Unexpected features and mechanism of heterodimer formation of a herpesvirus nuclear egress complex

'Getting Better'-Is It a Feasible Strategy of Broad Pan-Antiherpesviral Drug Targeting by Using the Nuclear Egress-Directed Mechanism?

The herpesviral nuclear egress represents an essential step of viral replication efficiency in host cells, as it defines the nucleocytoplasmic release of viral capsids. Due to the size limitation of the nuclear pores, viral nuclear capsids are unable to traverse the nuclear envelope without a destabilization of this natural host-specific barrier. To this end, herpesviruses evolved the regulatory nuclear egress complex (NEC), composed of a heterodimer unit of two conserved viral NEC proteins (core NEC) and a large-size extension of this complex including various viral and cellular NEC-associated proteins (multicomponent NEC). Notably, the NEC harbors the pronounced ability to oligomerize (core NEC hexamers and lattices), to multimerize into higher-order complexes, and, ultimately, to closely interact with the migrating nuclear capsids. Moreover, most, if not all, of these NEC proteins comprise regulatory modifications by phosphorylation, so that the responsible kinases, and additional enzymatic activities, are part of the multicomponent NEC. This sophisticated basis of NEC-specific structural and functional interactions offers a variety of different modes of antiviral interference by pharmacological or nonconventional inhibitors. Since the multifaceted combination of NEC activities represents a highly conserved key regulatory stage of herpesviral replication, it may provide a unique opportunity towards a broad, pan-antiherpesviral mechanism of drug targeting. This review presents an update on chances, challenges, and current achievements in the development of NEC-directed antiherpesviral strategies.

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

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

A small molecule exerts selective antiviral activity by targeting the human cytomegalovirus nuclear egress complex

Human cytomegalovirus (HCMV) is an important pathogen for which new antiviral drugs are needed. HCMV, like other herpesviruses, encodes a nuclear egress complex (NEC) composed of two subunits, UL50 and UL53, whose interaction is crucial for viral replication. To explore whether small molecules can exert selective antiviral activity by inhibiting NEC subunit interactions, we established a homogeneous time-resolved fluorescence (HTRF) assay of these interactions and used it to screen >200,000 compound-containing wells. Two compounds, designated GK1 and GK2, which selectively inhibited this interaction in the HTRF assay with GK1 also active in a co-immunoprecipitation assay, exhibited more potent anti-HCMV activity than cytotoxicity or activity against another herpesvirus. At doses that substantially reduced HCMV plaque formation, GK1 and GK2 had little or no effect on the expression of viral proteins and reduced the co-localization of UL53 with UL50 at the nuclear rim in a subset of cells. GK1 and GK2 contain an acrylamide moiety predicted to covalently interact with cysteines, and an analog without this potential lacked activity. Mass spectrometric analysis showed binding of GK2 to multiple cysteines on UL50 and UL53. Nevertheless, substitution of cysteine 214 of UL53 with serine (C214S) ablated detectable inhibitory activity of GK1 and GK2 in vitro, and the C214S substitution engineered into HCMV conferred resistance to GK1, the more potent of the two inhibitors. Thus, GK1 exerts selective antiviral activity by targeting the NEC. Docking studies suggest that the acrylamide tethers one end of GK1 or GK2 to C214 within a pocket of UL53, permitting the other end of the molecule to sterically hinder UL50 to prevent NEC formation. Our results prove the concept that targeting the NEC with small molecules can selectively block HCMV replication. Such compounds could serve as a foundation for development of anti-HCMV drugs and as chemical tools for studying HCMV.

The Knowns and Unknowns of Herpesvirus Nuclear Egress

Nuclear egress of herpesvirus capsids across the intact nuclear envelope is an exceptional vesicle-mediated nucleocytoplasmic translocation resulting in the delivery of herpesvirus capsids into the cytosol. Budding of the (nucleo)capsid at and scission from the inner nuclear membrane (INM) is mediated by the viral nuclear egress complex (NEC) resulting in a transiently enveloped virus particle in the perinuclear space followed by fusion of the primary envelope with the outer nuclear membrane (ONM). The dimeric NEC oligomerizes into a honeycomb-shaped coat underlining the INM to induce membrane curvature and scission. Mutational analyses complemented structural data defining functionally important regions. Questions remain, including where and when the NEC is formed and how membrane curvature is mediated, vesicle formation is regulated, and directionality is secured. The composition of the primary enveloped virion and the machinery mediating fusion of the primary envelope with the ONM is still debated. While NEC-mediated budding apparently follows a highly conserved mechanism, species and/or cell type-specific differences complicate understanding of later steps.

Human herpesvirus 6A nuclear matrix protein U37 interacts with heat shock transcription factor 1 and activates the heat shock response

Nascent nucleocapsids of herpesviruses acquire a primary envelope during their nuclear export by budding through the inner nuclear membrane into the perinuclear space between the inner and outer nuclear membranes. This process is mediated by a conserved viral heterodimeric complex designated the nuclear egress complex, which consists of the nuclear matrix protein and the nuclear membrane protein. In addition to its essential roles during nuclear egress, the nuclear matrix protein has been shown to interact with intracellular signaling pathway molecules including NF-κB and IFN-β to affect viral or cellular gene expression. The human herpesvirus 6A (HHV-6A) U37 gene encodes a nuclear matrix protein, the role of which has not been analyzed. Here, we show that HHV-6A U37 activates the heat shock element promoter and induces the accumulation of the molecular chaperone Hsp90. Mechanistically, HHV-6A U37 interacts with heat shock transcription factor 1 (HSF1) and induces its phosphorylation at Ser-326. We report that pharmacological inhibition of HSF1, Hsp70, or Hsp90 decreases viral protein accumulation and viral replication. Taken together, our results lead us to propose a model in which HHV-6A U37 activates the heat shock response to support viral gene expression and replication. IMPORTANCE Human herpesvirus 6A (HHV-6A) is a dsDNA virus belonging to the Roseolovirus genus within the Betaherpesvirinae subfamily. It is frequently found in patients with neuroinflammatory disease, although its pathogenetic role, if any, awaits elucidation. The heat shock response is important for cell survival under stressful conditions that disrupt homeostasis. Our results indicate that HHV-6A U37 activates the heat shock element promoter and leads to the accumulation of heat shock proteins. Next, we show that the heat shock response is important for viral replication. Overall, our findings provide new insights into the function of HHV-6A U37 in host cell signaling and identify potential cellular targets involved in HHV-6A pathogenesis and replication.

An antiviral targeting strategy based on the inducible interference with cytomegalovirus nuclear egress complex

The nucleocytoplasmic capsid egress of herpesviruses like the human cytomegalovirus (HCMV) is based on a uniquely regulated process. The core nuclear egress complex (NEC) of HCMV, represented by the pUL50-pUL53 heterodimer, is able to oligomerize and thus to build hexameric lattices. Recently, we and others validated the NEC as a novel target for antiviral strategies. So far, the experimental targeting approaches included the development of NEC-directed small molecules, cell-penetrating peptides and NEC-directed mutagenesis. Our postulate states that an interference with the hook-into-groove interaction of pUL50-pUL53 prevents NEC formation and strictly limits viral replication efficiency. Here, we provide an experimental proof-of-concept of the antiviral strategy: the inducible intracellular expression of a NLS-Hook-GFP construct exerted a pronounced level of antiviral activity. The data provide evidence for the following points: (i) generation of a primary fibroblast population with inducible NLS-Hook-GFP expression showed nuclear localization of the construct, (ii) interaction between NLS-Hook-GFP and the viral core NEC was found specific for cytomegaloviruses but not for other herpesviruses, (iii) construct overexpression exerted a strong antiviral activity against three strains of HCMV, (iv) confocal imaging demonstrated the interference with NEC nuclear rim formation in HCMV-infected cells, and (v) quantitative nuclear egress assay confirmed the block of viral nucleocytoplasmic transition and, consequently, an inhibitory effect onto viral cytoplasmic virion assembly complex (cVAC). Combined, data confirmed that the specific interference with protein-protein interaction of the HCMV core NEC represents an efficient antiviral targeting strategy.

The human cytomegalovirus decathlon: Ten critical replication events provide opportunities for restriction

Human cytomegalovirus (HCMV) is a ubiquitous human pathogen that can cause severe disease in immunocompromised individuals, transplant recipients, and to the developing foetus during pregnancy. There is no protective vaccine currently available, and with only a limited number of antiviral drug options, resistant strains are constantly emerging. Successful completion of HCMV replication is an elegant feat from a molecular perspective, with both host and viral processes required at various stages. Remarkably, HCMV and other herpesviruses have protracted replication cycles, large genomes, complex virion structure and complicated nuclear and cytoplasmic replication events. In this review, we outline the 10 essential stages the virus must navigate to successfully complete replication. As each individual event along the replication continuum poses as a potential barrier for restriction, these essential checkpoints represent potential targets for antiviral development.

The nuclear egress complex of Epstein-Barr virus buds membranes through an oligomerization-driven mechanism

During replication, herpesviral capsids are translocated from the nucleus into the cytoplasm by an unusual mechanism, termed nuclear egress, that involves capsid budding at the inner nuclear membrane. This process is mediated by the viral nuclear egress complex (NEC) that deforms the membrane around the capsid. Although the NEC is essential for capsid nuclear egress across all three subfamilies of the Herpesviridae, most studies to date have focused on the NEC homologs from alpha- and beta- but not gammaherpesviruses. Here, we report the crystal structure of the NEC from Epstein-Barr virus (EBV), a prototypical gammaherpesvirus. The structure resembles known structures of NEC homologs yet is conformationally dynamic. We also show that purified, recombinant EBV NEC buds synthetic membranes in vitro and forms membrane-bound coats of unknown geometry. However, unlike other NEC homologs, EBV NEC forms dimers in the crystals instead of hexamers. The dimeric interfaces observed in the EBV NEC crystals are similar to the hexameric interfaces observed in other NEC homologs. Moreover, mutations engineered to disrupt the dimeric interface reduce budding. Putting together these data, we propose that EBV NEC-mediated budding is driven by oligomerization into membrane-bound coats.

Herpesviruses, which infect most of the world’s population for life, translocate their capsids from the nucleus, where they are formed, into the cytoplasm, where they mature into infectious virions, by an unusual mechanism, termed nuclear egress. During nuclear budding, an early step in this process, the inner nuclear membrane is deformed around the capsid by the complex of two viral proteins termed the nuclear egress complex (NEC). The NEC is conserved across all three subfamilies of Herpesviruses and essential for nuclear egress. However, most studies to date have focused on the NEC homologs from alpha- and betaherpesviruses while less is known about the NEC from gammaherpesviruses. Here, we determined the crystal structure of the NEC from Epstein-Barr virus (EBV), a prototypical gammaherpesvirus, and investigated its membrane budding properties in vitro. Our data show that the ability to vesiculate membranes by forming membrane-bound coats and the structure are conserved across the NEC homologs from all three subfamilies. However, the EBV NEC may employ a distinct membrane-budding mechanism due to its structural flexibility and the ability to form coats of different geometry.

‘Come Together’—The Regulatory Interaction of Herpesviral Nuclear Egress Proteins Comprises both Essential and Accessory Functions

Herpesviral nuclear egress is a fine-tuned regulatory process that defines the nucleocytoplasmic release of viral capsids. Nuclear capsids are unable to traverse via nuclear pores due to the fact of their large size; therefore, herpesviruses evolved to develop a vesicular transport pathway mediating the transition across the two leaflets of the nuclear membrane. The entire process involves a number of regulatory proteins, which support the local distortion of the nuclear envelope. In the case of the prototype species of β-Herpesvirinae, the human cytomegalovirus (HCMV), the nuclear egress complex (NEC) is determined by the core proteins pUL50 and pUL53 that oligomerize, form capsid docking lattices and mediate multicomponent assembly with NEC-associated viral and cellular proteins. The NEC-binding principle is based on the hook-into-groove interaction through an N-terminal hook-like pUL53 protrusion that embraces an α-helical pUL50 binding groove. Thus far, the function and characteristics of herpesviral core NECs have been well studied and point to the groove proteins, such as pUL50, as the multi-interacting, major determinants of NEC formation and egress. This review provides closer insight into (i) sequence and structure conservation of herpesviral core NEC proteins, (ii) experimentation on cross-viral core NEC interactions, (iii) the essential functional roles of hook and groove proteins for viral replication, (iv) an establishment of assay systems for NEC-directed antiviral research and (v) the validation of NEC as putative antiviral drug targets. Finally, this article provides new insights into the conservation, function and antiviral targeting of herpesviral core NEC proteins and, into the complex regulatory role of hook and groove proteins during the assembly, egress and maturation of infectious virus.

The crystal structure of the varicella-zoster Orf24-Orf27 nuclear egress complex spotlights multiple determinants of herpesvirus subfamily specificity

Varicella-zoster virus (VZV) is a human pathogen from the α-subfamily of herpesviruses. The VZV Orf24-Orf27 complex represents the essential viral core nuclear egress complex (NEC) that orchestrates the egress of the preassembled virus capsids from the nucleus. While previous studies have primarily emphasized that the architecture of core NEC complexes is highly conserved among herpesviruses, the present report focuses on subfamily-specific structural and functional features that help explain the differences in the autologous versus nonautologous interaction patterns observed for NEC formation across herpesviruses. Here, we describe the crystal structure of the Orf24-Orf27 complex at 2.1 Å resolution. Coimmunoprecipitation and confocal imaging data show that Orf24-Orf27 complex formation displays some promiscuity in a herpesvirus subfamily-restricted manner. At the same time, analysis of thermodynamic parameters of NEC formation of three prototypical α-, β-, and γ herpesviruses, i.e., VZV, human cytomegalovirus (HCMV), and Epstein–Barr virus (EBV), revealed highly similar binding affinities for the autologous interaction with specific differences in enthalpy and entropy. Computational alanine scanning, structural comparisons, and mutational data highlight intermolecular interactions shared among α-herpesviruses that are clearly distinct from those seen in β- and γ-herpesviruses, including a salt bridge formed between Orf24-Arg167 and Orf27-Asp126. This interaction is located outside of the hook-into-groove interface and contributes significantly to the free energy of complex formation. Combined, these data explain distinct properties of specificity and permissivity so far observed in herpesviral NEC interactions. These findings will prove valuable in attempting to target multiple herpesvirus core NECs with selective or broad-acting drug candidates.

Human Cytomegalovirus Egress: Overcoming Barriers and Co-Opting Cellular Functions

The assembly of human cytomegalovirus (HCMV) and other herpesviruses includes both nuclear and cytoplasmic phases. During the prolonged replication cycle of HCMV, the cell undergoes remarkable changes in cellular architecture that include marked increases in nuclear size and structure as well as the reorganization of membranes in cytoplasm. Similarly, significant changes occur in cellular metabolism, protein trafficking, and cellular homeostatic functions. These cellular modifications are considered integral in the efficient assembly of infectious progeny in productively infected cells. Nuclear egress of HCMV nucleocapsids is thought to follow a pathway similar to that proposed for other members of the herpesvirus family. During this process, viral nucleocapsids must overcome structural barriers in the nucleus that limit transit and, ultimately, their delivery to the cytoplasm for final assembly of progeny virions. HCMV, similar to other herpesviruses, encodes viral functions that co-opt cellular functions to overcome these barriers and to bridge the bilaminar nuclear membrane. In this brief review, we will highlight some of the mechanisms that define our current understanding of HCMV egress, relying heavily on the current understanding of egress of the more well-studied α-herpesviruses, HSV-1 and PRV.

Herpesvirus Nuclear Egress across the Outer Nuclear Membrane

Herpesvirus capsids are assembled in the nucleus and undergo a two-step process to cross the nuclear envelope. Capsids bud into the inner nuclear membrane (INM) aided by the nuclear egress complex (NEC) proteins UL31/34. At that stage of egress, enveloped virions are found for a short time in the perinuclear space. In the second step of nuclear egress, perinuclear enveloped virions (PEVs) fuse with the outer nuclear membrane (ONM) delivering capsids into the cytoplasm. Once in the cytoplasm, capsids undergo re-envelopment in the Golgi/trans-Golgi apparatus producing mature virions. This second step of nuclear egress is known as de-envelopment and is the focus of this review. Compared with herpesvirus envelopment at the INM, much less is known about de-envelopment. We propose a model in which de-envelopment involves two phases: (i) fusion of the PEV membrane with the ONM and (ii) expansion of the fusion pore leading to release of the viral capsid into the cytoplasm. The first phase of de-envelopment, membrane fusion, involves four herpes simplex virus (HSV) proteins: gB, gH/gL, gK and UL20. gB is the viral fusion protein and appears to act to perturb membranes and promote fusion. gH/gL may also have similar properties and appears to be able to act in de-envelopment without gB. gK and UL20 negatively regulate these fusion proteins. In the second phase of de-envelopment (pore expansion and capsid release), an alpha-herpesvirus protein kinase, US3, acts to phosphorylate NEC proteins, which normally produce membrane curvature during envelopment. Phosphorylation of NEC proteins reverses tight membrane curvature, causing expansion of the membrane fusion pore and promoting release of capsids into the cytoplasm.

The Complex Regulatory Role of Cytomegalovirus Nuclear Egress Protein pUL50 in the Production of Infectious Virus

The regulation of the nucleocytoplasmic release of herpesviral capsids is defined by the process of nuclear egress. Due to their large size, nuclear capsids are unable to traverse via nuclear pores, so that herpesviruses evolved to develop a vesicular transport pathway mediating their transition through both leaflets of the nuclear membrane. This process involves regulatory proteins, which support the local distortion of the nuclear envelope. For human cytomegalovirus (HCMV), the nuclear egress complex (NEC) is determined by the pUL50-pUL53 core that initiates multicomponent assembly with NEC-associated proteins and capsids. Hereby, pUL50 serves as a multi-interacting determinant that recruits several viral and cellular factors by direct and indirect contacts. Recently, we generated an ORF-UL50-deleted recombinant HCMV in pUL50-complementing cells and obtained first indications of putative additional functions of pUL50. In this study, we produced purified ΔUL50 particles under both complementing (ΔUL50C) and non-complementing (ΔUL50N) conditions and performed a phenotypical characterization. Findings were as follows: (i) ΔUL50N particle preparations exhibited a clear replicative defect in qPCR-based infection kinetics compared to ΔUL50C particles; (ii) immuno-EM analysis of ΔUL50C did not reveal major changes in nuclear distribution of pUL53 and lamin A/C; (iii) mass spectrometry-based quantitative proteomics showed a large concordance of protein contents in the NIEP fractions of ΔUL50C and ΔUL50N particles, but virion fraction was close to the detection limit for ΔUL50N; (iv) confocal imaging of viral marker proteins of immediate early (IE) and later phases of ΔUL50N infection indicated a very low number of cells showing an onset of viral lytic protein expression; and, finally (v) quantitative measurements of encapsidated genomes provided evidence for a substantial reduction in the DNA contents in ΔUL50N compared to ΔUL50C particles. In summary, the results point to a complex and important regulatory role of the HCMV nuclear egress protein pUL50 in the maturation of infectious virus.

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

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

Role of the arginine cluster in the disordered domain of Herpes Simplex Virus 1 UL34 for the recruitment of ESCRT-III for viral primary envelopment

During the nuclear export of nascent nucleocapsids of herpesviruses, the nucleocapsids bud through the inner nuclear membrane (INM) by acquiring the INM as a primary envelope (primary envelopment). We recently reported that herpes simplex virus 1 (HSV-1) nuclear egress complex (NEC), which consists of UL34 and UL31, interacts with an ESCRT-III adaptor ALIX and recruits ESCRT-III machinery to the INM for efficient primary envelopment. In this study, we identified a cluster of six arginine residues in the disordered domain of UL34 as a minimal region required for the interaction with ALIX as well as the recruitment of ALIX and an ESCRT-III protein CHMP4B to the INM in HSV-1-infected cells. Mutations in the arginine cluster exhibited phenotypes similar to those with ESCRT-III inhibition reported previously, including the mis-localization of NEC, induction of membranous invagination structures containing enveloped virions, aberrant accumulation of enveloped virions in the invaginations and perinuclear space, and reduction of viral replication. We also showed that the effect of the arginine cluster in UL34 on HSV-1 replication was dependent primarily on ALIX. These results indicated that the arginine cluster in the disordered domain of UL34 was required for the interaction with ALIX and the recruitment of ESCRT-III machinery to the INM to promote primary envelopment. IMPORTANCE Herpesvirus UL34 homologs contain conserved amino-terminal domains that mediate vesicle formation through interactions with UL31 homologs during primary envelopment. UL34 homologs also comprise other domains adjacent to their membrane-anchoring regions, which differ in length, are variable in herpesviruses and do not form distinguished secondary structures. However, the role of these disordered domains in infected cells remains to be elucidated. In this study, we present data suggesting that the arginine cluster in the disordered domain of HSV-1 UL34 mediates the interaction with ALIX, thereby leading to the recruitment of ESCRT-III machinery to the INM for efficient primary envelopment. This is the first study to report the role of the disordered domain of a UL34 homolog in herpesvirus infections.

Highly Basic Clusters in the Herpes Simplex Virus 1 Nuclear Egress Complex Drive Membrane Budding by Inducing Lipid Ordering

During replication of herpesviruses, capsids escape from the nucleus into the cytoplasm by budding at the inner nuclear membrane. This unusual process is mediated by the viral nuclear egress complex (NEC) that deforms the membrane around the capsid by oligomerizing into a hexagonal, membrane-bound scaffold. Here, we found that highly basic membrane-proximal regions (MPRs) of the NEC alter lipid order by inserting into the lipid headgroups and promote negative Gaussian curvature. We also find that the electrostatic interactions between the MPRs and the membranes are essential for membrane deformation. One of the MPRs is phosphorylated by a viral kinase during infection, and the corresponding phosphomimicking mutations block capsid nuclear egress. We show that the same phosphomimicking mutations disrupt the NEC-membrane interactions and inhibit NEC-mediated budding in vitro, providing a biophysical explanation for the in vivo phenomenon. Our data suggest that the NEC generates negative membrane curvature by both lipid ordering and protein scaffolding and that phosphorylation acts as an off switch that inhibits the membrane-budding activity of the NEC to prevent capsid-less budding. IMPORTANCE Herpesviruses are large viruses that infect nearly all vertebrates and some invertebrates and cause lifelong infections in most of the world's population. During replication, herpesviruses export their capsids from the nucleus into the cytoplasm by an unusual mechanism in which the viral nuclear egress complex (NEC) deforms the nuclear membrane around the capsid. However, how membrane deformation is achieved is unclear. Here, we show that the NEC from herpes simplex virus 1, a prototypical herpesvirus, uses clusters of positive charges to bind membranes and order membrane lipids. Reducing the positive charge or introducing negative charges weakens the membrane deforming ability of the NEC. We propose that the virus employs electrostatics to deform nuclear membrane around the capsid and can control this process by changing the NEC charge through phosphorylation. Blocking NEC-membrane interactions could be exploited as a therapeutic strategy.

The HSV1 Tail-Anchored Membrane Protein pUL34 Contains a Basic Motif That Supports Active Transport to the Inner Nuclear Membrane Prior to Formation of the Nuclear Egress Complex

Herpes simplex virus type 1 nucleocapsids are released from the host nucleus by a budding process through the nuclear envelope called nuclear egress. Two viral proteins, the integral membrane proteins pUL34 and pUL31, form the nuclear egress complex at the inner nuclear membrane, which is critical for this process. The nuclear import of both proteins ensues separately from each other: pUL31 is actively imported through the central pore channel, while pUL34 is transported along the peripheral pore membrane. With this study, we identified a functional bipartite NLS between residues 178 and 194 of pUL34. pUL34 lacking its NLS is mislocalized to the TGN but retargeted to the ER upon insertion of the authentic NLS or a mimic NLS, independent of the insertion site. If co-expressed with pUL31, either of the pUL34-NLS variants is efficiently, although not completely, targeted to the nuclear rim where co-localization with pUL31 and membrane budding seem to occur, comparable to the wild-type. The viral mutant HSV1(17⁺)Lox-UL34-NLS mt is modestly attenuated but viable and associated with localization of pUL34-NLS mt to both the nuclear periphery and cytoplasm. We propose that targeting of pUL34 to the INM is facilitated by, but not dependent on, the presence of an NLS, thereby supporting NEC formation and viral replication.

Role of Vesicle-Associated Membrane Protein-Associated Proteins (VAP) A and VAPB in Nuclear Egress of the Alphaherpesvirus Pseudorabies Virus

The molecular mechanism affecting translocation of newly synthesized herpesvirus nucleocapsids from the nucleus into the cytoplasm is still not fully understood. The viral nuclear egress complex (NEC) mediates budding at and scission from the inner nuclear membrane, but the NEC is not sufficient for efficient fusion of the primary virion envelope with the outer nuclear membrane. Since no other viral protein was found to be essential for this process, it was suggested that a cellular machinery is recruited by viral proteins. However, knowledge on fusion mechanisms involving the nuclear membranes is rare. Recently, vesicle-associated membrane protein-associated protein B (VAPB) was shown to play a role in nuclear egress of herpes simplex virus 1 (HSV-1). To test this for the related alphaherpesvirus pseudorabies virus (PrV), we mutated genes encoding VAPB and VAPA by CRISPR/Cas9-based genome editing in our standard rabbit kidney cells (RK13), either individually or in combination. Single as well as double knockout cells were tested for virus propagation and for defects in nuclear egress. However, no deficiency in virus replication nor any effect on nuclear egress was obvious suggesting that VAPB and VAPA do not play a significant role in this process during PrV infection in RK13 cells.

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

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

Host and Viral Factors Involved in Nuclear Egress of Herpes Simplex Virus 1

Herpes simplex virus 1 (HSV-1) replicates its genome and packages it into capsids within the nucleus. HSV-1 has evolved a complex mechanism of nuclear egress whereby nascent capsids bud on the inner nuclear membrane to form perinuclear virions that subsequently fuse with the outer nuclear membrane, releasing capsids into the cytosol. The viral-encoded nuclear egress complex (NEC) plays a crucial role in this vesicle-mediated nucleocytoplasmic transport. Nevertheless, similar system mediates the movement of other cellular macromolecular complexes in normal cells. Therefore, HSV-1 may utilize viral proteins to hijack the cellular machinery in order to facilitate capsid transport. However, little is known about the molecular mechanisms underlying this phenomenon. This review summarizes our current understanding of the cellular and viral factors involved in the nuclear egress of HSV-1 capsids.

Exploring the Human Cytomegalovirus Core Nuclear Egress Complex as a Novel Antiviral Target: A New Type of Small Molecule Inhibitors

Nuclear egress is an essential process in the replication of human cytomegalovirus (HCMV), as it enables the migration of newly formed viral capsids from the nucleus into the cytoplasm. Inhibition of the HCMV core nuclear egress complex (core NEC), composed of viral proteins pUL50 and pUL53, has been proposed as a potential new target for the treatment of HCMV infection and disease. Here, we present a new type of small molecule inhibitors of HCMV core NEC formation, which inhibit the pUL50-pUL53 interaction at nanomolar concentrations. These inhibitors, i.e., verteporfin and merbromin, were identified through the screening of the Prestwick Chemical Library^® of approved drug compounds. The inhibitory effect of merbromin is both compound- and target-specific, as no inhibition was seen for other mercury-organic compounds. Furthermore, merbromin does not inhibit an unrelated protein-protein interaction either. More importantly, merbromin was found to inhibit HCMV infection of cells in three different assays, as well as to disrupt HCMV NEC nuclear rim formation. Thus, while not being an ideal drug candidate by itself, merbromin may serve as a blueprint for small molecules with high HCMV core NEC inhibitory potential, as candidates for novel anti-herpesviral drugs.

Properties of Oligomeric Interaction of the Cytomegalovirus Core Nuclear Egress Complex (NEC) and Its Sensitivity to an NEC Inhibitory Small Molecule

Herpesviral nuclear egress is a regulated process shared by all family members, ensuring the efficient cytoplasmic release of viral capsids. In the case of human cytomegalovirus (HCMV), the core of the nuclear egress complex (NEC) consists of the pUL50-pUL53 heterodimer that builds hexameric lattices for capsid binding and multicomponent interaction, including NEC-associated host factors. A characteristic feature of NEC interaction is the N-terminal hook structure of pUL53 that binds to an alpha-helical groove of pUL50, thus termed as hook-into-groove interaction. This central regulatory element is essential for viral replication and shows structural–functional conservation, which has been postulated as a next-generation target of antiviral strategies. However, a solid validation of this concept has been missing. In the present study, we focused on the properties of oligomeric HCMV core NEC interaction and the antiviral activity of specifically targeted prototype inhibitors. Our data suggest the following: (i) transiently expressed, variably tagged versions of HCMV NEC proteins exert hook-into-groove complexes, putatively in oligomeric assemblies that are distinguishable from heterodimers, as shown by in vitro assembly and coimmunoprecipitation approaches; (ii) this postulated oligomeric binding pattern was further supported by the use of a pUL50::pUL53 fusion construct also showing a pronounced multi-interaction potency; (iii) using confocal imaging cellular NEC-associated proteins were found partly colocalized with the tagged core NECs; (iv) a small inhibitory molecule, recently identified by an in vitro binding inhibition assay, was likewise active in blocking pUL50–pUL53 oligomeric assembly and in exerting antiviral activity in HCMV-infected fibroblasts. In summary, the findings refine the previous concept of HCMV core NEC formation and nominate this drug-accessible complex as a validated antiviral drug target.

Phenotypical Characterization of the Nuclear Egress of Recombinant Cytomegaloviruses Reveals Defective Replication upon ORF-UL50 Deletion but Not pUL50 Phosphosite Mutation

Nuclear egress is a common herpesviral process regulating nucleocytoplasmic capsid release. For human cytomegalovirus (HCMV), the nuclear egress complex (NEC) is determined by the pUL50-pUL53 core that regulates multicomponent assembly with NEC-associated proteins and capsids. Recently, NEC crystal structures were resolved for α-, β- and γ-herpesviruses, revealing profound structural conservation, which was not mirrored, however, by primary sequence and binding properties. The NEC binding principle is based on hook-into-groove interaction through an N-terminal hook-like pUL53 protrusion that embraces an α-helical pUL50 binding groove. So far, pUL50 has been considered as the major kinase-interacting determinant and massive phosphorylation of pUL50-pUL53 was assigned to NEC formation and functionality. Here, we addressed the question of phenotypical changes of ORF-UL50-mutated HCMVs. Surprisingly, our analyses did not detect a predominant replication defect for most of these viral mutants, concerning parameters of replication kinetics (qPCR), viral protein production (Western blot/CoIP) and capsid egress (confocal imaging/EM). Specifically, only the ORF-UL50 deletion rescue virus showed a block of genome synthesis during late stages of infection, whereas all phosphosite mutants exhibited marginal differences compared to wild-type or revertants. These results (i) emphasize a rate-limiting function of pUL50 for nuclear egress, and (ii) demonstrate that mutations in all mapped pUL50 phosphosites may be largely compensated. A refined mechanistic concept points to a multifaceted nuclear egress regulation, for which the dependence on the expression and phosphorylation of pUL50 is discussed.

Generation and characterization of monoclonal antibodies specific for the Pseudorabies Virus nuclear egress complex

During herpesvirus replication, newly synthesized nucleocapsids exit the nucleus by a vesicle-mediated transport, which requires the nuclear egress complex (NEC), composed of the conserved viral proteins designated as pUL31 and pUL34 in the alphaherpesviruses pseudorabies virus (PrV) and herpes simplex viruses. Oligomerization of the heterodimeric NEC at the inner nuclear membrane (INM) results in membrane bending and budding of virus particles into the perinuclear space. The INM-derived primary envelope then fuses with the outer nuclear membrane to release nucleocapsids into the cytoplasm. The two NEC components are necessary and sufficient for induction of vesicle budding and scission as shown after co-expression in eukaryotic cells or in synthetic membranes. However, where and when the NEC is formed, how membrane curvature is mediated and how it is regulated, remains unclear. While monospecific antisera raised against the different components of the PrV NEC aided in the characterization and intracellular localization of the individual proteins, no NEC specific tools have been described yet for any herpesvirus. To gain more insight into vesicle budding and scission, we aimed at generating NEC specific monoclonal antibodies (mAbs). To this end, mice were immunized with bacterially expressed soluble PrV NEC, which was previously used for structure determination. Besides pUL31- and pUL34-specific mAbs, we also identified mAbs, which reacted only in the presence of both proteins indicating specificity for the complex. Confocal microscopy with those NEC-specific mAbs revealed small puncta (approx. 0.064 μm²) along the nuclear rim in PrV wild type infected cells. In contrast, ca. 5-fold larger speckles (approx. 0.35 μm²) were detectable in cells infected with a PrV mutant lacking the viral protein kinase pUS3, which is known to accumulate primary enveloped virions in the PNS within large invaginations of the INM, or in cells co-expressing pUL31 and pUL34. Kinetic experiments showed that while the individual proteins were detectable already between 2-4 hours after infection, the NEC-specific mAbs produced significant staining only after 4-6 hours in accordance with timing of nuclear egress. Taken together, the data indicate that these mAbs specifically label the PrV NEC.

Nuclear Egress

During viral replication, herpesviruses utilize a unique strategy, termed nuclear egress, to translocate capsids from the nucleus into the cytoplasm. This initial budding step transfers a newly formed capsid from within the nucleus, too large to fit through nuclear pores, through the inner nuclear membrane to the perinuclear space. The perinuclear enveloped virion must then fuse with the outer nuclear membrane to be released into the cytoplasm for further maturation, undergoing budding once again at the trans-Golgi network or early endosomes, and ultimately exit the cell non-lytically to spread infection. This first budding process is mediated by two conserved viral proteins, UL31 and UL34, that form a heterodimer called the nuclear egress complex (NEC). This review focuses on what we know about how the NEC mediates capsid transport to the perinuclear space, including steps prior to and after this budding event. Additionally, we discuss the involvement of other viral proteins in this process and how NEC-mediated budding may be regulated during infection.

Human Cytomegalovirus Primary Infection and Reactivation: Insights From Virion-Carried Molecules

Human cytomegalovirus (HCMV), a ubiquitous beta-herpesvirus, is able to establish lifelong latency after initial infection. Periodical reactivation occurs after immunosuppression, remaining a major cause of death in immunocompromised patients. HCMV has to reach a structural and functional balance with the host at its earliest entry. Virion-carried mediators are considered to play pivotal roles in viral adaptation into a new cellular environment upon entry. Additionally, one clear difference between primary infection and reactivation is the idea that virion-packaged factors are already formed such that those molecules can be used swiftly by the virus. In contrast, virion-carried mediators have to be transcribed and translated; thus, they are not readily available during reactivation. Hence, understanding virion-carried molecules helps to elucidate HCMV reactivation. In this article, the impact of virion-packaged molecules on viral structure, biological behavior, and viral life cycle will be reviewed.

Nuclear Egress Complexes of HCMV and Other Herpesviruses: Solving the Puzzle of Sequence Coevolution, Conserved Structures and Subfamily-Spanning Binding Properties

Herpesviruses uniquely express two essential nuclear egress-regulating proteins forming a heterodimeric nuclear egress complex (core NEC). These core NECs serve as hexameric lattice-structured platforms for capsid docking and recruit viral and cellular NEC-associated factors that jointly exert nuclear lamina as well as membrane-rearranging functions (multicomponent NEC). The regulation of nuclear egress has been profoundly analyzed for murine and human cytomegaloviruses (CMVs) on a mechanistic basis, followed by the description of core NEC crystal structures, first for HCMV, then HSV-1, PRV and EBV. Interestingly, the highly conserved structural domains of these proteins stand in contrast to a very limited sequence conservation of the key amino acids within core NEC-binding interfaces. Even more surprising, although a high functional consistency was found when regarding the basic role of NECs in nuclear egress, a clear specification was identified regarding the limited, subfamily-spanning binding properties of core NEC pairs and NEC multicomponent proteins. This review summarizes the evolving picture of the relationship between sequence coevolution, structural conservation and properties of NEC interaction, comparing HCMV to α-, β- and γ-herpesviruses. Since NECs represent substantially important elements of herpesviral replication that are considered as drug-accessible targets, their putative translational use for antiviral strategies is discussed.

Differentiating the Roles of UL16, UL21, and Us3 in the Nuclear Egress of Herpes Simplex Virus Capsids

Viral proteins pUL16 and pUL21 are required for efficient nuclear egress of herpes simplex virus 2 capsids. To better understand the role of these proteins in nuclear egress, we established whether nuclear egress complex (NEC) distribution and/or function was altered in the absence of either pUL16 or pUL21. NEC distribution in cells infected with pUL16-deficient viruses was indistinguishable from that observed in cells infected with wild-type viruses. In contrast, NEC distribution was aberrant in cells infected with pUL21-deficient virus and, instead, showed some similarity to the aberrant NEC distribution pattern observed in cells infected with pUs3-deficient virus. These results indicated that pUL16 plays a role in nuclear egress that is distinct from that of pUL21 and pUs3. Higher-resolution examination of nuclear envelope ultrastructure in cells infected with pUL21-deficient viruses by transmission electron microscopy showed different types of nuclear envelope perturbations, including some that were not observed in cells infected with pUs3 deficient virus. The formation of the nuclear envelope perturbations observed in pUL21-deficient virus infections was dependent on a functional NEC, revealing a novel role for pUL21 in regulating NEC activity. The results of comparisons of nuclear envelope ultrastructure in cells infected with viruses lacking pUs3, pUL16, or both pUs3 and pUL16 were consistent with a role for pUL16 in advance of primary capsid envelopment and shed new light on how pUs3 functions in nuclear egress.IMPORTANCE The membrane deformation activity of the herpesvirus nuclear egress complex (NEC) allows capsids to transit through both nuclear membranes into the cytoplasm. NEC activity must be precisely controlled during viral infection, and yet our knowledge of how NEC activity is controlled is incomplete. To determine how pUL16 and pUL21, two viral proteins required for nuclear egress of herpes simplex virus 2, function in nuclear egress, we examined how the lack of each protein impacted NEC distribution. These analyses revealed a function of pUL16 in nuclear egress distinct from that of pUL21, uncovered a novel role for pUL21 in regulating NEC activity, and shed new light on how a viral kinase, pUs3, regulates nuclear egress. Nuclear egress of capsids is required for all herpesviruses. A complete understanding of all aspects of nuclear egress, including how viral NEC activity is controlled, may yield strategies to disrupt this process and aid the development of herpes-specific antiviral therapies.

The Cytomegalovirus Protein Kinase pUL97:Host Interactions, Regulatory Mechanisms and Antiviral Drug Targeting

Human cytomegalovirus (HCMV) expresses a variety of viral regulatory proteins that undergo close interaction with host factors including viral-cellular multiprotein complexes. The HCMV protein kinase pUL97 represents a viral cyclin-dependent kinase ortholog (vCDK) that determines the efficiency of HCMV replication via phosphorylation of viral and cellular substrates. A hierarchy of functional importance of individual pUL97-mediated phosphorylation events has been discussed; however, the most pronounced pUL97-dependent phenotype could be assigned to viral nuclear egress, as illustrated by deletion of the UL97 gene or pharmacological pUL97 inhibition. Despite earlier data pointing to a cyclin-independent functionality, experimental evidence increasingly emphasized the role of pUL97-cyclin complexes. Consequently, the knowledge about pUL97 involvement in host interaction, viral nuclear egress and additional replicative steps led to the postulation of pUL97 as an antiviral target. Indeed, validation experiments in vitro and in vivo confirmed the sustainability of this approach. Consequently, current investigations of pUL97 in antiviral treatment go beyond the known pUL97-mediated ganciclovir prodrug activation and henceforward include pUL97-specific kinase inhibitors. Among a number of interesting small molecules analyzed in experimental and preclinical stages, maribavir is presently investigated in clinical studies and, in the near future, might represent a first kinase inhibitor applied in the field of antiviral therapy.

Mutational Functional Analysis of the Pseudorabies Virus Nuclear Egress Complex-Nucleocapsid Interaction

Herpesvirus nucleocapsids leave the nucleus by a vesicle-mediated translocation mediated by the viral nuclear egress complex (NEC). The NEC is composed of two conserved viral proteins, designated pUL34 and pUL31 in the alphaherpesvirus pseudorabies virus (PrV). It is required for efficient nuclear egress and is sufficient for vesicle formation and scission from the inner nuclear membrane (INM). Structure-based mutagenesis identified a lysine at position 242 (K242) in pUL31, located in the most membrane distal part of the NEC, to be crucial for efficient nucleocapsid incorporation into budding vesicles. Replacing the lysine by alanine (K242A) resulted in accumulations of empty vesicles in the perinuclear space, despite the presence of excess nucleocapsids in the nucleus. However, it remained unclear whether the defect in capsid incorporation was due to interference with a direct, electrostatic interaction between the capsid and the NEC or structural restrictions. To test this, we replaced K242 with several amino acids, thereby modifying the charge, size, and side chain orientation. In addition, virus recombinants expressing pUL31-K242A were passaged and screened for second-site mutations. Compensatory mutations at different locations in pUL31 or pUL34 were identified, pointing to an inherent flexibility of the NEC. In summary, our data suggest that the amino acid at position 242 does not directly interact with the nucleocapsid but that rearrangements in the NEC coat are required for efficient nucleocapsid envelopment at the INM.IMPORTANCE Herpesviruses encode an exceptional vesicle formation and scission machinery, which operates at the inner nuclear membrane, translocating the viral nucleocapsid from the nucleus into the perinuclear space. The conserved herpesviral nuclear egress complex (NEC) orchestrates this process. High-resolution imaging approaches as well as the recently solved crystal structures of the NEC provided deep insight into the molecular details of vesicle formation and scission. Nevertheless, the molecular mechanism of nucleocapsid incorporation remained unclear. In accordance with structure-based predictions, a basic amino acid could be pinpointed in the most membrane-distal domain of the NEC (pUL31-K242), indicating that capsid incorporation might depend on a direct electrostatic interaction. Our follow-up study, described here, however, shows that the positive charge is not relevant but that the overall structure matters.

Patterns of Autologous and Nonautologous Interactions Between Core Nuclear Egress Complex (NEC) Proteins of α-, β- and γ-Herpesviruses

Nuclear egress is a regulated process shared by α-, β- and γ-herpesviruses. The core nuclear egress complex (NEC) is composed of the membrane-anchored protein homologs of human cytomegalovirus (HCMV) pUL50, murine cytomegalovirus (MCMV) pM50, Epstein-Barr virus (EBV) BFRF1 or varicella zoster virus (VZV) Orf24, which interact with the autologous NEC partners pUL53, pM53, BFLF2 or Orf27, respectively. Their recruitment of additional proteins leads to the assembly of a multicomponent NEC, coordinately regulating viral nucleocytoplasmic capsid egress. Here, the functionality of VZV, HCMV, MCMV and EBV core NECs was investigated by coimmunoprecipitation and confocal imaging analyses. Furthermore, a recombinant MCMV, harboring a replacement of ORF M50 by UL50, was analyzed both in vitro and in vivo. In essence, core NEC interactions were strictly limited to autologous NEC pairs and only included one measurable nonautologous interaction between the homologs of HCMV and MCMV. A comparative analysis of MCMV-WT versus MCMV-UL50-infected murine fibroblasts revealed almost identical phenotypes on the levels of protein and genomic replication kinetics. In infected BALB/c mice, virus spread to lung and other organs was found comparable between these viruses, thus stating functional complementarity. In conclusion, our study underlines that herpesviral core NEC proteins are functionally conserved regarding complementarity of core NEC interactions, which were found either virus-specific or restricted within subfamilies.

High-resolution crystal structures of two prototypical β- and γ-herpesviral nuclear egress complexes unravel the determinants of subfamily specificity

Herpesviruses uniquely express two essential nuclear egress-regulating proteins forming a heterodimeric basic structure of the nuclear egress complex (core NEC). These core NECs serve as a hexameric lattice-structured platform for capsid docking and recruit viral and cellular NEC-associated factors that jointly exert nuclear lamina- and membrane-rearranging functions (multicomponent NEC). Here, we report the X-ray structures of β- and γ-herpesvirus core NECs obtained through an innovative recombinant expression strategy based on NEC-hook::NEC-groove protein fusion constructs. This approach yielded the first structure of γ-herpesviral core NEC, namely the 1.56 Å structure of Epstein-Barr virus (EBV) BFRF1-BFLF2, as well as an increased resolution 1.48 Å structure of human cytomegalovirus (HCMV) pUL50-pUL53. Detailed analysis of these structures revealed that the prominent hook segment is absolutely required for core NEC formation and contributes approximately 80% of the interaction surface of the globular domains of NEC proteins. Moreover, using HCMV::EBV hook domain swap constructs, computational prediction of the roles of individual hook residues for binding, and quantitative binding assays with synthetic peptides presenting the HCMV- and EBV-specific NEC hook sequences, we characterized the unique hook-into-groove NEC interaction at various levels. Although the overall physicochemical characteristics of the protein interfaces differ considerably in these β- and γ-herpesvirus NECs, the binding free energy contributions of residues displayed from identical positions are similar. In summary, the results of our study reveal critical details of the molecular mechanism of herpesviral NEC interactions and highlight their potential as an antiviral drug target.

The Novel Nuclear Targeting and BFRF1-Interacting Domains of BFLF2 Are Essential for Efficient Epstein-Barr Virus Virion Release

Epstein-Barr virus (EBV) genomic DNA is replicated and packaged into procapsids in the nucleus to form nucleocapsids, which are then transported into the cytoplasm for tegumentation and final maturation. The process is facilitated by the coordination of the viral nuclear egress complex (NEC), which consists of BFLF2 and BFRF1. By expression alone, BFLF2 is distributed mainly in the nucleus. However, it colocalizes with BFRF1 at the nuclear rim and in cytoplasmic nuclear envelope-derived vesicles in coexpressing cells, suggesting temporal control of the interaction between BFLF2 and BFRF1 is critical for their proper function. The N-terminal sequence of BFLF2 is less conserved than that of alpha- and betaherpesvirus homologs. Here, we found that BFLF2 amino acids (aa) 2 to 102 are required for both nuclear targeting and its interaction with BFRF1. Coimmunoprecipitation and confocal analysis indicated that aa 82 to 106 of BFLF2 are important for its interaction with BFRF1. Three crucial amino acids (R47, K50, and R52) and several noncontinuous arginine and histidine residues within aa 59 to 80 function together as a noncanonical nuclear localization signal (NLS), which can be transferred onto yellow fluorescent protein (YFP)-LacZ for nuclear targeting in an importin β-dependent manner. Virion secretion is defective in 293 cells harboring a BFLF2 knockout EBV bacmid upon lytic induction and is restored by trans-complementation of wild-type BFLF2, but not NLS or BFRF1-interacting defective mutants. In addition, multiple domains of BFRF1 were found to bind BFLF2, suggesting multiple contact regions within BFRF1 and BFLF2 are required for proper nuclear egress of EBV nucleocapsids.IMPORTANCE Although Epstein-Barr virus (EBV) BFRF1 and BFLF2 are homologs of conserved viral nuclear egress complex (NEC) in all human herpesviruses, unique amino acid sequences and functions were identified in both proteins. In this study, the nuclear targeting and BFRF1-interacting domains were found within the N terminus of BFLF2. We showed that amino acids (aa) 82 to 106 are the major region required for BFLF2 to interact with BFRF1. However, the coimmunoprecipitation (Co-IP) data and glutathione transferase (GST) pulldown experiments revealed that multiple regions of both proteins contribute to reciprocal interactions. Different from the canonical nuclear localization signal (NLS) in other herpes viral homologs, BFLF2 contains a novel importin-dependent nuclear localization signal, including R47, K50, and R52 and several neighboring discontinuous arginine and histidine residues. Using a bacmid complementation system, we show that both the nuclear targeting and the novel nuclear localization signal within aa 82 to 106 of BFLF2 are required for virion secretion.

Functional Identification and Characterization of the Nuclear Egress Complex of a Gammaherpesvirus

The herpesvirus nuclear egress complex (NEC) is composed of two viral proteins. They play key roles in mediating the translocation of capsids from the nucleus to the cytoplasm by facilitating the budding of capsids into the perinuclear space (PNS). The NEC of alphaherpesvirus can induce the formation of virion-like vesicles from the nuclear membrane in the absence of other viral proteins. However, whether the NEC of gammaherpesvirus harbors the ability to do so in mammalian cells remains to be determined. In this study, we first constructed open reading frame 67 (ORF67)-null and ORF69-null mutants of murine gammaherpesvirus 68 (MHV-68) and demonstrated that both ORF67 and ORF69 play critical roles in nuclear egress and hence viral lytic replication. Biochemical and bioimaging analyses showed that ORF67 and ORF69 interacted with each other and were sufficient to induce the formation of virion-like vesicles from the nuclear membrane in mammalian cells. Thus, we designated ORF67 and ORF69 components of MHV-68 NEC. Furthermore, we identified amino acids critical for mediating the interaction between ORF67 and ORF69 through homology modeling and verified their function in nuclear egress, providing insights into the molecular basis of NEC formation in gammaherpesviruses.IMPORTANCE Increasing amounts of knowledge indicate that the nuclear egress complex (NEC) is critical for the nuclear egress of herpesvirus capsids, which can be viewed as a vesicle-mediated transport pathway through the nuclear membrane. In this study, we identified open reading frame 67 (ORF67) and ORF69 as components of the NEC in murine gammaherpesvirus 68 (MHV-68) and demonstrated that they efficiently induce virion-like vesicles from the nuclear membrane in mammalian cells. This is the first time that the NEC of a gammaherpesvirus has been found to demonstrate such an essential characteristic. In addition, we identified amino acids critical for mediating the interaction between ORF67 and ORF69 as well as nuclear egress. Notably, these amino acids are conserved in Kaposi's sarcoma-associated herpesvirus (KSHV) and Epstein-Barr virus (EBV), providing a structural basis to design antigammaherpesvirus drugs.

Roles of the Interhexamer Contact Site for Hexagonal Lattice Formation of the Herpes Simplex Virus 1 Nuclear Egress Complex in Viral Primary Envelopment and Replication

During the nuclear export of nascent nucleocapsids of herpes simplex virus 1 (HSV-1), the nucleocapsids acquire a primary envelope by budding through the inner nuclear membrane into the perinuclear space between the inner and outer nuclear membranes. This unique budding process, termed primary envelopment, is initiated by the nuclear egress complex (NEC), composed of the HSV-1 UL31 and UL34 proteins. Earlier biochemical approaches have shown that the NEC has an intrinsic ability to vesiculate membranes through the formation of a hexagonal lattice structure. The significance of intrahexamer interactions of the NEC in the primary envelopment of HSV-1-infected cells has been reported. In contrast, the contribution of lattice formation of the NEC hexamer to primary envelopment in HSV-1-infected cells remains to be elucidated. Therefore, we constructed and characterized a recombinant HSV-1 strain carrying an amino acid substitution in a UL31 residue that is an interhexamer contact site for the lattice formation of the NEC hexamer. This mutation was reported to destabilize the interhexamer interactions of the HSV-1 NEC. Here, we demonstrate that the mutation causes the aberrant accumulation of nucleocapsids in the nucleus and reduces viral replication in Vero and HeLa cells. Thus, the ability of HSV-1 to form the hexagonal lattice structure of the NEC was linked to an increase in primary envelopment and viral replication. Our results suggest that the lattice formation of the NEC hexamer has an important role in HSV-1 replication by regulating primary envelopment.IMPORTANCE The scaffolding proteins of several envelope viruses required for virion assembly form high-order lattice structures. However, information on the significance of their lattice formation in infected cells is limited. Herpesviruses acquire envelopes twice during their viral replication. The first envelop acquisition (primary envelopment) is one of the steps in the vesicle-mediated nucleocytoplasmic transport of nascent nucleocapsids, which is unique in biology. HSV-1 NEC, thought to be conserved in all members of the Herpesviridae family, is critical for primary envelopment and was shown to form a hexagonal lattice structure. Here, we investigated the significance of the interhexamer contact site for hexagonal lattice formation of the NEC in HSV-1-infected cells and present evidence suggesting that the lattice formation of the NEC hexamer has an important role in HSV-1 replication by regulating primary envelopment. Our results provide insights into the mechanisms of the envelopment of herpesviruses and other envelope viruses.

Structural Aspects of Betaherpesvirus-Encoded Proteins

Betaherpesvirus possesses a large genome DNA with a lot of open reading frames, indicating abundance in the variety of viral protein factors. Because the complicated pathogenicity of herpesvirus reflects the combined functions of these factors, analyses of individual proteins are the fundamental steps to comprehensively understand about the viral life cycle and the pathogenicity. In this chapter, structural aspects of the betaherpesvirus-encoded proteins are introduced. Betaherpesvirus-encoded proteins of which structural information is available were summarized and subcategorized into capsid proteins, tegument proteins, nuclear egress complex proteins, envelope glycoproteins, enzymes, and immune-modulating factors. Structure of capsid proteins are analyzed in capsid by electron cryomicroscopy at quasi-atomic resolution. Structural information of teguments is limited, but a recent crystallographic analysis of an essential tegument protein of human herpesvirus 6B is introduced. As for the envelope glycoproteins, crystallographic analysis of glycoprotein gB has been done, revealing the fine-tuned structure and the distribution of its antigenic domains. gH/gL structure of betaherpesvirus is not available yet, but the overall shape and the spatial arrangement of the accessory proteins are analyzed by electron microscopy. Nuclear egress complex was analyzed from the structural perspective in 2015, with the structural analysis of cytomegalovirus UL50/UL53. The category "enzymes" includes the viral protease, DNA polymerase and terminase for which crystallographic analyses have been done. The immune-modulating factors are viral ligands or receptors for immune regulating factors of host immune cells, and their communications with host immune molecules are demonstrated in the aspect of molecular structure.

Conformational Dynamics of Herpesviral NEC Proteins in Different Oligomerization States

All herpesviruses use a heterodimeric nuclear egress complex (NEC) to transport capsids out of host cell nuclei. Despite their overall similar structure, NECs may differ significantly in sequence between different viruses. Up to now, structural information is limited to isolated NEC heterodimers and to large hexagonal lattices made up of hexagonal ring-like structures ("Hexagons"). The present study aimed to expand the existing structural knowledge with information on the dynamics of NECs from different viruses and in different oligomerization states. For this task, comparative molecular dynamics simulations were performed of the free NEC heterodimers from three different viruses (HCMV (human cytomegalovirus), HSV-1 (herpes simplex virus 1), and PRV (pseudorabies virus)). In addition, higher oligomerization states comprising two or six NEC heterodimers were characterized for HCMV and HSV-1. The study revealed that the isolated NEC heterodimers from α- (HSV-1, PRV) and β-herpesviruses (HCMV) differ significantly in their dynamics, which can be attributed to a poorly conserved interface region between the NEC subdomains. These differences become smaller for higher oligomerization states, and both HCMV and HSV-1 individual Hexagons exhibit a common region of enhanced dynamics, which might be of functional relevance for the formation of curved vesicle structures or the recognition of hexameric capsid proteins.

Function of the Nonconserved N-Terminal Domain of Pseudorabies Virus pUL31 in Nuclear Egress

Nuclear egress of herpesvirus capsids is mediated by the conserved nuclear egress complex (NEC), composed of the membrane-anchored pUL34 and its nucleoplasmic interaction partner, pUL31. The recently solved crystal structures of the NECs from different herpesviruses show a high structural similarity, with the pUL34 homologs building a platform recruiting pUL31 to the inner nuclear membrane. Both proteins possess a central globular fold, while the conserved N-terminal portion of pUL31 forms an extension reaching around the core of pUL34. However, the extreme N terminus of the pUL31 homologs, which is highly variable in length and amino acid composition, had to be removed for crystallization. Several pUL31 homologs contain a classical nuclear localization signal (NLS) within this part mediating efficient nuclear import. In addition, membrane-binding activity, blocking premature interaction with pUL34, nucleocapsid trafficking, and regulation of NEC assembly and disassembly via phosphorylation were assigned to the extreme pUL31 N terminus. To test the functional importance in the alphaherpesvirus pseudorabies virus (PrV) pUL31, N-terminal truncations and site-specific mutations were generated, and the resulting proteins were tested for intracellular localization, interaction with pUL34, and functional complementation of PrV-ΔUL31. Our data show that neither the bipartite NLS nor the predicted phosphorylation sites are essential for pUL31 function during nuclear egress. Moreover, nearly the complete variable N-terminal part was dispensable for function as long as a stretch of basic amino acids was retained. Phosphorylation of this domain controls efficient nucleocapsid release from the perinuclear space.IMPORTANCE Nuclear egress of herpesvirus capsids is a unique vesicle-mediated nucleocytoplasmic transport. Crystal structures of the heterodimeric NECs from different herpesviruses provided important details of this viral nuclear membrane deformation and scission machinery but excluded the highly variable N terminus of the pUL31 component. We present here a detailed mutagenesis study of this important portion of pUL31 and show that basic amino acid residues within this domain play an essential role for proper targeting, complex formation, and function during nuclear egress, while phosphorylation modulates efficient release from the perinuclear space. Thus, our data complement previous structure-function assignments of the nucleocapsid-interacting component of the NEC.

WDR5 Facilitates Human Cytomegalovirus Replication by Promoting Capsid Nuclear Egress

WD repeat-containing protein 5 (WDR5) is essential for assembling the VISA-associated complex to induce a type I interferon antiviral response to Sendai virus infection. However, the roles of WDR5 in DNA virus infections are not well described. Here, we report that human cytomegalovirus exploits WDR5 to facilitate capsid nuclear egress. Overexpression of WDR5 in fibroblasts slightly enhanced the infectious virus yield. However, WDR5 knockdown dramatically reduced infectious virus titers with only a small decrease in viral genome replication or gene expression. Further investigation of late steps of viral replication found that WDR5 knockdown significantly impaired formation of the viral nuclear egress complex and induced substantially fewer infoldings of the inner nuclear membrane. In addition, fewer capsids were associated with these infoldings, and there were fewer capsids in the cytoplasm. Restoration of WDR5 partially reversed these effects. These results suggest that WDR5 knockdown impairs the nuclear egress of capsids, which in turn decreases virus titers. These findings reveal an important role for a host factor whose function(s) is usurped by a viral pathogen to promote efficient replication. Thus, WDR5 represents an interesting regulatory mechanism and a potential antiviral target.IMPORTANCE Human cytomegalovirus (HCMV) has a large (∼235-kb) genome with over 170 open reading frames and exploits numerous cellular factors to facilitate its replication. HCMV infection increases protein levels of WD repeat-containing protein 5 (WDR5) during infection, overexpression of WDR5 enhances viral replication, and knockdown of WDR5 dramatically attenuates viral replication. Our results indicate that WDR5 promotes the nuclear egress of viral capsids, the depletion of WDR5 resulting in a significant decrease in production of infectious virions. This is the first report that WDR5 favors HCMV, a DNA virus, replication and highlights a novel target for antiviral therapy.

Improving nuclear envelope dynamics by EBV BFRF1 facilitates intranuclear component clearance through autophagy

Although a vesicular nucleocytoplasmic transport system is believed to exist in eukaryotic cells, the features of this pathway are mostly unknown. Here, we report that the BFRF1 protein of the Epstein-Barr virus improves vesicular transport of nuclear envelope (NE) to facilitate the translocation and clearance of nuclear components. BFRF1 expression induces vesicles that selectively transport nuclear components to the cytoplasm. With the use of aggregation-prone proteins as tools, we found that aggregated nuclear proteins are dispersed when these BFRF1-induced vesicles are formed. BFRF1-containing vesicles engulf the NE-associated aggregates, exit through from the NE, and putatively fuse with autophagic vacuoles. Chemical treatment and genetic ablation of autophagy-related factors indicate that autophagosome formation and autophagy-linked FYVE protein-mediated autophagic proteolysis are involved in this selective clearance of nuclear proteins. Remarkably, vesicular transport, elicited by BFRF1, also attenuated nuclear aggregates accumulated in neuroblastoma cells. Accordingly, induction of NE-derived vesicles by BFRF1 facilitates nuclear protein translocation and clearance, suggesting that autophagy-coupled transport of nucleus-derived vesicles can be elicited for nuclear component catabolism in mammalian cells.-Liu, G.-T., Kung, H.-N., Chen, C.-K., Huang, C., Wang, Y.-L., Yu, C.-P., Lee, C.-P. Improving nuclear envelope dynamics by EBV BFRF1 facilitates intranuclear component clearance through autophagy.

Human Cytomegalovirus Nuclear Capsids Associate with the Core Nuclear Egress Complex and the Viral Protein Kinase pUL97

The nuclear phase of herpesvirus replication is regulated through the formation of regulatory multi-component protein complexes. Viral genomic replication is followed by nuclear capsid assembly, DNA encapsidation and nuclear egress. The latter has been studied intensely pointing to the formation of a viral core nuclear egress complex (NEC) that recruits a multimeric assembly of viral and cellular factors for the reorganization of the nuclear envelope. To date, the mechanism of the association of human cytomegalovirus (HCMV) capsids with the NEC, which in turn initiates the specific steps of nuclear capsid budding, remains undefined. Here, we provide electron microscopy-based data demonstrating the association of both nuclear capsids and NEC proteins at nuclear lamina budding sites. Specifically, immunogold labelling of the core NEC constituent pUL53 and NEC-associated viral kinase pUL97 suggested an intranuclear NEC-capsid interaction. Staining patterns with phospho-specific lamin A/C antibodies are compatible with earlier postulates of targeted capsid egress at lamina-depleted areas. Important data were provided by co-immunoprecipitation and in vitro kinase analyses using lysates from HCMV-infected cells, nuclear fractions, or infectious virions. Data strongly suggest that nuclear capsids interact with pUL53 and pUL97. Combined, the findings support a refined concept of HCMV nuclear trafficking and NEC-capsid interaction.

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

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

Lysine 242 within Helix 10 of the Pseudorabies Virus Nuclear Egress Complex pUL31 Component Is Critical for Primary Envelopment of Nucleocapsids

Newly assembled herpesvirus nucleocapsids are translocated from the nucleus to the cytosol by a vesicle-mediated process engaging the nuclear membranes. This transport is governed by the conserved nuclear egress complex (NEC), consisting of the alphaherpesviral pUL34 and pUL31 homologs. The NEC is not only required for efficient nuclear egress but also sufficient for vesicle formation from the inner nuclear membrane (INM), as well as from synthetic lipid bilayers. The recently solved crystal structures for the NECs from different herpesviruses revealed molecular details of this membrane deformation and scission machinery uncovering the interfaces involved in complex and coat formation. However, the interaction domain with the nucleocapsid remained undefined. Since the NEC assembles a curved hexagonal coat on the nucleoplasmic side of the INM consisting of tightly interwoven pUL31/pUL34 heterodimers arranged in hexamers, only the membrane-distal end of the NEC formed by pUL31 residues appears to be accessible for interaction with the nucleocapsid cargo. To identify the amino acids involved in capsid incorporation, we mutated the corresponding regions in the alphaherpesvirus pseudorabies virus (PrV). Site-specifically mutated pUL31 homologs were tested for localization, interaction with pUL34, and complementation of PrV-ΔUL31. We identified a conserved lysine residue at amino acid position 242 in PrV pUL31 located in the alpha-helical domain H10 exposed on the membrane-distal end of the NEC as a key residue for nucleocapsid incorporation into the nascent primary particle.IMPORTANCE Vesicular transport through the nuclear envelope is a focus of research but is still not well understood. Herpesviruses pioneered this mechanism for translocation of the newly assembled nucleocapsid from the nucleus into the cytosol via vesicles derived from the inner nuclear membrane which fuse in a well-tuned process with the outer nuclear membrane to release their content. The structure of the viral nuclear membrane budding and scission machinery has been solved recently, providing in-depth molecular details. However, how cargo is incorporated remained unclear. We identified a conserved lysine residue in the membrane-distal portion of the nuclear egress complex required for capsid uptake into inner nuclear membrane-derived vesicles.

Dynamic regulatory interaction between cytomegalovirus major tegument protein pp65 and protein kinase pUL97 in intracellular compartments, dense bodies and virions

Human cytomegalovirus (HCMV) is a ubiquitous pathogen of considerable clinical importance. Understanding the processes that are important for viral replication is essential for the development of therapeutic strategies against HCMV infection. The HCMV-encoded protein kinase pUL97 is an important multifunctional regulator of viral replication. Several viral and cellular proteins are phosphorylated by pUL97. The phosphoprotein pp65 is one important substrate of pUL97. It is the most abundant tegument protein of HCMV virions, mediating the upload of other virion constituents and contributing to particle integrity. Further to that, it interferes with host innate immune defences, thereby enabling efficient viral replication. By applying different approaches, we characterized the pp65-pUL97 interaction in various compartments. Specifically, the pUL97 interaction domain of pp65 was defined (282-415). A putative cyclin bridge that enhances pUL97-pp65 interaction was identified. The impact of pUL97 mutation on virion and dense body morphogenesis was addressed using pUL97 mutant viruses. Alterations in the proteome of viral particles were seen, especially with mutant viruses expressing cytoplasmic variants of pUL97. On the basis of these data we postulate a so far poorly recognized functional relationship between pp65 and pUL97, and present a refined model of pp65-pUL97 interaction.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

The Primary Enveloped Virion of Herpes Simplex Virus 1: Its Role in Nuclear Egress

Many viruses migrate between different cellular compartments for successive stages of assembly. The HSV-1 capsid assembles in the nucleus and then transfers into the cytoplasm. First, the capsid buds through the inner nuclear membrane, becoming coated with nuclear egress complex (NEC) protein. This yields a primary enveloped virion (PEV) whose envelope fuses with the outer nuclear membrane, releasing the capsid into the cytoplasm. We investigated the associated molecular mechanisms by isolating PEVs from US3-null-infected cells and imaging them by cryo-electron microscopy and tomography. (pUS3 is a viral protein kinase in whose absence PEVs accumulate in the perinuclear space.) Unlike mature extracellular virions, PEVs have very few glycoprotein spikes. PEVs are ~20% smaller than mature virions, and the little space available between the capsid and the NEC layer suggests that most tegument proteins are acquired later in the egress pathway. Previous studies have proposed that NEC is organized as hexamers in honeycomb arrays in PEVs, but we find arrays of heptameric rings in extracts from US3-null-infected cells. In a PEV, NEC contacts the capsid predominantly via the pUL17/pUL25 complexes which are located close to the capsid vertices. Finally, the NEC layer dissociates from the capsid as it leaves the nucleus, possibly in response to pUS3-mediated phosphorylation. Overall, nuclear egress emerges as a process driven by a program of multiple weak interactions.IMPORTANCE On its maturation pathway, the newly formed HSV-1 nucleocapsid must traverse the nuclear envelope, while respecting the integrity of that barrier. Nucleocapsids (125 nm in diameter) are too large to pass through the nuclear pore complexes that conduct most nucleocytoplasmic traffic. It is now widely accepted that the process involves envelopment/de-envelopment of a key intermediate-the primary enveloped virion. In wild-type infections, PEVs are short-lived, which has impeded study. Using a mutant that accumulates PEVs in the perinuclear space, we were able to isolate PEVs in sufficient quantity for structural analysis by cryo-electron microscopy and tomography. The findings not only elucidate the maturation pathway of an important human pathogen but also have implications for cellular processes that involve the trafficking of large macromolecular complexes.

Getting to and through the inner nuclear membrane during herpesvirus nuclear egress

Herpesviruses, like most DNA viruses, replicate and package their genomes into capsids in the host cell nucleus. Capsids then transit to the cytoplasm in a fascinating process called nuclear egress, which includes several unusual steps: Movement of capsids from the nuclear interior to the periphery, disruption of the nuclear lamina, capsid budding through the inner nuclear membrane, and fusion of enveloped particles with the outer nuclear membrane. Here, we review recent advances and emerging questions relating to herpesvirus nuclear egress, emphasizing controversies regarding mechanisms for capsid trafficking to the nuclear periphery, and implications of recent structures of the two-subunit, viral nuclear egress complex for the process, particularly at the step of budding through the inner nuclear membrane.

Herpesvirus Nuclear Egress

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

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

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