African swine fever virus infection disrupts centrosome assembly and function

African Swine Fever Virus Host-Pathogen Interactions

African swine fever virus is a complex double-stranded DNA virus that exhibits tropism for cells of the mononuclear phagocytic system. Virus replication is a multi-step process that involves the nucleus of the host cell as well the formation of large perinuclear sites where progeny virions are assembled prior to transport to, and budding through, the plasma membrane. Like many viruses, African swine fever virus reorganises the cellular architecture to facilitate its replication and has evolved multiple mechanisms to avoid the potential deleterious effects of host cell stress response pathways. However, how viral proteins and virus-induced structures trigger cellular stress pathways and manipulate the subsequent responses is still relatively poorly understood. African swine fever virus alters nuclear substructures, modulates autophagy, apoptosis and the endoplasmic reticulum stress response pathways. The viral genome encodes for at least 150 genes, of which approximately 70 are incorporated into the virion. Many of the non-structural genes have not been fully characterised and likely play a role in host range and modifying immune responses. As the field moves towards approaches that take a broader view of the effect of expression of individual African swine fever genes, we summarise how the different steps in virus replication interact with the host cell and the current state of knowledge on how it modulates the resulting stress responses.

Amoebal Tubulin Cleavage Late during Infection Is a Characteristic Feature of Mimivirus but Not of Marseillevirus

Mimivirus and Marseillevirus infections of Acanthamoeba castellanii, like most other viral infections, induce cytopathic effects (CPE). The details of how they bring about CPE and to what extent and how they modify the host cytoskeletal network are unclear. In this study, we compared the rearrangement of the host cytoskeletal network induced by Mimivirus and Marseillevirus upon infection. We show that while both Mimivirus and Marseillevirus infections of A. castellanii cells cause retraction of acanthopodia and depolymerization of the host actin filament network, the Mimivirus infection also results in characteristic cleavage of the host tubulin, a phenomenon not previously reported with any intracellular pathogens. Furthermore, we show that the amoebal tubulin cleavage during Mimivirus infection is a post-replicative event. Because time-lapse microscopy showed that Mimivirus infection leads to the bursting of cells, releasing the virus, we hypothesize that tubulin cleavage together with actin depolymerization during the later stages of Mimivirus assembly is essential for cell lysis due to apoptotic/necrotic cell death. We also characterize the Mimivirus-encoded gp560, a Zn metalloprotease, however, the purified gp560 protein was unable to cleave the commercially available porcine brain tubulin. While protein synthesis is essential for causing the morphological changes in the case of Mimivirus, the proteins which are packaged in the viral capsid along with the genome are sufficient to induce CPE in the case of Marseillevirus. IMPORTANCE In general, intracellular pathogens target the cytoskeletal network to enable their life cycle inside the host. Pathogen-induced changes in the host cell morphology usually accompany global changes in the cytoskeleton resulting in cytopathic effects. While viruses have been shown to use the host actin cytoskeleton for entry and transport during early infection, the role of microtubules in the viral life cycle is only beginning to emerge. Here, we show that the giant viruses Mimivirus and Marseillevirus both induce depolymerization of the actin filament, Mimivirus also causes a characteristic cleavage of tubulin not previously reported for any intracellular pathogen. Because tubulin cleavage occurs late during infection, we hypothesize that tubulin cleavage aids in cell death and lysis rather than establishing infection. The different strategies used by viruses with similar host niches may help them survive in competition.

A New Method for Detection African Swine Fever Virus: Time-resolved Fluorescence Immunoassay

African swine fever (ASF) has severely influenced the swine industry of the whole world. Fast and accurate African swine fever virus (ASFV) antigen detection is very important for ASF prevention. This study aims to establish a new detection method for detection ASFV antigen using time-resolved fluorescence immunoassay (TRFIA) in the nose and mouth discharge. A double antibody sandwich TRFIA method was optimized and established. Recombinant P30 recombinant antigen was captured by its antibodies immobilized on 96-well plate, and then banded together with another detection antibodies labeled with Europium(III) (Eu3+) chelates, finally time-resolved analyzer measured the fluorescence intensity. The performance of this TRFIA (sensitivity, specificity and accuracy) was evaluated using the clinical samples and compared with the nucleic acid testing method. The sensitivity of this TRFIA was 0.015 ng/mL (dynamic range 0.24-500 ng/mL) with high specificity. The recovery ranged from 92.00 to 103.62 %, the inter-assay CVs ranged from 5.50 to 11.96 %, and the intra-assay CVs was between 5.20 and 10.53 %. Additionally, the cutoff value was 0.016. TRFIA took only 45 min to generate results, and its detection capability comparable to the nucleic acid detection. This study developed a TRFIA method that could be used for qualitative/quantitative detection of ASFV antigen in pigs nasal discharge, which has high sensitivity, specificity and accuracy. This TRFIA provides a new method for rapidly screening ASFV infection in pigs industry.

Discovery of Viral Myosin Genes With Complex Evolutionary History Within Plankton

Nucleocytoplasmic large DNA viruses (NCLDVs) infect diverse eukaryotes and form a group of viruses with capsids encapsulating large genomes. Recent studies are increasingly revealing a spectacular array of functions encoded in their genomes, including genes for energy metabolisms, nutrient uptake, as well as cytoskeleton. Here, we report the discovery of genes homologous to myosins, the major eukaryotic motor proteins previously unrecognized in the virosphere, in environmental genomes of NCLDVs from the surface of the oceans. Phylogenetic analyses indicate that most viral myosins (named "virmyosins") belong to the Imitervirales order, except for one belonging to the Phycodnaviridae family. On the one hand, the phylogenetic positions of virmyosin-encoding Imitervirales are scattered within the Imitervirales. On the other hand, Imitervirales virmyosin genes form a monophyletic group in the phylogeny of diverse myosin sequences. Furthermore, phylogenetic trends for the virmyosin genes and viruses containing them were incongruent. Based on these results, we argue that multiple transfers of myosin homologs have occurred not only from eukaryotes to viruses but also between viruses, supposedly during co-infections of the same host. Like other viruses that use host motor proteins for their intracellular transport or motility, these viruses may use the virally encoded myosins for the intracellular trafficking of giant viral particles.

Impacts of virus-mediated manipulation of host Dynein

In general viruses' modus operandi to propagate is achieved by the co-opting host cell components, membranes, proteins, and machineries to their advantage. This is true for virtually every aspect of a virus' replication cycle from virus entry to the budding or release of progeny virus particles. In this chapter, we will discuss new information on the impacts of virus-mediated manipulation of Dynein motor complexes and associated machineries and factors. We will highlight how these host cell components impact on pathogenicity and immune responses, as many of the virus-mediated hijacked components also play pivotal roles in immune responses to pathogen insult. There are several comprehensive reviews that define virus–Dynein interactions including the first edition of this book that describes how viruses manipulate the host cell machineries their advantage. An updated table is included to summarize these virus–host interactions. Notably, barriers to intracellular translocation represent major hurdles to viral components during de novo infection and during active replication and the generation of progeny virus particles. Clearly, the subversion of host cell molecular motor protein activities takes advantage of constitutive and regulated membrane trafficking events and will target virus components to intracytoplasmic locales and membrane assembly. Broadening our understanding of the interplay between viruses, Dynein and the cytoskeleton will likely inform on new types of therapies. Continual enhancement of the breadth of new information on how viruses manipulate host cell biology will inevitably aid in the identification of new targets that can be poisoned to block old, new, and emerging viruses alike in their tracks.

Microtubule Regulation and Function during Virus Infection

Microtubules (MTs) form a rapidly adaptable network of filaments that radiate throughout the cell. These dynamic arrays facilitate a wide range of cellular processes, including the capture, transport, and spatial organization of cargos and organelles, as well as changes in cell shape, polarity, and motility. Nucleating from MT-organizing centers, including but by no means limited to the centrosome, MTs undergo rapid transitions through phases of growth, pause, and catastrophe, continuously exploring and adapting to the intracellular environment. Subsets of MTs can become stabilized in response to environmental cues, acquiring distinguishing posttranslational modifications and performing discrete functions as specialized tracks for cargo trafficking. The dynamic behavior and organization of the MT array is regulated by MT-associated proteins (MAPs), which include a subset of highly specialized plus-end-tracking proteins (+TIPs) that respond to signaling cues to alter MT behavior. As pathogenic cargos, viruses require MTs to transport to and from their intracellular sites of replication. While interactions with and functions for MT motor proteins are well characterized and extensively reviewed for many viruses, this review focuses on MT filaments themselves. Changes in the spatial organization and dynamics of the MT array, mediated by virus- or host-induced changes to MT regulatory proteins, not only play a central role in the intracellular transport of virus particles but also regulate a wider range of processes critical to the outcome of infection.

Proteasomes raise the microtubule dynamics in influenza A (H1N1) virus-infected LLC-MK2 cells

The dynamics of microtubule networks are known to have an impact on replication of influenza A virus in some cellular models. Here we present evidence suggesting that at late stages of LLC-MK2 cell infection by influenza A (H1N1) virus the ubiquitin-proteasome protein degradation system participates in destabilization of microtubules, and favours virus replication. Chemical inhibition of proteasome activity partially suppresses influenza A virus replication, while stimulation of proteasome activity favours influenza A virus replication. Conversely, in another cellular model, A549 cells, inhibitors and activators of proteasomes have a small effect on influenza A virus replication. These data suggest that influenza A virus might take selective advantage of proteasome functions in order to set up a favourable cytoskeletal “environment” for its replication and spread. Furthermore, the relationship between influenza virus and the host cell is likely to depend on both the cellular model and the virus strain.

Plus-end tracking proteins, CLASPs, and a viral Akt mimic regulate herpesvirus-induced stable microtubule formation and virus spread

Although microtubules (MTs) frequently form highly dynamic networks, subsets of MTs become stabilized in response to environmental cues and function as specialized tracks for vesicle and macromolecular trafficking. MT stabilization is controlled by specialized plus-end tracking proteins (+TIPs) whose accumulation at the MT ends is facilitated by the end-binding protein, EB1, and regulated by various signaling pathways. As cargoes themselves, viruses are dependent on MTs for their intracellular movement. Although many viruses affect MT organization, the potential contribution of MT stabilization by +TIPs to infection remains unknown. Here we show that early in infection of primary human fibroblasts, herpes simplex virus type 1 (HSV-1) disrupts the centrosome, the primary MT organizing center in many cell types. As infection progresses HSV-1 induces the formation of stable MT subsets through inactivation of glycogen synthase kinase 3beta by the viral Ser/Thr kinase, Us3. Stable MT formation is reduced in cells infected with Us3 mutants and those stable MTs that form cluster around the trans-Golgi network. Downstream of glycogen synthase kinase 3beta, cytoplasmic linker-associated proteins (CLASPs), specialized host +TIPs that control MT formation at the trans-Golgi network and cortical capture, are specifically required for virus-induced MT stabilization and HSV-1 spread. Our findings demonstrate the biological importance of +TIPs to viral infection and suggest that HSV-1 has evolved to exploit the trans-Golgi network as an alternate MT organizing center to facilitate virus spread.

Differing effects of herpes simplex virus 1 and pseudorabies virus infections on centrosomal function

Efficient intracellular transport of the capsid of alphaherpesviruses, such as herpes simplex virus 1 (HSV-1), is known to be dependent upon the microtubule (MT) network. Typically, the MT network radiates from an MT-organizing center (MTOC), which is, in most cases, the centrosome. During herpesvirus egress, it has been assumed that capsids travel first from the nucleus to the centrosome and then from the centrosome to the site of envelopment. Here we report that the centrosome is no longer a primary MTOC in HSV-1-infected cells, but it retains this function in cells infected by another alphaherpesvirus, pseudorabies virus (PrV). As a result, MTs formed at late times after infection with PrV grow from a major, centralized MTOC, while those formed after HSV-1 infection arise from dispersed locations in the cytoplasm, indicating the presence of alternative and minor MTOCs. Thus, loss of the principal MT nucleating center in cells following HSV-1 infection raises questions about the mechanism of HSV-1 capsid egress. It is possible that, rather than passing via the centrosome, capsids may travel directly to the site of envelopment after exiting the nucleus. We suggest that, in HSV-1-infected cells, the disruption of centrosomal functions triggers reorganization of the MT network to favor noncentrosomal MTs and promote efficient viral spread.

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.

Kinetic mechanism of the ssDNA recognition by the polymerase X from African Swine Fever Virus. Dynamics and energetics of intermediate formations

Kinetic mechanism of the ssDNA recognition by the polymerase X of African Swine Fever Virus (ASFV) and energetics of intermediate formations have been examined, using the fluorescence stopped-flow method. The association is a minimum three-step process PolX + ssDNA k(1) <-- --> k(-1) (P-ssDNA)(1) k(2) <-- --> k(-2) (P-ssDNA)(2) k(3) <-- --> k(-3) (P-ssDNA)(3). The nucleic acid makes the initial contact through the C-terminal domain, which generates most of the overall ΔG°. In the second step the nucleic acid engages the N-terminal domain, assuming the bent structure. In equilibrium, the complex exists in at least two different states. Apparent enthalpy and entropy changes, characterizing formations of intermediates, reflect association of the DNA with the C-terminal domain and gradual engagement of the catalytic domain by the nucleic acid. The intrinsic DNA-binding steps are entropy-driven processes accompanied by the net release of water molecules. The final conformational transition of the complex does not involve any large changes of the DNA topology, or the net release of the water molecules.

Interactions of the DNA polymerase X from African Swine Fever Virus with the ssDNA. Properties of the total DNA-binding site and the strong DNA-binding subsite

Interactions of the polymerase X from the African Swine Fever Virus with the ssDNA have been studied, using quantitative fluorescence titration and fluorescence resonance energy transfer techniques. The primary DNA-binding subsite of the enzyme, independent of the DNA conformation, is located on the C-terminal domain. Association of the bound DNA with the catalytic N-terminal domain finalizes the engagement of the total DNA-binding site of the enzyme and induces a large topological change in the structure of the bound ssDNA. The free energy of binding includes a conformational transition of the protein. Large positive enthalpy changes accompanying the ASFV pol X-ssDNA association indicate that conformational changes of the complex are induced by the engagement of the N-terminal domain. The enthalpy changes are offset by large entropy changes accompanying the DNA binding to the C-terminal domain and the total DNA-binding site, predominantly resulting from the release of water molecules.

Proteomic and functional analysis of the mitotic Drosophila centrosome

Regulation of centrosome structure, duplication and segregation is integrated into cellular pathways that control cell cycle progression and growth. As part of these pathways, numerous proteins with well-established non-centrosomal localization and function associate with the centrosome to fulfill regulatory functions. In turn, classical centrosomal components take up functional and structural roles as part of other cellular organelles and compartments. Thus, although a comprehensive inventory of centrosome components is missing, emerging evidence indicates that its molecular composition reflects the complexity of its functions. We analysed the Drosophila embryonic centrosomal proteome using immunoisolation in combination with mass spectrometry. The 251 identified components were functionally characterized by RNA interference. Among those, a core group of 11 proteins was critical for centrosome structure maintenance. Depletion of any of these proteins in Drosophila SL2 cells resulted in centrosome disintegration, revealing a molecular dependency of centrosome structure on components of the protein translation machinery, actin- and RNA-binding proteins. In total, we assigned novel centrosome-related functions to 24 proteins and confirmed 13 of these in human cells.

Microtubule-dependent and microtubule-independent steps in Crimean-Congo hemorrhagic fever virus replication cycle

Following binding and entry many viruses exploit the host cell cytoskeleton to ensure intracellular transport, assembly or egress. For Crimean-Congo hemorrhagic fever virus (CCHFV), the causative agent of a severe hemorrhagic disease, virus-host interactions are poorly investigated. In this study we demonstrated that drug-induced suppression of microtubule dynamics and especially microtubule disassembly, impaired CCHFV biogenesis. Our results showed that intact microtubules were required early during virus internalization, and late, during virus assembly and egress. Furthermore, disruption of microtubules resulted in reduced levels of viral RNA while preservation of microtubule dynamics was most important during viral egress. Finally, although CCHFV proteins were redistributed in drug-treated cells, the glycoprotein remained associated with the Golgi apparatus, the organelle of virus budding. Taken together, our results suggest that manipulation of microtubules affects CCHFV entry, replication, assembly and egress.

African swine fever virus

African swine fever virus (ASFV) is a large, intracytoplasmically-replicating DNA arbovirus and the sole member of the family Asfarviridae. It is the etiologic agent of a highly lethal hemorrhagic disease of domestic swine and therefore extensively studied to elucidate the structures, genes, and mechanisms affecting viral replication in the host, virus-host interactions, and viral virulence. Increasingly apparent is the complexity with which ASFV replicates and interacts with the host cell during infection. ASFV encodes novel genes involved in host immune response modulation, viral virulence for domestic swine, and in the ability of ASFV to replicate and spread in its tick vector. The unique nature of ASFV has contributed to a broader understanding of DNA virus/host interactions.

Foot-and-mouth disease virus, but not bovine enterovirus, targets the host cell cytoskeleton via the nonstructural protein 3Cpro

Foot-and-mouth disease virus (FMDV), a member of the Picornaviridae, is a pathogen of cloven-hoofed animals and causes a disease of major economic importance. Picornavirus-infected cells show changes in cell morphology and rearrangement of cytoplasmic membranes, which are a consequence of virus replication. We show here, by confocal immunofluorescence and electron microscopy, that the changes in morphology of FMDV-infected cells involve changes in the distribution of microtubule and intermediate filament components during infection. Despite the continued presence of centrosomes in infected cells, there is a loss of tethering of microtubules to the microtubule organizing center (MTOC) region. Loss of labeling for gamma-tubulin, but not pericentrin, from the MTOC suggests a targeting of gamma-tubulin (or associated proteins) rather than a total breakdown in MTOC structure. The identity of the FMDV protein(s) responsible was determined by the expression of individual viral nonstructural proteins and their precursors in uninfected cells. We report that the only viral nonstructural protein able to reproduce the loss of gamma-tubulin from the MTOC and the loss of integrity of the microtubule system is FMDV 3C(pro). In contrast, infection of cells with another picornavirus, bovine enterovirus, did not affect gamma-tubulin distribution, and the microtubule network remained relatively unaffected.

A guide to viral inclusions, membrane rearrangements, factories, and viroplasm produced during virus replication

Virus replication can cause extensive rearrangement of host cell cytoskeletal and membrane compartments leading to the “cytopathic effect” that has been the hallmark of virus infection in tissue culture for many years. Recent studies are beginning to redefine these signs of viral infection in terms of specific effects of viruses on cellular processes. In this chapter, these concepts have been illustrated by describing the replication sites produced by many different viruses. In many cases, the cellular rearrangements caused during virus infection lead to the construction of sophisticated platforms in the cell that concentrate replicase proteins, virus genomes, and host proteins required for replication, and thereby increase the efficiency of replication. Interestingly, these same structures, called virus factories, virus inclusions, or virosomes, can recruit host components that are associated with cellular defences against infection and cell stress. It is possible that cellular defence pathways can be subverted by viruses to generate sites of replication. The recruitment of cellular membranes and cytoskeleton to generate virus replication sites can also benefit viruses in other ways. Disruption of cellular membranes can, for example, slow the transport of immunomodulatory proteins to the surface of infected cells and protect against innate and acquired immune responses, and rearrangements to cytoskeleton can facilitate virus release.

Interactions of the DNA polymerase X of African swine fever virus with double-stranded DNA. Functional structure of the complex

Interactions of the polymerase X of African swine fever virus with the double-stranded DNA (dsDNA) have been studied with fluorescent dsDNA oligomers, using quantitative fluorescence titrations, analytical ultracentrifugation, and fluorescence energy transfer techniques. Studies with unmodified dsDNAs were performed, using competition titration method. ASV pol X binds the dsDNA with a site-size of n=10(+/-2) base-pairs, which is significantly shorter than the total site-size of 16(+/-2) nucleotides of the enzyme-ssDNA complex. The small site size indicates that the enzyme binds the dsDNA exclusively using the proper DNA-binding subsite. Fluorescence energy transfer studies between the tryptophan residue W92 and the acceptor, located at the 5' or 3' end of the dsDNA, suggest strongly that the proper DNA-binding subsite is located on the non-catalytic C-terminal domain. Moreover, intrinsic interactions with the dsDNA 10-mer or 20-mer are accompanied by the same net number of ions released, independent of the length of the DNA, indicating the same length of the DNA engaged in the complex. The dsDNA intrinsic affinity is about two orders of magnitude higher than the ssDNA affinity, indicating that the proper DNA-binding subsite is, in fact, the specific dsDNA-binding site. Surprisingly, ASFV pol X binds the dsDNA with significant positive cooperativity, which results from protein-protein interactions. Cooperative interactions are accompanied by the net ion release, with anions participating in the ion-exchange process. The significance of these results for ASFV pol X activity in the recognition of damaged DNA is discussed.

Centrosome and retroviruses: the dangerous liaisons

Centrosomes are the major microtubule organizing structures in vertebrate cells. They localize in close proximity to the nucleus for the duration of interphase and play major roles in numerous cell functions. Consequently, any deficiency in centrosome function or number may lead to genetic instability. Several viruses including retroviruses such as, Foamy Virus, HIV-1, JSRV, M-PMV and HTLV-1 have been shown to hamper centrosome functions for their own profit, but the outcomes are very different. Foamy viruses, HIV-1, JSRV, M-PMV and HTLV-1 use the cellular machinery to traffic towards the centrosome during early and/or late stages of the infection. In addition HIV-1 Vpr protein alters the cell-cycle regulation by hijacking centrosome functions. Enthrallingly, HTLV-1 Tax expression also targets the functions of the centrosome, and this event is correlated with centrosome amplification, aneuploidy and transformation.

Epstein-Barr virus thymidine kinase is a centrosomal resident precisely localized to the periphery of centrioles

The thymidine kinase (TK) encoded by Epstein-Barr virus (EBV) differs not only from that of the alphaherpesviruses but also from that of the gamma-2 herpesvirus subfamily. Because cellular location is frequently a determinant of regulatory function, to gain insight into additional role(s) of EBV TK and to uncover how the lymphocryptovirus and rhadinovirus enzymes differ, the subcellular localizations of EBV TK and the related cercopithecine herpesvirus-15 TK were investigated. We show that in contrast to those of the other family members, the gamma-1 herpesvirus TKs localize to the centrosome and even more precisely to the periphery of the centriole, tightly encircling the tubulin-rich centrioles in a microtubule-independent fashion. Centrosomal localization is observed in diverse cell types and occurs whether the protein is expressed independently or in the context of lytic EBV infection. Surprisingly, analysis of mutants revealed that the unique N-terminal domain was not critical for targeting to the centrosome, but rather, peptide sequences located C terminal to this domain were key. This is the first herpesvirus protein documented to reside in the centrosome, or microtubule-organizing center, an amembranous organelle that regulates the structural biology of the cell cycle through control of chromosome separation and cytokinesis. More recently, proteasome-mediated degradation of cell cycle regulatory proteins, production and loading of antigenic peptides onto HLA molecules, and transient homing of diverse virion proteins required for entry and/or egress have been shown to be coordinated at the centrosome. Potential implications of centrosomal localization for EBV TK function are discussed.

Loss of centrosome integrity induces p38-p53-p21-dependent G1-S arrest

Centrosomes organize the microtubule cytoskeleton for both interphase and mitotic functions. They are implicated in cell-cycle progression but the mechanism is unknown. Here, we show that depletion of 14 out of 15 centrosome proteins arrests human diploid cells in G1 with reduced Cdk2-cyclin A activity and that expression of a centrosome-disrupting dominant-negative construct gives similar results. Cell-cycle arrest is always accompanied by defects in centrosome structure and function (for example, duplication and primary cilia assembly). The arrest occurs from within G1, excluding contributions from mitosis and cytokinesis. The arrest requires p38, p53 and p21, and is preceded by p38-dependent activation and centrosomal recruitment of p53. p53-deficient cells fail to arrest, leading to centrosome and spindle dysfunction and aneuploidy. We propose that loss of centrosome integrity activates a checkpoint that inhibits G1-S progression. This model satisfies the definition of a checkpoint in having three elements: a perturbation that is sensed, a transducer (p53) and a receiver (p21).

African swine fever virus causes microtubule-dependent dispersal of the trans-golgi network and slows delivery of membrane protein to the plasma membrane

Viral interference with secretory cargo is a common mechanism for pathogen immune evasion. Selective down regulation of critical immune system molecules such as major histocompatibility complex (MHC) proteins enables pathogens to mask themselves from their host. African swine fever virus (ASFV) disrupts the trans-Golgi network (TGN) by altering the localization of TGN46, an organelle marker for the distal secretory pathway. Reorganization of membrane transport components may provide a mechanism whereby ASFV can disrupt the correct secretion and/or cell surface expression of host proteins. In the study reported here, we used the tsO45 temperature-sensitive mutant of the G protein of vesicular stomatitis virus to show that ASFV significantly reduces the rate at which the protein is delivered to the plasma membrane. This is linked to a general reorganization of the secretory pathway during infection and a specific, microtubule-dependent disruption of structural components of the TGN. Golgin p230 and TGN46 are separated into distinct vesicles, whereupon TGN46 is depleted. These data suggest that disruption of the TGN by ASFV can slow membrane traffic during viral infection. This may be functionally important because infection of macrophages with virulent isolates of ASFV increased the expression of MHC class I genes, but there was no parallel increase in MHC class I molecule delivery to the plasma membrane.

Infection with replication-deficient adenovirus induces changes in the dynamic instability of host cell microtubules

Adenovirus translocation to the nucleus occurs through a well characterized minus end-directed transport along microtubules. Here, we show that the adenovirus infection process has a significant impact on the stability and dynamic behavior of host cell microtubules. Adenovirus-infected cells had elevated levels of acetylated and detyrosinated microtubules compared with uninfected cells. The accumulation of modified microtubules within adenovirus-infected cells required active RhoA. Adenovirus-induced changes in microtubule dynamics were characterized at the centrosome and at the cell periphery in living cells. Adenovirus infection resulted in a transient enhancement of centrosomal microtubule nucleation frequency. At the periphery of adenovirus-infected cells, the dynamic instability of microtubules plus ends shifted toward net growth, compared with the nearly balanced growth and shortening observed in uninfected cells. In infected cells, microtubules spent more time in growth, less time in shortening, and underwent catastrophes less frequently compared with those in uninfected cells. Drug-induced inhibition of Rac1 prevented most of these virus-induced shifts in microtubule dynamic instability. These results demonstrate that adenovirus infection induces a significant stabilizing effect on host cell microtubule dynamics, which involve, but are not limited to, the activation of the RhoGTPases RhoA and Rac1.

Viral interactions with the cytoskeleton: a hitchhiker's guide to the cell

The actin and microtubule cytoskeleton play important roles in the life cycle of every virus. During attachment, internalization, endocytosis, nuclear targeting, transcription, replication, transport of progeny subviral particles, assembly, exocytosis, or cell-to-cell spread, viruses make use of different cellular cues and signals to enlist the cytoskeleton for their mission. Viruses induce rearrangements of cytoskeletal filaments so that they can utilize them as tracks or shove them aside when they represent barriers. Viral particles recruit molecular motors in order to hitchhike rides to different subcellular sites which provide the proper molecular environment for uncoating, replicating and packaging viral genomes. Interactions between subviral components and cytoskeletal tracks also help to orchestrate virus assembly, release and efficient cell-to-cell spread. There is probably not a single virus that does not use cytoskeletal and motor functions in its life cycle. Being well informed intracellular passengers, viruses provide us with unique tools to decipher how a particular cargo recruits one or several motors, how these are activated or tuned down depending on transport needs, and how cargoes switch from actin tracks to microtubules to nuclear pores and back.

DNA polymerase X from African swine fever virus: quantitative analysis of the enzyme-ssDNA interactions and the functional structure of the complex

Interactions of polymerase X from African swine fever virus with single-stranded DNA (ssDNA) have been studied, using quantitative fluorescence titration and analytical ultracentrifugation techniques. Experiments were performed with a fluorescent etheno-derivative of ssDNA oligomers. Studies of unmodified ssDNA oligomers were carried out using the competition titration method. The total site-size of the pol X-ssDNA complex is 16(+/-1) nucleotide residues. The large total ssDNA-binding site has a complex heterogeneous structure. It contains the proper ssDNA-binding site that encompasses only 7(+/-1) residues. As the length of the ssDNA increases, the enzyme engages an additional binding area in interactions with the DNA, at a distance of approximately 7-8 nucleotides from the proper site, which is located asymmetrically within the polymerase molecule. As a result, the net ion release accompanying the interactions with the DNA, increases from approximately 1 for the proper DNA-binding site to approximately 6 for the total DNA-binding site. Unlike in the case of the mammalian polymerase beta that belongs to the same polymerase X family, the DNA-binding areas within the total DNA-binding site of pol X are not autonomous. Consequently, the enzyme does not form different binding modes with different numbers of occluded nucleotide residues, although the interacting areas are structurally separated. The statistical thermodynamic model that accounts for the engagement of the proper and the total DNA-binding site in interactions with the DNA provides an excellent description of the binding process. Pol X binds the ssDNA without detectable cooperativity and with very modest base specificity.

Viral stop-and-go along microtubules: taking a ride with dynein and kinesins

Incoming viral particles move from the cell surface to sites of viral transcription and replication. By contrast, during assembly and egress, subviral nucleoprotein complexes and virions travel back to the plasma membrane. Because diffusion of large molecules is severely restricted in the cytoplasm, viruses use ATP-hydrolyzing molecular motors of the host for propelling along the microtubules, which are the intracellular highways. Recent studies have revealed that, besides travelling inside endocytic or exocytic vesicles, viral proteins interact directly with dynein or kinesin motors. Understanding the molecular mechanisms of cytoplasmic viral transport will aid in the construction of viral vectors for human gene therapy and the search for new antiviral targets.