Dynein light chain association sequences can facilitate nuclear protein import

Interaction proteomics analysis to provide insight into TFAMoplex-mediated transfection

In an earlier investigation, our group introduced the TFAMoplex, a transfection agent based on the mitochondrial transcription factor A (TFAM) protein, which complexes DNA into nanoparticles. The original TFAMoplex further contained a bacterial phospholipase to achieve endosomal escape, and the vaccinia-related kinase 1 (VRK1), which significantly boosted the transfection efficiency of the system by an unknown mechanism. This study aims at replacing VRK1 within the TFAMoplex with dynein light chain proteins, specifically RP3, to directly tether the complexes to the dynein motor complex for enhanced cytosolic transport. To confirm the interaction between the dynein complex and the resulting fusion protein, we examined the binding kinetics of TFAM-RP3 to the dynein intermediate chains 1 and 2. Furthermore, we established a proteomics-based assay to compare cytosolic protein interactions of different TFAMoplex variants, including the RP3-modified version and the original VRK1-containing system. In the group of the VRK1-containing TFAMoplex, significant shifts of protein interactors were observed, especially for nucleolar proteins. Leveraging this knowledge, we incorporated one of these nuclear proteins, leucine-rich repeat-containing protein 59 (LRRC59), into the TFAMoplex, resulting in a significant improvement of transfection properties compared to the RP3-modified system and comparable levels versus the original, VRK1-containing version. This study not only advances our comprehension of the TFAMoplex system but also offers broader insights into the potential of protein engineering for designing effective gene delivery systems.

Structural insights into the multifunctionality of rabies virus P3 protein

Viruses form extensive interfaces with host proteins to modulate the biology of the infected cell, frequently via multifunctional viral proteins. These proteins are conventionally considered as assemblies of independent functional modules, where the presence or absence of modules determines the overall composite phenotype. However, this model cannot account for functions observed in specific viral proteins. For example, rabies virus (RABV) P3 protein is a truncated form of the pathogenicity factor P protein, but displays a unique phenotype with functions not seen in longer isoforms, indicating that changes beyond the simple complement of functional modules define the functions of P3. Here, we report structural and cellular analyses of P3 derived from the pathogenic RABV strain Nishigahara (Nish) and an attenuated derivative strain (Ni-CE). We identify a network of intraprotomer interactions involving the globular C-terminal domain and intrinsically disordered regions (IDRs) of the N-terminal region that characterize the fully functional Nish P3 to fluctuate between open and closed states, whereas the defective Ni-CE P3 is predominantly open. This conformational difference appears to be due to the single mutation N226H in Ni-CE P3. We find that Nish P3, but not Ni-CE or N226H P3, undergoes liquid-liquid phase separation and this property correlates with the capacity of P3 to interact with different cellular membrane-less organelles, including those associated with immune evasion and pathogenesis. Our analyses propose that discrete functions of a critical multifunctional viral protein depend on the conformational arrangements of distant individual domains and IDRs, in addition to their independent functions.

Prospects of Using Protein Engineering for Selective Drug Delivery into a Specific Compartment of Target Cells

A large number of proteins are successfully used to treat various diseases. These include natural polypeptide hormones, their synthetic analogues, antibodies, antibody mimetics, enzymes, and other drugs based on them. Many of them are demanded in clinical settings and commercially successful, mainly for cancer treatment. The targets for most of the aforementioned drugs are located at the cell surface. Meanwhile, the vast majority of therapeutic targets, which are usually regulatory macromolecules, are located inside the cell. Traditional low molecular weight drugs freely penetrate all cells, causing side effects in non-target cells. In addition, it is often difficult to elaborate a small molecule that can specifically affect protein interactions. Modern technologies make it possible to obtain proteins capable of interacting with almost any target. However, proteins, like other macromolecules, cannot, as a rule, freely penetrate into the desired cellular compartment. Recent studies allow us to design multifunctional proteins that solve these problems. This review considers the scope of application of such artificial constructs for the targeted delivery of both protein-based and traditional low molecular weight drugs, the obstacles met on the way of their transport to the specified intracellular compartment of the target cells after their systemic bloodstream administration, and the means to overcome those difficulties.

Getting on the right track: Interactions between viruses and the cytoskeletal motor proteins

The cytoskeleton is an essential component of the cell and it is involved in multiple physiological functions, including intracellular organization and transport. It is composed of three main families of proteinaceous filaments; microtubules, actin filaments and intermediate filaments and their accessory proteins. Motor proteins, which comprise the dynein, kinesin and myosin superfamilies, are a remarkable group of accessory proteins that mainly mediate the intracellular transport of cargoes along with the cytoskeleton. Like other cellular structures and pathways, viruses can exploit the cytoskeleton to promote different steps of their life cycle through associations with motor proteins. The complexity of the cytoskeleton and the differences among viruses, however, has led to a wide diversity of interactions, which in most cases remain poorly understood. Unveiling the details of these interactions is necessary not only for a better comprehension of specific infections, but may also reveal new potential drug targets to fight dreadful diseases such as rabies disease and acquired immunodeficiency syndrome (AIDS). In this review, we describe a few examples of the mechanisms that some human viruses, that is, rabies virus, adenovirus, herpes simplex virus, human immunodeficiency virus, influenza A virus and papillomavirus, have developed to hijack dyneins, kinesins and myosins.

Microtubules in Influenza Virus Entry and Egress

Influenza viruses are respiratory pathogens that represent a significant threat to public health, despite the large-scale implementation of vaccination programs. It is necessary to understand the detailed and complex interactions between influenza virus and its host cells in order to identify successful strategies for therapeutic intervention. During viral entry, the cellular microenvironment presents invading pathogens with a series of obstacles that must be overcome to infect permissive cells. Influenza hijacks numerous host cell proteins and associated biological pathways during its journey into the cell, responding to environmental cues in order to successfully replicate. The cellular cytoskeleton and its constituent microtubules represent a heavily exploited network during viral infection. Cytoskeletal filaments provide a dynamic scaffold for subcellular viral trafficking, as well as virus-host interactions with cellular machineries that are essential for efficient uncoating, replication, and egress. In addition, influenza virus infection results in structural changes in the microtubule network, which itself has consequences for viral replication. Microtubules, their functional roles in normal cell biology, and their exploitation by influenza viruses will be the focus of this review.

Stimuli-Sensitive Cell Penetrating Peptide-Modified Nanocarriers

The integration of drugs into nanocarriers favorably altered their pharmacodynamics and pharmacokinetics compared to free drugs, and increased their therapeutic index. However, selective cellular internalization in diseased tissues rather than normal tissues still presents a formidable challenge. In this chapter I will cover solutions involving environment-responsive cell-penetrating peptides (CPPs). I will discuss properties of CPPs as universal cellular uptake enhancers, and the modifications imparted to CPP-modified nanocarriers to confine CPP activation to diseased tissues.

Exportin-1-Dependent Nuclear Export of DEAD-box Helicase DDX3X is Central to its Role in Antiviral Immunity

DEAD-box helicase 3, X-linked (DDX3X) regulates the retinoic acid-inducible gene I (RIG-I)-like receptor (RLR)-mediated antiviral response, but can also be a host factor contributing to the replication of viruses of significance to human health, such as human immunodeficiency virus type 1 (HIV-1). These roles are mediated in part through its ability to actively shuttle between the nucleus and the cytoplasm to modulate gene expression, although the trafficking mechanisms, and impact thereof on immune signaling and viral infection, are incompletely defined. We confirm that DDX3X nuclear export is mediated by the nuclear transporter exportin-1/CRM1, dependent on an N-terminal, leucine-rich nuclear export signal (NES) and the monomeric guanine nucleotide binding protein Ran in activated GTP-bound form. Transcriptome profiling and ELISA show that exportin-1-dependent export of DDX3X to the cytoplasm strongly impacts IFN-β production and the upregulation of immune genes in response to infection. That this is key to DDX3X's antiviral role was indicated by enhanced infection by human parainfluenza virus-3 (hPIV-3)/elevated virus production when the DDX3X NES was inactivated. Our results highlight a link between nucleocytoplasmic distribution of DDX3X and its role in antiviral immunity, with strong relevance to hPIV-3, as well as other viruses such as HIV-1.

Microtubules: Evolving roles and critical cellular interactions

Microtubules are cytoskeletal elements known as drivers of directed cell migration, vesicle and organelle trafficking, and mitosis. In this review, we discuss new research in the lens that has shed light into further roles for stable microtubules in the process of development and morphogenesis. In the lens, as well as other systems, distinct roles for characteristically dynamic microtubules and stabilized populations are coming to light. Understanding the mechanisms of microtubule stabilization and the associated microtubule post-translational modifications is an evolving field of study. Appropriate cellular homeostasis relies on not only one cytoskeletal element, but also rather an interaction between cytoskeletal proteins as well as other cellular regulators. Microtubules are key integrators with actin and intermediate filaments, as well as cell–cell junctional proteins and other cellular regulators including myosin and RhoGTPases to maintain this balance.

Impact statement

The role of microtubules in cellular functioning is constantly expanding. In this review, we examine new and exciting fields of discovery for microtubule’s involvement in morphogenesis, highlight our evolving understanding of differential roles for stabilized versus dynamic subpopulations, and further understanding of microtubules as a cellular integrator.

Dynein Light Chain DLC-1 Facilitates the Function of the Germline Cell Fate Regulator GLD-1 in Caenorhabditis elegans

Developmental transitions of germ cells are often regulated at the level of post-transcriptional control of gene expression. In the Caenorhabditis elegans germline, stem and progenitor cells exit the proliferative phase and enter meiotic differentiation to form gametes essential for fertility. The RNA binding protein GLD-1 is a cell fate regulator that promotes meiosis and germ cell differentiation during development by binding to and repressing translation of target messenger RNAs. Here, we discovered that some GLD-1 functions are promoted by binding to DLC-1, a small protein that functions as an allosteric regulator of multisubunit protein complexes. We found that DLC-1 is required to regulate a subset of GLD-1 target messenger RNAs and that DLC-1 binding GLD-1 prevents ectopic germ cell proliferation and facilitates gametogenesis in vivo Additionally, our results reveal a new requirement for GLD-1 in the events of oogenesis leading to ovulation. DLC-1 contributes to GLD-1 function independent of its role as a light chain component of the dynein motor. Instead, we propose that DLC-1 promotes assembly of GLD-1 with other binding partners, which facilitates formation of regulatory ribonucleoprotein complexes and may direct GLD-1 target messenger RNA selectivity.

Recognition by host nuclear transport proteins drives disorder-to-order transition in Hendra virus V

Hendra virus (HeV) is a paramyxovirus that causes lethal disease in humans, for which no vaccine or antiviral agent is available. HeV V protein is central to pathogenesis through its ability to interact with cytoplasmic host proteins, playing key antiviral roles. Here we use immunoprecipitation, siRNA knockdown and confocal laser scanning microscopy to show that HeV V shuttles to and from the nucleus through specific host nuclear transporters. Spectroscopic and small angle X-ray scattering studies reveal HeV V undergoes a disorder-to-order transition upon binding to either importin α/β1 or exportin-1/Ran-GTP, dependent on the V N-terminus. Importantly, we show that specific inhibitors of nuclear transport prevent interaction with host transporters, and reduce HeV infection. These findings emphasize the critical role of host-virus interactions in HeV infection, and potential use of compounds targeting nuclear transport, such as the FDA-approved agent ivermectin, as anti-HeV agents.

Peptides mediating DNA transport on microtubules and their impact on non-viral gene transfer efficiency

Synthetic vectors such as cationic polymers and cationic lipids remain attractive tools for non-viral gene transfer which is a complex process whose effectiveness relies on the ability to deliver a plasmid DNA (pDNA) into the nucleus of non-dividing cells. Once in the cytosol, the transport of pDNAs towards the nuclear envelope is strongly impaired by their very low cytosolic mobility due to their large size. To promote their movement towards the cell nucleus, few strategies have been implemented to exploit dynein, the microtubule’s (MT’s) motor protein, for propagation of cytosolic pDNA along the MTs towards the cell nucleus. In the first part of this review, an overview on MTs, dynein, dynein/virus interaction feature is presented followed by a summary of the results obtained by exploitation of LC8 and TCTEL1 dynein light chain association sequence (DLC-AS) for non-viral transfection. The second part dedicated to the adenoviral protein E3-14.7K, reports the transfection efficiency of polyplexes and lipoplexes containing the E3-14.7K-derived P79-98 peptide linked to pDNA. Here, several lines of evidence are given showing that dynein can be targeted to improve cytosolic pDNA mobility and accumulate pDNA near nuclear envelope in order to facilitate its transport through the nuclear pores. The linkage of various DLC-AS to pDNA carriers led to modest transfection improvements and their direct interaction with MTs was not demonstrated. In contrast, pDNA linked to the P79-98 peptide interacting with TCTEL1 via a cytosolic protein (fourteen seven K-interacting protein-1 (FIP-1)), interaction with MTs is evidenced in cellulo and transfection efficiency is improved.

C3G shows regulated nucleocytoplasmic exchange and represses histone modifications associated with euchromatin

C3G (RapGEF1), essential for mammalian embryonic development, shows dynamic nucleocytoplasmic exchange. Nuclear localization is regulated by NLSs, NES, and phosphorylation. C3G translocates to the nucleus in response to physiological stimuli and regulates chromatin modifications and gene expression.

C3G (RapGEF1) is a ubiquitously expressed guanine nucleotide exchange factor that functions in signaling pathways regulating cell proliferation, apoptosis, and actin reorganization. It is essential for differentiation and early embryonic development in mice. Overexpressed C3G shows predominant cytoplasmic localization, but endogenous C3G is a component of nuclear fractions in a variety of cell types. Coexpression of importin-α and inhibition of nuclear export by leptomycin B resulted in predominant nuclear localization of C3G. Functional NLSs, NES, and GSK3-β–dependent phosphorylation regulate its dynamic nuclear localization. C3G translocates to the nucleus in response to myogenic differentiation and sublethal dose of cisplatin. C3G is associated with chromatin and nuclear matrix fractions. Cells with C3G localized in the nucleus showed peripheralization of heterochromatin and reduced histone modifications associated with euchromatin. Short hairpin RNA–mediated depletion of C3G in epithelial cells resulted in reduced expression of CDK inhibitors and the histone demethylase KDM5A. Myoblast clones with CRISPR/Cas9-mediated knockout of C3G failed to show repression of histone marks and did not show up-regulation of myosin heavy chain and myotube formation when grown in differentiation medium. Our results document regulated nucleocytoplasmic exchange of C3G in response to physiological stimuli and provide insights into nuclear functions for C3G.

Dynein light chain DYNLL1 subunit facilitates porcine circovirus type 2 intracellular transports along microtubules

Microtubule (MT) and dynein motor proteins facilitate intracytoplasmic transport of cellular proteins. Various viruses utilize microtubules and dynein for their movement from the cell periphery to the nucleus. The aim of this study was to investigate the intracellular transport of porcine circovirus type 2 (PCV2) via 8 kDa dynein light chain (DYNLL1, LC8) subunit along the MTs. At 20 μM, vinblastine sulfate inhibited tubulin polymerization resulting in disorganized morphology. In PCV2-infected PK-15 cells, double immunofluorescent labeling showed that the viral particles appeared at the cell periphery and gradually moved to the microtubule organization center (MTOC) at 0-12 hour post inoculation (hpi) while at 20-24 hpi they accumulated in the nucleus. Co-localization between DYNLL1 and PCV2 particles was observed clearly at 8-12 hpi. At 20-24 hpi, most aggregated tubulin had a paracrystalline appearance at the MTOC around the nucleus in vinblastine-treated, PCV2-infected PK-15 cells. Between 12 and 24 hpi, PCV2 particles were still bound to DYNLL1 before they were translocated to the nucleus in both treatments, indicating that vinblastine sulfate had no effect on the protein-protein co-localization. The DYNLL1 binding motif, LRLQT, was found near the C-terminus of PCV2 capsid protein (Cap). Molecular docking analysis confirmed the specific interaction between these residues and the cargo binding site on DYNLL1. Our study clearly demonstrated that dynein, in particular DYNLL1, mediated PCV2 intracellular trafficking. The results could explain, at least in part, the viral transport mechanism by DYNLL1 via MT during PCV2 infection.

Nucleocytoplasmic trafficking of Nipah virus W protein involves multiple discrete interactions with the nuclear import and export machinery

Paramyxoviruses replicate in the cytoplasm with no obvious requirement to interact with the nucleus. Nevertheless, the W protein of the highly lethal bat-borne paramyxovirus Nipah virus (NiV) is known to undergo specific targeting to the nucleus, mediated by a single nuclear localisation signal (NLS) within the C-terminal domain. Here, we report for the first time that additional sites modulate nucleocytoplasmic localisation of W. We show that the N-terminal domain interacts with importin α1 and contributes to nuclear accumulation of W, indicative of a novel N-terminal NLS. We also find that W undergoes exportin-1 mediated nuclear export, dependent on a leucine at position 174. Together, these data enable significant revision of the generally accepted model of W trafficking, with implications for understanding of the mechanisms of NiV immune evasion.

Roles of nuclear trafficking in infection by cytoplasmic negative-strand RNA viruses: paramyxoviruses and beyond

Genome replication and virion production by most negative-sense RNA viruses (NSVs) occurs exclusively in the cytoplasm, but many NSV-expressed proteins undergo active nucleocytoplasmic trafficking via signals that exploit cellular nuclear transport pathways. Nuclear trafficking has been reported both for NSV accessory proteins (including isoforms of the rabies virus phosphoprotein, and V, W and C proteins of paramyxoviruses) and for structural proteins. Trafficking of the former is thought to enable accessory functions in viral modulation of antiviral responses including the type I IFN system, but the intranuclear roles of structural proteins such as nucleocapsid and matrix proteins, which have critical roles in extranuclear replication and viral assembly, are less clear. Nevertheless, nuclear trafficking of matrix protein has been reported to be critical for efficient production of Nipah virus and Respiratory syncytial virus, and nuclear localization of nucleocapsid protein of several morbilliviruses has been linked to mechanisms of immune evasion. Together, these data point to the nucleus as a significant host interface for viral proteins during infection by NSVs with otherwise cytoplasmic life cycles. Importantly, several lines of evidence now suggest that nuclear trafficking of these proteins may be critical to pathogenesis and thus could provide new targets for vaccine development and antiviral therapies.

The importance of immune evasion in the pathogenesis of rabies virus

Rabies is a zoonotic disease caused by the Lyssavirus rabies virus (RABV) that can infect most mammals, including humans, where it has a case-fatality rate of almost 100%. Although preventable by vaccination, rabies causes c. 59,000 human fatalities every year worldwide. Thus, there exists an urgent need to establish an effective therapy and/or improve dissemination of vaccines for humans and animals. These outcomes require greater understanding of the mechanisms of RABV pathogenesis to identify new molecular targets for the development of therapeutics and/or live vaccines with high levels of safety. Importantly, a number of studies in recent years have indicated that RABV specifically suppresses host immunity through diverse mechanisms and that this is a key process in pathogenicity. Here, we review current understanding of immune modulation by RABV, with an emphasis on its significance to pathogenicity and the potential exploitation of this knowledge to develop new vaccines and antivirals.

Nuclear Trafficking of the Rabies Virus Interferon Antagonist P-Protein Is Regulated by an Importin-Binding Nuclear Localization Sequence in the C-Terminal Domain

Rabies virus P-protein is expressed as five isoforms (P1-P5) which undergo nucleocytoplasmic trafficking important to roles in immune evasion. Although nuclear import of P3 is known to be mediated by an importin (IMP)-recognised nuclear localization sequence in the N-terminal region (N-NLS), the mechanisms underlying nuclear import of other P isoforms in which the N-NLS is inactive or has been deleted have remained unresolved. Based on the previous observation that mutation of basic residues K214/R260 of the P-protein C-terminal domain (P-CTD) can result in nuclear exclusion of P3, we used live cell imaging, protein interaction analysis and in vitro nuclear transport assays to examine in detail the nuclear trafficking properties of this domain. We find that the effect of mutation of K214/R260 on P3 is largely dependent on nuclear export, suggesting that nuclear exclusion of mutated P3 involves the P-CTD-localized nuclear export sequence (C-NES). However, assays using cells in which nuclear export is pharmacologically inhibited indicate that these mutations significantly inhibit P3 nuclear accumulation and, importantly, prevent nuclear accumulation of P1, suggestive of effects on NLS-mediated import activity in these isoforms. Consistent with this, molecular binding and transport assays indicate that the P-CTD mediates IMPα2/IMPβ1-dependent nuclear import by conferring direct binding to the IMPα2/IMPβ1 heterodimer, as well as to a truncated form of IMPα2 lacking the IMPβ-binding autoinhibitory domain (ΔIBB-IMPα2), and IMPβ1 alone. These properties are all dependent on K214 and R260. This provides the first evidence that P-CTD contains a genuine IMP-binding NLS, and establishes the mechanism by which P-protein isoforms other than P3 can be imported to the nucleus. These data underpin a refined model for P-protein trafficking that involves the concerted action of multiple NESs and IMP-binding NLSs, and highlight the intricate regulation of P-protein subcellular localization, consistent with important roles in infection.

The protein arginine methyltransferase PRMT6 inhibits HIV-1 Tat nucleolar retention

The human immunodeficiency virus (HIV)-1 transactivator protein Tat is known to play a key role in HIV infection, integrally related to its role in the host cell nucleus/nucleolus. Here we show for the first time that Tat localisation can be modulated by specific methylation, whereby overexpression of active but not catalytically inactive PRMT6 methyltransferase specifically leads to exclusion of Tat from the nucleolus. An R52/53A mutated Tat derivative does not show this redistribution, implying that R52/53, within Tat's nuclear/nucleolar localisation signal, are the targets of PRMT6 activity. Analysis using fluorescence recovery after photobleaching indicate that Tat nucleolar accumulation is largely through binding to nucleolar components, with methylation of Tat by PRMT6 preventing this. To our knowledge, this is the first report of specific protein methylation inhibiting nucleolar retention.

Fast track, dynein-dependent nuclear targeting of human immunodeficiency virus Vpr protein; impaired trafficking in a clinical isolate

Nuclear import of the accessory protein Vpr is central to infection by human immunodeficiency virus (HIV). We previously identified the Vpr F72L mutation in a HIV-infected, long-term non-progressor, showing that it resulted in reduced Vpr nuclear accumulation and altered cytoplasmic localisation. Here we demonstrate for the first time that the effects of nuclear accumulation of the F72L mutation are due to impairment of microtubule-dependent-enhancement of Vpr nuclear import. We use high resolution imaging approaches including fluorescence recovery after photobleaching and other approaches to document interaction between Vpr and the dynein light chain protein, DYNLT1, and impaired interaction of the F72L mutant with DYNLT1. The results implicate MTs/DYNLT1 as drivers of Vpr nuclear import and HIV infection, with important therapeutic implications.

Mechanisms Regulating Protein Localization

Cellular functions are dictated by protein content and activity. There are numerous strategies to regulate proteins varying from modulating gene expression to post-translational modifications. One commonly used mode of regulation in eukaryotes is targeted localization. By specifically redirecting the localization of a pool of existing protein, cells can achieve rapid changes in local protein function. Eukaryotic cells have evolved elegant targeting pathways to direct proteins to the appropriate cellular location or locations. Here, we provide a general overview of these localization pathways, with a focus on nuclear and mitochondrial transport, and present a survey of the evolutionarily conserved regulatory strategies identified thus far. We end with a description of several specific examples of proteins that exploit localization as an important mode of regulation.

High-Throughput Differentiation and Screening of a Library of Mutant Stem Cell Clones Defines New Host-Based Genes Involved in Rabies Virus Infection

We used a genomic library of mutant murine embryonic stem cells (ESCs) and report the methodology required to simultaneously culture, differentiate, and screen more than 3,200 heterozygous mutant clones to identify host-based genes involved in both sensitivity and resistance to rabies virus infection. Established neuronal differentiation protocols were miniaturized such that many clones could be handled simultaneously, and molecular markers were used to show that the resultant cultures were pan-neuronal. Next, we used a green fluorescent protein (GFP) labeled rabies virus to develop, validate, and implement one of the first host-based, high-content, high-throughput screens for rabies virus. Undifferentiated cell and neuron cultures were infected with GFP-rabies and live imaging was used to evaluate GFP intensity at time points corresponding to initial infection/uptake and early and late replication. Furthermore, supernatants were used to evaluate viral shedding potential. After repeated testing, 63 genes involved in either sensitivity or resistance to rabies infection were identified. To further explore hits, we used a completely independent system (siRNA) to show that reduction in target gene expression leads to the observed phenotype. We validated the immune modulatory gene Unc13d and the dynein adapter gene Bbs4 by treating wild-type ESCs and primary neurons with siRNA; treated cultures were resistant to rabies infection/replication. Overall, the potential of such in vitro functional genomics screens in stem cells adds additional value to other libraries of stem cells. This technique is applicable to any bacterial or virus interactome and any cell or tissue types that can be differentiated from ESCs.

Wongabel rhabdovirus accessory protein U3 targets the SWI/SNF chromatin remodeling complex

Wongabel virus (WONV) is an arthropod-borne rhabdovirus that infects birds. It is one of the growing array of rhabdoviruses with complex genomes that encode multiple accessory proteins of unknown function. In addition to the five canonical rhabdovirus structural protein genes (N, P, M, G, and L), the 13.2-kb negative-sense single-stranded RNA (ssRNA) WONV genome contains five uncharacterized accessory genes, one overlapping the N gene (Nx or U4), three located between the P and M genes (U1 to U3), and a fifth one overlapping the G gene (Gx or U5). Here we show that WONV U3 is expressed during infection in insect and mammalian cells and is required for efficient viral replication. A yeast two-hybrid screen against a mosquito cell cDNA library identified that WONV U3 interacts with the 83-amino-acid (aa) C-terminal domain of SNF5, a component of the SWI/SNF chromatin remodeling complex. The interaction was confirmed by affinity chromatography, and nuclear colocalization was established by confocal microscopy. Gene expression studies showed that SNF5 transcripts are upregulated during infection of mosquito cells with WONV, as well as West Nile virus (Flaviviridae) and bovine ephemeral fever virus (Rhabdoviridae), and that SNF5 knockdown results in increased WONV replication. WONV U3 also inhibits SNF5-regulated expression of the cytokine gene CSF1. The data suggest that WONV U3 targets the SWI/SNF complex to block the host response to infection.

IMPORTANCE

The rhabdoviruses comprise a large family of RNA viruses infecting plants, vertebrates, and invertebrates. In addition to the major structural proteins (N, P, M, G, and L), many rhabdoviruses encode a diverse array of accessory proteins of largely unknown function. Understanding the role of these proteins may reveal much about host-pathogen interactions in infected cells. Here we examine accessory protein U3 of Wongabel virus, an arthropod-borne rhabdovirus that infects birds. We show that U3 enters the nucleus and interacts with SNF5, a component of the chromatin remodeling complex that is upregulated in response to infection and restricts viral replication. We also show that U3 inhibits SNF5-regulated expression of the cytokine colony-stimulating factor 1 (CSF1), suggesting that it targets the chromatin remodeling complex to block the host response to infection. This study appears to provide the first evidence of a virus targeting SNF5 to inhibit host gene expression.

Overlapping binding sites for importin β1 and suppressor of fused (SuFu) on glioma-associated oncogene homologue 1 (Gli1) regulate its nuclear localization

The hedgehog signalling protein Gli1 has overlapping binding sites for the proteins importin β1 and SuFu at its N-terminus. These proteins compete to regulate the nuclear/cytoplasmic localization of Gli1, with importin β promoting nuclear import and SuFu preventing it.

Peptide vectors for gene delivery: from single peptides to multifunctional peptide nanocarriers

The therapeutic use of nucleic acids relies on the availability of sophisticated delivery systems for targeted and intracellular delivery of these molecules. Such a gene delivery should possess essential characteristics to overcome several extracellular and intracellular barriers. Peptides offer an attractive platform for nonviral gene delivery, as several functional peptide classes exist capable of overcoming these barriers. However, none of these functional peptide classes contain all the essential characteristics required to overcome all of the barriers associated with successful gene delivery. Combining functional peptides into multifunctional peptide vectors will be pivotal for improving peptide-based gene delivery systems. By using combinatorial strategies and high-throughput screening, the identification of multifunctional peptide vectors will accelerate the optimization of peptide-based gene delivery systems.

Nucleocytoplasmic shuttling of the Duchenne muscular dystrophy gene product dystrophin Dp71d is dependent on the importin α/β and CRM1 nuclear transporters and microtubule motor dynein

Even though the Duchenne muscular dystrophy (DMD) gene product Dystrophin Dp71d is involved in various key cellular processes through its role as a scaffold for structural and signalling proteins at the plasma membrane as well as the nuclear envelope, its subcellular trafficking is poorly understood. Here we map the nuclear import and export signals of Dp71d by truncation and point mutant analysis, showing for the first time that Dp71d shuttles between the nucleus and cytoplasm mediated by the conventional nuclear transporters, importin (IMP) α/β and the exportin CRM1. Binding was confirmed in cells using pull-downs, while in vitro binding assays showed direct, high affinity (apparent dissociation coefficient of c. 0.25nM) binding of Dp71d to IMPα/β. Interestingly, treatment of cells with the microtubule depolymerizing reagent nocodazole or the dynein inhibitor EHNA both decreased Dp71d nuclear localization, implying that Dp71d nuclear import may be facilitated by microtubules and the motor protein dynein. The role of Dp71d in the nucleus appears to relate in part to interaction with the nuclear envelope protein emerin, and maintenance of the integrity of the nuclear architecture. The clear implication is that Dp71d's previously unrecognised nuclear transport properties likely contribute to various, important physiological roles.

Rabies virus envelope glycoprotein targets lentiviral vectors to the axonal retrograde pathway in motor neurons

Rabies pseudotyped lentiviral vectors have great potential in gene therapy, not least because of their ability to transduce neurons following their distal axonal application. However, very little is known about the molecular processes that underlie their retrograde transport and cell transduction. Using multiple labeling techniques and confocal microscopy, we demonstrated that pseudotyping with rabies virus envelope glycoprotein (RV-G) enabled the axonal retrograde transport of two distinct subtypes of lentiviral vector in motor neuron cultures. Analysis of this process revealed that these vectors trafficked through Rab5-positive endosomes and accumulated within a non-acidic Rab7 compartment. RV-G pseudotyped vectors were co-transported with both the tetanus neurotoxin-binding fragment and the membrane proteins thought to mediate rabies virus endocytosis (neural cell adhesion molecule, nicotinic acetylcholine receptor, and p75 neurotrophin receptor), thus demonstrating that pseudotyping with RV-G targets lentiviral vectors for transport along the same pathway exploited by several toxins and viruses. Using motor neurons cultured in compartmentalized chambers, we demonstrated that axonal retrograde transport of these vectors was rapid and efficient; however, it was not able to transduce the targeted neurons efficiently, suggesting that impairment in processes occurring after arrival of the viral vector in the soma is responsible for the low transduction efficiency seen in vivo, which suggests a novel area for improvement of gene therapy vectors.

Cytoplasmic retention of a nucleocytoplasmic protein TBC1D3 by microtubule network is required for enhanced EGFR signaling

The hominoid oncogene TBC1D3 enhances epidermal growth factor receptor (EGFR) signaling and induces cell transformation. However, little is known regarding its spatio-temporal regulation and mechanism of tumorigenesis. In the current study, we identified the microtubule subunit β-tubulin as a potential interaction partner for TBC1D3 using affinity purification combined with mass spectrometry analysis. The interaction between TBC1D3 and β-tubulin was confirmed by co-immunoprecipitation. Using the same method, we also revealed that TBC1D3 co-precipitated with endogenous α-tubulin, another subunit of the microtubule. In agreement with these results, microtubule cosedimentation assays showed that TBC1D3 associated with the microtubule network. The β-tubulin-interacting site of TBC1D3 was mapped to amino acids 286∼353 near the C-terminus of the TBC domain. Deletion mutation within these amino acids was shown to abolish the interaction of TBC1D3 with β-tubulin. Interestingly, the deletion mutation caused a complete loss of TBC1D3 from the cytoplasmic filamentous and punctate structures, and TBC1D3 instead appeared in the nucleus. Consistent with this, wild-type TBC1D3 exhibited the same nucleocytoplasmic distribution in cells treated with the microtubule depolymerizing agent nocodazole, suggesting that the microtubule network associates with and retains TBC1D3 in the cytoplasm. We further found that deficiency in β-tubulin-interacting resulted in TBC1D3's inability to inhibit c-Cbl recruitment and EGFR ubiquitination, ultimately leading to dysregulation of EGFR degradation and signaling. Taken together, these studies indicate a novel model by which the microtubule network regulates EGFR stability and signaling through tubulin dimer/oligomer interaction with the nucleocytoplasmic protein TBC1D3.

An E3-14.7K peptide that promotes microtubules-mediated transport of plasmid DNA increases polyplexes transfection efficiency

Chemical vectors as cationic polymers and cationic lipids are promising alternatives to viral vectors for gene therapy. Beside endosome escape and nuclear import, plasmid DNA (pDNA) migration in the cytosol toward the nuclear envelope is also regarded as a limiting step for efficient DNA transfection with non-viral vectors. Here, the interaction between E3-14.7K and FIP-1 to favor migration of pDNA along microtubules is exploited. E3-14.7K is an early protein of human adenoviruses that interacts via FIP-1 (Fourteen.7K Interacting Protein 1) protein with the light-chain components of the human microtubule motor protein dynein (TCTEL1). This peptide is conjugated with pDNA and mediates interaction of pDNA in vitro with isolated microtubules as well as with microtubules in cellulo. Videomicroscopy and tracking treatment of images clearly demonstrate that P79-98/pDNA conjugate exhibits a linear transport with large amplitude along microtubules upon 2 h transfection with polyplexes whereas control pDNA conjugate exhibits small non-directional movements in the cytoplasm. Remarkably, P79-98/peGFP polyplexes enhance by a factor 2.5 (up to 76%) the number of transfected cells. The results demonstrate, for the first time, that the transfection efficiency of polyplexes can be drastically increased when the microtubules migration of pDNA is facilitated by a peptide allowing pDNA docking to TCTEL1. This is a real breakthrough in the non viral gene delivery field that opens hope to build artificial viruses.

Bovine ephemeral fever rhabdovirus α1 protein has viroporin-like properties and binds importin β1 and importin 7

Bovine ephemeral fever virus (BEFV) is an arthropod-borne rhabdovirus that is classified as the type species of the genus Ephemerovirus. In addition to the five canonical rhabdovirus structural proteins (N, P, M, G, and L), the large and complex BEFV genome contains several open reading frames (ORFs) between the G and L genes (α1, α2/α3, β, and γ) encoding proteins of unknown function. We show that the 10.5-kDa BEFV α1 protein is expressed in infected cells and, consistent with previous predictions based on its structure, has the properties of a viroporin. Expression of a BEFV α1-maltose binding protein (MBP) fusion protein in Escherichia coli was observed to inhibit cell growth and increase membrane permeability to hygromycin B. Increased membrane permeability was also observed in BEFV-infected mammalian cells (but not cells infected with an α1-deficient BEFV strain) and in cells expressing a BEFV α1-green fluorescent protein (GFP) fusion protein, which was shown by confocal microscopy to localize to the Golgi complex. Furthermore, the predicted C-terminal cytoplasmic domain of α1, which contains a strong nuclear localization signal (NLS), was translocated to the nucleus when expressed independently, and in an affinity chromatography assay employing a GFP trap, the full-length α1 was observed to interact specifically with importin β1 and importin 7 but not with importin α3. These data suggest that, in addition to its function as a viroporin, BEFV α1 may modulate components of nuclear trafficking pathways, but the specific role thereof remains unclear. Although rhabdovirus accessory genes occur commonly among arthropod-borne rhabdoviruses, little is known of their functions. Here, we demonstrate that the BEFV α1 ORF encodes a protein which has the structural and functional characteristics of a viroporin. We show that α1 localizes in the Golgi complex and increases cellular permeability. We also show that BEFV α1 binds importin β1 and importin 7, suggesting that it may have a yet unknown role in modulating nuclear trafficking. This is the first functional analysis of an ephemerovirus accessory protein and of a rhabdovirus viroporin.

Strategies on the nuclear-targeted delivery of genes

To improve the nuclear-targeted delivery of non-viral vectors, extensive effort has been carried out on the development of smart vectors which could overcome multiple barriers. The nuclear envelope presents a major barrier to transgene delivery. Viruses are capable of crossing the nuclear envelope to efficiently deliver their genome into the nucleus through the specialized protein components. However, non-viral vectors are preferred over viral ones because of the safety concerns associated with the latter. Non-viral delivery systems have been designed to include various types of components to enable nuclear translocation at the periphery of the nucleus. This review summarizes the progress of research regarding nuclear transport mechanisms. "Smart" non-viral vectors that have been modified by peptides and other small molecules are able to facilitate the nuclear translocation and enhance the efficacy of gene expression. The resulting technology may also enhance delivery of other macromolecules to the nucleus.

Viral interactions with microtubules: orchestrators of host cell biology?

Viral interaction with the microtubule (MT) cytoskeleton is critical to infection by many viruses. Most data regarding virus–MT interaction indicate key roles in the subcellular transport of virions/viral genomic material to sites of replication, assembly and egress. However, the MT cytoskeleton orchestrates diverse processes in addition to subcellular cargo transport, including regulation of signaling pathways, cell survival and mitosis, suggesting that viruses, expert manipulators of the host cell, may use the virus–MT interface to control multiple aspects of cell biology. Several lines of evidence support this idea, indicating that specific viral proteins can modify MT dynamics and/or structure and regulate processes such as apoptosis and innate immune signaling through MT-dependent mechanisms. Here, the authors review general aspects of virus–MT interactions, with emphasis on viral mechanisms that modify MT dynamics and functions to affect processes beyond virion transport. The emerging importance of discrete viral protein–MT interactions in pathogenic processes indicates that these interfaces may represent new targets for future therapeutics and vaccine development.

Particle tracking analysis for the intracellular trafficking of nanoparticles modified with African swine fever virus protein p54-derived peptide

Previous studies showed that the cytoplasmic transport of nanoparticles to the nucleus is driven by a vesicular sorting system. Artificial approaches for targeting a microtubule-associating motor complex is also a challenge. We describe herein the development of a liposomal nanoparticle, the surface of which is modified with stearylated octa-arginine (STR-R8), and a dynein light chain (LC8)-associated peptide derived from an African swine fever virus protein p54 (p54(149-161)) with polyethyleneglycol (PEG) as a spacer (p54(149-161)-PEG/R8-liposomal nanoparticles (LNPs)). The p54(149-161)-PEG/R8-LNPs preferentially gain access to the nucleus, resulting in a one- to two-order of magnitude higher transfection activity in comparison with p54(149-161)-free nanoparticles (PEG/R8-LNPs). Further studies of particle tracking in HeLa cells stably expressing green fluorescent protein (GFP)-tagged tubulin (GFP/Tub-HeLa) indicate that p54(149-161) stimulated the transport of nanoparticles along fibrous tubulin structures. Moreover, a part of the p54(149-161)-PEG/R8-LNPs appeared to undergo quasi-straight transport without sharing the tracks corresponding to PKH67, the plasma membrane of which had been prestained with a marker just before transfection, while corresponding movement was never observed in the case of PEG/R8-LNPs. These findings suggest that a portion of the p54(149-161)-modified nanoparticles can use microtubule-dependent transport without the need for an assist by a vesicular sorting system.

A novel nuclear trafficking module regulates the nucleocytoplasmic localization of the rabies virus interferon antagonist, P protein

Regulated nucleocytoplasmic transport of proteins is central to cellular function and dysfunction during processes such as viral infection. Active protein trafficking into and out of the nucleus is dependent on the presence within cargo proteins of intrinsic specific modular signals for nuclear import (nuclear localization signals, NLSs) and export (nuclear export signals, NESs). Rabies virus (RabV) phospho (P) protein, which is largely responsible for antagonising the host anti-viral response, is expressed as five isoforms (P1-P5). The subcellular trafficking of these isoforms is thought to depend on a balance between the activities of a dominant N-terminal NES (N-NES) and a distinct C-terminal NLS (C-NLS). Specifically, the N-NES-containing isoforms P1 and P2 are cytoplasmic, whereas the shorter P3-P5 isoforms, which lack the N-NES, are believed to be nuclear through the activity of the C-NLS. Here, we show for the first time that RabV P contains an additional strong NLS in the N-terminal region (N-NLS), which, intriguingly, overlaps with the N-NES. This arrangement represents a novel nuclear trafficking module where the N-NLS is inactive in P1 but becomes activated in P3, concomitant with truncation of the N-NES, to become the principal targeting signal conferring nuclear accumulation. Understanding this unique switch arrangement of overlapping, co-regulated NES/NLS sequences is vital to delineating the critical role of RabV P protein in viral infection.

Challenges in neuroprotective nanomedicine development: progress towards noninvasive gene therapy of glaucoma

Over the past decade the application of gene therapy of retinal diseases such as glaucoma has produced promising results. However, optic nerve regeneration and restoration of vision in patients with glaucoma is still far from reality. Neuroprotective approaches in the form of gene therapy may provide significant advantages, but are still limited by many factors both at the organ and cellular levels. In general, gene delivery systems for eye diseases range from simple eye drops and ointments to more advanced bio- and nanotechnology-based systems such as muco-adhesive systems, polymers, liposomes and ocular inserts. Most of these technologies were developed for front-of-the-eye ophthalmic therapies and are not applicable as back-of-the-eye delivery systems. Currently, only the invasive intravitreal injections are capable of successfully delivering genes to the retina. Here we review the challenges and possible strategies for the noninvasive gene therapy of glaucoma including the barriers in the eye and in neural cells, and present a cross-sectional view of gene delivery as it pertains to the prevention and treatment of glaucoma.

Conservation of a unique mechanism of immune evasion across the Lyssavirus genus

The evasion of host innate immunity by Rabies virus, the prototype of the genus Lyssavirus, depends on a unique mechanism of selective targeting of interferon-activated STAT proteins by the viral phosphoprotein (P-protein). However, the immune evasion strategies of other lyssaviruses, including several lethal human pathogens, are unresolved. Here, we show that this mechanism is conserved between the most distantly related members of the genus, providing important insights into the pathogenesis and potential therapeutic targeting of lyssaviruses.

Global enhancement of nuclear localization-dependent nuclear transport in transformed cells

Fundamental to eukaryotic cell function, nucleocytoplasmic transport can be regulated at many levels, including through modulation of the importin/exportin (Imp/Exp) nuclear transport machinery itself. Although Imps/Exps are overexpressed in a number of transformed cell lines and patient tumor tissues, the efficiency of nucleocytoplasmic transport in transformed cell types compared with nontransformed cells has not been investigated. Here we use quantitative live cell imaging of 3 isogenic nontransformed/transformed cell pairs to show that nuclear accumulation of nuclear localization signal (NLS)-containing proteins, but not their NLS-mutated derivatives, is increased up to 7-fold in MCF10CA1h human epithelial breast carcinoma cells and in simian virus 40 (SV40)-transformed fibroblasts of human and monkey origin, compared with their nontransformed counterparts. The basis for this appears to be a significantly faster rate of nuclear import in transformed cell types, as revealed by analysis using fluorescence recovery after photobleaching for the human MCF10A/MCF10CA1h cell pair. Nuclear accumulation of NLS/nuclear export signal-containing (shuttling) proteins was also enhanced in transformed cell types, experiments using the nuclear export inhibitor leptomycin B demonstrating that efficient Exp-1-mediated nuclear export was not impaired in transformed compared with nontransformed cells. Enhanced nuclear import and export efficiencies were found to correlate with 2- to 4-fold higher expression of specific Imps/Exps in transformed cells, as indicated by quantitative Western blot analysis, with ectopic expression of Imps able to enhance NLS nuclear accumulation levels up to 5-fold in nontransformed MCF10A cells. The findings indicate that transformed cells possess altered nuclear transport properties, most likely due to the overexpression of Imps/Exps. The findings have important implications for the development of tumor-specific drug nanocarriers in anticancer therapy.

Coupling viruses to dynein and kinesin-1

It is now clear that transport on microtubules by dynein and kinesin family motors has an important if not critical role in the replication and spread of many different viruses. Understanding how viruses hijack dynein and kinesin motors using a limited repertoire of proteins offers a great opportunity to determine the molecular basis of motor recruitment. In this review, we discuss the interactions of dynein and kinesin-1 with adenovirus, the α herpes viruses: herpes simplex virus (HSV1) and pseudorabies virus (PrV), human immunodeficiency virus type 1 (HIV-1) and vaccinia virus. We highlight where the molecular links to these opposite polarity motors have been defined and discuss the difficulties associated with identifying viral binding partners where the basis of motor recruitment remains to be established. Ultimately, studying microtubule-based motility of viruses promises to answer fundamental questions as to how the activity and recruitment of the dynein and kinesin-1 motors are coordinated and regulated during bi-directional transport.

Interferon in rabies virus infection

Rabies is among the longest known and most dangerous and feared infectious diseases for humans and animals and still is responsible for tenth of thousands of human deaths per year. The rabies virus (RABV) is a rather atypical member of the Rhabdoviridae family as it has completely adapted during evolution to warm-blooded hosts and is directly transmitted between them, whereas most other rhabdoviruses are transmitted by insect vectors. The virus is also unique with respect to its extremely broad host species range and a very narrow host organ range, namely its strict neurotropism. It is becoming increasingly clear that the host innate immune system, particularly the type I interferon system, and the viral counteractions profoundly shape this virus-host relationship. In the past few years, exciting new insight was obtained on how viruses are sensed by innate immune receptors, how the downstream signaling networks for activation of interferon are working, and how viruses can interfere with the system. While RABV 5'-triphosphate RNAs were identified as the major pathogen-associated molecular pattern sensed by cytoplasmic RIG-I-like receptors (RLR), the RABV phosphoprotein (P) has emerged as a potent multifunctional antagonist able to counteract the signaling cascades leading to transcriptional activation of interferon genes as well as interferon signaling pathways, thereby limiting expression of antiviral and immune-stimulatory genes.

The association of viral proteins with host cell dynein components during virus infection

After fusion with the cellular plasma membrane or endosomal membranes, viral particles are generally too large to diffuse freely within the crowded cytoplasm environment. Thus, they will never reach the cell nucleus or the perinuclear areas where replication or reverse transcription usually takes place. It has been proposed that many unrelated viruses are transported along microtubules in a retrograde manner using the cellular dynein machinery or, at least, some dynein components. A putative employment of the dynein motor in a dynein‐mediated transport has been suggested from experiments in which viral capsid proteins were used as bait in yeast two‐hybrid screens using libraries composed of cellular proteins and dynein‐associated chains were retrieved as virus‐interacting proteins. In most cases DYNLL1, DYNLT1 or DYNLRB1 were identified as the dynein chains that interact with viral proteins. The importance of these dynein–virus interactions has been supported, in principle, by the observation that in some cases the dynein‐interacting motifs of viral proteins altered by site‐directed mutagenesis result in non‐infective virions. Furthermore, overexpression of p50 dynamitin, which blocks the dynein–dynactin interaction, or incubation of infected cells with peptides that compete with viral polypeptides for dynein binding have been shown to alter the viral retrograde transport. Still, it remains to be proved that dynein light chains can bind simultaneously to incoming virions and to the dynein motor for retrograde transport to take place. In this review, we will analyse the association of viral proteins with dynein polypeptides and its implications for viral infection.

Resonance Assignments and Secondary Structure Analysis of Dynein Light Chain 8 by Magic Angle Spinning NMR Spectroscopy

Dynein light chain LC8 is the smallest subunit of the dynein motor complex and has been shown to play important roles in both dynein dependent and dynein independent physiological functions via its interaction with a number of its binding partners. It has also been linked to pathogenesis including roles in viral infections and tumorigenesis. Structural information for LC8-target proteins is critical to understanding the underlying function of LC8 in these complexes. However, some LC8-target interactions are not amenable for structural characterization by conventional structural biology techniques due to their large size, low solubility and crystallization difficulties. Here, we report magic angle spinning (MAS) NMR studies of the homodimeric apo-LC8 protein as a first effort in addressing more complex, multi-partner LC8-based protein assemblies. We have established site-specific backbone and side chain resonance assignments for the majority of the residues of LC8, and show TALOS+ predicted torsion angles ϕ and ψ in close agreement with most residues in the published LC8 crystal structure. Data obtained through these studies will provide the first step toward using MAS NMR to examine the LC8 structure, which will eventually be used to investigate protein-protein interactions in larger systems, which cannot be determined by conventional structural studies.

An AlphaScreen®-based assay for high-throughput screening for specific inhibitors of nuclear import

Specific viral proteins enter the nucleus of infected cells to perform essential functions, as part of the viral life cycle. The integrase (IN) molecule of human immunodeficiency virus (HIV)-1 is of particular interest in this context due to its integral role in integrating the HIV genome into that of the infected host cell. Most IN-based antiviral compounds target the IN/DNA interaction, but since IN must first enter the nucleus before it can perform these critical functions, nuclear transport of IN is also an attractive target for therapeutic intervention. Here the authors describe a novel high-throughput screening assay for identifying inhibitors of nuclear import, particularly IN, based on amplified luminescent proximity homogeneous assay (AlphaScreen(®)) technology, which is high throughput, requires low amounts of material, and is efficient and cost-effective. The authors use the assay to screen for specific inhibitors of the interaction between IN and its nuclear transport receptor importin α/β, successfully identifying several inhibitors of the IN/importin α/β interaction. Importantly, they demonstrate that one of the identified compounds, mifepristone, is effective in preventing active nuclear transport of IN in transfected cells and hence may represent a useful anti-HIV therapeutic. The screen also identified broad-spectrum importin α/β inhibitors such as ivermectin, which may represent useful tools for nuclear transport research in the future. The authors validate the activity and specificity of mifepristone and ivermectin in inhibiting nuclear protein import in living cells, underlining the utility of the screening approach.

Regulation of nucleocytoplasmic trafficking of viral proteins: an integral role in pathogenesis?

Signal-dependent targeting of proteins into and out of the nucleus is mediated by members of the importin (IMP) family of transport receptors, which recognise targeting signals within a cargo protein and mediate passage through the nuclear envelope-embedded nuclear pore complexes. Regulation of this process is paramount to processes such as cell division and differentiation, but is also critically important for viral replication and pathogenesis; phosphorylation appears to play a major role in regulating viral protein nucleocytoplasmic trafficking, along with other posttranslational modifications. This review focuses on viral proteins that utilise the host cell IMP machinery in order to traffic into/out of the nucleus, and in particular those where trafficking is critical to viral replication and/or pathogenesis, such as simian virus SV40 large tumour antigen (T-ag), human papilloma virus E1 protein, human cytomegalovirus processivity factor ppUL44, and various gene products from RNA viruses such as Rabies. Understanding of the mechanisms regulating viral protein nucleocytoplasmic trafficking is paramount to the future development of urgently needed specific and effective anti-viral therapeutics. This article was originally intended for the special issue "Regulation of Signaling and Cellular Fate through Modulation of Nuclear Protein Import". The Publisher apologizes for any inconvenience caused.

Structure, interactions with host cell and functions of rhabdovirus phosphoprotein

Rabies is an incurable albeit preventable disease that remains an important human health issue, with the number of deaths exceeding 50,000 people each year. Its causative agent, the rabies virus, is a negative-sense RNA virus, the genome of which encodes five proteins. Three of these proteins, the nucleoprotein, the phosphoprotein (P) and the large protein, are required to synthesize viral RNA in an efficient and regulated manner. P plays multiple roles during the transcription and replication of the RNA genome. It acts as a noncatalytic cofactor of the large protein polymerase and it chaperones nucleoprotein. Recent structural characterizations of rabies virus P revealed that P forms elongated and flexible dimers and uncovered the structural basis of its modular organization, revealing the existence of two independent structured domains and two long intrinsically disordered regions. In addition, recent studies also revealed that P interacts with nucleocytoplasmic trafficking carriers and with the host cell cytoskeleton, probably allowing viral components to be transported within the host cell and blocking the innate immune response by inhibiting different steps of the interferon pathway. With multiple binding sites for different viral and cellular partners located in either its structured or disordered regions, P appears to be a flexible ‘hub’ protein that connects viral or cellular proteins and allows their assembly into multimolecular complexes. These new findings shed light on the mechanism of replication of the virus and on the intimate interactions between the virus and its host cell, and will also help to identify new targets for the development of antiviral treatments.

The importance of being short: the role of rabies virus phosphoprotein isoforms assessed by differential IRES translation initiation

The rabies virus (RV) phosphoprotein P is a multifunctional protein involved in viral RNA synthesis and in counteracting host innate immune responses. We have previously shown that RV P gene expression levels can be regulated by using picornavirus internal ribosome entry site (IRES) elements. Here we exploited a particular feature of the foot-and-mouth disease virus (FMDV) IRES, namely, preferential initiation at a downstream initiation codon, to address the role of N-terminally truncated RV phosphoproteins usually generated in RV-infected cells through ribosomal leaky scanning. Recombinant RVs in which P synthesis was directed by the poliovirus or FMDV IRES produced full-length P (P1) or a truncated form (P2), as the dominant product, respectively. While the P2 overexpressing virus showed attenuated growth in interferon-incompetent cells, it was superior to the P1 overexpressing virus in preventing expression of host interferon-stimulated genes. This indicates that in RV infected cells the availability of the truncated P2 protein is critical for viral resistance to interferon.

Mechanism of microtubule-facilitated "fast track" nuclear import

Although the microtubule (MT) cytoskeleton has been shown to facilitate nuclear import of specific cancer-regulatory proteins including p53, retinoblastoma protein, and parathyroid hormone-related protein (PTHrP), the MT association sequences (MTASs) responsible and the nature of the interplay between MT-dependent and conventional importin (IMP)-dependent nuclear translocation are unknown. Here we used site-directed mutagenesis, live cell imaging, and direct IMP and MT binding assays to map the MTAS of PTHrP for the first time, finding that it is within a short modular region (residues 82-108) that overlaps with the IMPβ1-recognized nuclear localization signal (residues 66-108) of PTHrP. Importantly, fluorescence recovery after photobleaching experiments indicated that disruption of the MT network or mutation of the MTAS of PTHrP decreases the rate of nuclear import by 2-fold. Moreover, MTAS functions depend on mutual exclusivity of binding of PTHrP to MTs and IMPβ1 such that, following MT-dependent trafficking toward the nucleus, perinuclear PTHrP can be displaced from MTs by IMPβ1 prior to import into the nucleus. This is the first molecular definition of an MTAS that facilitates protein nuclear import as well as the first delineation of the mechanism whereby cargo is transferred directly from the cytoskeleton to the cellular nuclear import apparatus. The results have broad significance with respect to fundamental processes regulating cell physiology/transformation.

Characterization of a panel of anti-phosphoprotein monoclonal antibodies generated against the raccoon strain of rabies virus

The generation of a new panel of 32 monoclonal antibodies (MAbs) reactive with the P protein of the raccoon strain of rabies virus is described. Through a series of analyses employing competitive ELISA and immunoblotting, these MAbs were classified into eight groups, each defining an antigenic site, thereby increasing the number of sites now recognized along the length of the P protein. Studies on MAb reactivity with a collection of diverse lyssaviruses identified sites that were highly conserved, moderately conserved and highly variable. Several groups of MAbs were highly specific for the raccoon rabies virus (RRV) strain and may be useful for inclusion into panels used for antigenic typing of rabies viruses. The utility of these MAbs to detect truncated versions of the P product may facilitate more fundamental studies on the function of this rabies virus protein.

Role of interferon antagonist activity of rabies virus phosphoprotein in viral pathogenicity

The fixed rabies virus (RV) strain Nishigahara kills adult mice after intracerebral inoculation, whereas the chicken embryo fibroblast cell-adapted strain Ni-CE causes nonlethal infection in adult mice. We previously reported that the chimeric CE(NiP) strain, which has the phosphoprotein (P protein) gene from the Nishigahara strain in the genetic background of the Ni-CE strain, causes lethal infection in adult mice, indicating that the P gene is responsible for the different pathogenicities of the Nishigahara and Ni-CE strains. Previous studies demonstrated that RV P protein binds to the interferon (IFN)-activated transcription factor STAT1 and blocks IFN signaling by preventing its translocation to the nucleus. In this study, we examine the molecular mechanism by which RV P protein determines viral pathogenicity by comparing the IFN antagonist activities of the Nishigahara and Ni-CE P proteins. The results, obtained from both RV-infected cells and cells transfected to express P protein only, show that Ni-CE P protein is significantly impaired for its capacity to block IFN-activated STAT1 nuclear translocation and, consequently, inhibits IFN signaling less efficiently than Nishigahara P protein. Further, it was demonstrated that a defect in the nuclear export of Ni-CE P protein correlates with a defect in its ability to cause the mislocalization of STAT1. These data provide the first evidence that the capacity of the RV P protein to inhibit STAT1 nuclear translocation and IFN signaling correlates with the viral pathogenicity.

Development and application of in vivo molecular traps reveals that dynein light chain occupancy differentially affects dynein-mediated processes

The ability to rapidly and specifically regulate protein activity combined with in vivo functional assays and/or imaging can provide unique insight into underlying molecular processes. Here we describe the application of chemically induced dimerization of FKBP to create nearly instantaneous high-affinity bivalent ligands capable of sequestering cellular targets from their endogenous partners. We demonstrate the specificity and efficacy of these inducible, dimeric "traps" for the dynein light chains LC8 (Dynll1) and TcTex1 (Dynlt1). Both light chains can simultaneously bind at adjacent sites of dynein intermediate chain at the base of the dynein motor complex, yet their specific function with respect to the dynein motor or other interacting proteins has been difficult to dissect. Using these traps in cultured mammalian cells, we observed that induction of dimerization of either the LC8 or TcTex1 trap rapidly disrupted early endosomal and lysosomal organization. Dimerization of either trap also disrupted Golgi organization, but at a substantially slower rate. Using either trap, the time course for disruption of each organelle was similar, suggesting a common regulatory mechanism. However, despite the essential role of dynein in cell division, neither trap had a discernable effect on mitotic progression. Taken together, these studies suggest that LC occupancy of the dynein motor complex directly affects some, but not all, dynein-mediated processes. Although the described traps offer a method for rapid inhibition of dynein function, the design principle can be extended to other molecular complexes for in vivo studies.