Nucleocytoplasmic protein transport and recycling of Ran

Recent Advances in Nanotechnology-Based Targeted Delivery Systems of Active Constituents in Natural Medicines for Cancer Treatment

Owing to high efficacy and safety, natural medicines have found their way into the field of cancer therapy over the past few decades. However, the effective ingredients of natural medicines have shortcomings of poor solubility and low bioavailability. Nanoparticles can not only solve the problems above but also have outstanding targeting ability. Targeting preparations can be classified into three levels, which are target tissues, cells, and organelles. On the premise of clarifying the therapeutic purpose of drugs, one or more targeting methods can be selected to achieve more accurate drug delivery and consequently to improve the anti-tumor effects of drugs and reduce toxicity and side effects. The aim of this review is to summarize the research status of natural medicines' nano-preparations in tumor-targeting therapies to provide some references for further accurate and effective cancer treatments.

Membrane Fusion and SNAREs: Interaction with Ras Proteins

The superfamily of Ras proteins comprises different molecules belonging to the GTPase family. They normally cycle between an active state bound to GTP which activates effectors while the protein is membrane-associated, and an inactive GDP-bound state. They regulate the intracellular trafficking and other cellular processes. The family of Rab proteins includes several members and they have been found, among other Ras proteins, to be fundamental for important biological processes, such as endocytosis and exocytosis. SNARE proteins control the fusion of vesicles by forming quaternary complexes which are divided into two small groups on the two different compartments. Generally, the association of three SNARE proteins on the donor compartment with the one on the target compartment determines the formation of the SNARE complex, the opening of the fusion pore and the formation of one single bigger vesicle. Interestingly, novel interactions between other molecules involved in intracellular trafficking, endosomal fusion and maturation have recently been found, such as the interaction between invariant chain and the Qb SNARE vti1b, and more functional connections between Rab proteins and SNAREs are supposed to be fundamental for the regulation of membrane fusion.

Targeting of nucleo‑cytoplasmic transport factor exportin 1 in malignancy (Review)

Nuclear pore complexes (NPCs) regulate the entry and exit of molecules from the cell nucleus. Small molecules pass through NPCs by diffusion while large molecules enter and exit the nucleus by karyopherins, which serve as transport factors. Exportin-1 (XPO1) is a protein that is an important member of the karyopherin family and carries macromolecules from the nucleus to the cytoplasm. XPO1 is responsible for nuclear-cytoplasmic transport of protein, ribosomal RNA and certain required mRNAs for ribosomal biogenesis. Furthermore, XPO1-mediated nuclear export is associated with various types of disease, such as cancer, inflammation and viral infection. The key role of XPO1 in carcinogenesis and its potential as a therapeutic target has been demonstrated by previous studies. Clinical use of novel developed generation-specific XPO1 inhibitors and their combination with other agents to block XPO1-mediated nuclear export are a promising new treatment strategy. The aim of the present study was to explain the working mechanism of XPO1 and inhibitors that block XPO1-mediated nuclear export.

The non-canonical effects of heme oxygenase-1, a classical fighter against oxidative stress

The role of heme oxygenase-1 in resisting oxidative stress and cell protection has always been a hot research topic. With the continuous deepening of research, in addition to directly regulating redox by catalyzing the degradation of heme, HO-1 protein also participates in the gene expression level in a great diversity of methods, thereby initiating cell defense. Particularly the non-canonical nuclear-localized HO-1 and HO-1 protein interactions play the role of a warrior against oxidative stress. Besides, HO-1 may be a promising marker for disease prediction and detection in many clinical trials. Especially for malignant diseases, there may be new advances in the treatment of HO-1 by regulating abnormal ROS and metabolic signaling. The purpose of this review is to systematically sort out and describe several aspects of research to facilitate further detailed mechanism research and clinical application promotion in the future.

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The different subcellular localizations ofHO-1 implies that it has special functions.

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Nuclear HO-1 plays an indispensable role in gene regulation and other aspects.

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The interactions between HO-1 and others provide the possibility to participate in vital physiological processes.

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HO-1 may become a potential disease assessment marker.

Design, synthesis, and biological evaluation of a multifunctional neuropeptide-Y conjugate for selective nuclear delivery of radiolanthanides

Background

Targeting G protein-coupled receptors on the surface of cancer cells with peptide ligands is a promising concept for the selective tumor delivery of therapeutically active cargos, including radiometals for targeted radionuclide therapy (TRT). Recently, the radiolanthanide terbium-161 (¹⁶¹Tb) gained significant interest for TRT application, since it decays with medium-energy β-radiation but also emits a significant amount of conversion and Auger electrons with short tissue penetration range. The therapeutic efficiency of radiometals emitting Auger electrons, like ¹⁶¹Tb, can therefore be highly boosted by an additional subcellular delivery into the nucleus, in order to facilitate maximum dose deposition to the DNA. In this study, we describe the design of a multifunctional, radiolabeled neuropeptide-Y (NPY) conjugate, to address radiolanthanides to the nucleus of cells naturally overexpressing the human Y₁ receptor (hY₁R).

By using solid-phase peptide synthesis, the hY₁R-preferring [F⁷,P³⁴]-NPY was modified with a fatty acid, a cathepsin B-cleavable linker, followed by a nuclear localization sequence (NLS), and a DOTA chelator (compound pb12). In this proof-of-concept study, labeling was performed with either native terbium-159 (^natTb), as surrogate for ¹⁶¹Tb, or with indium-111 (¹¹¹In).

Results

[^natTb]Tb-pb12 showed a preserved high binding affinity to endogenous hY₁R on MCF-7 cells and was able to induce receptor activation and internalization similar to the hY₁R-preferring [F⁷,P³⁴]-NPY. Specific internalization of the ¹¹¹In-labeled conjugate into MCF-7 cells was observed, and importantly, time-dependent nuclear uptake of ¹¹¹In was demonstrated. Study of metabolic stability showed that the peptide is insufficiently stable in human plasma. This was confirmed by injection of [¹¹¹In]In-pb12 in nude mice bearing MCF-7 xenograft which showed specific uptake only at very early time point.

Conclusion

The multifunctional NPY conjugate with a releasable DOTA-NLS unit represents a promising concept for enhanced TRT with Auger electron-emitting radiolanthanides. Our research is now focusing on improving the reported concept with respect to the poor plasmatic stability of this promising radiopeptide.

Genome-Wide Identification, Expression Profile and Evolution Analysis of Karyopherin β Gene Family in Solanum tuberosum Group Phureja DM1-3 Reveals Its Roles in Abiotic Stresses

In eukaryotic cells, nucleocytoplasmic trafficking of macromolecules is largely mediated by Karyopherin β/Importin (KPNβ or Impβ) nuclear transport factors, and they import and export cargo proteins or RNAs via the nuclear pores across the nuclear envelope, consequently effecting the cellular signal cascades in response to pathogen attack and environmental cues. Although achievements on understanding the roles of several KPNβs have been obtained from model plant Arabidopsis thaliana, comprehensive analysis of potato KPNβ gene family is yet to be elucidated. In our genome-wide identifications, a total of 13 StKPNβ (Solanum tuberosum KPNβ) genes were found in the genome of the doubled monoploid S. tuberosum Group Phureja DM1-3. Sequence alignment and conserved domain analysis suggested the presence of importin-β N-terminal domain (IBN_N, PF08310) or Exporin1-like domain (XpoI, PF08389) at N-terminus and HEAT motif at the C-terminal portion in most StKPNβs. Phylogenetic analysis indicated that members of StKPNβ could be classified into 16 subgroups in accordance with their homology to human KPNβs, which was also supported by exon-intron structure, consensus motifs, and domain compositions. RNA-Seq analysis and quantitative real-time PCR experiments revealed that, except StKPNβ3d and StKPNβ4, almost all StKPNβs were ubiquitously expressed in all tissues analyzed, whereas transcriptional levels of several StKPNβs were increased upon biotic/abiotic stress or phytohormone treatments, reflecting their potential roles in plant growth, development or stress responses. Furthermore, we demonstrated that silencing of StKPNβ3a, a SA- and H₂O₂-inducible KPNβ genes led to increased susceptibility to environmental challenges, implying its crucial roles in plant adaption to abiotic stresses. Overall, our results provide molecular insights into StKPNβ gene family, which will serve as a strong foundation for further functional characterization and will facilitate potato breeding programs.

Nuclear Envelope Regulation of Oncogenic Processes: Roles in Pancreatic Cancer

Pancreatic cancer is an aggressive and intractable malignancy with high mortality. This is due in part to a high resistance to chemotherapeutics and radiation treatment conferred by diverse regulatory mechanisms. Among these, constituents of the nuclear envelope play a significant role in regulating oncogenesis and pancreatic tumor biology, and this review focuses on three specific components and their roles in cancer. The LINC complex is a nuclear envelope component formed by proteins with SUN and KASH domains that interact in the periplasmic space of the nuclear envelope. These interactions functionally and structurally couple the cytoskeleton to chromatin and facilitates gene regulation informed by cytoplasmic activity. Furthermore, cancer cell invasiveness is impacted by LINC complex biology. The nuclear lamina is adjacent to the inner nuclear membrane of the nuclear envelope and can actively regulate chromatin in addition to providing structural integrity to the nucleus. A disrupted lamina can impart biophysical compromise to nuclear structure and function, as well as form dysfunctional micronuclei that may lead to genomic instability and chromothripsis. In close relationship to the nuclear lamina is the nuclear pore complex, a large megadalton structure that spans both outer and inner membranes of the nuclear envelope. The nuclear pore complex mediates bidirectional nucleocytoplasmic transport and is comprised of specialized proteins called nucleoporins that are overexpressed in many cancers and are diagnostic markers for oncogenesis. Furthermore, recent demonstration of gene regulatory functions for discrete nucleoporins independent of their nuclear trafficking function suggests that these proteins may contribute more to malignant phenotypes beyond serving as biomarkers. The nuclear envelope is thus a complex, intricate regulator of cell signaling, with roles in pancreatic tumorigenesis and general oncogenic transformation.

Is unphosphorylated Rex, as multifunctional protein of HTLV-1, a fully intrinsically disordered protein? An in silico study

Intracellularlocation of a viral unspliced mRNA in host cell is a crucial factor for normal life of the virus. Rex is a neucleo-cytoplasmic shuffling protein of Human T-cell Leukemia Virus-1(HTLV-1)which has important role in active transport of cargo-containing RNA from nucleus to cytoplasm. Therefore, it plays a crucial role in the disease development by the virus. In spite of its importance, the 3d-structurephosphorylated and unphosphorylated of this protein has not been determined. In this study, first we predicted whether Rex protein is an ordered or disordered protein. In second step protein 3Dstructure of Rex was obtained. The content of disorder-promoting amino acids, flexibility, hydrophobicity, short linear motifs (SLiMs) and protein binding regions and probability of Rex crystallization were calculated by various In Silico methods. The3D models of Rex protein were obtained by various In Silico methods, such as homology modeling, threading and ab initio, including; I-TASSER, LOMETS, SPARSKS, ROBBETA and QUARK servers. By comparing and analyzing Qmean, z-scores and energy levels of selected models, the best structures with highest favored region in Ramachandran plot (higher than 90%) was refined with MODREFINER software. In silico analysis of Rex physicochemical properties and also predicted SLiMs and binding regions sites confirms that unphosphorylated Rex protein in HTLV-1 as Rev protin in HIV is wholly disordered protein belongs to the class of intrinsically disordered proteins with extended disorder (native coils, native pre-molten globules).

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Physico-chemical properties of Rex protein were confirmed unphosphorilated Rex protein is a wholly intrinsically disordered protein.

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The 3d-structure model of Rex protein was determined.

Role of a nuclear localization signal on the minor capsid proteins VP2 and VP3 in BKPyV nuclear entry

BK Polyomavirus (BKPyV) is a ubiquitous nonenveloped human virus that can cause severe disease in immunocompromised populations. After internalization into renal proximal tubule epithelial cells, BKPyV traffics through the ER and enters the cytosol. However, it is unclear how the virus enters the nucleus. In this study, we elucidate a role for the nuclear localization signal located on the minor capsid proteins VP2 and VP3 during infection. Site-directed mutagenesis of a single lysine in the basic region of the C-terminus of the minor capsid proteins abrogated their nuclear localization, and the analogous genomic mutation reduced infectivity. Additionally, through use of the inhibitor ivermectin and knockdown of importin β1, we found that the importin α/β pathway is involved during infection. Overall these data are the first to show the significance of the NLS of the BKPyV minor capsid proteins during infection in a natural host cell.

Inhibition of CRM1-dependent nuclear export sensitizes malignant cells to cytotoxic and targeted agents

Nuclear-cytoplasmic trafficking of proteins is a significant factor in the development of cancer and drug resistance. Subcellular localization of exported proteins linked to cancer development include those involved in cell growth and proliferation, apoptosis, cell cycle regulation, transformation, angiogenesis, cell adhesion, invasion, and metastasis. Here, we examined the basic mechanisms involved in the export of proteins from the nucleus to the cytoplasm. All proteins over 40kDa use the nuclear pore complex to gain entry or exit from the nucleus, with the primary nuclear export molecule involved in these processes being chromosome region maintenance 1 (CRM1, exportin 1 or XPO1). Proteins exported from the nucleus must possess a hydrophobic nuclear export signal (NES) peptide that binds to a hydrophobic groove containing an active-site Cys528 in the CRM1 protein. CRM1 inhibitors function largely by covalent modification of the active site Cys528 and prevent binding to the cargo protein NES. In the absence of a CRM1 inhibitor, CRM1 binds cooperatively to the NES of the cargo protein and RanGTP, forming a trimer that is actively transported out of the nucleus by facilitated diffusion. Nuclear export can be blocked by CRM1 inhibitors, NES peptide inhibitors or by preventing post-translational modification of cargo proteins. Clinical trials using the classic CRM1 inhibitor leptomycin B proved too toxic for patients; however, a new generation of less toxic small molecule inhibitors is being used in clinical trials in patients with both hematological malignancies and solid tumors. Additional trials are being initiated using small-molecule CRM1 inhibitors in combination with chemotherapeutics such as pegylated liposomal doxorubicin. In this review, we present evidence that combining the new CRM1 inhibitors with other classes of therapeutics may prove effective in the treatment of cancer. Potential combinatorial therapies discussed include the use of CRM1 inhibitors and the addition of alkylating agents (melphalan), anthracyclines (doxorubicin and daunomycin), BRAF inhibitors, platinum drugs (cisplatin and oxaliplatin), proteosome inhibitors (bortezomib and carfilzomib), or tyrosine-kinase inhibitors (imatinib). Also, the sequence of treatment may be important for combination therapy. We found that the most effective treatment regimen involved first priming the cancer cells with the CRM1 inhibitor followed by doxorubicin, bortezomib, carfilzomib, or melphalan. This order sensitized both de novo and acquired drug-resistant cancer cell lines.

Hypoxia enhances chondrogenesis and prevents terminal differentiation through PI3K/Akt/FoxO dependent anti-apoptotic effect

Hypoxia, a common environmental condition, influences cell signals and functions. Here, we compared the effects of hypoxia (1% oxygen) and normoxia (air) on chondrogenic differentiation of human mesenchymal stem cells (MSCs). For in vitro chondrogenic differentiation, MSCs were concentrated to form pellets and subjected to conditions appropriate for chondrogenic differentiation under normoxia and hypoxia, followed by the analysis for the expression of genes and proteins of chondrogenesis and endochondral ossification. MSCs induced for differentiation under hypoxia increased in chondrogenesis, but decreased in endochondral ossification compared to those under normoxia. MSCs induced for differentiation were more resistant to apoptosis under hypoxia compared to those under normoxia. The hypoxia-dependent protection of MSCs from chondrogenesis-induced apoptosis correlated with an increase in the activation of the phosphatidylinositol 3-kinase (PI3K)/Akt/FoxO pathway. These results suggest that the PI3K/Akt/FoxO survival pathway activated by hypoxia in MSCs enhances chondrogenesis and plays an important role in preventing endochondral ossification.

Biological significance of the importin-β family-dependent nucleocytoplasmic transport pathways

Importin-β family proteins (Imp-βs) are nucleocytoplasmic transport receptors (NTRs) that import and export proteins and RNAs through the nuclear pores. The family consists of 14-20 members depending on the biological species, and each member transports a specific group of cargoes. Thus, the Imp-βs mediate multiple, parallel transport pathways that can be regulated separately. In fact, the spatiotemporally differential expressions and the functional regulations of Imp-βs have been reported. Additionally, the biological significance of each pathway has been characterized by linking the function of a member of Imp-βs to a cellular consequence. Connecting these concepts, the regulation of the transport pathways conceivably induces alterations in the cellular physiological states. However, few studies have linked the regulation of an importin-β family NTR to an induced cellular response and the corresponding cargoes, despite the significance of this linkage in comprehending the biological relevance of the transport pathways. This review of recent reports on the regulation and biological functions of the Imp-βs highlights the significance of the transport pathways in physiological contexts and points out the possibility that the identification of yet unknown specific cargoes will reinforce the importance of transport regulation.

A novel tumor suppressor function of glycine N-methyltransferase is independent of its catalytic activity but requires nuclear localization

Glycine N-methyltransferase (GNMT), an abundant cytosolic enzyme, catalyzes the transfer of a methyl group from S-adenosylmethionine (SAM) to glycine generating S-adenosylhomocysteine and sarcosine (N-methylglycine). This reaction is regulated by 5-methyltetrahydrofolate, which inhibits the enzyme catalysis. In the present study, we observed that GNMT is strongly down regulated in human cancers and is undetectable in cancer cell lines while the transient expression of the protein in cancer cells induces apoptosis and results in the activation of ERK1/2 as an early pro-survival response. The antiproliferative effect of GNMT can be partially reversed by treatment with the pan-caspase inhibitor zVAD-fmk but not by supplementation with high folate or SAM. GNMT exerts the suppressor effect primarily in cells originated from malignant tumors: transformed cell line of non-cancer origin, HEK293, was insensitive to GNMT. Of note, high levels of GNMT, detected in regenerating liver and in NIH3T3 mouse fibroblasts, do not produce cytotoxic effects. Importantly, GNMT, a predominantly cytoplasmic protein, was translocated into nuclei upon transfection of cancer cells. The presence of GNMT in the nuclei was also observed in normal human tissues by immunohistochemical staining. We further demonstrated that the induction of apoptosis is associated with the GNMT nuclear localization but is independent of its catalytic activity or folate binding. GNMT targeted to nuclei, through the fusion with nuclear localization signal, still exerts strong antiproliferative effects while its restriction to cytoplasm, through the fusion with nuclear export signal, prevents these effects (in each case the protein was excluded from cytosol or nuclei, respectively). Overall, our study indicates that GNMT has a secondary function, as a regulator of cellular proliferation, which is independent of its catalytic role.

Gcse, a novel germ-cell-specific gene, is differentially expressed during meiosis and gametogenesis

Gametogenesis is a complex process wherein germ cells develop from primordial diploid cells into haploid gametes. To understand the mechanisms controlling gametogenesis, we identified a novel germ-cell-specific gene, Gcse. Gcse produces two major transcripts that are 1589 bp (Gcse-l) and 906 bp (Gcse-s) in length. Northern blotting and reverse transcription-polymerase chain reaction (RT-PCR) analyses of multiple tissues reveal that Gcse-l is expressed in both adult testes and ovaries, but Gcse-s is expressed only in adult testes. During female gonad development, Gcse-l is expressed from embryonic day 13.5 to adulthood, specifically in oocytes, and maintained in ovulated and fertilized eggs. However, Gcse-s signals were detected only in ovulated oocytes and fertilized eggs but not in adult ovary. During male gonad development, strong Gcse-l signals were detected in late pachytene spermatocytes and round spermatids. However, Gcse-s transcripts exist only in round spermatids. Furthermore, the expression of GCSE-L proteins and their subcellular localizations within cells are stage specific. GCSE-L is detected in the nucleus of late pachytene spermatocytes. During meiosis, GCSE-L is translocated to acrosome regions in spermatids and maintained in the acrosome of spermatozoa. GCSE-L colocalizes with acrosin and lectin peanut agglutinin in the Golgi apparatus. However, GCSE-S proteins are expressed only in the nucleus of spermatids. From these results, we suggest that GCSE proteins play roles in meiosis and may be involved in acrosome biogenesis during spermiogenesis.

Differences in the nuclear export mechanism between myocardin and myocardin-related transcription factor A

Myocardin (Mycd), a key factor in smooth muscle cell differentiation, is constitutively located in the nucleus, whereas myocardin-related transcription factors A and B (MRTF-A/B) reside mostly in the cytoplasm and translocate to the nucleus in a Rho-dependent manner. Here, we investigated the nuclear export of Mycd family members. They possess two leucine-rich sequences: L1 in the N terminus and L2 in the Gln-rich domain. Although L2 (but not L1) served as a CRM1-binding site for Mycd, CRM1-mediated nuclear export did not affect its subcellular localization. Serum response factor (SRF) competitively inhibited Mycd/CRM1 interaction. Furthermore, such interaction was autonomously inhibited. The N terminus of Mycd bound intramolecularly to Mycd, resulting in masking L2. In contrast, the CRM1-binding affinity of MRTF-A was much higher than that of Mycd because both L1 and L2 of MRTF-A served as functional CRM1-binding sites, and the autoinhibition observed in the Mycd/CRM1 interaction was absent in the MRTF-A/CRM1 interaction. Additionally, because the SRF-binding affinity of MRTF-A was lower than that of Mycd, the inhibitory effect of SRF on the MRTF-A/CRM1 interaction was weak. Thus, MRTF-A is much more likely to be exported from the nucleus. These differences could be the reason for the distinct subcellular localization of Mycd and MRTF-A.

The E3 ligase Cdh1-anaphase promoting complex operates upstream of the E3 ligase Smurf1 in the control of axon growth

Axon growth is an essential event during brain development and is extremely limited due to extrinsic and intrinsic inhibition in the adult brain. The E3 ubiquitin ligase Cdh1-anaphase promoting complex (APC) has emerged as an important intrinsic suppressor of axon growth. In this study, we identify in rodents the E3 ligase Smurf1 as a novel substrate of Cdh1-APC and that Cdh1 targets Smurf1 for degradation in a destruction box-dependent manner. We find that Smurf1 acts downstream of Cdh1-APC in axon growth and that the turnover of RhoA by Smurf1 is important in this process. In addition, we demonstrate that acute knockdown of Smurf1 in vivo in the developing cerebellar cortex results in impaired axonal growth and migration. Finally, we show that a stabilized form of Smurf1 overrides the inhibition of axon growth by myelin. Taken together, we uncovered a Cdh1-APC/Smurf1/RhoA pathway that mediates axonal growth suppression in the developing mammalian brain.

Nuclear export of proteins and drug resistance in cancer

The intracellular location of a protein is crucial to its normal functioning in a cell. Cancer cells utilize the normal processes of nuclear-cytoplasmic transport through the nuclear pore complex of a cell to effectively evade anti-neoplastic mechanisms. CRM1-mediated export is increased in various cancers. Proteins that are exported in cancer include tumor-suppressive proteins such as retinoblastoma, APC, p53, BRAC1, FOXO proteins, INI1/hSNF5, galectin-3, Bok, nucleophosmin, RASSF2, Merlin, p21(CIP), p27(KIP1), N-WASP/FAK, estradiol receptor and Tob, drug targets topoisomerase I and IIα and BCR-ABL, and the molecular chaperone protein Hsp90. Here, we review in detail the current processes and known structures involved in the export of a protein through the nuclear pore complex. We also discuss the export receptor molecule CRM1 and its binding to the leucine-rich nuclear export signal of the cargo protein and the formation of a nuclear export trimer with RanGTP. The therapeutic potential of various CRM1 inhibitors will be addressed, including leptomycin B, ratjadone, KOS-2464, and specific small molecule inhibitors of CRM1, N-azolylacrylate analogs, FOXO export inhibitors, valtrate, acetoxychavicol acetate, CBS9106, and SINE inhibitors. We will also discuss examples of how drug resistance may be reversed by targeting the exported proteins topoisomerase IIα, BCR-ABL, and galectin-3. As effective and less toxic CRM1 export inhibitors become available, they may be used as both single agents and in combination with current chemotherapeutic drugs. We believe that the future development of low-toxicity, small-molecule CRM1 inhibitors may provide a new approach to treating cancer.

Identification of a nuclear localization sequence in β-arrestin-1 and its functional implications

A mounting body of evidence suggests that β-arrestin-1 plays important roles in the nucleus, but how β-arrestin-1 enters the nucleus remains unclear because no nuclear import signal has been identified in the β-arrestins. We sought to characterize the cellular localization of wild type β-arrestin-1 and a series of N domain mutants to determine the structural basis and functional implications of β-arrestin-1 nuclear localization. A seven-residue candidate nuclear localization sequence (NLS) was identified based on sequence analysis. Mutation of the NLS led to a loss of β-arrestin-1 nuclear localization in transfected cells. Exogenous expression of wild type β-arrestin-1 enhanced the transcriptional activity of nuclear factor κB (NF-κB) induced by bradykinin, whereas mutation of the NLS reduced this effect by two-thirds relative to wild type controls. Loss of β-arrestin-1 nuclear localization was accompanied by reduced recruitment of the CREB-binding protein and altered post-translational modification profile of p65/RelA. Further mutational analysis identified Lys(157) within the putative NLS as being critical to nuclear localization of β-arrestin-1. Substitution of Lys(157) to Ala led to reduced nuclear localization, decreased promoter binding by p65/RelA and decreased IL-1β gene transcription. These results demonstrate a critical role for β-arrestin-1 nuclear localization in scaffolding and transcriptional regulation.

Regulation of Nuclear Import During Differentiation; The IMP alpha Gene Family and Spermatogenesis

Access to nuclear genes in eukaryotes is provided by members of the importin (IMP) superfamily of proteins, which are of α- or β-types, the best understood nuclear import pathway being mediated by a heterodimer of an IMP α and IMP β1. IMP α recognises specific targeting signals on cargo proteins, while IMP β1 mediates passage into, and release within, the nucleus by interacting with other components of the transport machinery, including the monomeric guanine nucleotide binding protein Ran. In this manner, hundreds of different proteins can be targeted specifically into the nucleus in a tightly regulated fashion. The IMP α gene family has expanded during evolution, with only a single IMP α (Srp1p) gene in budding yeast, and three (IMP α1, 2/pendulin and 3) and five (IMP α1, -2, -3, -4 and -6) IMP α genes in Drosophila melanogaster and mouse respectively, which fall into three phylogenetically distinct groups. The fact that IMP α3 and IMP α2 are only present in metazoans implies that they emerged during the evolution of multicellular animals to perform specialised roles in particular cells and tissues. This review describes what is known of the IMP α gene family in mouse and in D. melanogaster, including a comparitive examination of their mRNA expression profiles in a highly differentiated tissue, the testis. The clear implication of their highly regulated synthesis during the course of spermatogenesis is that the different IMP αs have distinct expression patterns during cellular differentiation, implying tissue/cell type-specific roles.

Expression of nucleocytoplasmic transport machinery: clues to regulation of spermatogenic development

Spermatogenesis is one example of a developmental process which requires tight control of gene expression to achieve normal growth and sustain function. This review is based on the principle that events in spermatogenesis are controlled by changes in the distribution of proteins between the nuclear and cytoplasmic compartments. Through analysis of the regulated production of nucleocytoplasmic transport machinery in mammalian spermatogenesis, this review addresses the concept that access to the nucleus is tightly controlled to enable and prevent differentiation. A broad review of nuclear transport components is presented, outlining the different categories of machinery required for import, export and non-nuclear functions. In addition, the complexity of nomenclature is addressed by the provision of a concise yet comprehensive listing of information that will aid in comparative studies of different transport proteins and the genes which encode them. We review a suite of existing transcriptional analyses which identify common and distinct patterns of transport machinery expression, showing how these can be linked with key events in spermatogenic development. The additional importance of this for human fertility is considered, in light of data that identify which importin and nuclear transport machinery components are present in testicular cancer specimens, while also providing an indication of how their presence (and absence) may be considered as potential mediators of oncogenesis. This article is part of a Special Issue entitled: Regulation of Signaling and Cellular Fate through Modulation of Nuclear Protein Import.

Pericentrin contains five NESs and an NLS essential for its nucleocytoplasmic trafficking during the cell cycle

Pericentrin, a conserved centrosomal component, provides the structural scaffold to anchor numerous centrosomal proteins, and thus plays an essential role in the organization and function of the centrosome and the mitotic spindle. Although pericentrin was shown to localize in the cytoplasm and reported to be sensitive to leptomycin B (LMB), a specific inhibitor of Crm1, the regions within pericentrin that serve as signals for transporting in and out of the nucleus have not yet been identified. In this study, we identified five novel nuclear export signals (NESs) in pericentrin with diverse export activities. All of the five NESs could bind to Crm1 in a LMB-sensitive way when mediating the nuclear export of pericentrin. We also demonstrated that the region of amino acids 8-42 in pericentrin contains a tripartite nuclear localization signal (NLS) consisting of three clusters of basic amino acids. The NLS of pericentrin binds to importin beta directly or via the adaptor importin alpha to form the import complex, which could be disrupted by RanQ69L, a dominant-negative Ran GTPase possessing high affinity for importin beta. Furthermore, we found that mutation of the NESs in full-length pericentrin results in both nuclear and cytoplasmic localization, and mutation of the NLS abolishes the nuclear import of pericentrin. On the basis of these results, we suggest that the NESs and NLS of pericentrin are essential for its subcellular localization and nucleocytoplasmic trafficking during the cell cycle.

Neurogenin 3 cellular and subcellular localization in the developing and adult hippocampus

Neurogenin 3 (Ngn3), a proneural gene controlled by the Notch receptor, is implicated in the control of dendrite morphology and synaptic plasticity of cultured hippocampal neurons. Here we report the localization and subcellular distribution of Ngn3 in the hippocampus in vivo and in neuronal cultures. In situ hybridization showed Ngn3 mRNA expression in the pyramidal layer and dentate gyrus of adult mouse hippocampus. Immunohistochemistry studies revealed that Ngn3 localization is mostly cytoplasmic in the hippocampal eminence at embryonic day (E)17 and postnatal day (P)0. At P10 it is cytoplasmic in CA1-CA3 pyramidal neurons and nuclear in granule cells of the dentate gyrus. In the adult hippocampus Ngn3 is localized in the nucleus and cytoplasm of both pyramidal neurons and granule cells. During development of cultured hippocampal neurons, Ngn3 mRNA expression is higher at stages of neuronal polarization, as judged by reverse-transcription polymerase chain reaction (RT-PCR), and it is mostly cytoplasmic. The tracking of the subcellular localization of Ngn3 in neurons infected with a virus expressing myc-Ngn3 suggests that the protein is quickly translocated to the cell nucleus after synthesis and then reexported to the cytoplasm. Treatment with leptomycin B, a potent and specific inhibitor of the exportin CRM1, induced its accumulation into the nucleus, suggesting that CRM1 mediates the nuclear export of Ngn3. These results suggest that Ngn3 may play a role in neuronal development by actions in the cytoplasm.

Identification of functional domains in TdIF1 and its inhibitory mechanism for TdT activity

TdT interacting factor 1 (TdIF1) was identified as a protein that binds to terminal deoxynucleotidyltransferase (TdT) to negatively regulate TdT activity. TdT is a template-independent DNA polymerase that catalyzes the incorporation of deoxynucleotides to the 3'-hydroxyl end of DNA templates to increase the junctional diversity of immunoglobulin or T-cell receptor (TcR) genes. Here, using bioinformatics analysis, we identified the TdT binding, DNA binding and dimerization regions, and nuclear localization signal (NLS) in TdIF1. TdIF1 bound to double-stranded DNA (dsDNA) through three DNA binding regions: residues 1-75, the AT-hook-like motif (ALM) and the predicted helix-turn-helix (HTH) motif. ALM in TdIF1 preferentially bound to AT-rich DNA regions. NLS was of the bipartite type and overlapped ALM. TdIF1 bound to the Pol beta-like region in TdT and blocked TdT access to DNA ends. In the presence of dsDNA, however, TdIF1 bound to dsDNA to release TdT from the TdIF1/TdT complex and to exhibit TdT activity, implying that active TdT released microenvironmentally concentrates around AT-rich DNA to synthesize DNA.

Nucleocytoplasmic Shuttling of the Zinc Finger Protein EZI Is Mediated by Importin-7-dependent Nuclear Import and CRM1-independent Export Mechanisms

Nucleocytoplasmic translocation constitutes a foundation for nuclear proteins to exert their proper functions and hence for various biological reactions to occur normally in eukaryotic cells. We reported previously that EZI/Zfp467, a 12 zinc finger motif-containing protein, localizes predominantly in the nucleus, yet the underlying mechanism still remains elusive. Here we constructed a series of mutant forms of EZI and examined their subcellular localization. The results delineated a non-canonical nuclear localization signal in the region covering the 9th to the 12th zinc fingers, which was necessary for nuclear accumulation of EZI as well as sufficient to confer nuclear localizing ability to a heterologous protein. We also found that the N-terminal domain of EZI is necessary for its nuclear export, the process of which was not sensitive to the CRM1 inhibitor leptomycin B. An interaction proteomics approach and the following co-immunoprecipitation experiments identified the nuclear import receptor importin-7 as a molecule that associated with EZI and, importantly, short interfering RNA-mediated knockdown of importin-7 expression completely abrogated nuclear accumulation of EZI. Taken together, these results identify EZI as a novel cargo protein for importin-7 and demonstrate a nucleocytoplasmic shuttling mechanism that is mediated by importin-7-dependent nuclear localization and CRM1-independent nuclear export.

Interphase nucleo-cytoplasmic shuttling and localization of SIRT2 during mitosis

The human NAD+-dependent protein deacetylase SIRT2 resides predominantly in the cytoplasm where it functions as a tubulin deacetylase. Here we report that SIRT2 maintains a largely cytoplasmic localization during interphase by active nuclear export in a Crm1-dependent manner. We identified a functional, leptomycin B-sensitive, nuclear export signal sequence within SIRT2. During the cell cycle, SIRT2 becomes enriched in the nucleus and is associated with mitotic structures, beginning with the centrosome during prophase, the mitotic spindle during metaphase, and the midbody during cytokinesis. Cells overexpressing wild-type or a catalytically inactive SIRT2 exhibit an increase in multinucleated cells. The findings suggest a novel mechanism of regulating SIRT2 function by nucleo-cytoplasmic shuttling, as well as a role for SIRT2 in the nucleus during interphase and throughout mitosis.

Poliovirus infection blocks ERGIC-to-Golgi trafficking and induces microtubule-dependent disruption of the Golgi complex

Cells infected with poliovirus exhibit a rapid inhibition of protein secretion and disruption of the Golgi complex. Neither the precise step at which the virus inhibits protein secretion nor the fate of the Golgi complex during infection has been determined. We find that transport-vesicle exit from the endoplasmic reticulum (ER) and trafficking to the ER-Golgi intermediate compartment (ERGIC) are unaffected in the poliovirus-infected cell. By contrast, poliovirus infection blocks transport from the ERGIC to the Golgi complex. Poliovirus infection also induces fragmentation of the Golgi complex resulting in diffuse distribution of both large and small vesicles throughout the cell. Pre-treatment with nocodazole prevents complete fragmentation, indicating that microtubules are required for poliovirus-induced Golgi dispersion. However, virally induced inhibition of the secretory pathway is not affected by nocodazole, and Golgi dispersion was found to occur during infection with mutant viruses with reduce ability to inhibit protein secretion. We conclude that the dispersion of the Golgi complex is not in itself the cause of inhibition of traffic between the ERGIC and the Golgi. Instead, these phenomena are independent effects of poliovirus infection on the host secretory complex.

G-protein-coupled receptors signalling at the cell nucleus: an emerging paradigm

G-protein-coupled receptors (GPCRs) comprise a wide family of monomeric heptahelical glycoproteins that recognize a broad array of extracellular mediators including cationic amines, lipids, peptides, proteins, and sensory agents. Thus far, much attention has been given towards the comprehension of intracellular signaling mechanisms activated by cell membrane GPCRs, which convert extracellular hormonal stimuli into acute, non-genomic (e.g., hormone secretion, muscle contraction, and cell metabolism) and delayed, genomic biological responses (e.g., cell division, proliferation, and apoptosis). However, with respect to the latter response, there is compelling evidence for a novel intracrine mode of genomic regulation by GPCRs that implies either the endocytosis and nuclear translocation of peripheral-liganded GPCR and (or) the activation of nuclearly located GPCR by endogenously produced, nonsecreted ligands. A noteworthy example of the last scenario is given by heptahelical receptors that are activated by bioactive lipoids (e.g., PGE(2) and PAF), many of which may be formed from bilayer membranes including those of the nucleus. The experimental evidence for the nuclear localization and signalling of GPCRs will be reviewed. We will also discuss possible molecular mechanisms responsible for the atypical compartmentalization of GPCRs at the cell nucleus, along with their role in gene expression.

Exportin-5 orthologues are functionally divergent among species

Exportin-5, an evolutionarily conserved nuclear export factor belonging to the importin-β family of proteins, is known to play a role in the nuclear export of small noncoding RNAs such as precursors of microRNA, viral minihelix RNA and a subset of tRNAs in mammalian cells. In this study, we show that the exportin-5 orthologues from different species such as human, fruit fly and yeast exhibit diverged functions. We found that Msn5p, a yeast exportin-5 orthologue, binds double-stranded RNAs and that it prefers a shorter 22 nt, double-stranded RNA to ∼80 nt pre-miRNA, even though both of these RNAs share a similar terminal structure. Furthermore, we found that Drosophila exportin-5 binds pre-miRNAs and that amongst the exportin-5 orthologues tested, it shows the highest affinity for tRNAs. The knockdown of Drosophila exportin-5 in cultured cells decreased the amounts of tRNA as well as miRNA, whereas the knock down of human exportin-5 in cultured cells affected only miRNA but not tRNA levels. These results indicate that double-stranded RNA binding ability is an inherited functional characteristic of the exportin-5 orthologues and that Drosophila exportin-5 functions as an exporter of tRNAs as well as pre-miRNAs in the fruit fly that lacks the orthologous gene for exportin-t.

Cell-intrinsic regulation of axonal morphogenesis by the Cdh1-APC target SnoN

Axonal growth is fundamental to the establishment of neuronal connectivity in the brain. However, the cell-intrinsic mechanisms that govern axonal morphogenesis remain to be elucidated. The ubiquitin ligase Cdh1-anaphase-promoting complex (Cdh1-APC) suppresses the growth of axons in postmitotic neurons. Here, we report that Cdh1-APC operates in the nucleus to inhibit axonal growth. We also identify the transcriptional corepressor SnoN as a key target of neuronal Cdh1-APC that promotes axonal growth. Cdh1 forms a physical complex with SnoN and stimulates the ubiquitin-dependent proteasomal degradation of SnoN in neurons. Knockdown of SnoN in neurons significantly reduces axonal growth and suppresses Cdh1 RNAi enhancement of axonal growth. In addition, SnoN knockdown in vivo suggests an essential function for SnoN in the development of granule neuron parallel fibers in the cerebellar cortex. These findings define Cdh1-APC and SnoN as components of a cell-intrinsic pathway that orchestrates axonal morphogenesis in a transcription-dependent manner in the mammalian brain.

Mechanism of the stress-induced collapse of the Ran distribution

The small GTPase Ran plays a central role in several key nuclear functions, including nucleocytoplasmic transport, cell cycle progression, and assembly of the nuclear envelope. In a previous study, we showed that cellular stress induces the nuclear accumulation of importin alpha, and that this appears to be triggered by a collapse in the Ran gradient, leading to the down-regulation of classical nuclear transport. We report here that a decrease in stress-induced ATP is associated with an increase in cytoplasmic Ran levels. A luciferin-luciferase assay showed that cellular stress decreased the intracellular levels of ATP. Treatment of the cells with ATP-depleting agents altered the distribution of Ran. Furthermore, when exogenous ATP was introduced in oxidative stress-treated cells, a normal distribution of Ran was restored. In addition, a pull-down experiment with an importin beta1 variant that binds to RanGTP showed that oxidative stress was accompanied by a decrease in intracellular RanGTP levels. These findings indicate that the decrease in ATP levels induced by cellular stress causes a decrease in RanGTP levels and a collapse of Ran distribution.

Camptothecin induces nuclear export of prohibitin preferentially in transformed cells through a CRM-1-dependent mechanism

Prohibitin is a growth-suppressive protein that has multiple functions in the nucleus and the mitochondria. Our earlier studies had shown that prohibitin represses the activity of E2F transcription factors while enhancing p53-mediated transcription. At the same time, prohibitin has been implicated in mediating the proper folding of mitochondrial proteins. We had found that treatment of cells with camptothecin, a topoisomerase 1 inhibitor, led to the export of prohibitin and p53 from the nucleus to the mitochondria. Here we show that the camptothecin-induced export of prohibitin occurs preferentially in transformed cell lines, but not in untransformed or primary cells. Cells that did not display the translocation of prohibitin were refractive to the apoptotic effects of camptothecin. The translocation was mediated by a putative nuclear export signal at the C-terminal region of prohibitin; fusion of the nuclear export signal (NES) of prohibitin to green fluorescence protein led to its export from the nucleus. Leptomycin B could inhibit the nuclear export of prohibitin showing that it was a CRM-1-dependent event driven by Ran GTPase. Confirming this, prohibitin was found to physically interact with CRM-1, and this interaction was significantly higher in transformed cells. Delivery of a peptide corresponding to the NES of prohibitin prevented the export of prohibitin to cytoplasm and protected cells from apoptosis. These results suggest that the regulated translocation of prohibitin from the nucleus to the mitochondria facilitates its pleiotropic functions and might contribute to its anti-proliferative and tumor suppressive properties.

Nucleolar localization of cirhin, the protein mutated in North American Indian childhood cirrhosis

Cirhin (NP_116219), the product of the CIRH1A gene is mutated in North American Indian childhood cirrhosis (NAIC/CIRH1A, OMIM 604901), a severe autosomal recessive intrahepatic cholestasis. It is a 686-amino-acid WD40-repeat containing protein of unknown function that is predicted to contain multiple targeting signals, including an N-terminal mitochondrial targeting signal, a C-terminal monopartite nuclear localization signal (NLS) and a bipartite nuclear localization signal (BNLS). We performed the direct determination of subcellular localization of cirhin as a crucial first step in unraveling its biological function. Using EGFP and His-tagged cirhin fusion proteins expressed in HeLa and HepG2, cells we show that cirhin is a nucleolar protein and that the R565W mutation, for which all NAIC patients are homozygous, has no effect on subcellular localization. Cirhin has an active C-terminal monopartite nuclear localization signal (NLS) and a unique nucleolar localization signal (NrLS) between residues 315 and 432. The nucleolus is not known to be important specifically for intrahepatic cholestasis. These observations provide a new dimension in the study of hereditary cholestasis.

Detailed mapping of the nuclear export signal in the Rous sarcoma virus Gag protein

The Rous sarcoma virus (RSV) Gag polyprotein undergoes transient nuclear trafficking as an intrinsic part of the virus assembly pathway. Nuclear export of Gag is crucial for the efficient production of viral particles and is accomplished through the action of a leptomycin B (LMB)-dependent nuclear export signal (NES) in the p10 domain (L. Z. Scheifele, R. A. Garbitt, J. D. Rhoads, and L. J. Parent, Proc. Natl. Acad. Sci. USA 99:3944-3949, 2002). We have now mapped the nuclear export activity to the C-terminal portion of the p10 sequence and identified the four hydrophobic amino acids within this region that comprise a leucine-rich NES. Alteration of these hydrophobic residues resulted in the accumulation of Gag proteins within the nucleus and a budding defect greater than that obtained with LMB treatment of cells expressing the wild-type Gag protein (Scheifele et al., Proc. Natl. Acad. Sci. USA 99:3944-3949, 2002). In addition, export of Gag from the nucleus was found to be a rate-limiting step in virus-like particle production. Consistent with a role for the NES sequence in viral replication, this cluster of hydrophobic residues in p10 is conserved across a wide range of avian retroviruses. Furthermore, naturally occurring substitutions within this region in related viruses maintained nuclear export activity and remained sensitive to the activity of LMB. Using gain-of-function approaches, we found that the hydrophobic motif in p10 was sufficient to promote the nuclear export of a heterologous protein and was positionally independent within the Gag polyprotein. Finally, the export pathway was further defined by the ability of specific nucleoporin inhibitors to prevent the egress of Gag from the nucleus, thereby identifying additional cellular mediators of RSV replication.

Zinc finger domain of Snail functions as a nuclear localization signal for importin β-mediated nuclear import pathway

Snail, a DNA-binding zinc finger protein, functions as a transcriptional repressor for genes including E-cadherin during development and the acquisition of tumor cell invasiveness. Human Snail is a 264-amino acid nuclear protein with an amino-terminal basic amino acid-rich domain (SNAG domain) and a carboxyl-terminal DNA-binding domain (zinc finger domain). A series of fusion proteins composed of green fluorescent protein (GFP) and portions of the Snail protein were generated, and their subcellular localization was examined. Fusion of the four zinc fingers to GFP led to the targeting of GFP to the nucleus, demonstrating that the zinc finger domain is sufficient for nuclear localization. Using an in vitro transport system, the nuclear import of Snail was reconstituted by importin (karyopherin) beta in the presence of Ran and NTF2. We further demonstrated that Snail binds directly to importin beta in a zinc finger domain-dependent manner. These results indicate that zinc finger domain of Snail functions as a nuclear localization signal and Snail can be transported into the nucleus in an importin beta-mediated manner.

Enhanced nuclear import and transfection efficiency of plasmid DNA using streptavidin-fused importin-β

In order to enhance the nuclear import of exogenous genes, novel plasmid DNA/importin-beta conjugates, which consist of a biotinylated plasmid DNA and a recombinant streptavidin-fused importin-beta, were prepared. The spacer length between plasmid DNA and biotin and the number of introduced biotin were adjusted. The microinjection of plasmid DNA/importin-beta conjugates into the cytoplasm of NIH3T3 cells resulted in the nuclear localization of conjugates and the higher expression efficiency, compared to intact plasmid DNA alone. These results indicate that plasmid DNA/importin-beta conjugates would be an important tool to enhance the nuclear localization of exogenous DNA in non-viral gene delivery system.

The ins and outs of FoxO shuttling: mechanisms of FoxO translocation and transcriptional regulation

FoxO (forkhead box O; forkhead members of the O class) are transcription factors that function under the control of insulin/insulin-like signalling. FoxO factors have been associated with a multitude of biological processes, including cell-cycle, cell death, DNA repair, metabolism and protection from oxidative stress. Central to the regulation of FoxO factors is a shuttling system, which confines FoxO factors to either the nucleus or the cytosol. Shuttling of FoxO requires protein phosphorylation within several domains, and association with 14-3-3 proteins and the nuclear transport machinery. Description of the FoxO-shuttling mechanism contributes to the understanding of FoxO function in relation to signalling and gene regulation.

Cellular stresses induce the nuclear accumulation of importin alpha and cause a conventional nuclear import block

We report here that importin α accumulates reversibly in the nucleus in response to cellular stresses including UV irradiation, oxidative stress, and heat shock. The nuclear accumulation of importin α appears to be triggered by a collapse in the Ran gradient, resulting in the suppression of the nuclear export of importin α. In addition, nuclear retention and the importin β/Ran-independent import of importin α also facilitate its rapid nuclear accumulation. The findings herein show that the classical nuclear import pathway is down-regulated via the removal of importin α from the cytoplasm in response to stress. Moreover, whereas the nuclear accumulation of heat shock cognate 70 is more sensitive to heat shock than the other stresses, importin α is able to accumulate in the nucleus at all the stress conditions tested. These findings suggest that the stress-induced nuclear accumulation of importin α can be involved in a common physiological response to various stress conditions.

DT-diaphorase: a target for new anticancer drugs

DT-diaphorase (DTD) is an obligate two-electron reductase which bioactivates chemotherapeutic quinones. DTD levels are elevated in a number of tumour types, including non-small cell lung carcinoma, colorectal carcinoma, liver cancers and breast carcinomas, when compared to the surrounding normal tissue. The differential in DTD between tumour and normal tissue should allow targeted activation of chemotherapeutic quinones in the tumour whilst minimising normal tissue toxicity. The prototypical bioreductive drug is Mitomycin C (MMC) which is widely used in clinical practice. However, MMC is actually a relatively poor substrate for DTD and its metabolism is pH-dependent. Other bioreductive drugs have failed because of poor solubility and inability to surpass other agents in use. RH1, a novel diaziridinylbenzoquinone, is a more efficient substrate for DTD. It has been demonstrated to have anti-tumour effects both in vitro and in vivo and demonstrates a relationship between DTD expression levels and drug response. RH1 has recently entered a phase I clinical trial in solid tumours under the auspices of Cancer Research UK. Recent work has demonstrated that DTD is present in the nucleus and is associated with both p53 and the heat shock protein, HSP-70. Furthermore, DTD is inducible by several non-toxic compounds and therefore much interest has focussed on increasing the differential in DTD levels between tumour and normal tissues.

Human topoisomerase IIalpha nuclear export is mediated by two CRM-1-dependent nuclear export signals

Resistance to chemotherapeutic drugs is a major obstacle in the treatment of leukemia and multiple myeloma. We have previously found that myeloma and leukemic cells in transition from low-density log phase conditions to high-density plateau phase conditions export substantial amounts of endogenous topoisomerase II alpha from the nucleus to the cytoplasm. In order for topoisomerase-targeted chemotherapy to function, the topoisomerase target must have access to the nuclear DNA. Therefore, the nuclear export of topoisomerase II alpha may contribute to drug resistance, and defining this mechanism may lead to methods to preclude this avenue of resistance. We have identified nuclear export signals for topoisomerase II alpha at amino acids 1017-1028 and 1054-1066, using FITC-labeled BSA-export signal peptide conjugates microinjected into the nuclei of HeLa cells. Functional confirmation of both signals (1017-1028 and 1054-1066) was provided by transfection of human myeloma cells with plasmids containing the gene for a full-length human FLAG-topoisomerase fusion protein, mutated at hydrophobic amino acid residues in the export signals. Of the six putative export signals tested, the two sites above were found to induce export into the cytoplasm. Export by both signals was blocked by treatment of the cells with leptomycin B, indicating that a CRM-1-dependent pathway mediates export. Site-directed mutagenesis of two central hydrophobic residues in either export signal in full-length human topoisomerase blocked export of recombinant FLAG-topoisomerase II alpha, indicating that both signals may be required for export. Interestingly, this pair of nuclear export signals (1017-1028 and 1054-1066) also defines a dimerization domain of the topoisomerase II alpha molecule.

Signals that dictate nuclear localization of human papillomavirus type 16 oncoprotein E6 in living cells

Human papillomavirus (HPV) type 16 E6 (16E6) is an oncogenic, multifunctional nuclear protein that induces p53 degradation and perturbs normal cell cycle control, leading to immortalization and transformation of infected keratinocytes and epithelial cells. Although it is unclear how 16E6 disrupts the epigenetic profile of host genes, its presence in the nucleus is a key feature. The present report describes intrinsic properties of 16E6 that influence its nuclear import in living cells. When the coding region of full-length 16E6 was inserted in frame into the C terminus of green fluorescent protein (GFP), it effectively prevented the 16E6 pre-mRNA from being spliced and led to the expression of a GFP-E6 fusion which localized predominantly to the nucleus. Further studies identified three novel nuclear localization signals (NLSs) in 16E6 that drive the protein to accumulate in the nucleus. We found that all three NLS sequences are rich in positively charged basic residues and that point mutations in these key residues could abolish the retention of 16E6 in the nucleus as well as the p53 degradation and cell immortalization activities of the protein. When inserted into corresponding regions of low-risk HPV type 6 E6, the three NLS sequences described for 16E6 functioned actively in converting the normally cytoplasmic HPV type 6 E6 into a nuclear protein. The separate NLS sequences, however, appear to play different roles in nuclear import and retention of HPV E6. The discovery of three unique NLS sequences in 16E6 provides new insights into the nuclear association of 16E6 which may reveal other novel activities of this important oncogenic protein.

A possible novel mechanism underlying temperature-dependent uptake of [3H]spermidine in nuclear fractions of murine brain

[3H]Spermidine (SPD) was accumulated in subcellular fractions enriched of the nucleus in a temperature-dependent manner with a saturable profile in murine brain. The accumulation reached a plateau within 30 min at 2 degrees C and 30 degrees C, while excess unlabeled SPD significantly inhibited the accumulation at 2 degrees C without markedly affecting that at 30 degrees C when added after equilibrium. The temperature-dependent accumulation of [3H]SPD was significantly inhibited by the triamine SPD and the tetraamine spermine, but not by the diamine putrescine. Phospholipases were invariably effective in significantly inhibiting the accumulation at 30 degrees C in a concentration-dependent manner. Amongst different discrete murine central structures examined, the temperature-dependent [3H]SPD accumulation was highest in neocortex with progressively lower activities in striatum, hypothalamus, spinal cord, medulla-pons, hippocampus, midbrain and cerebellum. These results suggest the possible presence of a hitherto unidentified nuclear transport system for particular polyamines in murine brain.