The capsid protein encoded by U(L)17 of herpes simplex virus 1 interacts with tegument protein VP13/14

Interferon-induced MXB protein restricts vimentin-dependent viral infection

Type I interferon (IFN) inhibits a wide spectrum of viruses through stimulating the expression of antiviral proteins. As an IFN-induced protein, myxovirus resistance B (MXB) protein was reported to inhibit multiple highly pathogenic human viruses. It remains to be determined whether MXB employs a common mechanism to restrict different viruses. Here, we find that IFN alters the subcellular localization of hundreds of host proteins, and this IFN effect is partially lost upon MXB depletion. The results of our mechanistic study reveal that MXB recognizes vimentin (VIM) and recruits protein kinase B (AKT) to phosphorylate VIM at amino acid S38, which leads to reorganization of the VIM network and impairment of intracellular trafficking of virus protein complexes, hence causing a restriction of virus infection. These results highlight a new function of MXB in modulating VIM-mediated trafficking, which may lead towards a novel broad-spectrum antiviral strategy to control a large group of viruses that depend on VIM for successful replication.

Proteomic Comparison of Three Wild-Type Pseudorabies Virus Strains and the Attenuated Bartha Strain Reveals Reduced Incorporation of Several Tegument Proteins in Bartha Virions

Pseudorabies virus (PRV) is a member of the alphaherpesvirus subfamily and the causative agent of Aujeszky's disease in pigs. Driven by the large economic losses associated with PRV infection, several vaccines and vaccine programs have been developed. To this day, the attenuated Bartha strain, generated by serial passaging, represents the golden standard for PRV vaccination. However, a proteomic comparison of the Bartha virion to wild-type (WT) PRV virions is lacking. Here, we present a comprehensive mass spectrometry-based proteome comparison of the attenuated Bartha strain and three commonly used WT PRV strains: Becker, Kaplan, and NIA3. We report the detection of 40 structural and 14 presumed nonstructural proteins through a combination of data-dependent and data-independent acquisition. Interstrain comparisons revealed that packaging of the capsid and most envelope proteins is largely comparable in-between all four strains, except for the envelope protein pUL56, which is less abundant in Bartha virions. However, distinct differences were noted for several tegument proteins. Most strikingly, we noted a severely reduced incorporation of the tegument proteins IE180, VP11/12, pUS3, VP22, pUL41, pUS1, and pUL40 in Bartha virions. Moreover, and likely as a consequence, we also observed that Bartha virions are on average smaller and more icosahedral compared to WT virions. Finally, we detected at least 28 host proteins that were previously described in PRV virions and noticed considerable strain-specific differences with regard to host proteins, arguing that the potential role of packaged host proteins in PRV replication and spread should be further explored. IMPORTANCE The pseudorabies virus (PRV) vaccine strain Bartha-an attenuated strain created by serial passaging-represents an exceptional success story in alphaherpesvirus vaccination. Here, we used mass spectrometry to analyze the Bartha virion composition in comparison to three established WT PRV strains. Many viral tegument proteins that are considered nonessential for viral morphogenesis were drastically less abundant in Bartha virions compared to WT virions. Interestingly, many of the proteins that are less incorporated in Bartha participate in immune evasion strategies of alphaherpesviruses. In addition, we observed a reduced size and more icosahedral morphology of the Bartha virions compared to WT PRV. Given that the Bartha vaccine strain elicits potent immune responses, our findings here suggest that differences in protein packaging may contribute to its immunogenicity. Further exploration of these observations could aid the development of efficacious vaccines against other alphaherpesvirus vaccines such as HSV-1/2 or EHV-1.

Duck enteritis virus pUL47, as a late structural protein localized in the nucleus, mainly depends on residues 40 to 50 and 768 to 777 and inhibits IFN-β signalling by interacting with STAT1

Duck enteritis virus (DEV) is a member of the Alphaherpesvirinae subfamily. The characteristics of some DEV genes have been reported. However, information regarding the DEV UL47 gene is limited. In this study, we identified the DEV UL47 gene encoding a late structural protein located in the nucleus of infected cells. We further found that two domains of DEV pUL47, amino acids (aa) 40 to 50 and 768 to 777, could function as nuclear localization sequence (NLS) to guide the nuclear localization of pUL47 and nuclear translocation of heterologous proteins, including enhanced green fluorescent protein (EGFP) and beta-galactosidase (β-Gal). Moreover, pUL47 significantly inhibited polyriboinosinic:polyribocytidylic acid [poly(I:C)]-induced interferon beta (IFN-β) production and downregulated interferon-stimulated gene (ISG) expression, such as Mx and oligoadenylate synthetase-like (OASL), by interacting with signal transducer and activator of transcription-1 (STAT1).

HSV-1 Cytoplasmic Envelopment and Egress

Herpes simplex virus type 1 (HSV-1) is a structurally complex enveloped dsDNA virus that has evolved to replicate in human neurons and epithelia. Viral gene expression, DNA replication, capsid assembly, and genome packaging take place in the infected cell nucleus, which mature nucleocapsids exit by envelopment at the inner nuclear membrane then de-envelopment into the cytoplasm. Once in the cytoplasm, capsids travel along microtubules to reach, dock, and envelope at cytoplasmic organelles. This generates mature infectious HSV-1 particles that must then be sorted to the termini of sensory neurons, or to epithelial cell junctions, for spread to uninfected cells. The focus of this review is upon our current understanding of the viral and cellular molecular machinery that enables HSV-1 to travel within infected cells during egress and to manipulate cellular organelles to construct its envelope.

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.

Dissecting the Herpesvirus Architecture by Targeted Proteolysis

Herpesvirus particles have a complex architecture consisting of an icosahedral capsid that is surrounded by a lipid envelope. Connecting these two components is a layer of tegument that consists of various amounts of 20 or more proteins. The arrangement of proteins within the tegument cannot easily be assessed and instead is inferred from tegument interactions identified in reductionist models. To better understand the tegument architecture, we have developed an approach to probe capsid-tegument interactions of extracellular viral particles by encoding tobacco etch virus (TEV) protease sites in viral structural proteins, along with distinct fluorescent tags in capsid and tegument components. In this study, TEV sites were engineered within the pUL36 large tegument protein, a critical structural element that is anchored directly on the capsid surface. Purified pseudorabies virus extracellular particles were permeabilized, and TEV protease was added to selectively cleave the exposed pUL36 backbone. Interactions with the capsid were assessed in situ by monitoring the fate of the fluorescent signals following cleavage. Although several regions of pUL36 are proposed to bind capsids, pUL36 was found stably anchored to the capsid exclusively at its carboxyl terminus. Two additional tegument proteins, pUL37 and pUS3, were tethered to the capsid via pUL36, whereas the pUL16, pUL47, pUL48, and pUL49 tegument proteins were not stably bound to the capsid.IMPORTANCE Neuroinvasive alphaherpesviruses produce diseases of clinical and economic significance in humans and veterinary animals but are predominantly associated with less serious recurrent disease. Like all viruses, herpesviruses assemble a metastable particle that selectively dismantles during initial infection. This process is made more complex by the presence of a tegument layer that resides between the capsid surface and envelope. Components of the tegument are essential for particle assembly and also serve as critical effectors that promote infection upon entry into cells. How this dynamic network of protein interactions is arranged within virions is largely unknown. We present a molecular approach to dissect the tegument, and with it we begin to tease apart the protein interactions that underlie this complex layer of the virion architecture.

Virus Assembly and Egress of HSV

The assembly and egress of herpes simplex virus (HSV) is a complicated multistage process that involves several different cellular compartments and the activity of many viral and cellular proteins. The process begins in the nucleus, with capsid assembly followed by genome packaging into the preformed capsids. The DNA-filled capsids (nucleocapsids) then exit the nucleus by a process of envelopment at the inner nuclear membrane followed by fusion with the outer nuclear membrane. In the cytoplasm nucleocapsids associate with tegument proteins, which form a complicated protein network that links the nucleocapsid to the cytoplasmic domains of viral envelope proteins. Nucleocapsids and associated tegument then undergo secondary envelopment at intracellular membranes originating from late secretory pathway and endosomal compartments. This leads to assembled virions in the lumen of large cytoplasmic vesicles, which are then transported to the cell periphery to fuse with the plasma membrane and release virus particles from the cell. The details of this multifaceted process are described in this chapter.

Vertex-Specific Proteins pUL17 and pUL25 Mechanically Reinforce Herpes Simplex Virus Capsids

Using atomic force microscopy imaging and nanoindentation measurements, we investigated the effect of the minor capsid proteins pUL17 and pUL25 on the structural stability of icosahedral herpes simplex virus capsids. pUL17 and pUL25, which form the capsid vertex-specific component (CVSC), particularly contributed to capsid resilience along the 5-fold and 2-fold but not along the 3-fold icosahedral axes. Our detailed analyses, including quantitative mass spectrometry of the protein composition of the capsids, revealed that both pUL17 and pUL25 are required to stabilize the capsid shells at the vertices. This indicates that herpesviruses withstand the internal pressure that is generated during DNA genome packaging by locally reinforcing the mechanical sturdiness of the vertices, the most stressed part of the capsids.IMPORTANCE In this study, the structural, material properties of herpes simplex virus 1 were investigated. The capsid of herpes simplex virus is built up of a variety of proteins, and we scrutinized the influence of two of these proteins on the stability of the capsid. For this, we used a scanning force microscope that makes detailed, topographic images of the particles and that is able to perform mechanical deformation measurements. Using this approach, we revealed that both studied proteins play an essential role in viral stability. These new insights support us in forming a complete view on viral structure and furthermore could possibly help not only to develop specific antivirals but also to build protein shells with improved stability for drug delivery purposes.

Assembly and Egress of an Alphaherpesvirus Clockwork

All viruses produce infectious particles that possess some degree of stability in the extracellular environment yet disassemble upon cell contact and entry. For the alphaherpesviruses, which include many neuroinvasive viruses of mammals, these metastable virions consist of an icosahedral capsid surrounded by a protein matrix (referred to as the tegument) and a lipid envelope studded with glycoproteins. Whereas the capsid of these viruses is a rigid structure encasing the DNA genome, the tegument and envelope are dynamic assemblies that orchestrate a sequential series of events that ends with the delivery of the genome into the nucleus. These particles are adapted to infect two different polarized cell types in their hosts: epithelial cells and neurons of the peripheral nervous system. This review considers how the virion is assembled into a primed state and is targeted to infect these cell types such that the incoming particles can subsequently negotiate the diverse environments they encounter on their way from plasma membrane to nucleus and thereby achieve their remarkably robust neuroinvasive infectious cycle.

Tegument Assembly and Secondary Envelopment of Alphaherpesviruses

Alphaherpesviruses like herpes simplex virus are large DNA viruses characterized by their ability to establish lifelong latent infection in neurons. As for all herpesviruses, alphaherpesvirus virions contain a protein-rich layer called "tegument" that links the DNA-containing capsid to the glycoprotein-studded membrane envelope. Tegument proteins mediate a diverse range of functions during the virus lifecycle, including modulation of the host-cell environment immediately after entry, transport of virus capsids to the nucleus during infection, and wrapping of cytoplasmic capsids with membranes (secondary envelopment) during virion assembly. Eleven tegument proteins that are conserved across alphaherpesviruses have been implicated in the formation of the tegument layer or in secondary envelopment. Tegument is assembled via a dense network of interactions between tegument proteins, with the redundancy of these interactions making it challenging to determine the precise function of any specific tegument protein. However, recent studies have made great headway in defining the interactions between tegument proteins, conserved across alphaherpesviruses, which facilitate tegument assembly and secondary envelopment. We summarize these recent advances and review what remains to be learned about the molecular interactions required to assemble mature alphaherpesvirus virions following the release of capsids from infected cell nuclei.

Role of Host Cell p32 in Herpes Simplex Virus 1 De-Envelopment during Viral Nuclear Egress

To clarify the function(s) of the herpes simplex virus 1 (HSV-1) major virion structural protein UL47 (also designated VP13/14), we screened cells overexpressing UL47 for UL47-binding cellular proteins. Tandem affinity purification of transiently expressed UL47 coupled with mass spectrometry-based proteomics technology and subsequent analyses showed that UL47 interacted with cell protein p32 in HSV-1-infected cells. Unlike in mock-infected cells, p32 accumulated at the nuclear rim in HSV-1-infected cells, and this p32 recruitment to the nuclear rim required UL47. p32 formed a complex(es) with HSV-1 proteins UL31, UL34, Us3, UL47, and/or ICP22 in HSV-1-infected cells. All these HSV-1 proteins were previously reported to be important for HSV-1 nuclear egress, in which nucleocapsids bud through the inner nuclear membrane (primary envelopment) and the enveloped nucleocapsids then fuse with the outer nuclear membrane (de-envelopment). Like viral proteins UL31, UL34, Us3, and UL47, p32 was detected in primary enveloped virions. p32 knockdown reduced viral replication and induced membranous invaginations adjacent to the nuclear rim containing primary enveloped virions and aberrant localization of UL31 and UL34 in punctate structures at the nuclear rim. These effects of p32 knockdown were reduced in the absence of UL47. Therefore, the effects of p32 knockdown in HSV-1 nuclear egress were similar to those of the previously reported mutation(s) in HSV-1 regulatory proteins for HSV-1 de-envelopment during viral nuclear egress. Collectively, these results suggested that p32 regulated HSV-1 de-envelopment and replication in a UL47-dependent manner. IMPORTANCE In this study, we have obtained data suggesting that (i) the HSV-1 major virion structural protein UL47 interacted with host cell protein p32 and mediated the recruitment of p32 to the nuclear rim in HSV-1-infected cells; (ii) p32 was a component of the HSV-1 nuclear egress complex (NEC), whose core components were UL31 and UL34; and (iii) p32 regulated HSV-1 de-envelopment during viral nuclear egress. It has been reported that p32 was a component of human cytomegalovirus NEC and was required for efficient disintegration of nuclear lamina, which has been thought to facilitate HSV-1 primary envelopment during viral nuclear egress. Thus, p32 appeared to be a core component of herpesvirus NECs, like UL31 and UL34 homologs in other herpesviruses, and to play multiple roles in herpesvirus nuclear egress.

Herpes simplex virus 1 UL47 interacts with viral nuclear egress factors UL31, UL34, and Us3 and regulates viral nuclear egress

Herpesviruses have evolved a unique mechanism for nuclear egress of nascent progeny nucleocapsids: the nucleocapsids bud through the inner nuclear membrane into the perinuclear space between the inner and outer nuclear membranes (primary envelopment), and enveloped nucleocapsids then fuse with the outer nuclear membrane to release nucleocapsids into the cytoplasm (de-envelopment). We have shown that the herpes simplex virus 1 (HSV-1) major virion structural protein UL47 (or VP13/VP14) is a novel regulator for HSV-1 nuclear egress. In particular, we demonstrated the following: (i) UL47 formed a complex(es) with HSV-1 proteins UL34, UL31, and/or Us3, which have all been reported to be critical for viral nuclear egress, and these viral proteins colocalized at the nuclear membrane in HSV-1-infected cells; (ii) the UL47-null mutation considerably reduced primary enveloped virions in the perinuclear space although capsids accumulated in the nucleus; and (iii) UL47 was detected in primary enveloped virions in the perinuclear space by immunoelectron microscopy. These results suggested that UL47 promoted HSV-1 primary envelopment, probably by interacting with the critical HSV-1 regulators for viral nuclear egress and by modulating their functions.

IMPORTANCE

Like other herpesviruses, herpes simplex virus 1 (HSV-1) has evolved a vesicle-mediated nucleocytoplasmic transport mechanism for nuclear egress of nascent progeny nucleocapsids. Although previous reports identified and characterized several HSV-1 and cellular proteins involved in viral nuclear egress, complete details of HSV-1 nuclear egress remain to be elucidated. In this study, we have presented data suggesting (i) that the major HSV-1 virion structural protein UL47 (or VP13/VP14) formed a complex with known viral regulatory proteins critical for viral nuclear egress and (ii) that UL47 played a regulatory role in HSV-1 primary envelopment. Thus, we identified UL47 as a novel regulator for HSV-1 nuclear egress.

A proteomic perspective of inbuilt viral protein regulation: pUL46 tegument protein is targeted for degradation by ICP0 during herpes simplex virus type 1 infection

Much like the host cells they infect, viruses must also regulate their life cycles. Herpes simples virus type 1 (HSV-1), a prominent human pathogen, uses a promoter-rich genome in conjunction with multiple viral trans-activating factors. Following entry into host cells, the virion-associated outer tegument proteins pUL46 and pUL47 act to increase expression of viral immediate-early (α) genes, thereby helping initiate the infection life cycle. Because pUL46 has gone largely unstudied, we employed a hybrid mass spectrometry-based approach to determine how pUL46 exerts its functions during early stages of infection. For a spatio-temporal characterization of pUL46, time-lapse microscopy was performed in live cells to define its dynamic localization from 2 to 24 h postinfection. Next, pUL46-containing protein complexes were immunoaffinity purified during infection of human fibroblasts and analyzed by mass spectrometry to investigate virus-virus and virus-host interactions, as well as post-translational modifications. We demonstrated that pUL46 is heavily phosphorylated in at least 23 sites. One phosphorylation site matched the consensus 14-3-3 phospho-binding motif, consistent with our identification of 14-3-3 proteins and host and viral kinases as specific pUL46 interactions. Moreover, we determined that pUL46 specifically interacts with the viral E3 ubiquitin ligase ICP0. We demonstrated that pUL46 is partially degraded in a proteasome-mediated manner during infection, and that the catalytic activity of ICP0 is responsible for this degradation. This is the first evidence of a viral protein being targeted for degradation by another viral protein during HSV-1 infection. Together, these data indicate that pUL46 levels are tightly controlled and important for the temporal regulation of viral gene expression throughout the virus life cycle. The concept of a structural virion protein, pUL46, performing nonstructural roles is likely to reflect a theme common to many viruses, and a better understanding of these functions will be important for developing therapeutics.

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.

Analysis of the interaction between the essential herpes simplex virus 1 tegument proteins VP16 and VP1/2

The incorporation of tegument proteins into the herpes simplex virus 1 (HSV-1) virion during virion assembly is thought to be a complex, multistage process occurring via numerous interactions between the tegument and the capsid, within the tegument, and between the tegument and the envelope. Here, we set out to examine if the direct interaction between two essential tegument proteins VP1/2 and VP16 is required for connecting the inner tegument with the outer tegument. By using glutathione S-transferase (GST) pulldowns, we identified an essential role of lysine 343 in VP16, mutation of which to a neutral amino acid abrogated the interaction between VP1/2 and VP16. When the K343A substitution was inserted into the gene encoding VP16 (UL48) of the viral genome, HSV-1 replicated successfully although its growth was delayed, and final titers were reduced compared to titers of wild-type virus. Surprisingly, the mutated VP16 was incorporated into virions at levels similar to those of wild-type VP16. However, the analysis of VP16 on cytoplasmic capsids by fluorescence microscopy showed that VP16 associated with cytoplasmic capsids less efficiently when the VP16-VP1/2 interaction was inhibited. This implies that the direct interaction between VP1/2 and VP16 is important for the efficiency/timing of viral assembly but is not essential for HSV-1 replication in cell culture. These data also support the notion that the incorporation of tegument proteins into the herpesviruses is a very complex process with significant redundancy.

Biochemical analysis of infected cell polypeptide (ICP)0, ICP4, UL7 and UL23 incorporated into extracellular herpes simplex virus type 1 virions

Herpes simplex virus type 1 (HSV-1) capsids assemble in the nucleus but acquire their teguments from various cellular compartments. Unfortunately, little is known about their exact arrangement and when they coat the newly produced capsids. The complexity of the virions is further highlighted by our recent proteomics analysis that detected the presence of several novel or controversial components in extracellular HSV-1 virions. The present study probes the localization and linkage to the virus particles of some of these incorporated proteins. We confirm the recently reported tight association of infected cell polypeptide (ICP)0 with the capsid and show that this property extends to ICP4. We also confirm our proteomics data and show biochemically that UL7 and UL23 are indeed mature virion tegument components that, unlike ICP0 and ICP4, are salt-extractable. Interestingly, treatment with N-ethylmaleimide, which covalently modifies reduced cysteines, strongly prevented the release of UL7 and UL23 by salts, but did not perturb the interactions of ICP0 and ICP4 with the virus particles. This hitheir at distinct biochemical properties of the virion constituents and the selective implication of reduced cysteines in their organization and dynamics. Finally, the data revealed, by two independent means, the presence of ICP0 and ICP4 on intranuclear capsids, consistent with the possibility that they may at least partially be recruited to the virus particles early on. These findings add significantly to our understanding of HSV-1 virion assembly and to the debate about the incorporation of ICP0 and ICP4 in virus particles.

Selection of HSV capsids for envelopment involves interaction between capsid surface components pUL31, pUL17, and pUL25

During egress from the nucleus, HSV capsids that contain DNA (termed C capsids) are preferentially enveloped at the inner nuclear membrane over capsid types lacking DNA. Using coimmunoprecipitation and biochemical analyses of wild-type and mutant capsids, we identify an interaction between a complex of pU(L)17/pU(L)25, termed the C capsid-specific complex (CCSC), and pU(L)31, a component of the nuclear egress complex (NEC). We also show that the interactions between these components are dependent on expression of all three proteins but occur independently of the pU(L)31 interacting protein and NEC component pU(L)34, as well as a kinase encoded by U(S)3 that phosphorylates both pU(L)31 and pU(L)34. The interaction between the CCSC and pU(L)31 in the NEC suggests a mechanism to conserve viral resources by promoting assembly of only those viral particles with the potential to become infectious.