dose-dependent manner. The growth inhibitory rates of SGC-7901 cells treated with 10, 20, 40, 80 and, 160 μmol/L celecoxib for 48 h were 9.8%, 30%, 58.9%, 76.3%, and 88.3%, respectively. The SGC7901 cells presented typical morphological features of apoptosis by TUNEL staining. By flow cytometry, the apoptosistic rates of SGC-7901 cells (4.23 ± 0.81%, 15.50 ± 2.10%, 24.35 ± 2.32%, 31.52±3.64%, and 45.82±5.92% for 10, 20, 40, 80 and 160 μmol/L celecoxib, respectively) were significantly higher than those inof the control group(1.85 ±0.31%, P < 0.01). The optical density value of ERK2 of SGC7901 cells (10, 20, 40, 80 and 160 μmol/L celecoxib: 2.96 ± 0.24, P >0.05; 2.56 ± 0.24, P<0.05; 2.04 ± 0.20, P < 0.01; 1.68 ± 0.16, P < 0.01; 1.52 ± 0.09, P < 0.01) were significantly lower than that ofin the control group (3.32 ± 0.28).

CONCLUSION: Celecoxib can inhibit the growth and induce the apoptosis of SGC7901 cells, and its mechanism may and induce the cell apoptosis. This effect may relate to the inhibition of ERK2 kinase signaling pathway.

Cytoplasmic retention of peroxide-activated ERK provides survival in primary cultures of rat hepatocytes

Reactive oxygen species (ROS) are implicated in tissue damage causing primary hepatic dysfunction following ischemia/reperfusion injury and during inflammatory liver diseases. A potential role of extracellular signal-regulated kinase (ERK) as a mediator of survival signals during oxidative stress was investigated in primary cultures of hepatocytes exposed to ROS. Hydrogen peroxide (H(2)O(2)) induced a dose-dependent activation of ERK, which was dependent on MEK activation. The ERK activation pattern was transient compared with the ERK activation seen after stimulation with epidermal growth factor (EGF). Nuclear accumulation of ERK was found after EGF stimulation, but not after H(2)O(2) exposure. A slow import/rapid export mechanism was excluded through the use of leptomycin B, an inhibitor of nuclear export sequence-dependent nuclear export. Reduced survival of hepatocytes during ROS exposure was observed when ERK activation was inhibited. Ribosomal S6 kinase (RSK), a cytoplasmic ERK substrate involved in cell survival, was activated and located in the nucleus of H(2)O(2)-exposed hepatocytes. The activation was abolished when ERK was inhibited with U0126. In conclusion, our results indicate that activity of ERK in the cytoplasm is important for survival during oxidative stress in hepatocytes and that RSK is activated downstream of ERK. Supplementary material for this article can be found on the HEPATOLOGY website (http://www.interscience.wiley.com/jpages/0270-9139/suppmat/index.html).

Analysis of nuclear transport signals in the human apurinic/apyrimidinic endonuclease (APE1/Ref1)

The mammalian abasic-endonuclease1/redox-factor1 (APE1/Ref1) is an essential protein whose subcellular distribution depends on the cellular physiological status. However, its nuclear localization signals have not been studied in detail. We examined nuclear translocation of APE1, by monitoring enhanced green fluorescent protein (EGFP) fused to APE1. APE1's nuclear localization was significantly decreased by deleting 20 amino acid residues from its N-terminus. Fusion of APE1's N-terminal 20 residues directed nuclear localization of EGFP. An APE1 mutant lacking the seven N-terminal residues (ND7 APE1) showed nearly normal nuclear localization, which was drastically reduced when the deletion was combined with the E12A/D13A double mutation. On the other hand, nearly normal nuclear localization of the full-length E12A/D13A mutant suggests that the first 7 residues and residues 8–13 can independently promote nuclear import. Both far-western analyses and immuno-pull-down assays indicate interaction of APE1 with karyopherin alpha 1 and 2, which requires the 20 N-terminal residues and implicates nuclear importins in APE1's nuclear translocation. Nuclear accumulation of the ND7 APE1(E12A/D13A) mutant after treatment with the nuclear export inhibitor leptomycin B suggests the presence of a previously unidentified nuclear export signal, and the subcellular distribution of APE1 may be regulated by both nuclear import and export.

Role of hsp90 and the hsp90-binding immunophilins in signalling protein movement

The ubiquitous protein chaperone hsp90 has been shown to regulate more than 100 proteins involved in cellular signalling. These proteins are called 'client proteins' for hsp90, and a multiprotein hsp90/hsp70-based chaperone machinery forms client protein.hsp90 heterocomplexes in the cytoplasm and the nucleus. In the case of signalling proteins that act as transcription factors, the client protein.hsp90 complexes also contain one of several TPR domain immunophilins or immunophilin homologs that bind to a TPR domain binding site on hsp90. Using several intracellular receptors and the tumor suppressor p53 as examples, we review evidence that dynamic assembly of heterocomplexes with hsp90 is required for rapid movement through the cytoplasm to the nucleus along microtubular tracks. The role of the immunophilin in this system is to connect the client protein.hsp90 complex to cytoplasmic dynein, the motor protein for retrograde movement toward the nucleus. Upon arrival at the nuclear pores, the receptor.hsp90.immunophilin complexes are transferred to the nuclear interior by importin-dependent facilitated diffusion. The unliganded receptors then distribute within the nucleus to diffuse patches from which they proceed in a ligand-dependent manner to discrete nuclear foci where chromatin binding occurs. We review evidence that dynamic assembly of heterocomplexes with hsp90 is required for movement to these foci and for the dynamic exchange of transcription factors between chromatin and the nucleoplasm.

Prostaglandin D2 induces nuclear import of the sex-determining factor SOX9 via its cAMP-PKA phosphorylation

During mammalian gonadal development, nuclear import/export of the transcription factor SOX9 is a critical step of the Sry-initiated testis-determining cascade. In this study, we identify a molecular mechanism contributing to the SOX9 nuclear translocation in NT2/D1 cells, which is mediated by the prostaglandin D2 (PGD2) signalling pathway via stimulation of its adenylcyclase-coupled DP1 receptor. We find that activation of cAMP-dependent protein kinase A (PKA) induces phosphorylation of SOX9 on its two S64 and S181 PKA sites, and its nuclear localization by enhancing SOX9 binding to the nucleocytoplasmic transport protein importin beta. Moreover, in embryonic gonads, we detect a male-specific prostaglandin D synthase expression and an active PGD2 signal at the time and place of SOX9 expression. We thus propose a new step in the sex-determining cascade where PGD2 acts as an autocrine factor inducing SOX9 nuclear translocation and subsequent Sertoli cell differentiation.

MOLECULAR AND EVOLUTIONARY BASIS OF THE CELLULAR STRESS RESPONSE

The cellular stress response is a universal mechanism of extraordinary physiological/pathophysiological significance. It represents a defense reaction of cells to damage that environmental forces inflict on macromolecules. Many aspects of the cellular stress response are not stressor specific because cells monitor stress based on macromolecular damage without regard to the type of stress that causes such damage. Cellular mechanisms activated by DNA damage and protein damage are interconnected and share common elements. Other cellular responses directed at re-establishing homeostasis are stressor specific and often activated in parallel to the cellular stress response. All organisms have stress proteins, and universally conserved stress proteins can be regarded as the minimal stress proteome. Functional analysis of the minimal stress proteome yields information about key aspects of the cellular stress response, including physiological mechanisms of sensing membrane lipid, protein, and DNA damage; redox sensing and regulation; cell cycle control; macromolecular stabilization/repair; and control of energy metabolism. In addition, cells can quantify stress and activate a death program (apoptosis) when tolerance limits are exceeded.

Evidence that C-terminal non-kinase domain of Pbs2p has a role in high osmolarity-induced nuclear localization of Hog1p

Mitogen-activated protein kinase (MAPK) cascade is a ubiquitous signaling module that transmits extracellular stimuli through the cytoplasm to the nucleus. In baker's yeast external high osmolarity activates high osmolarity glycerol (HOG) MAPK pathway which consists of two upstream branches (SHO1 and SLN1) and common downstream elements Pbs2p MAPKK and Hog1p MAPK. Activation of this pathway causes rapid nuclear accumulation of Hog1p, essentially leading to the expression of target genes. Previously we have isolated a PBS2 homologue (DPBS2) from osmo-tolerant and salt-tolerant yeast Debaryomyces hansenii that partially complemented pbs2 mutation in Saccharomyces cerevisiae. Here we show that by replacing C-terminal region of Dpbs2p with the homologous region of Pbs2p we could abrogate partial complementation exhibited by Dpbs2p and this was achieved due to increase in nuclear translocation of Hog1p. Thus, our result showed that in HOG pathway, MAPKK has important role in nuclear translocation of Hog1p.

Glutaredoxin: role in reversible protein s-glutathionylation and regulation of redox signal transduction and protein translocation

Reversible posttranslational modifications on specific amino acid residues can efficiently regulate protein functions. O-Phosphorylation is the prototype and analogue to the rapidly emerging mechanism of regulation known as S-glutathionylation. The latter is being recognized as a potentially widespread form of modulation of the activities of redox-sensitive thiol proteins, especially those involved in signal transduction pathways and translocation. The abundance of reduced glutathione in cells and the ready conversion of sulfenic acids and S-nitroso derivatives to S-glutathione mixed disulfides support the notion that reversible S-glutathionylation is likely to be the preponderant mode of redox signal transduction. The glutaredoxin enzyme has served as a focal point and important tool for evolution of this regulatory mechanism because of its characterization as a specific and efficient catalyst of protein-SSG de-glutathionylation (akin to phosphatases). Identification of specific mechanisms and enzyme(s) that catalyze formation of protein-SSG intermediates, however, is largely unknown and represents a prime objective for furthering understanding of this evolving mechanism of cellular regulation. Several proteomic approaches, including the use of cysteine-reactive fluorescent and radiolabel probes, have been developed to detect arrays of proteins whose cysteine residues are modified in response to oxidants, thus identifying them as potential interconvertible proteins to be regulated by redox signaling (glutathionylation). Specific criteria were used to evaluate current data on cellular regulation via S-glutathionylation. Among many proteins under consideration, actin, protein tyrosine phosphatase-1B, and Ras stand out as the best current examples for establishing this regulatory mechanism.

The LIM domain: from the cytoskeleton to the nucleus

First described 15 years ago as a cysteine-rich sequence that was common to a small group of homeodomain transcription factors, the LIM domain is now recognized as a tandem zinc-finger structure that functions as a modular protein-binding interface. LIM domains are present in many proteins that have diverse cellular roles as regulators of gene expression, cytoarchitecture, cell adhesion, cell motility and signal transduction. An emerging theme is that LIM proteins might function as biosensors that mediate communication between the cytosolic and the nuclear compartments.

The CRM1 nuclear export receptor controls pathological cardiac gene expression

Diverse pathological insults trigger a cardiac remodeling process during which myocytes undergo hypertrophy, with consequent decline in cardiac function and eventual heart failure. Multiple transcriptional regulators of pathological cardiac hypertrophy are controlled at the level of subcellular distribution. For example, prohypertrophic transcription factors belonging to the nuclear factor of activated T cells (NFAT) and GATA families are subject to CRM1-dependent nuclear export but are rapidly relocalized to the nucleus in response to cues for hypertrophic growth. Here, we demonstrate that the antihypertrophic chromatin-modifying enzyme histone deacetylase 5 (HDAC5) is shuttled out of the cardiomyocyte nucleus via a CRM1-mediated pathway in response to diverse signals for hypertrophy. CRM1 antagonists block the agonist-mediated nuclear export of HDAC 5 and repress pathological gene expression and associated hypertrophy of cultured cardiomyocytes. Conversely, CRM1 activity is dispensable for nonpathological cardiac gene activation mediated by thyroid hormone and insulin-like growth factor 1, agonists that fail to trigger the nuclear export of HDAC5. These results suggest a selective role for CRM1 in derepression of pathological cardiac genes via its neutralizing effects on antihypertrophic factors such as HDAC5. Pharmacological approaches targeting CRM1-dependent nuclear export in heart muscle may have salutary effects on cardiac function by suppressing maladaptive changes in gene expression evoked by stress signals.

Live Cell Imaging of ERK and MEK: simple binding equilibrium explains the regulated nucleocytoplasmic distribution of ERK

In response to epidermal growth factor (EGF), the mitogen-activated protein kinase ERK2 translocates into the nucleus. To probe the mechanisms regulating the subcellular localization of ERK2, we used live cell imaging to examine the interaction between MEK1 and ERK2. Fluorescence resonance energy transfer (FRET) studies show that MEK1 and ERK2 directly interact and demonstrate that this interaction in the cytoplasm is largely responsible for cytoplasmic retention of ERK2. Stimulation with EGF caused loss of FRET as ERK separated from MEK and moved into the nucleus. FRET was recovered as ERK returned to the cytosol, indicating ERK reassociation with MEK in the cytoplasm. The EGF-induced transit of ERK through the nucleus was complete within 20 min, and there was no significant movement of MEK into the nucleus. Fluorescence recovery after photobleaching experiments was used to assess the rate of movement of MEK and ERK. The steady-state rate of ERK entry into the nucleus in resting cells was energy-independent and greater than the rate of ERK entry upon EGF stimulation. This suggests that the rate constant for ERK transport across the nuclear membrane is not limiting nuclear entry. Thus, we suggest that the movement of ERK into and out of the nucleus in response to agonist occurs primarily by diffusion and is controlled by interactions with binding partners in the cytosol and nucleus. No evidence of ERK dimerization was detected by FRET methods; the kinetics for nucleocytoplasmic transport were unaffected by mutations in the ERK putative dimerization domain.

Brap2 functions as a cytoplasmic retention protein for p21 during monocyte differentiation

The cell cycle inhibitor p21 plays an important role in monocytic cell differentiation, during which it translocates from the nucleus to cytoplasm. This process involves the negative regulation of the p21 nuclear localization signal (NLS). Here, we sought to determine the relationship between the cytoplasmic translocation of p21 and another molecule, Brap2, a cytoplasmic protein which binds the NLS of BRCA1 and was recently reported to inactivate KSR in the Ras-activating signal pathway under the name of IMP. We report that p21 and Brap2 directly interact, both in vitro and in vivo, in a manner requiring the NLS of p21 and the C-terminal portion of Brap2. When it is cotransfected with Brap2, p21 is expressed in the cytoplasm. Monocytic differentiation of the promyelomonocytic cell lines U937 and HL60 is associated with the upregulation of Brap2 expression concomitantly with the upregulation and cytoplasmic relocalization of p21. Our results underscore the role played by Brap2 in the process of cytoplasmic translocation of p21 during monocyte differentiation.

Nesca, a novel adapter, translocates to the nuclear envelope and regulates neurotrophin-induced neurite outgrowth

We provide the first characterization of a novel signaling adapter, Nesca, in neurotrophic signal transduction. Nesca contains a RUN domain, a WW domain, a leucine zipper, a carboxyl-terminal SH3 domain, and several proline-rich regions. Nesca is highly expressed in the brain, is serine phosphorylated, and mobilizes from the cytoplasm to the nuclear membrane in response to neurotrophin, but not epidermal growth factor, stimulation in a MEK-dependent process. Overexpression studies in PC12 cells indicate that Nesca facilitates neurotrophin-dependent neurite outgrowth at nonsaturating doses of nerve growth factor (NGF). Similarly, short interfering RNA studies significantly reduce NGF-dependent neuritogenesis in PC12 cells. Mutational analyses demonstrate that the RUN domain is an important structural determinant for the nuclear translocation of Nesca and that the nuclear redistribution of Nesca is essential to its neurite outgrowth-promoting properties. Collectively, these works provide the first functional characterization of Nesca in the context of neurotrophin signaling and suggest that Nesca serves a novel, nuclear-dependent role in neurotrophin-dependent neurite outgrowth.

Inducible translocation trap: a system for detecting inducible nuclear translocation

Here we report a general system, inducible translocation trap (ITT), for identification of proteins that translocate into the nucleus following signal transduction from cell surface receptors. ITT consists of a retroviral cDNA expression library of fusion proteins consisting of a LexA DNA binding domain, the transactivation domain of a transcriptional activator, and proteins encoded by cDNA inserts. The retroviral library is then transduced into cell lines containing a reporter gene with LexA binding sites in its promoter. Cells expressing the reporter gene by extracellular stimuli are then selected by flow-cytometric sorting. By using ITT, we identified cDNA encoding Stat1 in a screen of proteins which translocate into the nucleus by IFNgamma, indicating that this system can be used for isolation of nuclear translocating proteins induced by extracellular stimuli. ITT may be a useful tool for dissecting dynamic translocation in various biological systems.

Asr1p, a novel yeast ring/PHD finger protein, signals alcohol stress to the nucleus

During fermentation, yeast cells are exposed to increasing amounts of alcohol, which is stressful and affects both growth and viability. On the molecular level, numerous aspects of alcohol stress signaling remain unresolved. We have identified a novel yeast Ring/PHD finger protein that constitutively shuttles between nucleus and cytoplasm but accumulates in the nucleus upon exposure to ethanol, 2-propanol, or 1-butanol. Subcellular localization of this protein is not altered by osmotic, oxidative, or heat stress or during nitrogen or glucose starvation. Because of its exclusive sensitivity to environmental alcohol, the protein was called Asr1p for Alcohol Sensitive Ring/PHD finger 1 protein. Nuclear accumulation of Asr1p is rapid, reversible, and requires a functional Ran/Gsp1p gradient. Asr1p contains two N terminally located leucine-rich nuclear export sequences (NES) required for nuclear export. Consistently, it accumulates in the nucleus of xpo1-1 cells at restrictive temperature and forms a trimeric complex with the exportin Xpo1p and Ran-GTP. Deletion of ASR1 leads to sensitivity in growth on medium containing alcohol or detergent, consistent with a function of Asr1p in alcohol-related signaling. Asr1p is the first reported protein that changes its subcellular localization specifically upon exposure to alcohol and therefore represents a key element in the analysis of alcohol-responsive signaling.

Glutathione, Altruistic Metabolite in Fungi

Glutathione (GSH; gamma-L-glutamyl-L-cysteinyl-glycine), a non-protein thiol with a very low redox potential (E'0 = 240 mV for thiol-disulfide exchange), is present in high concentration up to 10 mM in yeasts and filamentous fungi. GSH is concerned with basic cellular functions as well as the maintenance of mitochondrial structure, membrane integrity, and in cell differentiation and development. GSH plays key roles in the response to several stress situations in fungi. For example, GSH is an important antioxidant molecule, which reacts non-enzymatically with a series of reactive oxygen species. In addition, the response to oxidative stress also involves GSH biosynthesis enzymes, NADPH-dependent GSH-regenerating reductase, glutathione S-transferase along with peroxide-eliminating glutathione peroxidase and glutaredoxins. Some components of the GSH-dependent antioxidative defence system confer resistance against heat shock and osmotic stress. Formation of protein-SSG mixed disulfides results in protection against desiccation-induced oxidative injuries in lichens. Intracellular GSH and GSH-derived phytochelatins hinder the progression of heavy metal-initiated cell injuries by chelating and sequestering the metal ions themselves and/or by eliminating reactive oxygen species. In fungi, GSH is mobilized to ensure cellular maintenance under sulfur or nitrogen starvation. Moreover, adaptation to carbon deprivation stress results in an increased tolerance to oxidative stress, which involves the induction of GSH-dependent elements of the antioxidant defence system. GSH-dependent detoxification processes concern the elimination of toxic endogenous metabolites, such as excess formaldehyde produced during the growth of the methylotrophic yeasts, by formaldehyde dehydrogenase and methylglyoxal, a by-product of glycolysis, by the glyoxalase pathway. Detoxification of xenobiotics, such as halogenated aromatic and alkylating agents, relies on glutathione S-transferases. In yeast, these enzymes may participate in the elimination of toxic intermediates that accumulate in stationary phase and/or act in a similar fashion as heat shock proteins. GSH S-conjugates may also form in a glutathione S-transferases-independent way, e.g. through chemical reaction between GSH and the antifugal agent Thiram. GSH-dependent detoxification of penicillin side-chain precursors was shown in Penicillium sp. GSH controls aging and autolysis in several fungal species, and possesses an anti-apoptotic feature.

Gln3 phosphorylation and intracellular localization in nutrient limitation and starvation differ from those generated by rapamycin inhibition of Tor1/2 in Saccharomyces cerevisiae

The ability of the cell to sense environmental conditions and alter gene expression in response to them is critical to its survival. In Saccharomyces cerevisiae, the Tor1/2 serine/threonine kinases are global regulators situated at the top of a signal cascade reported to receive and transmit nutritional signals associated with the nitrogen supply of the cell. At the other end of that cascade is Gln3, one of two transcriptional activators responsible for most nitrogen catabolic gene expression. When nitrogen is in excess, Tor1/2 are active, and Gln3 is phosphorylated and localizes to the cytoplasm. If Tor1/2 are inhibited by rapamycin or mutation, Gln3 becomes dephosphorylated, accumulates in the nucleus, and mediates nitrogen catabolite repression (NCR)-sensitive transcription. The observations that Gln3 also accumulates in the nuclei of cells provided with poor nitrogen sources or during nitrogen starvation has led to the conclusion that Tor1/2 control intracellular Gln3 localization and NCR-sensitive transcription by regulating Gln3 phosphorylation/dephosphorylation. To test this model, we compared Gln3 phosphorylation states and intracellular localizations under a variety of physiological conditions known to elicit different levels of NCR-sensitive transcription. Our data indicate that: (i) observable Gln3 phosphorylation levels do not correlate in a consistent way with the quality or quantity of the nitrogen source provided, the intracellular localization of Gln3, or the capacity to support NCR-sensitive transcription. (ii) Gln3-Myc(13) is hyperphosphorylated during nitrogen and carbon starvation, but this uniform response does not correlate with Gln3 intracellular localization. (iii) Gln3-Myc(13) dephosphorylation and nuclear localization correlate with one another at early but not late times after rapamycin treatment. These data suggest that rapamycin treatment and growth with poor nitrogen sources bring about nuclear accumulation of Gln3 but likely do so by different mechanisms or by a common mechanism involving molecules other than Gln3 and/or other than the levels of Gln3-Myc(13) phosphorylation thus far detected by others and ourselves.

Mechanisms of alternative pre-messenger RNA splicing

Alternative pre-mRNA splicing is a central mode of genetic regulation in higher eukaryotes. Variability in splicing patterns is a major source of protein diversity from the genome. In this review, I describe what is currently known of the molecular mechanisms that control changes in splice site choice. I start with the best-characterized systems from the Drosophila sex determination pathway, and then describe the regulators of other systems about whose mechanisms there is some data. How these regulators are combined into complex systems of tissue-specific splicing is discussed. In conclusion, very recent studies are presented that point to new directions for understanding alternative splicing and its mechanisms.

Nuclear export of ERK3 by a CRM1-dependent mechanism regulates its inhibitory action on cell cycle progression

Extracellular signal-regulated kinase 3 (ERK3) is an atypical member of the mitogen-activated protein kinase family of serine/threonine kinases. Little is known on the regulation of ERK3 function. Here, we report that ERK3 is constitutively localized in the cytoplasmic and nuclear compartments. In contrast to other mitogen-activated protein kinases, the cellular distribution of ERK3 remains unchanged in response to common mitogenic or stress stimuli and is independent of the enzymatic activity or phosphorylation of the kinase. The cytoplasmic localization of ERK3 is directed by a CRM1-dependent nuclear export mechanism. Treatment of cells with leptomycin B causes the nuclear accumulation of ERK3 in a high percentage of cells. Moreover, ectopic expression of CRM1 promotes the cytoplasmic relocalization of ERK3, whereas overexpression of snurportin 1, which binds CRM1 with high affinity, inhibits the nuclear export of ERK3. We also show that CRM1 binds to ERK3 in vitro. Importantly, we show that enforced localization of ERK3 in the nucleus or cytoplasm markedly attenuates the ability of the kinase to induce cell cycle arrest in fibroblasts. Our results suggest that nucleocytoplasmic shuttling of ERK3 is required for its negative regulatory effect on cell cycle progression.

Caspase-activated PAK-2 is regulated by subcellular targeting and proteasomal degradation

p21-activated protein kinases (PAKs) are a family of serine/threonine protein kinases that are activated by binding of the p21 G proteins Cdc42 or Rac. The ubiquitous PAK-2 (gamma-PAK) is unique among the PAK isoforms because it is also activated through proteolytic cleavage by caspases or caspase-like proteases. In response to stress stimulants such as tumor necrosis factor alpha or growth factor withdrawal, PAK-2 is activated as a full-length enzyme and as a proteolytic PAK-2p34 fragment. Activation of full-length PAK-2 stimulates cell survival, whereas proteolytic activation of PAK-2p34 is involved in programmed cell death. Here we provide evidence that the proapoptotic effect of PAK-2p34 is regulated by subcellular targeting and degradation by the proteasome. Full-length PAK-2 is localized in the cytoplasm, whereas the proteolytic PAK-2p34 fragment translocates to the nucleus. Subcellular localization of PAK-2 is regulated by nuclear localization and nuclear export signal motifs. A nuclear export signal motif within the regulatory domain prevents nuclear localization of full-length PAK-2. Proteolytic activation removes most of the regulatory domain and disrupts the nuclear export signal. The activated PAK-2p34 fragment contains a nuclear localization signal and translocates to the nucleus. However, levels of activated PAK-2p34 are tightly regulated through ubiquitination and degradation by the proteasome. Inhibition of degradation by blocking polyubiquitination results in significantly increased levels of PAK-2p34 and as a consequence, in stimulation of programmed cell death. Therefore, nuclear targeting and inhibition of degradation appear to be critical for stimulation of the cell death response by PAK-2p34.

Nucleo-cytoplasmic partitioning of proteins in plants: implications for the regulation of environmental and developmental signalling

Considerable progress has been made in the past few years in characterising Arabidopsis nuclear transport receptors and in elucidating plant signal transduction pathways that employ nucleo-cytoplasmic partitioning of a member of the signal transduction chain. This review briefly introduces the major principles of nuclear transport of macromolecules across the nuclear envelope and the proteins involved, as they have been described in vertebrates and yeast. Proteins of the plant nuclear transport machinery that have been identified to date are discussed, the focus being on Importin beta-like nuclear transport receptors. Finally, the importance of nucleo-cytoplasmic partitioning as a regulatory tool for signalling is highlighted, and different plant signal transduction pathways that make use of this regulatory potential are presented.

Differential expression of rod photoreceptor cGMP-phosphodiesterase alpha and beta subunits: mRNA and protein levels

The catalytic core of photoreceptor-specific cGMP-phosphodiesterase (PDE) consists of two subunits, PDEalpha and PDEbeta, that are homologous and have similar domain organization but are encoded by different genes. We have examined the PDEalpha and PDEbeta mRNA steady-state and protein levels as well as the biosynthesis rate of these proteins in developing and fully differentiated retinas. We have also determined the translational efficiency of PDE subunits and the role of their mRNA structures in regulating protein synthesis. In mature retinas, PDEalpha and PDEbeta are represented by approximately 1.5 x 108 and 7.5 x 108 copies/microg retinal mRNA, respectively. The levels of these transcripts in developing photoreceptors (P10) are approximately 75% of those at P30. Quantification of protein concentration indicated that PDEalpha and PDEbeta are equally expressed in developing and fully differentiated photoreceptors. Furthermore, the PDEalpha/PDEbeta ratios obtained throughout a 2-h pulsechase period revealed a similar turnover rate for both subunits. The observed discordance between the mRNA and protein levels of PDEalpha and PDEbeta suggested post-transcriptional regulation of their expression. We found that PDEalpha mRNA is translated more efficiently than either of the two PDEbeta transcripts expressed in retina. Therefore, the lower level of PDEalpha mRNA is compensated by its more efficient translation to achieve equimolar expression with PDEbeta. We also analyzed the effect of PDEalpha and PDEbeta mRNA 5'- and 3'-untranslated regions as well as that of their coding regions on protein synthesis. We determined that the PDE-coding regions play a critical role in the differential translation of these subunits.

c-Jun is regulated by combination of enhanced expression and phosphorylation in acute-overloaded rat heart

The transient increase in the expression of transcription factors encoded by immediate-early genes has been considered to play a critical role in the coordination of early gene expression during the hypertrophic growth of cardiac myocytes. Here, we investigated the regulation of c-Jun and its upstream activators JNKs in the myocardium of rats subjected to acute pressure overload induced by transverse aortic constriction. Western blotting and immunohistochemistry analysis demonstrated that both JNK1 and JNK2 were transiently activated by pressure overload, but only JNK1 was activated at the nuclei of cardiac myocytes. JNK1 activation was paralleled by phosphorylation of c-Jun at serine-63 in the myocardial nuclear fraction and by an increase in c-Jun expression in cardiac myocytes. A consistent increase in DNA binding of activator protein-1 (AP-1) complex was observed after 10 and 30 min of pressure overload and Supershift assays confirmed that c-Jun was a major component of activated AP-1 complex. Moreover, experiments performed with the specific JNK inhibitor SP-600125 abolished c-Jun phosphorylation and markedly attenuated its expression as well as the expression of the fetal gene beta-myosin heavy chain. Overall, these findings demonstrate a molecular basis for load-induced activation of c-Jun in cardiac myocytes and its connection with the regulation of fetal gene, characteristic of the acute response to pressure overload.