Duck enteritis virus pUL47, as a late structural protein localized in the nucleus, mainly depends on residues 40 to 50 and 768 to 777 and inhibits IFN-β signalling by interacting with STAT1

Duck enteritis virus (DEV) is a member of the Alphaherpesvirinae subfamily. The characteristics of some DEV genes have been reported. However, information regarding the DEV UL47 gene is limited. In this study, we identified the DEV UL47 gene encoding a late structural protein located in the nucleus of infected cells. We further found that two domains of DEV pUL47, amino acids (aa) 40 to 50 and 768 to 777, could function as nuclear localization sequence (NLS) to guide the nuclear localization of pUL47 and nuclear translocation of heterologous proteins, including enhanced green fluorescent protein (EGFP) and beta-galactosidase (β-Gal). Moreover, pUL47 significantly inhibited polyriboinosinic:polyribocytidylic acid [poly(I:C)]-induced interferon beta (IFN-β) production and downregulated interferon-stimulated gene (ISG) expression, such as Mx and oligoadenylate synthetase-like (OASL), by interacting with signal transducer and activator of transcription-1 (STAT1).

Reduced expression of AtNUP62 nucleoporin gene affects auxin response in Arabidopsis

BACKGROUND

The plant nuclear pore complex has strongly attracted the attention of the scientific community during the past few years, in particular because of its involvement in hormonal and pathogen/symbiotic signalling. In Arabidopsis thaliana, more than 30 nucleoporins have been identified, but only a few of them have been characterized. Among these, AtNUP160, AtNUP96, AtNUP58, and AtTPR have been reported to modulate auxin signalling, since corresponding mutants are suppressors of the auxin resistance conferred by the axr1 (auxin-resistant) mutation. The present work is focused on AtNUP62, which is essential for embryo and plant development. This protein is one of the three nucleoporins (with AtNUP54 and AtNUP58) of the central channel of the nuclear pore complex.

RESULTS

AtNUP62 promoter activity was detected in many organs, and particularly in the embryo sac, young germinating seedlings and at the adult stage in stipules of cauline leaves. The atnup62-1 mutant, harbouring a T-DNA insertion in intron 5, was identified as a knock-down mutant. It displayed developmental phenotypes that suggested defects in auxin transport or responsiveness. Atnup62 mutant plantlets were found to be hypersensitive to auxin, at the cotyledon and root levels. The phenotype of the AtNUP62-GFP overexpressing line further supported the existence of a link between AtNUP62 and auxin signalling. Furthermore, the atnup62 mutation led to an increase in the activity of the DR5 auxin-responsive promoter, and suppressed the auxin-resistant root growth and leaf serration phenotypes of the axr1 mutant.

CONCLUSION

AtNUP62 appears to be a major negative regulator of auxin signalling. Auxin hypersensitivity of the atnup62 mutant, reminding that of atnup58 (and not observed with other nucleoporin mutants), is in agreement with the reported interaction between AtNUP62 and AtNUP58 proteins, and suggests closely related functions. The effect of AtNUP62 on auxin signalling likely occurs in relation to scaffold proteins of the nuclear pore complex (AtNUP160, AtNUP96 and AtTPR).

Broad targeting of resistance to apoptosis in cancer

Apoptosis or programmed cell death is natural way of removing aged cells from the body. Most of the anti-cancer therapies trigger apoptosis induction and related cell death networks to eliminate malignant cells. However, in cancer, de-regulated apoptotic signaling, particularly the activation of an anti-apoptotic systems, allows cancer cells to escape this program leading to uncontrolled proliferation resulting in tumor survival, therapeutic resistance and recurrence of cancer. This resistance is a complicated phenomenon that emanates from the interactions of various molecules and signaling pathways. In this comprehensive review we discuss the various factors contributing to apoptosis resistance in cancers. The key resistance targets that are discussed include (1) Bcl-2 and Mcl-1 proteins; (2) autophagy processes; (3) necrosis and necroptosis; (4) heat shock protein signaling; (5) the proteasome pathway; (6) epigenetic mechanisms; and (7) aberrant nuclear export signaling. The shortcomings of current therapeutic modalities are highlighted and a broad spectrum strategy using approaches including (a) gossypol; (b) epigallocatechin-3-gallate; (c) UMI-77 (d) triptolide and (e) selinexor that can be used to overcome cell death resistance is presented. This review provides a roadmap for the design of successful anti-cancer strategies that overcome resistance to apoptosis for better therapeutic outcome in patients with cancer.

Macromolecular transport between the nucleus and the cytoplasm: Advances in mechanism and emerging links to disease

Transport of macromolecules between the cytoplasm and the nucleus is critical for the function of all eukaryotic cells. Large macromolecular channels termed nuclear pore complexes that span the nuclear envelope mediate the bidirectional transport of cargoes between the nucleus and cytoplasm. However, the influence of macromolecular trafficking extends past the nuclear pore complex to transcription and RNA processing within the nucleus and signaling pathways that reach into the cytoplasm and beyond. At the Mechanisms of Nuclear Transport biennial meeting held from October 18 to 23, 2013 in Woods Hole, MA, researchers in the field met to report on their recent findings. The work presented highlighted significant advances in understanding nucleocytoplasmic trafficking including how transport receptors and cargoes pass through the nuclear pore complex, the many signaling pathways that impinge on transport pathways, interplay between the nuclear envelope, nuclear pore complexes, and transport pathways, and numerous links between transport pathways and human disease. The goal of this review is to highlight newly emerging themes in nuclear transport and underscore the major questions that are likely to be the focus of future research in the field.

Promising SINEs for embargoing nuclear-cytoplasmic export as an anticancer strategy

In cancer cells, the nuclear-cytoplasmic transport machinery is frequently disrupted, resulting in mislocalization and loss of function for many key regulatory proteins. In this review, the mechanisms by which tumor cells co-opt the nuclear transport machinery to facilitate carcinogenesis, cell survival, drug resistance, and tumor progression will be elucidated, with a particular focus on the role of the nuclear-cytoplasmic export protein. The recent development of a new generation of selective inhibitors of nuclear export (XPO1 antagonists) and how these novel anticancer drugs may bring us closer to the implementation of this therapeutic strategy in the clinic will be discussed.

Transportin acts to regulate mitotic assembly events by target binding rather than Ran sequestration

Transportin-specific molecular tools are used to show that the mitotic cell contains importin β and transportin “global positioning system” pathways that are mechanistically parallel. Transportin works to control where the spindle, nuclear membrane, and nuclear pores are formed by directly affecting assembly factor function.

The nuclear import receptors importin β and transportin play a different role in mitosis: both act phenotypically as spatial regulators to ensure that mitotic spindle, nuclear membrane, and nuclear pore assembly occur exclusively around chromatin. Importin β is known to act by repressing assembly factors in regions distant from chromatin, whereas RanGTP produced on chromatin frees factors from importin β for localized assembly. The mechanism of transportin regulation was unknown. Diametrically opposed models for transportin action are as follows: 1) indirect action by RanGTP sequestration, thus down-regulating release of assembly factors from importin β, and 2) direct action by transportin binding and inhibiting assembly factors. Experiments in Xenopus assembly extracts with M9M, a superaffinity nuclear localization sequence that displaces cargoes bound by transportin, or TLB, a mutant transportin that can bind cargo and RanGTP simultaneously, support direct inhibition. Consistently, simple addition of M9M to mitotic cytosol induces microtubule aster assembly. ELYS and the nucleoporin 107–160 complex, components of mitotic kinetochores and nuclear pores, are blocked from binding to kinetochores in vitro by transportin, a block reversible by M9M. In vivo, 30% of M9M-transfected cells have spindle/cytokinesis defects. We conclude that the cell contains importin β and transportin “global positioning system”or “GPS” pathways that are mechanistically parallel.

A novel mechanism for SUMO system control: regulated Ulp1 nucleolar sequestration

The small ubiquitin-related modifiers (SUMOs) are evolutionarily conserved polypeptides that are covalently conjugated to protein targets to modulate their subcellular localization, half-life, or activity. Steady-state SUMO conjugation levels increase in response to many different types of environmental stresses, but how the SUMO system is regulated in response to these insults is not well understood. Here, we characterize a novel mode of SUMO system control: in response to elevated alcohol levels, the Saccharomyces cerevisiae SUMO protease Ulp1 is disengaged from its usual location at the nuclear pore complex (NPC) and sequestered in the nucleolus. We further show that the Ulp1 region previously demonstrated to interact with the karyopherins Kap95 and Kap60 (amino acids 150 to 340) is necessary and sufficient for nucleolar targeting and that enforced sequestration of Ulp1 in the nucleolus significantly increases steady-state SUMO conjugate levels, even in the absence of alcohol. We have thus characterized a novel mechanism of SUMO system control in which the balance between SUMO-conjugating and -deconjugating activities at the NPC is altered in response to stress via relocalization of a SUMO-deconjugating enzyme.

A mechanism of nucleocytoplasmic trafficking for the homeodomain protein PRH

Proline-rich homeodomain (PRH)/hematopoietically expressed homeodomain (Hex) is a homeodomain protein that plays an important role in early embryonic patterning and hematopoiesis. PRH can act as either a tumor suppressor or an oncogene and its expression is dysregulated in certain types of lymphoid and myeloid leukemias. Aberrant exclusion of PRH from the nuclei has been associated with thyroid and breast cancers and a subset of myeloid leukemias. Accordingly, nuclear localization of PRH was found to be necessary for the inhibition of eIF4E-dependent transformation. Since PRH's nuclear-cytoplasmic localization has been associated with neoplastic transformation we sought to better understand how PRH is transported to the nuclear compartment. Here, we report an essential element that controls the mechanism of PRH nucleocytoplasmic trafficking, namely that it is imported into the nuclei by Karyopherin/Importin 7. Kap7 was identified as a binding partner for PRH in a GST-pull down from a HeLa cell protein lysate, followed by mass-spectrometry. The Kap7-PRH complex is dissociated in the presence of RanGTP, as expected for a nuclear import complex. Kap7 can bind directly to PRH in a GST-pull down assay with purified proteins, as well as mediates the transport of PRH to the nuclear compartment in a digitonin permeabilized cells assay. Finally, in vivo depletion of Kap7 dramatically reduces accumulation of PRH in the nucleus. Our data open the way for investigations of the mechanism of perturbed PRH localization in tumors and possible therapeutic interventions.

SN52, a novel nuclear factor-kappaB inhibitor, blocks nuclear import of RelB:p52 dimer and sensitizes prostate cancer cells to ionizing radiation

The activation of nuclear factor-kappaB (NF-kappaB) is thought to protect cancer cells against therapy-induced cytotoxicity. RelB, a member of the NF-kappaB family in the alternative pathway, is uniquely expressed at a high level in prostate cancer with high Gleason scores. Here, we show that ionizing radiation (IR) enhances nuclear import of RelB, leading to up-regulation of its target gene, manganese superoxide dismutase (MnSOD), and renders prostate cancer cells resistant to IR. To selectively block RelB nuclear import, we designed a cell-permeable SN52 peptide, a variant of the SN50 peptide that has been shown to block nuclear import of NF-kappaB family members in the classic pathway. Inhibition of IR-induced NF-kappaB activation by SN50 and SN52 was achieved by selectively interrupting the association of p50 and p52 with nuclear import factors importin-alpha1 and importin-beta1. Importantly, SN52 seems to be more efficient for radiosensitization of prostate cancer cells at clinically relevant radiation doses and has less cytotoxicity to normal prostate epithelial cells compared with the toxicity observed with SN50. These results suggest that targeting the alternative pathway is a promising approach to selectively radiosensitize prostate cancers and that SN52 may serve as a prototype biological agent for sensitizing prostate cancers to clinically relevant doses of IR.

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.

Beta-catenin control of T-cell transcription factor 4 (Tcf4) importation from the cytoplasm to the nucleus contributes to Tcf4-mediated transcription in 293 cells

Beta-catenin has essential roles in morphogenesis and human cancer, both as a subunit of adhesive complexes in the cell membrane and as a transcriptional coactivator in the Wnt signaling pathway. In addition, beta-catenin also has the ability to transport lymphoid enhancer binding factor-1 into the nucleus. In this study, we examined a constitutive active mutation, beta-catenin (T41A, S45A), for its potential as a nuclear import receptor for T-cell transcription factor 4 in 293 cells. Immunoblot analysis demonstrated that this constitutive active form of beta-catenin increased the amount of Tcf4 in the nucleus about 4-5-fold compared to controls. However, the overall expression of Tcf4 remained the same with or without over-expression of beta-catenin (T41A, S45A). T-cell transcription factor 4 reporter gene and electrophoretic mobility shift assay further indicated that the increase in Tcf4 in the nucleus was consistent with its accrued DNA binding capacity and transcription activity. Microscopic immunofluorescence examination showed that Tcf4 was mainly located in the cytoplasm and transported into the nucleus, without or with over-expression of beta-catenin (T41A, S45A), respectively. Our results suggest that beta-catenin might be a major factor regulating the import of Tcf4 from the cytoplasm into the nucleus, consequently controlling its transcription activity.

Functional and Physical Interactions between Autonomously Replicating Sequence-Binding Factor 1 and the Nuclear Transport Machinery

Autonomously replicating sequence-binding factor 1 (Abf1p) is a site-specific DNA binding protein in Saccharomyces cerevisiae that functions to regulate multiple nuclear events including DNA replication, transcriptional activation, and gene silencing. Previous work indicates that the multiple functions of Abf1p are conferred by the carboxy-terminus of the protein, which can be further dissected into two important clusters of amino acid residues (CS1 and CS2). Here we present genetic and cell biological evidence for a critical role of CS1 in proper nuclear localization of Abf1p. Mutations in CS1 cause severe defects in cell growth, nuclear translocation, and Abf1p-mediated gene regulation, which can be rescued by a heterologous nuclear localization sequence (NLS). In addition, the CS1-domain can mediate the import of a CS1-GFP fusion protein. Importantly, the CS1-mediated nuclear import depends on the Ran guanine nucleotide exchange factor Prp20p. Interestingly, a single amino acid change in CS1 (K625I) also causes the protein to be exported out of the nucleus via the Crm1p-dependent pathway. The temperature-sensitive growth phenotype of this particular mutant can be overcome by overexpression of Kap121p/Pse1p, a well-established nuclear transport receptor. Biochemical studies indicate that Pse1p binds to a region of Abf1p upstream of CS1 in a RanGTP-sensitive manner, suggesting that Abf1p has a second distinct NLS and can be imported into the nucleus by several overlapping pathways. We propose that the link between Abf1p and the nuclear transport machinery may also be important for partitioning multiple Abf1p-mediated nuclear processes.

Importin alpha1 (Rch1) mediates nuclear translocation of thioredoxin-binding protein-2/vitamin D(3)-up-regulated protein 1

Thioredoxin-binding protein-2 (TBP-2)/vitamin D(3) up-regulated protein 1 is an endogenous molecule interacting with thioredoxin (TRX), negatively regulating TRX function, and being implicated in the suppression of tumor development and metastasis. We found that TBP-2 ectopically expressed in the breast cancer cell line MCF-7 was localized predominantly in the nucleus exhibiting growth suppressive activity. The nuclear accumulation of endogenous TBP-2 protein was also demonstrated when the cells were treated with an anti-cancer drug, suberoylanilide hydroxamic acid. To investigate the mechanism underlying the nuclear localization, we performed a yeast two-hybrid screening and identified importin alpha(1) (Rch1) as a protein interacting with TBP-2. The physical interaction between TBP-2 and Rch1 was confirmed with a glutathione S-transferase pull-down assay. The interaction of TBP-2 was specific to Rch1 among other importin alpha subfamilies (Qip1 and NPI-1), and amino acids 1-227 of TBP-2 were sufficient for both the interaction with Rch1 and the nuclear localization, although there is no typical nuclear localization signal in this sequence. The expression of short interfering RNA of Rch1 suppressed suberoylanilide hydroxamic acid-induced nuclear accumulation of TBP-2. Collectively, our results strongly suggest that an interaction with importin system is required for TBP-2 nuclear translocation and growth control tightly associated with TRX-dependent redox regulation of transcription factors.

Peering through the pore: nuclear pore complex structure, assembly, and function

Nuclear pore complexes (NPCs) are large proteinaceous assemblies that provide the only known portals for exchanging macromolecules between the nucleus and cytoplasm. This includes the movement of small molecules and the selective, facilitated transport of large proteins and RNAs. Faithful, continuous NPC assembly is key for maintaining normal physiological function and is closely tied to proper cell division. This review focuses on the most outstanding issues involving NPC structure, assembly, and function.

Tamo selectively modulates nuclear import in Drosophila

BACKGROUND

The NF-kappaB/Rel pathway functions in the establishment of dorsal-ventral polarity and in the innate humoral and cellular immune response in Drosophila. An important aspect of all NF-kappaB/Rel pathways is the translocation of the Rel proteins from the cytoplasm to the nucleus, where they function as transcription factors.

RESULTS

We have identified a new protein, Tamo, which binds to Drosophila Rel protein Dorsal, but not to Dorsal lacking the nuclear localization sequence. Tamo does not bind to the other Drosophila Rel proteins, Dif and Relish. The Tamo-Dorsal complex forms in the cytoplasm and Tamo also interacts with a cytoplasmically orientated nucleoporin. In addition Tamo binds the Ras family small GTPase, Ran. Tamo functions during oogenesis and, based on phenotypic analysis, controls the levels of nuclear Dorsal in early embryos. It further regulates the accumulation of Dorsal in the nucleus after immune challenge.

CONCLUSIONS

Tamo has an essential function during oogenesis. Tamo interacts with Dorsal and proteins that are part of the nuclear import machinery. We propose that tamo modulates the levels of import of Dorsal and other proteins.

Domain analysis of the Saccharomyces cerevisiae heterogeneous nuclear ribonucleoprotein, Nab2p. Dissecting the requirements for Nab2p-facilitated poly(A) RNA export

Mature poly(A) RNA transcripts are exported from the nucleus in complex with heterogeneous nuclear ribonucleoproteins (hnRNPs). Nab2p is an essential Saccharomyces cerevisiae hnRNP protein that interacts with poly(A) RNA and shuttles between the nucleus and cytoplasm. Functional Nab2p is required for export of poly(A) RNA from the nucleus. The Nab2 protein consists of the following four domains: a unique N-terminal domain, a glutamine-rich domain, an arginine-glycine (RGG) domain, and a zinc finger domain. We generated Nab2p deletion mutants to analyze the contribution of each domain to the in vivo function of Nab2p. We first tested whether the deletion mutants could replace the essential NAB2 gene. We then examined the impact of these mutations on Nab2p localization, poly(A) RNA localization, and association of Nab2p with poly(A) RNA. Our analyses revealed that the N-terminal domain is required for nuclear export of both poly(A) RNA and Nab2p. We confirm that the RGG domain is important for Nab2p import in vivo. Finally, the zinc finger domain is critical for the interaction between Nab2p and poly(A) RNA in vivo. Our data support a model where Nab2p associates with poly(A) RNA in the nucleus through the zinc finger domain and facilitates the export of the poly(A) RNA through protein interactions mediated by the N-terminal domain.

An intracellular targeted NLS peptide inhibitor of karyopherin alpha:NF-kappa B interactions

The nuclear import of transcription factors involves proteins termed karyopherins. Previously, we described an intracellular targeted dual nuclear localization sequence (NLS) peptide inhibitor of processes dependent upon the transcription factor NF-kappa B. We have now developed a homogeneous solution based assay and show that NF-kappa B interacts with karyopherin alpha and that the dual NLS peptide inhibits this interaction. We also show that both L- and D-amino acid containing peptides bind well to karyopherin alpha 2, whereas, the L-amino acid peptides bind more efficiently than the D-amino acid peptide to karyopherin alpha1.

Binding dynamics of structural nucleoporins govern nuclear pore complex permeability and may mediate channel gating

The nuclear pore complex (NPC) is a permeable sieve that can dilate to facilitate the bidirectional translocation of a wide size range of receptor-cargo complexes. The binding of receptors to FG nucleoporin docking sites triggers channel gating by an unknown mechanism. Previously, we used deoxyglucose and chilling treatments to implicate Nup170p and Nup188p in the control of NPC sieving in Saccharomyces cerevisiae. Here, we report that aliphatic alcohols increase the permeability of wild-type and nup170Delta NPCs. In conjunction with increases in permeability, aliphatic alcohols, deoxyglucose, and chilling trigger the reversible dissociation of several nucleoporins from nup170Delta NPCs. These results are consistent with the hypothesis that NPC gating occurs when molecular latches composed of FG repeats and structural nucleoporins dissociate.