Identification of cytosolic factors required for nuclear location sequence-mediated binding to the nuclear envelope

Thermal Stress and Nuclear Transport

Nuclear transport is the basis for the biological reaction of eukaryotic cells, as it is essential to coordinate nuclear and cytoplasmic events separated by nuclear envelope. Although we currently understand the basic molecular mechanisms of nuclear transport in detail, many unexplored areas remain. For example, it is believed that the regulations and biological functions of the nuclear transport receptors (NTRs) highlights the significance of the transport pathways in physiological contexts. However, physiological significance of multiple parallel transport pathways consisting of more than 20 NTRs is still poorly understood, because our knowledge of each pathway, regarding their substrate information or how they are differently regulated, is still limited. In this report, we describe studies showing how nuclear transport systems in general are affected by temperature rises, namely, thermal stress or heat stress. We will then focus on Importin α family members and unique transport factor Hikeshi, because these two NTRs are affected in heat stress. Our present review will provide an additional view to point out the importance of diversity of the nuclear transport pathways in eukaryotic cells.

Dual control of formin-nucleated actin assembly by the chromatin and ER in mouse oocytes

The first asymmetric meiotic cell divisions in mouse oocytes are driven by formin 2 (FMN2)-nucleated actin polymerization around the spindle. In this study, we investigated how FMN2 is recruited to the spindle peripheral ER and how its activity is regulated in mouse meiosis I (MI) oocytes. We show that this process is regulated by the Ran GTPase, a conserved mediator of chromatin signal, and the ER-associated protein VAPA. FMN2 contains a nuclear localization sequence (NLS) within a domain (SLD) previously shown to be required for FMN2 localization to the spindle periphery. FMN2 NLS is bound to the importin α1/β complex, and the disruption of this interaction by RanGTP is required for FMN2 accumulation in the area proximal to the chromatin and the MI spindle. The importin-free FMN2 is then recruited to the surface of ER around the spindle through the binding of the SLD with the ER-membrane protein VAPA. We further show that FMN2 is autoinhibited through an intramolecular interaction between the SLD with the C-terminal formin homology 2 (FH2) domain that nucleates actin filaments. VAPA binding to SLD relieves the autoinhibition of FMN2, leading to localized actin polymerization. This dual control of formin-mediated actin assembly allows actin polymerization to initiate the movement of the meiotic spindle toward the cortex, an essential step in the maturation of the mammalian female gamete.

Regulating Phase Transition in Neurodegenerative Diseases by Nuclear Import Receptors

RNA-binding proteins (RBPs) with a low-complexity prion-like domain (PLD) can undergo aberrant phase transitions and have been implicated in neurodegenerative diseases such as ALS and FTD. Several nuclear RBPs mislocalize to cytoplasmic inclusions in disease conditions. Impairment in nucleocytoplasmic transport is another major event observed in ageing and in neurodegenerative disorders. Nuclear import receptors (NIRs) regulate the nucleocytoplasmic transport of different RBPs bearing a nuclear localization signal by restoring their nuclear localization. NIRs can also specifically dissolve or prevent the aggregation and liquid–liquid phase separation of wild-type or disease-linked mutant RBPs, due to their chaperoning activity. This review focuses on the LLPS of intrinsically disordered proteins and the role of NIRs in regulating LLPS in neurodegeneration. This review also discusses the implication of NIRs as therapeutic agents in neurogenerative diseases.

Distinct mutations in importin-β family nucleocytoplasmic transport receptors transportin-SR and importin-13 affect specific cargo binding

Importin-(Imp)β family nucleocytoplasmic transport receptors (NTRs) are supposed to bind to their cargoes through interaction between a confined interface on an NTR and a nuclear localization or export signal (NLS/NES) on a cargo. Although consensus NLS/NES sequence motifs have been defined for cargoes of some NTRs, many experimentally identified cargoes of those NTRs lack those motifs, and consensus NLSs/NESs have been reported for only a few NTRs. Crystal structures of NTR-cargo complexes have exemplified 3D structure-dependent binding of cargoes lacking a consensus NLS/NES to different sites on an NTR. Since only a limited number of NTR-cargo interactions have been studied, whether most cargoes lacking a consensus NLS/NES bind to the same confined interface or to various sites on an NTR is still unclear. Addressing this issue, we generated four mutants of transportin-(Trn)SR, of which many cargoes lack a consensus NLS, and eight mutants of Imp13, where no consensus NLS has been defined, and we analyzed their binding to as many as 40 cargo candidates that we previously identified by a nuclear import reaction-based method. The cargoes bind differently to the NTR mutants, suggesting that positions on an NTR contribute differently to the binding of respective cargoes.

Calcium Regulates the Nuclear Localization of Protein Arginine Deiminase 2

Protein arginine deiminases (PADs) are calcium-dependent enzymes that mediate the post-translational conversion of arginine into citrulline. Dysregulated PAD activity is associated with numerous autoimmune disorders and cancers. In breast cancer, PAD2 citrullinates histone H3R26 and activates the transcription of estrogen receptor target genes. However, PAD2 lacks a canonical nuclear localization sequence, and it is unclear how this enzyme is transported into the nucleus. Here, we show for the first time that PAD2 translocates into the nucleus in response to calcium signaling. Using BioID2, a proximity-dependent biotinylation method for identifying interacting proteins, we found that PAD2 preferentially associates with ANXA5 in the cytoplasm. Binding of calcium to PAD2 weakens this cytoplasmic interaction, which generates a pool of calcium-bound PAD2 that can interact with Ran. We hypothesize that this latter interaction promotes the translocation of PAD2 into the nucleus. These findings highlight a critical role for ANXA5 in regulating PAD2 and identify an unusual mechanism whereby proteins translocate between the cytosol and nucleus.

The coming-of-age of nucleocytoplasmic transport in motor neuron disease and neurodegeneration

The nuclear pore is the gatekeeper of nucleocytoplasmic transport and signaling through which a vast flux of information is continuously exchanged between the nuclear and cytoplasmic compartments to maintain cellular homeostasis. A unifying and organizing principle has recently emerged that cements the notion that several forms of amyotrophic lateral sclerosis (ALS), and growing number of other neurodegenerative diseases, co-opt the dysregulation of nucleocytoplasmic transport and that this impairment is a pathogenic driver of neurodegeneration. The understanding of shared pathomechanisms that underpin neurodegenerative diseases with impairments in nucleocytoplasmic transport and how these interface with current concepts of nucleocytoplasmic transport is bound to illuminate this fundamental biological process in a yet more physiological context. Here, I summarize unresolved questions and evidence and extend basic and critical concepts and challenges of nucleocytoplasmic transport and its role in the pathogenesis of neurodegenerative diseases, such as ALS. These principles will help to appreciate the roles of nucleocytoplasmic transport in the pathogenesis of ALS and other neurodegenerative diseases, and generate a framework for new ideas of the susceptibility of motoneurons, and possibly other neurons, to degeneration by dysregulation of nucleocytoplasmic transport.

Importin α: functions as a nuclear transport factor and beyond

Nucleocytoplasmic transport is an essential process in eukaryotes. The molecular mechanisms underlying nuclear transport that involve the nuclear transport receptor, small GTPase Ran, and the nuclear pore complex are highly conserved from yeast to humans. On the other hand, it has become clear that the nuclear transport system diverged during evolution to achieve various physiological functions in multicellular eukaryotes. In this review, we first summarize the molecular mechanisms of nuclear transport and how these were elucidated. Then, we focus on the diverse functions of importin α, which acts not merely an import factor but also as a multi-functional protein contributing to a variety of cellular functions in higher eukaryotes.

Nuclear Protein Transport in Digitonin Permeabilized Cells

The high concentration of cholesterol in the plasma membrane relative to the endomembranes of eukaryotic cells allows the selective permeabilization of the plasma membrane with the glycoside digitonin leaving the intracellular membrane bound organelles intact. In this chapter, we describe the basic method to use digitonin permeabilized cells to reconstitute the transport of proteins containing nuclear localization signals into the nucleus. The assay requires only a target cell line that can be permeabilized with digitonin, a source of soluble transport factors, typically provided by the cytosol fraction of cultured cells, and a cargo protein of interest. No other specialized equipment is required other than a fluorescence microscope. The assay can be used to identify transport factors required to transport specific proteins, to study the regulation of protein transport, or to study nuclear protein transport under different conditions.

Determination of the binding site of 2-aminothiazole derivative with importin β1 by UV-crosslinking experiment

Importin β1 (KPBN1) appears to be overexpressed in several cancer cells and siRNA-induced inhibition of KPNB1 shows significant inhibition of cancer cell proliferation, but do not affect normal cells. These results indicate that KPNB1 is a potential target and inhibition of KPNB1 can be used as a novel therapeutic approach for the treatment of cancer. Recently, we identified the aminothiazole derivative 1 as a KPNB1-targeted anticancer agent. Herein, we report that compound 1 binds strongly to KPNB1, in a pocket centered around serine-476, as shown by UV-crosslinking and tandem mass spectrometry experiments, and supported using a model derived from molecular docking.

The Pseudorabies Virus DNA Polymerase Accessory Subunit UL42 Directs Nuclear Transport of the Holoenzyme

Pseudorabies virus (PRV) DNA replication occurs in the nuclei of infected cells and requires the viral DNA polymerase. The PRV DNA polymerase comprises a catalytic subunit, UL30, and an accessory subunit, UL42, that confers processivity to the enzyme. Its nuclear localization is a prerequisite for its enzymatic function in the initiation of viral DNA replication. However, the mechanisms by which the PRV DNA polymerase holoenzyme enters the nucleus have not been determined. In this study, we characterized the nuclear import pathways of the PRV DNA polymerase catalytic and accessory subunits. Immunofluorescence analysis showed that UL42 localizes independently in the nucleus, whereas UL30 alone predominantly localizes in the cytoplasm. Intriguingly, the localization of UL30 was completely shifted to the nucleus when it was coexpressed with UL42, demonstrating that nuclear transport of UL30 occurs in an UL42-dependent manner. Deletion analysis and site-directed mutagenesis of the two proteins showed that UL42 contains a functional and transferable bipartite nuclear localization signal (NLS) at amino acids 354–370 and that K³⁵⁴, R³⁵⁵, and K³⁶⁷ are important for the NLS function, whereas UL30 has no NLS. Coimmunoprecipitation assays verified that UL42 interacts with importins α3 and α4 through its NLS. In vitro nuclear import assays demonstrated that nuclear accumulation of UL42 is a temperature- and energy-dependent process and requires both importins α and β, confirming that UL42 utilizes the importin α/β-mediated pathway for nuclear entry. In an UL42 NLS-null mutant, the UL42/UL30 heterodimer was completely confined to the cytoplasm when UL42 was coexpressed with UL30, indicating that UL30 utilizes the NLS function of UL42 for its translocation into the nucleus. Collectively, these findings suggest that UL42 contains an importin α/β-mediated bipartite NLS that transports the viral DNA polymerase holoenzyme into the nucleus in an in vitro expression system.

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.

Mechanistic Insights from Structural Analyses of Ran-GTPase-Driven Nuclear Export of Proteins and RNAs

Understanding how macromolecules are rapidly exchanged between the nucleus and the cytoplasm through nuclear pore complexes is a fundamental problem in biology. Exportins are Ran-GTPase-dependent nuclear transport factors that belong to the karyopherin-β family and mediate nuclear export of a plethora of proteins and RNAs, except for bulk mRNA nuclear export. Exportins bind cargo macromolecules in a Ran-GTP-dependent manner in the nucleus, forming exportin-cargo-Ran-GTP complexes (nuclear export complexes). Transient weak interactions between exportins and nucleoporins containing characteristic FG (phenylalanine-glycine) repeat motifs facilitate nuclear pore complex passage of nuclear export complexes. In the cytoplasm, nuclear export complexes are disassembled, thereby releasing the cargo. GTP hydrolysis by Ran promoted in the cytoplasm makes the disassembly reaction virtually irreversible and provides thermodynamic driving force for the overall export reaction. In the past decade, X-ray crystallography of some of the exportins in various functional states coupled with functional analyses, single-particle electron microscopy, molecular dynamics simulations, and small-angle solution X-ray scattering has provided rich insights into the mechanism of cargo binding and release and also begins to elucidate how exportins interact with the FG repeat motifs. The knowledge gained from structural analyses of nuclear export is being translated into development of clinically useful inhibitors of nuclear export to treat human diseases such as cancer and influenza.

Structure of importin-α bound to a non-classical nuclear localization signal of the influenza A virus nucleoprotein

A non-classical nuclear localization signal (ncNLS) of influenza A virus nucleoprotein (NP) is critical for nuclear import of viral genomic RNAs that transcribe and replicate in the nucleus of infected cells. Here we report a 2.3 Å resolution crystal structure of mouse importin-α1 in complex with NP ncNLS. The structure reveals that NP ncNLS binds specifically and exclusively to the minor NLS-binding site of importin-α. Structural and functional analyses identify key binding pockets on importin-α as potential targets for antiviral drug development. Unlike many other NLSs, NP ncNLS binds to the NLS-binding domain of importin-α weakly with micromolar affinity. These results suggest that a modest inhibitor with low affinity to importin-α could have anti-influenza activity with minimal cytotoxicity.

Nuclear localization signals for four distinct karyopherin-β nuclear import systems

The Karyopherin-β family of proteins mediates nuclear transport of macromolecules. Nuclear versus cytoplasmic localization of proteins is often suggested by the presence of NLSs (nuclear localization signals) or NESs (nuclear export signals). Import-Karyopherin-βs or Importins bind to NLSs in their protein cargos to transport them through nuclear pore complexes into the nucleus. Until recently, only two classes of NLS had been biochemically and structurally characterized: the classical NLS, which is recognized by the Importin-α/β heterodimer and the PY-NLS (proline-tyrosine NLS), which is recognized by Karyopherin-β2 or Transportin-1. Structures of two other Karyopherin-βs, Kap121 and Transportin-SR2, in complex with their respective cargos were reported for the first time recently, revealing two new distinct classes of NLSs. The present paper briefly describes the classical NLS, reviews recent literature on the PY-NLS and provides in-depth reviews of the two newly discovered classes of NLSs that bind Kap121p and Transportin-SR respectively.

Three cardiovirus Leader proteins equivalently inhibit four different nucleocytoplasmic trafficking pathways

Cardiovirus infections inhibit nucleocytoplasmic trafficking by Leader protein-induced phosphorylation of Phe/Gly-containing nucleoporins (Nups). Recombinant Leader from encephalomyocarditis virus, Theiler׳s murine encephalomyelitis virus and Saffold virus target the same subset of Nups, including Nup62 and Nup98, but not Nup50. Reporter cell lines with fluorescence mCherry markers for M9, RS and classical SV40 import pathways, as well as the Crm1-mediated export pathway, all responded to transfection with the full panel of Leader proteins, showing consequent cessation of path-specific active import/export. For this to happen, the Nups had to be presented in the context of intact nuclear pores and exposed to cytoplasmic extracts. The Leader phosphorylation cascade was not effective against recombinant Nup proteins. The findings support a model of Leader-dependent Nup phosphorylation with the purpose of disrupting Nup-transportin interactions.

Nuclear translocation of proteins and the effect of phosphatidic acid

Transport of proteins containing a nuclear localization signal (NLS) into the nucleus is mediated by nuclear transport receptors called importins, typically dimmers of a cargo-binding α-subunit and a β-subunit that mediates translocation through the nuclear pore complexes (NPCs). However, how proteins without canonical NLS move into the nucleus is not well understood. Recent results indicate that phospholipids, such as phosphatidic acid, play important roles in the intracellular translocation of proteins between the nucleus and cytoplasm.

Interaction of parvoviruses with the nuclear envelope

Parvoviruses are serious pathogens but also serve as platforms for gene therapy or for using their lytic activity in experimental cancer treatment. Despite of their growing importance during the last decade little is known on how the viral genome is transported into the nucleus of the infected cell, which is crucial for replication. As nucleic acids are not karyophilic per se nuclear import must be driven by proteins attached to the viral genome. In turn, presence and conformation of these proteins depend upon the entry pathway of the virus into the cell. This review focuses on the trafficking of the parvoviral genome from the cellular periphery to nucleus. Despite of the uncertainties in knowledge about the entry pathway we show that parvoviruses developed a unique strategy to pass the nuclear envelope by hijacking enzymes involved in mitosis.

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.

The Drosophila importin-α3 is required for nuclear import of notch in vivo and it displays synergistic effects with notch receptor on cell proliferation

The Notch signaling pathway controls diverse cell-fate specification events throughout development. The versatility of this pathway to influence different aspects of development comes from its multiple levels of regulation. Upon ligand-induced Notch activation, the Notch intracellular domain (Notch-ICD) is released from the membrane and translocates to the nucleus, where it transduces Notch signals by regulating the transcription of downstream target genes. But the exact mechanism of translocation of Notch-ICD into the nucleus is not clear. Here, we implicate Importin-α3 (also known as karyopherin-α3) in the nuclear translocation of Notch-ICD in Drosophila. Our present analyses reveal that Importin-α3 can directly bind to Notch-ICD and loss of Importin-α3 function results in cytoplasmic accumulation of the Notch receptor. Using MARCM (Mosaic Analysis with a Repressible Cell Marker) technique, we demonstrate that Importin-α3 is required for nuclear localization of Notch-ICD. These results reveal that the nuclear transport of Notch-ICD is mediated by the canonical Importin-α3/Importin-β transport pathway. In addition, co-expression of both Notch-ICD and Importin-α3 displays synergistic effects on cell proliferation. Taken together, our results suggest that Importin-α3 mediated nuclear import of Notch-ICD may play important role in regulation of Notch signaling.

IGFBP-2 nuclear translocation is mediated by a functional NLS sequence and is essential for its pro-tumorigenic actions in cancer cells

IGFBP-2 is highly expressed in both the serum and tumor tissues of most cancers, and is considered one of the most significant genes in the signature of major cancers. IGFBP-2 mainly modulates IGF actions in the pericellular space; however, there is considerable evidence to suggest that IGFBP-2 may also act independently of the IGFs. These IGF-independent actions of IGFBP-2 are exerted either via interactions at the cell surface or intracellularly, via interaction with cytoplasmic or nuclear-binding partners. The precise mechanism underlying the intracellular/intranuclear localization of IGFBP-2 remains unclear. In this study, we investigated IGFBP-2 nuclear localization in several common cancer cells with the aim of dissecting the mechanism of its nuclear trafficking. IGFBP-2 is detected in the nuclei of common cancer cells, including breast, prostate and several neuroblastoma cell lines, using cell fractionation and confocal microscopy. Via nuclear import assays, we show that nuclear entry of IGFBP-2 is mediated by the classical nuclear import mechanisms, primarily through importin-α, as demonstrated by the use of blocking, competition and co-immunoprecipitation assays. Bioinformatics analysis of the IGFBP-2 protein sequence with PSORT II identified a classical nuclear localization signal (cNLS) sequence at 179PKKLRPP185, within the IGFBP-2 linker domain, mutagenesis of which abolishes IGFBP-2 nuclear import. Accordingly, the NLSmutIGFBP-2 fails to activate the VEGF promoter, which would otherwise occur in the presence of wild-type IGFBP-2. As a consequence, no activation of angiogenic processes were observed in NLSmutIGFBP-2 expressing SHEP cells when implanted onto our in vivo quail chorio-allantoic membrane model. Taken together, these data show for the first time that IGFBP-2 possesses a functional NLS sequence and that IGFBP-2 actively translocates into the nucleus by a classical nuclear import mechanism, involving formation of IGFBP-2 complexes with importin-α. Nuclear IGFBP-2 is required for the activation of VEGF expression and consequent angiogenesis.

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.

Nucleocytoplasmic transport: a role for nonspecific competition in karyopherin-nucleoporin interactions

Nucleocytoplasmic transport occurs through the nuclear pore complex (NPC), which in yeast is a ~50 MDa complex consisting of ~30 different proteins. Small molecules can freely exchange through the NPC, but macromolecules larger than ~40 kDa must be aided across by transport factors, most of which belong to a related family of proteins termed karyopherins (Kaps). These transport factors bind to the disordered phenylalanine-glycine (FG) repeat domains in a family of NPC proteins termed FG nups, and this specific binding allows the transport factors to cross the NPC. However, we still know little in terms of the molecular and kinetic details regarding how this binding translates to selective passage of transport factors across the NPC. Here we show that the specific interactions between Kaps and FG nups are strongly modulated by the presence of a cellular milieu whose proteins appear to act as very weak competitors that nevertheless collectively can reduce Kap/FG nup affinities by several orders of magnitude. Without such modulation, the avidities between Kaps and FG nups measured in vitro are too tight to be compatible with the rapid transport kinetics observed in vivo. We modeled the multivalent interactions between the disordered repeat binding sites in the FG nups and multiple cognate binding sites on Kap, showing that they should indeed be sensitive to even weakly binding competitors; the introduction of such competition reduces the availability of these binding sites, dramatically lowering the avidity of their specific interactions and allowing rapid nuclear transport.

Ran-dependent nuclear export mediators: a structural perspective

Nuclear export is an essential eukaryotic activity. It proceeds through nuclear pore complexes (NPCs) and is mediated by soluble receptors that shuttle between nucleus and cytoplasm. RanGTPase-dependent export mediators (exportins) constitute the largest class of these carriers and are functionally highly versatile. All of these exportins load their substrates in response to RanGTP binding in the nucleus and traverse NPCs as ternary RanGTP-exportin-cargo complexes to the cytoplasm, where GTP hydrolysis leads to export complex disassembly. The different exportins vary greatly in their substrate range. Recent structural studies of both protein- and RNA-specific exporters have illuminated how exportins bind their cargoes, how Ran triggers cargo loading and how export complexes are disassembled in the cytoplasm. Here, we review the current state of knowledge and highlight emerging principles as well as prevailing questions.

Single molecule studies of nucleocytoplasmic transport

Molecular traffic between the cytoplasm and the nucleoplasm of eukaryotic cells is mediated by nuclear pore complexes (NPCs). Hundreds, if not thousands, of molecules interact with and transit through each NPC every second. The pore is blocked by a permeability barrier, which consists of a network of intrinsically unfolded polypeptides containing thousands of phenylalanine-glycine (FG) repeat motifs. This FG-network rejects larger molecules and admits smaller molecules or cargos bound to nuclear transport receptors (NTRs). For a cargo transport complex, minimally consisting of a cargo molecule plus an NTR, access to the permeability barrier is provided by interactions between the NTR and the FG repeat motifs. Numerous models have been postulated to explain the controlled accessibility and the transport characteristics of the FG-network, but the amorphous, flexible nature of this structure has hindered characterization. A relatively recent development is the ability to monitor the real-time movement of single molecules through individual NPCs via single molecule fluorescence (SMF) microscopy. A major advantage of this approach is that it can be used to continuously monitor a series of specific molecular interactions in an active pore with millisecond time resolution, which therefore allows one to distinguish between kinetic and thermodynamic control. Novel insights and prospects for the future are outlined in this review. This article is part of a Special Issue entitled: Regulation of Signaling and Cellular Fate through Modulation of Nuclear Protein Import.

Molecular basis for specificity of nuclear import and prediction of nuclear localization

Although proteins are translated on cytoplasmic ribosomes, many of these proteins play essential roles in the nucleus, mediating key cellular processes including but not limited to DNA replication and repair as well as transcription and RNA processing. Thus, understanding how these critical nuclear proteins are accurately targeted to the nucleus is of paramount importance in biology. Interaction and structural studies in the recent years have jointly revealed some general rules on the specificity determinants of the recognition of nuclear targeting signals by their specific receptors, at least for two nuclear import pathways: (i) the classical pathway, which involves the classical nuclear localization sequences (cNLSs) and the receptors importin-α/karyopherin-α and importin-β/karyopherin-β1; and (ii) the karyopherin-β2 pathway, which employs the proline-tyrosine (PY)-NLSs and the receptor transportin-1/karyopherin-β2. The understanding of specificity rules allows the prediction of protein nuclear localization. We review the current understanding of the molecular determinants of the specificity of nuclear import, focusing on the importin-α•cargo recognition, as well as the currently available databases and predictive tools relevant to nuclear localization. This article is part of a Special Issue entitled: Regulation of Signaling and Cellular Fate through Modulation of Nuclear Protein Import.

Identification of a small molecule inhibitor of importin β mediated nuclear import by confocal on-bead screening of tagged one-bead one-compound libraries

In eukaryotic cells, proteins and RNAs are transported between the nucleus and the cytoplasm by nuclear import and export receptors. Over the past decade, small molecules that inhibit the nuclear export receptor CRM1 have been identified, most notably leptomycin B. However, up to now no small molecule inhibitors of nuclear import have been described. Here we have used our automated confocal nanoscanning and bead picking method (CONA) for on-bead screening of a one-bead one-compound library to identify the first such import inhibitor, karyostatin 1A. Karyostatin 1A binds importin β with high nanomolar affinity and specifically inhibits importin α/β mediated nuclear import at low micromolar concentrations in vitro and in living cells, without perturbing transportin mediated nuclear import or CRM1 mediated nuclear export. Surface plasmon resonance binding experiments suggest that karyostatin 1A acts by disrupting the interaction between importin β and the GTPase Ran. As a selective inhibitor of the importin α/β import pathway, karyostatin 1A will provide a valuable tool for future studies of nucleocytoplasmic trafficking.

Shiga toxin is transported into the nucleoli of intestinal epithelial cells via a carrier-dependent process

Shiga toxin (Stx) produced by the invasive Shigella dysenteriae serotype 1 (S. dysenteriae1) causes gastrointestinal and kidney complications. It has been assumed that Stx is released intracellularly after enterocyte invasion by S. dysenteriae1. However, there is little information about Stx distribution inside S. dysenteriae1-infected enterocytes. Here, we use intestinal epithelial T84 cells to characterize the trafficking of Stx delivered into the cytosol, in ways that mimic aspects of S. dysenteriae1 infection. We find that cytoplasmic Stx is transported into nucleoli. Stx nucleolar movement is carrier- and energy-dependent. Stx binding to the nucleoli of normal human enterocytes in vitro supports possible roles for nucleolar trafficking in toxin-induced intestinal pathology.

Evasion of interferon responses by Ebola and Marburg viruses

The filoviruses, Ebola virus (EBOV) and Marburg virus (MARV), cause frequently lethal viral hemorrhagic fever. These infections induce potent cytokine production, yet these host responses fail to prevent systemic virus replication. Consistent with this, filoviruses have been found to encode proteins VP35 and VP24 that block host interferon (IFN)-alpha/beta production and inhibit signaling downstream of the IFN-alpha/beta and the IFN-gamma receptors, respectively. VP35, which is a component of the viral nucleocapsid complex and plays an essential role in viral RNA synthesis, acts as a pseudosubstrate for the cellular kinases IKK-epsilon and TBK-1, which phosphorylate and activate interferon regulatory factor 3 (IRF-3) and interferon regulatory factor 7 (IRF-7). VP35 also promotes SUMOylation of IRF-7, repressing IFN gene transcription. In addition, VP35 is a dsRNA-binding protein, and mutations that disrupt dsRNA binding impair VP35 IFN-antagonist activity while leaving its RNA replication functions intact. The phenotypes of recombinant EBOV bearing mutant VP35s unable to inhibit IFN-alpha/beta demonstrate that VP35 IFN-antagonist activity is critical for full virulence of these lethal pathogens. The structure of the VP35 dsRNA-binding domain, which has recently become available, is expected to provide insight into how VP35 IFN-antagonist and dsRNA-binding functions are related. The EBOV VP24 protein inhibits IFN signaling through an interaction with select host cell karyopherin-alpha proteins, preventing the nuclear import of otherwise activated STAT1. It remains to be determined to what extent VP24 may also modulate the nuclear import of other host cell factors and to what extent this may influence the outcome of infection. Notably, the Marburg virus VP24 protein does not detectably block STAT1 nuclear import, and, unlike EBOV, MARV infection inhibits STAT1 and STAT2 phosphorylation. Thus, despite their similarities, there are fundamental differences by which these deadly viruses counteract the IFN system. It will be of interest to determine how these differences influence pathogenesis.

Nucleoporin 153 arrests the nuclear import of hepatitis B virus capsids in the nuclear basket

Virtually all DNA viruses including hepatitis B viruses (HBV) replicate their genome inside the nucleus. In non-dividing cells, the genome has to pass through the nuclear pore complexes (NPCs) by the aid of nuclear transport receptors as e.g. importin β (karyopherin). Most viruses release their genome in the cytoplasm or at the cytosolic face of the NPC, as the diameter of their capsids exceeds the size of the NPC. The DNA genome of HBV is derived from reverse transcription of an RNA pregenome. Genome maturation occurs in cytosolic capsids and progeny capsids can deliver the genome into the nucleus causing nuclear genome amplification. The karyophilic capsids are small enough to pass the NPC, but nuclear entry of capsids with an immature genome is halted in the nuclear basket on the nuclear side of the NPC, and the genome remains encapsidated. In contrast, capsids with a mature genome enter the basket and consequently liberate the genome. Investigating the difference between immature and mature capsids, we found that mature capsids had to disintegrate in order to leave the nuclear basket. The arrest of a karyophilic cargo at the nuclear pore is a rare phenomenon, which has been described for only very few cellular proteins participating in nuclear entry. We analyzed the interactions causing HBV capsid retention. By pull-down assays and partial siRNA depletion, we showed that HBV capsids directly interact with nucleoporin 153 (Nup153), an essential protein of the nuclear basket which participates in nuclear transport via importin β. The binding sites of importin β and capsids were shown to overlap but capsid binding was 150-fold stronger. In cellulo experiments using digitonin-permeabilized cells confirmed the interference between capsid binding and nuclear import by importin β. Collectively, our findings describe a unique nuclear import strategy not only for viruses but for all karyophilic cargos.

Viral capsids facilitate protection of the enclosed viral genome and participate in the intracellular transport of the genome. At the site of replication capsids have to release the genome. The particular factors triggering genome liberation are not well understood. Like other karyophilic cargos, hepatitis B virus (HBV) capsids are transported through the nuclear pore using nuclear transport receptors of the importin ß superfamily. Unlike physiological cargos, HBV capsids become arrested within the nuclear basket, which is a filamentous structure on the nuclear side of the nuclear pore. Asking which interaction causes this unique strategy, we found that the capsids bind to a protein of the basket periphery, nucleoporin 153 (Nup153). The findings were confirmed in situ using digitonin-permeabilized cells that support physiological genome delivery into the nucleus. We observed that HBV capsids bound to Nup153 irrespective of the maturation of the encapsidated genome. But while capsids with an immature genome remained in arrested state, capsids with a mature genome disassembled and released their DNA.

Quantification of nuclear protein transport using induced heterodimerization

Trafficking of proteins between the cytoplasm and nucleus occurs exclusively across the nuclear pore complex of eucaryotic cells. Fundamental aspects of this process affect temporal and spatial parameters, the latter carried out by specific import [nuclear localization sequence (NLS)] and export [nuclear export sequence (NES)] sequences. In this study, we focused on the adaptation of a protein heterodimerization assay to kinetically measure Crm1-mediated nuclear export in living cells using the rapalog AP21967, a heterodimerizing agent and NLS- and NES-containing fusion proteins equipped with distinct AP21967-specific binding motifs. In HeLa cells, we observed rapid nuclear export of the NLS-containing fusion protein in the presence of AP21967, with the extent of this process being a function of the number of AP21967-binding motifs. AP21967-induced nuclear export was specifically inhibited by the Crm1-binding molecule leptomycin B. Half maximal export was achieved after approximately 10 min. We further applied protein heterodimerization in HeLa cells to study induced NLS-mediated nuclear import. Only in the presence of heterodimerizer AP21967 nuclear import of a cytoplasmically localizing fusion protein was observed. Induced protein heterodimerization is thus a valuable tool to quantitatively study nucleocytoplasmic protein trafficking in cultured cells, in a non-invasive, time-saving manner.

Transportin regulates major mitotic assembly events: from spindle to nuclear pore assembly

Mitosis in higher eukaryotes is marked by the sequential assembly of two massive structures: the mitotic spindle and the nucleus. Nuclear assembly itself requires the precise formation of both nuclear membranes and nuclear pore complexes. Previously, importin alpha/beta and RanGTP were shown to act as dueling regulators to ensure that these assembly processes occur only in the vicinity of the mitotic chromosomes. We now find that the distantly related karyopherin, transportin, negatively regulates nuclear envelope fusion and nuclear pore assembly in Xenopus egg extracts. We show that transportin-and importin beta-initiate their regulation as early as the first known step of nuclear pore assembly: recruitment of the critical pore-targeting nucleoporin ELYS/MEL-28 to chromatin. Indeed, each karyopherin can interact directly with ELYS. We further define the nucleoporin subunit targets for transportin and importin beta and find them to be largely the same: ELYS, the Nup107/160 complex, Nup53, and the FG nucleoporins. Equally importantly, we find that transportin negatively regulates mitotic spindle assembly. These negative regulatory events are counteracted by RanGTP. We conclude that the interplay of the two negative regulators, transportin and importin beta, along with the positive regulator RanGTP, allows precise choreography of multiple cell cycle assembly events.

Identification of protein cofactors necessary for sequence-specific plasmid DNA nuclear import

Although transfections are routinely used in the laboratory, the mechanism(s) by which exogenous DNA is transported into the nucleus is poorly understood. By improving our understanding of how vectors circumvent the numerous cellular barriers to gene transfer, more efficient gene delivery methods can be devised. We have begun to design plasmid constructs that enter the nucleus of specific cell types in the absence of cell division, thereby enhancing levels of expression. We have shown that inclusion of specific DNA sequences in plasmid constructs mediates nuclear import both in vitro and in vivo. Here, we use plasmid affinity chromatography, mass spectrometry (MS), and live-cell pulldowns of transfected plasmid constructs to identify protein cofactors that interact in a sequence-specific manner with these DNA nuclear targeting sequences (DTSs). Importin beta(1), importin 7, and the small guanosine triphosphatase Ran all demonstrate DTS-specific interaction in both MS and pull-down assays, consistent with our model of plasmid nuclear import. In addition, knockdown of importin beta(1) with small interfering RNA (siRNA) abrogates plasmid nuclear import, indicating that it is a necessary cofactor. Our discovery that specific karyopherins mediate plasmid nuclear import can be used to design more effective vectors for gene delivery.

The N-terminal nuclear localization sequences of liver X receptors alpha and beta bind to importin alpha and are essential for both nuclear import and transactivating functions

Liver X receptors (LXRs) alpha and beta are nuclear receptors, which form obligate heterodimers with the retinoid X receptor (RXR). The LXRs regulate both redundantly and non-redundantly the transcription of genes controlling cholesterol metabolism and transport as well as lipogenesis. Previously, we showed that mutations in putative N-terminal nuclear localization sequences (NLSs) within both LXRs inhibit nuclear import. Through in vitro studies, we show here that these NLSs bind importin alpha and are both necessary and sufficient for the nuclear import of LXRs. Imaging, transactivation, and electro-mobility shift experiments show that RXR rescues the nuclear import of the LXRalpha NLS mutant yet does not restore its transcriptional activity despite intact DNA binding. In contrast, RXR partially rescues the import of the LXRbeta NLS mutant, but has no effect on its transcriptional activity due to the loss of DNA binding. Experiments with NLS mutant RXR confirmed that RXR may dominate the nuclear import of the RXR/LXRalpha heterodimer, whereas LXRbeta dominates the nuclear import of the RXR/LXRbeta heterodimer. Intriguingly, our data indicate differences between LXRalpha and LXRbeta in their interaction with RXR and in the role their NLSs play in transactivating functions independent of nuclear import.

Nuclear import of Avian Sarcoma Virus integrase is facilitated by host cell factors

BACKGROUND

Integration of retroviral DNA into the host cell genome is an obligatory step in the virus life cycle. In previous reports we identified a sequence (amino acids 201-236) in the linker region between the catalytic core and C-terminal domains of the avian sarcoma virus (ASV) integrase protein that functions as a transferable nuclear localization signal (NLS) in mammalian cells. The sequence is distinct from all known NLSs but, like many, contains basic residues that are essential for activity.

RESULTS

Our present studies with digitonin-permeabilized HeLa cells show that nuclear import mediated by the NLS of ASV integrase is an active, saturable, and ATP-dependent process. As expected for transport through nuclear pore complexes, import is blocked by treatment of cells with wheat germ agglutinin. We also show that import of ASV integrase requires soluble cellular factors but does not depend on binding the classical adapter Importin-alpha. Results from competition studies indicate that ASV integrase relies on one or more of the soluble components that mediate transport of the linker histone H1.

CONCLUSION

These results are consistent with a role for ASV integrase and cytoplasmic cellular factors in the nuclear import of its viral DNA substrate, and lay the foundation for identification of host cell components that mediate this reaction.

Identification of a monopartite sequence in PU.1 essential for nuclear import, DNA-binding and transcription of myeloid-specific genes

The Ets transcription factor PU.1 is an essential regulator of normal hematopoiesis, especially within the myeloid lineage. As such, endogenous PU.1 predominantly localizes to the nucleus of mammalian cells to facilitate gene regulation. However, to date, little is known regarding the mechanisms of PU.1 nuclear transport. We found, using HeLa and RAW 264.7 macrophage cells, that PU.1 enters the nucleus via passive diffusion and active transport. The latter can be facilitated by: (i) the classical pathway requiring importin alpha and beta; (ii) the non-classical pathway requiring only importin beta; or (iii) direct interaction with nucleoporins. A group of six positively charged lysine or arginine residues within the Ets DNA-binding domain was determined to be crucial in active nuclear import. These residues directly interact with importin beta to facilitate a predominantly non-classical import pathway. Furthermore, luciferase reporter assays demonstrated that these same six amino acids are crucial for PU.1-mediated transcriptional activation of myeloid-specific genes. Indeed, these residues may represent a consensus sequence vital for nuclear import, DNA-binding and transcriptional activity of Ets family members. By identifying and characterizing the mechanisms of PU.1 nuclear import and the specific amino acids involved, this report may provide insights into the molecular basis of diseases.

Nucleocytoplasmic transport of proteins

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

Nuclear import in digitonin-permeabilized cells

Import of proteins into the nucleus of a cell is a complex process that can be reconstituted in vitro. Digitonin-permeabilized cells are washed free of cytosolic factors to provide competent nuclei. Cytosolic factors for import are provided by an extract of Xenopus ovarian cells. Fluorochrome-conjugated probes, cloned proteins fused to green fluorescent protein, or antibodies to the protein of interest are used to visualize nuclear import. The system can also be used to identify nuclear localization sequences (NLS) of imported proteins.

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

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

Ceramide regulation of nuclear protein import

Nucleocytoplasmic trafficking is an essential and responsive cellular mechanism that directly affects cell growth and proliferation, and its potential to address metabolic challenge is incompletely defined. Ceramide is an antiproliferative sphingolipid found within vascular smooth muscle cells in atherosclerotic plaques, but its mechanism of action remains unclear. The hypothesis that ceramide inhibits cell growth through nuclear transport regulation was tested. In smooth muscle cells, exogenously supplemented ceramide inhibited classical nuclear protein import that involved the activation of cytosolic p38 mitogen-activated protein kinase (MAPK). After application of SB 202190, a specific and potent pharmacological antagonist of p38 MAPK, sphingolipid impingement on nuclear transport was corrected. Distribution pattern assessments of two essential nuclear transport proteins, importin-alpha and Cellular Apoptosis Susceptibility, revealed ceramide-mediated relocalization that was reversed upon the addition of SB 202190. Furthermore, cell counts, nuclear cyclin A, and proliferating cell nuclear antigen expression, markers of cellular proliferation, were diminished after ceramide treatment and effectively rescued by the addition of inhibitor. Together, these data demonstrate, for the first time, the sphingolipid regulation of nuclear import that defines and expands the adaptive capacity of the nucleocytoplasmic transport machinery.

Involvement of importin-4 in the transport of transition protein 2 into the spermatid nucleus

Mammalian spermiogenesis is characterized by a unique chromatin-remodeling process in which histones are replaced by transition protein 1 (TP1), TP2, and TP4, which are further replaced by protamines. We showed previously that the import of TP2 into the haploid spermatid nucleus requires the components of cytosol and ATP. We have now carried out a detailed analysis to characterize the molecular components underlying the nuclear translocation of TP2. Real-time PCR analysis of the expression of different importins in testicular germ cells revealed that importin-4 and importin-beta3 are significantly up-regulated in tetraploid and haploid germ cells. We carried out physical interaction studies as well as an in vitro nuclear transport assay using recombinant TP2 and the nuclear localization signal of TP2 (TP2(NLS)) fused to glutathione S-transferase in digitonin-permeabilized, haploid, round spermatids and identified importin-4 to be involved in the import of TP2. A three-dimensional model of the importin-4 protein was generated using the crystal structure of importin-beta1 as the template. Molecular docking simulations of TP2(NLS) with the importin-4 structure led to the identification of a TP2(NLS) binding pocket spanning the three helices (helices 21 to 23) of importin-4, which was experimentally confirmed by in vitro interaction and import studies with different deletion mutants of importin-4. In contrast to TP2, TP1 import was accomplished through a passive diffusion process.

Mouse Disabled1 (DAB1) is a nucleocytoplasmic shuttling protein

Disabled1 (DAB1) is an intracellular mediator of the Reelin-signaling pathway and essential for correct neuronal positioning during brain development. So far, DAB1 has been considered a cytoplasmic protein. Here, we show that DAB1 is subject to nucleocytoplasmic shuttling. In its steady state, DAB1 is mainly located in the cytoplasm. However, treatment with leptomycine B, a specific inhibitor of the CRM1 (chromosomal region maintenance 1)-RanGTP-dependent nuclear export, resulted in nuclear accumulation of DAB1. By using deletion or substitutional mutants of DAB1 fused with enhanced green fluorescent protein, we have mapped a bipartite nuclear localization signal and two CRM1-dependent nuclear export signals. These targeting signals were functional in both Neuro2a cells and primary cerebral cortical neurons. Using purified recombinant proteins, we have shown that CRM1 binds to DAB1 directly in a RanGTP-dependent manner. We also show that tyrosine phosphorylation of DAB1, which is indispensable for the layer formation of the brain, by Fyn tyrosine kinase or Reelin stimulation did not affect the subcellular localization of DAB1 in vitro. These results suggest that DAB1 is a nucleocytoplasmic shuttling protein and raise the possibility that DAB1 plays a role in the nucleus as well as in the cytoplasm.

Passive and facilitated transport in nuclear pore complexes is largely uncoupled

Nuclear pore complexes provide the sole gateway for the exchange of material between nucleus and cytoplasm of interphase eukaryotic cells. They support two modes of transport: passive diffusion of ions, metabolites, and intermediate-sized macromolecules and facilitated, receptor-mediated translocation of proteins, RNA, and ribonucleoprotein complexes. It is generally assumed that both modes of transport occur through a single diffusion channel located within the central pore of the nuclear pore complex. To test this hypothesis, we studied the mutual effects between transporting molecules utilizing either the same or different modes of translocation. We find that the two modes of transport do not interfere with each other, but molecules utilizing a particular mode of transport do hinder motion of others utilizing the same pathway. We therefore conclude that the two modes of transport are largely segregated.

Dissection of a novel nuclear localization signal in open reading frame 29 of varicella-zoster virus

Open reading frame 29 (ORF29) of varicella-zoster virus (VZV) encodes a 120-kDa single-stranded DNA binding protein (ORF29p) that is not packaged in the virion and is expressed during latency. During lytic infection, ORF29p is localized primarily to infected cell nuclei. In contrast, ORF29p is found exclusively in the cytoplasm in neurons of the dorsal root ganglia obtained at autopsy from seropositive latently infected patients. ORF29p accumulates in the nuclei of neurons in dorsal root ganglia obtained at autopsy from patients with active zoster. The localization of this protein is, therefore, tightly correlated with the proposed VZV lytic/latent switch. In this report, we have investigated the nuclear import mechanism of ORF29p. We identified a novel nuclear targeting domain bounded by amino acids 9 to 154 of ORF29p that functions independent of other VZV-encoded factors. In vitro import assays in digitonin-permeabilized HeLa cells reveal that ORF29p is transported into the nucleus by a Ran-, karyopherin alpha- and beta-dependent mechanism. These data are further supported by the demonstration that a glutathione S-transferase-karyopherin alpha fusion interacts with ORF29p, but not with a protein containing a point mutation in its nuclear localization signal (NLS). Therefore, the region of ORF29p responsible for its nuclear targeting is also involved in the association with karyopherin alpha. As a result of this interaction, this noncanonical NLS appears to hijack the classical cellular nuclear import machinery. Elucidation of the mechanisms governing ORF29p nuclear targeting could shed light on the VZV reactivation process.

Cross-talk between snurportin1 subdomains

The initial steps of spliceosomal small nuclear ribonucleoprotein (snRNP) maturation take place in the cytoplasm. After formation of an Sm-core and a trimethylguanosine (TMG) cap, the RNPs are transported into the nucleus via the import adaptor snurportin1 (SPN) and the import receptor importin-beta. To better understand this process, we identified SPN residues that are required to mediate interactions with TMG caps, importin-beta, and the export receptor, exportin1 (Xpo1/Crm1). Mutation of a single arginine residue within the importin-beta binding domain (IBB) disrupted the interaction with importin-beta, but preserved the ability of SPN to bind Xpo1 or TMG caps. Nuclear transport assays showed that this IBB mutant is deficient for snRNP import but that import can be rescued by addition of purified survival of motor neurons (SMN) protein complexes. Conserved tryptophan residues outside of the IBB are required for TMG binding. However, SPN can be imported into the nucleus without cargo. Interestingly, SPN targets to Cajal bodies when U2 but not U1 snRNPs are imported as cargo. SPN also relocalizes to Cajal bodies upon treatment with leptomycin B. Finally, we uncovered an interaction between the N- and C-terminal domains of SPN, suggesting an autoregulatory function similar to that of importin-alpha.

Polyethylenimine-based non-viral gene delivery systems

Gene therapy has become a promising strategy for the treatment of many inheritable or acquired diseases that are currently considered incurable. Non-viral vectors have attracted great interest, as they are simple to prepare, rather stable, easy to modify and relatively safe, compared to viral vectors. Unfortunately, they also suffer from a lower transfection efficiency, requiring additional effort for their optimization. The cationic polymer polyethylenimine (PEI) has been widely used for non-viral transfection in vitro and in vivo and has an advantage over other polycations in that it combines strong DNA compaction capacity with an intrinsic endosomolytic activity. Here, we give some insight into strategies developed for PEI-based non-viral vectors to overcome intracellular obstacles, including the improvement of methods for polyplex preparation and the incorporation of endosomolytic agents or nuclear localization signals. In recent years, PEI-based non-viral vectors have been locally or systemically delivered, mostly to target gene delivery to tumor tissue, the lung or liver. This requires strategies to efficiently shield transfection polyplexes against non-specific interaction with blood components, extracellular matrix and untargeted cells and the attachment of targeting moieties, which allow for the directed gene delivery to the desired cell or tissue. In this context, materials, facilitating the design of novel PEI-based non-viral vectors are described.

Molecular basis for dissimilar nuclear trafficking of the actin-bundling protein isoforms T- and L-plastin

T- and L-plastin are highly similar actin-bundling proteins implicated in the regulation of cell morphology, lamellipodium protrusion, bacterial invasion and tumor progression. We show that T-plastin localizes predominantly to the cytoplasm, whereas L-plastin distributes between nucleus and cytoplasm in HeLa or Cos cells. T-plastin shows nuclear accumulation upon incubation of cells with the CRM1 antagonist leptomycin B (LMB). We identified a Rev-like nuclear export sequence (NES) in T-plastin that is able to export an otherwise nuclear protein in an LMB-dependent manner. Deletion of the NES promotes nuclear accumulation of T-plastin. Mutation of residues L17, F21 or L26 in the T-plastin NES inhibits nuclear efflux. L-plastin harbors a less conserved NES and lacks the F21 T-plastin residue. Insertion of a Phe residue in the L-plastin NES specifically enhances its export activity. These findings explain why both isoforms exhibit specific distribution patterns in eukaryotic cells.