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

Coupling viruses to dynein and kinesin-1

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

Adenovirus recruits dynein by an evolutionary novel mechanism involving direct binding to pH-primed hexon

Following receptor-mediated uptake into endocytic vesicles and escape from the endosome, adenovirus is transported by cytoplasmic dynein along microtubules to the perinuclear region of the cell. How motor proteins are recruited to viruses for their own use has begun to be investigated only recently. We review here the evidence for a role for dynein and other motor proteins in adenovirus infectivity. We also discuss the implications of recent studies on the mechanism of dynein recruitment to adenovirus for understanding the relationship between pathogenic and physiological cargo recruitment and for the evolutionary origins of dynein-mediated adenovirus transport.

Molecular organization and force-generating mechanism of dynein

Dynein, which is a minus-end-directed microtubule motor, is crucial to a range of cellular processes. The mass of its motor domain is about 10 times that of kinesin, the other microtubule motor. Its large size and the difficulty of expressing and purifying mutants have hampered progress in dynein research. Recently, however, electron microscopy, X-ray crystallography and single-molecule nanometry have shed light on several key unsolved questions concerning how the dynein molecule is organized, what conformational changes in the molecule accompany ATP hydrolysis, and whether two or three motor domains are coordinated in the movements of dynein. This minireview describes our current knowledge of the molecular organization and the force-generating mechanism of dynein, with emphasis on findings from electron microscopy and single-molecule nanometry.

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

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

Cell death-independent functions of granzymes: hit viruses where it hurts

Granule exocytosis by cytotoxic lymphocytes is the key mechanism of our immune response to eliminate virus-infected cells. These lytic granules contain the pore-forming protein perforin and a set of five serine proteases called granzymes (GrA, GrB, GrH, GrK, GrM) that display distinct substrate specificities. Granzymes have mostly been studied for their ability to induce cell death. However, viruses have evolved many inhibitors to effectively block apoptosis. Evidence is emerging that granzymes also use noncytotoxic strategies to inhibit viral replication and potential viral reactivation from latency. Granzymes directly cleave viral or host cell proteins that are required in the viral life cycle. Furthermore, granzymes induce a pro-inflammatory cytokine response to create an antiviral environment. In this review, we summarize and discuss these novel strategies by which the immune system counteracts viral infections, and we will address the potential therapeutic applications that could emerge from this intriguing mechanism.

Alphaherpesviruses and the cytoskeleton in neuronal infections

Following infection of exposed peripheral tissues, neurotropic alphaherpesviruses invade nerve endings and deposit their DNA genomes into the nuclei of neurons resident in ganglia of the peripheral nervous system. The end result of these events is the establishment of a life-long latent infection. Neuroinvasion typically requires efficient viral transmission through a polarized epithelium followed by long-distance transport through the viscous axoplasm. These events are mediated by the recruitment of the cellular microtubule motor proteins to the intracellular viral particle and by alterations to the cytoskeletal architecture. The focus of this review is the interplay between neurotropic herpesviruses and the cytoskeleton.

Single-particle tracking of hepatitis B virus-like vesicle entry into cells

HBsAg, the surface antigen of the hepatitis B virus (HBV), is used as a model to study the mechanisms and dynamics of a single-enveloped virus infecting living cells by imaging and tracking at the single-particle level. By monitoring the fluorescent indicator of HBsAg particles, it is found that HBsAg enters cells via a caveolin-mediated endocytic pathway. Tracking of individual HBsAg particles in living cells reveals the anomalously actin-dependent but not microtubule-dependent motility of the internalized HBsAg particle. The motility of HBsAg particles in living cells is also analyzed quantitatively. These results may settle the long-lasting debate of whether HBV directly breaks the plasma membrane barrier or relies on endocytosis to deliver its genome into the cell, and how the virus moves in the cell.

Pentagalloylglucose downregulates cofilin1 and inhibits HSV-1 infection

To investigate the anti-herpesvirus mechanism of pentagalloylglucose (PGG), we compared the proteomic changes between herpes simplex virus type 1 (HSV-1) infected MRC-5 cells with or without PGG-treatment, and between non-infected MRC-5 cells with or without PGG-treatment by 2-DE and MS-based analysis. Differentially expressed cellular proteins were mainly involved with actin cytoskeleton regulation. Significantly, PGG can down-regulate cofilin1, a key regulator of actin cytoskeleton dynamics. PGG can inhibit HSV-1-induced rearrangements of actin cytoskeleton which is important for infectivity. Furthermore, cofilin1 knockdown by siRNA also inhibited the HSV-1-induced actin-skeleton rearrangements. Both PGG-treatment and cofilin1 knockdown can reduce HSV-1 DNA, mRNA, protein synthesis and virus yields. Altogether, the results suggested that down-regulating cofilin1 plays a role in PGG inhibiting HSV-1 infection. PGG may be a promising anti-herpesvirus agent for drug development.

Noncytotoxic inhibition of cytomegalovirus replication through NK cell protease granzyme M-mediated cleavage of viral phosphoprotein 71

Granzyme M (GrM) is highly expressed in cytotoxic granules of NK cells, which provide the first line of defense against viral pathogens. GrM knockout mice show increased susceptibility toward murine CMV infection. Although GrM is a potent inducer of cell death, the mechanism by which GrM eliminates viruses remains elusive. In this paper, we show that purified human GrM in combination with the perforin-analog streptolysin O (SLO) strongly inhibited human CMV (HCMV) replication in fibroblasts in the absence of host cell death. In a proteomic approach, GrM was highly specific toward the HCMV proteome and most efficiently cleaved phosphoprotein 71 (pp71), an HCMV tegument protein that is critical for viral replication. Cleavage of pp71 occurred when viral lysates were incubated with purified GrM, when intact cells expressing recombinant pp71 were challenged with living cytotoxic effector cells, and when HCMV-infected fibroblasts were incubated with SLO and purified GrM. GrM directly cleaved pp71 after Leu(439), which coincided with aberrant cellular localization of both pp71 cleavage fragments as determined by confocal immunofluorescence. In a luciferase reporter assay, cleavage of pp71 after Leu(439) by GrM completely abolished the ability of pp71 to transactivate the HCMV major immediate-early promoter, which is indispensable for effective HCMV replication. Finally, GrM decreased immediate-early 1 protein expression in HCMV-infected fibroblasts. These results indicate that the NK cell protease GrM mediates cell death-independent antiviral activity by direct cleavage of a viral substrate.

Electron tomography of the supramolecular structure of virus-infected cells

Visualizing the viral life cycle in the host challenges us to extend our understanding of the viral infection mechanism. Three-dimensional images obtained by advanced electron tomographic imaging techniques, if resolved to molecular resolution, are helpful for bridging the atomic structural information of proteins to cellular events. Characteristic large structures appear in virus-infected host cells through the life cycle of various viruses. These structures are likely to provide clues to understanding viral infection mechanisms, such as how viruses move in host cells, how they are assembled, how they egress and how they spread cell-to-cell. Here we review recent advances in the studies of the molecular architecture of virus machinery involved in the mechanism of virus infection using comprehensive electron tomographic imaging techniques.

Small peptide inhibitors disrupt a high-affinity interaction between cytoplasmic dynein and a viral cargo protein

Several viruses target the microtubular motor system in early stages of the viral life cycle. African swine fever virus (ASFV) protein p54 hijacks the microtubule-dependent transport by interaction with a dynein light chain (DYNLL1/DLC8). This was shown to be a high-affinity interaction, and the residues gradually disappearing were mapped on DLC8 to define a putative p54 binding surface by nuclear magnetic resonance (NMR) spectroscopy. The potential of short peptides targeting the binding domain to disrupt this high-affinity protein-protein interaction was assayed, and a short peptide sequence was shown to bind and compete with viral protein binding to dynein. Given the complexity and number of proteins involved in cellular transport, the prevention of this viral-DLC8 interaction might not be relevant for successful viral infection. Thus, we tested the capacity of these peptides to interfere with viral infection by disrupting dynein interaction with viral p54. Using this approach, we report on short peptides that inhibit viral growth.

Interaction of viruses with host cell molecular motors

Viral particles are generally too large to diffuse freely within the crowded environment of the host cell cytoplasm. They depend on mammalian cell transport systems, in particular the microtubular molecular motor dynein, to deliver their nucleic acids to the vicinity of the nucleus. An understanding of how viruses interact with dynein, and its many accessory proteins, may reveal targets for drug discovery and will unlock the toolbox required to improve the performance of synthetic gene delivery systems.

Plus- and minus-end directed microtubule motors bind simultaneously to herpes simplex virus capsids using different inner tegument structures

Many viruses depend on host microtubule motors to reach their destined intracellular location. Viral particles of neurotropic alphaherpesviruses such as herpes simplex virus 1 (HSV1) show bidirectional transport towards the cell center as well as the periphery, indicating that they utilize microtubule motors of opposing directionality. To understand the mechanisms of specific motor recruitment, it is necessary to characterize the molecular composition of such motile viral structures. We have generated HSV1 capsids with different surface features without impairing their overall architecture, and show that in a mammalian cell-free system the microtubule motors dynein and kinesin-1 and the dynein cofactor dynactin could interact directly with capsids independent of other host factors. The capsid composition and surface was analyzed with respect to 23 structural proteins that are potentially exposed to the cytosol during virus assembly or cell entry. Many of these proteins belong to the tegument, the hallmark of all herpesviruses located between the capsid and the viral envelope. Using immunoblots, quantitative mass spectrometry and quantitative immunoelectron microscopy, we show that capsids exposing inner tegument proteins such as pUS3, pUL36, pUL37, ICP0, pUL14, pUL16, and pUL21 recruited dynein, dynactin, kinesin-1 and kinesin-2. In contrast, neither untegumented capsids exposing VP5, VP26, pUL17 and pUL25 nor capsids covered by outer tegument proteins such as vhs, pUL11, ICP4, ICP34.5, VP11/12, VP13/14, VP16, VP22 or pUS11 bound microtubule motors. Our data suggest that HSV1 uses different structural features of the inner tegument to recruit dynein or kinesin-1. Individual capsids simultaneously accommodated motors of opposing directionality as well as several copies of the same motor. Thus, these associated motors either engage in a tug-of-war or their activities are coordinately regulated to achieve net transport either to the nucleus during cell entry or to cytoplasmic membranes for envelopment during assembly.

Many viruses, particularly neurotropic alphaherpesviruses such as herpes simplex virus (HSV), require an intact microtubule network for efficient replication and pathogenesis. In living cells, host and viral cargo show rapid reversals in transport direction, suggesting that they can recruit motors of opposing directionality simultaneously. To elucidate the molecular mechanisms for specific motor-cargo recognition, it is necessary to characterize the surface of such cargos. We established a cell-free system that reconstitutes the binding of native, mammalian microtubule motors to intact tegumented HSV capsids. Our data suggest that the inbound motor dynein and the outbound motor kinesin-1 bind directly and independently of other host factors to the inner tegument that coats the capsids during cytosolic transport. Identifying viral receptors for the hosts' transport machinery will provide us on the one hand with new potential targets for antiviral therapy. On the other hand, such viral protein domains could be added to viral vectors or even to artificial nano carriers designed to deliver therapeutic genes or molecules to the nucleus or other subcellular destinations.

ICP0 dismantles microtubule networks in herpes simplex virus-infected cells

Infected-cell protein 0 (ICP0) is a RING finger E3 ligase that regulates herpes simplex virus (HSV) mRNA synthesis, and strongly influences the balance between latency and replication of HSV. For 25 years, the nuclear functions of ICP0 have been the subject of intense scrutiny. To obtain new clues about ICP0's mechanism of action, we constructed HSV-1 viruses that expressed GFP-tagged ICP0. To our surprise, both GFP-tagged and wild-type ICP0 were predominantly observed in the cytoplasm of HSV-infected cells. Although ICP0 is exclusively nuclear during the immediate-early phase of HSV infection, further analysis revealed that ICP0 translocated to the cytoplasm during the early phase where it triggered a previously unrecognized process; ICP0 dismantled the microtubule network of the host cell. A RING finger mutant of ICP0 efficiently bundled microtubules, but failed to disperse microtubule bundles. Synthesis of ICP0 proved to be necessary and sufficient to disrupt microtubule networks in HSV-infected and transfected cells. Plant and animal viruses encode many proteins that reorganize microtubules. However, this is the first report of a viral E3 ligase that regulates microtubule stability. Intriguingly, several cellular E3 ligases orchestrate microtubule disassembly and reassembly during mitosis. Our results suggest that ICP0 serves a dual role in the HSV life cycle, acting first as a nuclear regulator of viral mRNA synthesis and acting later, in the cytoplasm, to dismantle the host cell's microtubule network in preparation for virion synthesis and/or egress.

Trafficking of Sendai virus nucleocapsids is mediated by intracellular vesicles

Background

Paramyxoviruses are assembled at the plasma membrane budding sites after synthesis of all the structural components in the cytoplasm. Although viral ribonuclocapsid (vRNP) is an essential component of infectious virions, the process of vRNP translocation to assembly sites is poorly understood.

Methodology/Principal Findings

To analyze real-time trafficking of vRNPs in live infected cells, we created a recombinant Sendai virus (SeV), rSeVLeGFP, which expresses L protein fused to enhanced green fluorescent protein (eGFP). The rSeVLeGFP showed similar growth kinetics compared to wt SeV, and newly synthesized LeGFP could be detected as early as 8 h postinfection. The majority of LeGFP co-localized with other components of vRNPs, NP and P proteins, suggesting the fluorescent signals of LeGFP represent the locations of vRNPs. Analysis of LeGFP movement using time-lapse digital video microscopy revealed directional and saltatory movement of LeGFP along microtubules. Treatment of the cells with nocodazole restricted vRNP movement and reduced progeny virion production without affecting viral protein synthesis, suggesting the role of microtubules in vRNP trafficking and virus assembly. Further study with an electron microscope showed close association of vRNPs with intracellular vesicles present in infected cells. In addition, the vRNPs co-localized with Rab11a protein, which is known to regulate the recycling endocytosis pathway and Golgi-to-plasma membrane trafficking. Simultaneous movement between LeGFP and Rab11a was also observed in infected cells, which constitutively express mRFP-tagged Rab11a. Involvement of recycling endosomes in vRNP translocation was also suggested by the fact that vRNPs move concomitantly with recycling transferrin labeled with Alexa 594.

Conclusions/Significance

Collectively, our results strongly suggest a previously unrecognized involvement of the intracellular vesicular trafficking pathway in vRNP translocation and provide new insights into the transport of viral structural components to the assembly sites of enveloped viruses.

Preparation of BFV Gag antiserum and preliminary study on cellular distribution of BFV

Viruses (e.g. Human immunodeficiency virus, Human simplex virus and Prototype foamy virus) are obligate intracellular parasites and therefore depend on the cellular machinery for cellular trafficking. Bovine foamy virus (BFV) is a member of the Spumaretrovirinae subfamily of Retroviruses, however, details of its cellular trafficking remain unknown. In this study, we cloned the BFV gag gene into prokaryotic expression vector pET28a and purified the denaturalized Gag protein. The protein was used to immunize BALB/c mouse to produce antiserum, which could specifically recognize the BFV Gag protein in BFV-infected cells through western blot assay. Additionally, these results demonstrated that both the optimal and suboptimal cleavage of Gag protein occur in BFV-infected cells. Subsequently, the Gag antiserum was used to investigate subcellular localization of BFV. In immunofluorescence microscopy assays, colocalization microtubules (MTs) and assembling viral particles were clearly observed, which implied that BFV may transport along cellular MTs in host cells. Furthermore, MTs-depolymerizing assay indicated MTs were required for the efficient replication of BFV. In conclusion, our study suggests that BFV has evolved the mechanism to hijack the cellular cytoskeleton for its replication.

A quantitative measure for alterations in the actin cytoskeleton investigated with automated high-throughput microscopy

The actin cytoskeleton modulates a large variety of physiological and disease-related processes in the cell. For example, actin has been shown to be a crucial host factor for successful infection by HIV-1, but the underlying mechanistic details are still unknown. Automated approaches open up the perspective to clarify such an issue by processing many samples in a high-throughput manner. To analyze the alterations in the actin cytoskeleton within an automated setting, large-scale image acquisition and analysis were established for JC-53 cells stained for actin. As a quantitative measure in such an automated approach, we suggest a parameter called image coherency. We successfully benchmarked our analysis by calculating coherency for both a biophysical model of the actin cytoskeleton and for cells whose actin architecture had been disturbed pharmacologically by latrunculin B or cytochalasin D. We then tested the influence of HIV-1 infection on actin coherency, but observed no significant differences between uninfected and infected cells.

Microtubule-dependent retrograde transport of bovine immunodeficiency virus

Microtubules are essential components of the cytoskeleton that participate in a variety of cellular processes such as cell division and migration. In addition, there is a growing body of evidence implicating a role for microtubules in intracellular viral transport. In this study, we found that pharmacological disruption of microtubules remarkably blocked bovine immunodeficiency virus (BIV) movement from the cell periphery to the perinuclear region, a process known as retrograde transport. A similar effect was observed by inhibiting function of the microtubule-associated motor protein dynein. By yeast two-hybrid assay, we found that the capsid protein (CA) of BIV interacted with the dynein light-chain component LC8. Immunoprecipitation and GST-pulldown assays further demonstrated an interaction between CA and LC8 in mammalian cells. In addition, our data revealed LC8 as a linker between BIV particles and microtubules. Retrograde transport of BIV was significantly inhibited by knockdown of LC8 expression. Our findings present the first evidence that incoming BIV particles employ host microtubule/dynein machinery for transport towards the perinuclear region. In addition, our data indicate that the LC8-CA interaction is a potential target for the design of antiviral strategies.

Adenovirus transport via direct interaction of cytoplasmic dynein with the viral capsid hexon subunit

Early in infection, adenovirus travels to the nucleus as a naked capsid using the microtubule motor cytoplasmic dynein. How the dynein complex is recruited to viral cargo remains unclear. We find that cytoplasmic dynein and its associated proteins dynactin and NudE/NudEL, but not LIS1 or ZW10, colocalized with incoming, postendosomal adenovirus particles. However, in contrast to physiological cargos, dynein binding to adenovirus was independent of these dynein-associated proteins. Dynein itself directly interacted through its intermediate and light intermediate chains with the adenovirus capsid subunit hexon in a pH-dependent manner. Expression of hexon or injection of anti-hexon antibody inhibited virus transport but not physiological dynein function. These results identify hexon as a direct receptor for cytoplasmic dynein and demonstrate that hexon recruits dynein for transport to the nucleus by a mechanism distinct from that for physiological dynein cargo.

A role for heparan sulfate in viral surfing

Heparan sulfate (HS) moieties on cell surfaces are known to provide attachment sites for many viruses including herpes simplex virus type-1 (HSV-1). Here, we demonstrate that cells respond to HSV-1 infection by enhancing filopodia formation. Filopodia express HS and are subsequently utilized for the transport of HSV-1 virions to cell bodies in a surfing-like phenomenon, which is facilitated by the underlying actin cytoskeleton and is regulated by transient activation of a small Rho GTPase, Cdc42. We also demonstrate that interaction between a highly conserved herpesvirus envelope glycoprotein B (gB) and HS is required for surfing. A HSV-1 mutant that lacks gB fails to surf and quantum dots conjugated with gB demonstrate surfing-like movements. Our data demonstrates a novel use of a common receptor, HS, which could also be exploited by multiple viruses and quite possibly, many additional ligands for transport along the plasma membrane.

Directional transneuronal spread of α-herpesvirus infection

Most α-herpesviruses are pantropic, neuroinvasive pathogens that establish a reactivateable, latent infection in the PNS of their natural hosts. Various manifestations of herpes disease rely on extent and direction of the spread of infection between the surface epithelia and the nervous system components that innervate that surface. One aspect of such controlled spread of infection is the capacity for synaptically defined, transneuronal spread, a property that makes α-herpesviruses useful tools for determining the connectivity of neural circuits. The current understanding of intra-axonal transport and transneuronal spread of α-herpesviruses is reviewed, focusing on work with herpes simplex virus and pseudorabies virus, the available in vitro technology used to study viral transport and spread is evaluated and how certain viral mutants can be used to examine neural circuit architecture is described in this article.

Dual modes of rabies P-protein association with microtubules: a novel strategy to suppress the antiviral response

Conventional nuclear import is independent of the cytoskeleton, but recent data have shown that the import of specific proteins can be either facilitated or inhibited by microtubules (MTs). Nuclear import of the P-protein from rabies virus involves a MT-facilitated mechanism, but here, we show that P-protein is unique in that it also undergoes MT-inhibited import, with the mode of MT-interaction being regulated by the oligomeric state of the P-protein. This is the first demonstration that a protein can utilise both MT-inhibited and MT-facilitated import mechanisms, and can switch between these different modes of MT interaction to regulate its nuclear trafficking. Importantly, we show that the P-protein exploits MT-dependent mechanisms to manipulate host cell processes by switching the import of the interferon-activated transcription factor STAT1 from a conventional to a MT-inhibited mechanism. This prevents STAT1 nuclear import and signalling in response to interferon, which is vital to the host innate antiviral response. This is the first report of MT involvement in the viral subversion of interferon signalling that is central to virus pathogenicity, and identifies novel targets for the development of antiviral drugs or attenuated viruses for vaccine applications.

Rapid actin-dependent viral motility in live cells

During the course of an infection, viruses take advantage of a variety of mechanisms to travel in cells, ranging from diffusion within the cytosol to active transport along cytoskeletal filaments. To study viral motility within the intrinsically heterogeneous environment of the cell, we have developed a motility assay that allows for the global and unbiased analysis of tens of thousands of virus trajectories in live cells. Using this assay, we discovered that poliovirus exhibits anomalously rapid intracellular movement that was independent of microtubules, a common track for fast and directed cargo transport. Such rapid motion, with speeds of up to 5 microm/s, allows the virus particles to quickly explore all regions of the cell with the exception of the nucleus. The rapid, microtubule-independent movement of poliovirus was observed in multiple human-derived cell lines, but appeared to be cargo-specific. Other cargo, including a closely related picornavirus, did not exhibit similar motility. Furthermore, the motility is energy-dependent and requires an intact actin cytoskeleton, suggesting an active transport mechanism. The speed of this microtubule-independent but actin-dependent movement is nearly an order of magnitude faster than the fastest speeds reported for actin-dependent transport in animal cells, either by actin polymerization or by myosin motor proteins.

Equine herpesvirus type 1 (EHV-1) utilizes microtubules, dynein, and ROCK1 to productively infect cells

To initiate infection, equine herpesvirus type 1 (EHV-1) attaches to heparan sulfate on cell surfaces and then interacts with a putative glycoprotein D receptor(s). After attachment, virus entry occurs either by direct fusion of the virus envelope with the plasma membrane or via endocytosis followed by fusion between the virus envelope and an endosomal membrane. Upon fusion, de-enveloped virus particles are deposited into the cytoplasm and travel to the nucleus for viral replication. In this report, we examined the mechanism of EHV-1 intracellular trafficking and investigated the ability of EHV-1 to utilize specific cellular components to efficiently travel to the nucleus post-entry. Using a panel of microtubule-depolymerizing drugs and inhibitors of microtubule motor proteins, we show that EHV-1 infection is dependent on both the integrity of the microtubule network and the minus-end microtubule motor protein, dynein. In addition, we show that EHV-1 actively induces the acetylation of tubulin, a marker of microtubule stabilization, as early as 15 min post-infection. Finally, our data support a role for the cellular kinase, ROCK1, in virus trafficking to the nucleus.

Intracellular transport of human immunodeficiency virus type 1 genomic RNA and viral production are dependent on dynein motor function and late endosome positioning

Our earlier work indicated that the human immunodeficiency virus type 1 (HIV-1) genomic RNA (vRNA) is trafficked to the microtubule-organizing center (MTOC) when heterogeneous nuclear ribonucleoprotein A2/B1 is depleted from cells. Also, Rab7-interacting lysosomal protein promoted dynein motor complex, late endosome and vRNA clustering at the MTOC suggesting that the dynein motor and late endosomes were involved in vRNA trafficking. To investigate the role of the dynein motor in vRNA trafficking, dynein motor function was disrupted by small interference RNA-mediated depletion of the dynein heavy chain or by p50/dynamitin overexpression. These treatments led to a marked relocalization of vRNA and viral structural protein Gag to the cell periphery with late endosomes and a severalfold increase in HIV-1 production. In contrast, rerouting vRNA to the MTOC reduced virus production. vRNA localization depended on Gag membrane association as shown using both myristoylation and Gag nucleocapsid domain proviral mutants. Furthermore, the cytoplasmic localization of vRNA and Gag was not attributable to intracellular or internalized endocytosed virus particles. Our results demonstrate that dynein motor function is important for regulating Gag and vRNA egress on endosomal membranes in the cytoplasm to directly impact on viral production.

Ebolavirus VP35 interacts with the cytoplasmic dynein light chain 8

The viral protein VP35 of ebolavirus (EBOV) is implicated to have diverse roles in the viral life cycle. We employed a yeast two-hybrid screen to search for VP35 binding partners and identified the cytoplasmic dynein light chain (DLC8) as a protein that interacts with VP35. Mapping analysis unraveled a consensus motif, SQTQT, within VP35 through which VP35 binds to DLC8. The disruption of DLC8 binding does not affect the ability of VP35 to inhibit type I IFN production. Given that VP35 from various EBOV species interacts with DLC8, this interaction may have a role in regulating the EBOV life cycle.

Initiation of hepatitis C virus infection requires the dynamic microtubule network: role of the viral nucleocapsid protein

Early events leading to the establishment of hepatitis C virus (HCV) infection are not completely understood. We show that intact and dynamic microtubules play a key role in the initiation of productive HCV infection. Microtubules were required for virus entry into cells, as evidenced using virus pseudotypes presenting HCV envelope proteins on their surface. Studies carried out using the recent infectious HCV model revealed that microtubules also play an essential role in early, postfusion steps of the virus cycle. Moreover, low concentrations of vinblastin and nocodazol, microtubule-affecting drugs, and paclitaxel, which stabilizes microtubules, inhibited infection, suggesting that microtubule dynamic instability and/or treadmilling mechanisms are involved in HCV internalization and early transport. By protein chip and direct core-dependent pull-down assays, followed by mass spectrometry, we identified beta- and alpha-tubulin as cellular partners of the HCV core protein. Surface plasmon resonance analyses confirmed that core directly binds to tubulin with high affinity via amino acids 2-117. The interaction of core with tubulin in vitro promoted its polymerization and enhanced the formation of microtubules. Immune electron microscopy showed that HCV core associates, at least temporarily, with microtubules polymerized in its presence. Studies by confocal microscopy showed a juxtaposition of core with microtubules in HCV-infected cells. In summary, we report that intact and dynamic microtubules are required for virus entry into cells and for early postfusion steps of infection. HCV may exploit a direct interaction of core with tubulin, enhancing microtubule polymerization, to establish efficient infection and promote virus transport and/or assembly in infected cells.

AcMNPV EXON0 (AC141) which is required for the efficient egress of budded virus nucleocapsids interacts with beta-tubulin

The Autographa californica multiple nucleopolyhedrovirus (AcMNPV) encoded protein, EXON0 (AC141), is required for the efficient transport of nucleocapsids out of the nucleus for the production of budded virus (BV). To further elucidate the molecular mechanisms by which EXON0 regulates BV production, EXON0 was tagged at the N-terminus with 3x FLAG-6x His. Protein complexes were isolated by tandem affinity purification and potential EXON0 specific interacting protein partners were gel purified and identified by LC-MS/MS. This analysis showed that the cellular protein, beta-tubulin, co-purified with EXON0 which was confirmed by co-immunoprecipitation. In addition, immunofluorescence showed that EXON0 and beta-tubulin co-localized during virus infection. The microtubule inhibitors colchicine and nocodazole were used to treat AcMNPV infected Sf9 cells and results showed that BV production was reduced by over 85%. These data suggest that the egress of AcMNPV budded virus may be facilitated by the interaction of EXON0 with beta-tubulin and microtubules.

Translocation of incoming pseudorabies virus capsids to the cell nucleus is delayed in the absence of tegument protein pUL37

After fusion of the envelope of herpesvirus particles with the host cell plasma membrane, incoming nucleocapsids are transported to nuclear pores. Inner tegument proteins pUL36, pUL37, and pUS3 remain attached to the nucleocapsid after entry and therefore might mediate interactions between the nucleocapsid and cellular microtubule-associated motor proteins during transport. To assay for the role of pUL37 in this process, we constructed a pUL37-deleted pseudorabies virus mutant, PrV-DeltaUL37/UL35GFP, which expresses a fusion protein of green fluorescent protein (GFP) and the nonessential small capsid protein pUL35, resulting in the formation of fluorescently labeled capsids. Confocal laser-scanning microscopy of rabbit kidney cells infected with PrV-DeltaUL37/UL35GFP revealed that, whereas penetration was not affected in the absence of pUL37, nuclear translocation of incoming particles was delayed by approximately 1 h compared to PrV-UL35GFP, but not abolished. In contrast, phenotypically complemented pUL37-containing virions of PrV-DeltaUL37/UL35GFP exhibited wild type-like entry kinetics. Thus, the presence of pUL37 is required for rapid nuclear translocation of incoming nucleocapsids.

Transport of TMV movement protein particles associated with the targeting of RNA to plasmodesmata

The cell-to-cell movement of Tobacco mosaic virus through plasmodesmata (PD) requires virus-encoded movement protein (MP). The MP targets PD through the endoplasmic reticulum (ER)/actin network, whereas the intercellular movement of the viral RNA genome has been correlated with the association of the MP with mobile, microtubule-proximal particles in cells at the leading front of infection as well as the accumulation of the protein on the microtubule network during later infection stages. To understand how the associations of MP with ER and microtubules are functionally connected, we applied multiple marker three-dimensional confocal and time-lapse video microscopies to Nicotiana benthamiana cells expressing fluorescent MP, fluorescent RNA and fluorescent cellular markers. We report the reconstitution of MP-dependent RNA transport to PD in a transient assay. We show that transiently expressed MP occurs in association with small particles as observed during infection. The same MP accumulates in PD and mediates the transport of its messenger RNA transcript to the pore. In the cellular cortex, the particles occur at microtubule-proximal sites and can undergo ER-associated and latrunculin-sensitive movements between such sites. These and other observations suggest that the microtubule network performs anchorage and release functions for controlling the assembly and intracellular movement of MP-containing RNA transport particles in association with the ER.

Vimentin is required for dengue virus serotype 2 infection but microtubules are not necessary for this process

The present study investigated the effect of microtubules (MTs) and vimentin during dengue virus serotype 2 (DV2) infection. Immunostaining showed that DV2 infection induced MT and vimentin reorganization. Colocalization of DV2 antigens with MTs or vimentin were often observed in ECV304 cells. MT-disrupting agents could enhance DV2 release but did not affect other steps of virus replication. In contrast, disruption of vimentin inhibited DV2 infection. Our results suggest that an MT-dependent mechanism may not be necessary for DV2 infection, and MT disruption may promote DV2 release. However, vimentin is required for DV2 infection.

Harnessing biological motors to engineer systems for nanoscale transport and assembly

Living systems use biological nanomotors to build life's essential molecules--such as DNA and proteins--as well as to transport cargo inside cells with both spatial and temporal precision. Each motor is highly specialized and carries out a distinct function within the cell. Some have even evolved sophisticated mechanisms to ensure quality control during nanomanufacturing processes, whether to correct errors in biosynthesis or to detect and permit the repair of damaged transport highways. In general, these nanomotors consume chemical energy in order to undergo a series of shape changes that let them interact sequentially with other molecules. Here we review some of the many tasks that biomotors perform and analyse their underlying design principles from an engineering perspective. We also discuss experiments and strategies to integrate biomotors into synthetic environments for applications such as sensing, transport and assembly.

Transport and egress of herpes simplex virus in neurons

The mechanisms of axonal transport of the alphaherpesviruses, HSV and pseudorabies virus (PrV), in neuronal axons are of fundamental interest, particularly in comparison with other viruses, and offer potential sites for antiviral intervention or development of gene therapy vectors. These herpesviruses are transported rapidly along microtubules (MTs) in the retrograde direction from the axon terminus to the dorsal root ganglion and then anterogradely in the opposite direction. Retrograde transport follows fusion and deenvelopment of the viral capsid at the axonal membrane followed by loss of most of the tegument proteins and then binding of the capsid via one or more viral proteins (VPs) to the retrograde molecular motor dynein. The HSV capsid protein pUL35 has been shown to bind to the dynein light chain Tctex1 but is likely to be accompanied by additional dynein binding of an inner tegument protein. The mechanism of anterograde transport is much more controversial with different processes being claimed for PrV and HSV: separate transport of HSV capsid/tegument and glycoproteins versus PrV transport as an enveloped virion. The controversy has not been resolved despite application, in several laboratories, of confocal microscopy (CFM), real-time fluorescence with viruses dual labelled on capsid and glycoprotein, electron microscopy in situ and immuno-electron microscopy. Different processes for each virus seem counterintuitive although they are the most divergent in the alphaherpesvirus subfamily. Current hypotheses suggest that unenveloped HSV capsids complete assembly in the axonal growth cones and varicosities, whereas with PrV unenveloped capsids are only found travelling in a retrograde direction.

The Ezrin-radixin-moesin family member ezrin regulates stable microtubule formation and retroviral infection

We recently identified the cytoskeletal regulatory protein moesin as a novel gene that inhibits retroviral replication prior to reverse transcription by downregulation of stable microtubule formation. Here, we provide evidence that overexpression of ezrin, another closely related ezrin-radixin-moesin (ERM) family member, also blocks replication of both murine leukemia viruses and human immunodeficiency virus type 1 (HIV-1) in Rat2 fibroblasts before reverse transcription, while knockdown of endogenous ezrin increases the susceptibility of human cells to HIV-1 infection. Together, these results suggest that ERM proteins may be important determinants of retrovirus susceptibility through negative regulation of stable microtubule networks.

Lissencephaly 1 linking to multiple diseases: mental retardation, neurodegeneration, schizophrenia, male sterility, and more

Lissencephaly 1 (LIS1) was the first gene implicated in the pathogenesis of type-1 lissencephaly. More than a decade of research by multiple laboratories has revealed that LIS1 is a key node protein, which participates in several pathways, including association with the molecular motor cytoplasmic dynein, the reelin signaling pathway, and the platelet-activating factor pathway. Mutations in LIS1-interacting proteins, either in human, or in mouse models has suggested that LIS1 might play a role in the pathogenesis of numerous diseases such as male sterility, schizophrenia, neuronal degeneration, and viral infections.

Functions of human cytomegalovirus tegument proteins prior to immediate early gene expression

Proteins within the tegument layer of herpesviruses such as human cytomegalovirus (HCMV) are released into the cell upon entry when the viral envelope fuses with the cell membrane. These proteins are fully formed and active, and they mediate key events at the very start of the lytic infectious cycle, including the delivery of the viral genome to the nucleus and the initiation of viral gene expression. This review examines what is known about tegument protein function prior to the immediate early (IE) phase of the viral lytic replication cycle and identifies key questions that need to be answered to better understand how these proteins promote HCMV infection so that antiviral treatments that target these important viral regulators can be developed.

Aggresomes and pericentriolar sites of virus assembly: cellular defense or viral design?

Virus replication and virus assembly often occur in virus inclusions or virus factories that form at pericentriolar sites close to the microtubule organizing center or in specialized nuclear domains called ND10/PML bodies. Similar inclusions called aggresomes form in response to protein aggregation. Protein aggregates are toxic to cells and are transported along microtubules to aggresomes for immobilization and subsequent degradation by proteasomes and/or autophagy. The similarity between aggresomes and virus inclusions raises the possibility that viruses use aggresome pathways to concentrate cellular and viral proteins to facilitate replication and assembly. Alternatively, aggresomes may be part of an innate cellular response that recognizes virus components as foreign or misfolded and targets them for storage and degradation. Insights into the possible roles played by aggresomes during virus assembly are emerging from an understanding of how virus inclusions form and how viral proteins are targeted to them.

Actin and Rho GTPases in herpesvirus biology

Viruses have evolved a variety of interactions with host cells to create an optimal niche for viral replication, persistence and spread. The actin cytoskeleton of the host cell and actin-regulating Rho GTPase signaling pathways can be involved in several of these interactions. This review focuses on recent findings on herpesvirus interactions with actin and Rho GTPases during viral entry, replication in the nucleus and egress. Unraveling these often fascinating interactions might also provide additional insights into sometimes poorly known aspects of actin biology (e.g. its role in the nucleus) and in the development of novel antiviral therapies.

Retroviral proteins that interact with the host cell cytoskeleton

In the past decade, several lines of evidence have highlighted the importance of the host cell cytoskeleton in various stages of retroviral infection. To complete their lifecycle, retroviruses must penetrate the outer barrier of the cell membrane, and viral cores containing the viral genome must traverse the cytoplasm to the nucleus and then viral gene products must make the journey back to the cell surface in order to release new progeny. The presence of a dense cytoskeletal network and organelles in the cytoplasm creates an environment that greatly impedes diffusion of macromolecules such as viruses. As such, retroviruses have evolved means to hijack actin as well as microtubule cytoskeletal networks that regulate macromolecular movement within the host cell. Developing studies are discovering several host and viral factors that play important roles in retroviral trafficking.

Beta-tubulin cofactor D and ARL2 take part in apical junctional complex disassembly and abrogate epithelial structure

In epithelial cells, the apical junctional complex (AJC), composed of tight junctions (TJs) and adherens junctions (AJs), maintains cell-surface polarity by forming a fence that prevents lateral movement and diffusion of proteins and lipids between the apical and basolateral PM and holds the epithelial monolayer intact through cell-cell contacts. Disassembly of this complex is a prime event in development and cell transformation. Maintenance of the AJC has been shown to involve mainly the actin cytoskeleton. Recent findings also point to the involvement of the microtubule (MT) system. Here we show the first evidence that in polarized epithelial MDCK cells, ARF-like protein 2 (ARL2) and beta-tubulin cofactor D, known to be involved in MT dynamics, have a role in disassembly of the AJC followed by cell dissociation from the epithelial monolayer, which is not dependent on MT depolymerization. In addition, we show that beta-tubulin cofactor D is partially localized to the lateral PM through its 15 C-terminal amino acids and intact MTs. ARL2 inhibited beta-tubulin cofactor D-dependent cell dissociation from the monolayer and AJC disassembly. To our knowledge, this is the first evidence that beta-tubulin cofactor D plays a role in cells independent of its presumed role in folding tubulin heterodimers. We conclude that ARL2 and beta-tubulin cofactor D participate in AJC disassembly and epithelial depolarization.

A microtubule-facilitated nuclear import pathway for cancer regulatory proteins

Nuclear protein import is dependent on specific targeting signals within cargo proteins recognized by importins (IMPs) that mediate translocation through the nuclear pore. Recent evidence, however, implicates a role for the microtubule (MT) network in facilitating nuclear import of the cancer regulatory proteins parathyroid hormone-related protein (PTHrP) and p53 tumor suppressor. Here we assess the extent to which MT and actin integrity may be generally required for nuclear protein import for the first time. We examine 10 nuclear-localizing proteins with diverse IMP-dependent nuclear import pathways, our results indicating that the cytoskeleton does not have a general mechanistic role in nuclear localization sequence-dependent nuclear protein import. Of the proteins examined, only the p110(Rb) tumor suppressor protein Rb, together with p53 and PTHrP, was found to require MT integrity for optimal nuclear import. Fluorescence recovery after photobleaching experiments indicated that the MT-dependent nuclear transport pathway increases both the rate and extent of Rb nuclear import but does not affect Rb nuclear export. Dynamitin overexpression experiments implicate the MT motor dynein in the import process. The results indicate that, additional to IMP/diffusion-dependent processes, certain cancer regulatory proteins utilize an MT-enhanced pathway for accelerated nuclear import that is presumably required for their nuclear functions.

An ectromelia virus profilin homolog interacts with cellular tropomyosin and viral A-type inclusion protein

Background

Profilins are critical to cytoskeletal dynamics in eukaryotes; however, little is known about their viral counterparts. In this study, a poxviral profilin homolog, ectromelia virus strain Moscow gene 141 (ECTV-PH), was investigated by a variety of experimental and bioinformatics techniques to characterize its interactions with cellular and viral proteins.

Results

Profilin-like proteins are encoded by all orthopoxviruses sequenced to date, and share over 90% amino acid (aa) identity. Sequence comparisons show highest similarity to mammalian type 1 profilins; however, a conserved 3 aa deletion in mammalian type 3 and poxviral profilins suggests that these homologs may be more closely related. Structural analysis shows that ECTV-PH can be successfully modelled onto both the profilin 1 crystal structure and profilin 3 homology model, though few of the surface residues thought to be required for binding actin, poly(L-proline), and PIP₂ are conserved. Immunoprecipitation and mass spectrometry identified two proteins that interact with ECTV-PH within infected cells: alpha-tropomyosin, a 38 kDa cellular actin-binding protein, and the 84 kDa product of vaccinia virus strain Western Reserve (VACV-WR) 148, which is the truncated VACV counterpart of the orthopoxvirus A-type inclusion (ATI) protein. Western and far-western blots demonstrated that the interaction with alpha-tropomyosin is direct, and immunofluorescence experiments suggest that ECTV-PH and alpha-tropomyosin may colocalize to structures that resemble actin tails and cellular protrusions. Sequence comparisons of the poxviral ATI proteins show that although full-length orthologs are only present in cowpox and ectromelia viruses, an ~ 700 aa truncated ATI protein is conserved in over 90% of sequenced orthopoxviruses. Immunofluorescence studies indicate that ECTV-PH localizes to cytoplasmic inclusion bodies formed by both truncated and full-length versions of the viral ATI protein. Furthermore, colocalization of ECTV-PH and truncated ATI protein to protrusions from the cell surface was observed.

Conclusion

These results suggest a role for ECTV-PH in intracellular transport of viral proteins or intercellular spread of the virus. Broader implications include better understanding of the virus-host relationship and mechanisms by which cells organize and control the actin cytoskeleton.

Dynein light chain association sequences can facilitate nuclear protein import

Nuclear localization sequence (NLS)-dependent nuclear protein import is not conventionally held to require interaction with microtubules (MTs) or components of the MT motor, dynein. Here we report for the first time the role of sequences conferring association with dynein light chains (DLCs) in NLS-dependent nuclear accumulation of the rabies virus P-protein. We find that P-protein nuclear accumulation is significantly enhanced by its dynein light chain association sequence (DLC-AS), dependent on MT integrity and association with DLCs, and that P-protein-DLC complexes can associate with MT cytoskeletal structures. We also find that P-protein DLC-AS, as well as analogous sequences from other proteins, acts as an independent module that can confer enhancement of nuclear accumulation to proteins carrying the P-protein NLS, as well as several heterologous NLSs. Photobleaching experiments in live cells demonstrate that the MT-dependent enhancement of NLS-mediated nuclear accumulation by the P-protein DLC-AS involves an increased rate of nuclear import. This is the first report of DLC-AS enhancement of NLS function, identifying a novel mechanism regulating nuclear transport with relevance to viral and cellular protein biology. Importantly, this data indicates that DLC-ASs represent versatile modules to enhance nuclear delivery with potential therapeutic application.