Cytomegalovirus recruitment of cellular kinases to dissolve the nuclear lamina

Cytomegalovirus UL97 mutations in the era of ganciclovir and maribavir

Mutations in the human CMV UL97 kinase gene are a major mechanism of viral resistance to two anti-CMV drugs, ganciclovir (GCV) and maribavir (MBV). GCV, the most widely used and established therapy for CMV, is a substrate for the UL97 kinase. Well-characterised GCV-resistance mutations at UL97 codons 460, 520 and 590-607 impair the phosphorylation of GCV that is necessary for its antiviral activity, presumably by altering substrate recognition. In contrast, MBV is an inhibitor of the UL97 kinase and is the first new CMV therapy to reach later stage clinical trials in many years. No MBV-resistant CMV isolates have yet been detected in clinical trials, but after culture propagation under drug, UL97 mutations that confer moderate to high-level MBV resistance have been identified at codons 353, 397, 409 and 411. These mutations are located upstream of the GCV-resistance mutations and are close to the ATP-binding and catalytic domains common to all kinases, consistent with MBV acting as a small molecule ATP-competitive kinase inhibitor. So far, no UL97 mutations are known to confer resistance to both GCV and MBV.

Structure and formation of the cytomegalovirus virion

Transport and protection of the nuclear-replicating double-stranded DNA genome of herpesviruses is accomplished by the virion and its substructures. Studies of the composition, organization, and formation of these particles have provided insight into the molecular mechanisms of virus assembly, leads for antiviral strategies, and information about cellular processes that are required for, resemble, or antagonize virus replication. This chapter updates earlier reviews on the structure and formation human cytomegalovirus (HCMV) virions (Gibson 1996, 2006; Eickmann et al. 2006), and complements several other reviews on herpesvirus structure and replication presented in this volume (see the chapters by E. Murphy and T. Shenk, Z. Ruzsics and U. Koszinowski, R. Kalejta, and G.S. Pari) and elsewhere (Rixon 1993; Steven and Spear 1997; Brown et al. 2002; Varnum et al. 2004; Liu and Zhou 2007).

Control of cytoplasmic maturation events by cytomegalovirus tegument protein pp150

Cytomegalovirus replication depends upon a betaherpesvirus-conserved 150-kDa tegument phosphoprotein (pp150; encoded by UL32) that supports the final steps in virion maturation at cytoplasmic assembly compartments. Amino acid substitutions were introduced into conserved region 1 (CR1) and CR2 of pp150, affecting a region that may interact with nucleocapsids. Two independent CR2 point mutants (N201A and G207A) failed to support viral replication in evaluations by a transient complementation assay or after reconstruction into recombinant viruses. An assembly compartment-like cytoplasmic inclusion developed in UL32 mutant virus-infected cells that was similar to that of wild-type virus-infected cells. The cellular localization of the trans-Golgi marker Golgin-97 suggested differences in the organization of the assembly compartment compared to that of wild-type virus-infected cells. Replication-defective CR2 point mutants exhibited the same phenotype as that of a virus carrying a complete deletion of the UL32 open reading frame in these assays. Electron micrographs of fibroblasts at 3 or 5 days postinfection with a deletion mutant (DeltaUL32) grown on UL32-complementing cells showed a similar number and morphology of capsids in the nucleus, but the cytoplasmic region associated with virion assembly appeared highly vesiculated and contained few recognizable nucleocapsids or complete virus particles. These data demonstrate that the principle role of pp150 is to retain nucleocapsid organization through secondary envelopment at the assembly compartment.

Nuclear pore composition and gating in herpes simplex virus-infected cells

The mechanism by which herpes simplex virus (HSV) exits the nucleus remains a matter of controversy. The generally accepted route proposes that capsids exit via primary envelopment at the inner nuclear membrane and subsequent fusion of this primary particle with the outer nuclear membrane to gain capsid entry to the cytoplasm. However, recent observations indicate that HSV may induce gross morphological alterations of nuclear pores, resulting in the loss of normal pores and the appearance of dilated gaps in the nuclear membrane of up to several 100 nm. On this basis, it was proposed that a main route of capsid exit from the nucleus is directly through these altered pores. Here, we examine the biochemical composition of some of the major nuclear pore components in uninfected and HSV-infected cells. We show that total levels of major nucleoporins and their sedimentation patterns in density gradients remain largely unchanged up to 18 h after HSV infection. Some alteration in modification of one nucleoporin, Nup358/RanBP2, was observed during enrichment with anti-nucleoporin antibody and probing for O glycosylation. In addition, we examine functional gating within the nucleus in live cells, using microinjection of labeled dextran beads and a recombinant virus expressing GFP-VP16 to track the progress of infection. The nuclear permeability barrier for molecules bigger than 70 kDa remained intact throughout infection. Thus, in a functional assay in live cells, we find no evidence for gross perturbation to the gating of nuclear pores, although this might not exclude a small population of modified pores.

Herpes simplex virus induces extensive modification and dynamic relocalisation of the nuclear mitotic apparatus (NuMA) protein in interphase cells

The nuclear mitotic apparatus (NuMA) protein is a component of the nuclear matrix in interphase cells and an essential protein for the formation of mitotic spindle poles. We used herpes simplex virus (HSV), an enveloped DNA virus that replicates in the nucleus, to study the intra-nuclear dynamics of NuMA in infected cells. This study shows that NuMA is extensively modified following HSV infection, including phosphorylation of an unidentified site(s), and that it depends to an extent on viral DNA synthesis. Although NuMA is insoluble in uninfected interphase cells, HSV infection induced solubilisation and dynamic relocalisation of NuMA, whereupon the protein became excluded from viral replication compartments -- sites of virus transcription and replication. Live cell, confocal imaging showed that NuMA localisation dramatically changed from the early stages (diffusely nuclear, excluding nucleoli) to late stages of infection (central diminuition, but remaining near the inner nuclear peripheries). In addition, NuMA knockdown using siRNA suggested that NuMA is important for efficient viral growth. In summary, we suggest that NuMA is required for efficient HSV infection, and identify further areas of research that address how the virus challenges host cell barriers.

Remodelling of the nuclear lamina during human cytomegalovirus infection: role of the viral proteins pUL50 and pUL53

A fundamental step in the efficient production of human cytomegalovirus (HCMV) progeny is viral egress from the nucleus to the cytoplasm of infected cells. In the family Herpesviridae, this process involves alteration of nuclear lamina components by two highly conserved proteins, whose homologues in HCMV are named pUL50 and pUL53. This study showed that HCMV infection induced the mislocalization of nuclear lamins and that pUL50 and pUL53 play a role in this event. At late stages of infection, both lamin A/C and lamin B showed an irregular distribution on the nuclear rim, coincident with areas of pUL53 accumulation. No variations in the total amount of nuclear lamins could be detected, supporting the view that HCMV induces a qualitative, rather than a quantitative, alteration of these cellular components, as has been suggested previously for other herpesviruses. Interestingly, pUL53, in the absence of other viral products, localized diffusely in the nucleus, whilst the co-expression and interaction of pUL53 with its partner, pUL50, restored its nuclear rim localization in distinct patches, thus indicating that pUL50 is sufficient to induce the localization of pUL53 observed during virus infection. Importantly, analysis of the nuclear lamina in the presence of pUL50-pUL53 complexes at the nuclear boundary and in the absence of other viral products showed that the two viral proteins were sufficient to promote alterations of lamins, strongly resembling those observed during HCMV infection. These results suggest that pUL50 and pUL53 may play an important role in the exit of virions from the nucleus by inducing structural modifications of the nuclear lamina.

Partial functional complementation of a pseudorabies virus UL25 deletion mutant by herpes simplex virus type 1 pUL25 indicates overlapping functions of alphaherpesvirus pUL25 proteins

Homologs of the UL25 gene product of herpes simplex virus 1 (HSV-1) are highly conserved among the Herpesviridae. However, their exact function during viral replication is unknown. Current evidence suggests that in the alphaherpesvirus pseudorabies virus (PrV) the capsid-associated pUL25 plays a role in primary envelopment of DNA-containing mature capsids at the inner nuclear membrane. In the absence of pUL25, capsids were found in close association with the inner nuclear membrane, but nuclear egress was not observed (B. G. Klupp, H. Granzow, G. M. Keil, and T. C. Mettenleiter, J. Virol. 80:6235-6246, 2006). In contrast, HSV-1 pUL25 has been assigned a role in stable packaging of viral genomes (N. Stow, J. Virol. 75:10755-10765, 2001). Despite these apparently divergent functions, we wanted to assess whether the high sequence homology translates into functional homology. Therefore, we first analyzed a newly constructed HSV-1 UL25 deletion mutant in our assay system and observed a similar phenotype as in PrV. In the nuclei of infected cells, numerous electron-dense C capsids were detected, whereas primary envelopment of these capsids did not ensue. In agreement with results from PrV, vesicles were observed in the perinuclear space. Since these data indicated functional homology, we analyzed the ability of pUL25 of HSV-1 to complement a PrV UL25 deletion mutant and vice versa. Whereas a HSV-1 pUL25-expressing cell line partially complemented the pUL25 defect in PrV, reciprocal complementation of a HSV-1 UL25 deletion mutant by PrV pUL25 was not observed. Thus, our data demonstrate overlapping, although not identical functions of these two conserved herpesvirus proteins, and point to a conserved functional role in herpes virion formation.

Nuclear lamins: major factors in the structural organization and function of the nucleus and chromatin

Over the past few years it has become evident that the intermediate filament proteins, the types A and B nuclear lamins, not only provide a structural framework for the nucleus, but are also essential for many aspects of normal nuclear function. Insights into lamin-related functions have been derived from studies of the remarkably large number of disease-causing mutations in the human lamin A gene. This review provides an up-to-date overview of the functions of nuclear lamins, emphasizing their roles in epigenetics, chromatin organization, DNA replication, transcription, and DNA repair. In addition, we discuss recent evidence supporting the importance of lamins in viral infections.

Nuclear pore complex assembly through the cell cycle: regulation and membrane organization

In eukaryotes, all macromolecules traffic between the nucleus and the cytoplasm through nuclear pore complexes (NPCs), which are among the largest supramolecular assemblies in cells. Although their composition in yeast and metazoa is well characterized, understanding how NPCs are assembled and form the pore through the double membrane of the nuclear envelope and how both processes are controlled still remains a challenge. Here, we summarize what is known about the biogenesis of NPCs throughout the cell cycle with special focus on the membrane reorganization and the regulation that go along with NPC assembly.

Identification and characterization of the product encoded by ORF69 of Kaposi's sarcoma-associated herpesvirus

We report the identification and characterization of p33, the product of Kaposi's sarcoma-associated herpesvirus (KSHV) open reading frame 69 (ORF69), a positional homolog of the conserved herpesvirus protein UL31. p33 is expressed upon induction of viral lytic cycle with early kinetics. Immunofluorescence analysis revealed that in infected cell lines, the protein is localized in the nucleus, both in dotted spots and along the nuclear membrane. Nuclear fractionation experiments showed that p33 partitions with the nuclear matrix, and both immunoblotting of purified virions and immunoelectron microscopy indicated that the novel protein is not a component of the mature virus. Following ectopic expression in KSHV-negative cells, the protein was never associated with the nuclear membrane, suggesting that p33 needs to interact with additional viral proteins to reach the nuclear rim. In fact, after cotransfection with the ORF67 gene, the KSHV positional homolog of UL34, the p33 intranuclear signal changed and the two proteins colocalized on the nuclear membrane. A similar result was obtained when ORF69 was cotransfected with BFRF1, the Epstein-Barr virus (EBV) positional homolog of UL34 and ORF67. Finally, upon cotransfection, ORF69 significantly increased nuclear membrane reduplications induced by BFRF1. The above results indicate that KSHV p33 shares many similarities with its EBV homolog BFLF2 and suggest that functional cross-complementation is possible between members of the gammaherpesvirus subfamily.

Deletion of Epstein-Barr virus BFLF2 leads to impaired viral DNA packaging and primary egress as well as to the production of defective viral particles

Previous genetic and biochemical studies performed with several members of the Alphaherpesvirus subfamily have shown that the UL31 and UL34 proteins are essential components of the molecular machinery that mediates the primary egress of newly assembled capsids across the nuclear membrane. Further, there is substantial evidence that BFLF2 and BFRF1, the respective positional homologs of UL31 and UL34 in the Epstein-Barr virus (EBV) genome, are also their functional homologs, i.e., that the UL31/UL34 pathway is common to distant herpesviruses. However, the low degree of protein sequence identity between UL31 and BFLF2 would argue against such a hypothesis. To further clarify this issue, we have constructed a recombinant EBV strain devoid of BFLF2 (DeltaBFLF2) and show that BFLF2 is crucial for efficient virus production but not for lytic DNA replication or B-cell transformation. This defective phenotype could be efficiently restored by trans complementation with a BFLF2 expression plasmid. Detailed analysis of replicating cells by electron microscopy revealed that, as expected, DeltaBFLF2 viruses not only failed to egress from the nucleus but also showed defective DNA packaging. Nonfunctional primary egress did not, however, impair the production and extracellular release of enveloped but empty viral particles that comprised L particles containing tegument-like structures and a few virus-like particles carrying empty capsids. The DeltaBFLF2 and DeltaUL31 phenotypes therefore only partly overlap, from which we infer that BFLF2 and UL31 have substantially diverged during evolution to fulfil related but distinct functions.

Cytomegaloviral proteins pUL50 and pUL53 are associated with the nuclear lamina and interact with cellular protein kinase C

Human cytomegalovirus-encoded pUL50 and pUL53 belong to a group of conserved herpesviral nuclear proteins. This study describes: (i) the co-localization of pUL50 with components of the nuclear lamina such as lamins A/C and lamin B receptor by double immunofluorescent staining, (ii) a strong pUL50-mediated relocalization of pUL53 from a diffuse nuclear pattern towards a nuclear rim localization, (iii) a direct interaction between pUL50 and pUL53, as well as between pUL50 and protein kinase C (PKC), shown by yeast two-hybrid and co-immunoprecipitation analyses, (iv) in vitro phosphorylation of pUL50, which is highly suggestive of PKC activity, and finally (v) partial relocalization of PKC by pUL50/pUL53 from its main cytoplasmic localization to a marked nuclear lamina accumulation. These data suggest a role for pUL50 and pUL53 in the recruitment of PKC, an event that is considered to be important for cytomegalovirus-induced distortion of the nuclear lamina.

Actin and Rho GTPases in herpesvirus biology

Viruses have evolved a variety of interactions with host cells to create an optimal niche for viral replication, persistence and spread. The actin cytoskeleton of the host cell and actin-regulating Rho GTPase signaling pathways can be involved in several of these interactions. This review focuses on recent findings on herpesvirus interactions with actin and Rho GTPases during viral entry, replication in the nucleus and egress. Unraveling these often fascinating interactions might also provide additional insights into sometimes poorly known aspects of actin biology (e.g. its role in the nucleus) and in the development of novel antiviral therapies.

Emerin is hyperphosphorylated and redistributed in herpes simplex virus type 1-infected cells in a manner dependent on both UL34 and US3

Cells infected with wild-type herpes simplex virus type 1 (HSV-1) show disruption of the organization of the nuclear lamina that underlies the nuclear envelope. This disruption is reflected in changes in the localization and phosphorylation of lamin proteins. Here, we show that HSV-1 infection causes relocalization of the LEM domain protein emerin. In cells infected with wild-type virus, emerin becomes more mobile in the nuclear membrane, and in cells infected with viruses that fail to express UL34 protein (pUL34) and US3 protein (pUS3), emerin no longer colocalizes with lamins, suggesting that infection causes a loss of connection between emerin and the lamina. Infection causes hyperphosphorylation of emerin in a manner dependent upon both pUL34 and pUS3. Some emerin hyperphosphorylation can be inhibited by the protein kinase Cdelta (PKCdelta) inhibitor rottlerin. Emerin and pUL34 interact physically, as shown by pull-down and coimmunoprecipitation assays. Emerin expression is not, however, necessary for infection, since virus growth is not impaired in cells derived from emerin-null transgenic mice. The results suggest a model in which pUS3 and PKCdelta that has been recruited by pUL34 hyperphosphorylate emerin, leading to disruption of its connections with lamin proteins and contributing to the disruption of the nuclear lamina. Changes in emerin localization, nuclear shape, and lamin organization characteristic of cells infected with wild-type HSV-1 also occur in cells infected with recombinant virus that does not make viral capsids, suggesting that these changes occur independently of capsid envelopment.

Autographa californica multiple nucleopolyhedrovirus EXON0 (ORF141) is required for efficient egress of nucleocapsids from the nucleus

Autographa californica multiple nucleopolyhedrovirus (AcMNPV) exon0 (orf141) has been shown to be required for the efficient production of budded virus (BV). The deletion of exon0 reduces the level of BV production by up to 99% (X. Dai, T. M. Stewart, J. A. Pathakamuri, Q. Li, and D. A. Theilmann, J. Virol. 78:9633-9644, 2004); however, the function or mechanism by which EXON0 affects BV production is unknown. In this study, we further elucidated the function of EXON0 by investigating the localization of EXON0 in infected Sf9 cells and in virions and by identifying interactions between EXON0 and other viral proteins. In addition, electron microscopy was used to study the cellular localization of nucleocapsids in cells transfected with an exon0 knockout (KO) virus. The results showed that EXON0 was localized to both the cytoplasm and the nuclei of infected Sf9 cells throughout the infection. Western blotting results also showed that EXON0 was purified along with BV and occlusion-derived virus (ODV). The fractionation of BV into the nucleocapsid and envelope components showed that EXON0 localized to the BV nucleocapsid. Yeast two-hybrid screening, coimmunoprecipitation, and confocal microscopy revealed that it interacted with nucleocapsid proteins FP25 and BV/ODV-C42. Cells transfected with the exon0 KO virus exhibited normally appearing nucleocapsids in the nuclei in numbers equal to those in the nuclei of cells transfected with the EXON0 repaired virus. In contrast, the numbers of nucleocapsids in the cytoplasm of cells transfected with the exon0 KO virus were significantly lower than those in the cytoplasm of cells transfected with the repaired virus. These results support the conclusion that EXON0 is required in the BV pathway for the efficient egress of nucleocapsids from the nucleus to the cytoplasm.

Herpes simplex virus glycoproteins gB and gH function in fusion between the virion envelope and the outer nuclear membrane

Herpesviruses must traverse the nuclear envelope to gain access to the cytoplasm and, ultimately, to exit cells. It is believed that herpesvirus nucleocapsids enter the perinuclear space by budding through the inner nuclear membrane (NM). To reach the cytoplasm these enveloped particles must fuse with the outer NM and the unenveloped capsids then acquire a second envelope in the trans-Golgi network. Little is known about the process by which herpesviruses virions fuse with the outer NM. Here we show that a herpes simplex virus (HSV) mutant lacking both the two putative fusion glycoproteins gB and gH failed to cross the nuclear envelope. Enveloped virions accumulated in the perinuclear space or in membrane vesicles that bulged into the nucleoplasm (herniations). By contrast, mutants lacking just gB or gH showed only minor or no defects in nuclear egress. We concluded that either HSV gB or gH can promote fusion between the virion envelope and the outer NM. It is noteworthy that fusion associated with HSV entry requires the cooperative action of both gB and gH, suggesting that the two types of fusion (egress versus entry) are dissimilar processes.

Herpes simplex virus infection induces phosphorylation and delocalization of emerin, a key inner nuclear membrane protein

The inner nuclear membrane (INM) contains specialized membrane proteins that selectively interact with nuclear components including the lamina, chromatin, and DNA. Alterations in the organization of and interactions with INM and lamina components are likely to play important roles in herpesvirus replication and, in particular, exit from the nucleus. Emerin, a member of the LEM domain class of INM proteins, binds a number of nuclear components including lamins, the DNA-bridging protein BAF, and F-actin and is thought to be involved in maintaining nuclear integrity. Here we report that emerin is quantitatively modified during herpes simplex virus (HSV) infection. Modification begins early in infection, involves multiple steps, and is reversed by phosphatase treatment. Emerin phosphorylation during infection involves one or more cellular kinases but can also be influenced by the US3 viral kinase, a protein whose function is known to be involved in HSV nuclear egress. The results from biochemical extraction analyses and from immunofluorescence of the detergent-resistant population demonstrate that emerin association with the INM significantly reduced during infection. We propose that the induction of emerin phosphorylation in infected cells may be involved in nuclear egress and uncoupling interactions with targets such as the lamina, chromatin, or cytoskeletal components.

Vesicle formation from the nuclear membrane is induced by coexpression of two conserved herpesvirus proteins

Although the nuclear envelope is a dynamic structure that disassembles and reforms during mitosis, the formation of membranous vesicles derived from the nuclear envelope has not yet been described in noninfected cells. However, during herpesvirus maturation, intranuclear capsids initiate transit to the cytosol for final maturation by budding at the inner nuclear membrane. Two conserved herpesvirus proteins are required for this primary envelopment, designated in the alphaherpesviruses as pUL31 and pUL34. Here, we show that simultaneous expression of pUL31 and pUL34 of the alphaherpesvirus pseudorabies virus in stably transfected rabbit kidney cells resulted in the formation of vesicles in the perinuclear space that resemble primary envelopes without a nucleocapsid. They contain pUL31 and pUL34 as shown by immunolabeling and are derived from the nuclear envelope. Thus, coexpression of only two conserved herpesvirus proteins without any other viral factor is sufficient to induce the formation of vesicles from the nuclear membrane. This argues for the contribution of cellular factors in this process either recruited from their natural cytoplasmic location or not yet identified as components of the nuclear compartment.

Random screening for dominant-negative mutants of the cytomegalovirus nuclear egress protein M50

Inactivation of gene products by dominant-negative (DN) mutants is a powerful tool to assign functions to proteins. Here, we present a two-step procedure to establish a random screen for DN alleles, using the essential murine cytomegalovirus gene M50 as an example. First, loss-of-function mutants from a linker-scanning library were tested for inhibition of virus reconstitution with the help of FLP-mediated ectopic insertion of the mutants into the viral genome. Second, DN candidates were confirmed by conditional expression of the inhibitory proteins in the virus context. This allowed the quantification of the inhibitory effect, the identification of the morphogenesis block, and the construction of DN mutants with improved activity. Based on these observations a DN mutant of the homologous gene (UL50) in human cytomegalovirus was predicted and constructed. Our data suggest that a proline-rich sequence motif in the variable region of M50/UL50 represents a new functional site which is essential for nuclear egress of cytomegalovirus capsids.

Epstein-Barr virus BGLF4 kinase induces premature chromosome condensation through activation of condensin and topoisomerase II

Previous studies of Epstein-Barr virus (EBV) replication focused mainly on the viral and cellular factors involved in replication compartment assembly and controlling the cell cycle. However, little is known about how EBV reorganizes nuclear architecture and the chromatin territories. In EBV-positive nasopharyngeal carcinoma NA cells or Akata cells, we noticed that cellular chromatin becomes highly condensed upon EBV reactivation. In searching for the possible mechanisms involved, we found that transient expression of EBV BGLF4 kinase induces unscheduled chromosome condensation, nuclear lamina disassembly, and stress fiber rearrangements, independently of cellular DNA replication and Cdc2 activity. BGLF4 interacts with condensin complexes, the major components in mitotic chromosome assembly, and induces condensin phosphorylation at Cdc2 consensus motifs. BGLF4 also stimulates the decatenation activity of topoisomerase II, suggesting that it may induce chromosome condensation through condensin and topoisomerase II activation. The ability to induce chromosome condensation is conserved in another gammaherpesvirus kinase, murine herpesvirus 68 ORF36. Together, these findings suggest a novel mechanism by which gammaherpesvirus kinases may induce multiple premature mitotic events to provide more extrachromosomal space for viral DNA replication and successful egress of nucleocapsid from the nucleus.

The nuclear import of the human T lymphotropic virus type I (HTLV-1) tax protein is carrier- and energy-independent

HTLV-1 is the etiologic agent of the adult T cell leukemialymphoma (ATLL). The viral regulatory protein Tax plays a central role in leukemogenesis as a transcriptional transactivator of both viral and cellular gene expression, and this requires Tax activity in both the cytoplasm and the nucleus. In the present study, we have investigated the mechanisms involved in the nuclear localization of Tax. Employing a GFP fusion expression system and a range of Tax mutants, we could confirm that the N-terminal 60 amino acids, and specifically residues within the zinc finger motif in this region, are important for nuclear localization. Using an in vitro nuclear import assay, it could be demonstrated that the transportation of Tax to the nucleus required neither energy nor carrier proteins. Specific and direct binding between Tax and p62, a nucleoporin with which the importin beta family of proteins have been known to interact was also observed. The nuclear import activity of wild type Tax and its mutants and their binding affinity for p62 were also clearly correlated, suggesting that the entry of Tax into the nucleus involves a direct interaction with nucleoporins within the nuclear pore complex (NPC). The nuclear export of Tax was also shown to be carrier independent. It could be also demonstrated that Tax it self may have a carrier function and that the NF-kappaB subunit p65 could be imported into the nucleus by Tax. These studies suggest that Tax could alter the nucleocytoplasmic distribution of cellular proteins, and this could contribute to the deregulation of cellular processes observed in HTLV-1 infection.

Identification of transcripts and protein products of the UL31, UL37, UL46, UL47, UL48, UL49 and US4 gene homologues of avian infectious laryngotracheitis virus

In the present study, the transcription and protein expression of seven genes of infectious laryngotracheitis virus (ILTV) were investigated: UL31 and UL37 possess homologues in all known avian and mammalian herpesviruses, whereas UL46–UL49 and US4 are only conserved in most alphaherpesviruses. A peculiarity of the ILTV genome is the translocation of UL47 from the unique long region to a position upstream of US4 within the unique short region. Northern blot analyses revealed that all of the analysed genes were transcribed most abundantly during the late (γ) phase of replication, but the only true late (γ2) gene was UL47. Using monospecific rabbit antisera, the protein products of all of the genes could be detected and localized in ILTV-infected cells. Considerable amounts of the UL31, UL47 and UL48 gene products were found in the cell nuclei, whereas the other proteins were restricted largely to the cytoplasm. Like the respective tegument proteins of other herpesviruses, the UL37 and UL46–UL49 gene products of ILTV were incorporated into virus particles, whereas the UL31 protein and the glycoprotein encoded by US4 (gG) were not detectable in purified virions. It was also demonstrated that the UL48 protein of ILTV is able to activate an alphaherpesvirus immediate-early gene promoter, which is also a typical feature of other UL48 homologues. Taken together, these results indicate that the functions of all of the investigated ILTV proteins are related to those of their homologues in other alphaherpesviruses.

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.

Herpes simplex virus type 1 infection induces activation and recruitment of protein kinase C to the nuclear membrane and increased phosphorylation of lamin B

We report that herpes simplex virus type 1 (HSV-1) infection leads to the recruitment of protein kinase C (PKC) to the nuclear rim. In HEp-2 cells, PKC recruitment to the nuclear rim was initiated between 8 h and 12 h postinfection. PKCdelta, a proapoptotic kinase, was completely recruited to the nuclear rim upon infection with HSV-1. PKCalpha was less dramatically relocalized mostly at the nuclear rim upon infection, although some PKCalpha remained in the cytoplasm. PKCzeta-specific immunofluorescence was not significantly relocated to the nuclear rim. The UL34 and UL31 proteins, as well as their association, were each required for PKC recruitment to the nuclear rim. The HSV-1 US3 protein product, a kinase which regulates the phosphorylation state and localization of UL34, was not required for PKC recruitment to the nuclear rim; however, it was required for proper localization along the nuclear rim, as PKC appeared unevenly distributed along the nuclear rim of cells infected with US3 null and kinase-dead mutants. HSV-1 infection induced the phosphorylation of both lamin B and PKC. Elevated lamin B phosphorylation in HSV-1-infected cells was partially reduced by inhibitors of PKC. The data suggest a model in which kinases that normally disassemble the nuclear lamina during apoptosis are recruited to the nuclear membrane through functions requiring UL31 and UL34. We hypothesize that the recruitment of PKC functions to phosphorylate lamin B to help modify the nuclear lamina and promote budding of nucleocapsids at the inner nuclear membrane.

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.

Herpes simplex virus-type 2 infectivity and agents that block gap junctional intercellular communication

In rat liver epithelial (WB) cells, the protein kinase C inhibitor H7 blocked gap junctional intercellular communication (GJIC) and reduced virus infectivity. Octanol, 18-beta-glycyrrhetinic acid, and staurosporine, agents that reduce GJIC, had no effect upon virus infectivity. Previous studies demonstrated that herpes simplex virus-type 2 (HSV-2) infection was accompanied by attenuated GJIC. Of agents tested, only H7 reduced plaque forming unit (pfu) ability in a dose-dependent manner with 100% plaque reduction at 40 microM without evidence of cytotoxicity. Dye transfer indicated that H7 decreased GJIC, although Western blotting revealed that it did not alter phosphorylation of the gap junction protein, connexin 43 (Cx43). Using indirect immunofluorescence, Cx43 was found to localize in membrane plaques in uninfected cells and H7 did not alter this distribution. However, Cx43 was lost from the membrane at 24h in both H7-treated and untreated cells infected with HSV-2. Viral infection increased serine phosphorylation, particularly in the nuclear region, and this effect was reduced following H7 treatment. Thus, H7 attenuated both GJIC and infectivity of HSV-2 in WB cells but the anti-viral effects were due to reduced nuclear protein phosphorylation rather than alterations in phosphorylation or localization of Cx43.

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.

Common and specific properties of herpesvirus UL34/UL31 protein family members revealed by protein complementation assay

The proteins encoded by the UL34 and UL31 genes of herpes simplex virus are conserved among herpesviruses. They form a complex that is essential for the egress of the herpesvirus nucleocapsids from the nucleus. In previous work on the homologous protein complex in murine cytomegalovirus (MCMV), we defined their mutual binding domains. Here, we started to map binding domains within the UL34/UL31 proteins of alpha-, beta-, and gammaherpesviruses and to locate other functional properties. A protein complementation assay (PCA) using the TEM-1 beta-lactamase fragments fused to UL31 and UL34 protein homologues was used to study protein-protein interactions in cells. Wild-type MCMV M50 and M53 provided a strong reaction in the PCA, whereas mutants unable to form a complex did not. The homologous pairs of herpes simplex virus type 1, pseudorabies virus, human cytomegalovirus (HCMV), Epstein-Barr virus (EBV), and murine herpes virus 68 proteins also reacted, with the exception of the EBV proteins. Cross-complementation was found to be positive only within the same herpesvirus subfamily. Moreover, the HCMV homologues rescued replication-defective MCMV genomes lacking one or the other gene. We identified the binding site of M53 for M50 in the first conserved region (CR1) (M. Loetzerich, Z. Ruzsics, and U. H. Koszinowski, J. Virol. 80:73-84). Here we show that the CR1 of all tested UL31 proteins contains the UL34 binding site, and chimeric proteins carrying the subfamily-specific CR1 rescued the ability to cross-complement in the PCA.

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.

Structural changes in human cytomegalovirus cytoplasmic assembly sites in the absence of UL97 kinase activity

Studies of human cytomegalovirus (HCMV) UL97 kinase deletion mutant (DeltaUL97) indicated a multi-step role for this kinase in early and late phases of the viral life cycle, namely, in DNA replication, capsid maturation and nuclear egress. Here, we addressed its possible involvement in cytoplasmic steps of HCMV assembly. Using the DeltaUL97 and the UL97 kinase inhibitor NGIC-I, we demonstrate that the absence of UL97 kinase activity results in a modified subcellular distribution of the viral structural protein assembly sites, from compact structures impacting upon the nucleus to diffuse perinuclear structures punctuated by large vacuoles. Infection by either wild type or DeltaUL97 viruses induced a profound reorganization of wheat germ agglutinin (WGA)-positive Golgi-related structures. Importantly, the viral-induced Golgi remodeling along with the reorganization of the nuclear architecture was substantially altered in the absence of UL97 kinase activity. These findings suggest that UL97 kinase activity might contribute to organization of the viral cytoplasmic assembly sites.

Herpesvirus assembly: a tale of two membranes

Herpes virions are amongst the most complex virus particles: they comprise in excess of thirty virally encoded proteins, and also contain cellular components. Capsid formation and the cleavage and encapsidation of replicated viral DNA occur in the nucleus and resemble similar processes in tailed dsDNA (double-stranded DNA) bacteriophages, which indicates they might have common ancestry. In contrast, final virion maturation takes place in the cytoplasm. Nucleocapsids gain access to this compartment by envelopment at the inner nuclear membrane, which involves the interaction between viral and cellular proteins in order to locally alter nuclear architecture. Fusion of the primary viral envelope with the outer nuclear membrane results in translocation of the nucleocapsid to the cytoplasm. Here, the majority of the tegument - a structure, composed of a multitude of different proteins, that links the capsid and the envelope - is added to nucleocapsids, which obtain their final envelope by budding into glycoprotein-containing Golgi-derived vesicles. Thus, herpesvirus morphogenesis proceeds in two different cellular compartments, involving different viral and cellular proteins.

"Heads and tails" of intermediate filament phosphorylation: multiple sites and functional insights

Intermediate filaments (IFs) are major components of the mammalian cytoskeleton. They are among the most abundant cellular phosphoproteins; their phosphorylation typically involves multiple sites at repeat or unique motifs, preferentially within the "head" or "tail" domains. Phosphorylation and dephosphorylation are essential for the regulation of IF dynamics by modulating the intrinsic properties of IFs: solubility, conformation and filament organization, and, in addition, for the regulation of other IF post-translational modifications. These phosphorylation-regulated properties dictate generalized and context-dependent IF functions that reflect their tissue-specific expression. Most important among IF phosphorylation-mediated functions are the regulation of IF cellular or subcellular compartmentalization, levels and turnover, binding with associated proteins, susceptibility to cell stresses (including apoptosis), tissue-specific functions and IF-associated disease pathogenesis (where IF hyperphosphorylation also serves as a tissue-injury marker).

Induction of apoptosis limits cytomegalovirus cross-species infection

Cross-species infections are responsible for the majority of emerging and re-emerging viral diseases. However, little is known about the mechanisms that restrict viruses to a certain host species, and the factors viruses need to cross the species barrier and replicate in a different host. Cytomegaloviruses (CMVs) are representatives of the beta-herpesviruses that are highly species specific. They replicate only in cells of their own or a closely related species. In this study, the molecular mechanism underlying the cytomegalovirus species specificity was investigated. We show that infection of human cells with the murine cytomegalovirus (MCMV) triggers the intrinsic apoptosis pathway involving caspase-9 activation. MCMV can break the species barrier and replicate in human cells if apoptosis is blocked by Bcl-2 or a functionally analogous protein. A single gene of the human cytomegalovirus encoding a mitochondrial inhibitor of apoptosis is sufficient to allow MCMV replication in human cells. Moreover, the same principle facilitates replication of the rat cytomegalovirus in human cells. Thus, induction of apoptosis serves as an innate immune defense to inhibit cross-species infections of rodent CMVs.

Roles for herpes simplex virus type 1 UL34 and US3 proteins in disrupting the nuclear lamina during herpes simplex virus type 1 egress

Cells infected with wild type HSV-1 showed significant lamin A/C and lamin B rearrangement, while UL34-null virus-infected cells exhibited few changes in lamin localization, indicating that UL34 is necessary for lamin disruption. During HSV infection, US3 limited the development of disruptions in the lamina, since cells infected with a US3-null virus developed large perforations in the lamin layer. US3 regulation of lamin disruption does not correlate with the induction of apoptosis. Expression of either UL34 or US3 proteins alone disrupted lamin A/C and lamin B localization. Expression of UL34 and US3 together had little effect on lamin A/C localization, suggesting a regulatory interaction between the two proteins. The data presented in this paper argue for crucial roles for both UL34 and US3 in regulating the state of the nuclear lamina during viral infection.

Lamin A N-terminal phosphorylation is associated with myoblast activation: impairment in Emery-Dreifuss muscular dystrophy

BACKGROUND

Skeletal muscle disorders associated with mutations of lamin A/C gene include autosomal Emery-Dreifuss muscular dystrophy and limb girdle muscular dystrophy 1B. The pathogenic mechanism underlying these diseases is unknown. Recent data suggest an impairment of signalling mechanisms as a possible cause of muscle malfunction. A molecular complex in muscle cells formed by lamin A/C, emerin, and nuclear actin has been identified. The stability of this protein complex appears to be related to phosphorylation mechanisms.

OBJECTIVE

To analyse lamin A/C phosphorylation in control and laminopathic muscle cells.

METHODS

Lamin A/C N-terminal phosphorylation was determined in cultured mouse myoblasts using a specific antibody. Insulin treatment of serum starved myoblast cultures was carried out to evaluate involvement of insulin signalling in the phosphorylation pathway. Screening of four Emery-Dreifuss and one limb girdle muscular dystrophy 1B cases was undertaken to investigate lamin A/C phosphorylation in both cultured myoblasts and mature muscle fibres.

RESULTS

Phosphorylation of lamin A was observed during myoblast differentiation or proliferation, along with reduced lamin A/C phosphorylation in quiescent myoblasts. Lamin A N-terminus phosphorylation was induced by an insulin stimulus, which conversely did not affect lamin C phosphorylation. Lamin A/C was also hyperphosphorylated in mature muscle, mostly in regenerating fibres. Lamin A/C phosphorylation was strikingly reduced in laminopathic myoblasts and muscle fibres, while it was preserved in interstitial fibroblasts.

CONCLUSIONS

Altered lamin A/C interplay with a muscle specific phosphorylation partner might be involved in the pathogenic mechanism of Emery-Dreifuss muscular dystrophy and limb girdle muscular dystrophy 1B.

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.

Identification and functional evaluation of cellular and viral factors involved in the alteration of nuclear architecture during herpes simplex virus 1 infection

Herpes simplex virus 1 (HSV-1) replicates in the nucleus of host cells and radically alters nuclear architecture as part of its replication process. Replication compartments (RCs) form, and host chromatin is marginalized. Chromatin is later dispersed, and RCs spread past it to reach the nuclear edge. Using a lamin A-green fluorescent protein fusion, we provide direct evidence that the nuclear lamina is disrupted during HSV-1 infection and that the UL31 and UL34 proteins are required for this. We show nuclear expansion from 8 h to 24 h postinfection and place chromatin rearrangement and disruption of the lamina in the context of this global change in nuclear architecture. We show HSV-1-induced disruption of the localization of Cdc14B, a cellular protein and component of a putative nucleoskeleton. We also show that UL31 and UL34 are required for nuclear expansion. Studies with inhibitors of globular actin (G-actin) indicate that G-actin plays an essential role in nuclear expansion and chromatin dispersal but not in lamina alterations induced by HSV-1 infection. From analyses of HSV infections under various conditions, we conclude that nuclear expansion and chromatin dispersal are dispensable for optimal replication, while lamina rearrangement is associated with efficient replication.

Inhibition of protein kinases C prevents murine cytomegalovirus replication

For successful establishment of infection and initiation of the replication cycle, murine cytomegalovirus (MCMV) utilizes cellular structures and functions, including cell-membrane penetration, capsid dismantling and cytosolic transport of viral DNA into the nucleus. These early events of MCMV infections are dependent on cellular regulatory mechanisms, primarily protein phosphorylation. In the present study, protein kinase inhibitors were used to explore the role of protein phosphorylation mediated by protein kinases C (PKCs) in the very early events of MCMV infection. Inhibitory effects were determined by immunofluorescence and Western blot analysis of MCMV IE1 and E1 protein expression and by production of infectious virions in cell culture. It was found that H-7, a broadly specific inhibitor of cellular protein kinases, prevented virus replication in a dose-dependent and reversible manner, and that the block in replication occurred very early in infection. More specific PKC inhibitors (sangivamycin, calphostin C and bisindolylmaleimide II), Ca(2+)/calmodulin inhibitors (EDTA and W7) and phorbol esters (PMA) were used to dissect PKC-subclass contribution in the very early events of MCMV replication. The results indicate that the role of diacylglycerol/phorbol ester-dependent but calcium-independent PKCs is essential for establishment of MCMV infection in the host cell, starting at a very early stage of infection.

Cellular p32 recruits cytomegalovirus kinase pUL97 to redistribute the nuclear lamina

Replication of human cytomegalovirus is limited at the level of nucleocytoplasmic transport of viral capsids, a process that requires the disassembly of the nuclear lamina. Deletion of the protein kinase gene UL97 from the viral genome showed that the activity of pUL97 plays an important role for viral capsid egress. Here, we report that p32, a novel cellular interactor of the viral kinase pUL97, promotes the accumulation of pUL97 at the nuclear membrane by recruiting the p32-pUL97 complex to the lamin B receptor. Transfection of active pUL97, but not a catalytically inactive mutant, induced a redistribution of lamina components as demonstrated for recombinant lamin B receptor-green fluorescent protein and endogenous lamins A and C. Consistent with this, p32 itself and lamins were phosphorylated by pUL97. Importantly, overexpression of p32 in human cytomegalovirus-infected cells resulted in increased efficiency of viral replication and release of viral particles. Thus, it is highly suggestive that the cellular protein p32 recruits pUL97 to induce a dissolution of the nuclear lamina thereby facilitating the nuclear export of viral capsids.

Dissociation of heterochromatin protein 1 from lamin B receptor induced by human polyomavirus agnoprotein: role in nuclear egress of viral particles

The nuclear envelope is one of the chief obstacles to the translocation of macromolecules that are larger than the diameter of nuclear pores. Heterochromatin protein 1 (HP1) bound to the lamin B receptor (LBR) is thought to contribute to reassembly of the nuclear envelope after cell division. Human polyomavirus agnoprotein (Agno) has been shown to bind to HP1alpha and to induce its dissociation from LBR, resulting in destabilization of the nuclear envelope. Fluorescence recovery after photobleaching showed that Agno increased the lateral mobility of LBR in the inner nuclear membrane. Biochemical and immunofluorescence analyses showed that Agno is targeted to the nuclear envelope and facilitates the nuclear egress of polyomavirus-like particles. These results indicate that dissociation of HP1alpha from LBR and consequent perturbation of the nuclear envelope induced by polyomavirus Agno promote the translocation of virions out of the nucleus.

Characterization and intracellular localization of the Epstein-Barr virus protein BFLF2: interactions with BFRF1 and with the nuclear lamina

We have reported in the accompanying paper that the BFRF1 protein of Epstein-Barr virus (EBV) is important for efficient primary viral envelopment and egress (A. Farina, R. Feederle, S. Raffa, R. Gonnella, R. Santarelli, L. Frati, A. Angeloni, M. R. Torrisi, A. Faggioni, and H.-J. Delecluse, J. Virol. 79:3703-3712). Here we describe the characterization of the product of the EBV BFLF2 gene, which belongs to a family of conserved herpesviral genes which include the UL31 genes of herpes simplex virus and of pseudorabies virus and whose products are known to interact with UL34, the positional homolog of BFRF1. BFLF2 is an early transcript and is expressed in a variety of cell lines upon EBV lytic cycle activation. Western blotting of purified virion preparations showed that BFLF2 is a component of intracellular virions but is absent from mature extracellular virions. Coimmunoprecipitation experiments indicated that BFLF2 interacts with BFRF1, which was confirmed by immunofluorescence confocal microscopy showing that the two proteins colocalize on the nuclear membrane not only upon cotransfection in epithelial cells but also during viral replication. In cells carrying an EBV mutant with the BFRF1 gene deleted (293-BFRF1-KO cells) BFLF2 expression was low, and it was restored to wild-type levels upon treatment of the cells with the proteasome inhibitor MG132. Furthermore, recomplementing the 293-BFRF1-KO cells by BFRF1 transfection restored BFLF2 expression to the wild-type level. In addition, when expressed alone BFLF2 was localized diffusely inside the nucleus, whereas in the presence of BFRF1 the two proteins colocalized at the nuclear rim. Finally, 293 epithelial cells transfected with either protein or cotransfected were analyzed by electron microscopy to investigate potential alterations in the morphology of the nuclear membrane. The ultrastructural analysis revealed that (i) BFRF1 caused duplications of the nuclear membrane, similar to those reported to occur during the course of herpesviral replication, and (ii) while BFLF2 alone did not cause any apparent alteration, coexpression of the two proteins dramatically induced profound convolutions of the duplicated nuclear membrane. Both biochemical and morphological analysis showed association of the BFRF1-BFLF2 complex with a component of the nuclear lamina, lamin B. Taken together, these results and those of the accompanying paper (Farina et al., J. Virol. 79:3703-3712) indicate an important role of BFRF1 and BFLF2 in the early steps of EBV maturation at the nuclear membrane.

BFRF1 of Epstein-Barr virus is essential for efficient primary viral envelopment and egress

The molecular mechanisms that underlie maturation and egress of Epstein-Barr virus (EBV) virions are only partially characterized. We have recently shown that the BFRF1 gene, the EBV positional homolog of herpes simplex virus type 1 and pseudorabies virus UL34, is expressed early during EBV lytic replication and that it is found predominantly on the nuclear membrane (A. Farina, R. Santarelli, R. Gonnella, R. Bei, R. Muraro, G. Cardinali, S. Uccini, G. Ragona, L. Frati, A. Faggioni, and A. Angeloni, J. Virol. 74:3235-3244, 2000). These data suggest that the BFRF1 protein might be involved in viral primary envelopment. To precisely determine the function of this protein, we have constructed an EBV mutant devoid of the BFRF1 gene (BFRF1-KO). 293 cells carrying BFRF1-KO showed no differences in comparison with wild-type EBV in terms of DNA lytic replication or expression of late viral proteins upon induction of the lytic cycle. However, binding assays and infection experiments using cell lines or human cord blood lymphocytes showed a clear reduction in the viral mutant titers. Complementation experiments with BFRF1-KO and a BFRF1 expression vector restored viral titers to levels similar to those for the wild-type control, showing that the modifications that we introduced were limited to the BFRF1 gene. Electron microscopic observations showed that the reduction in viral titers was due to sequestration of EBV nucleocapsids in the nuclei of lytically induced cells. This suggests that BFRF1 is involved in transport of the maturing virion across the nuclear membrane. This hypothesis was further supported by the observation that BFRF1 is present in maturing intracellular virions but not in their extracellular counterparts. This implies that BFRF1 is a key protein for EBV maturation.

Human cytomegalovirus labeled with green fluorescent protein for live analysis of intracellular particle movements

Human cytomegalovirus (HCMV) replicates in the nuclei of infected cells. Successful replication therefore depends on particle movements between the cell cortex and nucleus during entry and egress. To visualize HCMV particles in living cells, we have generated a recombinant HCMV expressing enhanced green fluorescent protein (EGFP) fused to the C terminus of the capsid-associated tegument protein pUL32 (pp150). The resulting UL32-EGFP-HCMV was analyzed by immunofluorescence, electron microscopy, immunoblotting, confocal microscopy, and time-lapse microscopy to evaluate the growth properties of this virus and the dynamics of particle movements. UL32-EGFP-HCMV replicated similarly to wild-type virus in fibroblast cultures. Green fluorescent virus particles were released from infected cells. The fluorescence stayed associated with particles during viral entry, and fluorescent progeny particles appeared in the nucleus at 44 h after infection. Surprisingly, strict colocalization of pUL32 and the major capsid protein pUL86 within nuclear inclusions indicated that incorporation of pUL32 into nascent HCMV particles occurred simultaneously with or immediately after assembly of the capsid. A slow transport of nuclear particles towards the nuclear margin was demonstrated. Within the cytoplasm, most particles performed irregular short-distance movements, while a smaller fraction of particles performed centripetal and centrifugal long-distance movements. Although numerous particles accumulated in the cytoplasm, release of particles from infected cells was a rare event, consistent with a release rate of about 1 infectious unit per h per cell in HCMV-infected fibroblasts as calculated from single-step growth curves. UL32-EGFP-HCMV will be useful for further investigations into the entry, maturation, and release of this virus.

Human cytomegalovirus virion proteins

Human cytomegalovirus (HCMV) is the largest member of the family of human herpesviruses. The number of virus encoded proteins and the complexity of their functions in the life cycle of this virus are reflected in the size of its genome. There continues to be some controversy surrounding the exact protein coding capacity of the virus with estimates ranging from 160 open reading frames to more than 200 open reading frames. Very recent studies using mass spectrometry to determine the viral proteome suggests that the number of viral proteins may be even greater than previous estimates. The proteins of the virion capsid have readily identifiable homologous proteins in the capsid of the more extensively studied herpes simplex virus, likely because of similar capsid structure and assembly pathways. In contrast, the tegument and the envelope of HCMV contain a significant number of proteins that lack structural homology to proteins found in either alpha or gamma-herpesviruses. This brief overview discusses some of the general features and possible functions of the HCMV virion structural proteins in the replicative cycle of this virus.

Budding events in herpesvirus morphogenesis

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 a first budding event 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 two different budding steps, which are distinct not only in the subcellular compartments in which they occur but also by the viral proteins involved. This review summarizes recent advances in our understanding of the two herpesvirus budding events.

Selective alterations of the host cell architecture upon infection with parvovirus minute virus of mice

During a productive infection, the prototype strain of parvovirus minute virus of mice (MVMp) induces dramatic morphological alterations to the fibroblast host cell A9, resulting in cell lysis and progeny virus release. In order to understand the mechanisms underlying these changes, we characterized the fate of various cytoskeletal filaments and investigated the nuclear/cytoplasmic compartmentalization of infected cells. While most pronounced effects could be seen on micro- and intermediate filaments, manifest in dramatic rearrangements and degradation of filamentous (F-)actin and vimentin structures, only little impact could be seen on microtubules or the nuclear envelope during the entire monitored time of infection. To further analyze the disruption of the cytoskeletal structures, we investigated the viral impact on selective regulatory pathways. Thereby, we found a correlation between microtubule stability and MVM-induced phosphorylation of alpha/beta tubulin. In contrast, disassembly of actin filaments late in infection could be traced back to the disregulation of two F-actin associated proteins gelsolin and Wiscott-Aldrich Syndrome Protein (WASP). Thereby, an increase in the amount of gelsolin, an F-actin severing protein was observed during infection, accounting for the disruption of stress fibers upon infection. Concomitantly, the actin polymerization activity also diminished due to a loss of WASP, the activator protein of the actin polymerization machinery the Arp2/3 complex. No effects could be seen in amount and distribution of other F-actin regulatory factors such as cortactin, cofilin, and profilin. In summary, the selective attack of MVM towards distinct host cell cytoskeletal structures argues for a regulatory feature during infection, rather than a collapse of the host cell as a mere side effect of virus production.

The murine cytomegalovirus M73.5 gene, a member of a 3' co-terminal alternatively spliced gene family, encodes the gp24 virion glycoprotein

We have identified a novel family of five 3' co-terminal transcripts in murine cytomegalovirus (MCMV) arranged in a tail-to-tail orientation with respect to the MCMV glycoprotein H (gH) gene M75. They share the same exon 2 sequence but possess different exon 1 sequences. Two of these spliced transcripts (M73) encode the MCMV homolog of glycoprotein N (gN) entirely within exon 1. Two other transcripts designated M73.5 encode a previously described virion glycoprotein gp24 that shares its first 20 amino acids with gN, but which has another 64 amino acids encoded within exon 2. The fifth transcript, designated m60, has an 80-bp exon 1 near the MCMV oriLyt region 10.8 kb upstream of exon 2. Both MCMV M73.5 and m60 encode type II glycoproteins expressed at the cell surface. Their shared exon 2 coding sequences likely represent the highly conserved region of an as yet unidentified betaherpesvirus-specific glycoprotein species.

Conditional cytomegalovirus replication in vitro and in vivo

We have established a conditional gene expression system for cytomegalovirus which allows regulation of genes independently from the viral replication program. Due to the combination of all elements required for regulated expression in the same viral genome, conditional viruses can be studied in different cell lines in vitro and in the natural host in vivo. The combination of a self-sufficient tetracycline-regulated expression cassette and Flp recombinase-mediated insertion into the viral genome allowed fast construction of recombinant murine cytomegaloviruses carrying different conditional genes. The regulation of two reporter genes, the essential viral M50 gene and a dominant-negative mutant gene (m48.2) encoding the small capsid protein, was analyzed in more detail. In vitro, viral growth was regulated by the conditional expression of M50 by 3 orders of magnitude and up to a millionfold when the dominant-negative small capsid protein mutant was used. In vivo, viral growth of the dominant-negative mutant was reduced to detection limits in response to the presence of doxycycline in the organs of mice. We believe that this conditional expression system is applicable to genetic studies of large DNA viruses in general.

Essential function of the pseudorabies virus UL36 gene product is independent of its interaction with the UL37 protein

The large tegument protein encoded by the UL36 gene of pseudorabies virus (PrV) physically interacts with the product of the adjacent UL37 gene (B. G. Klupp, W. Fuchs, H. Granzow, R. Nixdorf, and T. C. Mettenleiter, J. Virol. 76:3065-3071, 2002). To analyze UL36 function, two PrV recombinants were generated by mutagenesis of an infectious PrV full-length clone in Escherichia coli: PrV-DeltaUL36F exhibited a deletion of virtually the complete UL36 coding region, whereas PrV-UL36BSF contained two in-frame deletions of 238 codons spanning the predicted UL37 binding domain. Coimmunoprecipitation experiments confirmed that the mutated gene product of PrV-UL36BSF did not interact with the UL37 protein. Like the previously described PrV-DeltaUL37 (B. G. Klupp, H. Granzow, and T. C. Mettenleiter, J. Virol. 75:8927-8936, 2001) but in contrast to PrV-DeltaUL36F, PrV-UL36BSF was able to replicate in rabbit kidney (RK13) cells, although maximum virus titers were reduced ca. 50-fold and plaque diameters were reduced by ca. 45% compared to wild-type PrV. PrV-DeltaUL36F was able to productively replicate after repair of the deleted gene or in a trans-complementing cell line. Electron microscopy of infected RK13 cells revealed that PrV-UL36BSF and phenotypically complemented PrV-DeltaUL36F were capable of nucleocapsid formation and egress from the nucleus by primary envelopment and deenvelopment at the nuclear membrane. However, reenvelopment of nucleocapsids in the cytoplasm was blocked. Only virus-like particles without capsids were released efficiently from cells. Interestingly, cytoplasmic nucleocapsids of PrV-UL36BSF but not of PrV-DeltaUL36F were found in large ordered structures similar to those which had previously been observed with PrV-DeltaUL37. In summary, our results demonstrate that the interaction between the UL36 and UL37 proteins is important but not strictly essential for the formation of secondary enveloped, infectious PrV particles. Furthermore, UL36 possesses an essential function during virus replication which is independent of its ability to bind the UL37 protein.

Global analysis of host cell gene expression late during cytomegalovirus infection reveals extensive dysregulation of cell cycle gene expression and induction of Pseudomitosis independent of US28 function

Replication of human cytomegalovirus (CMV) depends on host cell gene products working in conjunction with viral functions and leads to a dramatic dysregulation of cell cycle gene expression. Comprehensive transcriptional profiling was used to identify pathways most dramatically modulated by CMV at late times during infection and to determine the extent to which expression of the viral chemokine receptor US28 contributed to modulating cellular gene expression. Cells infected with the AD169 strain of virus or a fully replication competent US28-deficient derivative (RV101) were profiled throughout the late phase of infection (50, 72, and 98 h postinfection). Although sensitive statistical analysis showed striking global changes in transcript levels in infected cells compared to uninfected cells, the expression of US28 did not contribute to these alterations. CMV infection resulted in lower levels of transcripts encoding cytoskeletal, extracellular matrix, and adhesion proteins, together with small GTPases and apoptosis regulators, and in higher levels of transcripts encoding cell cycle, DNA replication, energy production, and inflammation-related gene products. Surprisingly, a large number of cellular transcripts encoding mitosis-related proteins were upmodulated at late times in infection, and these were associated with the formation of abnormal mitotic spindles and the appearance of pseudomitotic cells. These data extend our understanding of how broadly CMV alters the regulation of host cell cycle gene products and highlight the establishment of a mitosis-like environment in the absence of cellular DNA replication as important for viral replication and maturation.