Identification of functional domains within the essential large tegument protein pUL36 of pseudorabies virus

Small molecule screening identifies inhibitors of the Epstein-Barr virus deubiquitinating enzyme, BPLF1

Herpesviral deubiquitinating enzymes (DUBs) were discovered in 2005, are highly conserved across the family, and are proving to be increasingly important players in herpesviral infection. EBV's DUB, BPLF1, is known to regulate both cellular and viral target activities, yet remains largely unstudied. Our work has implicated BPLF1 in a wide range of processes including infectivity, viral DNA replication, and DNA repair. Additionally, knockout of BPLF1 delays and reduces human B-cell immortalization and lymphoma formation in humanized mice. These findings underscore the importance of BPLF1 in viral infectivity and pathogenesis and suggest that inhibition of EBV's DUB activity may offer a new approach to specific therapy for EBV infections. We set out to discover and characterize small molecule inhibitors of BPLF1 deubiquitinating activity through high-throughput screening. An initial small pilot screen resulted in discovery of 10 compounds yielding >80% decrease in BPLF1 DUB activity at a 10 μM concentration. Follow-up dose response curves of top hits identified several compounds with an IC₅₀ in the low micromolar range. Four of these hits were tested for their ability to cleave ubiquitin chains as well as their effects on viral infectivity and cell viability. Further characterization of the top hit, commonly known as suramin was found to not be selective yet decreased viral infectivity by approximately 90% with no apparent effects on cell viability. Due to the conserved nature of Herpesviral deubiquitinating enzymes, identification of an inhibitor of BPLF1 may prove to be an effective and promising new avenue of therapy for EBV and other herpesviral family members.

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.

The Functional Oligomeric State of Tegument Protein GP41 Is Essential for Baculovirus Budded Virion and Occlusion-Derived Virion Assembly

gp41, one of the baculovirus core genes, encodes the only recognized tegument (O-glycosylated) protein of the occlusion-derived virion (ODV) phenotype so far. A previous study using a temperature-sensitive Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV) mutant showed that GP41 plays a crucial role in budded virion (BV) formation. However, the precise function of GP41 in the baculovirus replication cycle remains unclear. In this study, AcMNPV GP41 was found to accumulate around the ring zone (RZ) region within the infected nucleus and finally assembled into both BVs and ODVs. Deletion of gp41 from the AcMNPV genome showed that BVs were no longer formed and ODVs were no longer assembled, suggesting the essential role of this gene in baculovirus virion morphogenesis. In infected cells, besides the 42-kDa monomers, dimers and trimers were detected under nonreducing conditions, whereas only trimeric GP41 forms were selectively incorporated into BVs or ODVs. Mutations of all five cysteines in GP41 individually had minor effects on GP41 oligomer formation, albeit certain mutations impaired infectious BV production, suggesting flexibility in the intermolecular disulfide bonding. Single mutations of key leucines within two predicted leucine zipper-like motifs did not interfere with GP41 oligomerization or BV and ODV formation, but double leucine mutations completely blocked oligomerization of GP41 and progeny BV production. In the latter case, the usual subcellular localization, especially RZ accumulation, of GP41 was abolished. The above findings clearly point out a close correlation between GP41 oligomerization and function and therefore highlight the oligomeric state as the functional form of GP41 in the baculovirus replication cycle.IMPORTANCE The tegument, which is sandwiched between the nucleocapsid and the virion envelope, is an important substructure of many enveloped viruses. It is composed of one or more proteins that have important functions during virus entry, replication, assembly, and egress. Unlike another large DNA virus (herpesvirus) that encodes an extensive set of tegument components, baculoviruses very likely exploit the major tegument protein, GP41, to execute functions in baculovirus virion morphogenesis and assembly. However, the function of this O-glycosylated baculovirus tegument protein remains largely unknown. In this study, we identified trimers as the functional structure of GP41 in baculovirus virion morphogenesis and showed that both disulfide bridging and protein-protein interactions via the two leucine zipper-like domains are involved in the formation of different oligomeric states. This study advances our understanding of the unique viral tegument protein GP41 participating in the life cycle of baculoviruses.

The Translesion Polymerase Pol η Is Required for Efficient Epstein-Barr Virus Infectivity and Is Regulated by the Viral Deubiquitinating Enzyme BPLF1

Epstein-Barr virus (EBV) infection and lytic replication are known to induce a cellular DNA damage response. We previously showed that the virally encoded BPLF1 protein interacts with and regulates several members of the translesion synthesis (TLS) pathway, a DNA damage tolerance pathway, and that these cellular factors enhance viral infectivity. BPLF1 is a late lytic cycle gene, but the protein is also packaged in the viral tegument, indicating that BPLF1 may function both early and late during infection. The BPLF1 protein expresses deubiquitinating activity that is strictly conserved across the Herpesviridae; mutation of the active site cysteine results in a loss of enzymatic activity. Infection with an EBV BPLF1 knockout virus results in decreased EBV infectivity. Polymerase eta (Pol η), a specialized DNA repair polymerase, functions in TLS and allows for DNA replication complexes to bypass lesions in DNA. Here we report that BPLF1 interacts with Pol η and that Pol η protein levels are increased in the presence of functional BPLF1. BPLF1 promotes a nuclear relocalization of Pol η molecules which are focus-like in appearance, consistent with the localization observed when Pol η is recruited to sites of DNA damage. Knockdown of Pol η resulted in decreased production of infectious virus, and further, Pol η was found to bind to EBV DNA, suggesting that it may allow for bypass of damaged viral DNA during its replication. The results suggest a mechanism by which EBV recruits cellular repair factors, such as Pol η, to sites of viral DNA damage via BPLF1, thereby allowing for efficient viral DNA replication.IMPORTANCE Epstein-Barr virus is the causative agent of infectious mononucleosis and infects approximately 90% of the world's population. It causes lymphomas in individuals with acquired and innate immune disorders and is strongly associated with Hodgkin's lymphoma, Burkitt's lymphoma, diffuse large B-cell lymphomas, nasopharyngeal carcinoma (NPC), and lymphomas that develop in organ transplant recipients. Cellular DNA damage is a major determinant in the establishment of oncogenic processes and is well studied, but there are few studies of endogenous repair of viral DNA. This work evaluates how EBV's BPLF1 protein and its conserved deubiquitinating activity regulate the cellular DNA repair enzyme polymerase eta and recruit it to potential sites of viral damage and replication, resulting in enhanced production of infectious virus. These findings help to establish how EBV enlists and manipulates cellular DNA repair factors during the viral lytic cycle, contributing to efficient infectious virion production.

Characterization of the Dynamic Transcriptome of a Herpesvirus with Long-read Single Molecule Real-Time Sequencing

Herpesvirus gene expression is co-ordinately regulated and sequentially ordered during productive infection. The viral genes can be classified into three distinct kinetic groups: immediate-early, early, and late classes. In this study, a massively parallel sequencing technique that is based on PacBio Single Molecule Real-time sequencing platform, was used for quantifying the poly(A) fraction of the lytic transcriptome of pseudorabies virus (PRV) throughout a 12-hour interval of productive infection on PK-15 cells. Other approaches, including microarray, real-time RT-PCR and Illumina sequencing are capable of detecting only the aggregate transcriptional activity of particular genomic regions, but not individual herpesvirus transcripts. However, SMRT sequencing allows for a distinction between transcript isoforms, including length- and splice variants, as well as between overlapping polycistronic RNA molecules. The non-amplified Isoform Sequencing (Iso-Seq) method was used to analyse the kinetic properties of the lytic PRV transcripts and to then classify them accordingly. Additionally, the present study demonstrates the general utility of long-read sequencing for the time-course analysis of global gene expression in practically any organism.

Herpes Simplex Virus Capsid Localization to ESCRT-VPS4 Complexes in the Presence and Absence of the Large Tegument Protein UL36p

UL36p (VP1/2) is the largest protein encoded by herpes simplex virus 1 (HSV-1) and resides in the innermost layer of tegument, the complex protein layer between the capsid and envelope. UL36p performs multiple functions in the HSV life cycle, including a critical but unknown role in capsid cytoplasmic envelopment. We tested whether UL36p is essential for envelopment because it is required to engage capsids with the cellular ESCRT/Vps4 apparatus. A green fluorescent protein (GFP)-fused form of the dominant negative ATPase Vps4-EQ was used to irreversibly tag ESCRT envelopment sites during infection by UL36p-expressing and UL36-null HSV strains. Using fluorescence microscopy and scanning electron microscopy, we quantitated capsid/Vps4-EQ colocalization and examined the ultrastructure of the corresponding viral assembly intermediates. We found that loss of UL36p resulted in a two-thirds reduction in the efficiency of capsid/Vps4-EQ association but that the remaining UL36p-null capsids were still able to engage the ESCRT envelopment apparatus. It appears that although UL36p helps to couple HSV capsids to the ESCRT pathway, this is likely not the sole reason for its absolute requirement for envelopment.

IMPORTANCE

Envelopment of the HSV capsid is essential for the assembly of an infectious virion and requires the complex interplay of a large number of viral and cellular proteins. Critical to envelope assembly is the virally encoded protein UL36p, whose function is unknown. Here we test the hypothesis that UL36p is essential for the recruitment of cellular ESCRT complexes, which are also known to be required for envelopment.

Knockout of Epstein-Barr virus BPLF1 retards B-cell transformation and lymphoma formation in humanized mice

BPLF1 of Epstein-Barr virus (EBV) is classified as a late lytic cycle protein but is also found in the viral tegument, suggesting its potential involvement at both initial and late stages of viral infection. BPLF1 possesses both deubiquitinating and deneddylating activity located in its N-terminal domain and is involved in processes that affect viral infectivity, viral DNA replication, DNA repair, and immune evasion. A recently constructed EBV BPLF1-knockout (KO) virus was used in conjunction with a humanized mouse model that can be infected with EBV, enabling the first characterization of BPLF1 function in vivo. Results demonstrate that the BPLF1-knockout virus is approximately 90% less infectious than wild-type (WT) virus. Transformation of human B cells, a hallmark of EBV infection, was delayed and reduced with BPLF1-knockout virus. Humanized mice infected with EBV BPLF1-knockout virus showed less weight loss and survived longer than mice infected with equivalent infectious units of WT virus. Additionally, splenic tumors formed in 100% of mice infected with WT EBV but in only 25% of mice infected with BPLF1-KO virus. Morphological features of spleens containing tumors were similar to those in EBV-induced posttransplant lymphoproliferative disease (PTLD) and were almost identical to cases seen in human diffuse large B-cell lymphoma. The presence of EBV genomes was detected in all mice that developed tumors. The results implicate BPLF1 in human B-cell transformation and tumor formation in humanized mice.

Epstein-Barr virus infects approximately 90% of the world’s population and is the causative agent of infectious mononucleosis. EBV also causes aggressive lymphomas in individuals with acquired and innate immune disorders and is strongly associated with diffuse large B-cell lymphomas, classical Hodgkin lymphoma, Burkitt lymphoma, and nasopharyngeal carcinoma (NPC). Typically, EBV initially infects epithelial cells in the oropharynx, followed by a lifelong persistent latent infection in B-cells, which may develop into lymphomas in immunocompromised individuals. This work is the first of its kind in evaluating the effects of EBV’s BPLF1 in terms of pathogenesis and lymphomagenesis in humanized mice and implicates BPLF1 in B-cell transformation and tumor development. Currently, there is no efficacious treatment for EBV, and therapeutic targeting of BPLF1 may lead to a new path to treatment for immunocompromised individuals or transplant recipients infected with EBV.

Herpes Simplex Virus Capsid-Organelle Association in the Absence of the Large Tegument Protein UL36p

UL36p (VP1/2) is the largest protein encoded by herpes simplex virus 1 (HSV-1) and resides in the innermost layer of the viral tegument, lying between the capsid and the envelope. UL36p performs multiple functions in the HSV life cycle, including an essential role in cytoplasmic envelopment. We earlier described the isolation of a virion-associated cytoplasmic membrane fraction from HSV-infected cells. Biochemical and ultrastructural analyses showed that the organelles in this buoyant fraction contain enveloped infectious HSV particles in their lumens and naked capsids docked to their cytoplasmic surfaces. These organelles can also recruit molecular motors and transport their cargo virions along microtubules in vitro. Here we examine the properties of these HSV-associated organelles in the absence of UL36p. We find that while capsid envelopment is clearly defective, a subpopulation of capsids nevertheless still associate with the cytoplasmic faces of these organelles. The existence of these capsid-membrane structures was confirmed by subcellular fractionation, immunocytochemistry, lipophilic dye fluorescence microscopy, thin-section electron microscopy, and correlative light and electron microscopy. We conclude that capsid-membrane binding can occur in the absence of UL36p and propose that this association may precede the events of UL36p-driven envelopment.

IMPORTANCE

Membrane association and envelopment of the HSV capsid are essential for the assembly of an infectious virion. Envelopment involves the complex interplay of a large number of viral and cellular proteins; however, the function of most of them is unknown. One example of this is the viral protein UL36p, which is clearly essential for envelopment but plays a poorly understood role. Here we demonstrate that organelles utilized for HSV capsid envelopment still accumulate surface-bound capsids in the absence of UL36p. We propose that UL36p-independent binding of capsids to organelles occurs prior to the function of UL36p in capsid envelopment.

Blocking ESCRT-mediated envelopment inhibits microtubule-dependent trafficking of alphaherpesviruses in vitro

Herpes simplex virus (HSV) and, as reported here, pseudorabies virus (PRV) utilize the ESCRT apparatus to drive cytoplasmic envelopment of their capsids. Here, we demonstrate that blocking ESCRT-mediated envelopment using the dominant-negative inhibitor Vps4A-EQ (Vps4A in which glutamate [E] at position 228 in the ATPase active site is replaced by a glutamine [Q]) reduced the ability of HSV and PRV particles to subsequently traffic along microtubules in vitro. HSV and PRV capsid-associated particles with bound green fluorescent protein (GFP)-labeled Vps4A-EQ were readily detected by fluorescence microscopy in cytoplasmic extracts of infected cells. These Vps4A-EQ-associated capsid-containing particles bound to microtubules in vitro but were unable to traffic along them. Using a PRV strain expressing a fluorescent capsid and a fluorescently tagged form of the envelope protein gD, we found that similar numbers of gD-positive and gD-negative capsid-associated particles accumulated in cytoplasmic extracts under our conditions. Both classes of PRV particle bound to microtubules in vitro with comparable efficiency, and similar results were obtained for HSV using anti-gD immunostaining. The gD-positive and gD-negative PRV capsids were both capable of trafficking along microtubules in vitro; however, motile gD-positive particles were less numerous and their trafficking was more sensitive to the inhibitory effects of Vps4A-EQ. We discuss our data in the context of microtubule-mediated trafficking of naked and enveloped alphaherpesvirus capsids.

IMPORTANCE

The alphaherpesviruses include several important human pathogens. These viruses utilize microtubule-mediated transport to travel through the cell cytoplasm; however, the molecular mechanisms of trafficking are not well understood. In this study, we have used a cell-free system to examine the requirements for microtubule trafficking and have attempted to distinguish between the movement of so-called "naked" and membrane-associated cytoplasmic alphaherpesvirus capsids.

Functional analysis of nuclear localization signals in VP1-2 homologues from all herpesvirus subfamilies

The herpes simplex virus (HSV) tegument protein VP1-2 contains an N-terminal nuclear localization signal (NLS) that is critical for capsid routing to the nuclear pore. Here we analyzed positionally conserved determinants in VP1-2 homologues from each of the alpha, beta, and gamma classes of human herpesviruses. The overall architectures of the VP1-2s were similar, with a conserved N-terminal ubiquitin-specific protease domain separated from an internal region by a linker that was quite poorly conserved in length and sequence. Within this linker region all herpesviruses contained a conserved, highly basic motif which nevertheless exhibited distinct class-specific features. The motif in HSV functioned as a monopartite NLS, while in varicella-zoster virus (VZV) activity required an adjacent basic section defining the motif as a bipartite NLS. Neither the beta- nor gammaherpesvirus VP1-2 motifs were identified by prediction algorithms, but they nevertheless functioned as efficient NLS motifs both in heterologous transfer assays and in HSV VP1-2. Furthermore, though with different efficiencies and with the exception of human herpesvirus 8 (HHV-8), these chimeric variants rescued the replication defect of an HSV mutant lacking its NLS motif. We demonstrate that the lysine at position 428 of HSV is critical for replication, with a single alanine substitution being sufficient to abrogate NLS function and virus growth. We conclude that the basic motifs of each of the VP1-2 proteins are likely to confer a similar function in capsid entry in the homologous setting and that while there is flexibility in the exact type of motif employed, specific individual residues are critical for function.

IMPORTANCE

To successfully infect cells, all herpesviruses, along with many other viruses, e.g., HIV, hepatitis B virus, and influenza virus, must navigate through the cytoplasmic environment and dock with nuclear pores for transport of their genomes into the nucleus. However, we still have a limited understanding of the detailed mechanisms involved. Insight into these events is needed and could offer opportunities for therapeutic intervention. This work investigated the role of a specific determinant in the structural protein VP1-2 in herpesvirus entry. We examined this determinant in representative VP1-2s from all herpesvirus subfamilies, demonstrated NLS function, dissected key residues, and showed functional relevance in rescuing replication of the mutant blocked in capsid navigation to the pore. The results are important and strongly support our conclusions of the generality that these motifs are crucial for entry of all herpesviruses. They also facilitate future analysis on selective host interactions and possible routes to disrupt function.

The Rad6/18 ubiquitin complex interacts with the Epstein-Barr virus deubiquitinating enzyme, BPLF1, and contributes to virus infectivity

Epstein-Barr virus (EBV) encodes BPLF1, a lytic cycle protein with deubiquitinating activity that is contained in its N-terminal domain and conserved across the Herpesviridae. EBV replication is associated with cellular DNA replication and repair factors, and initiation of EBV lytic replication induces a DNA damage response, which can be regulated at least in part by BPLF1. The cellular DNA repair pathway, translesion synthesis (TLS), is disrupted by BPLF1, which deubiquitinates the DNA processivity factor, PCNA, and inhibits the recruitment of the TLS polymerase, polymerase eta (Pol eta), after damage to DNA by UV irradiation. Here we showed that the E3 ubiquitin ligase, which activates TLS repair by monoubiquitination of PCNA, is also affected by BPLF1 deubiquitinating activity. First, BPLF1 interacts directly with Rad18, and overexpression of BPLF1 results in increased levels of the Rad18 protein, suggesting that it stabilizes Rad18. Next, expression of functionally active BPLF1 caused relocalization of Rad18 into nuclear foci, which is consistent with sites of cellular DNA replication that occur during S phase. Also, levels of Rad18 remain constant during lytic reactivation of wild-type virus, but reactivation of BPLF1 knockout virus resulted in decreased levels of Rad18. Finally, the contribution of Rad18 levels to infectious virus production was examined with small interfering RNA (siRNA) targeting Rad18. Results demonstrated that reducing levels of Rad18 decreased production of infectious virus, and infectious titers of BPLF1 knockout virus were partially restored by overexpression of Rad18. Thus, BPLF1 interacts with and maintains Rad18 at high levels during lytic replication, which assists in production of infectious virus.

IMPORTANCE

Characterization of EBV BPLF1's deubiquitinating activity and identification of its targets and subsequent functional effects remain little studied. All members of the Herpesviridae contain BPLF1 homologs with conserved enzymatic activity, and findings discovered with EBV BPLF1 are likely applicable to other members of the family. Discovery of new targets of BPLF1 will point to cellular pathways and viral processes regulated by the enzymatic activity of the EBV-encoded deubiquitinating enzyme. Here we determined the importance of the cellular ubiquitin ligase Rad18 in these processes and how it is affected by BPLF1. Our findings demonstrate that EBV can co-opt Rad18 as a novel accessory factor in the production of infectious virus.

The interaction of the HSV-1 tegument proteins pUL36 and pUL37 is essential for secondary envelopment during viral egress

The herpes simplex virus type 1 (HSV-1) tegument proteins pUL36 (VP1/2) and pUL37 are essential for viral egress. We previously defined a minimal domain in HSV-1 pUL36, residues 548-572, as important for binding pUL37. Here, we investigated the role of this region in binding to pUL37 and facilitating viral replication. We deleted residues 548-572 in frame in a virus containing a mRFP tag at the N-terminus of the capsid protein VP26 and an eGFP tag at the C-terminus of pUL37 (HSV-1pUL36∆548-572). This mutant virus was unable to generate plaques in Vero cells, indicating that deletion of this region of pUL36 blocks viral replication. Imaging of HSV-1pUL36∆548-572-infected Vero cells, in comparison to parental and resucant, revealed a block in secondary envelopment of cytoplasmic capsids. In addition, immunoblot analysis suggested that failure to bind pUL37 affected the stability of pUL36. This study provides further insight into the role of this essential interaction.

The herpesvirus VP1/2 protein is an effector of dynein-mediated capsid transport and neuroinvasion

Microtubule transport of herpesvirus capsids from the cell periphery to the nucleus is imperative for viral replication and, in the case of many alphaherpesviruses, transmission into the nervous system. Using the neuroinvasive herpesvirus, pseudorabies virus (PRV), we show that the viral protein 1/2 (VP1/2) tegument protein associates with the dynein/dynactin microtubule motor complex and promotes retrograde microtubule transport of PRV capsids. Functional activation of VP1/2 requires binding to the capsid protein pUL25 or removal of the capsid-binding domain. A proline-rich sequence within VP1/2 is required for the efficient interaction with the dynein/dynactin microtubule motor complex as well as for PRV virulence and retrograde axon transport in vivo. Additionally, in the absence of infection, functionally active VP1/2 is sufficient to move large surrogate cargoes via the dynein/dynactin microtubule motor complex. Thus, VP1/2 tethers PRV capsids to dynein/dynactin to enhance microtubule transport, neuroinvasion, and pathogenesis.

Epstein-Barr virus deubiquitinase downregulates TRAF6-mediated NF-κB signaling during productive replication

Epstein-Barr virus (EBV), a human oncogenic herpesvirus that establishes a lifelong latent infection in the host, occasionally enters lytic infection to produce progeny viruses. The EBV oncogene latent membrane protein 1 (LMP1), which is expressed in both latent and lytic infection, constitutively activates the canonical NF-κB (p65) pathway. Such LMP1-mediated NF-κB activation is necessary for proliferation of latently infected cells and inhibition of viral lytic cycle progression. Actually, canonical NF-κB target gene expression was suppressed upon the onset of lytic infection. TRAF6, which is activated by conjugation of polyubiquitin chains, associates with LMP1 to mediate NF-κB signal transduction. We have found that EBV-encoded BPLF1 interacts with and deubiquitinates TRAF6 to inhibit NF-κB signaling during lytic infection. HEK293 cells with BPLF1-deficient recombinant EBV exhibited poor viral DNA replication compared with the wild type. Furthermore, exogenous expression of BPLF1 or p65 knockdown in cells restored DNA replication of BPLF1-deficient viruses, indicating that EBV BPLF1 deubiquitinates TRAF6 to inhibit NF-κB signal transduction, leading to promotion of viral lytic DNA replication.

Viral and host control of cytomegalovirus maturation

Maturation in herpesviruses initiates in the nucleus of the infected cell, with encapsidation of viral DNA to form nucleocapsids, and concludes with envelopment in the cytoplasm to form infectious virions that egress the cell. The entire process of virus maturation is orchestrated by protein-protein interactions and enzymatic activities of viral and host origin. Viral tegument proteins play important roles in maintaining the structural stability of capsids and directing the acquisition of virus envelope. Envelopment occurs at modified host membranes and exploits host vesicular trafficking. In this review, we summarize current knowledge of and concepts in human cytomegalovirus (HCMV) maturation and their parallels in other herpesviruses, with an emphasis on viral and host factors that regulate this process.

A Nuclear localization signal in herpesvirus protein VP1-2 is essential for infection via capsid routing to the nuclear pore

To initiate infection, herpesviruses must navigate to and transport their genomes across the nuclear pore. VP1-2 is a large structural protein of the virion that is conserved in all herpesviruses and plays multiple essential roles in virus replication, including roles in early entry. VP1-2 contains an N-terminal basic motif which functions as an efficient nuclear localization signal (NLS). In this study, we constructed a mutant HSV strain, K.VP1-2ΔNLS, which contains a 7-residue deletion of the core NLS at position 475. This mutant fails to spread in normal cells but can be propagated in complementing cell lines. Electron microscopy (EM) analysis of infection in noncomplementing cells demonstrated capsid assembly, cytoplasmic envelopment, and the formation of extracellular enveloped virions. Furthermore, extracellular virions isolated from noncomplementing cells had similar profiles and abundances of structural proteins. Virions containing VP1-2ΔNLS were able to enter and be transported within cells. However, further progress of infection was prevented, with at least a 500- to 1,000-fold reduction in the efficiency of initiating gene expression compared to that in the revertant. Ultrastructural and immunofluorescence analyses revealed that the K.VP1-2ΔNLS mutant was blocked at the microtubule organizing center or immediately upstream of nuclear pore docking and prior to gene expression. These results indicate that the VP1-2 NLS is not required for the known assembly functions of the protein but is a key requirement for the early routing to the nuclear pore that is necessary for successful infection. Given its conservation, we propose that this motif may also be critical for entry of other classes of herpesviruses.

Epstein-Barr virus BPLF1 deubiquitinates PCNA and attenuates polymerase η recruitment to DNA damage sites

PCNA is monoubiquitinated in response to DNA damage and fork stalling and then initiates recruitment of specialized polymerases in the DNA damage tolerance pathway, translesion synthesis (TLS). Since PCNA is reported to associate with Epstein-Barr virus (EBV) DNA during its replication, we investigated whether the EBV deubiquitinating (DUB) enzyme encoded by BPLF1 targets ubiquitinated PCNA and disrupts TLS. An N-terminal BPLF1 fragment (a BPLF1 construct containing the first 246 amino acids [BPLF1 1-246]) associated with PCNA and attenuated its ubiquitination in response to fork-stalling agents UV and hydroxyurea in cultured cells. Moreover, monoubiquitinated PCNA was deubiquitinated after incubation with purified BPLF1 1-246 in vitro. BPLF1 1-246 dysregulated TLS by reducing recruitment of the specialized repair polymerase polymerase η (Polη) to the detergent-resistant chromatin compartment and virtually abolished localization of Polη to nuclear repair foci, both hallmarks of TLS. Expression of BPLF1 1-246 decreased viability of UV-treated cells and led to cell death, presumably through deubiquitination of PCNA and the inability to repair damaged DNA. Importantly, deubiquitination of PCNA could be detected endogenously in EBV-infected cells in comparison with samples expressing short hairpin RNA (shRNA) against BPLF1. Further, the specificity of the interaction between BPLF1 and PCNA was dependent upon a PCNA-interacting peptide (PIP) domain within the N-terminal region of BPLF1. Both DUB activity and PIP sequence are conserved in the members of the family Herpesviridae. Thus, deubiquitination of PCNA, normally deubiquitinated by cellular USP1, by the viral DUB can disrupt repair of DNA damage by compromising recruitment of TLS polymerase to stalled replication forks. PCNA is the first cellular target identified for BPLF1 and its deubiquitinating activity.

Nuclear egress of pseudorabies virus capsids is enhanced by a subspecies of the large tegument protein that is lost upon cytoplasmic maturation

Herpesviruses morphogenesis occurs stepwise both temporally and spatially, beginning in the nucleus and concluding with the emergence of an extracellular virion. The mechanisms by which these viruses interact with and penetrate the nuclear envelope and subsequent compartments of the secretory pathway remain poorly defined. In this report, a conserved viral protein (VP1/2; pUL36) that directs cytoplasmic stages of egress is identified to have multiple isoforms. Of these, a novel truncated VP1/2 species translocates to the nucleus and assists the transfer of DNA-containing capsids to the cytoplasm. The capsids are handed off to full-length VP1/2, which replaces the nuclear isoform on the capsids and is required for the final cytoplasmic stages of viral particle maturation. These results document that distinct VP1/2 protein species serve as effectors of nuclear and cytoplasmic egress.

Identification of a single amino acid residue which is critical for the interaction between HSV-1 inner tegument proteins pUL36 and pUL37

The herpes simplex virus type 1 (HSV-1) structural tegument proteins pUL36 and pUL37 are essential for secondary envelopment during the egress of viral particles. For this study, scanning alanine mutagenesis of HSV-1 pUL37, in combination with yeast two-hybrid, identified pUL37 residue D631 as a major determinant for binding of pUL36. Further analysis of the binding of this pUL37 mutant to pUL36 by coimmunoprecipitation assay confirmed the role of pUL37 D631 in mediating binding of pUL36. A trans-complementation assay using pUL37 deletion virus FRΔUL37 was then carried out, where pUL37 wild type or D631A were provided in trans. For pUL37 D631A, a significant reduction in virus titer was observed compared to that seen when pUL37 wild type was present. The results presented here underline the crucial role of the pUL36/pUL37 interaction in replication of HSV-1 and indicate a critical role for pUL37 D631 in mediating this interaction.

A wide extent of inter-strain diversity in virulent and vaccine strains of alphaherpesviruses

Alphaherpesviruses are widespread in the human population, and include herpes simplex virus 1 (HSV-1) and 2, and varicella zoster virus (VZV). These viral pathogens cause epithelial lesions, and then infect the nervous system to cause lifelong latency, reactivation, and spread. A related veterinary herpesvirus, pseudorabies (PRV), causes similar disease in livestock that result in significant economic losses. Vaccines developed for VZV and PRV serve as useful models for the development of an HSV-1 vaccine. We present full genome sequence comparisons of the PRV vaccine strain Bartha, and two virulent PRV isolates, Kaplan and Becker. These genome sequences were determined by high-throughput sequencing and assembly, and present new insights into the attenuation of a mammalian alphaherpesvirus vaccine strain. We find many previously unknown coding differences between PRV Bartha and the virulent strains, including changes to the fusion proteins gH and gB, and over forty other viral proteins. Inter-strain variation in PRV protein sequences is much closer to levels previously observed for HSV-1 than for the highly stable VZV proteome. Almost 20% of the PRV genome contains tandem short sequence repeats (SSRs), a class of nucleic acids motifs whose length-variation has been associated with changes in DNA binding site efficiency, transcriptional regulation, and protein interactions. We find SSRs throughout the herpesvirus family, and provide the first global characterization of SSRs in viruses, both within and between strains. We find SSR length variation between different isolates of PRV and HSV-1, which may provide a new mechanism for phenotypic variation between strains. Finally, we detected a small number of polymorphic bases within each plaque-purified PRV strain, and we characterize the effect of passage and plaque-purification on these polymorphisms. These data add to growing evidence that even plaque-purified stocks of stable DNA viruses exhibit limited sequence heterogeneity, which likely seeds future strain evolution.

Alphaherpesviruses such as herpes simplex virus (HSV) are ubiquitous in the human population. HSV causes oral and genital lesions, and has co-morbidities in acquisition and spread of human immunodeficiency virus (HIV). The lack of a vaccine for HSV hinders medical progress for both of these infections. A related veterinary alphaherpesvirus, pseudorabies virus (PRV), has long served as a model for HSV vaccine development, because of their similar pathogenesis, neuronal spread, and infectious cycle. We present here the first full genome characterization of a live PRV vaccine strain, Bartha, and reveal a spectrum of unique mutations that are absent from two divergent wild-type PRV strains. These mutations can now be examined individually for their contribution to vaccine strain attenuation and for potential use in HSV vaccine development. These inter-strain comparisons also revealed an abundance of short repetitive elements in the PRV genome, a pattern which is repeated in other herpesvirus genomes and even the unrelated Mimivirus. We provide the first global characterization of repeats in viruses, comparing both their presence and their variation among different viral strains and species. Repetitive elements such as these have been shown to serve as hotspots of variation between individuals or strains of other organisms, generating adaptations or even disease states through changes in length of DNA-binding sites, protein folding motifs, and other structural elements. These data suggest for the first time that similar mechanisms could be widely distributed in viral biology as well.

Proteomic characterization of pseudorabies virus extracellular virions

Pseudorabies virus (PRV), a member of the Alphaherpesvirinae, has a complex multilayered extracellular virion that is structurally conserved among other herpesviruses. PRV virions contain a double-stranded DNA genome within a proteinaceous capsid surrounded by the tegument, a layer of viral and cellular proteins. The envelope layer, which encloses the capsid and tegument, contains viral transmembrane proteins anchored in a phospholipid bilayer. The viral and host proteins contained within virions execute important functions during viral spread and pathogenesis, but a detailed understanding of the composition of PRV virions has been lacking. In this report, we present the first comprehensive proteomic characterization of purified PRV virions by mass spectrometry using two complementary approaches. To exclude proteins present in the extracellular medium that may nonspecifically associate with virions, we also analyzed virions treated with proteinase K and samples prepared from mock-infected cells. Overall, we identified 47 viral proteins associated with PRV virions, 40 of which were previously localized to the capsid, tegument, and envelope layers using traditional biochemical approaches. Additionally, we identified seven viral proteins that were previously undetected in virions, including pUL8, pUL20, pUL32, pUL40 (RR2), pUL42, pUL50 (dUTPase), and Rsp40/ICP22. Furthermore, although we did not enrich for posttranslational modifications, we detected phosphorylation of four virion proteins: pUL26, pUL36, pUL46, and pUL48. Finally, we identified 48 host proteins associated with PRV virions, many of which have known functions in important cellular pathways such as intracellular signaling, mRNA translation and processing, cytoskeletal dynamics, and membrane organization. This analysis extends previous work aimed at determining the composition of herpesvirus virions and provides novel insights critical for understanding the mechanisms underlying PRV entry, assembly, egress, spread, and pathogenesis.

A single mutation responsible for temperature-sensitive entry and assembly defects in the VP1-2 protein of herpes simplex virus

Evidence for an essential role of the herpes simplex virus type 1 (HSV-1) tegument protein VP1-2 originated from the analysis of the temperature-sensitive (ts) mutant tsB7. At the nonpermissive temperature (NPT), tsB7 capsids accumulate at the nuclear pore, with defective genome release and substantially reduced virus gene expression. We compared the UL36 gene of tsB7 with that of the parental strain HFEM or strain 17 and identified four amino acid substitutions, 1061D → G, 1453Y → H, 2273Y → H, and 2558T → I. We transferred the UL36 gene from tsB7, HFEM, or strain 17 into a KOS background. While KOS recombinants containing the HFEM or strain 17 UL36 gene exhibited no ts defect, recombinants containing the tsB7 UL36 VP1-2 exhibited a 5-log deficiency at the NPT. Incubation at the NPT resulted in little or no virus gene expression, though limited expression could be detected in a highly delayed fashion. Using shift-down regimes, gene expression recovered and recapitulated the time course normally observed, indicating that the initial block was in a reversible pathway. Using temperature shift-up regimes, a second defect later in the replication cycle was also observed in the KOS.ts viruses. We constructed a further series of recombinants which contained subsets of the four substitutions. A virus containing the wild-type (wt) residue at position 1453 and with the other three residues being from tsB7 VP1-2 exhibited wt plaquing efficiency. Conversely, a virus containing the three wt residues but the single Y → H change at position 1453 from tsB7 exhibited a 4- to 5-log drop in plaquing efficiency and was defective at both early and late stages of infection.

Random transposon-mediated mutagenesis of the essential large tegument protein pUL36 of pseudorabies virus

Homologs of the pseudorabies virus (PrV) essential large tegument protein pUL36 are conserved throughout the Herpesviridae. pUL36 functions during transport of the nucleocapsid to and docking at the nuclear pore as well as during virion formation after nuclear egress in the cytoplasm. Deletion analyses revealed several nonessential regions within the 3,084-amino-acid PrV pUL36 (S. Böttcher, B. G. Klupp, H. Granzow, W. Fuchs, K. Michael, and T. C. Mettenleiter, J. Virol. 80:9910-9915, 2006; S. Böttcher, H. Granzow, C. Maresch, B. Möhl, B. G. Klupp, and T. C. Mettenleiter, J. Virol. 81:13403-13411, 2007), while the C-terminal 62 amino acids are essential for virus replication (K. Coller, J. Lee, A. Ueda, and G. Smith, J. Virol. 81:11790-11797, 2007). To identify additional functional domains, we performed random mutagenesis of PrV pUL36 by transposon-mediated insertion of a 15-bp linker. By this approach, 26 pUL36 insertion mutants were selected and tested in transient transfection assays for their ability to complement one-step growth and/or viral spread of a PrV UL36 null mutant. Ten insertion mutants in the N-terminal half and 10 in the C terminus complemented both, whereas six insertion mutants clustering in the center of the protein did not complement in either assay. Interestingly, several insertions within conserved parts yielded positive complementation, including those located within the essential C-terminal 62 amino acids. For 15 mutants that mediated productive replication, stable virus recombinants were isolated and further characterized by plaque assay, in vitro growth analysis, and electron microscopy. Except for three mutant viruses, most insertion mutants replicated like wild-type PrV. Two insertion mutants, at amino acids (aa) 597 and 689, were impaired in one-step growth and viral spread and exhibited a defect in virion maturation in the cytoplasm. In contrast, one functional insertion (aa 1800) in a region which otherwise yielded only nonfunctional insertion mutants was impaired in viral spread but not in one-step growth without a distinctive ultrastructural phenotype. In summary, these studies extend and refine previous analyses of PrV pUL36 and demonstrate the different sensitivities of different regions of the protein to functional loss by insertion.

Cleavage specificity of the UL48 deubiquitinating protease activity of human cytomegalovirus and the growth of an active-site mutant virus in cultured cells

The human cytomegalovirus (HCMV) open reading frame UL48 encodes a 253-kDa tegument protein that is closely associated with the capsid and was recently shown to have ubiquitin-specific protease activity (J. Wang, A. N. Loveland, L. M. Kattenhorn, H. L. Ploegh, and W. Gibson, J. Virol. 80:6003-6012, 2006). Here, we examined the cleavage specificity of this deubiquitinase (DUB) and replication characteristics of an active-site mutant virus. The purified catalytic domain of the UL48 DUB (1 to 359 amino acids), corresponding to the herpes simplex virus UL36(USP) DUB (L. M. Kattenhorn, G. A. Korbel, B. M. Kessler, E. Spooner, and H. L. Ploegh, Mol. Cell 19:547-557, 2005), efficiently released ubiquitin but not ubiquitin-like modifications from a hemagglutinin peptide substrate. Mutating the active-site residues Cys24 or His162 (C24S and H162A, respectively) abolished this activity. The HCMV UL48 and HSV UL36(USP) DUBs cleaved both Lys48- and Lys63-linked ubiquitin dimers and oligomers, showing more activity toward Lys63 linkages. The DUB activity of the full-length UL48 protein immunoprecipitated from virus-infected cells also showed a better cleavage of Lys63-linked ubiquitinated substrates. An HCMV (Towne) mutant virus in which the UL48 DUB activity was destroyed [UL48(C24S)] produced 10-fold less progeny virus and reduced amounts of viral proteins compared to wild-type virus at a low multiplicity of infection. The mutant virus also produced perceptibly less overall deubiquitination than the wild-type virus. Our findings demonstrate that the HCMV UL48 DUB contains both a ubiquitin-specific carboxy-terminal hydrolase activity and an isopeptidase activity that favors ubiquitin Lys63 linkages and that these activities can influence virus replication in cultured cells.

Intracellular localization of the pseudorabies virus large tegument protein pUL36

Homologs of the essential large tegument protein pUL36 of herpes simplex virus 1 are conserved throughout the Herpesviridae, complex with pUL37, and form part of the capsid-associated "inner" tegument. pUL36 is crucial for transport of the incoming capsid to and docking at the nuclear pore early after infection as well as for virion maturation in the cytoplasm. Its extreme C terminus is essential for pUL36 function interacting with pUL25 on nucleocapsids to start tegumentation (K. Coller, J. Lee, A. Ueda, and G. Smith, J. Virol. 81:11790-11797, 2007). However, controversy exists about the cellular compartment in which pUL36 is added to the nascent virus particle. We generated monospecific rabbit antisera against four different regions spanning most of pUL36 of the alphaherpesvirus pseudorabies virus (PrV). By immunofluorescence and immunoelectron microscopy, we then analyzed the intracellular location of pUL36 after transient expression and during PrV infection. While reactivities of all four sera were comparable, none of them showed specific intranuclear staining during PrV infection. In immunoelectron microscopy, neither of the sera stained primary enveloped virions in the perinuclear cleft, whereas extracellular mature virus particles were extensively labeled. However, transient expression of pUL36 alone resulted in partial localization to the nucleus, presumably mediated by nuclear localization signals (NLS) whose functionality was demonstrated by fusion of the putative NLS to green fluorescent protein (GFP) and GFP-tagged pUL25. Since PrV pUL36 can enter the nucleus when expressed in isolation, the NLS may be masked during infection. Thus, our studies show that during PrV infection pUL36 is not detectable in the nucleus or on primary enveloped virions, correlating with the notion that the tegument of mature virus particles, including pUL36, is acquired in the cytosol.

Kaposi's sarcoma-associated herpesvirus encodes a viral deubiquitinase

Kaposi's sarcoma-associated herpesvirus (KSHV) is etiologically linked to Kaposi's sarcoma, primary effusion lymphomas, and multicentric Castleman's disease. Like other herpesviruses, KSHV can exist in either a lytic or a latent phase during its life cycle. We report that the lytic protein encoded by KSHV open reading frame 64 (Orf64) is a viral deubiquitinase (DUB) enzyme capable of deubiquitinating cellular proteins in vitro and in vivo. Orf64 DUB activity is effective against lysine 48 (K48)- and lysine 63 (K63)-linked ubiquitin chains. Thus, KSHV Orf64 is a viral DUB that does not show specificity toward K48 or K63 ubiquitin linkages. Orf64 DUB activity lies within the first 205 residues of the protein, and deubiquitination is dependent on a cysteine at position 29, since mutation of this residue ablated this activity. Cell fractionation studies revealed that the N terminus and the full-length protein localized to both the nuclear and cytoplasmic compartments. The function of Orf64 was tested by short interfering RNA (siRNA) knockdown studies on latently infected cells that were induced into lytic replication. We found that depletion of Orf64 by siRNA resulted in decreased viral lytic transcription and lytic protein expression. These experiments indicate that Orf64 plays a role in KSHV lytic replication.

Functional roles of the tegument proteins of herpes simplex virus type 1

Herpes virions consist of four morphologically distinct structures, a DNA core, capsid, tegument, and envelope. Tegument occupies the space between the nucleocapsid (capsid containing DNA core) and the envelope. A combination of genetic, biochemical and proteomic analysis of alphaherpes virions suggest the tegument contains in the order of 20 viral proteins. Historically the tegument has been described as amorphous but increasing evidence suggests there is an ordered addition of tegument during assembly. This review highlights the diverse roles, in addition to structural, that tegument plays during herpes viral replication using as an example herpes simplex virus type 1. Such diverse roles include: capsid transport during entry and egress; targeting of the capsid to the nucleus; regulation of transcription, translation and apoptosis; DNA replication; immune modulation; cytoskeletal assembly; nuclear egress of capsid; and viral assembly and final egress.

A herpesvirus encoded deubiquitinase is a novel neuroinvasive determinant

The neuroinvasive property of several alpha-herpesviruses underlies an uncommon infectious process that includes the establishment of life-long latent infections in sensory neurons of the peripheral nervous system. Several herpesvirus proteins are required for replication and dissemination within the nervous system, indicating that exploiting the nervous system as a niche for productive infection requires a specialized set of functions encoded by the virus. Whether initial entry into the nervous system from peripheral tissues also requires specialized viral functions is not known. Here we show that a conserved deubiquitinase domain embedded within a pseudorabies virus structural protein, pUL36, is essential for initial neural invasion, but is subsequently dispensable for transmission within and between neurons of the mammalian nervous system. These findings indicate that the deubiquitinase contributes to neurovirulence by participating in a previously unrecognized initial step in neuroinvasion.

Subsets of herpesviruses, such as herpes simplex virus and pseudorabies virus, are neuroinvasive pathogens. Upon infection, these viruses efficiently target peripheral nervous system tissue and establish a life-long infection for which there is no cure. Very few pathogens are known that invade the nervous system proficiently, and the mechanism by which herpesviruses achieve neuroinvasion is largely unknown. In this study, we demonstrate that a viral protease plays a critical and specific role allowing the virus to cross the threshold of the nervous system, but is dispensable for subsequent replication and encephalitic spread within the brain.

Herpesvirus assembly: an update

The order Herpesvirales contains viruses infecting animals from molluscs to men with a common virion morphology which have been classified into the families Herpesviridae, Alloherpesviridae and Malacoherpesviridae. Herpes virions are among the most complex virus particles containing a multitude of viral and cellular proteins which assemble into nucleocapsid, envelope and tegument. After autocatalytic assembly of the capsid and packaging of the newly replicated viral genome, a process which occurs in the nucleus and resembles head formation and genome packaging in the tailed double-stranded DNA bacteriophages, the nucleocapsid is translocated to the cytoplasm by budding at the inner nuclear membrane followed by fusion of the primary envelope with the outer nuclear membrane. Viral and cellular proteins are involved in mediating this 'nuclear egress' which entails substantial remodeling of the nuclear architecture. For final maturation within the cytoplasm tegument components associate with the translocated nucleocapsid, with themselves, and with the future envelope containing viral membrane proteins in a complex network of interactions resulting in the formation of an infectious herpes virion. The diverse interactions between the involved proteins exhibit a striking redundancy which is still insufficiently understood. In this review, recent advances in our understanding of the molecular processes resulting in herpes virion maturation will be presented and discussed as an update of a previous contribution [Mettenleiter, T.C., 2004. Budding events in herpesvirus morphogenesis. Virus Res. 106, 167-180].

Identification of structural protein-protein interactions of herpes simplex virus type 1

In this study we have defined protein-protein interactions between the structural proteins of herpes simplex virus type 1 (HSV-1) using a LexA yeast two-hybrid system. The majority of the capsid, tegument and envelope proteins of HSV-1 were screened in a matrix approach. A total of 40 binary interactions were detected including 9 out of 10 previously identified tegument-tegument interactions (Vittone, V., Diefenbach, E., Triffett, D., Douglas, M.W., Cunningham, A.L., and Diefenbach, R.J., 2005. Determination of interactions between tegument proteins of herpes simplex virus type 1. J. Virol. 79, 9566-9571). A total of 12 interactions involving the capsid protein pUL35 (VP26) and 11 interactions involving the tegument protein pUL46 (VP11/12) were identified. The most significant novel interactions detected in this study, which are likely to play a role in viral assembly, include pUL35-pUL37 (capsid-tegument), pUL46-pUL37 (tegument-tegument) and pUL49 (VP22)-pUS9 (tegument-envelope). This information will provide further insights into the pathways of HSV-1 assembly and the identified interactions are potential targets for new antiviral drugs.

Identification of envelope protein pORF81 of koi herpesvirus

Koi herpesvirus (KHV), an emerging pathogen causing mass mortality in koi and common carp, possesses the largest known herpesvirus genome of 295 kbp predicted to encode 156 different proteins. However, none of them has been identified or functionally characterized up to now. In this study, a rabbit antiserum was prepared against a bacterial fusion protein that permitted detection of the predicted type III membrane protein encoded by ORF81 of KHV. In Western blot analyses, the abundant ORF81 gene product of KHV exhibited an apparent mass of 26 kDa and appeared to be non-glycosylated. It could be localized in the cytoplasm of infected cells and in virion envelopes by indirect immunofluorescence and immunoelectron microscopy, respectively. The antiserum was also suitable for the detection of pORF81 in sections of gills, kidneys, hepatopancreas and skin of KHV-infected carp by immunohistochemistry.

Mutagenesis of the active-site cysteine in the ubiquitin-specific protease contained in large tegument protein pUL36 of pseudorabies virus impairs viral replication in vitro and neuroinvasion in vivo

Herpesviruses specify a ubiquitin-specific protease activity located within their largest tegument protein. Although its biological role is still largely unclear, mutation within the active site abolished deubiquitinating (DUB) activity and decreased virus replication in vitro and in vivo. To further elucidate the role of DUB activity for herpesvirus replication, the conserved active-site cysteine at amino acid position 26 within pUL36 of Pseudorabies virus (PrV) (Suid herpesvirus 1), a neurotropic alphaherpesvirus, was mutated to serine. Whereas one-step growth kinetics of the resulting mutant virus PrV-UL36(C(26)S) were moderately reduced, plaque size was decreased to 62% of that of the wild-type virus. Ultrastructural analysis revealed large accumulations of unenveloped nucleocapsids in the cytoplasm, but incorporation of the tegument protein pUL37 was not abolished. After intranasal infection with PrV-UL36(C(26)S) mice showed survival times two times longer than those of mice infected with wild-type or rescued virus. Thus, the DUB activity is important for PrV replication in vitro and for neuroinvasion in mice.