Evidence for distinct substrate specificities of importin alpha family members in nuclear protein import

Search for KPNA7 cargo proteins in human cells reveals MVP and ZNF414 as novel regulators of cancer cell growth

Karyopherin alpha 7 (KPNA7) belongs to a family of nuclear import proteins that recognize and bind nuclear localization signals (NLSs) in proteins to be transported to the nucleus. Previously we found that KPNA7 is overexpressed in a subset of pancreatic cancer cell lines and acts as a critical regulator of growth in these cells. This characteristic of KPNA7 is likely to be mediated by its cargo proteins that are still mainly unknown. Here, we used protein affinity chromatography in Hs700T and MIA PaCa-2 pancreatic cancer cell lines and identified 377 putative KPNA7 cargo proteins, most of which were known or predicted to localize to the nucleus. The interaction was confirmed for two of the candidates, MVP and ZNF414, using co-immunoprecipitation, and their transport to the nucleus was hindered by siRNA based KPNA7 silencing. Most importantly, silencing of MVP and ZNF414 resulted in marked reduction in Hs700T cell growth. In conclusion, these data uncover two previously unknown human KPNA7 cargo proteins with distinct roles as novel regulators of pancreatic cancer cell growth, thus deepening our understanding on the contribution of nuclear transport in cancer pathogenesis.

Knockdown of CSE1L Gene in Colorectal Cancer Reduces Tumorigenesis in Vitro

Human cellular apoptosis susceptibility (chromosomal segregation 1-like, CSE1L) gene plays a role in nuclear-to-cytoplasm transport and chromosome segregation during mitosis, cellular proliferation, and apoptosis. CSE1L is involved in colon carcinogenesis. CSE1L gene expression was assessed with three data sets using Affymetrix U133 + gene chips on normal human colonic mucosa (NR), adenomas (ADs), and colorectal carcinoma (CRC). CSE1L protein expression in CRC, AD, and NR from the same patients was measured by immunohistochemistry using a tissue microarray. We evaluated CSE1L expression in CRC cells (HCT116, SW480, and HT29) and its biological functions. CSE1L mRNA was significantly increased in all AD and CRC compared with NR (P < 0.001 and P = 0.02, respectivly). We observed a change in CSE1L staining intensity and cellular localization by immunohistochemistry. CSE1L was significantly increased during the transition from AD to CRC when compared with NR in a CRC tissue microarray (P = 0.01 and P < 0.001). HCT116, SW480, and HT29 cells also expressed CSE1L protein. CSE1L knockdown by shRNA inhibited protein, resulting in decreased cell proliferation, reduced colony formation in soft agar, and induction of apoptosis. CSE1L protein is expressed early and across all stages of CRC development. shRNA knockdown of CSE1L was associated with inhibition of tumorigenesis in CRC cells. CSE1L may represent a potential target for treatment of CRC.

Karyopherin Alpha 1 Regulates Satellite Cell Proliferation and Survival by Modulating Nuclear Import

Satellite cells are stem cells with an essential role in skeletal muscle repair. Precise regulation of gene expression is critical for proper satellite cell quiescence, proliferation, differentiation and self-renewal. Nuclear proteins required for gene expression are dependent on the nucleocytoplasmic transport machinery to access to nucleus, however little is known about regulation of nuclear transport in satellite cells. The best characterized nuclear import pathway is classical nuclear import which depends on a classical nuclear localization signal (cNLS) in a cargo protein and the heterodimeric import receptors, karyopherin alpha (KPNA) and beta (KPNB). Multiple KPNA1 paralogs exist and can differ in importing specific cNLS proteins required for cell differentiation and function. We show that transcripts for six Kpna paralogs underwent distinct changes in mouse satellite cells during muscle regeneration accompanied by changes in cNLS proteins in nuclei. Depletion of KPNA1, the most dramatically altered KPNA, caused satellite cells in uninjured muscle to prematurely activate, proliferate and undergo apoptosis leading to satellite cell exhaustion with age. Increased proliferation of satellite cells led to enhanced muscle regeneration at early stages of regeneration. In addition, we observed impaired nuclear localization of two key KPNA1 cargo proteins: p27, a cyclin-dependent kinase inhibitor associated with cell cycle control and lymphoid enhancer factor 1, a critical cotranscription factor for β-catenin. These results indicate that regulated nuclear import of proteins by KPNA1 is critical for satellite cell proliferation and survival and establish classical nuclear import as a novel regulatory mechanism for controlling satellite cell fate. Stem Cells 2016;34:2784-2797.

Nipah virus matrix protein: expert hacker of cellular machines

Nipah virus (NiV, Henipavirus) is a highly lethal emergent zoonotic paramyxovirus responsible for repeated human outbreaks of encephalitis in South East Asia. There are no approved vaccines or treatments, thus improved understanding of NiV biology is imperative. NiV matrix protein recruits a plethora of cellular machinery to scaffold and coordinate virion budding. Intriguingly, matrix also hijacks cellular trafficking and ubiquitination pathways to facilitate transient nuclear localization. While the biological significance of matrix nuclear localization for an otherwise cytoplasmic virus remains enigmatic, the molecular details have begun to be characterized, and are conserved among matrix proteins from divergent paramyxoviruses. Matrix protein appropriation of cellular machinery will be discussed in terms of its early nuclear targeting and later role in virion assembly.

Brain Corticosteroid Receptor Dynamics and Trafficking: Implications from Live Cell Imaging

Adrenal corticosteroids (cortisol in humans or corticosterone in rodents) exert numerous effects in the central nervous system that regulate the stress response, mood, learning and memory, and various neuroendocrine functions. Corticosterone actions in the brain are mediated by two corticosteroid receptors, glucocorticoid receptor (GR) and mineralocorticoid receptor (MR), and they show a high degree of colocalization in the hippocampal region. These receptors predominantly reside in the cytoplasm without ligand and are translocated into the nucleus upon ligand binding to act as transcriptional factors. Thus, their subcellualr localizations are an important component of their biological activity. Given the differential action of MR and GR in the central nervous system, it is important to elucidate how the trafficking of these receptors between the cytoplasm and the nucleus and their interactions are regulated by ligand or other molecules to exert transcriptional activity. In this review, the authors focus on the nucleocytoplasmic and subnuclear trafficking of GR and MR in neural cells and nonneural cells and discuss various factors affecting the dynamics of these receptors.

Nuclear import of prototype foamy virus transactivator Bel1 is mediated by KPNA1, KPNA6 and KPNA7

Bel1, a transactivator of the prototype foamy virus (PFV), plays pivotal roles in the replication of PFV. Previous studies have demonstrated that Bel1 bears a nuclear localization signal (NLS); however, its amino acid sequence remains unclear and the corresponding adaptor importins have not yet been identified. In this study, we inserted various fragments of Bel1 into an EGFP-GST fusion protein and investigated their subcellular localization by fluorescence microscopy. We found that the 215PRQKRPR221 fragment, which accords with the consensus sequence K(K/R)X(K/R) of the monopartite NLS, directed the nuclear translocation of Bel1. Point mutation experiments revealed that K218, R219 and R221 were essential for the nuclear localization of Bel1. The results of GST pull-down assay revealed that the Bel1 peptide 215-221, which bears the NLS, interacted with the nucleocytoplasmic transport receptors, karyopherin alpha 1 (importin alpha 5) (KPNA1), karyopherin alpha 6 (importin alpha 7) (KPNA6) and karyopherin alpha 7 (importin alpha 8) (KPNA7). Finally, in vitro nuclear import assays demonstrated that KPNA1, KPNA6 or KPNA7, along with other necessary nuclear factors, caused Bel1 to localize to the nucleus. Thus, the findings of our study indicate that KPNA1, KPNA6 and KPNA7 are involved in Bel1 nuclear distribution.

Expression of importin-α isoforms in human nasal mucosa: implication for adaptation of avian influenza A viruses to human host

BACKGROUND

Transportation into the host cell nucleus is crucial for replication and transcription of influenza virus. The classical nuclear import is regulated by specific cellular factor, importin-α. Seven isoforms of importin-α have been identified in human. The preference of importin-α3 of avian influenza virus and -α7 isoform of human strains during replication in human cells was previously identified. In addition, both avian and human influenza viruses were shown to use importin-α1 isoform for their replication.

FINDING

The mRNA levels of importin-α1, -α3, and -α7 isoforms in human respiratory tract was determined by real-time RT-PCR. The results indicate that mRNA level of importin-α7 was significantly higher than that of importin-α1 (p-value < 0.0001) and importin-α3 (p-value < 0.0001) isoforms in human nasal mucosa while importin-α1 was detected as the highest expression importin-α isoform in lung tissues.

CONCLUSIONS

These results may explain the preference of importin-α7 isoforms in seasonal influenza viruses in human upper respiratory tract and may suggest a selective pressure toward importin-α7 in human respiratory tract infection of an avian virus.

Nuclear import of the thyroid hormone receptor α1 is mediated by importin 7, importin β1, and adaptor importin α1

The thyroid hormone receptor α1 (TRα1) is a nuclear receptor for thyroid hormone that shuttles rapidly between the nucleus and cytoplasm. Our prior studies showed that nuclear import of TRα1 is directed by two nuclear localization signals, one in the N-terminal A/B domain and the other in the hinge domain. Here, we showed using in vitro nuclear import assays that TRα1 nuclear localization is temperature and energy-dependent and can be reconstituted by the addition of cytosol. In HeLa cells expressing green fluorescent protein (GFP)-tagged TRα1, knockdown of importin 7, importin β1 and importin α1 by RNA interference, or treatment with an importin β1-specific inhibitor, significantly reduced nuclear localization of TRα1, while knockdown of other importins had no effect. Coimmunoprecipitation assays confirmed that TRα1 interacts with importin 7, as well as importin β1 and the adapter importin α1, suggesting that TRα1 trafficking into the nucleus is mediated by two distinct pathways.

Molecular mechanism by which acyclic retinoid induces nuclear localization of transglutaminase 2 in human hepatocellular carcinoma cells

Nuclear accumulation of transglutaminase 2 (TG2) is an important step in TG2-dependent cell death. However, the underlying molecular mechanisms for nuclear translocation of TG2 are still poorly understood. In this study, we demonstrated that acyclic retinoid (ACR) induced nuclear accumulation of TG2 in JHH-7 cells, a hepatocellular carcinoma (HCC) leading to their apoptosis. We further demonstrated molecular mechanism in nuclear-cytoplasmic trafficking of TG2 and an effect of ACR on it. We identified a novel 14-amino acid nuclear localization signal (NLS) (466)AEKEETGMAMRIRV(479) in the 'C' domain and a leucine-rich nuclear export signal (NES) (657)LHMGLHKL(664) in the 'D' domain that allowed TG2 to shuttle between the nuclear and cytosolic milieu. Increased nuclear import of GAPDH myc-HIS fused with the identified NLS was observed, confirming its nuclear import ability. Leptomycin B, an inhibitor of exportin-1 as well as point mutation of all leucine residues to glutamine residues in the NES of TG2 demolished its nuclear export. TG2 formed a trimeric complex with importin-α and importin-β independently from transamidase activity which strongly suggested the involvement of a NLS-based translocation of TG2 to the nucleus. ACR accelerated the formation of the trimeric complex and that may be at least in part responsible for enhanced nuclear localization of TG2 in HCC cells treated with ACR.

Structural Biology and Regulation of Protein Import into the Nucleus

Proteins are translated in the cytoplasm, but many need to access the nucleus to perform their functions. Understanding how these nuclear proteins are transported through the nuclear envelope and how the import processes are regulated is therefore an important aspect of understanding cell function. Structural biology has played a key role in understanding the molecular events during the transport processes and their regulation, including the recognition of nuclear targeting signals by the corresponding receptors. Here, we review the structural basis of the principal nuclear import pathways and the molecular basis of their regulation. The pathways involve transport factors that are members of the β-karyopherin family, which can bind cargo directly (e.g., importin-β, transportin-1, transportin-3, importin-13) or through adaptor proteins (e.g., importin-α, snurportin-1, symportin-1), as well as unrelated transport factors such as Hikeshi, involved in the transport of heat-shock proteins, and NTF2, involved in the transport of RanGDP. Solenoid proteins feature prominently in these pathways. Nuclear transport factors recognize nuclear targeting signals on the cargo proteins, including the classical nuclear localization signals, recognized by the adaptor importin-α, and the PY nuclear localization signals, recognized by transportin-1. Post-translational modifications, particularly phosphorylation, constitute key regulatory mechanisms operating in these pathways.

Dynamic changes in nuclear import of a nuclear localisation signal-bearing substrate in 8-cell stage porcine embryos

Coordinated intracellular trafficking is critically important for proper timing of major cellular events during embryogenesis. Nuclear import mediated by the karyopherin α/β (importin α/β) heterodimer is perhaps the best characterised nuclear trafficking system in eukaryotic cells. Seven karyopherin α subtypes have been identified in the domestic pig, and although each karyopherin α subtype transports proteins bearing classical nuclear localisation signals (NLSs), individual karyopherin α subtypes have been shown to preferentially transport specific cargoes. The aim of the present study was to determine the mechanism by which BRN2, a transcription factor previously reported to be transported by the karyopherin α/β heterodimer, gains access to the nucleus in porcine oocytes and embryos. Using a combination of in vivo and in vitro assays, we tested the hypothesis that discrete karyopherin α subtypes transport BRN2 into the nuclei of porcine oocytes and cleavage stage embryos. Our results show that ectopically expressed BRN2 adopts a nuclear localisation in all nuclei through the 4-cell stage of development, whereas only a subset of blastomeres in 8-cell stage embryos possess nuclear BRN2. This pattern is unique to BRN2 because another ectopically expressed NLS-containing protein is able to adopt a nuclear localisation in all blastomeres of 8-cell stage embryos.

Up-Regulation of KPNB1 Involves in Neuronal Apoptosis Following Intracerebral Hemorrhage in Adult Rats

Kpnb1, also known as Importin β1, is a member of the Karyopherin protein family which plays a important role in nuclear import and export pathways. Its expression has been shown to be responsive to stress, such as heat shock, ethanol and oxidative stress. Previous studies demonstrated that Kpnb1 had anti-apoptotic in cervical cancer. These together prompted us to explore whether Kpnb1 has some association with neuron apoptosis in the pathophysiology of intracerebral hemorrhage (ICH). In our study, an ICH model was established by injecting into the right basal ganglia of adult rats with their autologous whole blood and assessed by behavioral tests. We found Kpnb1 were significantly up-regulated adjacent to the hematoma following ICH by Western blot and immunohistochemistry. Double immunofluorenscence manifested Kpnb1 was strikingly increased in neurons, not astrocytes or microglia. Furthermore, we also found that kpnb1 had co-localizations with active-caspase-3 which is a neuronal apoptosis marker suggesting its role in neuronal apoptosis. What's more, our in vitro study, using Kpnb1 RNA interference in PC12 cells, further indicated that Kpnb1 might exert its pro-apoptotic function on neuronal apoptosis. Therefore, Kpnb1 may play a role in the neuronal apoptosis following ICH.

Intracellular Dynamics of Synucleins: "Here, There and Everywhere"

Synucleins are small, soluble proteins expressed primarily in neural tissue and in certain tumors. The synuclein family consists of three members: α-, β-, and γ-synucleins present only in vertebrates. Members of the synuclein family have high sequence identity, especially in the N-terminal regions. The synuclein gene family came into the spotlight, when one of its members, α-synuclein, was found to be associated with Parkinson's disease and other neurodegenerative disorders, whereas γ-synuclein was linked to several forms of cancer. There are a lot of controversy and exciting debates concerning members of the synuclein family, including their normal functions, toxicity, role in pathology, transmission between cells and intracellular localization. Important findings which remain undisputable for many years are synuclein localization in synapses and their role in the regulation of synaptic vesicle trafficking, whereas their presence and function in mitochondria and nucleus is a debated topic. In this review, we present the data on the localization of synucleins in two intracellular organelles: the nucleus and mitochondria.

BIG3 Inhibits the Estrogen-Dependent Nuclear Translocation of PHB2 via Multiple Karyopherin-Alpha Proteins in Breast Cancer Cells

We recently reported that brefeldin A-inhibited guanine nucleotide-exchange protein 3 (BIG3) binds Prohibitin 2 (PHB2) in cytoplasm, thereby causing a loss of function of the PHB2 tumor suppressor in the nuclei of breast cancer cells. However, little is known regarding the mechanism by which BIG3 inhibits the nuclear translocation of PHB2 into breast cancer cells. Here, we report that BIG3 blocks the estrogen (E2)-dependent nuclear import of PHB2 via the karyopherin alpha (KPNA) family in breast cancer cells. We found that overexpressed PHB2 interacted with KPNA1, KPNA5, and KPNA6, thereby leading to the E2-dependent translocation of PHB2 into the nuclei of breast cancer cells. More importantly, knockdown of each endogenous KPNA by siRNA caused a significant inhibition of E2-dependent translocation of PHB2 in BIG3-depleted breast cancer cells, thereby enhancing activation of estrogen receptor alpha (ERα). These data indicated that BIG3 may block the KPNAs (KPNA1, KPNA5, and KPNA6) binding region(s) of PHB2, thereby leading to inhibition of KPNAs-mediated PHB2 nuclear translocation in the presence of E2 in breast cancer cells. Understanding this regulation of PHB2 nuclear import may provide therapeutic strategies for controlling E2/ERα signals in breast cancer cells.

Diversification of importin-α isoforms in cellular trafficking and disease states

The human genome encodes seven isoforms of importin α which are grouped into three subfamilies known as α1, α2 and α3. All isoforms share a fundamentally conserved architecture that consists of an N-terminal, autoinhibitory, importin-β-binding (IBB) domain and a C-terminal Arm (Armadillo)-core that associates with nuclear localization signal (NLS) cargoes. Despite striking similarity in amino acid sequence and 3D structure, importin-α isoforms display remarkable substrate specificity in vivo. In the present review, we look at key differences among importin-α isoforms and provide a comprehensive inventory of known viral and cellular cargoes that have been shown to associate preferentially with specific isoforms. We illustrate how the diversification of the adaptor importin α into seven isoforms expands the dynamic range and regulatory control of nucleocytoplasmic transport, offering unexpected opportunities for pharmacological intervention. The emerging view of importin α is that of a key signalling molecule, with isoforms that confer preferential nuclear entry and spatiotemporal specificity on viral and cellular cargoes directly linked to human diseases.

Suppression of the nuclear transporter-KPNβ1 expression inhibits tumor proliferation in hepatocellular carcinoma

BACKGROUND

Hepatocellular carcinoma (HCC) is one of the malignant tumors and leads to the highly death in the solid tumors, but its mechanism remains unclear. KPNβ1 is one of the soluble nuclear transport receptors, has been reported to act as an important role in the occurrence and development of tumor, such as cervical cancer, head and neck and lung cancer. However, the expression mechanisms and physiological significance of KPNβ1 in HCC is still unclear.

AIM

The expression of KPNβ1 and its involvement in HCC was studied.

METHODS

The expression of KPNβ1 protein was measured by Western blot and immunohistochemistry in HCC. We analyzed the effects of growth and interference of KPNβ1 in the cell cycle process by CCK8 and flow cytometrical analysis.

RESULTS

KPNβ1 protein level was up-regulated in HCC tissue samples. The KPNβ1 expression was significantly associated with histological differentiation. The levels of KPNβ1 were significantly correlated with histological grade (P = 0.03), metastasis (P = 0.01), vein invasion (P = 0.04) and tumor size (P = 0.01) in HCC samples. Serum starvation assay proved that KPNβ1 was arrested in G1 phase and was gradually reduced by refeeding serum. Moreover, the knockdown of KPNβ1 induced cell proliferation arrest in HepG2 cell. Western blot analyses showed that KPNβ1 was correlated with NF-кB signaling pathway.

CONCLUSIONS

Our datum showed that KPNβ1 expression was up-regulated in HCC tissue samples and increasing HCC cells growth and the KPNβ1 expression was associated with poor survival. KPNβ1 may take part in the pathogenesis of hepatocellular carcinoma via NF-кB signaling pathway and serve as an independent prognostic indicator and a novel therapeutic target for HCC.

Molecular determinants for nuclear import of influenza A PB2 by importin α isoforms 3 and 7

Influenza A virus polymerase subunit PB2 is a major virulence determinant implicated in pathogenicity and host adaptation. During cross-species virus transfer from avian to mammalian cells, PB2 switches specificity from importin α3 to α7. This specificity is not recapitulated in vitro, where PB2 binds all importin α isoforms with comparably high affinity. In this study, we investigated the structure, conformational dynamics, and autoinhibition of importin α isoforms 1, 3, and 7 in complex with PB2. Our data suggest that association of PB2 with α3 and α7 is favored by reduced autoinhibition of these isoforms and by the unique structure of the nuclear localization signal (NLS) domain of PB2. We propose that by recruiting importin α3 or α7 in the absence of importin β, PB2 reduces the complexity of adaptor-mediated import to a pseudo-bimolecular reaction, thereby acquiring a kinetic advantage over classical NLS cargos, which form an import complex only when importin α and β are simultaneously available.

Conserved arginines of bovine adenovirus-3 33K protein are important for transportin-3 mediated transport and virus replication

The L6 region of bovine adenovirus (BAdV)-3 encodes a spliced protein designated 33K. The 33K specific sera detected five major proteins and three minor proteins in transfected or virus infected cells, which could arise by internal initiation of translation and alternative splicing. The 33K protein is predominantly localized to the nucleus of BAdV-3 infected cells. The 33K nuclear transport utilizes both classical importin-α/-β and importin-β dependent nuclear import pathways and preferentially binds to importin-α5 and transportin-3 receptors, respectively. Analysis of mutant 33K proteins demonstrated that amino acids 201–240 of the conserved C-terminus of 33K containing RS repeat are required for nuclear localization and, binding to both importin-α5 and transportin-3 receptors. Interestingly, the arginine residues of conserved RS repeat are required for binding to transportin-3 receptor but not to importin-α5 receptor. Moreover, mutation of arginines residues of RS repeat proved lethal for production of progeny virus. Our results suggest that arginines of RS repeat are required for efficient nuclear transport of 33K mediated by transportin-3, which appears to be essential for replication and production of infectious virion.

Endothelial microRNAs and atherosclerosis

The vascular endothelium, a thin layer of endothelial cells (ECs) that line the inner surface of blood vessels, is a critical interface between blood and all tissues. EC activation, dysfunction, and vascular inflammation occur when the endothelium is exposed to various insults such as proinflammatory cytokines, oxidative stress, hypertension, hyperglycemia, aging, and shear stress. These insults lead to the pathogenesis of a range of disease states, including atherosclerosis. Several signaling pathways, especially nuclear factor κB mediated signaling, play crucial roles in these pathophysiological processes. Recently, microRNAs (miRNAs) have emerged as important regulators of EC function by fine-tuning gene expression. In this review, we discuss how miRNAs regulate EC function and vascular inflammation in response to a variety of pathophysiologic stimuli. An understanding of the role of miRNAs in EC activation and dysfunction may provide novel targets and therapeutic opportunities for controlling atherosclerosis and other chronic inflammatory disease states.

Conserved regions of bovine adenovirus-3 pVIII contain functional domains involved in nuclear localization and packaging in mature infectious virions

Adenoviruses are non-enveloped DNA viruses that replicate in the nucleus of infected cells. One of the core proteins, named pVIII, is a minor capsid protein connecting the core with the inner surface of the capsid. Here, we report the characterization of minor capsid protein pVIII encoded by the L6 region of bovine adenovirus (BAdV)-3. Anti-pVIII serum detected a 24 kDa protein at 12-48 h post-infection and an additional 8 kDa protein at 24-48 h post-infection. While the 24 kDa protein was detected in empty capsids, only the C-terminal-cleaved 8 kDa protein was detected in the mature virion, suggesting that amino acids147-216 of the conserved C-terminus of BAdV-3 pVIII are incorporated in mature virions. Detection of hexon protein associated with both precursor (24 kDa) and cleaved (8 kDa) forms of pVIII suggest that the C-terminus of pVIII interacts with the hexon. The pVIII protein predominantly localizes to the nucleus of BAdV-3-infected cells utilizing the classical importin α/β dependent nuclear import pathway. Analysis of mutant pVIII demonstrated that amino acids 52-72 of the conserved N-terminus bind to importin α-3 with high affinity and are required for the nuclear localization.

Identification of importin α 7 specific transport cargoes using a proteomic screening approach

The importin α:β complex is responsible for the nuclear import of proteins bearing classical nuclear localization signals. In mammals, several importin α subtypes are known to exist that are suggested to have individual functions. Importin α 7 was shown to play a crucial role in early embryonic development in mice. Embryos from importin α 7-depleted females stop at the two-cell stage and show disturbed zygotic genome activation. As there is evidence that individual importin α subtypes possess cargo specificities, we hypothesized that importin α 7 binds a unique set of intracellular proteins. With the use of a collection of in vitro and in vivo binding assays, importin α 7 interaction partners were identified that differed from proteins found to bind to importin α 2 and 3. One of the proteins preferentially binding importin α 7 was the maternal effect protein Brg1. However, Brg1 was localized in oocyte nuclei in importin α 7-deficient embryos, albeit in reduced amounts, suggesting additional modes of nuclear translocation of this factor. An additional SILAC-based screening approach identified Ash2l, Chd3, Mcm3, and Smarcc1, whose nuclear import seems to be disturbed in importin α 7-deficient fibroblasts.

Importins and exportins regulating allergic immune responses

Nucleocytoplasmic shuttling of macromolecules is a well-controlled process involving importins and exportins. These karyopherins recognize and bind to receptor-mediated intracellular signals through specific signal sequences that are present on cargo proteins and transport into and out of the nucleus through nuclear pore complexes. Nuclear localization signals (NLS) present on cargo molecules to be imported while nuclear export signals (NES) on the molecules to be exported are recognized by importins and exportins, respectively. The classical NLS are found on many transcription factors and molecules that are involved in the pathogenesis of allergic diseases. In addition, several immune modulators, including corticosteroids and vitamin D, elicit their cellular responses by regulating the expression and activity of importin molecules. In this review article, we provide a comprehensive list of importin and exportin molecules and their specific cargo that shuttled between cytoplasm and the nucleus. We also critically review the role and regulation of specific importin and exportin involved in the transport of activated transcription factors in allergic diseases, the underlying molecular mechanisms, and the potential target sites for developing better therapeutic approaches.

Identification of target genes using gene expression profile of granulocytes from patients with chronic myeloid leukemia treated with tyrosine kinase inhibitors

Differential gene expression analysis by suppression subtractive hybridization with correlation to the metabolic pathways involved in chronic myeloid leukemia (CML) may provide a new insight into the pathogenesis of CML. Among the overexpressed genes found in CML at diagnosis are SEPT5, RUNX1, MIER1, KPNA6 and FLT3, while PAN3, TOB1 and ITCH were decreased when compared to healthy volunteers. Some genes were identified and involved in CML for the first time, including TOB1, which showed a low expression in patients with CML during tyrosine kinase inhibitor treatment with no complete cytogenetic response. In agreement, reduced expression of TOB1 was also observed in resistant patients with CML compared to responsive patients. This might be related to the deregulation of apoptosis and the signaling pathway leading to resistance. Most of the identified genes were related to the regulation of nuclear factor κB (NF-κB), AKT, interferon and interleukin-4 (IL-4) in healthy cells. The results of this study combined with literature data show specific gene pathways that might be explored as markers to assess the evolution and prognosis of CML as well as identify new therapeutic targets.

Recombinant adeno-associated virus utilizes host cell nuclear import machinery to enter the nucleus

Recombinant adeno-associated viral (rAAV) vectors have garnered much promise in gene therapy applications. However, widespread clinical use has been limited by transduction efficiency. Previous studies suggested that the majority of rAAV accumulates in the perinuclear region of cells, presumably unable to traffic into the nucleus. rAAV nuclear translocation remains ill-defined; therefore, we performed microscopy, genetic, and biochemical analyses in vitro in order to understand this mechanism. Lectin blockade of the nuclear pore complex (NPC) resulted in inhibition of nuclear rAAV2. Visualization of fluorescently labeled particles revealed that rAAV2 localized to importin-β-dense regions of cells in late trafficking steps. Additionally, small interfering RNA (siRNA) knockdown of importin-β partially inhibited rAAV2 nuclear translocation and inhibited transduction by 50 to 70%. Furthermore, coimmunopreciptation (co-IP) analysis revealed that capsid proteins from rAAV2 could interact with importin-β and that this interaction was sensitive to the small GTPase Ran. More importantly, mutations to key basic regions in the rAAV2 capsid severely inhibited interactions with importin-β. We tested several other serotypes and found that the extent of importin-β interaction varied, suggesting that different serotypes may utilize alternative import proteins for nuclear translocation. Co-IP and siRNA analyses were used to investigate the role of other karyopherins, and the results suggested that rAAV2 may utilize multiple import proteins for nuclear entry. Taken together, our results suggest that rAAV2 interacts with importin-β alone or in complex with other karyopherins and enters the nucleus via the NPC. These results may lend insight into the design of novel AAV vectors that have an enhanced nuclear entry capability and transduction potential.

IMPORTANCE

Use of recombinant adeno-associated viral (rAAV) vectors for gene therapy applications is limited by relatively low transduction efficiency, in part due to cellular barriers that hinder successful subcellular trafficking to the nucleus, where uncoating and subsequent gene expression occur. Nuclear translocation of rAAV has been regarded as a limiting step for successful transduction but it remains ill-defined. We explored potential nuclear entry mechanisms for rAAV2 and found that rAAV2 can utilize the classical nuclear import pathway, involving the nuclear pore complex, the small GTPase Ran, and cellular karyopherins. These results could lend insight into the rational design of novel rAAV vectors that can more efficiently translocate to the nucleus, which may lead to more efficient transduction.

Role of miR-181 family in regulating vascular inflammation and immunity

The microRNA family, miR-181, plays diverse roles in regulating key aspects of cellular growth, development, and activation. Accumulating evidence supports a central role for the miR-181 family in vascular inflammation by controlling critical signaling pathways, such as downstream NF-κB signaling, and targets relevant to endothelial cell activation and immune cell homeostasis. This review examines the current knowledge of the miR-181 family's role in key cell types that critically control cardiovascular inflammation under pathological and physiological stimuli.

Importin alpha subtypes determine differential transcription factor localization in embryonic stem cells maintenance

We recently demonstrated that the expression of the importin α subtype is switched from α2 to α1 during neural differentiation in mouse embryonic stem cells (ESCs) and that this switching has a major impact on cell differentiation. In this study, we report a cell-fate determination mechanism in which importin α2 negatively regulates the nuclear import of certain transcription factors to maintain ESC properties. The nuclear import of Oct6 and Brn2 was inhibited via the formation of a transport-incompetent complex of the cargo bound to a nuclear localization signal binding site in importin α2. Unless this dominant-negative effect was downregulated upon ESC differentiation, inappropriate cell death was induced. We propose that although certain transcription factors are necessary for differentiation in ESCs, these factors are retained in the cytoplasm by importin α2, thereby preventing transcription factor activity in the nucleus until the cells undergo differentiation.

STATs get their move on

Understanding the mechanisms that regulate dynamic localization of a protein within a cell can provide critical insight to its functional molecular interactions. Signal transducers and activators of transcription (STATs) play essential roles in development, proliferation, and immune defense. However the consequences of STAT hyperactivity can predispose to diseases including autoimmunity and cancer. To function as transcription factors STATs must gain access to the nucleus, and knowledge of the mechanisms that regulate STAT nuclear trafficking can provide a means to control STAT action. This review presents a synopsis of some of the studies that address the nuclear dynamics of the STAT proteins. Evidence suggests that not all STATs are the same. Nuclear import of STAT1 and STAT4 appears linked to their tyrosine phosphorylation and the formation of parallel dimers via reciprocal phosphotyrosine and Src homology 2 domain interactions. This dimer arrangement generates a conformational nuclear localization signal. STAT2 is imported continually to the nucleus in an unphosphorylated state due to its association with IRF9, but the dominant nuclear export signal of STAT2 shuttles the complex back to the cytoplasm. Following STAT2 tyrosine phosphorylation, it can form dimers with STAT1 to affect nuclear import as the trimeric complex (ISGF3). Distinctly, STAT3, STAT5, and STAT6 are continually imported to the nucleus independent of tyrosine phosphorylation. Mutational studies indicate the nuclear localization signals in these STATs require the conformational structure of their coiled-coil domains. Increases in STAT nuclear accumulation following cytokine stimulation appear coordinate with their ability to bind DNA.

Downregulation of KPNA2 in non-small-cell lung cancer is associated with Oct4 expression

Background

Oct4 is a major transcription factor related to stem cell self-renewal and differentiation. To fulfill its functions, it must be able to enter the nucleus and remain there to affect transcription. KPNA2, a member of the karyopherin family, plays a central role in nucleocytoplasmic transport. The objective of the current study was to examine the association between Oct4 and KPNA2 expression levels with regard to both the clinicopathological characteristics and prognoses of patients with non-small-cell lung cancer (NSCLC).

Methods

Immunohistochemistry was used to detect the expression profile of Oct4 and KPNA2 in NSCLC tissues and adjacent noncancerous lung tissues. Real-time polymerase chain reaction and western blotting were used to detect the mRNA and protein expression profiles of Oct4 and KPNA2 in lung cancer cell lines. Small interfering RNAs were used to deplete Oct4 and KPNA2 expressions. Double immunofluorescence was used to detect Oct4 expression in KPNA2 knockdown cells. Co-immunoprecipitation was used to detect the interaction of Oct4 and KPNA2.

Results

Oct4 was overexpressed in 29 of 102 (28.4%) human lung cancer samples and correlated with differentiation (P = 0.002) and TNM stage (P = 0.003). KPNA2 was overexpressed in 56 of 102 (54.9%) human lung cancer samples and correlated with histology (P = 0.001) and differentiation (P = 0.045). Importantly, Oct4 and KPNA2 expression levels correlated significantly (P < 0.01). Expression of Oct4 and KPNA2 was associated with short overall survival. In addition, depleting Oct4 and KPNA2 expression using small interfering RNAs inhibited proliferation in lung cancer cell lines. Real-time polymerase chain reaction and western blotting analysis indicated that reduction of KPNA2 expression significantly reduced mRNA and nucleoprotein levels of Oct4. Double immunofluorescence analysis revealed that nuclear Oct4 signals were reduced significantly in KPNA2 knockdown cells. Co-immunoprecipitation experiments revealed that KPNA2 interacts with Oct4 in lung cancer cell lines.

Conclusion

Oct4 and KPNA2 play an important role in NSCLC progression. Oct4 nuclear localization may be mediated by its interaction with KPNA2.

Targeting nuclear import shuttles, importins/karyopherins alpha by a peptide mimicking the NFκB1/p50 nuclear localization sequence

Background

We recently reported that a bifunctional nuclear transport modifier (NTM), cSN50.1 peptide, reduced atherosclerosis, plasma cholesterol, triglycerides, and glucose along with liver fat and inflammatory markers, in a murine model of familial hypercholesterolemia. We determined that cSN50.1 improved lipid homeostasis by modulating nuclear transport of sterol regulatory element‐binding proteins through interaction with importin β. Previous studies established that cSN50.1 and related NTMs also modulate nuclear transport of proinflammatory transcription factors mediated by binding of their nuclear localization sequences (NLSs) to importins/karyopherins α. However, selectivity and specificity of NTMs for importins/karyopherins α were undetermined.

Methods and Results

We analyzed interaction of the NTM hydrophilic module, N50 peptide, derived from the NLS of NFκB1/p50, with endogenous human importins/karyopherins α to determine the mechanism of NTM modulation of importin α‐mediated nuclear transport. We show that N50 peptide forms stable complexes with multiple importins/karyopherins α. However, only interaction with importin α5 (Imp α5) displayed specific, high‐affinity binding. The 2:1 stoichiometry of the N50‐Imp α5 interaction (K_D1=73 nmol/L, K_D2=140 nmol/L) indicated occupancy of both major and minor NLS binding pockets. Utilizing in silico 3‐dimensional (3‐D) docking models and comparative structural analysis, we identified a structural component of the Imp α5 major NLS binding pocket that may stabilize N50 binding. Imp α5 also displayed rapid stimulus‐induced turnover, which could influence its availability for nuclear transport during the inflammatory response.

Conclusions

These results provide direct evidence that N50 peptide selectively targets Imp α5, encouraging further refinement of NLS‐derived peptides as new tools to modulate inflammatory disorders.

Chikungunya virus capsid protein contains nuclear import and export signals

BACKGROUND

Chikungunya virus (CHIKV) is an alphavirus of the Togaviridae family. After autoproteolytic cleavage, the CHIKV capsid protein (CP) is involved in RNA binding and assembly of the viral particle. The monomeric CP is approximately 30 kDa in size and is small enough for passive transport through nuclear pores. Some alphaviruses are found to harbor nuclear localization signals (NLS) and transport of these proteins between cellular compartments was shown to be energy dependent. The active nuclear import of cytoplasmic proteins is mediated by karyopherins and their export by exportins. As nuclear and cytoplasmic trafficking may play a role in the life cycle of CHIKV, we have sought to identify nuclear localization and nuclear export signals in CHIKV CP in a virus-free system.

METHODS

EGFP-fusion proteins of CHIKV CP and mutants thereof were created and used to monitor their intracellular localization. Binding of cellular proteins was confirmed in pull-down assays with purified CP using co-immuoprecipitation. Nuclear localization was demonstrated in a virus-free system using fluorescence microscopy.

RESULTS

Here we show that CHIKV CP is a nuclear-cytoplasmic shuttling protein with an active NLS that binds to karyopherin α (Karα) for its nuclear translocation. We also found that the Karα4 C-terminal NLS binding site is sufficient for this interaction. We further demonstrate that CHIKV CP interacts directly with the export receptor CRM1 to transport this viral protein out of the nucleus via a nuclear export signal (NES). The CHIKV CP NES was mapped between amino acids 143 and 155 of CP. Deduced from in silico analyses we found that the NES has a mode of binding similar to the snurportin-1 CRM1 complex.

CONCLUSIONS

We were able to show that in a virus-free system that the CHIKV capsid protein contains both, a NLS and a NES, and that it is actively transported between the cytoplasma and the nucleus. We conclude that CHIKV CP has the ability to shuttle via interaction with karyopherins for its nuclear import and, vice versa, by CRM1-dependent nuclear export.

KPNB1, XPO7 and IPO8 mediate the translocation ofNF-κB/p65 into the nucleus

NF-κB/p65 is retained in the cytoplasm until it is activated in response to stress. Nuclear import of p65 is regulated by importin α in a nuclear localization signal (NLS)-dependent manner. However, the role of importin β family members in the nuclear translocation of p65 is largely unclear. In this study, using high-content siRNA screening, we identified three of 17 importin β family members that are involved in the nuclear import of p65. Our data showed that knockdown of KPNB1, XPO7 and IPO8 reduced the amount of nuclear p65 following tumor necrosis factor-α (TNF-α) stimulation, resulting in lower NF-κB activity. KPNB1 was the major importin β receptor for p65 import, and this import was dependent on the NLS of p65. However, NLS-mutated p65 still entered the nucleus and bound to XPO7 and IPO8. Interestingly, among the six members of the importin α family, KPNA2 was most important for p65 import. Taken together, our results show that the import of p65 mainly relies on the canonical KPNA2/KPNB1 pathway; however, p65 is also imported by an alternative pathway that is independent of its NLS. Redundant importin receptors are likely to maintain the important function of p65 according to need.

Identification of cargo proteins specific for importin-β with importin-α applying a stable isotope labeling by amino acids in cell culture (SILAC)-based in vitro transport system

The human importin (Imp)-β family consists of 21 nucleocytoplasmic transport carrier proteins, which transport thousands of proteins (cargoes) across the nuclear envelope through nuclear pores in specific directions. To understand the nucleocytoplasmic transport in a physiological context, the specificity of cargoes for their cognate carriers should be determined; however, only a limited number of nuclear proteins have been linked to specific carriers. To address this biological question, we recently developed a novel method to identify carrier-specific cargoes. This method includes the following three steps: (i) the cells are labeled by stable isotope labeling by amino acids in cell culture (SILAC); (ii) the labeled cells are permeabilized, and proteins in the unlabeled cell extracts are transported into the nuclei of the permeabilized cells by a particular carrier; and (iii) the proteins in the nuclei are quantitatively identified by LC-MS/MS. The effectiveness of this method was demonstrated by the identification of transportin (Trn)-specific cargoes. Here, we applied this method to identify cargo proteins specific for Imp-β, which is a predominant carrier that exclusively utilizes Imp-α as an adapter for cargo binding. We identified candidate cargoes, which included previously reported and potentially novel Imp-β cargoes. In in vitro binding assays, most of the candidate cargoes bound to Imp-β in one of three binding modes: directly, via Imp-α, or via other cargoes. Thus, our method is effective for identifying a variety of Imp-β cargoes. The identified Imp-β and Trn cargoes were compared, ensuring the carrier specificity of the method and illustrating the complexity of these transport pathways.

Identification of Novel Stress Granule Components That Are Involved in Nuclear Transport

Background

Importin-α1 belongs to a subfamily of nuclear transport adaptors and participates in diverse cellular functions. Best understood for its role in protein transport, importin-α1 also contributes to other biological processes. For instance, arsenite treatment causes importin-α1 to associate with cytoplasmic stress granules (SGs) in mammalian cells. These stress-induced compartments contain translationally arrested mRNAs, small ribosomal subunits and numerous proteins involved in mRNA transport and metabolism. At present, it is not known whether members of all three importin-α subfamilies locate to SGs in response to stress.

Results

Here, we demonstrate that the oxidant diethyl maleate (DEM), arsenite and heat shock, promote the formation of cytoplasmic SGs that contain nuclear transport factors. Specifically, importin-α1, α4 and α5, which belong to distinct subfamilies, and importin-β1 were targeted by all of these stressors to cytoplasmic SGs, but not to P-bodies. Importin-α family members have been implicated in transcriptional regulation, which prompted us to analyze their ability to interact with poly(A)-RNA in growing cells. Our studies show that importin-α1, but not α4, α5, importin-β1 or CAS, associated with poly(A)-RNA under nonstress conditions. Notably, this interaction was significantly reduced when cells were treated with DEM. Additional studies suggest that importin-α1 is likely connected to poly(A)-RNA through an indirect interaction, as the adaptor did not bind homopolymer RNA specifically in vitro.

Significance

Our studies establish that members of three importin-α subfamilies are bona fide SG components under different stress conditions. Furthermore, importin-α1 is unique in its ability to interact with poly(A)-RNA in a stress-dependent fashion, and in vitro experiments indicate that this association is indirect. Collectively, our data emphasize that nuclear transport factors participate in a growing number of cellular activities that are modulated by stress.

Dynamic trafficking of STAT5 depends on an unconventional nuclear localization signal

Signal transducer and activator of transcription 5 (STAT5) is crucial for physiological processes that include hematopoiesis, liver metabolism and mammary gland development. However, aberrant continual activity of STAT5 has been causally linked to human leukemias and solid tumor formation. As a regulated transcription factor, precise cellular localization of STAT5 is essential. Conventional nuclear localization signals consist of short stretches of basic amino acids. In this study, we provide evidence that STAT5 nuclear import is dependent on an unconventional nuclear localization signal that functions within the conformation of an extensive coiled-coil domain. Both in vitro binding and in vivo functional assays reveal that STAT5 nuclear import is mediated by the importin-α3/β1 system independently of STAT5 activation by tyrosine phosphorylation. The integrity of the coiled-coil domain is essential for STAT5 transcriptional induction of the β-casein gene following prolactin stimulation as well as its ability to synergize with the glucocorticoid receptor. The glucocorticoid receptor accumulates in the nucleus in response to prolactin and this nuclear import is dependent on STAT5 nuclear import. STAT5 continually shuttles in and out of the nucleus and live cell imaging demonstrates that STAT5 nuclear export is mediated by both chromosome region maintenance 1 (Crm1)-dependent and Crm1-independent pathways. A Crm1-dependent nuclear export signal was identified within the STAT5 N-terminus. These findings provide insight into the fundamental mechanisms that regulate STAT5 nuclear trafficking and cooperation with the glucocorticoid receptor and provide a basis for clinical intervention of STAT5 function in disease.

Defective nuclear import of Tpr in Progeria reflects the Ran sensitivity of large cargo transport

Nuclear transport of large protein cargoes such as Tpr is more sensitive to the alteration of the ratio of nuclear to cytoplasmic Ran that occurs in Progeria.

The RanGTPase acts as a master regulator of nucleocytoplasmic transport by controlling assembly and disassembly of nuclear transport complexes. RanGTP is required in the nucleus to release nuclear localization signal (NLS)–containing cargo from import receptors, and, under steady-state conditions, Ran is highly concentrated in the nucleus. We previously showed the nuclear/cytoplasmic Ran distribution is disrupted in Hutchinson-Gilford Progeria syndrome (HGPS) fibroblasts that express the Progerin form of lamin A, causing a major defect in nuclear import of the protein, translocated promoter region (Tpr). In this paper, we show that Tpr import was mediated by the most abundant import receptor, KPNA2, which binds the bipartite NLS in Tpr with nanomolar affinity. Analyses including NLS swapping revealed Progerin did not cause global inhibition of nuclear import. Rather, Progerin inhibited Tpr import because transport of large protein cargoes was sensitive to changes in the Ran nuclear/cytoplasmic distribution that occurred in HGPS. We propose that defective import of large protein complexes with important roles in nuclear function may contribute to disease-associated phenotypes in Progeria.

Cell cycle-dependent binding modes of the ran exchange factor RCC1 to chromatin

The formation of an activity gradient of the small G-protein Ran around chromatin depends on the differential partitioning of the opposing enzyme activities of the Ran guanine nucleotide exchange factor RCC1 that resides on chromatin, and the cytoplasmic Ran GTPase activating protein RanGAP. We studied the time-dependent interaction kinetics between RCC1 and chromatin and the mobility of the Ran-RCC1 complex in living cells by fluorescence correlation spectroscopy to investigate whether binding of RCC1 to chromatin regulates the exchange activity of RCC1, and whether the stability of the RCC1-chromatin interaction is regulated during the cell cycle. We found that RCC1 mobility is dominated by two states: a highly mobile state that is trapped within chromatin, and a transiently immobilized state that is stabilized during mitosis. We show that only the immobilized state of RCC1 interacts with Ran and conclude that its guanine nucleotide exchange activity is restricted to specific sites on chromatin.

RNAi phenotypes are influenced by the genetic background of the injected strain

Background

RNA interference (RNAi) is a powerful tool to study gene function in organisms that are not amenable to classical forward genetics. Hence, together with the ease of comprehensively identifying genes by new generation sequencing, RNAi is expanding the scope of animal species and questions that can be addressed in terms of gene function. In the case of genetic mutants, the genetic background of the strains used is known to influence the phenotype while this has not been described for RNAi experiments.

Results

Here we show in the red flour beetle Tribolium castaneum that RNAi against Tc-importin α1 leads to different phenotypes depending on the injected strain. We rule out off target effects and show that sequence divergence does not account for this difference. By quantitatively comparing phenotypes elicited by RNAi knockdown of four different genes we show that there is no general difference in RNAi sensitivity between these strains. Finally, we show that in case of Tc-importin α1 the difference depends on the maternal genotype.

Conclusions

These results show that in RNAi experiments strain specific differences have to be considered and that a proper documentation of the injected strain is required. This is especially important for the increasing number of emerging model organisms that are being functionally investigated using RNAi. In addition, our work shows that RNAi is suitable to systematically identify the differences in the gene regulatory networks present in populations of the same species, which will allow novel insights into the evolution of animal diversity.

Hop-on hop-off: importin-α-guided tours to the nucleus in innate immune signaling

Nuclear translocation of immune regulatory proteins and signal transducers is an essential process in animal and plant defense signaling against pathogenic microbes. Import of proteins containing a nuclear localization signal (NLS) into the nucleus is mediated by nuclear transport receptors termed importins, typically dimers of a cargo-binding α-subunit and a β-subunit that mediates translocation through the nuclear pore complex. Here, we review recent reports of importin-α cargo specificity and mutant phenotypes in plant- and animal-microbe interactions. Using homology modeling of the NLS-binding cleft of nine predicted Arabidopsis α-importins and analyses of their gene expression patterns, we discuss functional redundancy and specialization within this transport receptor family. In addition, we consider how pathogen effector proteins that promote infection by manipulating host cell nuclear processes might compete with endogenous cargo proteins for nuclear uptake.

Controlling subcellular delivery to optimize therapeutic effect

This article focuses on drug targeting to specific cellular organelles for therapeutic purposes. Drugs can be delivered to all major organelles of the cell (cytosol, endosome/lysosome, nucleus, nucleolus, mitochondria, endoplasmic reticulum, Golgi apparatus, peroxisomes and proteasomes) where they exert specific effects in those particular subcellular compartments. Delivery can be achieved by chemical (e.g., polymeric) or biological (e.g., signal sequences) means. Unidirectional targeting to individual organelles has proven to be immensely successful for drug therapy. Newer technologies that accommodate multiple signals (e.g., protein switch and virus-like delivery systems) mimic nature and allow for a more sophisticated approach to drug delivery. Harnessing different methods of targeting multiple organelles in a cell will lead to better drug delivery and improvements in disease therapy.

Cellular cofactors of lentiviral integrase: from target validation to drug discovery

To accomplish their life cycle, lentiviruses make use of host proteins, the so-called cellular cofactors. Interactions between host cell and viral proteins during early stages of lentiviral infection provide attractive new antiviral targets. The insertion of lentiviral cDNA in a host cell chromosome is a step of no return in the replication cycle, after which the host cell becomes a permanent carrier of the viral genome and a producer of lentiviral progeny. Integration is carried out by integrase (IN), an enzyme playing also an important role during nuclear import. Plenty of cellular cofactors of HIV-1 IN have been proposed. To date, the lens epithelium-derived growth factor (LEDGF/p75) is the best studied cofactor of HIV-1 IN. Moreover, small molecules that block the LEDGF/p75-IN interaction have recently been developed for the treatment of HIV infection. The nuclear import factor transportin-SR2 (TRN-SR2) has been proposed as another interactor of HIV IN-mediating nuclear import of the virus. Using both proteins as examples, we will describe approaches to be taken to identify and validate novel cofactors as new antiviral targets. Finally, we will highlight recent advances in the design and the development of small-molecule inhibitors binding to the LEDGF/p75-binding pocket in IN (LEDGINs).

Mapping of nuclear import signal and importin α3 binding regions of 52K protein of bovine adenovirus-3

The L1 region of bovine adenovirus (BAdV)-3 encodes a non-structural protein designated 52K. Anti-52K serum detected a protein of 40kDa, which localized to the nucleus but not to the nucleolus in BAdV-3-infected or transfected cells. Analysis of mutant 52K proteins suggested that three basic residues ((105)RKR(107)) of the identified domain (amino acids (102)GMPRKRVLT(110)) are essential for nuclear localization of 52K. The nuclear import of a GST-52K fusion protein utilizes the classical importin α/β-dependent nuclear transport pathway. The 52K protein is preferentially bound to the cellular nuclear import receptor importin α3. Although deletion of amino acid 102-110 is sufficient to abrogate the nuclear localization of 52K, amino acid 90-133 are required for interaction with importin-α3 and localizing a cytoplasmic protein to the nucleus. These results suggest that 52K contains a bipartite NLS, which preferentially utilize an importin α3 nuclear import receptor-mediated pathway to transport 52K to the nucleus.

Identification of karyopherin α1 and α7 interacting proteins in porcine tissue

Specialized trafficking systems in eukaryotic cells serve a critical role in partitioning intracellular proteins between the nucleus and cytoplasm. Cytoplasmic proteins (including chromatin remodeling enzymes and transcription factors) must gain access to the nucleus to exert their functions to properly program fundamental cellular events ranging from cell cycle progression to gene transcription. Knowing that nuclear import mediated by members of the karyopherin α family of transport receptors plays a critical role in regulating development and differentiation, we wanted to determine the identity of proteins that are trafficked by this karyopherin α pathway. To this end, we performed a GST pull-down assay using porcine orthologs of karyopherin α1 (KPNA1) and karyopherin α7 (KPNA7) and prey protein derived from porcine fibroblast cells and used a liquid chromatography and tandem mass spectrometry (LC-MS/MS) approach to determine the identity of KPNA1 and KPNA7 interacting proteins. Our screen revealed that the proteins that interact with KPNA1 and KPNA7 are generally nuclear proteins that possess nuclear localization signals. We further validated two candidate proteins from this screen and showed that they are able to be imported into the nucleus in vivo and also interact with members of the karyopherin α family of proteins in vitro. Our results also reveal the utility of using a GST pull-down approach coupled with LC-MS/MS to screen for protein interaction partners in a non-traditional model system.

Nuclear localization signal in a cancer-related transcriptional regulator protein NAC1

Nucleus accumbens-associated protein 1 (NAC1) might have potential oncogenic properties and participate in regulatory networks for pluripotency. Although NAC1 is described as a transcriptional regulator, the nuclear import machinery of NAC1 remains unclear. We found, using a point mutant, that dimer formation was not committed to the nuclear localization of NAC1 and, using deletion mutants, that the amino-terminal half of NAC1 harbored a potential nuclear localization signal (NLS). Wild type, but not mutants of this region, alone was sufficient to drive the importation of green fluorescent protein (GFP) into the nucleus. Bimax1, a synthetic peptide that blocks the importin α/β pathway, impaired nuclear localization of NAC1 in cells. We also used the binding properties of importin to demonstrate that this region is an NLS. Furthermore, the transcriptional regulator function of NAC1 was dependent on its nuclear localization activity in cells. Taken together, these results show that the region with a bipartite motif constitutes a functional nuclear import sequence in NAC1 that is independent of NAC1 dimer formation. The identification of an NAC1 NLS thus clarifies the mechanism through which NAC1 translocates to the nucleus to regulate the transcription of genes involved in oncogenicity and pluripotency.

MicroRNA-181b regulates NF-κB-mediated vascular inflammation

EC activation and dysfunction have been linked to a variety of vascular inflammatory disease states. The function of microRNAs (miRNAs) in vascular EC activation and inflammation remains poorly understood. Herein, we report that microRNA-181b (miR-181b) serves as a potent regulator of downstream NF-κB signaling in the vascular endothelium by targeting importin-α3, a protein that is required for nuclear translocation of NF-κB. Overexpression of miR-181b inhibited importin-α3 expression and an enriched set of NF-κB-responsive genes such as adhesion molecules VCAM-1 and E-selectin in ECs in vitro and in vivo. In addition, treatment of mice with proinflammatory stimuli reduced miR-181b expression. Rescue of miR-181b levels by systemic administration of miR-181b "mimics" reduced downstream NF-κB signaling and leukocyte influx in the vascular endothelium and decreased lung injury and mortality in endotoxemic mice. In contrast, miR-181b inhibition exacerbated endotoxin-induced NF-κB activity, leukocyte influx, and lung injury. Finally, we observed that critically ill patients with sepsis had reduced levels of miR-181b compared with control intensive care unit (ICU) subjects. Collectively, these findings demonstrate that miR-181b regulates NF-κB-mediated EC activation and vascular inflammation in response to proinflammatory stimuli and that rescue of miR-181b expression could provide a new target for antiinflammatory therapy and critical illness.

The HIV-1 passage from cytoplasm to nucleus: the process involving a complex exchange between the components of HIV-1 and cellular machinery to access nucleus and successful integration

The human immunodeficiency virus 1 (HIV-1) synthesizes its genomic DNA in cytoplasm as soon as it enters the cell. The newly synthesized DNA remains associated with viral/cellular proteins as a high molecular weight pre-integration complex (PIC), which precludes passive diffusion across intact nuclear membrane. However, HIV-1 successfully overcomes nuclear membrane barrier by actively delivering its DNA into nucleus with the help of host nuclear import machinery. Such ability allows HIV-1 to productively infect non-dividing cells as well as dividing cells at interphase. Further, HIV-1 nuclear import is also found important for the proper integration of viral DNA. Thus, nuclear import plays a crucial role in establishment of infection and disease progression. While several viral components, including matrix, viral protein R, integrase, capsid, and central DNA flap are implicated in HIV-1 nuclear import, their molecular mechanism remains poorly understood. In this review, we will elaborate the role of individual viral factors and some of current insights on their molecular mechanism(s) associated with HIV-1 nuclear import. In addition, we will discuss the importance of nuclear import for subsequent step of viral DNA integration. Hereby we aim to further our understanding on molecular mechanism of HIV-1 nuclear import and its potential usefulness for anti-HIV-1 strategies.

Regulated nucleocytoplasmic transport during gametogenesis

Gametogenesis is the process by which sperm or ova are produced in the gonads. It is governed by a tightly controlled series of gene expression events, with some common and others distinct for males and females. Nucleocytoplasmic transport is of central importance to the fidelity of gene regulation that is required to achieve the precisely regulated germ cell differentiation essential for fertility. In this review we discuss the physiological importance for gamete formation of the molecules involved in classical nucleocytoplasmic protein transport, including importins/karyopherins, Ran and nucleoporins. To address what functions/factors are conserved or specialized for these developmental processes between species, we compare knowledge from mice, flies and worms. The present analysis provides evidence of the necessity for and specificity of each nuclear transport factor and for nucleoporins during germ cell differentiation. This article is part of a Special Issue entitled: Nuclear Transport and RNA Processing.

KPNA7, an oocyte- and embryo-specific karyopherin?subtype, is required for porcine embryo development

Coordinated partitioning of intracellular cargoes between nuclear and cytoplasmic compartments is critical for cell survival and differentiation. The karyopherin α/β heterodimer functions to import cytoplasmic proteins that possess classical nuclear localisation signals into the nucleus. Seven karyopherin α subtypes have been identified in mammals. The aim of this study was to determine the relative abundance of transcripts encoding seven karyopherin α subtypes in porcine oocytes and embryos at discrete stages of cleavage development, and to determine the developmental requirements of karypopherin α 7 (KPNA7), an oocyte and cleavage stage embryo-specific karyopherin α subtype. We hypothesised that knockdown of KPNA7 would negatively affect porcine cleavage development. To test this hypothesis, in vitro matured and fertilised porcine oocytes were injected with a double-stranded interfering RNA molecule that targeted KPNA7; nuclei were counted in all embryos 6 days after fertilisation. Embryos injected with KPNA7-interfering RNAs possessed significantly lower cell numbers than their respective control groups (P < 0.05). In vitro binding assays also suggest that KPNA7 may transport intracellular proteins that possess unique nuclear localisation signals. Our data suggest that embryos have differential requirements for individual karyopherin α subtypes and that these karyopherin α subtypes differentially transport intracellular cargo during cleavage development.

Specific Cooperation Between Imp-α2 and Imp-β/Ketel in Spindle Assembly During Drosophila Early Nuclear Divisions

The multifunctional factors Imp-α and Imp-β are involved in nuclear protein import, mitotic spindle dynamics, and nuclear membrane formation. Furthermore, each of the three members of the Imp-α family exerts distinct tasks during development. In Drosophila melanogaster, the imp-α2 gene is critical during oogenesis for ring canal assembly; specific mutations, which allow oogenesis to proceed normally, were found to block early embryonic mitosis. Here, we show that imp-α2 and imp-β genetically interact during early embryonic development, and we characterize the pattern of defects affecting mitosis in embryos laid by heterozygous imp-α2(D14) and imp-β(KetRE34) females. Embryonic development is arrested in these embryos but is unaffected in combinations between imp-β(KetRE34) and null mutations in imp-α1 or imp-α3. Furthermore, the imp-α2(D14)/imp-β(KetRE34) interaction could only be rescued by an imp-α2 transgene, albeit not imp-α1 or imp-α3, showing the exclusive imp-α2 function with imp-β. Use of transgenes carrying modifications in the major Imp-α2 domains showed the critical requirement of the nuclear localization signal binding (NLSB) site in this process. In the mutant embryos, we found metaphase-arrested mitoses made of enlarged spindles, suggesting an unrestrained activity of factors promoting spindle assembly. In accordance with this, we found that Imp-β(KetRE34) and Imp-β(KetD) bind a high level of RanGTP/GDP, and a deletion decreasing RanGTP level suppresses the imp-β(KetRE34) phenotype. These data suggest that a fine balance among Imp-α2, Imp-β, RanGTP, and the NLS cargos is critical for mitotic progression during early embryonic development.

Nucleoporin Nup50 stabilizes closed conformation of armadillo repeat 10 in importin α5

The human genome encodes six isoforms of importin α that show greater than 60% sequence similarity and remarkable substrate specificity. The isoform importin α5 can bind phosphorylated cargos such as STAT1 and Epstein-Barr Virus Nuclear Antigen 1, as well as the influenza virus polymerase subunit PB2. In this work, we have studied the interaction of the nucleoporin Nup50 with importin α5. We show that the first 47 residues of Nup50 bind to the C terminus of importin α5 like a "clip," stabilizing the closed conformation of ARM 10. In vitro, Nup50 binds with high affinity either to empty importin α5 or to a preassembled complex of importin α5 bound to the C-terminal domain of the import cargo PB2, resulting in a trimeric complex. By contrast, PB2 can only bind with high affinity to importin α5 in the absence of Nup50. This suggests that Nup50 primary function may not be to actively displace the import cargo from importin α5 but rather to prevent cargo rebinding in preparation for recycling. This is the first evidence for a nucleoporin modulating the import reaction by directly altering the three-dimensional structure of an import adaptor.