herpes viruses remodel host membranes for virus egress. johnson dc, baines jd. nature rev 2011

The three subfamilies of herpesviruses, alphaherpesviruses (including herpes simplex virus (HSV) and varicella zoster virus (VZV)), betaherpesviruses (including human cytomegalovirus (HCMV)) and gammaherpesviruses (including Epstein–Barr virus (EBV) and Kaposi’s sarcomaassociated herpes virus (KSHV)), share many strategies for replication. Importantly, all herpesviruses establish lifelong latent infections. Primary infection of the host normally begins with entry into epithelial cells, such as mucosal cells in the oropharynx or genitalia, followed by spread to numerous other cell types. Virus entry into host cells involves viral proteins, including glycoprotein B (gB), gD and gH–gL, that bind to cell surface molecules, including heparan sulphate glycos aminoglycans and other specific receptors, and then produce fusion between the virion lipid envelope and the cellular membrane1,2. This fusion delivers the viral nucleo capsids into the cytoplasm. The capsids latch onto micro tubules, which allow the capsids to move to the nuclear envelope, where they bind to nuclear pores and inject the viral DNA into the nucleoplasm. The large herpesvirus genomes, ranging from the 125 kb VZV genome to the 235 kb HCMV genome (which encodes over 200 proteins), can be immediately transcribed to produce early mRNAs, which are translated to produce viral proteins that promote further transcription and replication of the viral DNA. During a productive infection, viral DNAs are replicated in the nucleus through the actions of viral polymerases and other viral replicative machinery. Newly synthesized viral DNAs are used as templates for late gene transcription, and late mRNAs

produce viral structural proteins. Late in the infection, replicated DNAs are also packaged into capsids in the nucleoplasm. This assembly of viral nucleocapsids is a fascinating process, involving viral proteins that recognize and cleave packaging signals at genomic termini, dock at a unique portal located at one vertex of a preformed capsid and pump the DNA inside with enough force to result in packaged DNA of liquid crystalline density.

Once herpesvirus genomes are packaged into capsids, there begins the tortuous egress pathway, by which capsids move from the nucleus to extracellular spaces (FIG. 1), and it is this pathway that we focus on in this Review. Herpesvirus egress is best understood for the alphaherpesviruses4,5, but betaherpesviruses and gamma herpesviruses appear to use similar mechanisms, although certain distinctions are emerging6–8. During egress, all herpesviruses face several obstacles, which they solve by extensively restructuring host membranes. The first major barrier is the nuclear envelope. The viral nucleocapids in the nucleus (FIG. 2a) are too large, at 125 nm, to pass across nuclear pores, and nuclear pores are not grossly perturbed by viral movement across the nuclear envelope9. Instead, herpesviruses dissolve the nuclear lamina, a dense meshwork underlying the inner nuclear membrane (INM), and then become enveloped at the INM, so that enveloped particles bud into the perinuclear space (FIGS 1,2b). This process is known as primary envelopment. Perinuclear virus particles (FIG. 2c) with an envelope containing viral glycoproteins (FIG. 1) undergo deenvelopment, which involves membrane fusion between the virion envelope

*Molecular Microbiology & Immunology, Oregon Health & Science University, Portland, Oregon 97219, USA.‡Microbiology and Immunology, New York College of Veterinary Medicine, Cornell University, Ithaca, New York 14853, USA.Correspondence to J.D.B. e‑mail: [email protected]:10.1038/nrmicro2559

Herpesviruses remodel host membranes for virus egressDavid C. Johnson* and Joel D. Baines‡

Abstract | Herpesviruses replicate their DNA and package this DNA into capsids in the nucleus. These capsids then face substantial obstacles to their release from cells. Unlike other DNA viruses, herpesviruses do not depend on disruption of nuclear and cytoplasmic membranes for their release. Enveloped particles are formed by budding through inner nuclear membranes, and then these perinuclear enveloped particles fuse with outer nuclear membranes. Unenveloped capsids in the cytoplasm are decorated with tegument proteins and then undergo secondary envelopment by budding into trans-Golgi network membranes, producing infectious particles that are released. In this Review, we describe the remodelling of host membranes that facilitates herpesvirus egress.

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Nature Reviews | Microbiology

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and the outer nuclear membrane (ONM). This delivers capsids into the cytoplasm, where they are coated with a layer of 16–35 proteins that form the tegument, which ultimately fills the space between the capsid and the virion envelope. These tegument proteins promote the earliest stages of herpesvirus replication, including shutting off of host protein synthesis and transactivation of early viral genes. Tegumentcoated capsids then undergo secondary envelopment in the cytoplasm by budding into cytoplasmic membranes such as the Golgi, the trans Golgi network (TGN) and endosomes (FIGS 1,2d,e), producing mature, infectious virions inside a cellular vesicle. For simplicity, the TGN is considered to be the site of secondary envelopment in this Review. Cytoplasmic vesicles carrying enveloped virions are then transported to the cell surface, where they fuse with the plasma membrane to release the virions (FIG. 2f). This pathway of primary envelopment followed by deenvelopment and subsequent reenvelopment is supported by substantial genetic and biochemical data, although other models remain a possibility (reviewed in REFS 4,5).

The foregoing description of egress does not consider the fact that all herpesviruses replicate at some stage in polarized cells. Epithelial cells serve as portals for virus entry, as well as egress of the virus into saliva, milk or urine for spread to other hosts. Moreover, alphaherpesviruses infect neurons, the host target cells in which latency is established. Enveloped virions produced in the TGN can be packaged into transport vesicles that are sorted to specific cell surfaces in these polarized cells, and this can promote celltocell spread across epithelial cell junctions10 or transport into neuronal axons for spread to peripheral tissues11. In this Review, we describe pathways by which herpesviruses cross nuclear and cytoplasmic membranes for release from cells, and discuss how progeny virions can be sorted towards specific cell surfaces in order to promote virus spread.

Primary envelopment The first step in nuclear egress is primary envelopment, in which capsids are wrapped in the INM4,5 (FIGS 2b,3). Primary envelopment of HSV requires a nuclear envelopment complex (NEC) containing the viral proteins pUL31 and pUL34 (REFS 12–14); orthologues of HSV pUL31 and pUL34 include proteins encoded by the UL53 and UL51 genes in betaherpesviruses and the BFLF2 and BFRF1 genes in EBV. pUL31 is a nuclear phosphoprotein that is held in close apposition to the inner surface of the INM through an interaction with pUL34 (REFS. 15,16) (FIG. 3). pUL34 and its orthologues are type II integral membrane proteins that are targeted to the INM and have only five residues extending into the endoplasmic reticulum (ER) lumen or perinuclear space17,18. Both pUL31 and pUL34 are incorporated into perinuclear virus particles16,19, suggesting that these proteins interact directly or indirectly with the capsid surface. Remarkably, in some cell types the expression of the pUL31 and pUL34 orthologues from pseudorabies virus (PRV) is sufficient to generate perinuclear vesicles with a diameter similar to that of virions without expression of other viral proteins20. Thus, the NEC is sufficient for budding from the INM, even in the absence of capsids. However, these enveloped particles lacking capsids are rare, implying that capsids trigger budding at the INM by a process that involves interactions between the NEC and the capsid21, and possibly other viral proteins.

Crossing the nuclear lamina. To engage the INM, capsids must bypass the nuclear lamina, a dense meshwork of type V microfilaments that lines the inner surface of the INM to form a major structural component of the nucleus22. The lamina is composed of three lamin proteins: lamin A, lamin B and lamin C. Lamin A and lamin C are produced from alternatively spliced mRNAs of the LMNA gene, whereas the two lamin B proteins, lamin B1 and lamin B2, are each encoded by different

Figure 1 | Overview of the egress pathway of herpesviruses. After capsids are formed in the nucleus, they bud into the inner nuclear membrane (INM) (primary envelopment) to form an enveloped particle in the perinuclear space. These particles fuse with the outer nuclear membrane (ONM) (de-envelopment) and are released into the cytoplasm, leaving the envelope in the ONM. In the cytosol, capsids bind onto and bud into cytoplasmic membranes (secondary envelopment), and enveloped virions are secreted from cells (release). TGN, trans-Golgi network.

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genes. The nuclear lamina is maintained by interactions between lamins and several lamin receptors, which are integral membrane proteins. During mitosis and apoptosis, the lamina and associated nuclear membrane are disassembled. Disassembly during mitosis correlates with the phosphorylation of lamins by cellular kinases such as cell division cycle 2 (CDC2; also known as CDK1). During telophase, lamin B helps mediate reassembly of the nuclear membrane by promoting fusion of lamin Bladen vesicles in a process that correlates with lamin B dephosphorylation. During apoptosis, disassembly of the lamins involves their phosphorylation by protein kinase Cδ (PKCδ), followed by caspasemediated degradation.

The nuclear lamina is disrupted locally in a pUL31 and pUL34dependent manner through multiple mechanisms during HSV infection23–26. First, the virus manipulates several types of PKC molecules. The novel

PKCδ and the conventional PKCα are recruited to the nuclear membrane in a NECdependent manner27. Lamin B is phosphorylated by one or both of these kinases, possibly leading to thinning of the lamina to promote capsid budding28. Consistent with this hypothesis, blocking the conventional and novel PKCs impairs nuclear egress of capsids28. Conventional and novel PKCs are also recruited to the nuclear membrane in cells infected with murine cytomegalovirus, probably for similar purposes29. Second, the alphaherpesvirus kinase pUS3 can phosphorylate lamin A and lamin C at multiple sites26. Purified pUS3 can render lamins soluble when added to permeabilized nuclei, and this solubilization requires the pUS3 kinase activity26. Third, the genes UL13, UL97 and BGLF4 (found in alphaherpesviruses, betaherpesviruses and gammaherpesviruses, respectively) encode the conserved herpesviral protein kinases (CHPKs)30,31. The CHPKs of HCMV and EBV are critical for nuclear egress32,33, although this is not the case for alphaherpesviruses that also encode pUS3 kinases. The CHPKs of HSV2, HCMV and EBV phosphorylate lamins, leading to redistribution of nuclear lamina components in the case of HCMV and EBV, and conformational alteration of nuclear lamina components in the case of HSV2 (REFS 30,34–36). Moreover, when phosphorylation of lamin A and lamin C by BGLF4 is prevented by a mutation in the lamins, nuclear egress of capsids is impeded30, and knockdown of lamin A and lamin C partially rescues BGLF4null mutants of HSV2 and EBV34. The CHPKs may disrupt the nuclear lamina by other mechanisms as well. For example, HCMV pUL97 induces a novel binding site for the host peptidylprolyl cis–trans isomerase PIN1 in lamin A and lamin C, leading to PIN1 recruitment to the nuclear rim37. The activity of PIN1 might then further alter the conformation of the lamina. Third, binding of pUL31 and pUL34 to lamin A and lamin C may compete with lamin–lamin inter actions to locally perforate the lamina23,38. In relation to this, overexpression of pUL31 can disrupt the nuclear lamina, having similar effects to overexpression of dominantnegative forms of lamins23,39. Moreover, perforations in the lamina are highly coincident with local high concentrations of pUL31 and pUL34 (REF. 25,26). Fourth, phosphorylation of lamin receptors by viral or cellular kinases may decrease the affinity of lamins for their receptors, such as emerin, helping to sever the connections between lamins and the INM40,41.

Selecting mature capsids. Mature capsids containing viral DNA — known as C capsids — are preferentially enveloped at the INM at the expense of capsids lacking DNA42. This selection probably involves surface changes on the capsid that are recognized directly or indirectly by the NEC. A complex of pUL17 and pUL25, termed the C capsidspecific complex (CCSC), could be the structure involved, because the CCSC is highly enriched in C capsids and is located on the capsid surface43,44. The CCSC links the pentons, which are the 12 fivefoldsymmetrical vertices, to adjacent hexons that comprise the 20 planar faces44. pUL17 and pUL25 promote insertion and retention of viral DNA, respectively45,46; capsids lacking pUL17

Figure 2 | Electron micrographs of the steps of herpesvirus egress. a | Capsids in the nucleus. b | Primary envelopment, showing the close apposition of the capsid and the inner nuclear membrane (INM). c | Enveloped capsids present within the perinuclear space. The membrane surrounding the capsid is derived from the INM. d | Initial steps of secondary envelopment. The unenveloped capsids in the cytoplasm interact with trans-Golgi network (TGN) membranes and become wrapped in these membranes (white arrow). e | Final steps of secondary envelopment. Enveloped particles (white arrow) are present within the lumen of TGN-derived membranes. f | Release of virions. Enveloped virions are transported to cell surfaces and released or remain bound to the plasma membrane. ONM, outer nuclear membrane.

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or pUL25 are retained in the nucleus45–47. Moreover, recent observations suggest that pUL25 in the CCSC and pUL31 in the NEC interact in infected cells, and that pUL25 is required for the association of pUL31 with capsids (J.D.B. and K. Yang, unpublished observations). Together, these observations suggest that the interaction of pUL25 with pUL31 helps select DNAcontaining capsids for envelopment.

Role of tegument proteins in primary envelopment. Perinuclear HSV particles also contain the tegument proteins VP22 (encoded by UL49), virion host shutoff (vhs; encoded by UL41), VP16 (encoded by UL48), pUL11 and the kinase pUS3 (REFS 16,48–51). None of these tegument proteins is essential for primary envelopment, and the tegument of perinuclear HSV and PRV particles is considerably less dense than that of mature HSV and PRV virions52,53, suggesting that the bulk of the tegument is added to capsids in the cytoplasm.

Glycoproteins in primary envelopment. HSV glycoproteins gM, gB, gH–gL and gD are found in the INM and in perinuclear virus particles54–57. Moreover, the NEC promotes recruitment of gM and gD to the INM58. The portion of the HSV1 gD protein that is in the cytosol or nucleus interacts with pUL34, and this interaction might have a role in the recruitment of gD to, or its retention in, the INM58. Moreover, it is possible that herpesvirus glycoproteins play a part in anchoring the INM onto the capsid during primary envelopment, perhaps in a redundant manner, as is the case in secondary envelopment (see below). However, there is currently no evidence for a role for glycoproteins in primary envelopment. Certainly, gB and gH must be incorporated into perinuclear virus particles, because the proteins are important

for deenvelopment (see below). Unlike the glycoproteins of HSV, those of PRV were not detected in the INM and perinuclear particles59.

De-envelopmentIn the second stage of nuclear egress, called de envelopment, enveloped particles in the perinuclear space (FIG. 2c) fuse with the ONM, delivering capsids into the cytoplasm (FIG. 4). pUL31 and pUL34 are associated with virus particles in the perinuclear space, but pUL34 is not detected in cytoplasmic particles by immuno electron microscopy, suggesting that the protein remains in the ONM after deenvelopment16,60. These observations, and the results of experiments that showed that HSV glycoproteins are retained in the nuclear ER membranes61, provide important support for the envelopment–deenvelopment–reenvelopment pathway that is depicted in FIG. 1.

Glycoproteins and de-envelopment. During de envelopment, the virion envelope, which is studded with viral glycoproteins, fuses with the ONM (FIG. 4). In an apparently similar process, herpesvirus entry into cells involves fusion between the virion envelope and either the plasma membrane or endosomes. Three HSV glycoproteins, gB, gD and the heterodimer gH–gL, are required for fusion during entry2. These glycoproteins are also found in perinuclear particles, as well as in the INM and ONM54,56,57. For all of the herpesviruses that have been investigated thus far, the core entry fusion machinery includes gB and gH–gL homologues. HSV gB shows structural similarity to vesicular stomatitis virus G protein, which mediates fusion during entry2, and gB can insert directly into liposomes62. Evidence that HSV gB and gH–gL can also participate in deenvelopment came from studies showing that enveloped virus particles accumulated in large numbers in perinuclear spaces during infection with an HSV mutant lacking both gB and gH56. Alternatively, HSV lacking gB and gH formed herniations (membrane vesicles derived from the INM and protruding into the nucleoplasm) in some cells. Similar herniations were also observed in cells infected with HSV mutants lacking US3 (REFS 16,63). However, HSV lacking just gB displayed more minor defects in nuclear egress. By contrast, fusion during HSV entry requires both gB and gH–gL. Another important distinction between entry and deenvelopment is that deenvelopment fusion involves two membranes (the virion envelope and the ONM) that both contain viral glycoproteins (FIG. 4), whereas entry fusion involves only one membrane (the virion envelope) that contains glycoproteins. gB apparently acts directly in deenvelopment fusion, rather than at some upstream step, because mutant forms of gB with alterations in the socalled ‘fusion loops’ (internal bipartite domains composed of hydrophobic and hydrophilic residues that are thought to interact with membranes to promote fusion62) did not support deenvelopment64. gBmediated fusion with the ONM may also be important for other herpes viruses, as EBV and KSHV mutants lacking gB also exhibit defects in egress or no perinuclear particles65,66.

Figure 3 | Primary envelopment. The herpes simplex virus (HSV) nuclear envelopment complex (NEC), including pUL31 and pUL34, induces a thinning of the nuclear lamina. The NEC additionally recruits other viral proteins that are incorporated into the enveloping particle. Viral proteins pUL17 and pUL25 form a complex on the surface of the capsid that is important in selecting mature, DNA-filled capsids for primary envelopment. Part of the tegument is already bound to the capsid, including pUS3, which has an important role in later stages of egress. INM, inner nuclear membrane.

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Regulation of gB de-envelopment fusion. It seems highly probable that the fusogenic activities of HSV gB and gH–gL are tightly regulated inside cells. Promiscuous fusion of cytoplasmic or nuclear membranes with each other would rapidly homogenize cellular membranes, thereby degrading cellular architecture and reducing viral replication. During HSV entry, gB and gH–gL are triggered for fusion by a third glycoprotein, gD, which binds gD receptors2. Although there is currently no evidence implicating gD in deenvelopment fusion, there is evidence for other mechanisms by which gB is triggered for deenvelopment fusion. The cytoplasmic domain of gB, which is located within the tegument layer of virus particles, can regulate the fusogenic activity of gB67. HSV pUS3, a component of the tegument, phosphorylates threonine 887 in the gB cytoplasmic domain; mutation of this threonine impairs pUS3mediated gB phosphorylation and gBmediated deenvelopment at the ONM68. Thus, pUS3 and the cytoplasmic domain of gB are brought together in the tegument of perinuclear particles, and pUS3mediated phosphorylation of the gB cytoplasmic domain may activate gB for deenvelopment fusion. pUS3 also phosphorylates pUL31, and mutations that impair this phosphorylation reduce deenvelopment and enhance herniations, whereas pseudophosphorylation of pUL31 precludes herniations69. Therefore, pUS3 might act by several mechanisms to promote deenvelopment.

Alternative mechanisms for de-envelopment. Herpesviruses probably transit across nuclear membranes by alternative mechanisms that do not require gB and gH–gL. Deletion of HSV gB and gH increases the number of enveloped HSV particles in the perinuclear space and in herniations (by 20–50fold compared with the number for wildtype HSV), but there are still considerable numbers of enveloped particles found on cell surfaces (decreased by threefold to fivefold as compared with the number for wildtype HSV)56,68. Other HSV membrane proteins, including gD and gM, might promote fusion

with the ONM, although a recently constructed mutant lacking the gB and gD proteins displayed no defects in nuclear egress147. In addition, PRV double mutants lacking gB and gH, gD and gH, or gD and gB are not blocked in nuclear egress59, suggesting that there are alternative mechanisms of deenvelopment that do not involve glycoproteins used during entry. The NEC might also promote deenvelopment, as expression of PRV pUL31 and pUL34 in host cells produces enveloped perinuclear vesicles, some of which fuse with the ONM20. However, it is difficult to understand how pUL31 and pUL34 could direct deenvelopment fusion by themselves, as pUL31 does not extend from the virion surface and pUL34 extends only five amino acids from the surface. Host proteins, including ESCRT (endosomal sorting complex required for transport) proteins or others, might also be involved in deenvelopment, perhaps affecting the curvature of the ONM or pinching off membranes as the virus buds into the perinuclear space.

Negative regulation of de-envelopment. Two other HSV membrane proteins, pUL20 and gK, seem to negatively regulate fusion events at the ONM and at cytoplasmic membranes. Initial interest in HSV gK focused on substitution mutations that produced the syncytial phenotype, in which infected cells aberrantly fuse with one another70,71. Similarly, an HSV UL20 mutant also lacking the UL20.5 gene induced aberrant cell–cell fusion and accumulated enveloped particles in the peri nuclear space72. However, a different UL20 mutant (which retained an intact UL20.5 gene) primarily accumulated as cytoplasmic, unenveloped capsids73. Deletions affecting gK also produced defects in nuclear and cytoplasmic egress74,75. gK is primarily localized to ER and nuclear membranes76 and may not be present in extracellular virions77. Importantly, overexpression of gK in virusinfected cells greatly increased the number of enveloped perinuclear particles compared with the number in infected wildtype cells74. These results are consistent with a model in which gK and pUL20 inhibit fusion with

Figure 4 | De-envelopment. After primary envelopment, herpes simplex virus (HSV) particles, with an envelope that contains viral glycoproteins and pUL31 and pUL34, reside in the perinuclear space. HSV glycoproteins gB and gH–gL mediate fusion of the viral envelope with the outer nuclear membrane (ONM), delivering unenveloped capsids into the cytoplasm and leaving pUL31 and pUL34 behind in the ONM.

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both nuclear and cytoplasmic membranes. It was also suggested that pUL20 and gK do not inhibit fusion but instead promote secondary envelopment75. However, recent evidence that HSV gK and pUL20 interact with the fusion protein gB78 is more consistent with the hypothesis that gK and pUL20 act to negatively regulate premature fusion of nuclear and cytoplasmic membranes. The VP16 and pUL51 tegument proteins might also positively affect deenvelopment, either directly or indirectly, as HSV mutants lacking VP16 or pUL51 accumulate enveloped particles in the perinuclear space79,80.

Assembly of tegument on capsidsThe tegument layer of herpesvirus particles is complex. Extracellular HSV virions contain at least 23 different proteins, ranging in abundance from 200 copies to 2,000 copies77. HCMV has a larger tegument layer (with an increased diameter compared with that of HSV), including 50% of the virion mass and with twice as much of the major tegument protein pp65 compared with the major capsid protein81. The tegument can be divided into inner and outer layers based on whether proteins are intimately associated with capsid surfaces4,5. In HSV and HCMV virions, inner tegument proteins form filamentous, highly ordered structures primarily through interactions with the pentonic vertices of the capsid82,83. Inner tegument proteins are defined as proteins that pellet with capsids after extraction with nonionic detergents. For example, HSV VP12 (the product of the UL36 gene) and its partner pUL37 bind directly to capsids84,85, and virus particles from which most tegument proteins were extracted contained fibres made up of VP12 and pUL37 emanating from the penton vertices86. Moreover, VP12 and pUL37 remain attached to capsids following virus entry into cells, unlike other tegument proteins87, providing further evidence for direct and stable binding to capsid surfaces. An interaction between the carboxyl terminus of HSV VP12 and the capsid protein pUL25 is consistent with these observations44,88. Furthermore, the microtubulebased transport of nascent capsids during virus egress requires PRV VP12 (REF. 87), suggesting that VP12 might participate directly in binding microtubule motors. On the other hand, loss of VP12 could potentially reduce binding of other tegument proteins that directly interact with microtubules. In contrast to VP12 and pUL37, the vast majority of other tegument proteins (and all 21 of the 23 HSV proteins) might be considered outer tegument proteins, as many probably do not contact capsid surfaces directly and are relatively less ordered in structure82,83.

During egress, the tegument proteins are added to the virion in several locations. Herpesvirus capsids acquire a subset of the tegument proteins in the nucleus, including VP12, pUL37, vhs, VP22 and VP16 for HSV, and, for HCMV, the abundant tegument phosphoprotein pp150 (REFS 4,5). Additional quantities of these tegument proteins are added in the cytoplasm. For example, VP12, pUL37 and VP22 are added onto capsids in the nucleus but are also found on the cytoplasmic surfaces of TGN membranes and are added onto capsids during reenvelopment in this organelle as well89,90. Other

tegument proteins are found exclusively in the cytoplasm and can be added to capsids either as they are transported from nuclear membranes to the TGN by microtubule motors87 or after the capsids reach the TGN membranes91. Extensive and redundant protein–protein interactions serve to assemble layers of tegument proteins onto capsids or onto the cytoplasmic surfaces of TGN membranes. Subsets of these tegument proteins then promote bridging of capsids onto TGN membranes, followed by budding, which is the wrapping of the membrane around the capsid.

One example of outer tegument assembly is the interaction of the HSV and PRV tegument proteins VP22, VP16 and vhs with each other and with membraneanchored glycoproteins (FIG. 5). These interactions affect the assembly of the tegument proteins into nascent virions, alter the functions of these proteins, and promote envelopment of the capsid. Mutants lacking VP22 assemble and release 105–106 fewer infectious extracellular particles than wildtype viruses, and incorporate less ICP0, gD and heteromeric glycoprotein gE–gI into virions92,93. The reduced incorporation of HSV gD and gE–gI into virions lacking VP22 is especially revealing because HSV gD and gE–gI are essential for secondary envelopment (see below). VP22 localizes to the cytoplasmic membranes of infected cells, and gE–gI apparently contributes to this localization, although VP22 expressed without other HSV proteins can also accumulate in perinuclear structures, consistent with localization in the TGN90,92,94,95. HSV VP22 also interacts with VP16 (REF. 96), which is both a transcription factor and a major structural component of tegument. HSV and PRV mutants lacking VP16 exhibit major defects in virus egress, as they accumulate unenveloped capsids in the cytoplasm and enveloped particles in the perinuclear space19,79. VP16, in turn, interacts with the virion host shut off protein vhs97. vhs is incorporated into tegument and has the capacity to degrade cellular mRNAs when virus particles enter host cells98. During the late stages of infection, VP16 silences vhs activity, allowing accumulation of late viral mRNAs97,99. A mutant form of vhs that cannot bind VP16 displays reduced incorporation into virions98, supporting a role for VP16 in vhs packaging. Both VP22 and VP16 stabilize vhs in transfected cells, but vhs binds directly to only VP16 (REF. 100). Therefore, a complex of VP22, VP16 and vhs forms in association with capsids and TGN membranes (FIG. 5). Bonds between these proteins not only ensure that all the components of the complex are incorporated into virions, but also apparently help drive secondary envelopment by promoting linkages between the surfaces of capsids (perhaps containing VP16 and vhs) and membranes that contain VP22 and membrane glycoproteins.

A second example of tegument assembly on capsids and on membranes involves the HSV and PRV proteins pUL11, pUL16 and pUL21. pUL11 homologues are present in all herpesviruses. HSV pUL11 is a small (96 amino acid) protein that is modified with myristate and palmitate, which promote association with the cytoplasmic faces of the Golgi apparatus or TGN membranes, in conjunction with a dileucine motif and a cluster

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of acidic residues50,91,101. HSV, PRV or HCMV mutants lacking pUL11 homologues accumulate unenveloped, cytoplasmic capsids and produce 102fold to 103fold

fewer infectious extracellular virions than their wildtype counterparts102–104. HSV pUL11 binds a second tegument protein, pUL16, which is bound to the surface of cytoplasmic capsids105,106. HSV and PRV pUL16 proteins interact with a third capsidassociated tegument protein, pUL21 (REFS 107,108). Thus, one simple model suggests that the interactions between membranebound pUL11 and the capsid decorated with pUL16 and pUL21 connects the envelope to the capsid to drive secondary envelopment109.

Secondary envelopment (re-envelopment)Tegumentcoated capsids in the cytoplasm acquire a virion envelope by budding into cytoplasmic membranes (FIGS 2d,e,5). The cytoplasmic membranes used for secondary envelopment by different herpesviruses have been defined as the cisGolgi, the medial Golgi, the TGN and endosomes7,110–115. Studies involving a recombinant HSV that produced fluorescent proteins fused with glycoproteins, tegument proteins and capsid proteins showed colocalization of all three classes of HSV structural proteins in the TGN116. However, during late stages of herpesvirus replication there is dramatic rearrangement of Golgi, TGN and endosomal membranes, so their subcellular localization and boundaries blur. For example, HSV causes reorganization of both the Golgi apparatus and the TGN, such that Golgi markers are more uniformly distributed throughout the cytoplasm117 and TGN markers appear in the plasma membrane112.

HCMV replicates much more slowly than HSV and, over the course of days, dramatically reorganizes almost all cytoplasmic membranes into socalled ‘assembly compartments’, which appear as large cytoplasmic inclusions adjacent to nuclear membranes. The assembly compartments appear as concentric rings, with TGN and endosomederived membrane vesicles located more centrally

and surrounded by Golgiderived membranes that are, in turn, ringed by ER membranes; and all of these rings are arranged around the microtubule organizing centre6,7. Host dynein, BIP (also known as GRP78; the ER chaperone) and SUNdomain proteins are all important for construction of the HCMV assembly compartment8,113. These membrane rearrangements may promote assembly by bringing viral structural components into juxtaposition or by ordering interactions between various components as in a factory assembly line7.

Herpesvirus membrane glycoproteins often contain tyrosine and dileucine motifs, acidic clusters, and phosphorylated amino acids or oligosaccharides that promote transport from the Golgi to the TGN or recycling from the plasma membrane and endosomes to the TGN118. In this, the virus mimics sorting sequences found on cellular TGN proteins. These sorting mechanisms lead to the concentration of viral membrane proteins at sites of secondary envelopment, and this ultimately promotes assembly and egress10. For example, the accumulation of HSV and HCMV gB in the TGN depends on dileucine, tyrosine and acidic motifs in the gB cyto plasmic domain119,120. HSV gE–gI and gD and VZV gE–gI also contain dileucine and tyrosine motifs and certain of these glycoproteins are also modified with mannose6phosphate (M6P), which is bound by M6P receptors that sort proteins to endosomal compartments111,121. A second group of viral glycoproteins, without endosome or TGNspecific sorting motifs, can be recruited to the TGN through interactions with viral membrane proteins that do contain these motifs122. A third group of glycoproteins, such as HSV gJ, gK and gN, may not be recruited to sites of secondary envelopment and, as a result, may not be found in extracellular virions77.

For most enveloped viruses, the cytoplasmic domains of viral membrane glycoproteins interact with capsids or with matrix or tegument proteins, bridging membranes onto capsids to promote envelopment or budding (FIG. 5). Membranebound tegument proteins, such as VP22, can contribute to secondary envelopment as described above, but there is also evidence that herpesvirus glyco proteins play important roles in secondary envelopment by bridging tegumentcoated capsids to membrane surfaces (FIG. 5). Whereas HSV mutants lacking single glyco proteins (gB, gD, gC, gE, gI, gH or gL) have little or no defect in secondary envelopment, HSV mutants lacking gD and gE–gI123, as well as PRV mutants lacking both gE–gI and gM124, accumulate large aggregates of unenveloped capsids in the cytoplasm. PRV and HSV mutants lacking just the gE cytoplasmic domain and gM (PRV) or gD (HSV) are similarly defective for secondary envelopment123,124. Thus, as with deenvelopment, either of two membrane glycoproteins can function to promote relatively normal levels of secondary envelopment. Therefore, gD and gE–gI might be described as having redundant or overlapping functions in secondary envelopment. There is also evidence that HSV gD, gE and gM can bind to or are associated with VP22 and pUL11 and that these interactions may promote secondary envelopment94,125,126. However, these associations, which were measured in pulldown experiments, should be viewed with some

Figure 5 | Secondary envelopment. In the cytosol, capsids coated with numerous tegument proteins, including a complex of pVP16, the virion host shut-off protein (vhs) and pVP22, bind onto the surfaces of trans-Golgi network (TGN) membranes that contain herpes simplex virus (HSV) glycoproteins, including gE–gI, gD, gB, gH–gL and others. Interactions between pVP16–vhs–pVP22 and gE–gI and gD promote envelopment (that is, the wrapping of membrane around capsids). Interactions between other tegument and membrane proteins also have important roles in envelopment.

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caution. For example, HSV VP22 and pUL11 were coprecipitated with gD and gE–gI molecules that lack the cytoplasmic domains126. It seems unlikely that VP22 and pUL11 could interact with gE or gD lacking cytoplasmic domains, given that VP22 and pUL11 are mainly found on cytoplasmic surfaces. This coprecipitation was unrelated to insolubility of the tegument proteins in detergents and, thus, VP22 and pUL11 might be viewed as ‘sticky’ proteins that bind to other proteins nonspecifically. This ‘sticky’ nature may be important for the assembly of tegument. Nonetheless, an HSV mutant lacking VP22 incorporates less gE–gI and less gD into the virion93, and a mutant form of VP22 that is unable to bind gE–gI is mislocalized and not concentrated in perinuclear membranes94. These results strongly support the hypothesis that VP22, gE–gI and gD interact to promote secondary envelopment.

Herpesviruses can also produce enveloped particles that lack capsids. HSV and PRV produce L (light) particles composed of a virion envelope that is studded with viral glycoproteins and encloses most or all of the tegument proteins, and these L particles are secreted from some cells127. HCMV releases dense particles composed principally of the tegument protein pp65 surrounded by a viral envelope81,128. The existence of such particles illustrates that strong interactions between tegument proteins and viral glycoproteins are sufficient to drive envelopment, whether or not capsids are present. However, cellular proteins probably also have important roles in late stages of herpesvirus assembly, such as in the generation of membrane curvature for budding, because dominantnegative forms of ESCRT proteins block replication of HSV and HCMV129,130.

In summary, the assembly of tegument is a complex process involving numerous protein–protein interactions that occur on the surfaces of capsids and on the cytoplasmic surfaces of TGN membranes. The deposition of tegument on the capsids is intimately associated with secondary envelopment, and it seems likely that interactions between multiple tegument proteins and multiple membrane glycoproteins help drive budding.

Directed transport of herpesviruses to cell surfacesSecondary envelopment produces enveloped virions within TGNderived membrane vesicles (FIGS 2d,e,5). Both the virion envelope and the surrounding vesicle contain numerous glycoproteins. In the case of the surrounding vesicle, the glycoproteins are oriented with their sorting motifs located in the cytoplasm. As TGN membranes recycle to endosomes that, in turn, fuse with the plasma membrane, normal exocytosis probably ferries enveloped herpesvirus particles to the cell surface. Fusion between transport vesicles and the plasma membrane releases viruses into the extracellular space. This fusion is probably mediated by cellular exocytic fusion machinery rather than by viral fusion proteins because the viral proteins are in the wrong orientation. After fusion, most enveloped virus particles are found attached to the outer surface of the plasma membrane, rather than being released from the cell (FIG. 2f). Herpesviruses also extensively alter

TGN trafficking in late phases of infection112,117. For example, both cellular TGN proteins and nascent enveloped HSV virions within TGN vesicles are redistributed to the plasma membrane late during the infection112. This suggests that HSV actively disrupts TGN recycling loops — for example, by blocking backward transport from endosomes to the TGN — in order to promote forward transport of virions to the plasma membrane and the extracellular space. In approaching the plasma membrane, herpesviruses face a major barrier to secretion in the form of the cortical actin that is present on the cytosolic surface of the plasma membrane. To deal with this, HSV activates and exploits the movement of myosin Va to carry virion and viral glycoproteinladen vesicles along cortical actin to the plasma membrane131, similar to the secretion of melanosomes and secretory granules that is also promoted by myosin Va.

This simple picture of the late stages of herpesvirus egress does not take into account that many herpesviruses infect polarized cells, such as epithelial and neuronal cells, in which viral progeny are sorted to specific cell surfaces. There is evidence that HSV can direct viral progeny to cell–cell junctions in polarized epithelial cells to promote spread across these junctions to adjacent cells. For example, HSV particles assembled in epithelial cells are sorted in the TGN so that enveloped virions are delivered to lateral cell surfaces rather than to apical surfaces132 (FIG. 6a). Virions that arrive at cell–cell junctions are positioned in direct contact with adjacent cells, thereby promoting cell–cell spread of the virus. HSV gE–gI contributes to this sorting: mutations in gE, gI or just the cytoplasmic domain of gE all reduce the number of HSV particles at cell junctions and cause particles to accumulate on apical cell surfaces132. This misrouting reduces epithelial celltocell spread, causing smaller plaques in cultured epithelial cells, and markedly reducing the spread of HSV mutants lacking gE and gI in corneal epithelial tissues33. One model10 suggests that gE–gI, probably collaborating with other HSV proteins, accumulates in subdomains of the TGN that are ultimately sorted to basolateral cell surfaces or cell–cell junctions (FIG. 6a). This accumulation of gE–gI promotes assembly in TGN subdomains that leads to sorting of nascent progeny to cell–cell junctions. Thus, HSV can both disrupt TGN recycling pathways to promote exocytosis, and sort virions to cell–cell junctions. The capacity of herpesviruses to remain cell associated and to move rapidly across cell–cell junctions appears to have fundamentally important implications for a family of viruses that establish lifelong latency and engender strong immune responses. For example, by moving across cell–cell junctions, herpesviruses can escape the effects of antibodies and complement. This model fits with observations that the severity of HSV disease and the time to recrudescence do not correlate with the titres of HSVneutralizing antibodies in patients134.

There is also evidence for directed egress of alphaherpesviruses from neurons. Following reactivation from latency, HSV particles are transported in the anterograde direction — from neuronal cell bodies towards axon tips — driven by kinesin motors that move on

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HSV glycoprotein receptor

HSV glycoproteinEpithelial cell

Nucleus

Tight junction

TGN

HSV glycoproteins or synaptic proteins

HSV capsid

+–

Kinesin

Microtubule

Cell body

Dendrite

Neuron

Epithelial cell

Axon terminus

Epithelial cell

Nucleus

Glycoprotein

Capsid

Cell body

Golgi

Mature virion

Separate

a

b

c dMarried

Axon Axon terminus

microtubules (FIG. 6b). Earlier studies suggested that HSV capsids are transported in axons separately from membrane vesicles containing viral glycoproteins135–137. According to this model, known as the ‘Separate model’ (FIG. 6c), secondary envelopment occurs at axon termini. Supporting this model, colocalization of HSV glycoproteins and capsids was observed in axon tips but not

in axons136–138. By contrast, PRV is transported by what is termed the ‘Married model’ (FIG. 6d), in which secondary envelopment occurs in neuron cell bodies, and envel oped virions (present within membrane vesicles) are transported toward axon tips139,140. Some reports have suggested that HSV particles can be transported in axons, entirely as Married particles141 or as a mixture

Figure 6 | Sorting of virions within host cells. Herpesvirus particles can be sorted to specific cell surfaces in polarized cells. a | In epithelial cells, cellular proteins are often sorted to basolateral versus apical surfaces in the trans-Golgi network (TGN). Herpes simplex virus (HSV) and other herpesviruses direct viral structural proteins to subdomains of the TGN that ultimately sort mature virions to lateral cell surfaces (cell–cell junctions). This increases rates of cell-to-cell spread and allows the virus to escape certain host immune responses, including antibodies. b | Transport of alphaherpesviruses in neurons. HSV capsids and glycoproteins (present in membrane vesicles) are ferried from neuronal cell bodies to the axon terminus on kinesin motors that move on microtubules. c | Two models have been proposed for how HSV capsids are transported in neuronal axons from cell bodies to axon termini. In the ‘Separate model’, capsids and the viral glycoproteins are transported separately and secondary envelopment takes place at the axon terminus. d | In the ‘Married model’, the mature virion is formed in the cell body and then transported to the axon terminus. There is evidence for both pathways occurring in the same axon.

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of Married and Separate particles142. In recent studies involving a recombinant HSV expressing fluorescent capsids and glycoproteins, carried out in one of our laboratories, both Married and Separate transport of HSV was observed, although separate transport was the most common148. The extent to which alphaherpesviruses undergo secondary envelopment at cell bodies versus axon tips may vary with different viruses and different neurons. Collectively, the results suggest the interesting prospect that there must be multiple mechanisms for tethering three different alphaherpesvirus structural components onto kinesin motors: vesicles containing glycoproteins, unenveloped capsids and fully enveloped virions inside vesicles.

The alphaherpesvirus membrane proteins gE–gI and pUS9 promote anterograde transport of enveloped virions, capsids and glycoproteins143–145. As with other steps in egress, gE–gI and pUS9 apparently act in a cooperative manner because deletion of gE, gI or pUS9 reduces, but does not completely abolish, transport of capsids and glycoproteins. One model describing how gE–gI and pUS9 promote anterograde transport suggests that the gE–gI and pUS9 cytoplasmic domains extend from the surfaces of transport vesicles containing virions or glycoproteins to tether these vesicles onto kinesin motors146. However, given the evidence for separate transport of HSV capsids and glyco proteins, it is difficult to see how gE–gI and pUS9 can promote capsid transport136,144. Therefore, another model was proposed144, which suggests that gE–gI and pUS9 function in neuronal cell bodies, and not in axons, to facilitate the loading of viral structural components (capsids and glycoprotein vesicles) onto microtubule motors for subsequent axonal transport. This fits with the model defined for epithelial cells, in which gE, gI and pUS9 accumulate in TGN membranes to promote assembly of virion and glycoprotein containing vesicles and the loading of these vesicles onto microtubule motors.

Conclusions and future directionsHerpesviruses extensively manipulate host membranes to gain access to extracellular spaces and spread to other cells. Unlike poxviruses, which produce capsids in the cytoplasm, herpesviruses must traverse two nuclear membranes to reach the cytoplasm. Herpesviruses use membrane proteins and kinases to disrupt the nuclear lamina and bud into the INM, then fuse with the ONM. This method of transfer into the cytoplasm differs from that used by certain nonenveloped DNA viruses that replicate in the nucleus, such as the polyoma viruses, which squeeze through nuclear pores, and the adenoviruses, which disrupt the nuclear envelope. Once in the cytoplasm, herpesviruses bud into cytoplasmic membranes, much like other enveloped viruses that produce capsids in the cytoplasm. However, there is evidence that herpesviruses can discriminate between different cytoplasmic membranes in polarized cells in order to direct progeny virions to specific cell surfaces, promoting viral spread. In the future, it will be important to better understand how pUL34 and pUL31 participate in primary envelopment, how C capsids are preferentially selected for envelopment, and whether viral glycoproteins participate in primary envelopment, as is the case for secondary envelopment. There appear to be mechanisms for deenvelopment that do not require the fusogenic gB and gH–gL proteins, and these mechanisms need to be elucidated. Key information is missing on how the tegument layer is assembled and how tegument proteins interact with the cytoplasmic domains of glycoproteins to drive secondary envelopment. For the betaherpesviruses, major questions remain about how assembly compartments are constructed in the cytoplasm and how these compartments promote assembly. For the alphaherpesviruses, there is intense interest in the molecular mechanisms by which particles are transported in neuronal axons, specifically regarding which kinesin motors propel viruses toward axon tips and which viral proteins bind these motors.

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148. Wisner, T. W., Sugimoto, K., Howard, P. W., Kawaguchi, Y. & Johnson, D. C. Anterograde transport of herpes simplex virus capsids in neurons by both Seperate and Married mechanisms. J. Virol. 30 Mar 2011 (doi:10.1128/JVI.00116-11)

AcknowledgementsStudies in the Baines laboratory were supported by US National Institutes of Health (NIH) grants R01 AI52341 and R01 GM50740. The work in the Johnson laboratory was supported by NIH grants R01 EY018755 and R01 AI081517. We thank T. Howard for helping to create the figures, and M. Webb for the electron microscopy images in figure 2.

Competing interests statementThe authors declare no competing financial interests.

FURTHER INFORMATIONDavid C. Johnson’s homepage: http://www.ohsu.edu/microbiology/johnson.shtml Joel E. Baines’s homepage: http://www.vet.cornell.edu/microbiology/faculty/Baines/

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