Adenovirus core protein pVII is translocated into the nucleus by multiple import receptor pathways

Mechanistic Insights Into an Ancient Adenovirus Precursor Protein VII Show Multiple Nuclear Import Receptor Pathways

Adenoviral pVII proteins are multifunctional, highly basic, histone-like proteins that can bind to and transport the viral genome into the host cell nucleus. Despite the identification of several nuclear localization signals (NLSs) in the pVII protein of human adenovirus (HAdV)2, the mechanistic details of nuclear transport are largely unknown. Here we provide a full characterization of the nuclear import of precursor (Pre-) pVII protein from an ancient siadenovirus, frog siadenovirus 1 (FrAdV1), using a combination of structural, functional, and biochemical approaches. Two strong NLSs (termed NLSa and NLSd) interact with importin (IMP)β1 and IMPα, respectively, and are the main drivers of nuclear import. A weaker NLS (termed NLSb) also contributes, together with an additional signal (NLSc) which we found to be important for nucleolar targeting and intranuclear binding. Expression of wild-type and NLS defective derivatives Pre-pVII in the presence of selective inhibitors of different nuclear import pathways revealed that, unlike its human counterpart, FrAdV1 Pre-pVII nuclear import is dependent on IMPα/β1 and IMPβ1, but not on transportin-1 (IMPβ2). Clearly, AdVs evolved to maximize the nuclear import pathways for the pVII proteins, whose subcellular localization is the result of a complex process. Therefore, our results pave the way for an evolutionary comparison of the interaction of different AdVs with the host cell nuclear transport machinery.

Role of Protein VII in the Production of Infectious Bovine Adenovirus-3 Virion

Bovine adenovirus (BAdV)-3 genome encodes a 26 kDa core protein designated as protein VII, which localizes to the nucleus/nucleolus. The requirement of a protein VII-complementing cell line for the replication of VII-deleted BAdV-3 suggests that protein VII is required for the production of infectious progeny virions. An analysis of the BAV.VIId+ virus (only phenotypically positive for protein VII) detected no noticeable differences in the expression and incorporation of viral proteins in the virions. Moreover, protein VII does not appear to be essential for the formation of mature BAV.VIId+. However, protein VII appeared to be required for the efficient assembly of mature BAV.VIId- virions. An analysis of the BAV.VIId- virus (genotypically and phenotypically negative for protein VII) in non-complementing cells detected the inefficient release of virions from endosomes, which affected the expression of viral proteins or DNA replication. Moreover, the absence of protein VII altered the proteolytic cleavage of protein VI of BAV.VIId-. Our results suggest that BAdV-3 protein VII appears to be required for efficient production of mature virions. Moreover, the absence of protein VII produces non-infectious BAdV-3 by altering the release of BAdV-3 from endosomes/vesicles.

Regions of Bovine Adenovirus-3 Protein VII Involved in Interactions with Viral and Cellular Proteins

The L 1 region of bovine adenovirus (BAdV)-3 encodes a multifunctional protein named protein VII. Anti-protein VII sera detected a protein of 26 kDa in transfected or BAdV-3-infected cells, which localizes to nucleus and nucleolus of infected/transfected cells. Analysis of mutant protein VII identified four redundant overlapping nuclear/nucleolar localization signals as deletion of all four potential nuclear/nucleolar localization signals localizes protein VII predominantly to the cytoplasm. The nuclear import of protein VII appears to use importin α (α-1), importin-β (β-1) and transportin-3 nuclear transport receptors. In addition, different nuclear transport receptors also require part of protein VII outside nuclear localization sequences for efficient interaction. Proteomic analysis of protein complexes purified from recombinant BAdV-3 expressing protein VII containing Strep Tag II identified potential viral and cellular proteins interacting with protein VII. Here, we confirm that protein VII interacts with IVa2 and protein VIII in BAdV-3-infected cells. Moreover, amino acids 91-101 and 126-137, parts of non-conserved region of protein VII, are required for interaction with IVa2 and protein VIII, respectively.

An optimised protocol for the expression and purification of adenovirus core protein VII

Adenovirus protein VII (pVII) is a highly basic core protein, bearing resemblance to mammalian histones. Despite its diverse functions, a comprehensive understanding of its structural intricacies and the mechanisms underlying its functions remain elusive, primarily due to the complexity of producing a good amount of soluble pVII. This study aimed to optimise the expression and purification of recombinant pVII from four different adenoviruses with a simple vector construct. This study successfully determined the optimal conditions for efficiently purifying pVII across four adenovirus species, revealing the differential preference for bacterial expression systems. The One Shot BL21 Star (DE3) proved favourable over Rosetta 2 (DE3) pLysS with consistent levels of expression between IPTG-induced and auto-induction. We demonstrated that combining chemical and mechanical cell lysis is possible and highly effective. Other noteworthy benefits were observed in using RNase during sample processing. The addition of RNase has significantly improved the quality and quantity of the purified protein as confirmed by chromatographic and western blot analyses. These findings established a solid groundwork for pVII purification methodologies and carry the significant potential to assist in unveiling the core structure of pVII, its arrangement within the core, DNA condensation intricacies, and potential pathways for nuclear transport.

Structural and functional characterization of siadenovirus core protein VII nuclear localization demonstrates the existence of multiple nuclear transport pathways

Adenovirus protein VII (pVII) plays a crucial role in the nuclear localization of genomic DNA following viral infection and contains nuclear localization signal (NLS) sequences for the importin (IMP)-mediated nuclear import pathway. However, functional analysis of pVII in adenoviruses to date has failed to fully determine the underlying mechanisms responsible for nuclear import of pVII. Therefore, in the present study, we extended our analysis by examining the nuclear trafficking of adenovirus pVII from a non-human species, psittacine siadenovirus F (PsSiAdV). We identified a putative classical (c)NLS at pVII residues 120-128 (120PGGFKRRRL128). Fluorescence polarization and electrophoretic mobility shift assays demonstrated direct, high-affinity interaction with both IMPα2 and IMPα3 but not IMPβ. Structural analysis of the pVII-NLS/IMPα2 complex confirmed a classical interaction, with the major binding site of IMPα occupied by K124 of pVII-NLS. Quantitative confocal laser scanning microscopy showed that PsSiAdV pVII-NLS can confer IMPα/β-dependent nuclear localization to GFP. PsSiAdV pVII also localized in the nucleus when expressed in the absence of other viral proteins. Importantly, in contrast to what has been reported for HAdV pVII, PsSiAdV pVII does not localize to the nucleolus. In addition, our study demonstrated that inhibition of the IMPα/β nuclear import pathway did not prevent PsSiAdV pVII nuclear targeting, indicating the existence of alternative pathways for nuclear localization, similar to what has been previously shown for human adenovirus pVII. Further examination of other potential NLS signals, characterization of alternative nuclear import pathways, and investigation of pVII nuclear targeting across different adenovirus species is recommended to fully elucidate the role of varying nuclear import pathways in the nuclear localization of pVII.

Adenovirus entry: Stability, uncoating, and nuclear import

Adenoviruses (AdVs) are widespread in vertebrates. They infect the respiratory and gastrointestinal tracts, the eyes, heart, liver, and kidney, and are lethal to immunosuppressed people. Mastadenoviruses infecting mammals comprise several hundred different types, and many specifically infect humans. Human adenoviruses are the most widely used vectors in clinical applications, including cancer treatment and COVID-19 vaccination. AdV vectors are physically and genetically stable and generally safe in humans. The particles have an icosahedral coat and a nucleoprotein core with a DNA genome. We describe the concept of AdV cell entry and highlight recent advances in cytoplasmic transport, uncoating, and nuclear import of the viral DNA. We highlight a recently discovered "linchpin" function of the virion protein V ensuring cytoplasmic particle stability, which is relaxed at the nuclear pore complex by cues from the E3 ubiquitin ligase Mind bomb 1 (MIB1) and the proteasome triggering disruption. Capsid disruption by kinesin motor proteins and microtubules exposes the linchpin and renders protein V a target for MIB1 ubiquitination, which dissociates V from viral DNA and enhances DNA nuclear import. These advances uncover mechanisms controlling capsid stability and premature uncoating and provide insight into nuclear transport of nucleic acids.

Adeno-associated virus (AAV) cell entry: structural insights

Adeno-associated virus (AAV) is the leading vector in emerging treatments of inherited diseases. Higher transduction efficiencies and cellular specificity are required for broader clinical application, motivating investigations of virus-host molecular interactions during cell entry. High-throughput methods are identifying host proteins more comprehensively, with subsequent molecular studies revealing unanticipated complexity and serotype specificity. Cryogenic electron microscopy (cryo-EM) provides a path towards structural details of these sometimes heterogeneous virus-host complexes, and is poised to illuminate more fully the steps in entry. Here presented, is progress in understanding the distinct steps of glycan attachment, and receptor-mediated entry/trafficking. Comparison with structures of antibody complexes provides new insights on immune neutralization with implications for the design of improved gene therapy vectors.

CRM1 Promotes Capsid Disassembly and Nuclear Envelope Translocation of Adenovirus Independently of Its Export Function

After receptor-mediated endocytosis and endosomal escape, adenoviral capsids can travel via microtubule organizing centers to the nuclear envelope. Upon capsid disassembly, viral genome import into nuclei of interphase cells then occurs through nuclear pore complexes, involving the nucleoporins Nup214 and Nup358. Import also requires the activity of the classic nuclear export receptor CRM1, as it is blocked by the selective inhibitor leptomycin B. We have now used artificially enucleated as well as mitotic cells to analyze the role of an intact nucleus in different steps of the viral life cycle. In enucleated U2OS cells, viral capsids traveled to the microtubule organizing center, whereas their removal from this complex was blocked, suggesting that this step required nuclear factors. In mitotic cells, on the other hand, CRM1 promoted capsid disassembly and genome release, suggesting a role of this protein that does not require intact nuclear envelopes or nuclear pore complexes and is distinct from its function as a nuclear export receptor. Similar to enucleation, inhibition of CRM1 by leptomycin B also leads to an arrest of adenoviral capsids at the microtubule organizing center. In a small-scale screen using leptomycin B-resistant versions of CRM1, we identified a mutant, CRM1 W142A P143A, that is compromised with respect to adenoviral capsid disassembly in both interphase and mitotic cells. Strikingly, this mutant is capable of exporting cargo proteins out of the nucleus of living cells or digitonin-permeabilized cells, pointing to a role of the mutated region that is not directly linked to nuclear export. IMPORTANCE A role of nucleoporins and of soluble transport factors in adenoviral genome import into the nucleus of infected cells in interphase has previously been established. The nuclear export receptor CRM1 promotes genome import, but its precise function is not known. Using enucleated and mitotic cells, we showed that CRM1 does not simply function by exporting a crucial factor out of the nucleus that would then trigger capsid disassembly and genome import. Instead, CRM1 has an export-independent role, a notion that is also supported by a mutant, CRM1 W142A P143A, which is export competent but deficient in viral capsid disassembly, in both interphase and mitotic cells.

A viral ubiquitination switch attenuates innate immunity and triggers nuclear import of virion DNA and infection

Antiviral defense and virus exclusion from the cell nucleus restrict foreign nucleic acid influx and infection. How the genomes of DNA viruses evade cytosolic pattern recognition and cross the nuclear envelope is incompletely understood. Here, we show that the virion protein V of adenovirus functions as a linchpin between the genome and the capsid, thereby securing particle integrity. Absence of protein V destabilizes cytoplasmic particles and promotes premature genome release, raising cytokine levels through the DNA sensor cGAS. Non-ubiquitinable V yields stable virions, genome misdelivery to the cytoplasm, and increased cytokine levels. In contrast, normal protein V is ubiquitinated at the nuclear pore complex, dissociates from the virion depending on the E3 ubiquitin ligase Mib1 and the proteasome, and allows genome delivery into the nucleus for infection. Our data uncover previously unknown cellular and viral mechanisms of viral DNA nuclear import in pathogenesis, vaccination, gene therapy, and synthetic biology.

Adenovirus Core Proteins: Structure and Function

Adenoviruses have served as a model for investigating viral-cell interactions and discovering different cellular processes, such as RNA splicing and DNA replication. In addition, the development and evaluation of adenoviruses as the viral vectors for vaccination and gene therapy has led to detailed investigations about adenovirus biology, including the structure and function of the adenovirus encoded proteins. While the determination of the structure and function of the viral capsid proteins in adenovirus biology has been the subject of numerous reports, the last few years have seen increased interest in elucidating the structure and function of the adenovirus core proteins. Here, we provide a review of research about the structure and function of the adenovirus core proteins in adenovirus biology.

Emerging antiviral therapeutics for human adenovirus infection: Recent developments and novel strategies

Human adenoviruses (HAdV) are ubiquitous human pathogens that cause a significant burden of respiratory, ocular, and gastrointestinal illnesses. Although HAdV infections are generally self-limiting, pediatric and immunocompromised individuals are at particular risk for developing severe disease. Currently, no approved antiviral therapies specific to HAdV exist. Recent outbreaks underscore the need for effective antiviral agents to treat life-threatening infections. In this review we will focus on recent developments in search of potential therapeutic agents for controlling HAdV infections, with a focus on those targeting post-entry stages of the virus replicative cycle.

Cargo transport through the nuclear pore complex at a glance

Bidirectional transport of macromolecules across the nuclear envelope is a hallmark of eukaryotic cells, in which the genetic material is compartmentalized inside the nucleus. The nuclear pore complex (NPC) is the major gateway to the nucleus and it regulates nucleocytoplasmic transport, which is key to processes including transcriptional regulation and cell cycle control. Accordingly, components of the nuclear transport machinery are often found to be dysregulated or hijacked in diseases. In this Cell Science at a Glance article and accompanying poster, we provide an overview of our current understanding of cargo transport through the NPC, from the basic transport signals and machinery to more emerging aspects, all from a 'cargo perspective'. Among these, we discuss the transport of large cargoes (>15 nm), as well as the roles of different cargo properties to nuclear transport, from size and number of bound nuclear transport receptors (NTRs), to surface and mechanical properties.

Nuclear and Nucleolar Localization of Bovine Adenovirus-3 Protein V

The L2 region of bovine adenovirus-3 (BAdV-3) encodes a Mastadenovirus genus-specific protein, designated as pV, which is important for the production of progeny viruses. Here, we demonstrate that BAdV-3 pV, expressed as 55 kDa protein, localizes to the nucleus and specifically targets nucleolus of the infected cells. Analysis of deletion mutants of pV suggested that amino acids 81–120, 190–210, and 380–389 act as multiple nuclear localization signals (NLS), which also appear to serve as the binding sites for importin α-3 protein, a member of the importin α/β nuclear import receptor pathway. Moreover, pV amino acids 21–50 and 380–389 appear to act as nucleolar localization signals (NoLs). Interestingly, amino acids 380–389 appear to act both as NLS and as NoLS. The presence of NoLS is essential for the production of infectious progeny virions, as deletion of both NoLs are lethal for the production of infectious BAdV-3. Analysis of mutant BAV.pVd1d3 (isolated in pV completing CRL cells) containing deletion/mutation of both NoLS in non-complementing CRL cells not only revealed the altered intracellular localization of mutant pV but also reduced the expression of some late proteins. However, it does not appear to affect the incorporation of viral proteins, including mutant pV, in BAV.pVd1d3 virions. Further analysis of CsCl purified BAV.pVd1d3 suggested the presence of thermo-labile virions with disrupted capsids, which appear to affect the infectivity of the progeny virions. Our results suggest that pV contains overlapping and non-overlapping NoLS/NLS. Moreover, the presence of both NoLS appear essential for the production of stable and infectious progeny BAV.pVd1d3 virions.

E3 ubiquitin ligase Mindbomb 1 facilitates nuclear delivery of adenovirus genomes

The journey from plasma membrane to nuclear pore is a critical step in the lifecycle of DNA viruses, many of which must successfully deposit their genomes into the nucleus for replication. Viral capsids navigate this vast distance through the coordinated hijacking of a number of cellular host factors, many of which remain unknown. We performed a gene-trap screen in haploid cells to identify host factors for adenovirus (AdV), a DNA virus that can cause severe respiratory illness in immune-compromised individuals. This work identified Mindbomb 1 (MIB1), an E3 ubiquitin ligase involved in neurodevelopment, as critical for AdV infectivity. In the absence of MIB1, we observed that viral capsids successfully traffic to the proximity of the nucleus but ultimately fail to deposit their genomes within. The capacity of MIB1 to promote AdV infection was dependent on its ubiquitination activity, suggesting that MIB1 may mediate proteasomal degradation of one or more negative regulators of AdV infection. Employing complementary proteomic approaches to characterize proteins proximal to MIB1 upon AdV infection and differentially ubiquitinated in the presence or absence of MIB1, we observed an intersection between MIB1 and ribonucleoproteins (RNPs) largely unexplored in mammalian cells. This work uncovers yet another way that viruses utilize host cell machinery for their own replication, highlighting a potential target for therapeutic interventions that counter AdV infection.

The Role of Protein Disorder in Nuclear Transport and in Its Subversion by Viruses

The transport of host proteins into and out of the nucleus is key to host function. However, nuclear transport is restricted by nuclear pores that perforate the nuclear envelope. Protein intrinsic disorder is an inherent feature of this selective transport barrier and is also a feature of the nuclear transport receptors that facilitate the active nuclear transport of cargo, and the nuclear transport signals on the cargo itself. Furthermore, intrinsic disorder is an inherent feature of viral proteins and viral strategies to disrupt host nucleocytoplasmic transport to benefit their replication. In this review, we highlight the role that intrinsic disorder plays in the nuclear transport of host and viral proteins. We also describe viral subversion mechanisms of the host nuclear transport machinery in which intrinsic disorder is a feature. Finally, we discuss nuclear import and export as therapeutic targets for viral infectious disease.

Nup358 and Transportin 1 Cooperate in Adenoviral Genome Import

Nuclear import of viral genomes is an important step during the life cycle of adenoviruses (AdV), requiring soluble cellular factors as well as proteins of the nuclear pore complex (NPC). We addressed the role of the cytoplasmic nucleoporin Nup358 during adenoviral genome delivery by performing depletion/reconstitution experiments and time-resolved quantification of adenoviral genome import. Nup358-depleted cells displayed reduced efficiencies of nuclear import of adenoviral genomes, and the nuclear import receptor transportin 1 became rate limiting under these conditions. Furthermore, we identified a minimal N-terminal region of Nup358 that was sufficient to compensate for the import defect. Our data support a model where Nup358 functions as an assembly platform that promotes the formation of transport complexes, allowing AdV to exploit a physiological protein import pathway for accelerated transport of its DNA.IMPORTANCE Nuclear import of viral genomes is an essential step to initiate productive infection for several nuclear replicating DNA viruses. On the other hand, DNA is not a physiological nuclear import substrate; consequently, viruses have to exploit existing physiological transport routes. Here, we show that adenoviruses use the nucleoporin Nup358 to increase the efficiency of adenoviral genome import. In its absence, genome import efficiency is reduced and the transport receptor transportin 1 becomes rate limiting. We show that the N-terminal half of Nup358 is sufficient to drive genome import and identify a transportin 1 binding region. In our model, adenovirus genome import exploits an existing protein import pathway and Nup358 serves as an assembly platform for transport complexes.

Regions of bovine adenovirus-3 IVa2 involved in nuclear/nucleolar localization and interaction with pV

Bovine adenovirus-3 (BAdV-3) is a non enveloped, icosahedral DNA virus containing a genome of 34446 bps. The intermediate region of BAdV-3 encodes pIX and IVa2 proteins. Here, we report the characterization of BAdV-3 IVa2. Anti-IVa2 serum detected a 50 kDa protein at 24-48 h post infection in BAdV-3 infected cells. The IVa2 localizes to nucleus and nucleolus of BAdV-3 infected cells. Analysis of mutant IVa2 demonstrated that amino acids 1-25 and 373-448 are required for nuclear and nucleolar localization of IVa2, respectively. The nuclear import of IVa2 utilize importin α -1 of importin nuclear import pathway. Although deletion/substitution of amino acids 4-18 is sufficient to abrogate the nuclear localization of IVa2, amino acids 1-25 are required for nuclear localization of a cytoplasmic protein. Furthermore, we demonstrate that amino acids 1-25 and 120-140 of IVa2 interact with importin α-1 and pV proteins, respectively in BAdV-3 infected cells.

Adenovirus flow in host cell networks

Viruses are obligatory parasites that take advantage of intracellular niches to replicate. During infection, their genomes are carried in capsids across the membranes of host cells to sites of virion production by exploiting cellular behaviour and resources to guide and achieve all aspects of delivery and the downstream virus manufacturing process. Successful entry hinges on execution of a precisely tuned viral uncoating program where incoming capsids disassemble in consecutive steps to ensure that genomes are released at the right time, and in the right place for replication to occur. Each step of disassembly is cell-assisted, involving individual pathways that transmit signals to regulate discrete functions, but at the same time, these signalling pathways are organized into larger networks, which communicate back and forth in complex ways in response to the presence of virus. In this review, we consider the elegant strategy by which adenoviruses (AdVs) target and navigate cellular networks to initiate the production of progeny virions. There are many remarkable aspects about the AdV entry program; for example, the virus gains targeted control of a large well-defined local network neighbourhood by coupling several interacting processes (including endocytosis, autophagy and microtubule trafficking) around a collective reference state centred on the interactional topology and multifunctional nature of protein VI. Understanding the network targeting activity of protein VI, as well as other built-in mechanisms that allow AdV particles to be efficient at navigating the subsystems of the cell, can be used to improve viral vectors, but also has potential to be incorporated for use in entirely novel delivery systems.

Imaging the adenovirus infection cycle

Incoming adenoviruses seize control of cytosolic transport mechanisms to relocate their genome from the cell periphery to specialized sites in the nucleoplasm. The nucleus is the site for viral gene expression, genome replication, and the production of progeny for the next round of infection. By taking control of the cell, adenoviruses also suppress cell-autonomous immunity responses. To succeed in their production cycle, adenoviruses rely on well-coordinated steps, facilitated by interactions between viral proteins and cellular factors. Interactions between virus and host can impose remarkable morphological changes in the infected cell. Imaging adenoviruses has tremendously influenced how we delineate individual steps in the viral life cycle, because it allowed the development of specific optical markers to label these morphological changes in space and time. As technology advances, innovative imaging techniques and novel tools for specimen labeling keep uncovering previously unseen facets of adenovirus biology emphasizing why imaging adenoviruses is as attractive today as it was in the past. This review will summarize past achievements and present developments in adenovirus imaging centered on fluorescence microscopy approaches.

Adenovirus Entry: From Infection to Immunity

More than 80 different adenovirus (AdV) types infect humans through the respiratory, ocular, or gastrointestinal tracts. They cause acute clinical mani-festations or persist under humoral and cell-based immunity. Immuno-suppressed individuals are at risk of death from an AdV infection. Concepts about cell entry of AdV build on strong foundations from molecular and cellular biology-and increasingly physical virology. Here, we discuss how virions enter and deliver their genome into the nucleus of epithelial cells. This process breaks open the virion at distinct sites because the particle has nonisometric mechanical strength and reacts to specific host factors along the entry pathway. We further describe how macrophages and dendritic cells resist AdV infection yet enhance productive entry into polarized epithelial cells. A deep understanding of the viral mechanisms and cell biological and biophysical principles will continue to unravel how epithelial and antigen-presenting cells respond to AdVs and control inflammation and persistence in pathology and therapy.

Viral Appropriation: Laying Claim to Host Nuclear Transport Machinery

Protein nuclear transport is an integral process to many cellular pathways and often plays a critical role during viral infection. To overcome the barrier presented by the nuclear membrane and gain access to the nucleus, virally encoded proteins have evolved ways to appropriate components of the nuclear transport machinery. By binding karyopherins, or the nuclear pore complex, viral proteins influence their own transport as well as the transport of key cellular regulatory proteins. This review covers how viral proteins can interact with different components of the nuclear import machinery and how this influences viral replicative cycles. We also highlight the effects that viral perturbation of nuclear transport has on the infected host and how we can exploit viruses as tools to study novel mechanisms of protein nuclear import. Finally, we discuss the possibility that drugs targeting these transport pathways could be repurposed for treating viral infections.

Capsid-Incorporation Strategy To Display Antigens for an Alternative Adenoviral Vector Vaccine Approach

The adenovirus (Ad) is widely used as a vaccine because of its ability to induce a cellular and humoral immune response. In addition, human clinical trials have validated the safety and efficacy of Ad as a vaccine vector. The traditional approach for employing the adenovirus as vaccine is to configure the antigen genes into the expression cassette of the Ad genome. An alternative method for inducing an immune response is the "capsid-incorporation" strategy. This strategy is based upon the incorporation of proteins or peptides into the capsid proteins. This review will focus on the established uses of this approach as well as highlighting the new developments regarding the capsid-incorporation strategy.

In Vivo Labelling of Adenovirus DNA Identifies Chromatin Anchoring and Biphasic Genome Replication

Viruses must deliver their genomes to host cells to ensure replication and propagation. Characterizing the fate of viral genomes is crucial to understand the viral life cycle and the fate of virus-derived vector tools. Here, we integrated the ANCHOR3 system, an in vivo DNA-tagging technology, into the adenoviral genome for real-time genome detection. ANCHOR3 tagging permitted the in vivo visualization of incoming genomes at the onset of infection and of replicated genomes at late phases of infection. Using this system, we show viral genome attachment to condensed host chromosomes during mitosis, identifying this mechanism as a mode of cell-to-cell transfer. We characterize the spatiotemporal organization of adenovirus replication and identify two kinetically distinct phases of viral genome replication. The ANCHOR3 system is the first technique that allows the continuous visualization of adenoviral genomes during the entire virus life cycle, opening the way for further in-depth study.

Adenoviruses are DNA viruses with a lytic infection cycle. Following the fate of incoming as well as recently replicated genomes during infections is a challenge. In this study, we used the ANCHOR3 technology based on a bacterial partitioning system to establish a versatile in vivo imaging system for adenoviral genomes. The system allows the visualization of both individual incoming and newly replicated genomes in real time in living cells. We demonstrate that incoming adenoviral genomes are attached to condensed cellular chromatin during mitosis, facilitating the equal distribution of viral genomes in daughter cells after cell division. We show that the formation of replication centers occurs in conjunction with in vivo genome replication and determine replication rates. Visualization of adenoviral DNA revealed that adenoviruses exhibit two kinetically distinct phases of genome replication. Low-level replication occurred during early replication, while high-level replication was associated with late replication phases. The transition between these phases occurred concomitantly with morphological changes of viral replication compartments and with the appearance of virus-induced postreplication (ViPR) bodies, identified by the nucleolar protein Mybbp1A. Taken together, our real-time genome imaging system revealed hitherto uncharacterized features of adenoviral genomes in vivo. The system is able to identify novel spatiotemporal aspects of the adenovirus life cycle and is potentially transferable to other viral systems with a double-stranded DNA phase.

IMPORTANCE Viruses must deliver their genomes to host cells to ensure replication and propagation. Characterizing the fate of viral genomes is crucial to understand the viral life cycle and the fate of virus-derived vector tools. Here, we integrated the ANCHOR3 system, an in vivo DNA-tagging technology, into the adenoviral genome for real-time genome detection. ANCHOR3 tagging permitted the in vivo visualization of incoming genomes at the onset of infection and of replicated genomes at late phases of infection. Using this system, we show viral genome attachment to condensed host chromosomes during mitosis, identifying this mechanism as a mode of cell-to-cell transfer. We characterize the spatiotemporal organization of adenovirus replication and identify two kinetically distinct phases of viral genome replication. The ANCHOR3 system is the first technique that allows the continuous visualization of adenoviral genomes during the entire virus life cycle, opening the way for further in-depth study.

Domains of bovine adenovirus-3 protein 22K involved in interacting with viral protein 52K and cellular importins α-5/α-7

The L6 region of bovine adenovirus-3 (BAdV-3) encodes unspliced and spliced proteins named 22K and 33K, respectively. Earlier, anti-22K sera detected two proteins of 42- and 37-kDa in infected cells and 42-kDa protein in transfected cells. Here, we demonstrate that 22K protein localizes to the nucleus of BAdV-3 infected or transfected cells. Analysis of mutant 22K proteins suggested that amino acids 231-250 of non-conserved C-terminus of 22K are required for nuclear localization. The nuclear import of 22K appears to utilize multiple importin (α-5 and α-7) of importin α/β nuclear import pathway. Mutational analysis of 22K identified four basic residues ²³⁸RRRK²⁴¹, which apparently are essential for the nuclear localization of 22K. Our results suggest that the nuclear localization of 22K appear essential for virus replication and production of progeny BAdV-3. Furthermore, we demonstrate that N-terminus amino acid 35-65 conserved in 22K and 33K interact with 52K protein in BAdV-3 infected cells.

Human Adenovirus Infection Causes Cellular E3 Ubiquitin Ligase MKRN1 Degradation Involving the Viral Core Protein pVII

Human adenoviruses (HAdVs) are common human pathogens encoding a highly abundant histone-like core protein, VII, which is involved in nuclear delivery and protection of viral DNA as well as in sequestering immune danger signals in infected cells. The molecular details of how protein VII acts as a multifunctional protein have remained to a large extent enigmatic. Here we report the identification of several cellular proteins interacting with the precursor pVII protein. We show that the cellular E3 ubiquitin ligase MKRN1 is a novel precursor pVII-interacting protein in HAdV-C5-infected cells. Surprisingly, the endogenous MKRN1 protein underwent proteasomal degradation during the late phase of HAdV-C5 infection in various human cell lines. MKRN1 protein degradation occurred independently of the HAdV E1B55K and E4orf6 proteins. We provide experimental evidence that the precursor pVII protein binding enhances MKRN1 self-ubiquitination, whereas the processed mature VII protein is deficient in this function. Based on these data, we propose that the pVII protein binding promotes MKRN1 self-ubiquitination, followed by proteasomal degradation of the MKRN1 protein, in HAdV-C5-infected cells. In addition, we show that measles virus and vesicular stomatitis virus infections reduce the MKRN1 protein accumulation in the recipient cells. Taken together, our results expand the functional repertoire of the HAdV-C5 precursor pVII protein in lytic virus infection and highlight MKRN1 as a potential common target during different virus infections.IMPORTANCE Human adenoviruses (HAdVs) are common pathogens causing a wide range of diseases. To achieve pathogenicity, HAdVs have to counteract a variety of host cell antiviral defense systems, which would otherwise hamper virus replication. In this study, we show that the HAdV-C5 histone-like core protein pVII binds to and promotes self-ubiquitination of a cellular E3 ubiquitin ligase named MKRN1. This mutual interaction between the pVII and MKRN1 proteins may prime MKRN1 for proteasomal degradation, because the MKRN1 protein is efficiently degraded during the late phase of HAdV-C5 infection. Since MKRN1 protein accumulation is also reduced in measles virus- and vesicular stomatitis virus-infected cells, our results signify the general strategy of viruses to target MKRN1.

Viral mechanisms for docking and delivering at nuclear pore complexes

Some viruses possess the remarkable ability to transport their genomes across nuclear pore complexes (NPCs) for replication inside the host cell's intact nuclear compartment. Viral mechanisms for crossing the restrictive NPC passageway are highly complex and astonishingly diverse, requiring in each case stepwise interaction between incoming virus particles and components of the nuclear transport machinery. Exactly how a large viral genome loaded with accessory proteins is able to pass through the relatively narrow central channel of the NPC without causing catastrophic structural damage is not yet fully understood. It appears likely, however, that the overall structure of the NPC changes in response to the cargo. Translocation may result in nucleic acids being misdelivered to the cytoplasm. Here we consider in detail the diverse strategies that viruses have evolved to target and subvert NPCs during infection. For decades, this process has both captivated and confounded researchers in the fields of virology, cell biology, and structural biology.

Intracytoplasmic Transport of Hepatitis B Virus Capsids

The early steps of HBV entry remain largely unknown despite the recent discovery of an HBV-specific entry receptor. Following entry HBV capsids have to be transported through the cytoplasm to the nuclear periphery, followed by nuclear entry. These steps have to take place in a coordinated manner to allow delivery of the genome into the nucleus. Due to the viscosity of the cytoplasm, the intracytoplasmic translocation has to be active and directed.Here, we describe protocols that can be applied to investigations of the HBV capsid with the cytoplasmic transport systems. We have chosen to present two independent experimental approaches, which allow avoiding artifacts. Aside of the specific capsid detection system, the protocols can be applied to any other viral structure.

Principles of Virus Uncoating: Cues and the Snooker Ball

Viruses are spherical or complex shaped carriers of proteins, nucleic acids and sometimes lipids and sugars. They are metastable and poised for structural changes. These features allow viruses to communicate with host cells during entry, and to release the viral genome, a process known as uncoating. Studies have shown that hundreds of host factors directly or indirectly support this process. The cell provides molecules that promote stepwise virus uncoating, and direct the virus to the site of replication. It acts akin to a snooker player who delivers accurate and timely shots (cues) to the ball (virus) to score. The viruses, on the other hand, trick (snooker) the host, hijack its homeostasis systems, and dampen innate immune responses directed against danger signals. In this review, we discuss how cellular cues, facilitators, and built‐in viral mechanisms promote uncoating. Cues come from receptors, enzymes and chemicals that act directly on the virus particle to alter its structure, trafficking and infectivity. Facilitators are defined as host factors that are involved in processes which indirectly enhance entry or uncoating. Unraveling the mechanisms of virus uncoating will continue to enhance understanding of cell functions, and help counteracting infections with chemicals and vaccines.

Functions of DNA damage machinery in the innate immune response to DNA virus infection

DNA is potently immunostimulatory, and self-DNA is packaged in the nucleus or mitochondria allowing it to remain silent to cell-intrinsic sensors. However, damaged or mislocalised self-DNA is sensed by our innate immune systems, resulting in the production of type I interferons (IFNI), chemokines and inflammatory cytokines. During DNA virus infection the detection of viral DNA genomes by pattern recognition receptors (PRRs) is essential for the initiation of IFNI responses and host defence against these pathogens. It is intriguing that a number of molecular mechanisms have been found to be common to both of these DNA-induced stress responses and this has potentially important consequences for both sides of the host/pathogen arms race.

Misdelivery at the Nuclear Pore Complex-Stopping a Virus Dead in Its Tracks

Many viruses deliver their genomes into the host cell’s nucleus before they replicate. While onco-retroviruses and papillomaviruses tether their genomes to host chromatin upon mitotic breakdown of the nuclear envelope, lentiviruses, such as human immunodeficiency virus, adenoviruses, herpesviruses, parvoviruses, influenza viruses, hepatitis B virus, polyomaviruses, and baculoviruses deliver their genomes into the nucleus of post-mitotic cells. This poses the significant challenge of slipping a DNA or RNA genome past the nuclear pore complex (NPC) embedded in the nuclear envelope. Quantitative fluorescence imaging is shedding new light on this process, with recent data implicating misdelivery of viral genomes at nuclear pores as a bottleneck to virus replication. Here, we infer NPC functions for nuclear import of viral genomes from cell biology experiments and explore potential causes of misdelivery, including improper virus docking at NPCs, incomplete translocation, virus-induced stress and innate immunity reactions. We conclude by discussing consequences of viral genome misdelivery for viruses and host cells, and lay out future questions to enhance our understanding of this phenomenon. Further studies into viral genome misdelivery may reveal unexpected aspects about NPC structure and function, as well as aid in developing strategies for controlling viral infections to improve human health.

Viruses and the nuclear envelope

Viruses encounter and manipulate almost all aspects of cell structure and metabolism. The nuclear envelope (NE), with central roles in cell structure and genome function, acts and is usurped in diverse ways by different viruses. It can act as a physical barrier to infection that must be overcome, as a functional barrier that restricts infection by various mechanisms and must be counteracted or indeed as a positive niche, important or even essential for virus infection or production of progeny virions. This review summarizes virus-host interactions at the NE, highlighting progress in understanding the replication of viruses including HIV-1, Influenza, Herpes Simplex, Adenovirus and Ebola, and molecular insights into hitherto unknown functional pathways at the NE.

Nuclear entry of DNA viruses

DNA viruses undertake their replication within the cell nucleus, and therefore they must first deliver their genome into the nucleus of their host cells. Thus, trafficking across the nuclear envelope is at the basis of DNA virus infections. Nuclear transport of molecules with diameters up to 39 nm is a tightly regulated process that occurs through the nuclear pore complex (NPC). Due to the enormous diversity of virus size and structure, each virus has developed its own strategy for entering the nucleus of their host cells, with no two strategies alike. For example, baculoviruses target their DNA-containing capsid to the NPC and subsequently enter the nucleus intact, while the hepatitis B virus capsid crosses the NPC but disassembles at the nuclear side of the NPC. For other viruses such as herpes simplex virus and adenovirus, although both dock at the NPC, they have each developed a distinct mechanism for the subsequent delivery of their genome into the nucleus. Remarkably, other DNA viruses, such as parvoviruses and human papillomaviruses, access the nucleus through an NPC-independent mechanism. This review discusses our current understanding of the mechanisms used by DNA viruses to deliver their genome into the nucleus, and further presents the experimental evidence for such mechanisms.

Old foes, new understandings: nuclear entry of small non-enveloped DNA viruses

The nuclear import of viral genomes is an important step of the infectious cycle for viruses that replicate in the nucleus of their host cells. Although most viruses use the cellular nuclear import machinery or some components of this machinery, others have developed sophisticated ways to reach the nucleus. Some of these have been known for some time; however, recent studies have changed our understanding of how some non-enveloped DNA viruses access the nucleus. For example, parvoviruses enter the nucleus through small disruptions of the nuclear membranes and nuclear lamina, and adenovirus tugs at the nuclear pore complex, using kinesin-1, to disassemble their capsids and deliver viral proteins and genomes into the nucleus. Here we review recent findings of the nuclear import strategies of three small non-enveloped DNA viruses, including adenovirus, parvovirus, and the polyomavirus simian virus 40.

Nonenveloped capsid: mechanisms of viral entry

ABSTRACT 

Viruses are a diverse class of nanoparticles. However, they have evolved a few common mechanisms that enable successful infection of their host cells. The first stage of this process involves entry into the cell. For enveloped viruses this process has been well characterized. For nonenveloped viruses, the focus of this review, the entry mechanisms are less well understood. For these viruses, a typical pathway involves receptor attachment followed by internalization into cellular vesicles and subsequent viral escape to the cytosol and transport to the site of genome replication. Significantly, these viruses have evolved numerous mechanisms to fulfill this seemingly simple infection scheme. We focus on the latest observations for several families of nonenveloped viruses and highlight specific members for eukaryotic families: Adenoviridae, Papillomaviridae, Parvoviridae, Picornaviridae, Polyomaviridae and Reoviridae; and prokaryotic families: Microviridae, Myoviridae, Podoviridae and Siphoviridae.

Nuclear import of adenovirus DNA involves direct interaction of hexon with an N-terminal domain of the nucleoporin Nup214

In this study, we characterized the molecular basis for binding of adenovirus (AdV) to the cytoplasmic face of the nuclear pore complex (NPC), a key step during delivery of the viral genome into the nucleus. We used RNA interference (RNAi) to deplete cells of either Nup214 or Nup358, the two major Phe-Gly (FG) repeat nucleoporins localized on the cytoplasmic side of the NPC, and evaluated the impact on hexon binding and AdV infection. The accumulation of purified hexon trimers or partially disassembled AdV at the nuclear envelope (NE) was observed in digitonin-permeabilized cells in the absence of cytosolic factors. Both in vitro hexon binding and in vivo nuclear import of the AdV genome were strongly reduced in Nup214-depleted cells but still occurred in Nup358-depleted cells, suggesting that Nup214 is a major binding site of AdV during infection. The expression of an NPC-targeted N-terminal domain of Nup214 in Nup214-depleted cells restored the binding of hexon at the NE and the nuclear import of protein VII (pVII), indicating that this region is sufficient to allow AdV binding. We further narrowed the binding site to a 137-amino-acid segment in the N-terminal domain of Nup214. Together, our results have identified a specific region within the N terminus of Nup214 that acts as a direct NPC binding site for AdV.

IMPORTANCE

AdVs, which have the largest genome of nonenveloped DNA viruses, are being extensively explored for use in gene therapy, especially in alternative treatments for cancers that are refractory to traditional therapies. In this study, we characterized the molecular basis for binding of AdV to the cytoplasmic face of the NPC, a key step for delivery of the viral genome into the nucleus. Our data indicate that a 137-amino-acid region of the nucleoporin Nup214 is a binding site for the major AdV capsid protein, hexon, and that this interaction is required for viral DNA import. These findings provide additional insight on how AdV exploits the nuclear transport machinery for infection. The results could promote the development of new strategies for gene transfer and enhance understanding of the nuclear import of other viral DNA genomes, such as those of papillomavirus or hepatitis B virus that induce specific cancers.

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

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

Transportin-1 and Transportin-2: protein nuclear import and beyond

Nearly 20 years after its identification as a new β-karyopherin mediating the nuclear import of the RNA-binding protein hnRNP A1, Transportin-1 is still commonly overlooked in comparison with its best known cousin, Importin-β. Transportin-1 is nonetheless a considerable player in nucleo-cytoplasmic transport. Over the past few years, significant progress has been made in the characterization of the nuclear localization signals (NLSs) that Transportin-1 recognizes, thereby providing the molecular basis of its diversified repertoire of cargoes. The recent discovery that mutations in the Transportin-dependent NLS of FUS cause mislocalization of this protein and result in amyotrophic lateral sclerosis illustrates the importance of Transportin-dependent import for human health. Besides, new functions of Transportin-1 are emerging in processes other than nuclear import. Here, we summarize what is known about Transportin-1 and the related β-karyopherin Transportin-2.

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

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

IMPORTANCE

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

SPOC1-mediated antiviral host cell response is antagonized early in human adenovirus type 5 infection

Little is known about immediate phases after viral infection and how an incoming viral genome complex counteracts host cell defenses, before the start of viral gene expression. Adenovirus (Ad) serves as an ideal model, since entry and onset of gene expression are rapid and highly efficient, and mechanisms used 24–48 hours post infection to counteract host antiviral and DNA repair factors (e.g. p53, Mre11, Daxx) are well studied. Here, we identify an even earlier host cell target for Ad, the chromatin-associated factor and epigenetic reader, SPOC1, recently found recruited to double strand breaks, and playing a role in DNA damage response. SPOC1 co-localized with viral replication centers in the host cell nucleus, interacted with Ad DNA, and repressed viral gene expression at the transcriptional level. We discovered that this SPOC1-mediated restriction imposed upon Ad growth is relieved by its functional association with the Ad major core protein pVII that enters with the viral genome, followed by E1B-55K/E4orf6-dependent proteasomal degradation of SPOC1. Mimicking removal of SPOC1 in the cell, knock down of this cellular restriction factor using RNAi techniques resulted in significantly increased Ad replication, including enhanced viral gene expression. However, depletion of SPOC1 also reduced the efficiency of E1B-55K transcriptional repression of cellular promoters, with possible implications for viral transformation. Intriguingly, not exclusive to Ad infection, other human pathogenic viruses (HSV-1, HSV-2, HIV-1, and HCV) also depleted SPOC1 in infected cells. Our findings provide a general model for how pathogenic human viruses antagonize intrinsic SPOC1-mediated antiviral responses in their host cells. A better understanding of viral entry and early restrictive functions in host cells should provide new perspectives for developing antiviral agents and therapies. Conversely, for Ad vectors used in gene therapy, counteracting mechanisms eradicating incoming viral DNA would increase Ad vector efficacy and safety for the patient.

Viruses have acquired functions that target and modulate host cell signaling and diverse regulatory cascades, leading to efficient viral propagation. During the course of productive infection, Ad gene products manipulate destruction pathways to prevent viral clearance or cell death prior to viral genome amplification and release of progeny. Recently, we reported that chromatin formation and cellular SWI/SNF chromatin remodeling processes play a key role in Ad transcriptional regulation. Here, we observe for the first time that SPOC1, identified as a regulator of DNA damage response and chromatin structure, plays an essential role in restricting Ad gene expression and progeny production. This host cell antiviral mechanism is efficiently counteracted by tight association with the major core protein pVII bound to the incoming viral genome. Subsequently, SPOC1 undergoes proteasomal degradation via the Ad E1B-55K/E4orf6-dependent, Cullin-based E3 ubiquitin ligase complex. We also show that other viruses from RNA and DNA families also induce efficient degradation of SPOC1. These analyses of evasion strategies acquired by viruses and other human pathogens should provide important insights into factors manipulating the epigenetic environment to potentially inactivate, or amplify host cell immune responses, since detailed molecular mechanisms and the full repertoire of cellular targets still remain elusive.

Adenovirus precursor pVII protein stability is regulated by its propeptide sequence

Adenovirus encodes for the pVII protein, which interacts and modulates virus DNA structure in the infected cells. The pVII protein is synthesized as the precursor protein and undergoes proteolytic processing by viral proteinase Avp, leading to release of a propeptide sequence and accumulation of the mature VII protein. Here we elucidate the molecular functions of the propeptide sequence present in the precursor pVII protein. The results show that the propeptide is the destabilizing element targeting the precursor pVII protein for proteasomal degradation. Our data further indicate that the propeptide sequence and the lysine residues K26 and K27 regulate the precursor pVII protein stability in a co-dependent manner. We also provide evidence that the Cullin-3 E3 ubiquitin ligase complex alters the precursor pVII protein stability by association with the propeptide sequence. In addition, we show that inactivation of the Cullin-3 protein activity reduces adenovirus E1A gene expression during early phase of virus infection. Collectively, our results indicate a novel function of the adenovirus propeptide sequence and involvement of Cullin-3 in adenovirus gene expression.

Bovine adenovirus 3 core protein precursor pVII localizes to mitochondria, and modulates ATP synthesis, mitochondrial Ca2+ and mitochondrial membrane potential

Viruses modulate the functions of mitochondria by translocating viral proteins to the mitochondria. Subcellular fractionation and sensitivity to proteinase K/Triton X-100 treatment of mitochondrial fractions of bovine adenovirus (BAdV)-3-infected/transfected cells suggested that core protein pVII localizes to the mitochondria and contains a functional mitochondrial localization signal. Moreover, mitochondrial localization of BAdV-3 pVII appears to help in the retention of mitochondrial Ca(2+), inducing a significant increase in the levels of ATP and maintaining the mitochondrial membrane potential (MMP) in transfected cells. In contrast, mitochondrial localization of BAdV-3 pVII has no significant effect on the levels of cytoplasmic Ca(2+) and reactive oxygen species production in the transfected cells. Consistent with these results, expression of pVII in transfected cells treated with staurosporine decreased significantly the activation of caspase-3. Our results suggested that BAdV-3 pVII localizes to mitochondria, and interferes with apoptosis by inhibiting loss of the MMP and by increasing mitochondrial Ca(2+) and ATP production.

Viruses challenge selectivity barrier of nuclear pores

Exchange between the nucleus and the cytoplasm occurs through nuclear pore complexes (NPCs) embedded in the double membrane of the nuclear envelope. NPC permeability barrier restricts the entry of inert molecules larger than 5 nm in diameter but allows facilitated entry of selected cargos, whose size can reach up to 39 nm. The translocation of large molecules is facilitated by nuclear transport receptors (NTRs) that have affinity to proteins of NPC permeability barrier. Viruses that enter the nucleus replicate evolved strategies to overcome this barrier. In this review, we will discuss the functional principles of NPC barrier and nuclear transport machinery, as well as the various strategies viruses use to cross the selective barrier of NPCs.

Kinesin-1-mediated capsid disassembly and disruption of the nuclear pore complex promote virus infection

Many viruses deliver their genomes into the host cell nucleus for replication. However, the size restrictions of the nuclear pore complex (NPC), which regulates the passage of proteins, nucleic acids, and solutes through the nuclear envelope, require virus capsid uncoating before viral DNA can access the nucleus. We report a microtubule motor kinesin-1-mediated and NPC-supported mechanism of adenovirus uncoating. The capsid binds to the NPC filament protein Nup214 and kinesin-1 light-chain Klc1/2. The nucleoporin Nup358, which is bound to Nup214/Nup88, interacts with the kinesin-1 heavy-chain Kif5c to indirectly link the capsid to the kinesin motor. Kinesin-1 disrupts capsids docked at Nup214, which compromises the NPC and dislocates nucleoporins and capsid fragments into the cytoplasm. NPC disruption increases nuclear envelope permeability as indicated by the nuclear influx of large cytoplasmic dextran polymers. Thus, kinesin-1 uncoats viral DNA and compromises NPC integrity, allowing viral genomes nuclear access to promote infection.

Adenovirus-triggered innate signalling pathways

Adenoviruses are important infectious agents and also emerging vectors in different biomedical applications. These viruses elicit a strong innate and adaptive immune response, which influences both the course of disease and the success of the applied vectors. Several Toll-like Receptor (TLR)-dependent and -independent mechanisms contribute to these responses. Understanding of the involved viral and cellular factors is crucial for the treatment of various adenovirus diseases and the optimal design of adenovirus vector applications. Here we summarize our current understanding of the complex nature of adenovirus-induced innate immune mechanisms.

Conservation of complex nuclear localization signals utilizing classical and non-classical nuclear import pathways in LANA homologs of KSHV and RFHV

ORF73 latency-associated nuclear antigen (LANA) of the Kaposi's sarcoma-associated herpesvirus (KSHV) is targeted to the nucleus of infected cells where it binds to chromatin and mediates viral episome persistence, interacts with cellular proteins and plays a role in latency and tumorigenesis. A structurally related LANA homolog has been identified in the retroperitoneal fibromatosis herpesvirus (RFHV), the macaque homolog of KSHV. Here, we report the evolutionary and functional conservation of a novel bi-functional nuclear localization signal (NLS) in KSHV and RFHV LANA. N-terminal peptides from both proteins were fused to EGFP or double EGFP fusions to examine their ability to induce nuclear transport of a heterologous protein. In addition, GST-pull down experiments were used to analyze the ability of LANA peptides to interact with members of the karyopherin family of nuclear transport receptors. Our studies revealed that both LANA proteins contain an N-terminal arginine/glycine (RG)-rich domain spanning a conserved chromatin-binding motif, which binds directly to importin β1 in a RanGTP-sensitive manner and serves as an NLS in the importin β1-mediated non-classical nuclear import pathway. Embedded within this domain is a conserved lysine/arginine-(KR)-rich bipartite motif that binds directly to multiple members of the importin α family of nuclear import adaptors in a RanGTP-insensitive manner and serves as an NLS in the classical importin α/β-mediated nuclear import pathway. The positioning of a classical bipartite kr-NLS embedded within a non-classical rg-NLS is a unique arrangement in these viral proteins, whose nuclear localization is critical to their functionality and to the virus life cycle. The ability to interact with multiple import receptors provides alternate pathways for nuclear localization of LANA. Since different import receptors can import cargo to distinct subnuclear compartments, a multifunctional NLS may provide LANA with an increased ability to interact with different nuclear components in its multifunctional role to maintain viral latency.

Functional genetic and biophysical analyses of membrane disruption by human adenovirus

The identification of the adenovirus (AdV) protein that mediates endosome penetration during infection has remained elusive. Several lines of evidence from previous studies suggest that the membrane lytic factor of AdV is the internal capsid protein VI. While these earlier results imply a role for protein VI in endosome disruption, direct evidence during cell entry has not been demonstrated. To acquire more definitive proof, we engineered random mutations in a critical N-terminal amphipathic α-helix of VI in an attempt to generate AdV mutants that lack efficient membrane penetration and infection. Random mutagenesis within the context of the AdV genome was achieved via the development of a novel technique that incorporates both error-prone PCR and recombineering. Using this system, we identified a single mutation, L40Q, that significantly reduced infectivity and selectively impaired endosome penetration. Furthermore, we obtained biophysical data showing that the lack of efficient endosomalysis is associated with reduced insertion of the L40Q mutation in protein VI (VI-L40Q) into membranes. Our studies indicate that protein VI is the critical membrane lytic factor of AdV during cellular entry and reveal the biochemical basis for its membrane interactions.

How viruses access the nucleus

Many viruses depend on nuclear proteins for replication. Therefore, their viral genome must enter the nucleus of the host cell. In this review we briefly summarize the principles of nucleocytoplasmic transport, and then describe the diverse strategies used by viruses to deliver their genomes into the host nucleus. Some of the emerging mechanisms include: (1) nuclear entry during mitosis, when the nuclear envelope is disassembled, (2) viral genome release in the cytoplasm followed by entry of the genome through the nuclear pore complex (NPC), (3) capsid docking at the cytoplasmic side of the NPC, followed by genome release, (4) nuclear entry of intact capsids through the NPC, followed by genome release, and (5) nuclear entry via virus-induced disruption of the nuclear envelope. Which mechanism a particular virus uses depends on the size and structure of the virus, as well as the cellular cues used by the virus to trigger capsid disassembly and genome release. This article is part of a Special Issue entitled: Regulation of Signaling and Cellular Fate through Modulation of Nuclear Protein Import.

Nuclear import by karyopherin-βs: recognition and inhibition

Proteins in the karyopherin-β family mediate the majority of macromolecular transport between the nucleus and the cytoplasm. Eleven of the 19 known human karyopherin-βs and 10 of the 14S. cerevisiae karyopherin-βs mediate nuclear import through recognition of nuclear localization signals or NLSs in their cargos. This receptor-mediated process is essential to cellular viability as proteins are translated in the cytoplasm but many have functional roles in the nucleus. Many known karyopherin-β-cargo interactions were discovered through studies of the individual cargos rather than the karyopherins, and this information is thus widely scattered in the literature. We consolidate information about cargos that are directly recognized by import-karyopherin-βs and review common characteristics or lack thereof among cargos of different import pathways. Knowledge of karyopherin-β-cargo interactions is also critical for the development of nuclear import inhibitors and the understanding of their mechanisms of inhibition. This article is part of a Special Issue entitled: Regulation of Signaling and Cellular Fate through Modulation of Nuclear Protein Import.