The Epstein-Barr virus BFRF1 and BFLF2 proteins interact and coexpression alters their cellular localization

The universal suppressor mutation restores membrane budding defects in the HSV-1 nuclear egress complex by stabilizing the oligomeric lattice

Nuclear egress is an essential process in herpesvirus replication whereby nascent capsids translocate from the nucleus to the cytoplasm. This initial step of nuclear egress-budding at the inner nuclear membrane-is coordinated by the nuclear egress complex (NEC). Composed of the viral proteins UL31 and UL34, NEC deforms the membrane around the capsid as the latter buds into the perinuclear space. NEC oligomerization into a hexagonal membrane-bound lattice is essential for budding because NEC mutants designed to perturb lattice interfaces reduce its budding ability. Previously, we identified an NEC suppressor mutation capable of restoring budding to a mutant with a weakened hexagonal lattice. Using an established in-vitro budding assay and HSV-1 infected cell experiments, we show that the suppressor mutation can restore budding to a broad range of budding-deficient NEC mutants thereby acting as a universal suppressor. Cryogenic electron tomography of the suppressor NEC mutant lattice revealed a hexagonal lattice reminiscent of wild-type NEC lattice instead of an alternative lattice. Further investigation using x-ray crystallography showed that the suppressor mutation promoted the formation of new contacts between the NEC hexamers that, ostensibly, stabilized the hexagonal lattice. This stabilization strategy is powerful enough to override the otherwise deleterious effects of mutations that destabilize the NEC lattice by different mechanisms, resulting in a functional NEC hexagonal lattice and restoration of membrane budding.

Extragenic suppression of an HSV-1 UL34 nuclear egress mutant reveals role for pUS9 as an inhibitor of epithelial cell-to-cell spread

IMPORTANCE

Herpesviruses are able to disseminate in infected hosts despite development of a strong immune response. Their ability to do this relies on a specialized process called cell-to-cell spread in which newly assembled virus particles are trafficked to plasma membrane surfaces that abut adjacent uninfected cells. The mechanism of cell-to-cell spread is obscure, and little is known about whether or how it is regulated in different cells. We show here that a viral protein with a well-characterized role in promoting spread from neurons has an opposite, inhibitory role in other cells.

Application of functional proteomics in understanding RNA virus-mediated infection

Together with the expansion of genome sequencing research, the number of protein sequences whose function is yet unknown is increasing dramatically. The primary goals of functional proteomics, a developing area of study in the realm of proteomic science, are the elucidation of the biological function of unidentified proteins and the molecular description of cellular systems at the molecular level. RNA viruses have emerged as the cause of several human infectious diseases with large morbidity and fatality rates. The introduction of high-throughput sequencing tools and genetic-based screening approaches over the last few decades has enabled researchers to find previously unknown and perplexing elements of RNA virus replication and pathogenesis on a scale never feasible before. Viruses, on the other hand, frequently disrupt cellular proteostasis, macromolecular complex architecture or stoichiometry, and post-translational changes to take over essential host activities. Because of these consequences, structural and global protein and proteoform monitoring is highly necessiated. Mass spectrometry (MS) has the potential to elucidate key details of virus-host interactions and speed up the identification of antiviral targets, giving precise data on the stoichiometry of cellular and viral protein complexes as well as mechanistic insights, has lately emerged as a key part of the RNA virus biology toolbox as a functional proteomics approach. Affinity-based techniques are primarily employed to identify interacting proteins in stable complexes in living organisms. A protein's biological role is strongly suggested by its relationship with other members of a certain protein complex that is involved in a particular process. With a particular emphasis on the most recent advancements in defining host responses and their translational implications to uncover novel tractable antiviral targets, this chapter provides insight on several functional proteomics techniques in RNA virus biology.

DISTINCT ROLES OF VIRAL US3 AND UL13 PROTEIN KINASES IN HERPES VIRUS SIMPLEX TYPE 1 (HSV-1) NUCLEAR EGRESS

Herpesviruses transport nucleocapsids from the nucleus to the cytoplasm by capsid envelopment into the inner nuclear membrane and de-envelopment from the outer nuclear membrane, a process that is coordinated by nuclear egress complex (NEC) proteins, pUL34, and pUL31. Both pUL31 and pUL34 are phosphorylated by the virus-encoded protein kinase, pUS3, and phosphorylation of pUL31 regulates NEC localization at the nuclear rim. pUS3 also controls apoptosis and many other viral and cellular functions in addition to nuclear egress, and the regulation of these various activities in infected cells is not well understood. It has been previously proposed that pUS3 activity is selectively regulated by another viral protein kinase, pUL13 such that its activity in nuclear egress is pUL13-dependent, but apoptosis regulation is not, suggesting that pUL13 might regulate pUS3 activity on specific substrates. We compared HSV-1 UL13 kinase-dead and US3 kinase-dead mutant infections and found that pUL13 kinase activity does not regulate the substrate choice of pUS3 in any defined classes of pUS3 substrates and that pUL13 kinase activity is not important for promoting de-envelopment during nuclear egress. We also find that mutation of all pUL13 phosphorylation motifs in pUS3, individually or in aggregate, does not affect the localization of the NEC, suggesting that pUL13 regulates NEC localization independent of pUS3. Finally, we show that pUL13 co-localizes with pUL31 inside the nucleus in large aggregates, further suggesting a direct effect of pUL13 on the NEC and suggesting a novel mechanism for both UL31 and UL13 in the DNA damage response pathway. IMPORTANCE Herpes simplex virus infections are regulated by two virus-encoded protein kinases, pUS3 and pUL13, which each regulate multiple processes in the infected cell, including capsid transport from the nucleus to the cytoplasm. Regulation of the activity of these kinases on their various substrates is poorly understood, but importantly, kinases are attractive targets for the generation of inhibitors. It has been previously suggested that pUS3 activity on specific substrates is differentially regulated by pUL13 and, specifically, that pUL13 regulates capsid egress from the nucleus by phosphorylation of pUS3. In this study, we determined that pUL13 and pUS3 have different effects on nuclear egress and that pUL13 may interact directly with the nuclear egress apparatus with implications both for virus assembly and egress and, possibly, the host cell DNA- damage response.

Functional diversity: update of the posttranslational modification of Epstein-Barr virus coding proteins

Epstein-Barr virus (EBV), a human oncogenic herpesvirus with a typical life cycle consisting of latent phase and lytic phase, is associated with many human diseases. EBV can express a variety of proteins that enable the virus to affect host cell processes and evade host immunity. Additionally, these proteins provide a basis for the maintenance of viral infection, contribute to the formation of tumors, and influence the occurrence and development of related diseases. Posttranslational modifications (PTMs) are chemical modifications of proteins after translation and are very important to guarantee the proper biological functions of these proteins. Studies in the past have intensely investigated PTMs of EBV-encoded proteins. EBV regulates the progression of the latent phase and lytic phase by affecting the PTMs of its encoded proteins, which are critical for the development of EBV-associated human diseases. In this review, we summarize the PTMs of EBV-encoded proteins that have been discovered and studied thus far with focus on their effects on the viral life cycle.

Cell Culture Evolution of a Herpes Simplex Virus 1 (HSV-1)/Varicella-Zoster Virus (VZV) UL34/ORF24 Chimeric Virus Reveals Novel Functions for HSV Genes in Capsid Nuclear Egress

Herpes simplex virus (HSV) and varicella-zoster virus (VZV) are both members of the alphaherpesvirus subfamily but belong to different genera. Substitution of the HSV-1 UL34 coding sequence with that of its VZV homolog, open reading frame 24 (ORF24), results in a virus that has defects in viral growth, spread, capsid egress, and nuclear lamina disruption very similar to those seen in a UL34-null virus despite normal interaction between ORF24 protein and HSV pUL31 and proper localization of the nuclear egress complex at the nuclear envelope. Minimal selection for growth in cell culture resulted in viruses that grew and spread much more efficiently that the parental chimeric virus. These viruses varied in their ability to support nuclear lamina disruption, normal nuclear egress complex localization, and capsid de-envelopment. Single mutations that suppress the growth defect were mapped to the coding sequences of ORF24, ICP22, and ICP4, and one virus carried single mutations in each of the ICP22 and US3 coding sequences. The phenotypes of these viruses support a role for ICP22 in nuclear lamina disruption and a completely unexpected role for the major transcriptional regulator, ICP4, in capsid nuclear egress. IMPORTANCE Interactions among virus proteins are critical for assembly and egress of virus particles, and such interactions are attractive targets for antiviral therapy. Identification of critical functional interactions can be slow and tedious. Capsid nuclear egress of herpesviruses is a critical event in the assembly and egress pathway and is mediated by two proteins, pUL31 and pUL34, that are conserved among herpesviruses. Here, we describe a cell culture evolution approach to identify other viral gene products that functionally interact with pUL34.

Prazoles Targeting Tsg101 Inhibit Release of Epstein-Barr Virus following Reactivation from Latency

Epstein-Barr virus (EBV) is a ubiquitous herpesvirus responsible for several diseases, including cancers of lymphoid and epithelial cells. EBV cancers typically exhibit viral latency; however, the production and release of EBV through its lytic phase are essential for cancer development. Antiviral agents that specifically target EBV production do not currently exist. Previously, we reported that the proton pump inhibitor tenatoprazole, which blocks the interaction of ubiquitin with the ESCRT-1 factor Tsg101, inhibits production of several enveloped viruses, including EBV. Here, we show that three structurally distinct prazoles impair mature particle formation postreactivation and identify the impact on stages of replication. The prazoles did not impair expression of lytic genes representative of the different kinetic classes but interfered with capsid maturation in the nucleus as well as virion transport from the nucleus. Replacement of endogenous Tsg101 with a mutant Tsg101 refractory to prazole-mediated inhibition rescued EBV release. These findings directly implicate Tsg101 in EBV nuclear egress and identify prazoles as potential therapeutic candidates for conditions that rely on EBV replication, such as chronic active EBV infection and posttransplant lymphoproliferative disorders. IMPORTANCE Production of virions is necessary for the ubiquitous Epstein-Barr virus (EBV) to persist in humans and can set the stage for development of EBV cancers in at-risk individuals. In our attempts to identify inhibitors of the EBV lytic phase, we previously found that a prazole proton pump inhibitor, known to block the interaction of ubiquitin with the ESCRT-1 factor Tsg101, blocks production of EBV. We now find that three structurally distinct prazoles impair maturation of EBV capsids and virion transport from the nucleus and, by interfering with Tsg101, prevent EBV release from lytically active cells. Our findings not only implicate Tsg101 in EBV production but also identify widely used prazoles as candidates to prevent development of posttransplant EBV lymphomas.

Mechanism of Nuclear Lamina Disruption and the Role of pUS3 in HSV-1 Nuclear Egress

Herpes simplex virus capsid envelopment at the nuclear membrane is coordinated by nuclear egress complex (NEC) proteins, pUL34 and pUL31, and is accompanied by alteration in the nuclear architecture and local disruption of nuclear lamina. Here, we examined the role of capsid envelopment in the changes of the nuclear architecture by characterizing HSV-1 recombinants that do not form capsids. Typical changes in nuclear architecture and disruption of the lamina were observed in the absence of capsids, suggesting that disruption of the nuclear lamina occurs prior to capsid envelopment. Surprisingly, in the absence of capsid envelopment, lamin A/C becomes concentrated at the nuclear envelope in a pUL34-independent and cell type-specific manner, suggesting that ongoing nuclear egress may be required for the dispersal of lamins observed in wild-type infection. Mutation of virus-encoded protein kinase, pUS3, on a wild-type virus background has been shown to cause accumulation of perinuclear enveloped capsids, formation of NEC aggregates, and exacerbated lamina disruption. We observed that mutation of US3 in the absence of capsids results in identical NEC aggregation and lamina disruption phenotypes, suggesting that they do not result from accumulation of perinuclear virions. TEM analysis revealed that, in the absence of capsids, NEC aggregates correspond to multi-folded nuclear membrane structures, suggesting that pUS3 may control NEC self-association and membrane deformation. To determine the significance of the pUS3 nuclear egress function for virus growth, the replication of single and double UL34 and US3 mutants was measured, showing that the significance of pUS3 nuclear egress function is cell-type specific.ImportanceThe nuclear lamina is an important player in infection by viruses that replicate in the nucleus. Herpesviruses alter the structure of the nuclear lamina to facilitate transport of capsids from the nucleus to the cytoplasm and use both viral and cellular effectors to disrupt the protein-protein interactions that maintain the lamina. Here we explore the role of capsid envelopment and the virus-encoded protein kinase, pUS3, in the disruption of lamina structure. We show that capsid envelopment is not necessary for the lamina disruption, or for US3 mutant phenotypes, including exaggerated lamina disruption, that accompany nuclear egress. These results clarify the mechanisms behind alteration of nuclear lamina structure and support a function for pUS3 in regulating the aggregation state of the nuclear egress machinery.

Epstein-Barr Virus Early Protein BFRF1 Suppresses IFN-β Activity by Inhibiting the Activation of IRF3

Epstein-Barr virus (EBV) is the causative agent of infectious mononucleosis that is closely associated with several human malignant diseases, while type I interferon (IFN-I) plays an important role against EBV infection. As we all know, EBV can encode some proteins to inhibit the production of IFN-I, but it’s not clear whether other proteins also take part in this progress. EBV early lytic protein BFRF1 is shown to be involved in viral maturation, however, whether BFRF1 participates in the host innate immune response is still not well known. In this study, we found BFRF1 could down-regulate sendai virus-induced IFN-β promoter activity and mRNA expression of IFN-β and ISG54 during BFRF1 plasmid transfection and EBV lytic infection, but BFRF1 could not affect the promoter activity of NF-κB or IRF7. Specifically, BFRF1 could co-localize and interact with IKKi. Although BFRF1 did not interfere the interaction between IKKi and IRF3, it could block the kinase activity of IKKi, which finally inhibited the phosphorylation, dimerization, and nuclear translocation of IRF3. Taken together, BFRF1 may play a critical role in disrupting the host innate immunity by suppressing IFN-β activity during EBV lytic cycle.

Selective Targeting of Virus Replication by Proton Pump Inhibitors

Two proton pump inhibitors, tenatoprazole and esomeprazole, were previously shown to inhibit HIV-1 egress by blocking the interaction between Tsg101, a member of the ESCRT-I complex, and ubiquitin. Here, we deepen our understanding of prazole budding inhibition by studying a range of viruses in the presence of tenatoprazole. Furthermore, we investigate the relationship between the chemistry of prodrug activation and HIV-1 inhibition for diverse prazoles currently on the market. We report that tenatoprazole is capable of inhibiting the replication of members of the enveloped filo, alpha, and herpes virus families but not the flavivirus group and not the non-enveloped poliovirus. Another key finding is that prazole prodrugs must be activated inside the cell, while their rate of activation in vitro correlated to their efficacy in cells. Our study lays the groundwork for future efforts to repurpose prazole-based compounds as antivirals that are both broad-spectrum and selective in nature.

Seeking Closure: How Do Herpesviruses Recruit the Cellular ESCRT Apparatus?

The Herpesviridae are structurally complex DNA viruses whose capsids undergo primary envelopment at the inner nuclear membrane and secondary envelopment at organelles in the cytoplasm. In both locations, there is evidence that envelope formation and scission involve the participation of multiple viral proteins and also the cellular ESCRT apparatus. It nevertheless appears that the best-understood viral strategies for ESCRT recruitment, those adopted by the retroviruses and many other families of enveloped RNA viruses, are not utilized by the Herpesviridae, at least during envelopment in the cytoplasm. Thus, although a large number of herpesvirus proteins have been assigned roles in envelopment, there is a dearth of candidates for the acquisition of the ESCRT complex and the control of envelope scission. This review summarizes our current understanding of ESCRT association by enveloped viruses, examines what is known of herpesvirus ESCRT utilization in the nucleus and cytoplasm, and identifies candidate cellular and viral proteins that could link enveloping herpesviruses to cellular ESCRT components.

Venture from the Interior-Herpesvirus pUL31 Escorts Capsids from Nucleoplasmic Replication Compartments to Sites of Primary Envelopment at the Inner Nuclear Membrane

Herpesviral capsid assembly is initiated in the nucleoplasm of the infected cell. Size constraints require that newly formed viral nucleocapsids leave the nucleus by an evolutionarily conserved vescular transport mechanism called nuclear egress. Mature capsids released from the nucleoplasm are engaged in a membrane-mediated budding process, composed of primary envelopment at the inner nuclear membrane and de-envelopment at the outer nuclear membrane. Once in the cytoplasm, the capsids receive their secondary envelope for maturation into infectious virions. Two viral proteins conserved throughout the herpesvirus family, the integral membrane protein pUL34 and the phosphoprotein pUL31, form the nuclear egress complex required for capsid transport from the infected nucleus to the cytoplasm. Formation of the nuclear egress complex results in budding of membrane vesicles revealing its function as minimal virus-encoded membrane budding and scission machinery. The recent structural analysis unraveled details of the heterodimeric nuclear egress complex and the hexagonal coat it forms at the inside of budding vesicles to drive primary envelopment. With this review, I would like to present the capsid-escort-model where pUL31 associates with capsids in nucleoplasmic replication compartments for escort to sites of primary envelopment thereby coupling capsid maturation and nuclear egress.

Characterization of the subcellular localization of Epstein-Barr virus encoded proteins in live cells

Epstein-Barr virus (EBV) is the pathogenic factor of numerous human tumors, yet certain of its encoded proteins have not been studied. As a first step for functional identification, we presented the construction of a library of expression constructs for most of the EBV encoded proteins and an explicit subcellular localization map of 81 proteins encoded by EBV in mammalian cells. Viral open reading frames were fused with enhanced yellow fluorescent protein (EYFP) tag in eukaryotic expression plasmid then expressed in COS-7 live cells, and protein localizations were observed by fluorescence microscopy. As results, 34.57% (28 proteins) of all proteins showed pan-nuclear or subnuclear localization, 39.51% (32 proteins) exhibitted pan-cytoplasmic or subcytoplasmic localization, and 25.93% (21 proteins) were found in both the nucleus and cytoplasm. Interestingly, most envelope proteins presented pan-cytoplasmic or membranous localization, and most capsid proteins displayed enriched or complete localization in the nucleus, indicating that the subcellular localization of specific proteins are associated with their roles during viral replication. Taken together, the subcellular localization map of EBV proteins in live cells may lay the foundation for further illustrating the functions of EBV-encoded genes in human diseases especially in its relevant tumors.

The human cytomegalovirus nuclear egress complex unites multiple functions: Recruitment of effectors, nuclear envelope rearrangement, and docking to nuclear capsids

BACKGROUND

Nuclear replication represents a common hallmark of herpesviruses achieved by a number of sequentially unrolled regulatory processes. A rate-limiting step is provided by nucleo-cytoplasmic capsid export, for which a defined multiregulatory protein complex, namely, the nuclear egress complex (NEC), is assembled comprising both viral and cellular components. The NEC regulates at least 3 aspects of herpesviral nuclear replication: (1) multimeric recruitment of NEC-associated effector proteins, (2) reorganization of the nuclear lamina and membranes, and (3) the docking to nuclear capsids. Here, we review published data and own experimental work that characterizes the NEC of HCMV and other herpesviruses.

METHODS

A systematic review of information on nuclear egress of HCMV compared to selected alpha-, beta-, and gamma-herpesviruses: proteomics-based approaches, high-resolution imaging techniques, and functional investigations.

RESULTS

A large number of reports on herpesviral NECs have been published during the last two decades, focusing on protein-protein interactions, nuclear localization, regulatory phosphorylation, and functional validation. The emerging picture provides an illustrated example of well-balanced and sophisticated protein networking in virus-host interaction.

CONCLUSIONS

Current evidence refined the view about herpesviral NECs. Datasets published for HCMV, murine CMV, herpes simplex virus, and Epstein-Barr virus illustrate the marked functional consistency in the way herpesviruses achieve nuclear egress. However, this compares with only limited sequence conservation of core NEC proteins and a structural conservation restricted to individual domains. The translational use of this information might help to define a novel antiviral strategy on the basis of NEC-directed small molecules.

Herpesvirus Nuclear Egress

Herpesviruses assemble and package their genomes into capsids in the nucleus, but complete final assembly of the mature virion in the cell cytoplasm. This requires passage of the genome-containing capsid across the double-membrane nuclear envelope. Herpesviruses have evolved a mechanism that relies on a pair of conserved viral gene products to shuttle the capsids from the nucleus to the cytoplasm by way of envelopment and de-envelopment at the inner and outer nuclear membranes, respectively. This complex process requires orchestration of the activities of viral and cellular factors to alter the architecture of the nuclear membrane, select capsids at the appropriate stage for egress, and accomplish efficient membrane budding and fusion events. The last few years have seen major advances in our understanding of the membrane budding mechanism and helped clarify the roles of viral and cellular proteins in the other, more mysterious steps. Here, we summarize and place into context this recent research and, hopefully, clarify both the major advances and major gaps in our understanding.

The absence of p53 during Human Cytomegalovirus infection leads to decreased UL53 expression, disrupting UL50 localization to the inner nuclear membrane, and thereby inhibiting capsid nuclear egress

Our electron microscopy study (Kuan et al., 2016) found HCMV nuclear capsid egress was significantly reduced in p53 knockout cells (p53KOs), correlating with inhibited formation of infoldings of the inner nuclear membrane (IINMs). Molecular examination of these phenomena has found p53KOs expressed UL97 and phosphorylated lamins, however the lamina failed to remodel. The nuclear egress complex (NEC) protein UL50 was expressed in almost all cells. UL50 re-localized to the inner nuclear membrane (INM) in ~90% of wt cells, but only ~35% of p53KOs. UL53 expression was significantly reduced in p53KOs, and cells lacking UL50 nuclear staining, expressed no UL53. Re-introduction of p53 into p53KOs largely recovered UL53 positivity and UL50 nuclear re-localization. Nuclear rim located UL50/53 puncta, which co-localized with the major capsid protein, were largely absent in p53KOs. We believe these puncta were IINMs. In the absence of p53, UL53 expression was inhibited, disrupting formation of the NEC/IINMs, and reducing functional virion secretion.

Epstein-Barr virus glycoprotein gM can interact with the cellular protein p32 and knockdown of p32 impairs virus

The Epstein-Barr virus glycoprotein complex gMgN has been implicated in assembly and release of fully enveloped virus, although the precise role that it plays has not been elucidated. We report here that the long predicted cytoplasmic tail of gM is not required for complex formation and that it interacts with the cellular protein p32, which has been reported to be involved in nuclear egress of human cytomegalovirus and herpes simplex virus. Although redistribution of p32 and colocalization with gM was not observed in virus infected cells, knockdown of p32 expression by siRNA or lentivirus-delivered shRNA recapitulated the phenotype of a virus lacking expression of gNgM. A proportion of virus released from cells sedimented with characteristics of virus lacking an intact envelope and there was an increase in virus trapped in nuclear condensed chromatin. The observations suggest the possibility that p32 may also be involved in nuclear egress of Epstein-Barr virus.

Identification of molecular determinants for the nuclear import of pseudorabies virus UL31

Herpes simplex virus 1 (HSV-1) UL31 is a multifunctional protein and important for HSV-1 infection. Pseudorabies virus (PRV) UL31 is a late protein homologous to HSV-1 UL31. Previous studies showed that PRV UL31 is predominantly localized to nucleus, however, the molecular determinants for its nuclear import were unclear to date. Here, by utilizing live cells fluorescent microscopy, UL31 fused with enhanced yellow fluorescent protein was transiently expressed in live cells and confirmed to exclusively target to the nucleus in the absence of other viral proteins. Furthermore, the nuclear import of UL31 was found to be dependent on the Ran-, importin α1-, α3-, α5-, α7-, β1-and transportin-1-mediated pathway. Therefore, these results would open up new avenues for depicting the biological functions of UL31 during PRV infection.

EBV glycoproteins: where are we now?

Glycoproteins are critical to virus entry, to spread within and between hosts and can modify the behavior of cells. Many viruses carry only a few, most found in the virion envelope. EBV makes more than 12, providing flexibility in how it colonizes its human host. Some are dedicated to getting the virus through the cell membrane and on toward the nucleus of the cell, some help guide the virus back out and on to the next cell in the same or a new host. Yet others undermine host defenses helping the virus persist for a lifetime, maintaining a presence that is mostly tolerated and serves to perpetuate EBV as one of the most common infections of man.

Characterization of the nuclear import and export signals of pseudorabies virus UL31

The pseudorabies virus (PRV) UL31 protein (pUL31) is a homologue of the herpes simplex virus 1 pUL31, which is a multifunctional protein that is important for HSV-1 infection. However, little is known concerning the subcellular localization signal of PRV UL31. Here, by transfection with a series of PRV UL31 deletion mutants fused to an enhanced yellow fluorescent protein (EYFP) gene, a bipartite nuclear localization signal (NLS) and a PY motif NLS of UL31 were identified and mapped to amino acids (aa) 4 to 20 (RRRLLRRKSSAARRKTL) and aa 21 to 34 (TRAARDRYAPYFAY), respectively. Additionally, the predicted nuclear export signal (NES) was shown to be nonfunctional. Taken together, this information opens up new avenues for investigating the biological functions of UL31 during PRV infection.

Herpesvirus nuclear egress: Pseudorabies Virus can simultaneously induce nuclear envelope breakdown and exit the nucleus via the envelopment-deenvelopment-pathway

Herpesvirus replication takes place in the nucleus and in the cytosol. After entering the cell, nucleocapsids are transported to nuclear pores where viral DNA is released into the nucleus. After gene expression and DNA replication new nucleocapsids are assembled which have to exit the nucleus for virion formation in the cytosol. Since nuclear pores are not wide enough to allow passage of the nucleocapsid, nuclear egress occurs by vesicle-mediated transport through the nuclear envelope. To this end, nucleocapsids bud at the inner nuclear membrane (INM) recruiting a primary envelope which then fuses with the outer nuclear membrane (ONM). In the absence of this regulated nuclear egress, mutants of the alphaherpesvirus pseudorabies virus have been described that escape from the nucleus after virus-induced nuclear envelope breakdown. Here we review these exit pathways and demonstrate that both can occur simultaneously under appropriate conditions.

A single herpesvirus protein can mediate vesicle formation in the nuclear envelope

Herpesviruses assemble capsids in the nucleus and egress by unconventional vesicle-mediated trafficking through the nuclear envelope. Capsids bud at the inner nuclear membrane into the nuclear envelope lumen. The resulting intralumenal vesicles fuse with the outer nuclear membrane, delivering the capsids to the cytoplasm. Two viral proteins are required for vesicle formation, the tail-anchored pUL34 and its soluble interactor, pUL31. Whether cellular proteins are involved is unclear. Using giant unilamellar vesicles, we show that pUL31 and pUL34 are sufficient for membrane budding and scission. pUL34 function can be bypassed by membrane tethering of pUL31, demonstrating that pUL34 is required for pUL31 membrane recruitment but not for membrane remodeling. pUL31 can inwardly deform membranes by oligomerizing on their inner surface to form buds that constrict to vesicles. Therefore, a single viral protein can mediate all events necessary for membrane budding and abscission.

Functional characterization of nuclear trafficking signals in pseudorabies virus pUL31

The herpesviral nuclear egress complex (NEC), consisting of pUL31 and pUL34 homologs, mediates efficient translocation of newly synthesized capsids from the nucleus to the cytosol. The tail-anchored membrane protein pUL34 is autonomously targeted to the nuclear envelope, while pUL31 is recruited to the inner nuclear membrane (INM) by interaction with pUL34. A nuclear localization signal (NLS) in several pUL31 homologs suggests importin-mediated translocation of the protein. Here we demonstrate that deletion or mutation of the NLS in pseudorabies virus (PrV) pUL31 resulted in exclusively cytosolic localization, indicating active nuclear export. Deletion or mutation of a predicted nuclear export signal (NES) in mutant constructs lacking a functional NLS resulted in diffuse nuclear and cytosolic localization, indicating that both signals are functional. pUL31 molecules lacking the complete NLS or NES were not recruited to the INM by pUL34, while site-specifically mutated proteins formed the NEC and partially complemented the defect of the UL31 deletion mutant. Our data demonstrate that the N terminus of pUL31, encompassing the NLS, is required for efficient nuclear targeting but not for pUL34 interaction, while the C terminus, containing the NES but not necessarily the NES itself, is required for complex formation and efficient budding of viral capsids at the INM. Moreover, pUL31-ΔNLS displayed a dominant negative effect on wild-type PrV replication, probably by diverting pUL34 to cytoplasmic membranes.

IMPORTANCE

The molecular details of nuclear egress of herpesvirus capsids are still enigmatic. Although the key players, homologs of herpes simplex virus pUL34 and pUL31, which interact and form the heterodimeric nuclear egress complex, are well known, the molecular basis of this interaction and the successive budding, vesicle formation, and scission from the INM, as well as capsid release into the cytoplasm, remain largely obscure. Here we show that classical cellular targeting signals for nuclear import and export are important for proper localization and function of the NEC, thus regulating herpesvirus nuclear egress.

The first endogenous herpesvirus, identified in the tarsier genome, and novel sequences from primate rhadinoviruses and lymphocryptoviruses

Herpesviridae is a diverse family of large and complex pathogens whose genomes are extremely difficult to sequence. This is particularly true for clinical samples, and if the virus, host, or both genomes are being sequenced for the first time. Although herpesviruses are known to occasionally integrate in host genomes, and can also be inherited in a Mendelian fashion, they are notably absent from the genomic fossil record comprised of endogenous viral elements (EVEs). Here, we combine paleovirological and metagenomic approaches to both explore the constituent viral diversity of mammalian genomes and search for endogenous herpesviruses. We describe the first endogenous herpesvirus from the genome of the Philippine tarsier, belonging to the Roseolovirus genus, and characterize its highly defective genome that is integrated and flanked by unambiguous host DNA. From a draft assembly of the aye-aye genome, we use bioinformatic tools to reveal over 100,000 bp of a novel rhadinovirus that is the first lemur gammaherpesvirus, closely related to Kaposi's sarcoma-associated virus. We also identify 58 genes of Pan paniscus lymphocryptovirus 1, the bonobo equivalent of human Epstein-Barr virus. For each of the viruses, we postulate gene function via comparative analysis to known viral relatives. Most notably, the evidence from gene content and phylogenetics suggests that the aye-aye sequences represent the most basal known rhadinovirus, and indicates that tumorigenic herpesviruses have been infecting primates since their emergence in the late Cretaceous. Overall, these data show that a genomic fossil record of herpesviruses exists despite their extremely large genomes, and expands the known diversity of Herpesviridae, which will aid the characterization of pathogenesis. Our analytical approach illustrates the benefit of intersecting evolutionary approaches with metagenomics, genetics and paleovirology.

Herpesviridae is a family of DNA viruses that have characteristically large and complex genomes. This defining feature is also responsible for bioinformatic challenges that complicate herpesvirus genomics, and why an endogenous herpesvirus remains elusive. Given that several species of herpesvirus are clinically relevant to humans, there is a pressing demand for techniques capable of generating and managing large quantities of herpesvirus genome data. This is coupled with a need to explore herpesvirus diversity in order to understand pathogenesis within an evolutionary context. Lessons from the study of ancient viral integrations have also highlighted the need to include information offered by paleoviruses. Using perspectives from paleovirology and metagenomics, we identify three herpesviruses within the genome data of their primate hosts, including the first endogenous herpesvirus. All three viruses are closely related to important human pathogens and two of them are entirely new species. Both comparative molecular biology and evolutionary analysis were applied to examine our results for their clinical relevance. Furthermore, we demonstrate how this analytical approach was also used for the data collection itself, by treating nucleotide databases in their entirety as a single metagenomic resource.

Association of herpes simplex virus pUL31 with capsid vertices and components of the capsid vertex-specific complex

pU(L)34 and pU(L)31 of herpes simplex virus (HSV) comprise the nuclear egress complex (NEC) and are required for budding at the inner nuclear membrane. pU(L)31 also associates with capsids, suggesting it bridges the capsid and pU(L)34 in the nuclear membrane to initiate budding. Previous studies showed that capsid association of pU(L)31 was precluded in the absence of the C terminus of pU(L)25, which along with pU(L)17 comprises the capsid vertex-specific complex, or CVSC. The present studies show that the final 20 amino acids of pU(L)25 are required for pU(L)31 capsid association. Unexpectedly, in the complete absence of pU(L)25, or when pU(L)25 capsid binding was precluded by deletion of its first 50 amino acids, pU(L)31 still associated with capsids. Under these conditions, pU(L)31 was shown to coimmunoprecipitate weakly with pU(L)17. Based on these data, we hypothesize that the final 20 amino acids of pU(L)25 are required for pU(L)31 to associate with capsids. In the absence of pU(L)25 from the capsid, regions of capsid-associated pU(L)17 are bound by pU(L)31. Immunogold electron microscopy revealed that pU(L)31 could associate with multiple sites on a single capsid in the nucleus of infected cells. Electron tomography revealed that immunogold particles specific to pU(L)31 protein bind to densities at the vertices of the capsid, a location consistent with that of the CVSC. These data suggest that pU(L)31 loads onto CVSCs in the nucleus to eventually bind pU(L)34 located within the nuclear membrane to initiate capsid budding.

IMPORTANCE

This study is important because it localizes pU(L)1, a component previously known to be required for HSV capsids to bud through the inner nuclear membrane, to the vertex-specific complex of HSV capsids, which comprises the unique long region 25 (U(L)25) and U(L)17 gene products. It also shows this interaction is dependent on the C terminus of U(L)25. This information is vital for understanding how capsids bud through the inner nuclear membrane.

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.

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.

Mouse cytomegalovirus egress protein pM50 interacts with cellular endophilin-A2

The herpesvirus replication cycle comprises maturation processes in the nucleus and cytoplasm of the infected cells. After their nuclear assembly viral capsids translocate via primary envelopment towards the cytoplasm. This event is mediated by the nuclear envelopment complex, which is composed by two conserved viral proteins belonging to the UL34 and UL31 protein families. Here, we generated recombinant viruses, which express affinity-tagged pM50 and/or pM53, the pUL34 and pUL31 homologues of the murine cytomegalovirus. We extracted pM50- and pM53-associated protein complexes from infected cells and analysed their composition after affinity purification by mass spectrometry. We observed reported interaction partners and identified new putative protein-protein interactions for both proteins. Endophilin-A2 was observed as the most prominent cellular partner of pM50. We found that endophilin-A2 binds to pM50 directly, and this interaction seems to be conserved in the pUL34 family.

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.

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.

Nuclear actin and lamins in viral infections

Lamins are the best characterized cytoskeletal components of the cell nucleus that help to maintain the nuclear shape and participate in diverse nuclear processes including replication or transcription. Nuclear actin is now widely accepted to be another cytoskeletal protein present in the nucleus that fulfills important functions in the gene expression. Some viruses replicating in the nucleus evolved the ability to interact with and probably utilize nuclear actin for their replication, e.g., for the assembly and transport of capsids or mRNA export. On the other hand, lamins play a role in the propagation of other viruses since nuclear lamina may represent a barrier for virions entering or escaping the nucleus. This review will summarize the current knowledge about the roles of nuclear actin and lamins in viral infections.

Regulatory roles of protein kinases in cytomegalovirus replication

Viral replication is a complex process relying on a network of interacting viral and cellular proteins, in which particularly protein kinases play an important regulatory role. The specific phosphorylation of substrate proteins induces activation, inactivation, or other functional modification and thus determines virus-host cell interregulation. During herpesviral infections, both viral and cellular protein kinases are expressed and provide activities crucial for the efficiency of virus replication. The protein kinase pUL97 encoded by human cytomegalovirus (HCMV) is a multifunctional regulatory enzyme which exerts strong regulatory effects on early and late steps of the viral replication cycle. A number of interacting proteins and substrates of pUL97 have been described, including retinoblastoma (Rb) protein, nuclear lamins and viral pUL69. Recently, it was demonstrated that pUL97 has structural and functional resemblance to cyclin-dependent protein kinases (CDKs) and thus represents a CDK ortholog. pUL97 can phosphorylate and inactivate Rb, resulting in a stimulation of cell cycle progression. In addition, the association of pUL97 activity with nucleocytoplasmic export of viral capsids has been demonstrated by several investigators. We could show that pUL97 is able to phosphorylate nuclear lamins and to contribute to the HCMV-induced reorganization of the nuclear lamina. On the basis of very recent findings, it is becoming increasingly clear that pUL97 is a component of a multiprotein nuclear egress complex (NEC). The NEC contains a small number of egress proteins involved in the recruitment of protein kinases, such as pUL97 and cellular protein kinase C (PKC), to specific sites of the nuclear lamina. Current information about the composition, function, and regulatory complexity of the NEC leads to a mechanistic concept which may set the key features of HCMV nuclear egress in a new light.

Reconstitution of the Kaposi's sarcoma-associated herpesvirus nuclear egress complex and formation of nuclear membrane vesicles by coexpression of ORF67 and ORF69 gene products

The Kaposi's sarcoma-associated herpesvirus nuclear egress complex is composed of two proteins, ORF67 and ORF69. In this study, we have recapitulated the KSHV complex by coexpression of these two proteins in insect cells using expression from recombinant baculoviruses. The proteins form a complex at the nuclear membrane as judged by live-cell analysis of protein fusions tagged with green fluorescent protein (GFP) and mCherry. Ultrastructural analysis of infected cells showed that ORF67 expression results in reduplication of the nuclear membrane. When the two proteins are expressed together, numerous virion-size nuclear membrane-derived vesicles were evident at the nuclear margins.

Intragenic and extragenic suppression of a mutation in herpes simplex virus 1 UL34 that affects both nuclear envelope targeting and membrane budding

Late in infection herpesviruses move DNA-filled capsids from the nucleus to the cytoplasm by enveloping DNA-containing capsids at the inner nuclear membrane (INM) and deenveloping them at the outer nuclear membrane. This process requires two conserved herpesvirus proteins, pUL31 and pUL34. Interaction between pUL34 and pUL31 is essential for targeting both proteins to the nuclear envelope (NE), and sequences that mediate the targeting interaction have been mapped in both proteins. Here, we show that a mutation in the INM-targeting domain of pUL34 fails to support production of infectious virus or plaque formation. The mutation results in multiple defects, including impaired interaction between pUL34 and pUL31, poor NE targeting of pUL34, and misregulated, capsid-independent budding of the NE. The mutant defects in virus production, plaque formation, and pUL31 interaction can be suppressed by other mutations in the INM-targeting domain of pUL31 and by additional mutations in the pUL34 coding sequence.

A physical link between the pseudorabies virus capsid and the nuclear egress complex

Following their assembly, herpesvirus capsids exit the nucleus by budding at the inner nuclear membrane. Two highly conserved viral proteins are required for this process, pUL31 and pUL34. In this report, we demonstrate that the pUL31 component of the pseudorabies virus nuclear egress complex is a conditional capsid-binding protein that is unmasked in the absence of pUL34. The interaction between pUL31 and capsids was confirmed through fluorescence microscopy and Western blot analysis of purified intranuclear capsids. Three viral proteins were tested for their abilities to mediate the pUL31-capsid interaction: the minor capsid protein pUL25, the portal protein pUL6, and the terminase subunit pUL33. Despite the requirement for each protein in nuclear egress, none of these viral proteins were required for the pUL31-capsid interaction. These findings provide the first formal evidence that a herpesvirus nuclear egress complex interacts with capsids and have implications for how DNA-containing capsids are selectively targeted for nuclear egress.

Functional characterization of the essential tail anchor of the herpes simplex virus type 1 nuclear egress protein pUL34

Release of herpes simplex virus type 1 (HSV-1) nucleocapsids from the host nucleus relies on the nuclear egress complex consisting of the two essential proteins pUL34 and pUL31. The cytoplasmically exposed N-terminal region of pUL34 interacts with pUL31, while a hydrophobic region followed by a short luminal part mediates membrane association. Based on its domain organization, pUL34 was postulated to be a tail-anchor (TA) protein. We performed a coupled in vitro transcription/translation assay to show that membrane insertion of pUL34 occurs post-translationally. Transient transfection and localization experiments in mammalian cells were combined with HSV-1 bacterial artificial chromosome mutagenesis to reveal the functional properties of the essential pUL34 TA. Our data show that a minimal tail length of 15 residues is sufficient for nuclear envelope targeting and pUL34 function. Permutations of the pUL34 TA with orthologous regions of human cytomegalovirus pUL50 or Epstein-Barr virus pBFRF1 as well as the heterologous HSV-1 TA proteins pUL56 or pUS9 or the cellular TA proteins Bcl-2 and Vamp2 revealed that nuclear egress tolerates TAs varying in sequence and hydrophobicity, while a non-α-helical membrane anchor failed to complement the pUL34 function. In conclusion, this study provides the first mechanistic insights into the particular role of the TA of pUL34 in membrane curving and capsid egress from the host nucleus.

Herpes simplex virus 1 pUL34 plays a critical role in cell-to-cell spread of virus in addition to its role in virus replication

Herpes simplex virus (HSV) pUL34 plays a critical role in virus replication by mediating egress of nucleocapsids from the infected cell nucleus. We have identified a mutation in pUL34 (Y68A) that produces a major defect in virus replication and impaired nuclear egress but also profoundly inhibits cell-to-cell spread and trafficking of gE. Virion release to the extracellular medium is not affected by the Y68A mutation, indicating that the mutation specifically inhibits cell-to-cell spread. We isolated extragenic suppressors of the Y68A plaque formation defect and mapped them by a combination of high-throughput Illumina sequencing and PCR-based screening. We found that suppression is highly correlated with a nonsense mutation in the US9 gene, which plays a critical role in cell-to-cell spread of HSV-1 in neurons. The US9 mutation alone is not sufficient to suppress the Y68A spread phenotype, indicating a likely role for multiple viral factors.

Nuclear envelope breakdown can substitute for primary envelopment-mediated nuclear egress of herpesviruses

Herpesvirus nucleocapsids assemble in the nucleus but mature to infectious virions in the cytoplasm. To gain access to this cellular compartment, nucleocapsids are translocated to the cytoplasm by primary envelopment at the inner nuclear membrane and subsequent fusion of the primary envelope with the outer nuclear membrane. The conserved viral pUL34 and pUL31 proteins play a crucial role in this process. In their absence, viral replication is strongly impaired but not totally abolished. We used the residual infectivity of a pUL34-deleted mutant of the alphaherpesvirus pseudorabies virus (PrV) for reversion analysis. To this end, PrV-ΔUL34 was serially passaged in rabbit kidney cells until final titers of the mutant virus PrV-ΔUL34Pass were comparable to those of wild-type PrV. PrV-ΔUL34Pass produced infectious progeny independently of the pUL34/pUL31 nuclear egress complex and the pUS3 protein kinase. Ultrastructural analyses demonstrated that this effect was due to virus-induced disintegration of the nuclear envelope, thereby releasing immature and mature capsids into the cytosol for secondary envelopment. Our data indicate that nuclear egress primarily serves to transfer capsids through the intact nuclear envelope. Immature and mature intranuclear capsids are competent for further virion maturation once they reach the cytoplasm. However, nuclear egress exhibits a strong bias for nucleocapsids, thereby also functioning as a quality control checkpoint which is abolished by herpesvirus-induced nuclear envelope breakdown.

Role of tegument proteins in herpesvirus assembly and egress

Morphogenesis and maturation of viral particles is an essential step of viral replication. An infectious herpesviral particle has a multilayered architecture, and contains a large DNA genome, a capsid shell, a tegument and an envelope spiked with glycoproteins. Unique to herpesviruses, tegument is a structure that occupies the space between the nucleocapsid and the envelope and contains many virus encoded proteins called tegument proteins. Historically the tegument has been described as an amorphous structure, but increasing evidence supports the notion that there is an ordered addition of tegument during virion assembly, which is consistent with the important roles of tegument proteins in the assembly and egress of herpesviral particles. In this review we first give an overview of the herpesvirus assembly and egress process. We then discuss the roles of selected tegument proteins in each step of the process, i.e., primary envelopment, de-envelopment, secondary envelopment and transport of viral particles. We also suggest key issues that should be addressed in the near future.

Escape of herpesviruses from the nucleus

The nuclear envelope of eukaryotic cells is composed of double lipid-bilayer membranes, the membrane-connected nuclear pore complexes and an underlying nuclear lamina network. The nuclear pore complexes serve as gates for regulating the transport of macromolecules between cytoplasm and nucleus. The nuclear lamina not only provides an intact meshwork for maintaining the nuclear stiffness but also presents a natural barrier against most DNA viruses. Herpesviruses are large DNA viruses associated with multiple human and animal diseases. The complex herpesviral virion contains more than 30 viral proteins. After viral DNA replication, the newly synthesised genome is packaged into the pre-assembled intranuclear capsid. The nucleocapsid must then transverse through the nuclear envelope to the cytoplasm for the subsequent maturation process. Information regarding how nucleocapsid breaches the rigid nuclear lamina barrier and accesses the inner nuclear membrane for primary envelopment has emerged recently. From the point of view of both viral components and nuclear structure, this review summarises recent advances in the complicated protein-protein interactions and the phosphorylation regulations involved in the nuclear egress of herpesviral nucleocapsids.

Dominant negative mutants of the murine cytomegalovirus M53 gene block nuclear egress and inhibit capsid maturation

The alphaherpesvirus proteins UL31 and UL34 and their homologues in other herpesvirus subfamilies cooperate at the nuclear membrane in the export of nascent herpesvirus capsids. We studied the respective betaherpesvirus proteins M53 and M50 in mouse cytomegalovirus (MCMV). Recently, we established a random approach to identify dominant negative (DN) mutants of essential viral genes and isolated DN mutants of M50 (B. Rupp, Z. Ruzsics, C. Buser, B. Adler, P. Walther and U. H. Koszinowski, J. Virol 81:5508-5517). Here, we report the identification and phenotypic characterization of DN alleles of its partner, M53. While mutations in the middle of the M53 open reading frame (ORF) resulted in DN mutants inhibiting MCMV replication by approximately 100-fold, mutations at the C terminus resulted in up to 1,000,000-fold inhibition of virus production. C-terminal DN mutants affected nuclear distribution and steady-state levels of the nuclear egress complex and completely blocked export of viral capsids. In addition, they induced a marked maturation defect of viral capsids, resulting in the accumulation of nuclear capsids with aberrant morphology. This was associated with a two-thirds reduction in the total amount of unit length genomes, indicating an accessory role for M53 in DNA packaging.

Analysis of a charge cluster mutation of herpes simplex virus type 1 UL34 and its extragenic suppressor suggests a novel interaction between pUL34 and pUL31 that is necessary for membrane curvature around capsids

Interaction between pUL34 and pUL31 is essential for targeting both proteins to the inner nuclear membrane (INM). Sequences mediating the targeting interaction have been mapped by others with both proteins. We have previously reported identification of charge cluster mutants of herpes simplex virus type 1 UL34 that localize properly to the inner nuclear membrane, indicating interaction with UL31, but fail to complement a UL34 deletion. We have characterized one mutation (CL04) that alters a charge cluster near the N terminus of pUL34 and observed the following. (i) The CL04 mutant has a dominant-negative effect on pUL34 function, indicating disruption of some critical interaction. (ii) In infections with CL04 pUL34, capsids accumulate in close association with the INM, but no perinuclear enveloped viruses, cytoplasmic capsids, or virions or cell surface virions were observed, suggesting that CL04 UL34 does not support INM curvature around the capsid. (iii) Passage of UL34-null virus on a stable cell line that expresses CL04 resulted in selection of extragenic suppressor mutants that grew efficiently using the mutant pUL34. (iv) All extragenic suppressors contained an R229-->L mutation in pUL31 that was sufficient to suppress the CL04 phenotype. (v) Immunolocalization and coimmunoprecipitation experiments with truncated forms of pUL34 and pUL31 confirm that N-terminal sequences of pUL34 and a C-terminal domain of pUL31 mediate interaction but not nuclear membrane targeting. pUL34 and pUL31 may make two essential interactions-one for the targeting of the complex to the nuclear envelope and another for nuclear membrane curvature around capsids.

Connecting viral with cellular interactomes

Genome-scale screens for intraviral and virus–host protein interactions and the analysis of literature-curated datasets are able to provide a novel, comprehensive perspective of viruses, and virus-infected cells. Until now, large-scale interaction screens were predominantly performed with the yeast-two-hybrid (Y2H) system; however, alternative high-throughput technologies detecting binary protein interactions or protein complexes have been developed. Although many of the previous studies suffer from a rather poor validation of the results and few biological implications, these technologies potentially lead to a plethora of novel hypotheses. Here, we will give an overview of current approaches and their technical limitations, present recent examples and novel developments.

Molecular cloning and characterization of the UL31 gene from duck enteritis virus

Using a combination of bioinformation analysis and Dot blot technique, a gene, designated hereafter as the duck enteritis virus (DEV) UL31 gene (GenBank accession number EF643559), was identified from the DEV CHv genomic library. Then, the UL31 gene was cloned and sequenced, which was composed of 933 nucleotides encoding 310 amino acids. Multiple sequence alignment suggested that the UL31 gene was highly conserved in Alphaherpesvirinae and similar to the other herpesviral UL31. Phylogenetic analysis showed that the gene had a close evolutionary relationship with the avian herperviruses, and DEV should be placed into a single cluster within the subfamily Alphaherpesvirinae. Antigen prediction indicated that several potential B-cell epitopes sites located in the UL31 protein. To further study, the UL31 gene was cloned into a pET prokaryotic expression vector and transformed into Escherichia coli BL21 (DE3). A 55 kDa fusion protein was induced by the further culture at 37 degrees C after addition of 0.8 mM IPTG. Polyclonal antibody raised against the recombinant UL31 from rabbit was prepared. A protein about 55 kDa that reacted with the antibody was detected in immunoblots of bacterial proteins, suggesting that the 55 kDa protein was the product of the UL31 gene. Immunofluorescence analysis revealed that the protein was localized in very fine punctate forms in the nuclei of infected cells. Our results may provide some insight for further research about the gene and also enrich the database of herpesvirus.

Efficient production of infectious viruses requires enzymatic activity of Epstein-Barr virus protein kinase

The Epstein-Barr virus (EBV) BGLF4 gene product is the only protein kinase encoded by the virus genome. In order to elucidate its physiological roles in viral productive replication, we here established a BGLF4-knockout mutant and a revertant virus. While the levels of viral DNA replication of the deficient mutant were equivalent to those of the wild-type and the revertant, virus production was significantly impaired. Expression of the BGLF4 protein in trans fully complemented the low yield of the mutant virus, while expression of a kinase-dead (K102I) form of the protein failed to restore the virus titer. These results demonstrate that BGLF4 plays a significant role in production of infectious viruses and that the kinase activity is crucial.

Phenotypic similarities and differences between UL37-deleted pseudorabies virus and herpes simplex virus type 1

In the absence of the tegument protein pUL37, virion formation of pseudorabies virus (PrV) and herpes simplex virus type 1 (HSV-1) is severely impaired. Non-enveloped nucleocapsids accumulate in clusters in the cytoplasm, whereas only a few enveloped particles can be detected. Although a contribution of pUL37 to nuclear egress of HSV-1 has been suggested, the nuclear stages of morphogenesis are not impaired in PrV-DeltaUL37-infected cells. Moreover, HSV-1 pUL37 has been described as essential for replication, whereas PrV is able to replicate productively without pUL37, although to lower titres than wild-type virus. Thus, there may be functional differences between the respective pUL37 proteins. This study compared the phenotypes of UL37-deleted PrV and HSV-1 in parallel assays, using a novel pUL37 deletion mutant of HSV-1 strain KOS, HSV-1DeltaUL37[86-1035]. Aggregates of seemingly 'naked' nucleocapsids were present in the cytoplasm of African green monkey (Vero) or rabbit kidney (RK13) cells infected with HSV-1DeltaUL37[86-1035] or PrV-DeltaUL37. Nuclear retention of nucleocapsids was not observed in either virus. However, in contrast to PrV-DeltaUL37, HSV-1DeltaUL37[86-1035] was unable to replicate productively in, and to form plaques on, either Vero or RK13 cells. Trans-complementation of respective deletion mutants with the heterologous pUL37 did not ensue. These data demonstrate that the conserved pUL37 in HSV-1 and PrV have similar but distinct functions.

Expression and characterization of the UL31 protein from duck enteritis virus

Background

Previous studies indicate that the UL31 protein and its homology play similar roles in nuclear egress of all herpesviruses. However, there is no report on the UL31 gene product of DEV. In this study, we expressed and presented the basic properties of the DEV UL31 product.

Results

The entire ORF of the UL31 was cloned into pET 32a (+) prokaryotic expression vector. Escherichia coli BL21(DE3) competent cells were transformed with the construct followed by the induction of protein expression by the addition of IPTG. Band corresponding to the predicted sizes (55 kDa) was produced on the SDS-PAGE. Over expressed 6×His-UL31 fusion protein was purified by nickel affinity chromatography. The DEV UL31 gene product has been identified by using a rabbit polyclonal antiserum raised against the purified protein. A protein of approximate 35 kDa that reacted with the antiserum was detected in immunoblots of DEV-infected cellular lysates, suggesting that the 35 kDa protein was the primary translation product of the UL31 gene. RT-PCR analyses revealed that the UL31 gene was transcribed most abundantly during the late phase of replication. Subsequently, Immunofluorescence analysis revealed that the protein was widespread speckled structures in the nuclei of infected cells. Western blotting of purified virion preparations showed that UL31 was a component of intracellular virions but was absent from mature extracellular virions. Finally, an Immunofluorescence assay was established to study the distribution of the UL31 antigen in tissues of artificially DEV infected ducks. The results showed that the UL31 antigen was primarily located in the cells of digestive organs and immunological organs.

Conclusion

In this work, we present the basic properties of the DEV UL31 product. The results indicate that DEV UL31 shares many similarities with its HSV or PRV homolog UL31 and suggest that functional cross-complementation is possible between members of the Alphaherpesvirus subfamily. Furthermore, in vivo experiments with ducks infected with UL31-defective isolates of DEV will also be of importance in order to assess the possible role of the UL31 protein in viral pathogenesis. These properties of the UL31 protein provide a prerequisite for further functional analysis of this gene.

Characterization of pseudorabies virus (PrV) cleavage-encapsidation proteins and functional complementation of PrV pUL32 by the homologous protein of herpes simplex virus type 1

Cleavage and encapsidation of newly replicated herpes simplex virus type 1 (HSV-1) DNA requires several essential viral gene products that are conserved in sequence within the Herpesviridae. However, conservation of function has not been analyzed in greater detail. For functional characterization of the UL6, UL15, UL28, UL32, and UL33 gene products of pseudorabies virus (PrV), the respective deletion mutants were generated by mutagenesis of the virus genome cloned as a bacterial artificial chromosome (BAC) in Escherichia coli and propagated in transgenic rabbit kidney cells lines expressing the deleted genes. Neither of the PrV mutants was able to produce plaques or infectious progeny in noncomplementing cells. DNA analyses revealed that the viral genomes were replicated but not cleaved into monomers. By electron microscopy, only scaffold-containing immature but not DNA-containing mature capsids were detected in the nuclei of noncomplementing cells infected with either of the mutants. Remarkably, primary envelopment of empty capsids at the nuclear membrane occasionally occurred, and enveloped tegument-containing light particles were formed in the cytoplasm and released into the extracellular space. Immunofluorescence analyses with monospecific antisera of cells transfected with the respective expression plasmids indicated that pUL6, pUL15, and pUL32 were able to enter the nucleus. In contrast, pUL28 and pUL33 were predominantly found in the cytoplasm. Only pUL6 could be unequivocally identified and localized in PrV-infected cells and in purified virions, whereas the low abundance or immunogenicity of the other proteins hampered similar studies. Yeast two-hybrid analyses revealed physical interactions between the PrV pUL15, pUL28, and pUL33 proteins, indicating that, as in HSV-1, a tripartite protein complex might catalyze cleavage and encapsidation of viral DNA. Whereas the pUL6 protein is supposed to form the portal for DNA entry into the capsid, the precise role of the UL32 gene product during this process remains to be elucidated. Interestingly, the defect of UL32-negative PrV could be completely corrected in trans by the homologous protein of HSV-1, demonstrating similar functions. However, trans-complementation of UL32-negative HSV-1 by the PrV protein was not observed.

Biochemical, biophysical, and mutational analyses of subunit interactions of the human cytomegalovirus nuclear egress complex

Nuclear egress, the trafficking of herpesvirus nucleocapsids from the nucleus to the cytoplasm, involves two conserved viral proteins that form a complex at the nuclear envelope, referred to as the nuclear egress complex. In human cytomegalovirus, these two proteins are called UL50 and UL53. To study UL50 and UL53 in molecular detail, these proteins were expressed in bacteria and purified. To obtain highly expressed, pure proteins, it was necessary to truncate both constructs based on sequence conservation and predicted secondary structural elements. Size exclusion chromatography and analytical ultracentrifugation studies indicated that the truncated form of UL50 is a monomer in solution, that the truncated form of UL53 is a homodimer, and that, when mixed, the two proteins form a heterodimer. To identify residues of UL53 crucial for homodimerization and for heterodimerization with UL50, we constructed and expressed mutant forms of UL53 containing alanine substitutions in a predicted helix. Isothermal titration calorimetry was used to measure the binding affinities of the UL53 mutants to UL50. UL53 residues, the replacement of which reduced binding to UL50, form a surface on one face of the predicted helix. Moreover, most of the substitutions that reduce UL53-UL50 interactions also reduced homodimerization. Substitutions that reduced the interaction between UL50 and UL53 in vitro also reduced colocalization of full-length UL50 and UL53 at the nuclear rim in transfected cells. These results demonstrate direct protein-protein interactions between these proteins that are likely to be mediated by a helix, and they have implications for understanding nuclear egress and for drug discovery.