Budding events in herpesvirus morphogenesis

Herpes simplex virus protein UL11 but not UL51 is associated with lipid rafts

The UL11 and UL51 gene products of herpes simplex virus (HSV) are membrane-associated tegument proteins that are incorporated into the HSV virion. UL11 and UL51 are conserved throughout the herpesvirus family. Both UL11 and UL51, either singly or in combination, are involved in virion envelopment and/or egress. Both proteins are fatty acylated: UL11 is both acylated by myristoic and palmitoic acids and UL51 is monoacylated by palmitoic acid. Using confocal microscopy and sucrose gradient fractionations in transfected or HSV-infected cells, we found that HSV-2 UL11 but not UL51 was associated with lipid rafts. The dual acylation of UL11 was necessary for lipid raft association, as mutations in the myristoylation or palmitoylation sites prevented lipid raft association. These differences in lipid raft association may contribute to the functional differences between UL11 and UL51.

Intracellular trafficking and maturation of herpes simplex virus type 1 gB and virus egress require functional biogenesis of multivesicular bodies

The biogenesis of multivesicular bodies (MVBs) is topologically equivalent to virion budding. Hence, a number of viruses exploit the MVB pathway to build their envelope and exit from the cell. By expression of dominant negative forms of Vps4 and Vps24, two components of the MVB pathway, we observed an impairment in infectious herpes simplex virus (HSV) assembly/egress, in agreement with a recent report showing the involvement in HSV envelopment of Vps4, the MVB-specific ATPase (C. M. Crump, C. Yates, and T. Minson, J. Virol. 81:7380-7387). Furthermore, HSV infection resulted in morphological changes to MVBs. Glycoprotein B (gB), one of the most highly conserved glycoproteins across the Herpesviridae family, was sorted to MVB membranes. In cells expressing the dominant negative form of Vps4, the site of intracellular gB accumulation was altered; part of gB accumulated as an endoglycosidase H-sensitive immature form at a calreticulin-positive compartment, indicating that gB traffic was dependent on a functional MVB pathway. gB was ubiquitinated in both infected and transfected cells. Ubiquitination was in part dependent on ubiquitin lysine 63, a signal for cargo sorting to MVBs. Partial deletion of the gB cytoplasmic tail resulted in a dramatic reduction of ubiquitination, as well as of progeny virus assembly and release to the extracellular compartment. Thus, HSV envelopment/egress and gB intracellular trafficking are dependent on functional MVB biogenesis. Our data support the view that the sorting of gB to MVB membranes may represent a critical step in HSV envelopment and egress and that modified MVB membranes constitute a platform for HSV cytoplasmic envelopment or that MVB components are recruited to the site(s) of envelopment.

The ESCRT machinery is not required for human cytomegalovirus envelopment

The human cytomegalovirus (HCMV) has been proposed to complete its final envelopment on cytoplasmic membranes prior to its release to the extracellular medium. The nature of these membranes and the mechanisms involved in virus envelopment and release are poorly understood. Here we show by immunogold-labelling and electron microscopy that CD63, a marker of multivesicular bodies (MVBs), is incorporated into the viral envelope, supporting the notion that HCMV uses endocytic membranes for its envelopment. We therefore investigated a possible role for the cellular endosomal sorting complex required for transport (ESCRT) machinery in HCMV envelopment. Depletion of tumour suppressor gene 101 and ALIX/AIP1 with small interfering RNAs (siRNAs) in HCMV-infected cells did not affect virus production. In contrast, siRNAs against the vacuolar protein sorting 4 (VPS4) proteins silenced the expression of VPS4A and VPS4B, inhibited the sorting of epidermal growth factor to lysosomes, the formation of HIV Gag-derived virus-like particles and vesicular stomatitis virus infection, but enhanced the number of HCMV viral particles produced. Treatment of infected cells with protease inhibitors also increased viral production. These studies indicate that, in contrast to some enveloped RNA viruses, HCMV does not require the cellular ESCRT machinery to complete its envelopment.

Residues F593 and E596 of HSV-1 tegument protein pUL36 (VP1/2) mediate binding of tegument protein pUL37

The herpes simplex virus type 1 (HSV-1) structural tegument proteins pUL36 (VP1/2) and pUL37 are essential for secondary envelopment during the egress of viral particles. Our laboratory has previously shown that HSV-1 pUL36(512-767) fragment interacts with full-length pUL37. A number of single and double amino acid changes of conserved residues in the pUL36(512-767) fragment were generated using alanine-scanning site-directed mutagenesis. The interaction of pUL36(512-767) and pUL37 was then assessed using a combination of yeast two-hybrid and coimmunoprecipitation assays. Single changes to alanine of pUL36 residues F593 and E596 impaired binding of pUL37 with the greatest effect observed for the substitution E596A. Double mutations involving either of these residues in combination with the substitution E580A essentially blocked binding of pUL37. This information will provide the basis for generation of viral mutants to further define the importance of the pUL36/pUL37 interaction in assembly of HSV-1.

Murine gammaherpesvirus 68 ORF52 encodes a tegument protein required for virion morphogenesis in the cytoplasm

The tegument, a semiordered matrix of proteins overlying the nucleocapsid and underlying the virion envelope, in viruses in the gamma subfamily of Herpesviridae is poorly understood. Murine gammaherpesvirus 68 (MHV-68) is a robust model for studying gammaherpesvirus virion structure, assembly, and composition, as MHV-68 efficiently completes the lytic phase and productively infects cultured cells. We have found that MHV-68 ORF52 encodes an abundant tegument protein conserved among gammaherpesviruses. Detergent sensitivity experiments revealed that the MHV-68 ORF52 protein is more tightly bound to the virion nucleocapsid than the ORF45 tegument protein but could be dissociated from particles that retained the ORF65 small capsomer protein. ORF52, tagged with enhanced green fluorescent protein or FLAG epitope, localized to the cytoplasm. A recombinant MHV-68 bacterial artificial chromosome mutant with a nonsense mutation incorporated into ORF52 exhibited viral DNA replication, expression of late lytic genes, and capsid assembly and packaging at levels near those of the wild type. However, the MHV-68 ORF52-null virus was deficient in the assembly and release of infectious virion particles. Instead, partially tegumented capsids produced by the ORF52-null mutant accumulated in the cytoplasm, containing conserved capsid proteins, the ORF64 and ORF67 tegument proteins, but virtually no ORF45 tegument protein. Thus, ORF52 is essential for the tegumentation and egress of infectious MHV-68 particles in the cytoplasm, suggesting an important conserved function in gammaherpesvirus virion morphogenesis.

Relevance of the interaction between alphaherpesvirus UL3.5 and UL48 proteins for virion maturation and neuroinvasion

The UL3.5 and UL48 genes, which are conserved in most alphaherpesvirus genomes, are important for maturation of pseudorabies virus (PrV) particles in the cytoplasm of infected cells (W. Fuchs, B. G. Klupp, H. J. Rziha, and T. C. Mettenleiter, J. Virol. 70:3517-3527, 1996; W. Fuchs, H. Granzow, B. G. Klupp, M. Kopp and T. C. Mettenleiter, J. Virol. 76:6729-6742, 2002). In bovine herpesvirus 1 (BoHV-1), the homologous gene products pUL3.5 and pUL48 have been demonstrated to interact physically (N. Lam and G. Letchworth, J. Virol. 74:2876-2884, 2000). Moreover, BoHV-1 pUL3.5 partially complemented a pUL3.5 defect in PrV (W. Fuchs, H. Granzow, and T. C. Mettenleiter, J. Virol. 71:8886-8892, 1997). By using coimmunoprecipitation and yeast two-hybrid studies, we observed a similar interaction between pUL3.5 and pUL48 of PrV, as well as a heterologous interaction between the PrV and BoHV-1 gene products. The relevant domain could be confined to the first 43 amino acids of PrV pUL3.5. Unlike its BoHV-1 homologue, PrV pUL3.5 is processed by proteolytic cleavage, and only an abundant 14-kDa fragment consisting of amino acids 1 to >or=116 could be detected by peptide mass fingerprint analysis of purified wild-type PrV particles, which also contain the pUL48 tegument component. To determine the biological relevance of the protein-protein interaction, pUL3.5-, pUL48-, and double-negative PrV mutants were analyzed in parallel. All deletion mutants were replication competent but exhibited significantly reduced plaque sizes and virus titers in cultured rabbit kidney cells compared to wild-type and rescued viruses, which correlated with a delayed neuroinvasion in intranasally infected mice. Remarkably, the defects of the double-negative mutant were similar to those of pUL48-negative virus. Electron microscopy of cells infected with either deletion mutant revealed the retention of naked nucleocapsids in the cytoplasm and the absence of mature virus particles. In summary, our studies for the first time demonstrate the relevance of the pUL3.5-pUL48 interaction for secondary envelopment of an alphaherpesvirus, give a molecular basis for the observed trans-complementation between the PrV and BHV-1 pUL3.5 homologs, yield conclusive evidence for the incorporation of a proteolytically processed pUL3.5 into PrV virions, and demonstrate the importance of both proteins for neuroinvasion and neurovirulence of PrV.

Membrane and protein dynamics in live plant nuclei infected with Sonchus yellow net virus, a plant-adapted rhabdovirus

Sonchus yellow net virus (SYNV) serves as the paradigm for the cell biology of plant-adapted rhabdoviruses. Fluorescence recovery after photobleaching (FRAP) demonstrated that SYNV-induced intranuclear membranes are contiguous with the endomembrane system. Fluorescence intensity measurements of a green fluorescent protein-tagged nuclear envelope marker were consistent with electron microscopy studies, which suggest that infection by SYNV results in invagination of the inner nuclear membrane. Fusions of a red fluorescent protein to five SYNV-encoded proteins were used to determine the relationship between virus-induced intranuclear membranes and the localization of viral proteins. These data establish definitively that localization in the context of infected cells provides a superior means to predict protein function compared with localization studies conducted in mock-inoculated cells. Substructure has been identified within the viroplasm, the putative site of virus replication, which suggests that the nucleocapsid (N) protein occupies a region at the junction between the viroplasm and intranuclear membranes that largely excludes the phosphoprotein. Within virus-infected nuclei, the SYNV matrix (M) protein and glycoprotein (G) were associated predominantly with membranes, whereas sc4, the predicted movement protein, accumulated primarily at punctate loci on the periphery of cells. Coexpression of differently tagged SYNV protein fusions in combination with FRAP analyses suggest a model whereby the replication and morphogenesis of SYNV are spatially separated events. Finally, an M protein-containing complex was discovered that appears to bud from the nucleus and that moves on ER membranes. Taken together, these data represent the most comprehensive analyses of rhabdoviral protein localization conducted in the context of infected cells.

Viral regulation of the long distance axonal transport of herpes simplex virus nucleocapsid

Many membranous organelles and protein complexes are normally transported anterograde within axons to the presynaptic terminal, and details of the motors, adaptors and cargoes have received significant attention. Much less is known about the transport in neurons of non-membrane bound particles, such as mRNAs and their associated proteins. We propose that herpes simplex virus type 1 (HSV) can be used to study the detailed mechanisms regulating long distance transport of particles in axons. A critical step in the transmission of HSV from one infected neuron to the next is the polarized anterograde axonal transport of viral DNA from the host infected nerve cell body to the axon terminal. Using the in vivo mouse retinal ganglion cell model infected with wild type virus or a mutant strain that lacks the protein Us9, we found that Us9 protein was necessary for long distance anterograde axonal transport of viral nucleocapsid (DNA surrounded by capsid proteins), but unnecessary for transport of virus envelope. Thus, we conclude that nucleocapsid can be transported independently down axons via a Us9-dependent mechanism.

Epstein-Barr virus-encoded protein kinase (BGLF4) is involved in production of infectious virus

The Epstein-Barr virus (EBV) BGLF4 gene product is a protein kinase (PK). Although this kinase has been characterized and several of its targets have been identified, its biological role remains enigmatic. We have generated and assessed a BGLF4 knockdown phenotype by means of RNA interference and report the following: (i) BGLF4-targeting small interfering RNA effectively inhibited the expression of its product, the viral PK, during lytic reactivation, (ii) BGLF4 knockdown partially inhibited viral DNA replication and expression of selected late viral genes, (iii) the absence of EBV PK resulted in retention of the viral nucleocapsids in the nuclei, and (iv) as a result of the nuclear retention, release of infectious virions is significantly retarded. Our results provide evidence that EBV PK plays an important role in nuclear egress of the virus and ultimately is crucial for lytic virus replication.

Deubiquitination in virus infection

Post-translational modification of proteins and peptides by ubiquitin, a highly evolutionarily conserved 76 residue protein, and ubiquitin-like modifiers has emerged as a major regulatory mechanism in various cellular activities. Eukaryotic viruses are known to modulate protein ubiquitination to their advantage in various ways. At the same time, the evidence for the importance of deubiquitination as a viral target also is growing. This review centers on known viral interactions with protein deubiquitination, on viral enzymes for which deubiquitinating activities were recently demonstrated, and on the roles of viral ubiquitin-like sequences.

Functional domains of murine cytomegalovirus nuclear egress protein M53/p38

Two conserved herpes simplex virus 1 proteins, UL31 and UL34, form a complex at the inner nuclear membrane which governs primary envelopment and nuclear egress of the herpesvirus nucleocapsids. In mouse cytomegalovirus, a member of the betaherpesvirus subfamily, the homologous proteins M53/p38 and M50/p35 form the nuclear egress complex (NEC). Since the interaction of these proteins is essential for functionality, the definition of the mutual binding sites is a prerequisite for further analysis. Using a comprehensive random mutagenesis procedure, we have mapped the M53/p38 binding site of M50/p35 (A. Bubeck, M. Wagner, Z. Ruzsics, M. Lötzerich, M. Iglesias, I. R. Singh, and U. H. Koszinowski, J. Virol. 78:8026-8035). Here we describe a corresponding analysis for the UL31 homolog M53/p38. A total of 72 individual mutants were reinserted into the genome to test the complementation of the lethal M53 null phenotype. The mutants were also studied for colocalization and for coprecipitation with M50/p35. The analysis revealed that the nonconserved N-terminal one-third of M53/p38 provides the nuclear localization signal as an essential function. The collective results for many mutants localized the binding site for M50/p35 to amino acids (aa) 112 to 137. No single aa exchange for alanine could destroy NEC formation, but virus attenuation revealed a major role for aa K128, Y129, and L130. The lethal phenotype of several insertion and stop mutants indicated the functional importance of the C terminus of the protein.

The pseudorabies virus VP1/2 tegument protein is required for intracellular capsid transport

Transport of capsids in cells is critical to alphaherpesvirus infection and pathogenesis; however, viral factors required for transport have yet to be identified. Here we provide a detailed examination of capsid dynamics during the egress phase of infection in Vero cells infected with pseudorabies virus. We demonstrate that the VP1/2 tegument protein is required for processive microtubule-based transport of capsids in the cytoplasm. A second tegument protein that binds to VP1/2, UL37, was necessary for wild-type transport but was not essential for this process. Both proteins were also required for efficient nuclear egress of capsids to the cytoplasm.

UL20 protein functions precede and are required for the UL11 functions of herpes simplex virus type 1 cytoplasmic virion envelopment

Egress of herpes simplex virus type 1 (HSV-1) from the nucleus of the infected cell to extracellular spaces involves a number of distinct steps, including primary envelopment by budding into the perinuclear space, de-envelopment into the cytoplasm, cytoplasmic reenvelopment, and translocation of enveloped virions to extracellular spaces. UL20/gK-null viruses are blocked in cytoplasmic virion envelopment and egress, as indicated by an accumulation of unenveloped or partially enveloped capsids in the cytoplasm. Similarly, UL11-null mutants accumulate unenveloped capsids in the cytoplasm. To assess whether UL11 and UL20/gK function independently or synergistically in cytoplasmic envelopment, recombinant viruses having either the UL20 or UL11 gene deleted were generated. In addition, a recombinant virus containing a deletion of both UL20 and UL11 genes was constructed using the HSV-1(F) genome cloned into a bacterial artificial chromosome. Ultrastructural examination of virus-infected cells showed that both UL20- and UL11-null viruses accumulated unenveloped capsids in the cytoplasm. However, the morphology and distribution of the accumulated capsids appeared to be distinct, with the UL11-null virions forming aggregates of capsids having diffuse tegument-derived material and the UL20-null virus producing individual capsids in close juxtaposition to cytoplasmic membranes. The UL20/UL11 double-null virions appeared morphologically similar to the UL20-null viruses. Experiments on the kinetics of viral replication revealed that the UL20/UL11 double-null virus replicated in a manner similar to the UL20-null virus. Additional experiments revealed that transiently expressed UL11 localized to the trans-Golgi network (TGN) independently of either gK or UL20. Furthermore, virus infection with the UL11/UL20 double-null virus did not alter the TGN localization of transiently expressed UL11 or UL20 proteins, indicating that these proteins did not interact. Taken together, these results show that the intracellular transport and TGN localization of UL11 is independent of UL20/gK functions, and that UL20/gK are required and function prior to UL11 protein in virion cytoplasmic envelopment.

Complete replication cycle and acquisition of tegument in nucleus of human herpesvirus 6A in astrocytes and in T-cells

The ultrastructural replication cycle of human herpesvirus 6A and 6B, both T-lymphotropic viruses, with tropism for the central nervous system, was compared by electron microscopy in the same cells, that is, in the T-lymphoblastoid cell line SupT-1 and in human astrocytes. Both HHV-6A and HHV-6B replicated efficiently in SupT-1 and formed viral particles. The tegument is the least characterized structure of the herpesviral particle and both variants were able to form intranuclear membrane compartments called tegusomes in SupT-1 where tegumentation occurred. Also, tegumentation occurred in HHV-6A infected cells in the nucleoplasm without the presence of a tegusome. This suggests that there is more than one possible route of tegumentation. Differences in the replication cycles between HHV-6A and HHV-6B were also observed in the cytoplasm. One such difference was that prominent annulate lamellae were only found in the cytoplasm of HHV-6A infected cells. In astrocytes a successful formation of viral particles was only seen with the HHV-6A variant. The HHV-6A virus life cycle in astrocytes resembled the life cycle in the T-cell line SupT-1, except that no annulate lamellae were found. Complete viral particles were found extracellularly around the astrocytes and the supernatant of infected astrocytes were able to re-infect SupT-1 cells. This suggests that HHV-6A infection in astrocytes can generate complete, viable, and infectious viral particles. The HHV-6 variants behave differently in the same type of cells and have different tropisms for astrocytes, supporting the notion that the variants might induce different diseases.

Cytomegalovirus primary envelopment occurs at large infoldings of the inner nuclear membrane

We have investigated the morphogenesis of human and murine cytomegalovirus by transmission electron microscopy after high-pressure freezing, freeze substitution, and plastic embedding. We observed large tubular infoldings of the inner nuclear membrane that were free of lamina and active in primary envelopment and subsequent transport of capsids to the nuclear periphery. Semiquantitative determinations of the enlarged inner nuclear membrane area and the location of the primary envelopment of nucleocapsids demonstrated that this structure represents a virus-induced specialized membrane domain at which the particles are preferentially enveloped. This is a previously undescribed structural element relevant in cytomegalovirus morphogenesis.

Role of the TSG101 gene in Epstein-Barr virus late gene transcription

Rta, an Epstein-Barr virus (EBV)-encoded immediate-early protein, governs the reactivation of the viral lytic program by transactivating a cascade of lytic gene expression. Cellular transcription factors such as Sp1, ATF2, E2F, and Akt have been demonstrated to mediate Rta transactivation of lytic genes. We report herein that Rta associates with another potent transcription factor, tumor susceptibility gene 101 (TSG101), to promote the activation of EBV late genes. Results from an EBV cDNA array reveal that depletion of TSG101 by siRNA potently inhibits the transcription of five Rta-responsive EBV late genes, BcLF1, BDLF3, BILF2, BLLF1, and BLRF2. Depletion of TSG101 impairs the Rta transactivation of these late promoters severely. Moreover, a concordant augmentation of Rta transactivating activity is observed when TSG101 is overexpressed following ectopic transfection. Mechanistically, Rta interaction with TSG101 causes the latter to accumulate principally in the nuclei, wherein the proteins colocalize and are recruited to the viral promoters. Of note, TSG101 is crucial for the efficient binding of Rta to these late promoters. As a result, cells with defective TSG101 fail to express late viral proteins, leading to a decrease in the yield of virus particles. Thus, the contribution of TSG101 to Rta-mediated late gene activation is of great importance for completion of the EBV productive lytic cycle. These observations consolidate a role for TSG101 in the replication of EBV, a DNA virus, that differs from what is observed for RNA viruses, where TSG101 aids mainly in the endosomal sorting of enveloped late viral proteins for assembly at the plasma membrane.

Packaging of the virion host shutoff (Vhs) protein of herpes simplex virus: two forms of the Vhs polypeptide are associated with intranuclear B and C capsids, but only one is associated with enveloped virions

The virion host shutoff (Vhs) protein (UL41) is a minor component of herpes simplex virus virions which, following penetration, accelerates turnover of host and viral mRNAs. Infected cells contain 58-kDa and 59.5-kDa forms of Vhs, which differ in the extent of phosphorylation, yet only a 58-kDa polypeptide is incorporated into virions. In pulse-chase experiments, the primary Vhs translation product comigrated in sodium dodecyl sulfate-polyacrylamide gel electrophoresis with the 58-kDa virion polypeptide, and could be chased to 59.5 kDa. While both 59.5-kDa and 58-kDa forms were found in nuclear and cytoplasmic fractions, the 59.5-kDa form was significantly enriched in the nucleus. Both forms were associated with intranuclear B and C capsids, yet only the 58-kDa polypeptide was found in enveloped cytoplasmic virions. A 58-kDa form, but not the 59.5-kDa form, was found in L particles, noninfectious particles that contain an envelope and tegument but no capsid. The data suggest that virions contain two populations of Vhs that are packaged by different pathways. In the first pathway, the primary translation product is processed to 59.5 kDa, is transported to the nucleus, binds intranuclear capsids, and is converted to 58 kDa at some stage prior to final envelopment. The second pathway does not involve the 59.5-kDa form or interactions between Vhs and capsids. Instead, the primary translation product is phosphorylated to the 58-kDa virion form and packaged through interactions with other tegument proteins in the cytoplasm or viral envelope proteins at the site of final envelopment.

Two modes of herpesvirus trafficking in neurons: membrane acquisition directs motion

Alphaherpesvirus infection of the mammalian nervous system is dependent upon the long-distance intracellular transport of viral particles in axons. How viral particles are effectively trafficked in axons to either sensory ganglia following initial infection or back out to peripheral sites of innervation following reactivation remains unknown. The mechanism of axonal transport has, in part, been obscured by contradictory findings regarding whether capsids are transported in axons in the absence of membrane components or as enveloped virions. By imaging actively translocated viral structural components in living peripheral neurons, we demonstrate that herpesviruses use two distinct pathways to move in axons. Following entry into cells, exposure of the capsid to the cytosol resulted in efficient retrograde transport to the neuronal cell body. In contrast, progeny virus particles moved in the anterograde direction following acquisition of virion envelope proteins and membrane lipids. Retrograde transport was effectively shut down in this membrane-bound state, allowing for efficient delivery of progeny viral particles to the distal axon. Notably, progeny viral particles that lacked a membrane were misdirected back to the cell body. These findings show that cytosolic capsids are trafficked to the neuronal cell body and that viral egress in axons occurs after capsids are enshrouded in a membrane envelope.

Reconstitution of herpes simplex virus type 1 nuclear capsid egress in vitro

Newly assembled herpesvirus capsids travel from the nucleus to the plasma membrane by a mechanism that is poorly understood. Furthermore, the contribution of cellular proteins to this egress has yet to be clarified. To address these issues, an in vitro nuclear egress assay that reproduces the exit of herpes simplex virus type 1 (HSV-1) capsids from nuclei isolated from infected cells was established. As expected, the assay has all the hallmarks of intracellular transport assays, namely, a dependence on time, energy, and temperature. Surprisingly, it is also dependent on cytosol and was slightly enhanced by infected cytosol, suggesting an implication of both host and viral proteins in the process. The capsids escaped these nuclei by budding through the inner nuclear membrane, accumulated as enveloped capsids between the two nuclear membranes, and were released in cytosol exclusively as naked capsids, exactly as in intact cells. This is most consistent with the view that the virus escapes by crossing the two nuclear membranes rather than through nuclear pores. Unexpectedly, nuclei isolated at the nonpermissive temperature from cells infected with a U(L)26 thermosensitive protease mutant (V701) supported capsid egress. Although electron microscopy, biochemical, and PCR analyses hinted at a likely reconstitution of capsid maturation, DNA encapsidation could not be confirmed by a traditional SQ test. This assay should prove very useful for identification of the molecular players involved in HSV-1 nuclear egress.

Epstein-Barr virus BNRF1 protein allows efficient transfer from the endosomal compartment to the nucleus of primary B lymphocytes

Epstein-Barr virus (EBV) is a tumor virus with marked B lymphotropism. After crossing the B-cell membrane, the virus enters cytoplasmic vesicles, where decapsidation takes place to allow transfer of the viral DNA to the cell nucleus. BNRF1 has been characterized as the EBV major tegument protein, but its precise function is unknown. We have constructed a viral mutant that lacks the BNRF1 gene and report here its in vitro phenotype. A recombinant virus devoid of BNRF1 (DeltaBNRF1) showed efficient DNA replication and production of mature viral particles. B cells infected with the DeltaBNRF1 mutant presented viral lytic antigens as efficiently as B cells infected with wild-type or BNRF1 trans-complemented DeltaBNRF1 viruses. Antigen presentation in B cells infected with either wild-type (EBV-wt) or DeltaBNRF1 virus was blocked by leupeptin addition, showing that both viruses reach the endosome/lysosome compartment. These data were confirmed by direct observation of the mutant virus in endosomes of infected B cells by electron microscopy. However, we observed a 20-fold reduction in the number of B cells expressing the nuclear protein EBNA2 after infection with a DeltaBNRF1 virus compared to wild-type infection. Likewise, DeltaBNRF1 viruses transformed primary B cells much less efficiently than EBV-wt or BNRF1 trans-complemented viruses. We conclude from these findings that BNRF1 plays an important role in viral transport from the endosomes to the nucleus.

Varicella

Varicella-zoster virus, a herpesvirus, causes varicella (chickenpox) and, after endogenous reactivation, herpes zoster (shingles). Varicella, which is recognised by a characteristic vesicular rash, arises mainly in young children, although older individuals can be affected. In immunocompetent patients, symptoms are usually mild to moderate, but an uncomplicated severe case can have more than 1000 lesions and severe constitutional symptoms. Serious complications--including central nervous system involvement, pneumonia, secondary bacterial infections, and death--are sometimes seen. Varicella can be prevented by vaccination. Vaccine is about 80-85% effective against all disease and highly (more than 95%) effective in prevention of severe disease. In the USA, a routine childhood immunisation programme has reduced disease incidence, complications, hospital admissions, and deaths in children and in the general population, indicating strong herd immunity. Similar immunisation programmes have been adopted by some other countries, including Uruguay, Germany, Taiwan, Canada, and Australia, and are expected to be implemented more widely in future.

The cycle of human herpes simplex virus infection: virus transport and immune control

After infection of skin or mucosa, herpes simplex virus enters the sensory nerve endings and is conveyed by retrograde axonal transport to the dorsal root ganglion, where the virus develops lifelong latency. Intermittent reactivation, which is spontaneous in humans, leads to anterograde transport of virus particles and proteins to the skin or mucosa, where the virus is shed and/or causes disease. Immune control of viral infection and replication occurs at the level of skin or mucosa during initial or recurrent infection and also within the dorsal root ganglion, where immune mechanisms control latency and reactivation. This article examines current views on the mechanisms of retrograde and anterograde transport of the virus in axons and the mechanisms of innate and adaptive immunity that control infection in the skin or mucosa and in the dorsal root ganglion--in particular, the role of interferons, myeloid and plasmacytoid dendritic cells, CD4(+) and CD8(+) T cells, and interferon- gamma and other cytokines, including their significance in the development of vaccines for genital herpes.

Cytoplasmic residues of herpes simplex virus glycoprotein gE required for secondary envelopment and binding of tegument proteins VP22 and UL11 to gE and gD

The final assembly of herpes simplex virus (HSV) involves binding of tegument-coated capsids to viral glycoprotein-enriched regions of the trans-Golgi network (TGN) as enveloped virions bud into TGN membranes. We previously demonstrated that HSV glycoproteins gE/gI and gD, acting in a redundant fashion, are essential for this secondary envelopment. To define regions of the cytoplasmic (CT) domain of gE required for secondary envelopment, HSVs lacking gD and expressing truncated gE molecules were constructed. A central region (amino acids 470 to 495) of the gE CT domain was important for secondary envelopment, although more C-terminal residues also contributed. Tandem affinity purification (TAP) proteins including fragments of the gE CT domain were used to identify tegument proteins VP22 and UL11 as binding partners, and gE CT residues 470 to 495 were important in this binding. VP22 and UL11 were precipitated from HSV-infected cells in conjunction with full-length gE and gE molecules with more-C-terminal residues of the CT domain. gD also bound VP22 and UL11. Expression of VP22 and gD or gE/gI in cells by use of adenovirus (Ad) vectors provided evidence that other viral proteins were not necessary for tegument/glycoprotein interactions. Substantial quantities of VP22 and UL11 bound nonspecifically onto or were precipitated with gE and gD molecules lacking all CT sequences, something that is very unlikely in vivo. VP16 was precipitated equally whether gE/gI or gD was present in extracts or not. These observations illustrated important properties of tegument proteins. VP22, UL11, and VP16 are highly prone to binding nonspecifically to other proteins, and this did not represent insolubility during our assays. Rather, it likely reflects an inherent "stickiness" related to the formation of tegument. Nevertheless, assays involving TAP proteins and viral proteins expressed by HSV and Ad vectors supported the conclusion that VP22 and UL11 interact specifically with the CT domains of gD and gE.

Functional characterization of Kaposi's sarcoma-associated herpesvirus ORF45 by bacterial artificial chromosome-based mutagenesis

Open reading frame 45 (ORF45) of Kaposi's sarcoma-associated herpesvirus (KSHV) encodes an immediate-early protein. This protein is also present in virions as a tegument protein. ORF45 protein interacts with interferon regulatory factor 7 (IRF-7) and inhibits virus-induced type I interferon production by blocking activation of IRF-7. To define further the function of ORF45 and the mechanism underlying its action, we constructed an ORF45-null recombinant virus genome (BAC-stop45) by using a bacterial artificial chromosome (BAC) system. Stable 293T cells carrying the BAC36 (wild type) and BAC-stop45 genomes were generated. When monolayers of 293T BAC36 and 293T BAC-stop45 cells were induced with 12-O-tetradecanoylphorbol-13-acetate and sodium butyrate, no significant difference was found between them in overall viral gene expression and lytic DNA replication, but induced 293T BAC-stop45 cells released 10-fold fewer virions to the medium than did 293T BAC36 cells. When ORF45-null virus was used to infect cells, lower infectivity was observed than for wild-type BAC36. These results suggest that KSHV ORF45 plays roles in both early and late stages of viral infection, probably in viral ingress and egress.

A conserved region of the herpes simplex virus type 1 tegument protein VP22 facilitates interaction with the cytoplasmic tail of glycoprotein E (gE)

Herpes simplex virus type 1 (HSV-1) virions, contain a proteinaceous layer termed the tegument that lies between the nucleocapsid and viral envelope. Current evidence suggests that viral glycoprotein tails play a role in the recruitment of tegument-coated capsids to the site of final envelopment; vesicles derived from the trans-Golgi network. We have identified an interaction between VP22, an abundant tegument protein and the cytoplasmic tail of glycoprotein E (gE). This interaction was identified by coimmunoprecipitation studies and confirmed by a glutathione-S-transferase (GST) pulldown from infected cell lysates. Truncation mutagenesis suggests that residues 165-270 of VP22 facilitate the interaction with the cytoplasmic tail of gE. In fact, this region of VP22 is sufficient to bind to gE in the absence of additional viral proteins. Using a transfection/infection-based virion incorporation assay, residues 165-270 of VP22 fused to GFP competed efficiently with wild-type VP22 for packaging into assembling virus particles.

The UL4 gene of pseudorabies virus encodes a minor infected-cell protein that is dispensable for virus replication

Although homologues of the open reading frame (ORF) UL4 of herpes simplex virus 1 (Human herpesvirus 1) have been found in the genomes of all hitherto-analysed alphaherpesviruses, little is known about their function. In a project to analyse systematically, in an isogenic and standardized assay system, the gene products of the alphaherpesvirus pseudorabies virus (PrV; Suid herpesvirus 1), the PrV UL4 gene product was identified using a monospecific rabbit antiserum prepared against a bacterial fusion protein. Western blot and immunofluorescence analyses revealed that the 146 codon UL4 ORF of PrV was translated into a nuclear 15 kDa protein which was detectable from 6 h after infection of rabbit kidney cells, but was not found in purified virus particles. For functional analysis, a UL4-negative virus recombinant (PrV-DeltaUL4F) was generated by mutagenesis of an infectious full-length clone of the PrV genome in E. coli. PrV-DeltaUL4F was replication-competent in rabbit kidney cells, and plaque formation was not affected by the mutation. However, maximum virus titres of PrV-DeltaUL4F were decreased about fivefold compared with wild-type PrV, and electron microscopy of infected cells demonstrated an impairment of release of mature virions. This growth defect of PrV-DeltaUL4F could be corrected completely by propagation in UL4-expressing cells. Correlating with the inconspicuous in vitro phenotype, neurovirulence of PrV-DeltaUL4F was also not affected significantly. Thus, UL4 encodes a non-structural protein of PrV that enhances virion formation but is not essential for PrV replication in vitro or in vivo.

Betaherpesvirus-conserved cytomegalovirus tegument protein ppUL32 (pp150) controls cytoplasmic events during virion maturation

The UL32 gene of human cytomegalovirus (CMV) encodes a prominent betaherpesvirus-conserved virion tegument protein, called pp150 (basic phosphoprotein/ppUL32), that accumulates within a cytoplasmic inclusion adjacent to the nucleus at late times during infection. Using a UL32 deletion mutant (DeltaUL32-BAC) (where BAC is bacterial artificial chromosome), we demonstrate that pp150 is critical for virion maturation in the cytoplasmic compartment. Cotransfection of a pp150 expression plasmid with DeltaUL32-BAC DNA led to complementation of the replication defect with focus formation due to secondary spread. Deletion of the amino terminus of pp150 or disruption of the betaherpesvirus conserved regions, CR1 and CR2, revealed these regions to be critical for replication. In contrast, deletion of the carboxyl terminus only partially compromised maturation while disruption of glycosylation sites had no effect. An African green monkey CMV UL32 homolog complemented DeltaUL32-BAC replication but murine CMV M32 failed to complement, consistent with evolutionary divergence of rodent and primate cytomegaloviruses. Infection with DeltaUL32-BAC showed normal expression of all kinetic classes of viral genes and replication of viral DNA, with accumulation of viral DNA-containing particles in the cytoplasm; however, mutant virus did not spread to adjacent cells. In contrast to this block in virion infectivity, cell-to-cell transfer of pp65-containing particles was observed, suggesting that release of dense bodies continued in the absence of pp150. These observations demonstrate that pp150 is critical for virion egress, possibly at the stage of final envelopment.

Herpes simplex virus capsids are transported in neuronal axons without an envelope containing the viral glycoproteins

Electron micrographic studies of neuronal axons have produced contradictory conclusions on how alphaherpesviruses are transported from neuron cell bodies to axon termini. Some reports have described unenveloped capsids transported on axonal microtubules with separate transport of viral glycoproteins within membrane vesicles. Others have observed enveloped virions in proximal and distal axons. We characterized transport of herpes simplex virus (HSV) in human and rat neurons by staining permeabilized neurons with capsid- and glycoprotein-specific antibodies. Deconvolution microscopy was used to view 200-nm sections of axons. HSV glycoproteins were very rarely associated with capsids (3 to 5%) and vice versa. Instances of glycoprotein/capsid overlap frequently involved nonconcentric puncta and regions of axons with dense viral protein concentrations. Similarly, HSV capsids expressing a VP26-green fluorescent protein fusion protein (VP26/GFP) did not stain with antiglycoprotein antibodies. Live-cell imaging experiments with VP26/GFP-labeled capsids demonstrated that capsids moved in a saltatory fashion, and very few stalled for more than 1 to 2 min. To determine if capsids could be transported down axons without glycoproteins, neurons were treated with brefeldin A (BFA). However, BFA blocked both capsid and glycoprotein transport. Glycoproteins were transported into and down axons normally when neurons were infected with an HSV mutant that produces immature capsids that are retained in the nucleus. We concluded that HSV capsids are transported in axons without an envelope containing viral glycoproteins, with glycoproteins transported separately and assembling with capsids at axon termini.

Packaging determinants in the UL11 tegument protein of herpes simplex virus type 1

The UL11 gene of herpes simplex virus type 1 encodes a 96-amino-acid tegument protein that is myristylated, palmitylated, and phosphorylated and is found on the cytoplasmic faces of nuclear, Golgi apparatus-derived, and plasma membranes of infected cells. Although this protein is thought to play a role in virus budding, its specific function is unknown. Purified virions were found to contain approximately 700 copies of the UL11 protein per particle, making it an abundant component of the tegument. Moreover, comparisons of cell-associated and virion-associated UL11 showed that packaging is selective for underphosphorylated forms, as has been reported for several other tegument proteins. Although the mechanism by which UL11 is packaged is unknown, previous studies have identified several sequence motifs in the protein that are important for membrane binding, intracellular trafficking, and interaction with UL16, another tegument protein. To ascertain whether any of these motifs are needed for packaging, a transfection/infection-based assay was used in which mutant forms of the protein must compete with the wild type. In this assay, the entire C-terminal half of UL11 was found to be dispensable. In the N-terminal half, the sites of myristylation and palmitylation, which enable membrane-binding and Golgi apparatus-specific targeting, were found to be essential for efficient packaging. The acidic cluster motif, which is not needed for Golgi apparatus-specific targeting but is involved in recycling the protein from the plasma membrane and for the interaction with UL16, was found to be essential, too. Thus, something other than mere localization of UL11 to Golgi apparatus-derived membranes is needed for packaging. The critical factor is unlikely to be the interaction with UL16 because other mutants that fail to bind this protein (due to removal of the dileucine-like motif or substitutions with foreign acidic clusters) were efficiently packaged. Collectively, these results suggest that UL11 packaging is not driven by a passive mechanism but instead requires trafficking through a specific pathway.

The capsid-associated UL25 protein of the alphaherpesvirus pseudorabies virus is nonessential for cleavage and encapsidation of genomic DNA but is required for nuclear egress of capsids

Homologs of the UL25 gene product of herpes simplex virus (HSV) have been identified in all three subfamilies of the Herpesviridae. However, their exact function during viral replication is not yet known. Whereas earlier studies indicated that the UL25 protein of HSV-1 is not required for cleavage of newly replicated viral DNA but is necessary for stable encapsidation (A. R. McNab, P. Desai, S. Person, L. Roof, D. R. Thompson, W. W. Newcomb, J. C. Brown, and F. L. Homa, J. Virol. 72:1060-1070, 1998), viral DNA packaging has recently been demonstrated to occur in the absence of UL25, although at significantly decreased levels compared to wild-type HSV-1 (N. Stow, J. Virol. 75:10755-10765 2001). To clarify the functional role of UL25 we analyzed the homologous protein of the alphaherpesvirus pseudorabies virus (PrV). PrV UL25 was found to be essential for viral replication, as a mutant virus lacking the UL25 protein required UL25-expressing cells for productive propagation. In the absence of the UL25 protein, newly replicated PrV DNA was cleaved and DNA-containing C-type capsids were detected in infected cell nuclei. However, although capsids were frequently found in close association with the inner nuclear membrane, nuclear egress was not observed. Consequently, no capsids were found in the cytoplasm, resulting in an inhibition of virion morphogenesis. In contrast, the formation of capsidless enveloped tegument structures (L particles) in the cytoplasm was readily observed. Thus, our data demonstrate that the PrV UL25 protein is not essential for cleavage and encapsidation of viral genomes, although both processes occur more efficiently in the presence of the protein. However, the presence of the PrV UL25 protein is a prerequisite for nuclear egress. By immunoelectron microscopy, we detected UL25-specific label on DNA-containing C capsids but not on other intranuclear immature or defective capsid forms. Thus, the PrV UL25 protein may represent the hitherto missing trigger that allows primary envelopment preferably of DNA-filled C capsids.

Eclipse phase of herpes simplex virus type 1 infection: Efficient dynein-mediated capsid transport without the small capsid protein VP26

Cytoplasmic dynein,together with its cofactor dynactin, transports incoming herpes simplex virus type 1 (HSV-1) capsids along microtubules (MT) to the MT-organizing center (MTOC). From the MTOC, capsids move further to the nuclear pore, where the viral genome is released into the nucleoplasm. The small capsid protein VP26 can interact with the dynein light chains Tctex1 (DYNLT1) and rp3 (DYNLT3) and may recruit dynein to the capsid. Therefore, we analyzed nuclear targeting of incoming HSV1-DeltaVP26 capsids devoid of VP26 and of HSV1-GFPVP26 capsids expressing a GFPVP26 fusion instead of VP26. To compare the cell entry of different strains, we characterized the inocula with respect to infectivity, viral genome content, protein composition, and particle composition. Preparations with a low particle-to-PFU ratio showed efficient nuclear targeting and were considered to be of higher quality than those containing many defective particles, which were unable to induce plaque formation. When cells were infected with HSV-1 wild type, HSV1-DeltaVP26, or HSV1-GFPVP26, viral capsids were transported along MT to the nucleus. Moreover, when dynein function was inhibited by overexpression of the dynactin subunit dynamitin, fewer capsids of HSV-1 wild type, HSV1-DeltaVP26, and HSV1-GFPVP26 arrived at the nucleus. Thus, even in the absence of the potential viral dynein receptor VP26, HSV-1 used MT and dynein for efficient nuclear targeting. These data suggest that besides VP26, HSV-1 encodes other receptors for dynein or dynactin.

Viral interactions with the cytoskeleton: a hitchhiker's guide to the cell

The actin and microtubule cytoskeleton play important roles in the life cycle of every virus. During attachment, internalization, endocytosis, nuclear targeting, transcription, replication, transport of progeny subviral particles, assembly, exocytosis, or cell-to-cell spread, viruses make use of different cellular cues and signals to enlist the cytoskeleton for their mission. Viruses induce rearrangements of cytoskeletal filaments so that they can utilize them as tracks or shove them aside when they represent barriers. Viral particles recruit molecular motors in order to hitchhike rides to different subcellular sites which provide the proper molecular environment for uncoating, replicating and packaging viral genomes. Interactions between subviral components and cytoskeletal tracks also help to orchestrate virus assembly, release and efficient cell-to-cell spread. There is probably not a single virus that does not use cytoskeletal and motor functions in its life cycle. Being well informed intracellular passengers, viruses provide us with unique tools to decipher how a particular cargo recruits one or several motors, how these are activated or tuned down depending on transport needs, and how cargoes switch from actin tracks to microtubules to nuclear pores and back.

Varicella-zoster virus open reading frame 10 is a virulence determinant in skin cells but not in T cells in vivo

The open reading frame 10 (ORF10) of varicella-zoster virus (VZV) encodes a tegument protein that enhances transactivation of VZV genes and has homology to herpes simplex virus type 1 (HSV-1) VP16. While VP16 is essential for HSV replication, ORF10 is dispensable for vaccine OKA (VOKA) growth in vitro. We used parent OKA (POKA) cosmids to delete ORF10, producing POKA delta10; point mutations that disrupted the acidic activation domain and the putative motif for binding human cellular factor 1 (HCF-1) in ORF10 protein yielded POKA10-Phe28Ala, POKA10-Phe28Ser, and POKA10-mHCF viruses. Deleting ORF10 or mutating these two functional domains had no effect on VZV replication, immediate-early gene transcription, or virion assembly in vitro. However, deleting ORF10 reduced viral titers and the extent of cutaneous lesions significantly in SCIDhu skin xenografts in vivo compared to POKA. Epidermal cells infected with POKA delta10 had significantly fewer DNA-containing nucleocapsids and complete virions compared to POKA; extensive aggregates of intracytoplasmic viral particles were also observed. Altering the activation or the putative HCF-1 domains of ORF10 protein had no consequences for VZV replication in vivo. Thus, the decreased pathogenic potential of POKA delta10 in skin could not be attributed to absence of these ORF10 protein functions. In contrast to skin cells, deleting ORF10 did not impair VZV T-cell tropism in vivo, as assessed by infectious virus yields. We conclude that ORF10 protein is required for efficient VZV virion assembly and is a specific determinant of VZV virulence in epidermal and dermal cells in vivo.

The alpha-TIF (VP16) homologue (ETIF) of equine herpesvirus 1 is essential for secondary envelopment and virus egress

The equine herpesvirus 1 (EHV-1) alpha-trans-inducing factor homologue (ETIF; VP16-E) is a 60-kDa virion component encoded by gene 12 (ORF12) that transactivates the immediate-early gene promoter. Here we report on the function of EHV-1 ETIF in the context of viral infection. An ETIF-null mutant from EHV-1 strain RacL11 (vL11deltaETIF) was constructed and analyzed. After transfection of vL11deltaETIF DNA into RK13 cells, no infectious virus could be reconstituted, and only single infected cells or small foci containing up to eight infected cells were detected. In contrast, after transfection of vL11deltaETIF DNA into a complementing cell line, infectious virus could be recovered, indicating the requirement of ETIF for productive virus infection. The growth defect of vL11deltaETIF could largely be restored by propagation on the complementing cell line, and growth on the complementing cell line resulted in incorporation of ETIF in mature and secreted virions. Low- and high-multiplicity infections of RK13 cells with phenotypically complemented vL11deltaETIF virus resulted in titers of virus progeny similar to those used for infection, suggesting that input ETIF from infection was recycled. Ultrastructural studies of vL11deltaETIF-infected cells demonstrated a marked defect in secondary envelopment at cytoplasmic membranes, resulting in very few enveloped virions in transport vesicles or extracellular space. Taken together, our results demonstrate that ETIF has an essential function in the replication cycle of EHV-1, and its main role appears to be in secondary envelopment.

Herpes simplex virus type 1 accumulation, envelopment, and exit in growth cones and varicosities in mid-distal regions of axons

The mechanism of anterograde transport of alphaherpesviruses in axons remains controversial. This study examined the transport, assembly, and egress of herpes simplex virus type 1 (HSV-1) in mid- and distal axons of infected explanted human fetal dorsal root ganglia using confocal microscopy and transmission electron microscopy (TEM) at 19, 24, and 48 h postinfection (p.i.). Confocal-microscopy studies showed that although capsid (VP5) and tegument (UL37) proteins were not uniformly present in axons until 24 h p.i., they colocalized with envelope (gG) proteins in axonal varicosities and in growth cones at 24 and 48 h p.i. TEM of longitudinal sections of axons in situ showed enveloped and unenveloped capsids in the axonal varicosities and growth cones, whereas in the midregion of the axons, predominantly unenveloped capsids were observed. Partially enveloped capsids, apparently budding into vesicles, were observed in axonal varicosities and growth cones, but not during viral attachment and entry into axons. Tegument proteins (VP22) were found associated with vesicles in growth cones, either alone or together with envelope (gD) proteins, by transmission immunoelectron microscopy. Extracellular virions were observed adjacent to axonal varicosities and growth cones, with some virions observed in crescent-shaped invaginations of the axonal plasma membrane, suggesting exit at these sites. These findings suggest that varicosities and growth cones are probable sites of HSV-1 envelopment of at least a proportion of virions in the mid- to distal axon. Envelopment probably occurs by budding of capsids into vesicles with associated tegument and envelope proteins. Virions appear to exit from these sites by exocytosis.

The inner tegument promotes herpes simplex virus capsid motility along microtubules in vitro

After viral fusion, capsids of the neurotropic herpes simplex virus are transported along microtubules (MT) to the nuclear pores for viral genome uncoating, nuclear transcription and replication. After assembly and egress from the nucleus, cytosolic capsids are transported to host membranes for secondary envelopment or to the axon terminal for further viral spread. Using GFP-tagged capsids, Cy3-labelled MT and cytosol, we have reconstituted viral capsid transport in vitro. In the presence of ATP, capsids moved along MT up to 30 microm. Blocking the function of dynactin, a cofactor of dynein and kinesin-2, inhibited the transport. Removing outer tegument proteins from the capsids increased in vitro motility. In contrast, capsids isolated from infected nuclei that were devoid of inner as well as outer tegument proteins showed little interaction with dynein and its cofactor dynactin. Our data suggest that the inner tegument of alphaherpesviruses contains viral receptors for MT motors.

Herpes simplex virus tegument protein VP16 is a component of primary enveloped virions

Immunogold electron microscopy was used to determine whether the tegument proteins VP13/14, VP22, and VP16 of herpes simplex virus type 1 (HSV1) are components of primary enveloped virions. Whereas VP13/14 and VP22 were not detected in virus particles in the perinuclear space and were present in only mature extracellular virions, VP16 was acquired prior to primary envelopment of the virus at the inner nuclear membrane. This finding highlights potential similarities and differences between HSV1 and the related alphaherpesvirus, pseudorabies virus, in which the homologues of all three of these tegument proteins are not incorporated into the virion until secondary envelopment.

Composition of pseudorabies virus particles lacking tegument protein US3, UL47, or UL49 or envelope glycoprotein E

Proteins located in the tegument layer of herpesvirus particles play important roles in the replicative cycle at both early and late times after infection. As major constituents of the virion, they execute important functions in particular during formation of progeny virions. These functions have mostly been elucidated by construction and analysis of mutant viruses deleted in single or multiple tegument protein-encoding genes (reviewed in the work of T. C. Mettenleiter, Virus Res. 106:167-180, 2004). However, since tegument proteins have been shown to be involved in numerous protein-protein interactions, the impact of single protein deletions on the composition of the virus particle is unknown, but they could impair correct interpretation of the results. To analyze how the absence of single virion constituents influences virion composition, we established a procedure to assay relative amounts of virion structural proteins in deletion mutants of the alphaherpesvirus Pseudorabies virus (PrV) in comparison to wild-type particles. The assay is based on the mass spectrometric quantitation of virion protein-derived peptides carrying stable isotope mass tags. After deletion of the US3, UL47, UL49, or glycoprotein E gene, relative amounts of a capsid protein (UL38), a capsid-associated protein (UL25), several tegument proteins (UL36 and UL47, if present), and glycoprotein H were unaffected, whereas the content of other tegument proteins (UL46, UL48, and UL49, if present) varied significantly. In the case of the UL48 gene product, a specific increase in incorporation of a smaller isoform was observed after deletion of the UL47 or UL49 gene, whereas a larger isoform remained unaffected. The cellular protein actin was enriched in virions of mutants deficient in any of the tegument proteins UL47, UL49, or US3. By two-dimensional gel electrophoresis multiple isoforms of host cell-derived heat shock protein 70 and annexins A1 and A2 were also identified as structural components of PrV virions.

Intriguing interplay between viral proteins during herpesvirus assembly or: the herpesvirus assembly puzzle

Herpes virions are complex particles that consist of more than 30 different virally encoded proteins. The molecular basis of how this complicated structure is assembled is only recently beginning to emerge. After replication in the host cell nucleus viral DNA is incorporated into preformed capsids which leave the nucleus by budding at the inner nuclear membrane resulting in the formation of primary enveloped virions in the perinuclear space. The primary envelope then fuses with the outer leaflet of the nuclear membrane, thereby releasing nucleocapsids into the cytoplasm. Final envelopment including the acquisition of more than 15 tegument and more than 10 envelope (glyco)proteins occurs by budding into Golgi-derived vesicles. Mature virions are released after fusion of the vesicle membrane with the plasma membrane of the cell. Thus, herpesvirus morphogenesis requires a sequence of envelopment--de-envelopment--re-envelopment processes which are distinct not only in the subcellular compartments in which they occur but also in the viral proteins involved. This review summarizes recent advances in our understanding of the complex protein-protein interactions involved in herpesvirus assembly and egress.

Herpesvirus interference with virus-specific antibodies: bridging antibodies, internalizing antibodies, and hiding from antibodies

Herpesviruses have developed different tools to thwart efficient antibody-dependent neutralisation and lysis of virions and elimination of infected cells. This overview will briefly summarize different of these tools, including (i) viral Fc receptors and the resulting process of antibody bridging, (ii) internalization of individual viral proteins and clustered antibody-antigen complexes from the plasma membrane of infected cells, and (iii) directed egress of virus particles to sites of intimate cell-cell contact that are difficult to access for antibodies.

Molecular biology of pseudorabies virus: impact on neurovirology and veterinary medicine

Pseudorabies virus (PRV) is a herpesvirus of swine, a member of the Alphaherpesvirinae subfamily, and the etiological agent of Aujeszky's disease. This review describes the contributions of PRV research to herpesvirus biology, neurobiology, and viral pathogenesis by focusing on (i) the molecular biology of PRV, (ii) model systems to study PRV pathogenesis and neurovirulence, (iii) PRV transsynaptic tracing of neuronal circuits, and (iv) veterinary aspects of pseudorabies disease. The structure of the enveloped infectious particle, the content of the viral DNA genome, and a step-by-step overview of the viral replication cycle are presented. PRV infection is initiated by binding to cellular receptors to allow penetration into the cell. After reaching the nucleus, the viral genome directs a regulated gene expression cascade that culminates with viral DNA replication and production of new virion constituents. Finally, progeny virions self-assemble and exit the host cells. Animal models and neuronal culture systems developed for the study of PRV pathogenesis and neurovirulence are discussed. PRV serves asa self-perpetuating transsynaptic tracer of neuronal circuitry, and we detail the original studies of PRV circuitry mapping, the biology underlying this application, and the development of the next generation of tracer viruses. The basic veterinary aspects of pseudorabies management and disease in swine are discussed. PRV infection progresses from acute infection of the respiratory epithelium to latent infection in the peripheral nervous system. Sporadic reactivation from latency can transmit PRV to new hosts. The successful management of PRV disease has relied on vaccination, prevention, and testing.

Identification, subviral localization, and functional characterization of the pseudorabies virus UL17 protein

Homologs of the UL17 gene of the alphaherpesvirus herpes simplex virus 1 (HSV-1) are conserved in all three subfamilies of herpesviruses. However, only the HSV-1 protein has so far been characterized in any detail. To analyze UL17 of pseudorabies virus (PrV) the complete 597-amino-acid protein was expressed in Escherichia coli and used for rabbit immunization. The antiserum recognized a 64-kDa protein in PrV-infected cell lysates and purified virions, identifying PrV UL17 as a structural virion component. In indirect immunofluorescence analyses of PrV-infected cells the protein was predominantly found in the nucleus. In electron microscopic studies after immunogold labeling of negatively stained purified virion preparations, UL17-specific label was detected on single, mostly damaged capsids, whereas complete virions and the majority of capsids were free of label. In ultrathin sections of infected cells, label was primarily found dispersed around scaffold-containing B-capsids, whereas on DNA-filled C-capsids it was located in the center. Empty intranuclear A-capsids were free of label, as were extracellular capsid-less L-particles. Functional characterization of PrV-DeltaUL17F, a deletion mutant lacking codons 23 to 444, demonstrated that cleavage of viral DNA into unit-length genomes was inhibited in the absence of UL17. In electron microscopic analyses of PrV-DeltaUL17F-infected RK13 cells, DNA-containing capsids were not detected, while numerous capsidless L-particles were observed. In summary, our data indicate that the PrV UL17 protein is an internal nucleocapsid protein necessary for DNA cleavage and packaging but suggest that the protein is not a prominent part of the tegument.

T-cell tropism and the role of ORF66 protein in pathogenesis of varicella-zoster virus infection

The pathogenesis of varicella-zoster virus (VZV) involves a cell-associated viremia during which infectious virus is carried from sites of respiratory mucosal inoculation to the skin. We now demonstrate that VZV infection of T cells is associated with robust virion production and modulation of the apoptosis and interferon pathways within these cells. The VZV serine/threonine protein kinase encoded by ORF66 is essential for the efficient replication of VZV in T cells. Preventing ORF66 protein expression by stop codon insertion (pOka66S) impaired the growth of the parent Oka (pOka) strain in T cells in SCID-hu T-cell xenografts in vivo and reduced formation of VZV virions. The lack of ORF66 protein also increased the susceptibility of infected T cells to apoptosis and reduced the capacity of the virus to interfere with induction of the interferon (IFN) signaling pathway following exposure to IFN-gamma. However, preventing ORF66 protein expression only slightly reduced growth in melanoma cells in culture and did not diminish virion formation in these cells. The pOka66S virus showed only a slight defect in growth in SCID-hu skin implants compared with intact pOka. These observations suggest that the ORF66 kinase plays a unique role during infection of T cells and supports VZV T-cell tropism by contributing to immune evasion and enhancing survival of infected T cells.

A deubiquitinating enzyme encoded by HSV-1 belongs to a family of cysteine proteases that is conserved across the family Herpesviridae

We have discovered a ubiquitin (Ub)-specific cysteine protease encoded within the N-terminal approximately 500 residues of the UL36 gene product, the largest (3164 aa) tegument protein of herpes simplex virus 1 (HSV-1). Enzymatic activity of this fragment, UL36USP, is detectable only after cleavage of UL36USP from full-length UL36 and occurs late during viral replication. UL36USP bears no homology to known deubiquitinating enzymes (DUBs) or Ub binding proteins. Sequence alignment of the large tegument proteins across the family Herpesviridae indicates conservation of key catalytic residues amongst these viruses. Recombinant UL36USP exhibits hydrolytic activity toward Ub-AMC and ubiquitinated branched peptides in vitro. In addition, recombinant UL36USP can cleave polyUb chains and appears to be specific for Lys48 linkages. Mutation of the active site cysteine residue (Cys65) to alanine abolishes this enzymatic activity. The lack of homology between UL36USP and eukaryotic DUBs makes this new family of herpesvirus ubiquitin-specific proteases attractive targets for selective inhibition.

Genetic and molecular in vivo analysis of herpes simplex virus assembly in murine visual system neurons

Herpes simplex virus (HSV) infects both epithelial cells and neuronal cells of the human host. Although HSV assembly has been studied extensively for cultured epithelial and neuronal cells, cultured neurons are biochemically, physiologically, and anatomically significantly different than mature neurons in vivo. Therefore, it is imperative that viral maturation and assembly be studied in vivo. To study viral assembly in vivo, we inoculated wild-type and replication-defective viruses into the posterior chamber of mouse eyes and followed infection in retinal ganglion cell bodies and axons. We used PCR techniques to detect viral DNA and RNA and electron microscopy immunohistochemistry and Western blotting to detect viral proteins in specific portions of the optic tract. This approach has shown that viral DNA replication is necessary for viral DNA movement into axons. Movement of viral DNA along ganglion cell axons occurs within capsid-like structures at the speed of fast axonal transport. These studies show that the combined use of intravitreal injections of replication-defective viruses and molecular probes allows the genetic analysis of essential viral replication and maturation processes in neurons in vivo. The studies also provide novel direct evidence for the axonal transport of viral DNA and support for the subassembly hypothesis of viral maturation in situ.

Determination of interactions between tegument proteins of herpes simplex virus type 1

The aim of this study was to elucidate protein-protein interactions between tegument proteins of herpes simplex virus type 1 (HSV-1). To do so, we have cloned and expressed in the LexA yeast (Saccharomyces cerevisiae) two-hybrid system, 13 of the 21 currently known tegument proteins of HSV-1. These included the tegument proteins essential for replication in cell lines, UL17, UL36, UL37, UL48, and UL49, and the nonessential tegument proteins US11, UL11, UL14, UL16, UL21, UL41, UL46, and UL47. A total of 104 combinations were screened in the yeast two-hybrid assay, with 9 interactions identified. These included: UL11-UL16, UL36-UL37, UL36-UL48, UL46-UL48, UL47-UL48, and UL48-UL49. The remaining interactions consisted of self-associations that were observed for US11, UL37, and UL49. The interactions UL36-UL37, UL36-UL48, UL37-UL37, UL46-UL48, and UL47-UL48 have not been previously reported for HSV-1. The interaction of UL46-UL48 was verified using an in vitro pull-down assay. The interactions of UL36-UL37 and UL37-UL37 were verified with a coimmunoprecipitation assay. Knowledge of HSV-1 tegument protein-protein interactions will provide insights into the pathways of tegument assembly, and the identified interactions are potential targets for new antiviral drugs.

Herpes simplex virus type 1 capsids transit by the trans-Golgi network, where viral glycoproteins accumulate independently of capsid egress

Egress of herpes capsids from the nucleus to the plasma membrane is a complex multistep transport event that is poorly understood. The current model proposes an initial envelopment at the inner nuclear membrane of capsids newly assembled in the nucleus. The capsids are then released in cytosol by fusion with the outer nuclear membrane. They are finally reenveloped at a downstream organelle before traveling to the plasma membrane for their extracellular release. Although the trans-Golgi network (TGN) is often cited as a potential site of reenvelopment, other organelles have also been proposed, including the Golgi, endoplasmic reticulum-Golgi intermediate compartment, aggresomes, tegusomes, and early or late endosomes. To clarify this important issue, we followed herpes simplex virus type 1 egress by immunofluorescence under conditions that slowed intracellular transport and promoted the accumulation of the otherwise transient reenvelopment intermediate. The data show that the capsids transit by the TGN and point to this compartment as the main reenvelopment site, although a contribution by endosomes cannot formally be excluded. Given that viral glycoproteins are expected to accumulate where capsids acquire their envelope, we examined this prediction and found that all tested could indeed be detected at the TGN. Moreover, this accumulation occurred independently of capsid egress. Surprisingly, capsids were often found immediately adjacent to the viral glycoproteins at the TGN.

Identification and characterization of the pseudorabies virus UL43 protein

Among the least characterized herpesvirus membrane proteins are the homologs of UL43 of herpes simplex virus 1 (HSV-1). To identify and characterize the UL43 protein of pseudorabies virus (PrV), part of the open reading frame was expressed in Escherichia coli and used for immunization of a rabbit. The antiserum recognized in Western blots a 34-kDa protein in lysates of PrV infected cells and purified virions, demonstrating that the UL43 protein is a virion component. In indirect immunofluorescence analysis, the antiserum labeled vesicular structures in PrV infected cells which also contained glycoprotein B. To functionally analyze UL43, a deletion mutant was constructed lacking amino acids 23-332 of the 373aa protein. This mutant was only slightly impaired in replication as assayed by one-step growth kinetics, measurement of plaque sizes, and electron microscopy. Interestingly, the PrV UL43 protein was able to inhibit fusion induced by PrV glycoproteins in a transient expression-fusion assay to a similar extent as gM. Double mutant viruses lacking, in addition to UL43, the multiply membrane spanning glycoproteins K or M did not show a phenotype beyond that observed in the gK and gM single deletion mutants.