Herpesvirus assembly: an update

Crystal structure of the herpesvirus inner tegument protein UL37 supports its essential role in control of viral trafficking

In cells infected with herpesviruses, two capsid-associated, or inner tegument, proteins, UL37 and UL36, control cytosolic trafficking of capsids by as yet poorly understood mechanisms. Here, we report the crystal structure of the N-terminal half of UL37 from pseudorabies virus, an alphaherpesvirus closely related to herpes simplex viruses and varicella-zoster virus. The structure--the first for any alphaherpesvirus inner tegument protein--reveals an elongated molecule of a complex architecture rich in helical bundles. To explore the function of the UL37 N terminus, we used the three-dimensional framework provided by the structure in combination with evolutionary trace analysis to pinpoint several surface-exposed regions of potential functional importance and test their importance using mutagenesis. This approach identified a novel functional region important for cell-cell spread. These results suggest a novel role for UL37 in intracellular virus trafficking that promotes spread of viral infection, a finding that expands the repertoire of UL37 functions. Supporting this, the N terminus of UL37 shares structural similarity with cellular multisubunit tethering complexes (MTCs), which control vesicular trafficking in eukaryotic cells by tethering transport vesicles to their destination membranes. Our results suggest that UL37 could be the first viral MTC mimic and provide a structural rationale for the importance of UL37 for viral trafficking. We propose that herpesviruses may have co-opted the MTC functionality of UL37 to bring capsids to cytoplasmic budding destinations and further on to cell junctions for spread to nearby cells.

IMPORTANCE

To move within an infected cell, viruses encode genes for proteins that interact with host trafficking machinery. In cells infected with herpesviruses, two capsid-associated proteins control the cytosolic movement of capsids by as yet poorly understood mechanisms. Here, we report the crystal structure for the N-terminal half of one of these proteins, UL37. Structure-based mutagenesis revealed a novel function for UL37 in virus trafficking to cell junctions for cell-cell spread. The unexpected structural similarity to components of cellular multisubunit tethering complexes, which control vesicular traffic, suggests that UL37 could be the first viral MTC mimic and provides a structural basis for the importance of UL37 for virus trafficking.

Herpes simplex virus 1 protein UL37 interacts with viral glycoprotein gK and membrane protein UL20 and functions in cytoplasmic virion envelopment

We have shown that glycoprotein K (gK) and its interacting partner, the UL20 protein, play crucial roles in virion envelopment. Specifically, virions lacking either gK or UL20 fail to acquire an envelope, thus causing accumulation of capsids in the cytoplasm of infected cells. The herpes simplex virus 1 (HSV-1) UL37 protein has also been implicated in cytoplasmic virion envelopment. To further investigate the role of UL37 in virion envelopment, the recombinant virus DC480 was constructed by insertion of a 12-amino-acid protein C (protC) epitope tag within the UL37 amino acid sequence immediately after amino acid 480. The DC480 mutant virus expressed full-size UL37 as detected by the anti-protC antibody in Western immunoblots, accumulated unenveloped capsids in the cytoplasm of infected cells, and produced very small plaques on African green monkey kidney (Vero) cells that were similar in size to those produced by the UL20-null and UL37-null viruses. The DC480 virus replicated nearly 4 log less efficiently than the parental wild-type virus when grown on Vero cells. However, DC480 mutant virus titers increased nearly 20-fold when the virus was grown on FRT cells engineered to express the UL20 gene in comparison to the titers on Vero cells, while the UL37-null virus replicated approximately 20-fold less efficiently than the DC480 virus on FRT cells. Coimmunoprecipitation experiments and proximity ligation assays showed that gK and UL20 interact with the UL37 protein in infected cells. Collectively, these results indicate that UL37 interacts with the gK-UL20 protein complex to facilitate cytoplasmic virion envelopment.

IMPORTANCE

Herpes simplex viruses acquire their final envelopes by budding into cytoplasmic membranes derived from the trans-Golgi network (TGN). The tegument proteins UL36 and UL37 are known to be transported to the TGN sites of virus envelopment and to function in virion envelopment, since mutants lacking UL37 accumulate capsids in the cytoplasm that are unable to bud into TGN membranes. Viral glycoprotein K (gK) also functions in cytoplasmic envelopment, in a protein complex with the membrane-associated protein UL20 (UL20mp). This work shows for the first time that the UL37 protein functionally interacts with gK and UL20 to facilitate cytoplasmic virion envelopment. This work may lead to the design of specific drugs that can interrupt UL37 interactions with the gK-UL20 protein complex, providing new ways to combat herpesviral infections.

Nelfinavir inhibits maturation and export of herpes simplex virus 1

Nelfinavir (NFV) is an HIV-1 protease inhibitor with demonstrated antiviral activity against herpes simplex virus 1 (HSV-1) and several other herpesviruses. However, the stages of HSV-1 replication inhibited by NFV have not been explored. In this study, we investigated the effects of NFV on capsid assembly and envelopment. We confirmed the inhibitory effects of NFV on HSV-1 replication by plaque assay and found that treatment with NFV did not affect capsid assembly, activity of the HSV-1 maturational protease, or formation of DNA-containing capsids in the nucleus. Confocal and electron microscopy studies showed that these capsids were transported to the cytoplasm but failed to complete secondary envelopment and subsequent exit from the cell. Consistent with the microscopy results, a light-scattering band corresponding to enveloped virions was not evident following sucrose gradient rate-velocity separation of lysates from drug-treated cells. Evidence of a possibly related effect of NFV on viral glycoprotein maturation was also discovered. NFV also inhibited the replication of an HSV-1 thymidine kinase mutant resistant to nucleoside analogues such as acyclovir. Given that NFV is neither a nucleoside mimic nor a known inhibitor of nucleic acid synthesis, this was expected and suggests its potential as a coinhibitor or alternate antiviral therapeutic agent in cases of resistance.

IMPORTANCE

Nelfinavir (NFV) is a clinically important antiviral drug that inhibits production of infectious HIV. It was reported to inhibit herpesviruses in cell culture. Herpes simplex virus 1 (HSV-1) infections are common and often associated with several diseases. The studies we describe here confirm and extend earlier findings by investigating how NFV interferes with HSV-1 replication. We show that early steps in virus formation (e.g., assembly of DNA-containing capsids in the nucleus and their movement into the cytoplasm) appear to be unaffected by NFV, whereas later steps (e.g., final envelopment in the cytoplasm and release of infectious virus from the cell) are severely restricted by the drug. Our findings provide the first insight into how NFV inhibits HSV-1 replication and suggest that this drug may have applications for studying the herpesvirus envelopment process. Additionally, NFV may have therapeutic value alone or in combination with other antivirals in treating herpesvirus infections.

Role of human cytomegalovirus tegument proteins in virion assembly

Like other herpesviruses, human cytomegalovirus (HCMV) contains a unique proteinaceous layer between the virion envelope and capsid, termed the tegument. Upon infection, the contents of the tegument layer are delivered to the host cell, along with the capsid and the viral genome, where they facilitate the initial stages of virus replication. The tegument proteins also play important roles in virion assembly and this dual nature makes them attractive potential targets for antiviral therapies. While our knowledge regarding tegument protein function during the initiation of infection has been the subject of intense study, their roles in assembly are much less well understood. In this review, we will focus on recent studies that highlight the functions of HCMV tegument proteins during assembly, and pose key questions for further investigation.

Murine gammaherpesvirus-68 ORF38 encodes a tegument protein and is packaged into virions during secondary envelopment

Tegument is the unique structure of a herpesvirion which occupies the space between nucleocapsid and envelope. Accumulating data have indicated that interactions among tegument proteins play a key role in virion morphogenesis. Morphogenesis of gammaherpesviruses including Kaposi’s sarcoma-associated herpesvirus (KSHV) and Epstein-Barr virus (EBV) is poorly understood due to the lack of efficient de novo lytic replication in cell culture. Murine gammaherpesvirus-68 (MHV-68) is genetically related to these two human herpesviruses and serves as an effective model to study the lytic replication of gammaherpesviruses. We previously showed that ORF33 of MHV-68 encodes a tegument protein and plays an essential role in virion maturation in the cytoplasm. However, the molecular mechanism of how ORF33 participates in virion morphogenesis has not been elucidated. In this study we demonstrated that ORF38 of MHV-68 is also a tegument protein and is localized to cytoplasmic compartments during both transient transfection and viral infection. Immuno-gold labeling assay showed that ORF38 is only present on virions that have entered the cytoplasmic vesicles, indicating that ORF38 is packaged into virions during secondary envelopment. We further showed that ORF38 co-localizes with ORF33 during viral infection; therefore, the interaction between ORF38 and ORF33 is conserved among herpesviruses. Notably, we found that although ORF33 by itself is distributed in both the nucleus and the cytoplasm, in the presence of ORF38, ORF33 is co-localized to trans-Golgi network (TGN), a site where secondary envelopment takes place.

A replication competent HSV-1(McKrae) with a mutation in the amino-terminus of glycoprotein K (gK) is unable to infect mouse trigeminal ganglia after cornea infection

PURPOSE

To determine the role of the amino terminus of herpes simplex virus-1 (HSV-1) glycoprotein K (gK) in corneal infection, neuroinvasion, and establishment of virus latency in trigeminal ganglia of mice.

METHODS

The recombinant virus HSV-1 (McKΔgK31-68) was constructed by engineering gK genes encoding gK lacking 38 amino acids immediately after the gK signal sequence. A rescued virus was also produced. Mouse eyes were scarified and infected with 10(5) plaque forming units (PFU) in each eye. Clinical signs of ocular disease were monitored daily. Thirty days postinfection trigeminal ganglia were collected and processed for quantitative PCR (qPCR) analysis of viral DNA and recovery of infectious virions by cell culture of ganglionic tissues.

RESULTS

Deletion of the amino terminus of gK encoded by the McKΔgK31-68 mutant virus did not substantially affect its replication kinetics on African green monkey kidney cells (Vero), while it reduced cell-to-cell spread. McK viral infection of scarified mouse corneas with 10(5) PFU produced severe ocular disease. In contrast, McKΔgK31-68 viral infection with 10(5) PFU produced no significant ocular disease symptoms. All ganglia from mice infected with the McK virus produced high numbers of infectious virions upon explant culture in Vero cells, in agreement with qPCR results detecting high number of HSV-1 viral DNA in ganglionic tissues. In contrast, qPCR failed to detect any viral genomes in McKΔgK31-68 ganglia, while two of the ten ganglia revealed the presence of low numbers of infectious virions upon explant culture in Vero cells.

CONCLUSIONS

The results show that the amino terminus of gK is essential for neuroinvasiveness and acute herpes keratitis in the mouse eye model. It is likely that gK is involved in efficient infection of axonal termini, since mouse eye scarification provided a direct access to the high density of neuronal axons innervating mouse corneas.

Basics of virology

Viruses are important pathogens of the nervous system. Here we describe the basic properties of viruses and the principles of virus classification, evolution, structure, and replication, with a focus on neurotropic viruses that are important neuropathogens of humans. These properties then provide the background for introductions to pathogenesis of viral diseases of the nervous system, host immune responses to virus infection, and the diagnosis and treatment of virus infections of the nervous system.

Proteomic characterization of murid herpesvirus 4 extracellular virions

Gammaherpesvirinae, such as the human Epstein-Barr virus (EBV) and the Kaposi’s sarcoma associated herpesvirus (KSHV) are highly prevalent pathogens that have been associated with several neoplastic diseases. As EBV and KSHV are host-range specific and replicate poorly in vitro, animal counterparts such as Murid herpesvirus-4 (MuHV-4) have been widely used as models. In this study, we used MuHV-4 in order to improve the knowledge about proteins that compose gammaherpesviruses virions. To this end, MuHV-4 extracellular virions were isolated and structural proteins were identified using liquid chromatography tandem mass spectrometry-based proteomic approaches. These analyses allowed the identification of 31 structural proteins encoded by the MuHV-4 genome which were classified as capsid (8), envelope (9), tegument (13) and unclassified (1) structural proteins. In addition, we estimated the relative abundance of the identified proteins in MuHV-4 virions by using exponentially modified protein abundance index analyses. In parallel, several host proteins were found in purified MuHV-4 virions including Annexin A2. Although Annexin A2 has previously been detected in different virions from various families, its role in the virion remains controversial. Interestingly, despite its relatively high abundance in virions, Annexin A2 was not essential for the growth of MuHV-4 in vitro. Altogether, these results extend previous work aimed at determining the composition of gammaherpesvirus virions and provide novel insights for understanding MuHV-4 biology.

Fluorescent tagging of VP22 in N-terminus reveals that VP22 favors Marek's disease virus (MDV) virulence in chickens and allows morphogenesis study in MD tumor cells

Marek’s disease virus (MDV) is an alpha-herpesvirus causing Marek’s disease in chickens, mostly associated with T-cell lymphoma. VP22 is a tegument protein abundantly expressed in cells during the lytic cycle, which is essential for MDV spread in culture. Our aim was to generate a pathogenic MDV expressing a green fluorescent protein (EGFP) fused to the N-terminus of VP22 to better decipher the role of VP22 in vivo and monitor MDV morphogenesis in tumors cells. In culture, rRB-1B EGFP22 led to 1.6-fold smaller plaques than the parental virus. In chickens, the rRB-1B EGFP22 virus was impaired in its ability to induce lymphoma and to spread in contact birds. The MDV genome copy number in blood and feathers during the time course of infection indicated that rRB-1B EGFP22 reached its two major target cells, but had a growth defect in these two tissues. Therefore, the integrity of VP22 is critical for an efficient replication in vivo, for tumor formation and horizontal transmission. An examination of EGFP fluorescence in rRB-1B EGFP22-induced tumors showed that about 0.1% of the cells were in lytic phase. EGFP-positive tumor cells were selected by cytometry and analyzed for MDV morphogenesis by transmission electron microscopy. Only few particles were present per cell, and all types of virions (except mature enveloped virions) were detected unequivocally inside tumor lymphoid cells. These results indicate that MDV morphogenesis in tumor cells is more similar to the morphorgenesis in fibroblastic cells in culture, albeit poorly efficient, than in feather follicle epithelial cells.

Herpes simplex virus 1 counteracts tetherin restriction via its virion host shutoff activity

The interferon-inducible membrane protein tetherin (Bst-2, or CD317) is an antiviral factor that inhibits enveloped virus release by cross-linking newly formed virus particles to the producing cell. The majority of viruses that are sensitive to tetherin restriction appear to be those that acquire their envelopes at the plasma membrane, although many viruses, including herpesviruses, envelope at intracellular membranes, and the effect of tetherin on such viruses has been less well studied. We investigated the tetherin sensitivity and possible countermeasures of herpes simplex virus 1 (HSV-1). We found that overexpression of tetherin inhibits HSV-1 release and that HSV-1 efficiently depletes tetherin from infected cells. We further show that the virion host shutoff protein (Vhs) is important for depletion of tetherin mRNA and protein and that removal of tetherin compensates for defects in replication and release of a Vhs-null virus. Vhs is known to be important for HSV-1 to evade the innate immune response in vivo. Taken together, our data suggest that tetherin has antiviral activity toward HSV-1 and that the removal of tetherin by Vhs is important for the efficient replication and dissemination of HSV-1.

Elucidation of the block to herpes simplex virus egress in the absence of tegument protein UL16 reveals a novel interaction with VP22

UL16 is a tegument protein of herpes simplex virus (HSV) that is conserved among all members of the Herpesviridae, but its function is poorly understood. Previous studies revealed that UL16 is associated with capsids in the cytoplasm and interacts with the membrane protein UL11, which suggested a "bridging" function during cytoplasmic envelopment, but this conjecture has not been tested. To gain further insight, cells infected with UL16-null mutants were examined by electron microscopy. No defects in the transport of capsids to cytoplasmic membranes were observed, but the wrapping of capsids with membranes was delayed. Moreover, clusters of cytoplasmic capsids were often observed, but only near membranes, where they were wrapped to produce multiple capsids within a single envelope. Normal virion production was restored when UL16 was expressed either by complementing cells or from a novel position in the HSV genome. When the composition of the UL16-null viruses was analyzed, a reduction in the packaging of glycoprotein E (gE) was observed, which was not surprising, since it has been reported that UL16 interacts with this glycoprotein. However, levels of the tegument protein VP22 were also dramatically reduced in virions, even though this gE-binding protein has been shown not to depend on its membrane partner for packaging. Cotransfection experiments revealed that UL16 and VP22 can interact in the absence of other viral proteins. These results extend the UL16 interaction network beyond its previously identified binding partners to include VP22 and provide evidence that UL16 plays an important function at the membrane during virion production.

Cyprinid herpesvirus 3: an interesting virus for applied and fundamental research

Cyprinid herpesvirus 3 (CyHV-3), a member of the family Alloherpesviridae is the causative agent of a lethal, highly contagious and notifiable disease in common and koi carp. The economic importance of common and koi carp industries together with the rapid spread of CyHV-3 worldwide, explain why this virus became soon after its isolation in the 1990s a subject of applied research. In addition to its economic importance, an increasing number of fundamental studies demonstrated that CyHV-3 is an original and interesting subject for fundamental research. In this review, we summarized recent advances in CyHV-3 research with a special interest for studies related to host-virus interactions.

Marek's disease virus morphogenesis

Marek's disease virus (MDV) is a highly contagious virus that induces T-lymphoma in chicken. This viral infection still circulates in poultry flocks despite the use of vaccines. With the emergence of new virulent strains in the field over time, MDV remains a serious threat to the poultry industry. More than 40 yr after MDV identification as a herpesvirus, the visualization and purification of fully enveloped infectious particles remain a challenge for biologists. The various strategies used to detect such hidden particles by electron microscopy are reviewed herein. It is now generally accepted that the production of cell-free virions only occurs in the feather follicle epithelium and is associated with viral, cellular, or both molecular determinants expressed in this tissue. This tissue is considered the only source of efficient virus shedding into the environment and therefore the origin of successful transmission in birds. In other avian tissues or permissive cell cultures, MDV replication only leads to a very low number of intracellular enveloped virions. In the absence of detectable extracellular enveloped virions in cell culture, the nature of the transmitted infectious material and its mechanisms of spread from cell to cell remain to be deciphered. An attempt is made to bring together the current knowledge on MDV morphogenesis and spread, and new approaches that could help understand MDV morphogenesis are discussed.

Glycoproteins gB and gH are required for syncytium formation but not for herpesvirus-induced nuclear envelope breakdown

Herpesvirus nucleocapsids are assembled in the nucleus, whereas maturation into infectious virions takes place in the cytosol. Since, due to their size, nucleocapsids cannot pass the nuclear pores, they traverse the nuclear envelope by vesicle-mediated transport. Nucleocapsids bud at the inner nuclear membrane into the perinuclear space, forming primary enveloped particles and are released into the cytosol after fusion of the primary envelope with the outer nuclear membrane. The nuclear egress complex (NEC), consisting of the conserved herpesvirus proteins (p)UL31 and pUL34, is required for this process, whereas the viral glycoproteins gB and gH, which are essential for fusion during penetration, are not. We recently described herpesvirus-induced nuclear envelope breakdown (NEBD) as an alternative egress pathway used in the absence of the NEC. However, the molecular details of this pathway are still unknown. It has been speculated that glycoproteins involved in fusion during entry might play a role in NEBD. By deleting genes encoding glycoproteins gB and gH from the genome of NEBD-inducing pseudorabies viruses, we demonstrate that these glycoproteins are not required for NEBD but are still necessary for syncytium formation, again emphasizing fundamental differences in herpesvirus-induced alterations at the nuclear envelopes and plasma membranes of infected cells.

The Us2 gene product of herpes simplex virus 2 is a membrane-associated ubiquitin-interacting protein

The Us2 gene encodes a tegument protein that is conserved in most members of the Alphaherpesvirinae. Previous studies on the pseudorabies virus (PRV) Us2 ortholog indicated that it is prenylated, associates with membranes, and spatially regulates the enzymatic activity of the MAP (mitogen-activated protein) kinase ERK (extracellular signal-related kinase) through direct binding and sequestration of ERK at the cytoplasmic face of the plasma membrane. Here we present an analysis of the herpes simplex virus 2 (HSV-2) Us2 ortholog and demonstrate that, like PRV Us2, HSV-2 Us2 is a virion component and that, unlike PRV Us2, it does not interact with ERK in yeast two-hybrid assays. HSV-2 Us2 lacks prenylation signals and other canonical membrane-targeting motifs yet is tightly associated with detergent-insoluble membranes and localizes predominantly to recycling endosomes. Experiments to identify cellular proteins that facilitate HSV-2 Us2 membrane association were inconclusive; however, these studies led to the identification of HSV-2 Us2 as a ubiquitin-interacting protein, providing new insight into the functions of HSV-2 Us2.

Effect of black tea extract on herpes simplex virus-1 infection of cultured cells

Background

The purpose of this investigation was to determine if black tea extract (BTE), consisting primarily of flavanol compounds called theaflavins, could inhibit herpes simplex virus type-1 (HSV-1) infection in cultured A549 (human epithelial) and Vero cells.

Methods

The effect of BTE both on A549 and Vero cultured cells and on HSV-1 was assessed by using phase contrast and fluorescent microscopy, and cell viability and proliferation assays. After establishing the maximum non-cytotoxic concentration of BTE, A549 and Vero cells and HSV-1 virions were treated with varying concentrations of BTE, respectively. A549 and Vero cells were infected with HSV-1 with green fluorescent protein (GFP) insert at the UL46 gene. The effect of infectivity was determined by viral DNA extraction followed by PCR, plaque assays, adsorption assays, and electrophoresis of PCR products.

Results

BTE was not cytotoxic to A549 and Vero cells, as confirmed by cell viability and proliferation assays, in which BTE treated groups paralleled the positive control group. For both cell lines, plaque assays and fluorescent microscopy indicated an inverse relationship between BTE concentration (from 0.14 μM – 1.4 mM) and HSV-1 infectivity. Specifically, PCR and electrophoresis showed a reduction in the viral genome following treatment with BTE. In addition, there was a noticeable decrease in the amount of viral plaques for BTE treated samples in the adsorption assays.

Conclusions

BTE consisting primarily of theaflavins is not cytotoxic and can reduce or block the production of infectious HSV-1 virions in cultured A549 and Vero cells, thus inhibiting the infectivity of the virus by interfering in the attachment, penetration and viral DNA replication of HSV-1 particles. These findings indicate that BTE enriched with theaflavins has the potential to be developed as a safe, therapeutic antiviral agent to prevent the spread of HSV-1.

Role of immediate early protein ICP27 in the differential sensitivity of herpes simplex viruses 1 and 2 to leptomycin B

Leptomycin B (LMB) is a highly specific inhibitor of CRM1, a cellular karyopherin-β that transports nuclear export signal-containing proteins from the nucleus to the cytoplasm. Previous work has shown that LMB blocks herpes simplex virus 1 (HSV-1) replication in Vero cells and that certain mutations in viral immediate early protein ICP27 can confer LMB resistance. However, little is known of the molecular mechanisms involved. Here we report that HSV-2, a close relative of HSV-1, is naturally resistant to LMB. To see whether the ICP27 gene determines this phenotypic difference, we generated an HSV-1 mutant that expresses the HSV-2 ICP27 instead of the HSV-1 protein. This recombinant was fully sensitive to LMB, indicating that one or more other viral genes must be important in determining HSV-2's LMB-resistant phenotype. In additional work, we report several findings that shed light on how HSV-1 ICP27 mutations can confer LMB resistance. First, we show that LMB treatment of HSV-1-infected cells leads to suppression of late viral protein synthesis and a block to progeny virion release. Second, we identify a novel type of ICP27 mutation that can confer LMB resistance, that being the addition of a 100-residue amino-terminal affinity purification tag. Third, by studying infections where both LMB-sensitive and LMB-resistant forms of ICP27 are present, we show that HSV-1's sensitivity to LMB is dominant to its resistance. Together, our results suggest a model in which the N-terminal portion of ICP27 mediates a nonessential activity that interferes with HSV-1 replication when CRM1 is inactive. We suggest that LMB resistance mutations weaken or abrogate this activity.

The cytomegalovirus egress proteins pUL50 and pUL53 are translocated to the nuclear envelope through two distinct modes of nuclear import

The nucleocytoplasmic export of cytomegaloviral capsids is regulated by formation of a multi-component nuclear egress complex (NEC), essentially based on viral proteins pUL50 and pUL53. In this study, the generation of recombinant human cytomegaloviruses, expressing tagged versions of pUL50 and pUL53, enabled the investigation of NEC formation in infected primary fibroblasts. For these recombinant viruses, a wild-type-like mode of pUL50-pUL53 interaction and recruitment of both proteins to the nuclear envelope could be demonstrated. Importantly, pUL50 was translocated from an initial cytoplasmic distribution to the nuclear rim, whereas pUL53 accumulated in the nucleus before attaining overall rim colocalization with pUL50. Specified experimental settings illustrated that pUL50 and pUL53 were subject to different pathways of intracellular trafficking. Importantly, a novel nuclear localization signal (NLS) could be identified and functionally verified for pUL53 (amino acids 18-27), whereas no NLS was present in pUL50. Analysis of amino acid replacement mutants further illustrated the differential modes of nuclear import of the two essential viral egress proteins. Taken together, our findings suggest a combination of classical nuclear import (pUL53) and interaction-mediated recruitment (pUL50) as the driving forces for core NEC formation and viral nuclear egress.

Polarized cell migration during cell-to-cell transmission of herpes simplex virus in human skin keratinocytes

In addition to transmission involving extracellular free particles, a generally accepted model of virus propagation is one wherein virus replicates in one cell, producing infectious particles that transmit to the next cell via cell junctions or induced polarized contacts. This mechanism of spread is especially important in the presence of neutralizing antibody, and the concept underpins analysis of virus spread, plaque size, viral and host functions, and general mechanisms of virus propagation. Here, we demonstrate a novel process involved in cell-to-cell transmission of herpes simplex virus (HSV) in human skin cells that has not previously been appreciated. Using time-lapse microscopy of fluorescent viruses, we show that HSV infection induces the polarized migration of skin cells into the site of infection. In the presence of neutralizing antibody, uninfected skin cells migrate to the initial site of infection and spread over infected cells to become infected in a spatially confined cluster containing hundreds of cells. The cells in this cluster do not undergo cytocidal cell lysis but harbor abundant enveloped particles within cells and cell-free virus within interstitial regions below the cluster surface. Cells at the base and outside the cluster were generally negative for virus immediate-early expression. We further show, using spatially separated monolayer assays, that at least one component of this induced migration is the paracrine stimulation of a cytotactic response from infected cells to uninfected cells. The existence of this process changes our concept of virus transmission and the potential functions, virus, and host factors involved.

Differential protein partitioning within the herpesvirus tegument and envelope underlies a complex and variable virion architecture

The herpesvirus virion is a multilayered structure consisting of a DNA-filled capsid, tegument, and envelope. Detailed reconstructions of the capsid are possible based on its icosahedral symmetry, but the surrounding tegument and envelope layers lack regular architecture. To circumvent limitations of symmetry-based ultrastructural reconstruction methods, a fluorescence approach was developed using single-particle imaging combined with displacement measurements at nanoscale resolution. An analysis of 11 tegument and envelope proteins defined the composition and plasticity of symmetric and asymmetric elements of the virion architecture. The resulting virion protein map ascribes molecular composition to density profiles previously acquired by traditional ultrastructural methods, and provides a way forward to examine the dynamics of the virion architecture during infection.

Interactions of the Kaposi's Sarcoma-associated herpesvirus nuclear egress complex: ORF69 is a potent factor for remodeling cellular membranes

All herpesviruses encode a complex of two proteins, referred to as the nuclear egress complex (NEC), which together facilitate the exit of assembled capsids from the nucleus. Previously, we showed that the Kaposi's sarcoma-associated herpesvirus (KSHV) NEC specified by the ORF67 and ORF69 genes when expressed in insect cells using baculoviruses for protein expression forms a complex at the nuclear membrane and remodels these membranes to generate nuclear membrane-derived vesicles. In this study, we have analyzed the functional domains of the KSHV NEC proteins and their interactions. Site-directed mutagenesis of gammaherpesvirus conserved residues revealed functional domains of these two proteins, which in many cases abolish the formation of the NEC and remodeling of nuclear membranes. Small in-frame deletions within ORF67 in all cases result in loss of the ability of the mutant protein to induce cellular membrane proliferation as well as to interact with ORF69. Truncation of the C terminus of ORF67 that resides in the perinuclear space does not impair the functions of ORF67; however, deletion of the transmembrane domain of ORF67 produces a protein that cannot induce membrane proliferation but can still interact with ORF69 in the nucleus and can be tethered to the nuclear membrane by virtue of its interaction with the wild-type-membrane-anchored ORF67. In-frame deletions in ORF69 have varied effects on NEC formation, but all abolish remodeling of nuclear membranes into circular structures. One mutant interacts with ORF67 as well as the wild-type protein but cannot function in membrane curvature and fission events that generate circular vesicles. These studies genetically confirm that ORF67 is required for cellular membrane proliferation and that ORF69 is the factor required to remodel these duplicated membranes into circular-virion-size vesicles. Furthermore, we also investigated the NEC encoded by Epstein-Barr virus (EBV). The EBV complex comprised of BFRF1 and BFLF2 was visualized at the nuclear membrane using autofluorescent protein fusions. BFRF1 is a potent inducer of membrane proliferation; however, BFLF2 cannot remodel these membranes into circular structures. What was evident is the superior remodeling activity of ORF69, which could convert the host membrane proliferations induced by BFRF1 into circular structures.

A36-dependent actin filament nucleation promotes release of vaccinia virus

Cell-to-cell transmission of vaccinia virus can be mediated by enveloped virions that remain attached to the outer surface of the cell or those released into the medium. During egress, the outer membrane of the double-enveloped virus fuses with the plasma membrane leaving extracellular virus attached to the cell surface via viral envelope proteins. Here we report that F-actin nucleation by the viral protein A36 promotes the disengagement of virus attachment and release of enveloped virus. Cells infected with the A36(YdF) virus, which has mutations at two critical tyrosine residues abrogating localised actin nucleation, displayed a 10-fold reduction in virus release. We examined A36(YdF) infected cells by transmission electron microscopy and observed that during release, virus appeared trapped in small invaginations at the plasma membrane. To further characterise the mechanism by which actin nucleation drives the dissociation of enveloped virus from the cell surface, we examined recombinant viruses by super-resolution microscopy. Fluorescently-tagged A36 was visualised at sub-viral resolution to image cell-virus attachment in mutant and parental backgrounds. We confirmed that A36(YdF) extracellular virus remained closely associated to the plasma membrane in small membrane pits. Virus-induced actin nucleation reduced the extent of association, thereby promoting the untethering of virus from the cell surface. Virus release can be enhanced via a point mutation in the luminal region of B5 (P189S), another virus envelope protein. We found that the B5(P189S) mutation led to reduced contact between extracellular virus and the host membrane during release, even in the absence of virus-induced actin nucleation. Our results posit that during release virus is tightly tethered to the host cell through interactions mediated by viral envelope proteins. Untethering of virus into the surrounding extracellular space requires these interactions be relieved, either through the force of actin nucleation or by mutations in luminal proteins that weaken these interactions.

ORF11 protein interacts with the ORF9 essential tegument protein in varicella-zoster virus infection

The tegument proteins encoded by ORF11 and ORF9 of varicella-zoster virus (VZV) are conserved among all alphaherpesvirus. We previously demonstrated that the ORF9 gene is essential, whereas ORF11 is dispensable in vitro but its deletion severely impairs VZV infection of skin xenografts in the SCID mouse model in vivo. Here we report that ORF11 protein interacts with ORF9 protein in infected cells as well as in the absence of other viral proteins, and we have mapped the ORF11 protein domain involved in their interaction. Although ORF11 is an RNA binding protein, the interaction between ORF11 and ORF9 proteins was not mediated by RNA or DNA bridging. VZV recombinants with mutations preventing ORF11 protein binding to ORF9 protein had no effect on 6-day growth kinetics based on plaque numbers, but plaque sizes were reduced in vitro. However, disruption of the ORF11 and ORF9 protein interaction was associated with failure to replicate in skin xenografts in vivo. Further, we demonstrate that in the absence of their interaction, the ORF9 protein displays an identical cellular localization, accumulating in the trans-Golgi region, whereas the ORF11 protein exhibits aberrant localization, dispersing throughout the cytoplasm. Overall, our observations suggest that while complete tegument assembly may not be necessary for VZV replication in vitro, the interaction between the ORF11 and ORF9 proteins appears to be critical for the proper localization of ORF11 protein to the assembly complex and for production of infectious virus during VZV pathogenesis in skin.

Analysis of the early steps of herpes simplex virus 1 capsid tegumentation

Herpes simplex virus type 1 particles are multilayered structures with a DNA genome surrounded by a capsid, tegument, and envelope. While the protein content of mature virions is known, the sequence of addition of the tegument and the intracellular compartments where this occurs are intensely debated. To probe this process during the initial stages of egress, we used two approaches: an in vitro nuclear egress assay, which reconstitutes the exit of nuclear capsids to the cytoplasm, and a classical nuclear capsid sedimentation assay. As anticipated, in vitro cytoplasmic capsids did not harbor UL34, UL31, or viral glycoproteins but contained US3. In agreement with previous findings, both nuclear and in vitro capsids were positive for ICP0 and ICP4. Unexpectedly, nuclear C capsids and cytoplasmic capsids produced in vitro without any cytosolic viral proteins also scored positive for UL36 and UL37. Immunoelectron microscopy confirmed that these tegument proteins were closely associated with nuclear capsids. When cytosolic viral proteins were present in the in vitro assay, no additional tegument proteins were detected on the capsids. As previously reported, the tegument was sensitive to high-salt extraction but, surprisingly, was stabilized by exogenous proteins. Finally, some tegument proteins seemed partially lost during egress, while others possibly were added at multiple steps or modified along the way. Overall, an emerging picture hints at the early coating of capsids with up to 5 tegument proteins at the nuclear stage, the shedding of some viral proteins during nuclear egress, and the acquisition of others tegument proteins during reenvelopment.

Directional spread of alphaherpesviruses in the nervous system

Alphaherpesviruses are pathogens that invade the nervous systems of their mammalian hosts. Directional spread of infection in the nervous system is a key component of the viral lifecycle and is critical for the onset of alphaherpesvirus-related diseases. Many alphaherpesvirus infections originate at peripheral sites, such as epithelial tissues, and then enter neurons of the peripheral nervous system (PNS), where lifelong latency is established. Following reactivation from latency and assembly of new viral particles, the infection typically spreads back out towards the periphery. These spread events result in the characteristic lesions (cold sores) commonly associated with herpes simplex virus (HSV) and herpes zoster (shingles) associated with varicella zoster virus (VZV). Occasionally, the infection spreads transsynaptically from the PNS into higher order neurons of the central nervous system (CNS). Spread of infection into the CNS, while rarer in natural hosts, often results in severe consequences, including death. In this review, we discuss the viral and cellular mechanisms that govern directional spread of infection in the nervous system. We focus on the molecular events that mediate long distance directional transport of viral particles in neurons during entry and egress.

Mapping of sequences in Pseudorabies virus pUL34 that are required for formation and function of the nuclear egress complex

The nuclear egress complex (NEC) is required for efficient translocation of newly synthesized herpesvirus nucleocapsids from the nucleus to the cytosol. It consists of the type II membrane protein pUL34 which interacts with pUL31 at the inner nuclear membrane (INM). To map regions within pUL34 required for nuclear membrane targeting and pUL31 interaction, we constructed deletion/substitution mutations. Previously, we showed that 50 C-terminal amino acids (aa) of pseudorabies virus (PrV) pUL34, including the transmembrane domain, could be functionally replaced by cellular lamina-associated polypeptide 2β (Lap2β) sequences. In contrast, replacement of the C-terminal 100 aa abrogated complementation but not pUL31 interaction. To further delineate essential sequences within this region, C-terminal pUL34 truncations of 60, 70, 80, 85, and 90 aa fused to Lap2β sequences were generated. While truncations up to 85 aa were functional, deletion of the C-terminal 90 aa abrogated function, which indicates that the important region is located between aa 171 and 176. Amino acids 173 to 175 represent RQR, a motif suggested to mediate INM targeting. Mutagenesis to RQG revealed that the mutant protein exhibited pronounced Golgi localization, but a fraction still reached the INM. Deletion mutations in the N-terminal domain of pUL34 demonstrated that absence of the first 4 aa was tolerated, while removal of 9 or more residues resulted in a nonfunctional protein. In addition, mutation of three conserved cysteines did not abrogate pUL34 function, whereas alteration of a conserved glutamine/tyrosine sequence yielded a nonfunctional protein.

Characterization of herpes simplex virus type 1 L-particle assembly and egress in hippocampal neurones by electron cryo-tomography

Visualizing virus-host interactions in situ inside infected cells by electron cryo-tomography provides unperturbed snapshots of the infection process. Here we focus on the assembly and egress pathway of herpesviruses. Cells infected with herpes simplex virus 1 produce and release not only infective virions but also non-infectious light particles (L-particles). L-particles are devoid of viral capsids and genomes. In this study, we analysed L-particle assembly and egress pathways in cultured dissociated hippocampus neurones by electron cryo-tomography. Virion and L-particle formation occurred in close proximity, suggesting shared assembly and exit pathways. Clathrin-like coats were occasionally associated with L-particle and virion assembly sites. Further, we compared the three-dimensional ultrastructure of intracellular and extracellular L-particles and quantified their diameters and the abundance of inclusion bodies contained.

An endothelial cell-specific requirement for the UL133-UL138 locus of human cytomegalovirus for efficient virus maturation

Human cytomegalovirus (HCMV) infects a variety of cell types in humans, resulting in a varied pathogenesis in the immunocompromised host. Endothelial cells (ECs) are considered an important target of HCMV infection that may contribute to viral pathogenesis. Although the viral determinants important for entry into ECs are well defined, the molecular determinants regulating postentry tropism in ECs are not known. We previously identified the UL133-UL138 locus encoded within the clinical strain-specific ULb' region of the HCMV genome as important for the latent infection in CD34(+) hematopoietic progenitor cells (HPCs). Interestingly, this locus, while dispensable for replication in fibroblasts, was required for efficient replication in ECs infected with the TB40E or fusion-inducing factor X (FIX) HCMV strains. ECs infected with a virus lacking the entire locus (UL133-UL138(NULL) virus) complete the immediate-early and early phases of infection but are defective for infectious progeny virus production. ECs infected with UL133-UL138(NULL) virus exhibited striking differences in the organization of intracellular membranes and in the assembly of mature virions relative to ECs infected with wild-type (WT) virus. In UL133-UL138(NULL) virus-infected ECs, Golgi stacks were disrupted, and the viral assembly compartment characteristic of HCMV infection failed to form. Further, progeny virions in UL133-UL138(NULL) virus-infected ECs inefficiently acquired the virion tegument and secondary envelope. These defects were specific to infection in ECs and not observed in fibroblasts infected with UL133-UL138(NULL) virus, suggesting an EC-specific requirement for the UL133-UL138 locus for late stages of replication. To our knowledge, the UL133-UL138 locus represents the first cell-type-dependent, postentry tropism determinant required for viral maturation.

Cytosolic herpes simplex virus capsids not only require binding inner tegument protein pUL36 but also pUL37 for active transport prior to secondary envelopment

As the inner tegument proteins pUL36 and pUL37 of alphaherpesviruses may contribute to efficient intracellular transport of viral particles, we investigated their role in cytosolic capsid motility during assembly of herpes simplex virus type 1 (HSV1). As reported previously for pUL36, untagged pUL37 and UL37GFP bound to cytosolic capsids before these acquired outer tegument and envelope proteins. Capsids tagged with CheVP26 analysed by live cell imaging were capable of directed long-distance cytoplasmic transport during the assembly of wild-type virions, while capsids of the HSV1-ΔUL37 or HSV1-ΔUL36 deletion mutants showed only random, undirected motion. The HSV1-ΔUL37 phenotype was restored when UL37GFP had been overexpressed prior to infection. Quantitative immunoelectron microscopy revealed that capsids of HSV1-ΔUL37 still recruited pUL36, whereas pUL37 did not colocalize with capsids of HSV1-ΔUL36. Nevertheless, the cytosolic capsids of neither mutant could undergo secondary envelopment. Our data suggest that pUL36 and pUL37 are important prior to their functions in linking the inner to the outer tegument. Efficient capsid transport to the organelle of secondary envelopment requires recruitment ofpUL37 onto capsids, most likely via its interaction with pUL36, while capsid-associated pUL36 alone is insufficient.

Role of the small GTPase Rab27a during herpes simplex virus infection of oligodendrocytic cells

BACKGROUND

The morphogenesis of herpes simplex virus type 1 (HSV-1) comprises several events, of which some are not completely understood. It has been shown that HSV-1 glycoproteins accumulate in the trans-Golgi network (TGN) and in TGN-derived vesicles. It is also accepted that HSV-1 acquires its final morphology through a secondary envelopment by budding into TGN-derived vesicles coated with viral glycoproteins and tegument proteins. Nevertheless, several aspects of this process remain elusive. The small GTPase Rab27a has been implicated in regulated exocytosis, and it seems to play a key role in certain membrane trafficking events. Rab27a also seems to be required for human cytomegalovirus assembly. However, despite the involvement of various Rab GTPases in HSV-1 envelopment, there is, to date, no data reported on the role of Rab27a in HSV-1 infection.

RESULTS

Herein, we show that Rab27a colocalized with GHSV-UL46, a tegument-tagged green fluorescent protein-HSV-1, in the TGN. In fact, this small GTPase colocalized with viral glycoproteins gH and gD in that compartment. Functional analysis through Rab27a depletion showed a significant decrease in the number of infected cells and viral production in Rab27a-silenced cells.

CONCLUSIONS

Altogether, our results indicate that Rab27a plays an important role in HSV-1 infection of oligodendrocytic cells.

Function of glycoprotein E of herpes simplex virus requires coordinated assembly of three tegument proteins on its cytoplasmic tail

Glycoprotein E (gE) of HSV plays a key role in cell-to-cell spread and virus-induced cell fusion. Here, we report that this function of gE requires the cooperation of tegument proteins UL11, UL16, and UL21. We found that the four proteins come together with very high efficiency to form a complex in transfected cells and in a manner that is regulated and coordinated. In particular, the inefficient interaction of UL16 with each membrane protein (UL11 and gE) observed in pairwise transfections became efficient when other binding partners were present. The significance of these interactions was revealed in studies of viral mutants, which showed that each of these tegument proteins is critical for processing, transport, and biological activity of gE. These findings provide insights into the mechanisms of how gE executes its function and also have implications in understanding HSV assembly and budding.

The way out: what we know and do not know about herpesvirus nuclear egress

Herpesvirus capsids are assembled in the nucleus of infected cells whereas final maturation occurs in the cytosol. To access the final maturation compartment, intranuclear capsids have to cross the nuclear envelope which represents a formidable barrier. They do so by budding at the inner nuclear membrane, thereby forming a primary enveloped particle residing in the perinuclear cleft. Formation of primary envelopes is driven by a heterodimeric complex of two conserved herpesviral proteins, designated in the herpes simplex virus nomenclature as pUL34, a tail-anchored transmembrane protein located in the nuclear envelope, and pUL31. This nuclear egress complex recruits viral and cellular kinases to soften the nuclear lamina and allowing access of capsids to the inner nuclear membrane. How capsids are recruited to the budding site and into the primary virus particle is still not completely understood, nor is the composition of the primary enveloped virion in the perinuclear cleft. Fusion of the primary envelope with the outer nuclear membrane then results in translocation of the capsid to the cytosol. This fusion event is clearly different from fusion during infectious entry of free virions into target cells in that it does not require the conserved essential core herpesvirus fusion machinery. Nuclear egress can thus be viewed as a vesicle (primary envelope)-mediated transport of cargo (capsids) through thenuclear envelope, a process which had been unique in cell biology. Only recently has a similar process been identified in Drosophila for nuclear egress of large ribonucleoprotein complexes. Thus, herpesviruses appear to subvert a hitherto cryptic cellular pathway for translocation of capsids from the nucleus to the cytosol.

Characterization of conserved region 2-deficient mutants of the cytomegalovirus egress protein pM53

Dominant-negative (DN) mutants are powerful tools for studying essential protein-protein interactions. A systematic genetic screen of the essential murine cytomegalovirus (MCMV) protein pM53 identified the accumulation of inhibitory mutations within conserved region 2 (CR2) and CR4. The strong inhibitory potential of these CR4 mutants is characterized by a particular phenotype. The DN effect of the small insertion mutations in CR2 was too weak to analyze (M. Popa, Z. Ruzsics, M. Lötzerich, L. Dölken, C. Buser, P. Walther, and U. H. Koszinowski, J. Virol. 84:9035-9046, 2010); therefore, the present study describes the construction of M53 alleles lacking CR2 (either completely or partially) and subsequent examination of the DN effect on MCMV replication upon conditional expression. Overexpression of CR2-deficient pM53 inhibited virus production by about 10,000-fold. This was due to interference with capsid export from the nucleus and viral genome cleavage/packaging. In addition, the fate of the nuclear envelopment complex in the presence of DN pM53 overexpression was analyzed. The CR2 mutants were able to bind to pM50, albeit to a lesser extent than the wild-type protein, and relocalized the wild-type nuclear envelope complex in infected cells. Unlike the CR4 DN, the CR2 DN mutants did not affect the stability of pM50.

A network of protein interactions around the herpes simplex virus tegument protein VP22

Assembly of the herpesvirus tegument is poorly understood but is believed to involve interactions between outer tegument proteins and the cytoplasmic domains of envelope glycoproteins. Here, we present the detailed characterization of a multicomponent glycoprotein-tegument complex found in herpes simplex virus 1 (HSV-1)-infected cells. We demonstrate that the tegument protein VP22 bridges a complex between glycoprotein E (gE) and glycoprotein M (gM). Glycoprotein I (gI), the known binding partner of gE, is also recruited into this gE-VP22-gM complex but is not required for its formation. Exclusion of the glycoproteins gB and gD and VP22's major binding partner VP16 demonstrates that recruitment of virion components into this complex is highly selective. The immediate-early protein ICP0, which requires VP22 for packaging into the virion, is also assembled into this gE-VP22-gM-gI complex in a VP22-dependent fashion. Although subcomplexes containing VP22 and ICP0 can be formed when either gE or gM are absent, optimal complex formation requires both glycoproteins. Furthermore, and in line with complex formation, neither of these glycoproteins is individually required for VP22 or ICP0 packaging into the virion, but deletion of gE and gM greatly reduces assembly of both VP22 and ICP0. Double deletion of gE and gM also results in small plaque size, reduced virus yield, and defective secondary envelopment, similar to the phenotype previously shown for pseudorabies virus. Hence, we suggest that optimal gE-VP22-gM-gI-ICP0 complex formation correlates with efficient virus morphogenesis and spread. These data give novel insights into the poorly understood process of tegument acquisition.

MicroRNAs and unusual small RNAs discovered in Kaposi's sarcoma-associated herpesvirus virions

It is widely held that any given virus uses only one type of nucleic acid for genetic information storage. However, this consensus has been challenged slightly by several recent studies showing that many RNA species are present within a range of DNA viruses that include Kaposi's sarcoma-associated herpesvirus (KSHV). RNAs extracted from purified DNA virus particles exhibit great diversity in terms of length, abundance, temporal expression, cellular localization, and coding capacity during viral infection. In addition to known RNA species, the current study showed that small regulatory RNAs were present in KSHV virions. A large number of viral and cellular microRNAs (miRNAs), as well as unusual small RNAs (usRNAs), were detected in KSHV virions by using deep sequencing. Both viral and host miRNAs detected in small RNAs extracted from KSHV virions were further shown to colocalize with KSHV virions directly by in situ hybridization (ISH)-electron microscopy (EM) (ISH-EM). Some of these miRNAs were differentially present in the host cells and KSHV virions, suggesting that they are not randomly present in KSHV virions. The virional miRNAs could be transported into host cells, and they are biologically functional during de novo viral infection. Our study revealed miRNAs and usRNAs as a novel group of components in KSHV virions.

The ESCRT machinery is recruited by the viral BFRF1 protein to the nucleus-associated membrane for the maturation of Epstein-Barr Virus

The cellular endosomal sorting complex required for transport (ESCRT) machinery participates in membrane scission and cytoplasmic budding of many RNA viruses. Here, we found that expression of dominant negative ESCRT proteins caused a blockade of Epstein-Barr virus (EBV) release and retention of viral BFRF1 at the nuclear envelope. The ESCRT adaptor protein Alix was redistributed and partially colocalized with BFRF1 at the nuclear rim of virus replicating cells. Following transient transfection, BFRF1 associated with ESCRT proteins, reorganized the nuclear membrane and induced perinuclear vesicle formation. Multiple domains within BFRF1 mediated vesicle formation and Alix recruitment, whereas both Bro and PRR domains of Alix interacted with BFRF1. Inhibition of ESCRT machinery abolished BFRF1-induced vesicle formation, leading to the accumulation of viral DNA and capsid proteins in the nucleus of EBV-replicating cells. Overall, data here suggest that BFRF1 recruits the ESCRT components to modulate nuclear envelope for the nuclear egress of EBV.

Improper tagging of the non-essential small capsid protein VP26 impairs nuclear capsid egress of herpes simplex virus

To analyze the subcellular trafficking of herpesvirus capsids, the small capsid protein has been labeled with different fluorescent proteins. Here, we analyzed the infectivity of several HSV1(17(+)) strains in which the N-terminal region of the non-essential small capsid protein VP26 had been tagged at different positions. While some variants replicated with similar kinetics as their parental wild type strain, others were not infectious at all. Improper tagging resulted in the aggregation of VP26 in the nucleus, prevented efficient nuclear egress of viral capsids, and thus virion formation. Correlative fluorescence and electron microscopy showed that these aggregates had sequestered several other viral proteins, but often did not contain viral capsids. The propensity for aggregate formation was influenced by the type of the fluorescent protein domain, the position of the inserted tag, the cell type, and the progression of infection. Among the tags that we have tested, mRFPVP26 had the lowest tendency to induce nuclear aggregates, and showed the least reduction in replication when compared to wild type. Our data suggest that bona fide monomeric fluorescent protein tags have less impact on proper assembly of HSV1 capsids and nuclear capsid egress than tags that tend to dimerize. Small chemical compounds capable of inducing aggregate formation of VP26 may lead to new antiviral drugs against HSV infections.

Analysis of herpes simplex virus type I nuclear particles by flow cytometry

Flow cytometry has been instrumental to characterize cell populations and examine their inner molecules and processes. In most instances, whole cells are analyzed, and hence, particle size is not an issue. Viruses are 2-3 orders of magnitude smaller than cells so flow cytometry has typically been used to study viral markers within whole infected cells. However, the ability to separate and purify viral particles representing different maturation stages within a viral life cycle would be a useful tool to analyze them in details and characterize the host proteins they associate with. Herpes simplex virus Type 1 is a 250 nm enveloped DNA virus that replicates in the nucleus where it assembles new viral particles called capsids. These capsids eventually travel to the cell surface and are modified along the way, producing several intermediate particles. In the nucleus, three types of stable nonenveloped 125 nm nuclear capsids exist that differ in protein composition and genome content. This includes so-called nuclear C-capsids that are the precursors of mature extracellular virions. We report that we can apply flow cytometry to sort these nuclear C-capsid intermediates by labeling the viral genome with Syto 13, a fluorescent marker that binds to nucleic acids. This is the first time flow cytometry has been used not only to detect but also to purify an intracellular viral maturation intermediate. This opens new research avenues in virology to study capsid assembly, maturation and egress, analyze mutant phenotypes, and define host factors associated with specific viral intermediates.

Internal catalase protects herpes simplex virus from inactivation by hydrogen peroxide

Herpes simplex virus 1 (HSV-1) was shown to contain catalase, an enzyme able to detoxify hydrogen peroxide by converting it to water and oxygen. Studies with a catalase inhibitor indicated that virus-associated catalase can have a role in protecting the virus from oxidative inactivation. HSV-1 was found to be more sensitive to killing by hydrogen peroxide in the presence of a catalase inhibitor than in its absence. The results suggest a protective role for catalase during the time HSV-1 spends in the oxidizing environment outside a host cell.

Proteomic characterization of bovine herpesvirus 4 extracellular virions

Gammaherpesviruses are important pathogens in human and animal populations. During early events of infection, these viruses manipulate preexisting host cell signaling pathways to allow successful infection. The different proteins that compose viral particles are therefore likely to have critical functions not only in viral structures and in entry into target cell but also in evasion of the host's antiviral response. In this study, we analyzed the protein composition of bovine herpesvirus 4 (BoHV-4), a close relative of the human Kaposi's sarcoma-associated herpesvirus. Using mass spectrometry-based approaches, we identified 37 viral proteins associated with extracellular virions, among which 24 were resistant to proteinase K treatment of intact virions. Analysis of proteins associated with purified capsid-tegument preparations allowed us to define protein localization. In parallel, in order to identify some previously undefined open reading frames, we mapped peptides detected in whole virion lysates onto the six frames of the BoHV-4 genome to generate a proteogenomic map of BoHV-4 virions. Furthermore, we detected important glycosylation of three envelope proteins: gB, gH, and gp180. Finally, we identified 38 host proteins associated with BoHV-4 virions; 15 of these proteins were resistant to proteinase K treatment of intact virions. Many of these have important functions in different cellular pathways involved in virus infection. This study extends our knowledge of gammaherpesvirus virions composition and provides new insights for understanding the life cycle of these viruses.

Herpesviruses exploit several host compartments for envelopment

Enveloped viruses acquire their host-derived membrane at a variety of intracellular locations. Herpesviruses are complex entities that undergo several budding and fusion events during an infection. All members of this large family are believed to share a similar life cycle. However, they seemingly differ in terms of acquisition of their mature envelope. Herpes simplex virus is often believed to bud into an existing intracellular compartment, while the related cytomegalovirus may acquire its final envelope from a novel virus-induced assembly compartment. This review focuses on recent advances in the characterization of cellular compartment(s) potentially contributing to herpes virion final envelopment. It also examines the common points between seemingly distinct envelopment pathways and highlights the dynamic nature of intracellular compartments in the context of herpesvirus infections.

NEX-TRAP, a novel method for in vivo analysis of nuclear export of proteins

Transport of proteins between cytoplasm and nucleus is mediated by transport factors of the importin α- and β-families and occurs along a gradient of the small GTPase Ran. To date, in vivo analysis as well as prediction of protein nuclear export remain tedious and difficult. We generated a novel bipartite assay called NEX-TRAP (Nuclear EXport Trapped by RAPamycin) for in vivo analysis of protein nuclear export. The assay is based on the rapamycin-induced dimerization of the modules FRB (FK506-rapamycin (FR)-binding domain) and FKBP (FK506-binding protein-12): a potential nuclear export cargo is fused to FRB, to EYFP for direct visualization as well as to an SV40-derived nuclear localization signal (NLS) for constitutive nuclear import. An integral membrane protein that resides at the trans Golgi network (TGN) is fused to a cytoplasmically exposed FKBP and serves as reporter. EYFP-NLS-FRB fusion proteins with export activity accumulate in the nucleus at steady state but continuously shuttle between nucleus and cytoplasm. Rapamycin-induced dimerization of FRB and FKBP at the TGN traps the shuttling protein outside of the nucleus, making nuclear export permanent. Using several example cargoes, we show that the NEX-TRAP is superior to existing assays owing to its ease of use, its sensitivity and accuracy. Analysis of large numbers of export cargoes is facilitated by recombinational cloning. The NEX-TRAP holds the promise of applicability in automated fluorescence imaging for systematic analysis of nuclear export, thereby improving in silico prediction of nuclear export sequences.

Proteomic footprints of a member of Glossinavirus (Hytrosaviridae): an expeditious approach to virus control strategies in tsetse factories

The Glossinavirus (Glossina pallidipes salivary gland hypertrophy virus (GpSGHV)) is a rod-shaped enveloped insect virus containing a 190,032 bp-long, circular dsDNA genome. The virus is pathogenic for the tsetse fly Glossina pallidipes and has been associated with the collapse of selected mass-reared colonies. Maintenance of productive fly colonies is critical to tsetse and trypanosomiasis eradication in sub-Saharan Africa using the Sterile Insect Technique. Proteomics, an approach to define the expressed protein complement of a genome, was used to further our understanding of the protein composition, morphology, morphogenesis and pathology of GpSGHV. Additionally, this approach provides potential targets for novel and sustainable molecular-based antiviral strategies to control viral infections in tsetse colonies. To achieve this goal, identification of key protein partners involved in virus transmission is required. In this review, we integrate the available data on GpSGHV proteomics to assess the impact of viral infections on host metabolism and to understand the contributions of such perturbations to viral pathogenesis. The relevance of the proteome findings to tsetse and trypanosomiasis management in sub-Sahara Africa is also considered.

A herpes simplex virus 1 (McKrae) mutant lacking the glycoprotein K gene is unable to infect via neuronal axons and egress from neuronal cell bodies

We have shown that the herpes simplex virus 1 (HSV-1) gK gene is essential for efficient replication and spread in the corneal epithelium and trigeminal ganglion neuroinvasion in mice (A. T. David, A. Baghian, T. P. Foster, V. N. Chouljenko, and K. G. Kousoulas, Curr. Eye Res. 33:455-467, 2008). To further investigate the role of gK in neuronal infection, we utilized a microfluidic chamber system separating neuronal cell bodies and axonal termini. HSV-1 (McKrae) engineered virus constitutively expressing enhanced green fluorescence protein (GFP) was efficiently transmitted in both a retrograde and an anterograde manner. These results were corroborated by expression of virion structural proteins in either chamber, as well as detection of viral genomes and infectious viruses. In contrast, efficient infection of either chamber with a gK-null virus did not result in infection of the apposed chamber. These results show that gK is an important determinant in virion axonal infection. Moreover, the inability of the gK-null virus to be transmitted in an anterograde manner suggests that virions acquire cytoplasmic envelopes prior to entering axons.

IMPORTANCE

Herpes simplex virus 1 (HSV-1) enters mucosal epithelial cells and neurons via fusion of the viral envelope with cellular membranes, mediated by viral glycoprotein B (gB) in cooperation with other viral glycoproteins. Retrograde transport of virions to neuronal cell bodies (somata) establishes lifelong latent infection in ganglionic neurons. We have previously reported that gK binds gB and is required for gB-mediated membrane fusion (Jambunatathan et al., J. Virol. 85:12910-12918, 2011; V. N. Chouljenko, A. V. Iyer, S. Chowdhury, J. Kim, and K. G. Kousoulas, J. Virol. 84:8596-8606, 2010). In the current study, we constructed a recombinant virus with the gK gene deleted in the highly virulent ocular HSV-1 strain McKrae. This recombinant virus failed to infect rat ganglionic neuronal axons alone or cocultured with Vero cells in microfluidic chambers. In addition, lack of gK expression prevented anterograde transmission of virions. These results suggest that gK is a critical determinant for neuronal infection and transmission.

Making the case: married versus separate models of alphaherpes virus anterograde transport in axons

Alphaherpesvirus virions infect neurons and are transported in axons for long distance spread within the host nervous system. The assembly state of newly made herpesvirus particles during anterograde transport in axons is an essential question in alphaherpesvirus biology. The structure of the particle has remained both elusive and controversial for the past two decades, with conflicting evidence from EM, immunofluorescence, and live cell imaging studies. Two opposing models have been proposed-the Married and Separate Models. Under the Married Model, infectious virions are assembled in the neuronal cell body before sorting into axons and then traffic inside a transport vesicle. Conversely, the Separate Model postulates that vesicles containing viral membrane proteins are sorted into axons independent of capsids, with final assembly of mature virions occurring at a distant egress site. Recently, a complementary series of studies employing high-resolution EM and live cell fluorescence microscopy have provided evidence consistent with the Married Model, whereas other studies offer evidence supporting the Separate Model. In this review, we compare and discuss the published data and attempt to reconcile divergent findings and interpretations as they relate to these models.

Characterization and intracellular trafficking of Epstein-Barr virus BBLF1, a protein involved in virion maturation

Epstein-Barr virus (EBV) BBLF1 shares 13 to 15% amino acid sequence identities with the herpes simplex virus 1 UL11 and cytomegalovirus UL99 tegument proteins, which are involved in the final envelopment during viral maturation. This study demonstrates that BBLF1 is a myristoylated and palmitoylated protein, as are UL11 and UL99. Myristoylation of BBLF1 both facilitates its membrane anchoring and stabilizes it. BBLF1 is shown to localize to the trans-Golgi network (TGN) along with gp350/220, a site where final envelopment of EBV particles takes place. The localization of BBLF1 at the TGN requires myristoylation and two acidic clusters, which interact with PACS-1, a cytosolic protein, to mediate retrograde transport from the endosomes to the TGN. Knockdown of the expression of BBLF1 during EBV lytic replication reduces the production of virus particles, demonstrating the requirement of BBLF1 to achieve optimal production of virus particles. BBLF1 is hypothesized to facilitate the budding of tegumented capsid into glycoprotein-embedded membrane during viral maturation.

Herpesvirus transport to the nervous system and back again

Herpes simplex virus, varicella zoster virus, and pseudorabies virus are neurotropic pathogens of the Alphaherpesvirinae subfamily of the Herpesviridae. These viruses efficiently invade the peripheral nervous system and establish lifelong latency in neurons resident in peripheral ganglia. Primary and recurrent infections cycle virus particles between neurons and the peripheral tissues they innervate. This remarkable cycle of infection is the topic of this review. In addition, some of the distinguishing hallmarks of the infections caused by these viruses are evaluated in terms of their underlying similarities.

Specific residues of a conserved domain in the N terminus of the human cytomegalovirus pUL50 protein determine its intranuclear interaction with pUL53

Herpesviral capsids are assembled in the host cell nucleus and are subsequently translocated to the cytoplasm. During this process it has been demonstrated that the human cytomegalovirus proteins pUL50 and pUL53 interact and form, together with other viral and cellular proteins, the nuclear egress complex at the nuclear envelope. In this study we provide evidence that specific residues of a conserved N-terminal region of pUL50 determine its intranuclear interaction with pUL53. In silico evaluation and biophysical analyses suggested that the conserved region forms a regular secondary structure adopting a globular fold. Importantly, site-directed replacement of individual amino acids by alanine indicated a strong functional influence of specific residues inside this globular domain. In particular, mutation of the widely conserved residues Glu-56 or Tyr-57 led to a loss of interaction with pUL53. Consistent with the loss of binding properties, mutants E56A and Y57A showed a defective function in the recruitment of pUL53 to the nuclear envelope in expression plasmid-transfected and human cytomegalovirus-infected cells. In addition, in silico analysis suggested that residues 3-20 form an amphipathic α-helix that appears to be conserved among Herpesviridae. Point mutants revealed a structural role of this N-terminal α-helix for pUL50 stability rather than a direct role in the binding of pUL53. In contrast, the central part of the globular domain including Glu-56 and Tyr-57 is directly responsible for the functional interaction with pUL53 and thus determines formation of the basic nuclear egress complex.

Entry of herpes simplex virus type 1 (HSV-1) into the distal axons of trigeminal neurons favors the onset of nonproductive, silent infection

Following productive, lytic infection in epithelia, herpes simplex virus type 1 (HSV-1) establishes a lifelong latent infection in sensory neurons that is interrupted by episodes of reactivation. In order to better understand what triggers this lytic/latent decision in neurons, we set up an organotypic model based on chicken embryonic trigeminal ganglia explants (TGEs) in a double chamber system. Adding HSV-1 to the ganglion compartment (GC) resulted in a productive infection in the explants. By contrast, selective application of the virus to distal axons led to a largely nonproductive infection that was characterized by the poor expression of lytic genes and the presence of high levels of the 2.0-kb major latency-associated transcript (LAT) RNA. Treatment of the explants with the immediate-early (IE) gene transcriptional inducer hexamethylene bisacetamide, and simultaneous co-infection of the GC with HSV-1, herpes simplex virus type 2 (HSV-2) or pseudorabies virus (PrV) helper virus significantly enhanced the ability of HSV-1 to productively infect sensory neurons upon axonal entry. Helper-virus-induced transactivation of HSV-1 IE gene expression in axonally-infected TGEs in the absence of de novo protein synthesis was dependent on the presence of functional tegument protein VP16 in HSV-1 helper virus particles. After the establishment of a LAT-positive silent infection in TGEs, HSV-1 was refractory to transactivation by superinfection of the GC with HSV-1 but not with HSV-2 and PrV helper virus. In conclusion, the site of entry appears to be a critical determinant in the lytic/latent decision in sensory neurons. HSV-1 entry into distal axons results in an insufficient transactivation of IE gene expression and favors the establishment of a nonproductive, silent infection in trigeminal neurons.

Upon primary infection of the oronasal mucosa, herpes simplex virus type 1 (HSV-1) rapidly reaches the ganglia of the peripheral nervous system via axonal transport and establishes lifelong latency in surviving neurons. Central to the establishment of latency is the ability of HSV-1 to reliably switch from productive, lytic spread in epithelia to nonproductive, latent infection in sensory neurons. It is not fully understood what specifically disposes incoming particles of a highly cytopathogenic, fast-replicating alphaherpesvirus to nonproductive, latent infection in sensory neurons. The present study shows that selective entry of HSV-1 into the distal axons of trigeminal neurons strongly favors the establishment of a nonproductive, latent infection, whereas nonselective infection of neurons still enables HSV-1 to induce lytic gene expression. Our data support a model of latency establishment in which the site of entry is an important determinant of the lytic/latent decision in the infected neuron. Productive infection of the neuron ensues if particles enter the soma of the neuron directly. In contrast, previous retrograde axonal transport of incoming viral particles creates a distinct scenario that abrogates VP16-dependent transactivation of immediate-early gene expression and precludes the expression of lytic genes to an extent sufficient to prevent the initiation of massive productive infection of trigeminal neurons.

Human herpesvirus 6 glycoprotein M is essential for virus growth and requires glycoprotein N for its maturation

Human herpesvirus 6 (HHV-6) is a T-lymphotropic virus belonging to the betaherpesvirus family. Several HHV-6-encoded glycoproteins are required for cell entry and virion maturation. Glycoprotein M (gM) is conserved among all herpesviruses, and therefore thought to have important functions; however, the HHV-6 g has not been characterized. Here, we examined the expression of HHV-6 g, and examined its function in viral replication, using a mutant and revertant gM. HHV-6 g was expressed on virions as a glycoprotein modified with complex N-linked oligosaccharides. As in other herpesviruses, HHV-6 g formed a complex with glycoprotein N (gN), and was transported from the endoplasmic reticulum to the trans-Golgi network only when part of this complex. Finally, a gM mutant virus in which the gM start codon was destroyed was not reconstituted, although its revertant was, indicating that HHV-6 g is essential for virus production, unlike the gM of alphaherpesviruses.