Remodeling of host membranes during herpesvirus assembly and egress

Palmitoylation of KSHV pORF55 is required for Golgi localization and efficient progeny virion production

Kaposi's sarcoma-associated herpesvirus (KSHV) is a double-stranded DNA virus etiologically associated with multiple malignancies. Both latency and sporadic lytic reactivation contribute to KSHV-associated malignancies, however, the specific roles of many KSHV lytic gene products in KSHV replication remain elusive. In this study, we report that ablation of ORF55, a late gene encoding a tegument protein, does not impact KSHV lytic reactivation but significantly reduces the production of progeny virions. We found that cysteine 10 and 11 (C10 and C11) of pORF55 are palmitoylated, and the palmytoilation is essential for its Golgi localization and secondary envelope formation. Palmitoylation-defective pORF55 mutants are unstable and undergo proteasomal degradation. Notably, introduction of a putative Golgi localization sequence to these palmitoylation-defective pORF55 mutants restores Golgi localization and fully reinstates KSHV progeny virion production. Together, our study provides new insight into the critical role of pORF55 palmitoylation in KSHV progeny virion production and offers potential therapeutic targets for the treatment of related malignancies.

Comprehensive Analysis of the Tegument Proteins Involved in Capsid Transport and Virion Morphogenesis of Alpha, Beta and Gamma Herpesviruses

Herpesviruses are enveloped and have an amorphous protein layer surrounding the capsid, which is termed the tegument. Tegument proteins perform critical functions throughout the viral life cycle. This review provides a comprehensive and comparative analysis of the roles of specific tegument proteins in capsid transport and virion morphogenesis of selected, well-studied prototypes of each of the three subfamilies of Herpesviridae i.e., human herpesvirus-1/herpes simplex virus-1 (Alphaherpesvirinae), human herpesvirus-5/cytomegalovirus (Betaherpesvirinae) and human herpesvirus -8/Kaposi's sarcomavirus (Gammaherpesvirinae). Most of the current knowledge is based on alpha herpesviruses, in particular HSV-1. While some tegument proteins are released into the cytoplasm after virus entry, several tegument proteins remain associated with the capsid and are responsible for transport to and docking at the nucleus. After replication and capsid formation, the capsid is enveloped at the nuclear membrane, which is referred to as primary envelopment, followed by de-envelopment and release into the cytoplasm. This requires involvement of at least three tegument proteins. Subsequently, multiple interactions between tegument proteins and capsid proteins, other tegument proteins and glycoproteins are required for assembly of the virus particles and envelopment at the Golgi, with certain tegument proteins acting as the central hub for these interactions. Some redundancy in these interactions ensures appropriate morphogenesis.

The Knowns and Unknowns of Herpesvirus Nuclear Egress

Nuclear egress of herpesvirus capsids across the intact nuclear envelope is an exceptional vesicle-mediated nucleocytoplasmic translocation resulting in the delivery of herpesvirus capsids into the cytosol. Budding of the (nucleo)capsid at and scission from the inner nuclear membrane (INM) is mediated by the viral nuclear egress complex (NEC) resulting in a transiently enveloped virus particle in the perinuclear space followed by fusion of the primary envelope with the outer nuclear membrane (ONM). The dimeric NEC oligomerizes into a honeycomb-shaped coat underlining the INM to induce membrane curvature and scission. Mutational analyses complemented structural data defining functionally important regions. Questions remain, including where and when the NEC is formed and how membrane curvature is mediated, vesicle formation is regulated, and directionality is secured. The composition of the primary enveloped virion and the machinery mediating fusion of the primary envelope with the ONM is still debated. While NEC-mediated budding apparently follows a highly conserved mechanism, species and/or cell type-specific differences complicate understanding of later steps.

Liquid-liquid phase separation mediates the formation of herpesvirus assembly compartments

Virus assembly, which takes place during the late stage of viral replication, is essential for virus propagation. However, the underlying mechanisms remain poorly understood, especially for viruses with complicated structures. Here, we use correlative light and electron microscopy to examine the formation of cytoplasmic virion assembly compartments (cVACs) during infection by a γ-herpesvirus. These cVACs are membraneless organelles with liquid-like properties. Formation of cVACs during virus infection is mediated by ORF52, an abundant tegument protein. ORF52 undergoes liquid-liquid phase separation (LLPS), which is promoted by both DNA and RNA. Disrupting ORF52 phase separation blocks cVACs formation and virion production. These results demonstrate that phase separation of ORF52 is critical for cVACs formation. Our work defines herpesvirus cVACs as membraneless compartments that are generated through a process of LLPS mediated by a tegument protein and adds to the cellular processes that are facilitated by phase separation.

Antiviral Activity and Mechanisms of Seaweeds Bioactive Compounds on Enveloped Viruses-A Review

In the last decades, the interest in seaweed has significantly increased. Bioactive compounds from seaweed’s currently receive major attention from pharmaceutical companies as they express several interesting biological activities which are beneficial for humans. The structural diversity of seaweed metabolites provides diverse biological activities which are expressed through diverse mechanisms of actions. This review mainly focuses on the antiviral activity of seaweed’s extracts, highlighting the mechanisms of actions of some seaweed molecules against infection caused by different types of enveloped viruses: influenza, Lentivirus (HIV-1), Herpes viruses, and coronaviruses. Seaweed metabolites with antiviral properties can act trough different pathways by increasing the host’s defense system or through targeting and blocking virus replication before it enters host cells. Several studies have already established the large antiviral spectrum of seaweed’s bioactive compounds. Throughout this review, antiviral mechanisms and medical applications of seaweed’s bioactive compounds are analyzed, suggesting seaweed’s potential source of antiviral compounds for the formulation of novel and natural antiviral drugs.

‘Come Together’—The Regulatory Interaction of Herpesviral Nuclear Egress Proteins Comprises both Essential and Accessory Functions

Herpesviral nuclear egress is a fine-tuned regulatory process that defines the nucleocytoplasmic release of viral capsids. Nuclear capsids are unable to traverse via nuclear pores due to the fact of their large size; therefore, herpesviruses evolved to develop a vesicular transport pathway mediating the transition across the two leaflets of the nuclear membrane. The entire process involves a number of regulatory proteins, which support the local distortion of the nuclear envelope. In the case of the prototype species of β-Herpesvirinae, the human cytomegalovirus (HCMV), the nuclear egress complex (NEC) is determined by the core proteins pUL50 and pUL53 that oligomerize, form capsid docking lattices and mediate multicomponent assembly with NEC-associated viral and cellular proteins. The NEC-binding principle is based on the hook-into-groove interaction through an N-terminal hook-like pUL53 protrusion that embraces an α-helical pUL50 binding groove. Thus far, the function and characteristics of herpesviral core NECs have been well studied and point to the groove proteins, such as pUL50, as the multi-interacting, major determinants of NEC formation and egress. This review provides closer insight into (i) sequence and structure conservation of herpesviral core NEC proteins, (ii) experimentation on cross-viral core NEC interactions, (iii) the essential functional roles of hook and groove proteins for viral replication, (iv) an establishment of assay systems for NEC-directed antiviral research and (v) the validation of NEC as putative antiviral drug targets. Finally, this article provides new insights into the conservation, function and antiviral targeting of herpesviral core NEC proteins and, into the complex regulatory role of hook and groove proteins during the assembly, egress and maturation of infectious virus.

Role of the Orphan Transporter SLC35E1 in the Nuclear Egress of Herpes Simplex Virus 1

This study developed a system consisting of two rounds of screening cellular proteins involved in the nuclear egress of herpes simplex virus 1 (HSV-1). Using this system, we first screened cellular proteins that interacted with the HSV-1 nuclear egress complex (NEC) consisting of UL34 and UL31 in HSV-1-infected cells, which are critical for the nuclear egress of HSV-1, by tandem affinity purification coupled with mass spectrometry-based proteomics technology. Next, we performed CRISPR/Cas9-based screening of live HSV-1-infected reporter cells under fluorescence microscopy using single guide RNAs targeting the cellular proteins identified in the first proteomic screening to detect the mislocalization of the lamin-associated protein emerin, which is a phenotype for defects in HSV-1 nuclear egress. This study focused on a cellular orphan transporter SLC35E1, one of the cellular proteins identified by the screening system. Knockout of SLC35E1 reduced HSV-1 replication and induced membranous invaginations containing perinuclear enveloped virions (PEVs) adjacent to the nuclear membrane (NM), aberrant accumulation of PEVs in the perinuclear space between the inner and outer NMs and the invagination structures, and mislocalization of the NEC. These effects were similar to those of previously reported mutation(s) in HSV-1 proteins and depletion of cellular proteins that are important for HSV-1 de-envelopment, one of the steps required for HSV-1 nuclear egress. Our newly established screening system enabled us to identify a novel cellular protein required for efficient HSV-1 de-envelopment. IMPORTANCE The identification of cellular protein(s) that interact with viral effector proteins and function in important viral procedures is necessary for enhancing our understanding of the mechanics of various viral processes. In this study, we established a new system consisting of interactome screening for the herpes simplex virus 1 (HSV-1) nuclear egress complex (NEC), followed by loss-of-function screening to target the identified putative NEC-interacting cellular proteins to detect a defect in HSV-1 nuclear egress. This newly established system identified SLC35E1, an orphan transporter, as a novel cellular protein required for efficient HSV-1 de-envelopment, providing an insight into the mechanisms involved in this viral procedure.

Optineurin in ocular herpes infection

Herpes Simplex Virus-1 (HSV-1) is a neurotropic virus that can infect humans in the eye and travel to the trigeminal ganglion to establish latency. HSV-1 causes various disease states in both the primary and secondary sites of infection including the eye and the nervous system. This DNA virus exploits various adaptive measures to infect host cells, hijack host cell proteins, evade host immune response and spread from cell-to-cell to avoid immune detection. Recent data suggest that Optineurin (OPTN), a host protein, is a key restriction factor that prevents cell-to-cell spread of HSV-1 and guards against serious damage to the nervous system during infection. In recent years OPTN has gained increased attention because of its involvement in cellular mechanisms that promote homeostasis and prevent neurodegeneration. At the center of it all is the role OPTN plays as a receptor for selective autophagy. This review summarizes our latest understanding of the viral lifecycle, disease pathologies, and OPTN-mediated protective mechanisms during HSV-1 infection of the eye and the nervous system. We specifically highlight recent discoveries that implicate OPTN as crucial in the prevention of ocular and neurodegenerative diseases.

How Viruses Hijack and Modify the Secretory Transport Pathway

Eukaryotic cells contain dynamic membrane-bound organelles that are constantly remodeled in response to physiological and environmental cues. Key organelles are the endoplasmic reticulum, the Golgi apparatus and the plasma membrane, which are interconnected by vesicular traffic through the secretory transport route. Numerous viruses, especially enveloped viruses, use and modify compartments of the secretory pathway to promote their replication, assembly and cell egression by hijacking the host cell machinery. In some cases, the subversion mechanism has been uncovered. In this review, we summarize our current understanding of how the secretory pathway is subverted and exploited by viruses belonging to Picornaviridae, Coronaviridae, Flaviviridae,Poxviridae, Parvoviridae and Herpesviridae families.

Multifaceted Roles of ICP22/ORF63 Proteins in the Life Cycle of Human Herpesviruses

Herpesviruses are extremely successful parasites that have evolved over millions of years to develop a variety of mechanisms to coexist with their hosts and to maintain host-to-host transmission and lifelong infection by regulating their life cycles. The life cycle of herpesviruses consists of two phases: lytic infection and latent infection. During lytic infection, active replication and the production of numerous progeny virions occur. Subsequent suppression of the host immune response leads to a lifetime latent infection of the host. During latent infection, the viral genome remains in an inactive state in the host cell to avoid host immune surveillance, but the virus can be reactivated and reenter the lytic cycle. The balance between these two phases of the herpesvirus life cycle is controlled by broad interactions among numerous viral and cellular factors. ICP22/ORF63 proteins are among these factors and are involved in transcription, nuclear budding, latency establishment, and reactivation. In this review, we summarized the various roles and complex mechanisms by which ICP22/ORF63 proteins regulate the life cycle of human herpesviruses and the complex relationships among host and viral factors. Elucidating the role and mechanism of ICP22/ORF63 in virus-host interactions will deepen our understanding of the viral life cycle. In addition, it will also help us to understand the pathogenesis of herpesvirus infections and provide new strategies for combating these infections.

Conquering the Nuclear Envelope Barriers by EBV Lytic Replication

The nuclear envelope (NE) of eukaryotic cells has a highly structural architecture, comprising double lipid-bilayer membranes, nuclear pore complexes, and an underlying nuclear lamina network. The NE structure is held in place through the membrane-bound LINC (linker of nucleoskeleton and cytoskeleton) complex, spanning the inner and outer nuclear membranes. The NE functions as a barrier between the nucleus and cytoplasm and as a transverse scaffold for various cellular processes. Epstein-Barr virus (EBV) is a human pathogen that infects most of the world's population and is associated with several well-known malignancies. Within the nucleus, the replicated viral DNA is packaged into capsids, which subsequently egress from the nucleus into the cytoplasm for tegumentation and final envelopment. There is increasing evidence that viral lytic gene expression or replication contributes to the pathogenesis of EBV. Various EBV lytic proteins regulate and modulate the nuclear envelope structure in different ways, especially the viral BGLF4 kinase and the nuclear egress complex BFRF1/BFRF2. From the aspects of nuclear membrane structure, viral components, and fundamental nucleocytoplasmic transport controls, this review summarizes our findings and recently updated information on NE structure modification and NE-related cellular processes mediated by EBV.

Untargeted LC-MS based metabolomic profiling of iPAMs to investigate lipid metabolic pathways alternations induced by different Pseudorabies virus strains

Owing to viral recombination, interspecies transmission, and evolution, variant pseudorabies virus (PRV) strains exhibit different biological characteristics and pathogenicity. To improve the understanding of common and specific metabolic changes that occur upon infection by different PRV strains, we herein describe the comprehensive analysis of metabolites of PRV vaccine strain (Bartha K61), classical strain (EA) and variant strain (HNX) infection in immortalized porcine alveolar macrophage cells. Compared with uninfected cells, cells infected with Bartha K61, EA and HNX had 246, 225, and 272 differing metabolites, respectively. In the three types of PRV-strain-infected cells, lipids and lipid-like molecules accounted for over 50 % of the altered metabolites. As these viruses are enveloped, viral replication, assembly and release occur on cellular membranes primed through the manipulation of the host metabolism. We analyzed the potential relationship between virus replication and the virus-mediated host metabolism. Our study resulted in the first reconstruction of the major lipid metabolic pathways involved in PRV infection, including those of glycerophospholipids, sphingolipids, glycerolipids, and fatty acyls. In addition, the metabolic perturbations caused by different PRV strain infections are consistent across many species, however, our results also revealed many specific metabolic alterations during HNX infection, such as the enrichment of phosphatidylinositol and 15R-PGE2 methyl ester 15-acetate, and the diminishment of phosphatidylethanolamine, phosphatidic acid, and ceramides. These strain-specific altered metabolites may be linked to the unique biological characteristics and pathogenicity of the HNX strain.

Modulation of Endosome Function, Vesicle Trafficking and Autophagy by Human Herpesviruses

Human herpesviruses are a ubiquitous family of viruses that infect individuals of all ages and are present at a high prevalence worldwide. Herpesviruses are responsible for a broad spectrum of diseases, ranging from skin and mucosal lesions to blindness and life-threatening encephalitis, and some of them, such as Kaposi’s sarcoma-associated herpesvirus (KSHV) and Epstein–Barr virus (EBV), are known to be oncogenic. Furthermore, recent studies suggest that some herpesviruses may be associated with developing neurodegenerative diseases. These viruses can establish lifelong infections in the host and remain in a latent state with periodic reactivations. To achieve infection and yield new infectious viral particles, these viruses require and interact with molecular host determinants for supporting their replication and spread. Important sets of cellular factors involved in the lifecycle of herpesviruses are those participating in intracellular membrane trafficking pathways, as well as autophagic-based organelle recycling processes. These cellular processes are required by these viruses for cell entry and exit steps. Here, we review and discuss recent findings related to how herpesviruses exploit vesicular trafficking and autophagy components by using both host and viral gene products to promote the import and export of infectious viral particles from and to the extracellular environment. Understanding how herpesviruses modulate autophagy, endolysosomal and secretory pathways, as well as other prominent trafficking vesicles within the cell, could enable the engineering of novel antiviral therapies to treat these viruses and counteract their negative health effects.

Viral use and subversion of membrane organization and trafficking

Membrane trafficking is an essential cellular process conserved across all eukaryotes, which regulates the uptake or release of macromolecules from cells, the composition of cellular membranes and organelle biogenesis. It influences numerous aspects of cellular organisation, dynamics and homeostasis, including nutrition, signalling and cell architecture. Not surprisingly, malfunction of membrane trafficking is linked to many serious genetic, metabolic and neurological disorders. It is also often hijacked during viral infection, enabling viruses to accomplish many of the main stages of their replication cycle, including entry into and egress from cells. The appropriation of membrane trafficking by viruses has been studied since the birth of cell biology and has helped elucidate how this integral cellular process functions. In this Review, we discuss some of the different strategies viruses use to manipulate and take over the membrane compartments of their hosts to promote their replication, assembly and egress.

Why Cells and Viruses Cannot Survive without an ESCRT

Intracellular organelles enwrapped in membranes along with a complex network of vesicles trafficking in, out and inside the cellular environment are one of the main features of eukaryotic cells. Given their central role in cell life, compartmentalization and mechanisms allowing their maintenance despite continuous crosstalk among different organelles have been deeply investigated over the past years. Here, we review the multiple functions exerted by the endosomal sorting complex required for transport (ESCRT) machinery in driving membrane remodeling and fission, as well as in repairing physiological and pathological membrane damages. In this way, ESCRT machinery enables different fundamental cellular processes, such as cell cytokinesis, biogenesis of organelles and vesicles, maintenance of nuclear–cytoplasmic compartmentalization, endolysosomal activity. Furthermore, we discuss some examples of how viruses, as obligate intracellular parasites, have evolved to hijack the ESCRT machinery or part of it to execute/optimize their replication cycle/infection. A special emphasis is given to the herpes simplex virus type 1 (HSV-1) interaction with the ESCRT proteins, considering the peculiarities of this interplay and the need for HSV-1 to cross both the nuclear-cytoplasmic and the cytoplasmic-extracellular environment compartmentalization to egress from infected cells.

The Roles of Envelope Glycoprotein M in the Life Cycle of Some Alphaherpesviruses

The envelope glycoprotein M (gM), a surface virion component conserved among alphaherpesviruses, is a multiple-transmembrane domain-containing glycoprotein with a complex N-linked oligosaccharide. The gM mediates a diverse range of functions during the viral life cycle. In this review, we summarize the biological features of gM, including its characterization and function in some specicial alphaherpesviruses. gM modulates the virus-induced membrane fusion during virus invasion, transports other proteins to the appropriate intracellular membranes for primary and secondary envelopment during virion assembly, and promotes egress of the virus. The gM can interact with various viral and cellular components, and the focus of recent research has also been on interactions related to gM. And we will discuss how gM participates in the life cycle of alphaherpesviruses.

Stunning symmetries involved in the self-assembly of the HSV-1 capsid

Herpes simplex virus-1 (HSV-1) is an enveloped dsDNA virus, infecting ~ 67% of humans. Here, we present the essential components of the HSV-1, focusing on stunning symmetries on the capsid. However, little is known about how the symmetries are involved dynamically in the self-assembly process. We suggest small angle X-ray scattering as a suitable method to capture the dynamics of self-assembly. Furthermore, our understanding of the viruses can be expanded by using an integrative approach that combines heterogeneous types of data, thus promoting new diagnostic tools and a cure for viral infections.

Virus Budding

Enveloped viruses exit producer cells and acquire their external lipid envelopes by budding through limiting cellular membranes. Most viruses encode multifunctional structural proteins that coordinate the processes of virion assembly, membrane envelopment, budding, and maturation. In many cases, the cellular ESCRT pathway is recruited to facilitate the membrane fission step of budding, but alternative strategies are also employed. Recently, many viruses previously considered to be non-enveloped have been shown to exit cells non-lytically within vesicles, adding further complexity to the intricacies of virus budding and egress.

Mass Spectrometric Characterization of HSV-1 L-Particles From Human Dendritic Cells and BHK21 Cells and Analysis of Their Functional Role

Herpes simplex virus type 1 (HSV-1) is a very common human pathogenic virus among the world’s population. The lytic replication cycle of HSV-1 is, amongst others, characterized by a tripartite viral gene expression cascade, the assembly of nucleocapsids involving their subsequent nuclear egress, tegumentation, re-envelopment and the final release of progeny viral particles. During productive infection of a multitude of different cell types, HSV-1 generates not only infectious heavy (H-) particles, but also non-infectious light (L-) particles, lacking the capsid. In monocyte-derived mature dendritic cells (mDCs), HSV-1 causes a non-productive infection with the predominant release of L-particles. Until now, the generation and function of L-particles is not well understood, however, they are described as factors transferring viral components to the cellular microenvironment. To obtain deeper insights into the L-particle composition, we performed a mass-spectrometry-based analysis of L-particles derived from HSV-1-infected mDCs or BHK21 cells and H-particles from the latter one. In total, we detected 63 viral proteins in both H- and L-particle preparations derived from HSV-1-infected BHK21 cells. In L-particles from HSV-1-infected mDCs we identified 41 viral proteins which are differentially distributed compared to L-particles from BHK21 cells. In this study, we present data suggesting that L-particles modify mDCs and suppress their T cell stimulatory capacity. Due to the plethora of specific viral proteins incorporated into and transmitted by L-particles, it is tempting to speculate that L-particles manipulate non-infected bystander cells for the benefit of the virus.

Herpes Simplex Virus Organizes Cytoplasmic Membranes To Form a Viral Assembly Center in Neuronal Cells

Herpes simplex virus (HSV) is a neuroinvasive virus that has been used as a model organism for studying common properties of all herpesviruses. HSV induces host organelle rearrangement and forms multiple, dispersed assembly compartments in epithelial cells, which complicates the study of HSV assembly. In this study, we show that HSV forms a visually distinct unitary cytoplasmic viral assembly center (cVAC) in both cancerous and primary neuronal cells that concentrates viral structural proteins and is a major site of capsid envelopment. The HSV cVAC also concentrates host membranes that are important for viral assembly, such as Golgi- and recycling endosome-derived membranes. Finally, we show that HSV cVAC formation and/or maintenance depends on an intact microtubule network and a viral tegument protein, pUL51. Our observations suggest that the neuronal cVAC is a uniquely useful model to study common herpesvirus assembly pathways and cell-specific pathways for membrane reorganization.IMPORTANCE Herpesvirus particles are complex and contain many different proteins that must come together in an organized and coordinated fashion. Many viruses solve this coordination problem by creating a specialized assembly factory in the host cell, and the formation of such factories provides a promising target for interfering with virus production. Herpes simplex virus 1 (HSV-1) infects several types of cells, including neurons, but has not previously been shown to form such an organized factory in the nonneuronal cells in which its assembly has been best studied. Here, we show that HSV-1 forms an organized assembly factory in neuronal cells, and we identify some of the viral and host cell factors that are important for its formation.

Visualizing Nudivirus Assembly and Egress

The dynamics of nuclear envelope has a critical role in multiple cellular processes. However, little is known regarding the structural changes occurring inside the nucleus or at the inner and outer nuclear membranes. For viruses assembling inside the nucleus, remodeling of the intranuclear membrane plays an important role in the process of virion assembly. Here, we monitored the changes associated with viral infection in the case of nudiviruses. Our data revealed dramatic membrane remodeling inside the nuclear compartment during infection with Oryctes rhinoceros nudivirus, an important biocontrol agent against coconut rhinoceros beetle, a devastating pest for coconut and oil palm trees. Based on these findings, we propose a model for nudivirus assembly in which nuclear packaging occurs in fully enveloped virions.

Enveloped viruses hijack cellular membranes in order to provide the necessary material for virion assembly. In particular, viruses that replicate and assemble inside the nucleus have developed special approaches to modify the nuclear landscape for their advantage. We used electron microscopy to investigate cellular changes occurring during nudivirus infection and we characterized a unique mechanism for assembly, packaging, and transport of new virions across the nuclear membrane and through the cytoplasm. Our three-dimensional reconstructions describe the complex remodeling of the nuclear membrane necessary to release vesicle-associated viruses into the cytoplasm. This is the first report of nuclear morphological reconfigurations that occur during nudiviral infection.

Nuclear Egress

During viral replication, herpesviruses utilize a unique strategy, termed nuclear egress, to translocate capsids from the nucleus into the cytoplasm. This initial budding step transfers a newly formed capsid from within the nucleus, too large to fit through nuclear pores, through the inner nuclear membrane to the perinuclear space. The perinuclear enveloped virion must then fuse with the outer nuclear membrane to be released into the cytoplasm for further maturation, undergoing budding once again at the trans-Golgi network or early endosomes, and ultimately exit the cell non-lytically to spread infection. This first budding process is mediated by two conserved viral proteins, UL31 and UL34, that form a heterodimer called the nuclear egress complex (NEC). This review focuses on what we know about how the NEC mediates capsid transport to the perinuclear space, including steps prior to and after this budding event. Additionally, we discuss the involvement of other viral proteins in this process and how NEC-mediated budding may be regulated during infection.

Epstein-Barr Virus Exploits the Secretory Pathway to Release Virions

Herpesvirus egress mechanisms are strongly associated with intracellular compartment remodeling processes. Previously, we and other groups have described that intracellular compartments derived from the Golgi apparatus are the maturation sites of Epstein-Barr virus (EBV) virions. However, the mechanism by which these virions are released from the host cell to the extracellular milieu is poorly understood. Here, I adapted two independent induction systems of the EBV lytic cycle in vitro, in the context of Rab GTPase silencing, to characterize the EBV release pathway. Immunofluorescence staining revealed that p350/220, the major EBV glycoprotein, partially co-localized with three Rab GTPases: Rab8a, Rab10, and Rab11a. Furthermore, the knockdown of these Rab GTPases promoted the intracellular accumulation of viral structural proteins by inhibiting its distribution to the plasma membrane. Finally, the knockdown of the Rab8a, Rab10, and Rab11a proteins suppressed the release of EBV infectious virions. Taken together, these findings support the hypothesis that mature EBV virions are released from infected cells to the extracellular milieu via the secretory pathway, as well as providing new insights into the EBV life cycle.

Host BAG3 Is Degraded by Pseudorabies Virus pUL56 C-Terminal 181L-185L and Plays a Negative Regulation Role during Viral Lytic Infection

Bcl2-associated athanogene (BAG) 3, which is a chaperone-mediated selective autophagy protein, plays a pivotal role in modulating the life cycle of a wide variety of viruses. Both positive and negative modulations of viruses by BAG3 were reported. However, the effects of BAG3 on pseudorabies virus (PRV) remain unknown. To investigate whether BAG3 could modulate the PRV life cycle during a lytic infection, we first identified PRV protein UL56 (pUL56) as a novel BAG3 interactor by co-immunoprecipitation and co-localization analyses. The overexpression of pUL56 induced a significant degradation of BAG3 at protein level via the lysosome pathway. The C-terminal mutations of ¹⁸¹L/A, ¹⁸⁵L/A, or ¹⁸¹L/A-¹⁸⁵L/A in pUL56 resulted in a deficiency in pUL56-induced BAG3 degradation. In addition, the pUL56 C-terminal mutants that lost Golgi retention abrogated pUL56-induced BAG3 degradation, which indicates a Golgi retention-dependent manner. Strikingly, BAG3 was not observed to be degraded in either wild-type or UL56-deleted PRV infected cells as compared to mock infected ones, whereas the additional two adjacent BAG3 cleaved products were found in the infected cells in a species-specific manner. Overexpression of BAG3 significantly suppressed PRV proliferation, while knockdown of BAG3 resulted in increased viral yields in HEK293T cells. Thus, these data indicated a negative regulation role of BAG3 during PRV lytic infection. Collectively, our findings revealed a novel molecular mechanism on host protein degradation induced by PRV pUL56. Moreover, we identified BAG3 as a host restricted protein during PRV lytic infection in cells.

Duck plague virus gE serves essential functions during the virion final envelopment through influence capsids budding into the cytoplasmic vesicles

Duck plague virus (DPV), a member of the alphaherpesviruses subfamily, causes massive ducks death and results in a devastating hit to duck industries in China. It is of great significance for us to analyze the functions of DPV genes for controlling the outbreak of duck plague. Thus, glycoproteins E (gE) of DPV, which requires viral cell-to-cell spreading and the final envelopment in herpes simplex virus 1 (HSV-1) and pseudorabies virus (PRV), was chosen herein. The gE mutant virus BAC-CHv-ΔgE was constructed by using a markerless two-step Red recombination system implemented on the DPV genome cloned into a bacterial artificial chromosome (BAC). Viral plaques on duck embryo fibroblast (DEF) cells of BAC-CHv-ΔgE were on average approximately 60% smaller than those produced by BAC-CHv virus. Viral replication kinetics showed that BAC-CHv-ΔgE grew to lower titers than BAC-CHv virus did in DEF cells. Electron microscopy confirmed that deleting of DPV gE resulted in a large number of capsids accumulating around vesicles and very few of them could bud into vesicles. The drastic inhibition of virion formation in the absence of the DPV gE indicated that it played an important role in virion morphogenesis before the final envelopment of intracytoplasmic nucleocapsids.

Viruses Reveal the Secrets of Plasmodesmal Cell Biology

Plasmodesmata (PD) are essential for intercellular trafficking of molecules required for plant life, from small molecules like sugars and ions to macromolecules including proteins and RNA molecules that act as signals to regulate plant development and defense. As obligate intracellular pathogens, plant viruses have evolved to manipulate this communication system to facilitate the initial cell-to-cell and eventual systemic spread in their plant hosts. There has been considerable interest in how viruses manipulate the PD that connect the protoplasts of neighboring cells, and viruses have yielded invaluable tools for probing the structure and function of PD. With recent advances in biochemistry and imaging, we have gained new insights into the composition and structure of PD in the presence and absence of viruses. Here, we first discuss viral strategies for manipulating PD for their intercellular movement and examine how this has shed light on our understanding of native PD function. We then address the controversial role of the cytoskeleton in trafficking to and through PD. Finally, we address how viruses could alter PD structure and consider possible mechanisms of the phenomenon described as 'gating'. This discussion supports the significance of virus research in elucidating the properties of PD, these persistently enigmatic plant organelles.

Functional Identification and Characterization of the Nuclear Egress Complex of a Gammaherpesvirus

The herpesvirus nuclear egress complex (NEC) is composed of two viral proteins. They play key roles in mediating the translocation of capsids from the nucleus to the cytoplasm by facilitating the budding of capsids into the perinuclear space (PNS). The NEC of alphaherpesvirus can induce the formation of virion-like vesicles from the nuclear membrane in the absence of other viral proteins. However, whether the NEC of gammaherpesvirus harbors the ability to do so in mammalian cells remains to be determined. In this study, we first constructed open reading frame 67 (ORF67)-null and ORF69-null mutants of murine gammaherpesvirus 68 (MHV-68) and demonstrated that both ORF67 and ORF69 play critical roles in nuclear egress and hence viral lytic replication. Biochemical and bioimaging analyses showed that ORF67 and ORF69 interacted with each other and were sufficient to induce the formation of virion-like vesicles from the nuclear membrane in mammalian cells. Thus, we designated ORF67 and ORF69 components of MHV-68 NEC. Furthermore, we identified amino acids critical for mediating the interaction between ORF67 and ORF69 through homology modeling and verified their function in nuclear egress, providing insights into the molecular basis of NEC formation in gammaherpesviruses.IMPORTANCE Increasing amounts of knowledge indicate that the nuclear egress complex (NEC) is critical for the nuclear egress of herpesvirus capsids, which can be viewed as a vesicle-mediated transport pathway through the nuclear membrane. In this study, we identified open reading frame 67 (ORF67) and ORF69 as components of the NEC in murine gammaherpesvirus 68 (MHV-68) and demonstrated that they efficiently induce virion-like vesicles from the nuclear membrane in mammalian cells. This is the first time that the NEC of a gammaherpesvirus has been found to demonstrate such an essential characteristic. In addition, we identified amino acids critical for mediating the interaction between ORF67 and ORF69 as well as nuclear egress. Notably, these amino acids are conserved in Kaposi's sarcoma-associated herpesvirus (KSHV) and Epstein-Barr virus (EBV), providing a structural basis to design antigammaherpesvirus drugs.

Mechanisms Mediating Nuclear Trafficking Involved in Viral Propagation by DNA Viruses

Typical viral propagation involves sequential viral entry, uncoating, replication, gene transcription and protein synthesis, and virion assembly and release. Some viral proteins must be transported into host nucleus to facilitate viral propagation, which is essential for the production of mature virions. During the transport process, nuclear localization signals (NLSs) play an important role in guiding target proteins into nucleus through the nuclear pore. To date, some classical nuclear localization signals (cNLSs) and non-classical NLSs (ncNLSs) have been identified in a number of viral proteins. These proteins are involved in viral replication, expression regulation of viral genes and virion assembly. Moreover, other proteins are transported into nucleus with unknown mechanisms. This review highlights our current knowledge about the nuclear trafficking of cellular proteins associated with viral propagation.

Identification of the Capsid Binding Site in the Herpes Simplex Virus 1 Nuclear Egress Complex and Its Role in Viral Primary Envelopment and Replication

During nuclear egress of nascent progeny herpesvirus nucleocapsids, the nucleocapsids acquire a primary envelope by budding through the inner nuclear membrane of infected cells into the perinuclear space between the inner and outer nuclear membranes. Herpes simplex virus 1 (HSV-1) U_L34 and U_L31 proteins form a nuclear egress complex (NEC) and play critical roles in this budding process, designated primary envelopment. To clarify the role of NEC binding to progeny nucleocapsids in HSV-1 primary envelopment, we established an assay system for HSV-1 NEC binding to nucleocapsids and capsid proteins in vitro Using this assay system, we showed that HSV-1 NEC bound to nucleocapsids and to capsid protein U_L25 but not to the other capsid proteins tested (i.e., VP5, VP23, and U_L17) and that HSV-1 NEC binding of nucleocapsids was mediated by the interaction of NEC with U_L25. U_L31 residues arginine-281 (R281) and aspartic acid-282 (D282) were required for efficient NEC binding to nucleocapsids and U_L25. We also showed that alanine substitution of U_L31 R281 and D282 reduced HSV-1 replication, caused aberrant accumulation of capsids in the nucleus, and induced an accumulation of empty vesicles that were similar in size and morphology to primary envelopes in the perinuclear space. These results suggested that NEC binding via U_L31 R281 and D282 to nucleocapsids, and probably to U_L25 in the nucleocapsids, has an important role in HSV-1 replication by promoting the incorporation of nucleocapsids into vesicles during primary envelopment.IMPORTANCE Binding of HSV-1 NEC to nucleocapsids has been thought to promote nucleocapsid budding at the inner nuclear membrane and subsequent incorporation of nucleocapsids into vesicles during nuclear egress of nucleocapsids. However, data to directly support this hypothesis have not been reported thus far. In this study, we have present data showing that two amino acids in the membrane-distal face of the HSV-1 NEC, which contains the putative capsid binding site based on the solved NEC structure, were in fact required for efficient NEC binding to nucleocapsids and for efficient incorporation of nucleocapsids into vesicles during primary envelopment. This is the first report showing direct linkage between NEC binding to nucleocapsids and an increase in nucleocapsid incorporation into vesicles during herpesvirus primary envelopment.

Current and Emerging Therapies for Ocular Herpes Simplex Virus Type-1 Infections

Herpes simplex virus type-1 (HSV-1) is a neurotropic, double-stranded DNA virus that can cause a wide variety of diseases, including many ocular pathologies. It is one of the leading causes of infectious blindness in the United States. Because of its ubiquitous nature and its potential to cause serious ocular maladies, there is a significant need for more effective antiviral therapies against ocular HSV-1. In this review, we discuss the lifecycle of HSV-1 as it pertains to corneal infections and the clinically approved as well as emerging treatments to combat HSV-1 infections. We also highlight some newly identified host targets for the antiviral drug development.

Roles of the Interhexamer Contact Site for Hexagonal Lattice Formation of the Herpes Simplex Virus 1 Nuclear Egress Complex in Viral Primary Envelopment and Replication

During the nuclear export of nascent nucleocapsids of herpes simplex virus 1 (HSV-1), the nucleocapsids acquire a primary envelope by budding through the inner nuclear membrane into the perinuclear space between the inner and outer nuclear membranes. This unique budding process, termed primary envelopment, is initiated by the nuclear egress complex (NEC), composed of the HSV-1 UL31 and UL34 proteins. Earlier biochemical approaches have shown that the NEC has an intrinsic ability to vesiculate membranes through the formation of a hexagonal lattice structure. The significance of intrahexamer interactions of the NEC in the primary envelopment of HSV-1-infected cells has been reported. In contrast, the contribution of lattice formation of the NEC hexamer to primary envelopment in HSV-1-infected cells remains to be elucidated. Therefore, we constructed and characterized a recombinant HSV-1 strain carrying an amino acid substitution in a UL31 residue that is an interhexamer contact site for the lattice formation of the NEC hexamer. This mutation was reported to destabilize the interhexamer interactions of the HSV-1 NEC. Here, we demonstrate that the mutation causes the aberrant accumulation of nucleocapsids in the nucleus and reduces viral replication in Vero and HeLa cells. Thus, the ability of HSV-1 to form the hexagonal lattice structure of the NEC was linked to an increase in primary envelopment and viral replication. Our results suggest that the lattice formation of the NEC hexamer has an important role in HSV-1 replication by regulating primary envelopment.IMPORTANCE The scaffolding proteins of several envelope viruses required for virion assembly form high-order lattice structures. However, information on the significance of their lattice formation in infected cells is limited. Herpesviruses acquire envelopes twice during their viral replication. The first envelop acquisition (primary envelopment) is one of the steps in the vesicle-mediated nucleocytoplasmic transport of nascent nucleocapsids, which is unique in biology. HSV-1 NEC, thought to be conserved in all members of the Herpesviridae family, is critical for primary envelopment and was shown to form a hexagonal lattice structure. Here, we investigated the significance of the interhexamer contact site for hexagonal lattice formation of the NEC in HSV-1-infected cells and present evidence suggesting that the lattice formation of the NEC hexamer has an important role in HSV-1 replication by regulating primary envelopment. Our results provide insights into the mechanisms of the envelopment of herpesviruses and other envelope viruses.