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

Emerging roles of the Protein Phosphatase 1 (PP1) in the context of viral infections

Protein Phosphatase 1 (PP1) is a major serine/threonine phosphatase in eukaryotes, participating in several cellular processes and metabolic pathways. Due to their low substrate specificity, PP1's catalytic subunits do not exist as free entities but instead bind to Regulatory Interactors of Protein Phosphatase One (RIPPO), which regulate PP1's substrate specificity and subcellular localization. Most RIPPOs bind to PP1 through combinations of short linear motifs (4-12 residues), forming highly specific PP1 holoenzymes. These PP1-binding motifs may, hence, represent attractive targets for the development of specific drugs that interfere with a subset of PP1 holoenzymes. Several viruses exploit the host cell protein (de)phosphorylation machinery to ensure efficient virus particle formation and propagation. While the role of many host cell kinases in viral life cycles has been extensively studied, the targeting of phosphatases by viral proteins has been studied in less detail. Here, we compile and review what is known concerning the role of PP1 in the context of viral infections and discuss how it may constitute a putative host-based target for the development of novel antiviral strategies.

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.

Tegument proteins of Epstein-Barr virus: Diverse functions, complex networks, and oncogenesis

The tegument is the structure between the envelope and nucleocapsid of herpesvirus particles. Viral (and cellular) proteins accumulate to create the layers of the tegument. Some Epstein-Barr virus (EBV) tegument proteins are conserved widely in Herpesviridae, but others are shared only by members of the gamma-herpesvirus subfamily. As the interface to envelope and nucleocapsid, the tegument functions in virion morphogenesis and budding of the nucleocapsid during progeny production. When a virus particle enters a cell, enzymes such as kinase and deubiquitinase, and transcriptional activators are released from the virion to promote virus infection. Moreover, some EBV tegument proteins are involved in oncogenesis. Here, we summarize the roles of EBV tegument proteins, in comparison to those of other herpesviruses.

Tegument protein UL21 of alpha-herpesvirus inhibits the innate immunity by triggering CGAS degradation through TOLLIP-mediated selective autophagy

Alpha-herpesvirus causes lifelong infections and serious diseases in a wide range of hosts and has developed multiple strategies to counteract the host defense. Here, we demonstrate that the tegument protein UL21 (unique long region 21) in pseudorabies virus (PRV) dampens type I interferon signaling by triggering the degradation of CGAS (cyclic GMP-AMP synthase) through the macroautophagy/autophagy-lysosome pathway. Mechanistically, the UL21 protein scaffolds the E3 ligase UBE3C (ubiquitin protein ligase E3C) to catalyze the K27-linked ubiquitination of CGAS at Lys384, which is recognized by the cargo receptor TOLLIP (toll interacting protein) and degraded in the lysosome. Additionally, we show that the N terminus of UL21 in PRV is dominant in destabilizing CGAS-mediated innate immunity. Moreover, viral tegument protein UL21 in herpes simplex virus type 1 (HSV-1) also displays the conserved inhibitory mechanisms. Furthermore, by using PRV, we demonstrate the roles of UL21 in degrading CGAS to promote viral infection in vivo. Altogether, these findings describe a distinct pathway where alpha-herpesvirus exploits TOLLIP-mediated selective autophagy to evade host antiviral immunity, highlighting a new interface of interplay between the host and DNA virus.Abbreviations: 3-MA: 3-methyladenine; ACTB: actin beta; AHV-1: anatid herpesvirus 1; ATG7: autophagy related 7; ATG13: autophagy related 13; ATG101: autophagy related 101; BHV-1: bovine alphaherpesvirus 1; BNIP3L/Nix: BCL2 interacting protein 3 like; CALCOCO2/NDP52: calcium binding and coiled-coil domain 2; CCDC50: coiled-coil domain containing 50; CCT2: chaperonin containing TCP1 subunit 2; CGAS: cyclic GMP-AMP synthase; CHV-2: cercopithecine herpesvirus 2; co-IP: co-immunoprecipitation; CQ: chloroquine; CRISPR: clustered regulatory interspaced short palindromic repeat; Cas9: CRISPR-associated system 9; CTD: C-terminal domain; Ctrl: control; DAPI: 4',6-diamidino-2-phenylindole; DBD: N-terminal DNA binding domain; DMSO: dimethyl sulfoxide; DYNLRB1: dynein light chain roadblock-type 1; EHV-1: equine herpesvirus 1; gB: glycoprotein B; GFP: green fluorescent protein; H&E: hematoxylin and eosin; HSV-1: herpes simplex virus 1; HSV-2: herpes simplex virus 2; IB: immunoblotting; IRF3: interferon regulatory factor 3; lenti: lentivirus; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; MARCHF9: membrane associated ring-CH-type finger 9; MG132: cbz-leu-leu-leucinal; NBR1: NBR1 autophagy cargo receptor; NC: negative control; NEDD4L: NEDD4 like E3 ubiquitin protein ligase; NH4Cl: ammonium chloride; OPTN: optineurin; p-: phosphorylated; PFU: plaque-forming unit; Poly(dA:dT): Poly(deoxyadenylic-deoxythymidylic) acid; PPP1: protein phosphatase 1; PRV: pseudorabies virus; RB1CC1/FIP200: RB1 inducible coiled-coil 1; RNF126: ring finger protein 126; RT-PCR: real-time polymerase chain reaction; sgRNA: single guide RNA; siRNA: small interfering RNA; SQSTM1/p62: sequestosome 1; STING1: stimulator of interferon response cGAMP interactor 1; TBK1: TANK binding kinase 1; TOLLIP: toll interacting protein; TRIM33: tripartite motif containing 33; UL16: unique long region 16; UL21: unique long region 21; UL54: unique long region 54; Ub: ubiquitin; UBE3C: ubiquitin protein ligase E3C; ULK1: unc-51 like autophagy activating kinase 1; Vec: vector; VSV: vesicular stomatitis virus; VZV: varicella-zoster virus; WCL: whole-cell lysate; WT: wild-type; Z-VAD: carbobenzoxy-valyl-alanyl-aspartyl-[O-methyl]-fluoromethylketone.

Effects of US3 protein kinase activity on localization of UL31/UL34 protein and nucleocapsids egress of duck plague virus

Duck plague virus (DPV) is a pathogen causing duck plague and has caused huge economic losses in poultry industry. In our previous report, US3 gene deletion from DPV genome seriously impaired virus replication. In this study, we constructed a US3 kinase-inactive mutant (US3K213A) to further explore the function of US3 protein (pUS3) in DPV. Our results showed that the loss of pUS3 kinase activity caused lower viral titers, smaller plaque sizes and a blockage of capsids nuclear egress including primary enveloped virion (PEV) accumulation compared to the parental virus infection. It indicates that the effects of DPV pUS3 on viral propagation depended on its kinase activity. In addition, we conducted electron microscopy analysis to show the outer nuclear membrane (ONM) evaginations and the nuclear envelope (NE) deep invagination in US3K213A-infected cells. Finally, an irregular distribution of pUL31/pUL34 in the NE in △US3- and US3K213A-infected cells and an interaction of pUS3 and pUL31 were found, which suggests that pUS3 potentially targets pUL31 and regulates the localization of pUL31/pUL34 to promote nucleocapsids egress through its kinase activity.

Herpes simplex virus 1 protein pUL21 alters ceramide metabolism by activating the interorganelle transport protein CERT

Herpes simplex virus (HSV)-1 dramatically alters the architecture and protein composition of cellular membranes during infection, but its effects upon membrane lipid composition remain unclear. HSV-1 pUL21 is a virus-encoded protein phosphatase adaptor that promotes dephosphorylation of multiple cellular and virus proteins, including the cellular ceramide (Cer) transport protein CERT. CERT mediates nonvesicular Cer transport from the endoplasmic reticulum to the trans-Golgi network, whereupon Cer is converted to sphingomyelin (SM) and other sphingolipids that play important roles in cellular proliferation, signaling, and membrane trafficking. Here, we use click chemistry to profile the kinetics of sphingolipid metabolism, showing that pUL21-mediated dephosphorylation activates CERT and accelerates Cer-to-SM conversion. Purified pUL21 and full-length CERT interact with submicromolar affinity, and we solve the solution structure of the pUL21 C-terminal domain in complex with the CERT Pleckstrin homology and steroidogenic acute regulatory-related lipid transfer domains using small-angle X-ray scattering. We identify a single amino acid mutation on the surface of pUL21 that disrupts CERT binding in vitro and in cultured cells. This residue is highly conserved across the genus Simplexvirus. In addition, we identify a pUL21 residue essential for binding to HSV-1 pUL16. Sphingolipid profiling demonstrates that Cer-to-SM conversion is severely diminished in the context of HSV-1 infection, a defect that is compounded when infecting with a virus encoding the mutated form of pUL21 that lacks the ability to activate CERT. However, virus replication and spread in cultured keratinocytes or epithelial cells is not significantly altered when pUL21-mediated CERT dephosphorylation is abolished. Collectively, we demonstrate that HSV-1 modifies sphingolipid metabolism via specific protein-protein interactions.

Role of the Orphan Transporter SLC35E1 in the Nuclear Egress of Herpes Simplex Virus 1

This study developed a system consisting of two rounds of screening cellular proteins involved in the nuclear egress of herpes simplex virus 1 (HSV-1). Using this system, we first screened cellular proteins that interacted with the HSV-1 nuclear egress complex (NEC) consisting of UL34 and UL31 in HSV-1-infected cells, which are critical for the nuclear egress of HSV-1, by tandem affinity purification coupled with mass spectrometry-based proteomics technology. Next, we performed CRISPR/Cas9-based screening of live HSV-1-infected reporter cells under fluorescence microscopy using single guide RNAs targeting the cellular proteins identified in the first proteomic screening to detect the mislocalization of the lamin-associated protein emerin, which is a phenotype for defects in HSV-1 nuclear egress. This study focused on a cellular orphan transporter SLC35E1, one of the cellular proteins identified by the screening system. Knockout of SLC35E1 reduced HSV-1 replication and induced membranous invaginations containing perinuclear enveloped virions (PEVs) adjacent to the nuclear membrane (NM), aberrant accumulation of PEVs in the perinuclear space between the inner and outer NMs and the invagination structures, and mislocalization of the NEC. These effects were similar to those of previously reported mutation(s) in HSV-1 proteins and depletion of cellular proteins that are important for HSV-1 de-envelopment, one of the steps required for HSV-1 nuclear egress. Our newly established screening system enabled us to identify a novel cellular protein required for efficient HSV-1 de-envelopment. IMPORTANCE The identification of cellular protein(s) that interact with viral effector proteins and function in important viral procedures is necessary for enhancing our understanding of the mechanics of various viral processes. In this study, we established a new system consisting of interactome screening for the herpes simplex virus 1 (HSV-1) nuclear egress complex (NEC), followed by loss-of-function screening to target the identified putative NEC-interacting cellular proteins to detect a defect in HSV-1 nuclear egress. This newly established system identified SLC35E1, an orphan transporter, as a novel cellular protein required for efficient HSV-1 de-envelopment, providing an insight into the mechanisms involved in this viral procedure.

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.

Disruption of the Interaction between ORF33 and the Conserved Carboxyl-Terminus of ORF45 Abolishes Progeny Virion Production of Kaposi Sarcoma-Associated Herpesvirus

The Open Reading Frame 45 (ORF45) of Kaposi sarcoma-associated herpesvirus (KSHV) is a gammaherpesvirus-specific, immediate-early, tegument protein required for efficient viral replication and virion production. We have previously shown that ORF45 interacts with the conserved herpesviral protein ORF33 through the highly conserved C-terminal 19 amino acids (C19) of ORF45. Because the deletion of C19 abolished ORF33 accumulation and viral production, we reasoned that this interaction could be critical for viral production and explored as an antiviral target for gammaherpesviruses. In work described in this article, we characterize this interaction in further detail, first by revealing that this interaction is conserved among gammaherpesviruses, then by identifying residues in C19 critical for its interaction with and stabilization of ORF33. More importantly, we show that disruption of the interaction, either by mutating key residues (W403A or W405A) in C19 or by using competing cell penetration peptide TAT-C19, dramatically reduce the yield of KSHV progeny viruses. Our results not only reveal critical roles of this interaction to viral production but also provide a proof of concept for targeting the ORF33-ORF45 interaction as a novel antiviral strategy against KSHV and other gammaherpesviruses.

pUL21 regulation of pUs3 kinase activity influences the nature of nuclear envelope deformation by the HSV-2 nuclear egress complex

It is well established that the herpesvirus nuclear egress complex (NEC) has an intrinsic ability to deform membranes. During viral infection, the membrane-deformation activity of the NEC must be precisely regulated to ensure efficient nuclear egress of capsids. One viral protein known to regulate herpes simplex virus type 2 (HSV-2) NEC activity is the tegument protein pUL21. Cells infected with an HSV-2 mutant lacking pUL21 (ΔUL21) produced a slower migrating species of the viral serine/threonine kinase pUs3 that was shown to be a hyperphosphorylated form of the enzyme. Investigation of the pUs3 substrate profile in ΔUL21-infected cells revealed a prominent band with a molecular weight consistent with that of the NEC components pUL31 and pUL34. Phosphatase sensitivity and retarded mobility in phos-tag SDS-PAGE confirmed that both pUL31 and pUL34 were hyperphosphorylated by pUs3 in the absence of pUL21. To gain insight into the consequences of increased phosphorylation of NEC components, the architecture of the nuclear envelope in cells producing the HSV-2 NEC in the presence or absence of pUs3 was examined. In cells with robust NEC production, invaginations of the inner nuclear membrane were observed that contained budded vesicles of uniform size. By contrast, nuclear envelope deformations protruding outwards from the nucleus, were observed when pUs3 was included in transfections with the HSV-2 NEC. Finally, when pUL21 was included in transfections with the HSV-2 NEC and pUs3, decreased phosphorylation of NEC components was observed in comparison to transfections lacking pUL21. These results demonstrate that pUL21 influences the phosphorylation status of pUs3 and the HSV-2 NEC and that this has consequences for the architecture of the nuclear envelope.

During all herpesvirus infections, the nuclear envelope undergoes deformation in order to enable viral capsids assembled within the nucleus of the infected cell to gain access to the cytoplasm for further maturation and spread to neighbouring cells. These nuclear envelope deformations are orchestrated by the viral nuclear egress complex (NEC), which, in HSV, is composed of two viral proteins, pUL31 and pUL34. How the membrane-deformation activity of the NEC is controlled during infection is incompletely understood. The studies in this communication reveal that the phosphorylation status of pUL31 and pUL34 can determine the nature of nuclear envelope deformations and that the viral protein pUL21 can modulate the phosphorylation status of both NEC components. These findings provide an explanation for why HSV-2 strains lacking pUL21 are defective in nuclear egress. A thorough understanding of how NEC activity is controlled during infection may yield strategies to disrupt this fundamental step in the herpesvirus lifecycle, providing the basis for novel antiviral strategies.

pUL21 is a viral phosphatase adaptor that promotes herpes simplex virus replication and spread

The herpes simplex virus (HSV)-1 protein pUL21 is essential for efficient virus replication and dissemination. While pUL21 has been shown to promote multiple steps of virus assembly and spread, the molecular basis of its function remained unclear. Here we identify that pUL21 is a virus-encoded adaptor of protein phosphatase 1 (PP1). pUL21 directs the dephosphorylation of cellular and virus proteins, including components of the viral nuclear egress complex, and we define a conserved non-canonical linear motif in pUL21 that is essential for PP1 recruitment. In vitro evolution experiments reveal that pUL21 antagonises the activity of the virus-encoded kinase pUS3, with growth and spread of pUL21 PP1-binding mutant viruses being restored in adapted strains where pUS3 activity is disrupted. This study shows that virus-directed phosphatase activity is essential for efficient herpesvirus assembly and spread, highlighting the fine balance between kinase and phosphatase activity required for optimal virus replication.

Herpes simplex virus (HSV)-1 is a highly prevalent human virus that causes life-long infections. While the most common symptom of HSV-1 infection is orofacial lesions (‘cold sores’), HSV-1 infection can also cause fatal encephalitis and it is a leading cause of infectious blindness. The HSV-1 genome encodes many proteins that dramatically remodel the environment of infected cells to promote virus replication and spread, including enzymes that add phosphate groups (kinases) to cellular and viral proteins in order to fine-tune their function. Here we identify that pUL21 is an HSV-1 protein that binds directly to protein phosphatase 1 (PP1), a highly abundant cellular enzyme that removes phosphate groups from proteins. We demonstrate that pUL21 stimulates the specific dephosphorylation of both cellular and viral proteins, including a component of the viral nuclear egress complex that is essential for efficient assembly of new HSV-1 particles. Furthermore, our in vitro evolution experiments demonstrate that pUL21 antagonises the activity of the HSV-1 kinase pUS3. Our work highlights the precise control that herpesviruses exert upon the protein environment within infected cells, and specifically the careful balance of kinase and phosphatase activity that HSV-1 requires for optimal replication and spread.

Phosphoregulation of a Conserved Herpesvirus Tegument Protein by a Virally Encoded Protein Kinase in Viral Pathogenicity and Potential Linkage between Its Evolution and Viral Phylogeny

Us3 proteins of herpes simplex virus 1 (HSV-1) and HSV-2 are multifunctional serine-threonine protein kinases. Here, we identified an HSV-2 tegument protein, UL7, as a novel physiological substrate of HSV-2 Us3. Mutations in HSV-2 UL7, which precluded Us3 phosphorylation of the viral protein, significantly reduced mortality, viral replication in the vagina, and development of vaginal disease in mice following vaginal infection. These results indicated that Us3 phosphorylation of UL7 in HSV-2 was required for efficient viral replication and pathogenicity in vivo Of note, this phosphorylation was conserved in UL7 of chimpanzee herpesvirus (ChHV), which phylogenetically forms a monophyletic group with HSV-2 and the resurrected last common ancestral UL7 for HSV-2 and ChHV. In contrast, the phosphorylation was not conserved in UL7s of HSV-1, which belongs to a sister clade of the monophyletic group, the resurrected last common ancestor for HSV-1, HSV-2, and ChHV, and other members of the genus Simplexvirus that are phylogenetically close to these viruses. Thus, evolution of Us3 phosphorylation of UL7 coincided with the phylogeny of simplex viruses. Furthermore, artificially induced Us3 phosphorylation of UL7 in HSV-1, in contrast to phosphorylation in HSV-2, had no effect on viral replication and pathogenicity in mice. Our results suggest that HSV-2 and ChHV have acquired and maintained Us3 phosphoregulation of UL7 during their evolution because the phosphoregulation had an impact on viral fitness in vivo, whereas most other simplex viruses have not because the phosphorylation was not necessary for efficient fitness of the viruses in vivo IMPORTANCE It has been hypothesized that the evolution of protein phosphoregulation drives phenotypic diversity across species of organisms, which impacts fitness during their evolution. However, there is a lack of information regarding linkage between the evolution of viral phosphoregulation and the phylogeny of virus species. In this study, we clarified the novel HSV-2 Us3 phosphoregulation of UL7 in infected cells, which is important for viral replication and pathogenicity in vivo We also showed that the evolution of Us3 phosphoregulation of UL7 was linked to the phylogeny of viruses that are phylogenetically close to HSV-2 and to the phosphorylation requirements for the efficient in vivo viral fitness of HSV-2 and HSV-1, which are representative of viruses that have and have not evolved phosphoregulation, respectively. This study reports the first evidence showing that evolution of viral phosphoregulation coincides with phylogeny of virus species and supports the hypothesis regarding the evolution of viral phosphoregulation during viral evolution.