Deletion of UL21 causes a delay in the early stages of the herpes simplex virus 1 replication cycle

The loss of both pUL16 and pUL21 in HSV-1 infected cells abolishes cytoplasmic envelopment

Previously, we had developed synthetic genomics methods to assemble an infectious clone of herpes simplex virus type-1 (HSV-1). To do this, the genome was assembled from 11 separate cloned fragments in yeast using transformation associated recombination. The eleven fragments or "parts" spanned the 152 kb genome and recombination was achieved because of the overlapping homologous sequences between each fragment. To demonstrate the robustness of this genome assembly method for reverse genetics, we engineered different mutations that were located in distant loci on the genome and built a collection of HSV-1 genomes that contained single and different combination of mutations in 5 conserved HSV-1 genes. The five genes: UL7, UL11, UL16, UL21 and UL51 encode virion structural proteins and have varied functions in the infected cell. Each is dispensable for virus replication in cell culture, however, combinatorial analysis of deletions in the five genes revealed "synthetic-lethality" of some of the genetic mutations. Thus, it was discovered that any virus that carried a UL21 mutation in addition to the other gene was unable to replicate in Vero cells. Replication was restored in a complementing cell line that provided pUL21 in trans. One particular combination (UL16-UL21) was of interest because the proteins encoded by these genes are known to physically interact and are constituents of the tegument structure. Furthermore, their roles in HSV-1 infected cells are unclear. Both are dispensable for HSV-1 replication, however, in HSV-2 their mutation results in nuclear retention of assembled capsids. We thus characterized these viruses that carry the single and double mutant. What we discovered is that in cells where both pUL16 and pUL21 are absent, cytoplasmic capsids were evident but did not mature into enveloped particles. The capsid particles isolated from these cells showed significantly lower levels of incorporation of both VP16 and pUL37 when compared to the wild-type capsids. These data now show that of the tegument proteins, like the essential pUL36, pUL37 and VP16; the complex of pUL16 and pUL21 should be considered as important mediators of cytoplasmic maturation of the particle.

Disruption of herpes simplex virus type 2 pUL21 phosphorylation impairs secondary envelopment of cytoplasmic nucleocapsids

The multifunctional tegument protein pUL21 of HSV-2 is phosphorylated in infected cells. We have identified two residues in the unstructured linker region of pUL21, serine 251 and serine 253, as phosphorylation sites. Both phosphorylation sites are absent in HSV-1 pUL21, which likely explains why phosphorylated pUL21 was not detected in cells infected with HSV-1. Cells infected with HSV-2 strain 186 viruses deficient in pUL21 phosphorylation exhibited reductions in both cell-cell spread of virus infection and virus replication. Defects in secondary envelopment of cytoplasmic nucleocapsids were also observed in cells infected with viruses deficient in pUL21 phosphorylation as well as in cells infected with multiple strains of HSV-2 and HSV-1 deleted for pUL21. These results confirm a role for HSV pUL21 in the secondary envelopment of cytoplasmic nucleocapsids and indicate that phosphorylation of HSV-2 pUL21 is required for this activity. Phosphorylation of pUL21 was substantially reduced in cells infected with HSV-2 strain 186 mutants lacking the viral serine/threonine kinase pUL13, indicating a requirement for pUL13 in pUL21 phosphorylation.

IMPORTANCE

It is well known that post-translational modification of proteins by phosphorylation can regulate protein function. Here, we determined that phosphorylation of the multifunctional HSV-2 tegument protein pUL21 requires the viral serine/threonine kinase pUL13. In addition, we identified serine residues within HSV-2 pUL21 that can be phosphorylated. Phenotypic analysis of mutant HSV-2 strains with deficiencies in pUL21 phosphorylation revealed reductions in both cell-cell spread of virus infection and virus replication. Deficiencies in pUL21 phosphorylation also compromised the secondary envelopment of cytoplasmic nucleocapsids, a critical final step in the maturation of all herpes virions. Unlike HSV-2 pUL21, phosphorylation of HSV-1 pUL21 was not detected. This fundamental difference between HSV-2 and HSV-1 may underlie our previous observations that the requirements for pUL21 differ between HSV species.

The precise function of alphaherpesvirus tegument proteins and their interactions during the viral life cycle

Alphaherpesvirus is a widespread pathogen that causes diverse diseases in humans and animals and can severely damage host health. Alphaherpesvirus particles comprise a DNA core, capsid, tegument and envelope; the tegument is located between the nuclear capsid and envelope. According to biochemical and proteomic analyses of alphaherpesvirus particles, the tegument contains at least 24 viral proteins and plays an important role in the alphaherpesvirus life cycle. This article reviews the important role of tegument proteins and their interactions during the viral life cycle to provide a reference and inspiration for understanding alphaherpesvirus infection pathogenesis and identifying new antiviral strategies.

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.

Pathogenicity and immunogenicity of a quadruple gene-deleted pseudorabies virus variant as a vaccine candidate

Since late 2011, the PRV variants have emerged in China, characterized by the increased virulence. The traditional attenuated vaccines have proven insufficient in providing complete protection, resulting in substantial economic losses to swine industry. In this study, a vaccine candidate strain, ZJ01-ΔgI/gE/TK/UL21, carrying the quadruple gene deletion was derived from the previously generated three gene-deleted virus ZJ01-ΔgI/gE/TK. As anticipated, piglets inoculated with ZJ01-ΔgI/gE/TK/UL21 exhibited normal body temperatures and showed no viral shedding, consistent with the observations from piglets treated with ZJ01-ΔgI/gE/TK. Importantly, a significant higher level of interferon induction was observed among piglets in the ZJ01-ΔgI/gE/TK/UL21 group compared to those in the ZJ01-ΔgI/gE/TK group. Upon challenge with the PRV variant ZJ01, piglets immunized with ZJ01-ΔgI/gE/TK/UL21 exhibited reduced viral shedding compared to the ZJ01-ΔgI/gE/TK group. Furthermore, piglets vaccinated with ZJ01-ΔgI/gE/TK/UL21 exhibited minimal pathological lesions in brain tissues, similar to those in the ZJ01-ΔgI/gE/TK group. These results underscore the potential of ZJ01-ΔgI/gE/TK/UL21 as a promising vaccine for controlling PRV infection.

The Herpes Simplex Virus pUL16 and pUL21 Proteins Prevent Capsids from Docking at Nuclear Pore Complexes

After entry into cells, herpes simplex virus (HSV) nucleocapsids dock at nuclear pore complexes (NPCs) through which viral genomes are released into the nucleoplasm where viral gene expression, genome replication, and early steps in virion assembly take place. After their assembly, nucleocapsids are translocated to the cytoplasm for final virion maturation. Nascent cytoplasmic nucleocapsids are prevented from binding to NPCs and delivering their genomes to the nucleus from which they emerged, but how this is accomplished is not understood. Here we report that HSV pUL16 and pUL21 deletion mutants accumulate empty capsids at the cytoplasmic face of NPCs late in infection. Additionally, prior expression of pUL16 and pUL21 prevented incoming nucleocapsids from docking at NPCs, delivering their genomes to the nucleus and initiating viral gene expression. Both pUL16 and pUL21 localized to the nuclear envelope, placing them in an appropriate location to interfere with nucleocapsid/NPC interactions.

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.

The herpes simplex virus tegument protein pUL21 is required for viral genome retention within capsids

During virion morphogenesis herpes simplex virus nucleocapsids transit from the nucleoplasm to the cytoplasm, through a process called nuclear egress, where the final stages of virion assembly occur. Coupled to nuclear egress is a poorly understood quality-control mechanism that preferentially selects genome-containing C-capsids, rather than A- and B-capsids that lack genomes, for transit to the cytoplasm. We and others have reported that cells infected with HSV strains deleted for the tegument protein pUL21 accumulate both empty A-capsids and C-capsids in the cytoplasm of infected cells. Quantitative microscopy experiments indicated that C-capsids were preferentially selected for envelopment at the inner nuclear membrane and that nuclear integrity remained intact in cells infected with pUL21 mutants, prompting alternative explanations for the accumulation of A-capsids in the cytoplasm. More A-capsids were also found in the nuclei of cells infected with pUL21 mutants compared to their wild type (WT) counterparts, suggesting pUL21 might be required for optimal genome packaging or genome retention within capsids. In support of this, more viral genomes were prematurely released into the cytoplasm during pUL21 mutant infection compared to WT infection and led to enhanced activation of cellular cytoplasmic DNA sensors. Mass spectrometry and western blot analysis of WT and pUL21 mutant capsids revealed an increased association of the known pUL21 binding protein, pUL16, with pUL21 mutant capsids, suggesting that premature and/or enhanced association of pUL16 with capsids might result in capsid destabilization. Further supporting this idea, deletion of pUL16 from a pUL21 mutant strain rescued genome retention within capsids. Taken together, these findings suggest that pUL21 regulates pUL16 addition to nuclear capsids and that premature, and/or, over-addition of pUL16 impairs HSV genome retention within capsids.

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.

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.

An improved animal model for herpesvirus encephalitis in humans

Herpesviral encephalitis caused by Herpes Simplex Virus 1 (HSV-1) is one of the most devastating diseases in humans. Patients present with fever, mental status changes or seizures and when untreated, sequelae can be fatal. Herpes Simplex Encephalitis (HSE) is characterized by mainly unilateral necrotizing inflammation effacing the frontal and mesiotemporal lobes with rare involvement of the brainstem. HSV-1 is hypothesized to invade the CNS via the trigeminal or olfactory nerve, but viral tropism and the exact route of infection remain unclear. Several mouse models for HSE have been developed, but they mimic natural infection only inadequately. The porcine alphaherpesvirus Pseudorabies virus (PrV) is closely related to HSV-1 and Varicella Zoster Virus (VZV). While pigs can control productive infection, it is lethal in other susceptible animals associated with severe pruritus leading to automutilation. Here, we describe the first mutant PrV establishing productive infection in mice that the animals are able to control. After intranasal inoculation with a PrV mutant lacking tegument protein pUL21 and pUS3 kinase activity (PrV-ΔUL21/US3Δkin), nearly all mice survived despite extensive infection of the central nervous system. Neuroinvasion mainly occurred along the trigeminal pathway. Whereas trigeminal first and second order neurons and autonomic ganglia were positive early after intranasal infection, PrV-specific antigen was mainly detectable in the frontal, mesiotemporal and parietal lobes at later times, accompanied by a long lasting lymphohistiocytic meningoencephalitis. Despite this extensive infection, mice showed only mild to moderate clinical signs, developed alopecic skin lesions, or remained asymptomatic. Interestingly, most mice exhibited abnormalities in behavior and activity levels including slow movements, akinesia and stargazing. In summary, clinical signs, distribution of viral antigen and inflammatory pattern show striking analogies to human encephalitis caused by HSV-1 or VZV not observed in other animal models of disease.

In developed countries, more than 50% of humans are seropositive for the neurotropic Herpes Simplex Virus 1 (HSV-1) and two to four million cases of Herpes simplex encephalitis (HSE) are reported per year worldwide. Primary infection with HSV-1 takes place via the skin or the oral mucosa followed by intraaxonal retrograde spread to sensory ganglia of the peripheral nervous system where HSV-1 usually establishes latency. Further spread to the central nervous system results in HSE, a necrotizing encephalitis effacing predominantly the temporal and frontal lobes of the brain. Mice infected with HSV-1 develop encephalitis, but do not show the typical lesions and exhibit high mortality rates. Here we demonstrate that mice infected with a mutant pseudorabies virus lacking the tegument protein pUL21 and an active viral kinase pUS3 were able to survive the productive infection but developed lymphohistiocytic encephalitis with viral antigen distribution, inflammation and associated behavioral changes comparable to HSE in humans. These striking analogies offer new perspectives to study herpesviral encephalitis in a suitable animal model.

The Carboxyl Terminus of Tegument Protein pUL21 Contributes to Pseudorabies Virus Neuroinvasion

Following its entry into cells, pseudorabies virus (PRV) utilizes microtubules to deliver its nucleocapsid to the nucleus. Previous studies have shown that PRV VP1/2 is an effector of dynein-mediated capsid transport. However, the mechanism of PRV for recruiting microtubule motor proteins for successful neuroinvasion and neurovirulence is not well understood. Here, we provide evidence that PRV pUL21 is an inner tegument protein. We tested its interaction with the cytoplasmic light chains using a bimolecular fluorescence complementation (BiFC) assay and observed that PRV pUL21 interacts with Roadblock-1. This interaction was confirmed by coimmunoprecipitation (co-IP) assays. We also determined the efficiency of retrograde and anterograde axonal transport of PRV strains in explanted neurons using a microfluidic chamber system and investigated pUL21's contribution to PRV neuroinvasion in vivo Further data showed that the carboxyl terminus of pUL21 is essential for its interaction with Roadblock-1, and this domain contributes to PRV retrograde axonal transport in vitro and in vivo Our findings suggest that the carboxyl terminus of pUL21 contributes to PRV neuroinvasion.IMPORTANCE Herpesviruses are a group of DNA viruses that infect both humans and animals. Alphaherpesviruses are distinguished by their ability to establish latent infection in peripheral neurons. After entering neurons, the herpesvirus capsid interacts with cellular motor proteins and undergoes retrograde transport on axon microtubules. This elaborate process is vital to the herpesvirus lifecycle, but the underlying mechanism remains poorly understood. Here, we determined that pUL21 is an inner tegument protein of pseudorabies virus (PRV) and that it interacts with the cytoplasmic dynein light chain Roadblock-1. We also observed that pUL21 promotes retrograde transport of PRV in neuronal cells. Furthermore, our findings confirm that pUL21 contributes to PRV neuroinvasion in vivo Importantly, the carboxyl terminus of pUL21 is responsible for interaction with Roadblock-1, and this domain contributes to PRV neuroinvasion. This study offers fresh insights into alphaherpesvirus neuroinvasion and the interaction between virus and host during PRV infection.

CRISPR/Cas9 Mutagenesis of UL21 in Multiple Strains of Herpes Simplex Virus Reveals Differential Requirements for pUL21 in Viral Replication

Studies from multiple laboratories using different strains or species of herpes simplex virus (HSV) with deletions in UL21 have yielded conflicting results regarding the necessity of pUL21 in HSV infection. To resolve this discrepancy, we utilized CRISPR/Cas9 mutagenesis to isolate pUL21 deficient viruses in multiple HSV backgrounds, and performed a side-by-side comparison of the cell-to-cell spread and replication phenotypes of these viruses. These analyses confirmed previous studies implicating the involvement of pUL21 in cell-to-cell spread of HSV. Cell-to-cell spread of HSV-2 was more greatly affected by the lack of pUL21 than HSV-1, and strain-specific differences in the requirement for pUL21 in cell-to-cell spread were also noted. HSV-2 strain 186 lacking pUL21 was particularly crippled in both cell-to-cell spread and viral replication in non-complementing cells, in comparison to other HSV strains lacking pUL21, suggesting that the strict requirement for pUL21 by strain 186 may not be representative of the HSV-2 species as a whole. This work highlights CRISPR/Cas9 technology as a useful tool for rapidly constructing deletion mutants of alphaherpesviruses, regardless of background strain, and should find great utility whenever strain-specific differences need to be investigated.

The UL21 Tegument Protein of Herpes Simplex Virus 1 Is Differentially Required for the Syncytial Phenotype

The initial goal of this study was to reexamine the requirement of UL21 for herpes simplex virus 1 (HSV-1) replication. Previous studies suggested that UL21 is dispensable for replication in cell cultures, but a recent report on HSV-2 challenges those findings. As was done for the HSV-2 study, a UL21-null virus was made and propagated on complementing cells to discourage selection of compensating mutations. This HSV-1 mutant was able to replicate in noncomplementing cells, even at a low multiplicity of infection (MOI), though a reduction in titer was observed. Also, increased proportions of empty capsids were observed in the cytoplasm, suggesting a role for UL21 in preventing their exit from the nucleus. Surprisingly, passage of the null mutant resulted in rapid outgrowth of syncytial (Syn) variants. This was unexpected because UL21 has been shown to be required for the Syn phenotype. However, earlier experiments made use of only the A855V syncytial mutant of glycoprotein B (gB), and the Syn phenotype can also be produced by substitutions in glycoprotein K (gK), UL20, and UL24. Sequencing of the syncytial variants revealed mutations in the gK locus, but UL21 was shown to be dispensable for UL20^Syn and UL24^Syn To test whether UL21 is needed only for the A855V mutant, additional gB^Syn derivatives were examined in the context of the null virus, and all produced lytic rather than syncytial sites of infection. Thus, UL21 is required only for the gB^Syn phenotype. This is the first example of a differential requirement for a viral protein across the four syn loci.IMPORTANCE UL21 is conserved among alphaherpesviruses, but its role is poorly understood. This study shows that HSV-1 can replicate without UL21, although the virus titers are greatly reduced. The null virus had greater proportions of empty (DNA-less) capsids in the cytoplasm of infected cells, suggesting that UL21 may play a role in retaining them in the nucleus. This is consistent with reports showing UL21 to be capsid associated and localized to the nuclei of infected cells. UL21 also appears to be needed for viral membrane activities. It was found to be required for virus-mediated cell fusion, but only for mutants that harbor syncytial mutations in gB (not variants of gK, UL20, or UL24). The machinery needed for syncytial formation is similar to that needed for direct spread of the virus through cell junctions, and these studies show that UL21 is required for cell-to-cell spread even in the absence of syncytial mutations.

Bovine herpesvirus 1 tegument protein UL21 plays critical roles in viral secondary envelopment and cell-to-cell spreading

Bovine herpesvirus 1 (BoHV-1) UL21 is a tegument protein thought to be indispensable for efficient viral growth but its precise function in BoHV-1 is currently unknown. To determine the function of UL21 in BoHV-1 replication, we constructed a mutant virus bearing a UL21 deletion (vBoHV-1-∆UL21) and its revertant virus, vBoHV-1-∆UL21R, in which the UL21 gene was restored using a bacterial artificial chromosome system. The replication of vBoHV-1-∆UL21 was 1,000-fold lower and its plaque size was 85% smaller than those of the wild-type virus (BoHV-1). An ultrastructural analysis showed that deletion of UL21 led to an un-enveloped capsid accumulation in the cytoplasm, whereas nucleocapsid egress was not impaired, suggesting that UL21 is critical for secondary envelopment in BoHV-1. Co-immunoprecipitation assays revealed that HA-tagged UL21 pulled down UL16, suggesting that these two proteins form a complex, and this was further confirmed by a co-immunofluorescence assay. Taken together, these data provide evidence that UL21 plays critical roles in BoHV-1 secondary envelopment, and UL16 is likely to be involved in these activities.

Genome-wide engineering of an infectious clone of herpes simplex virus type 1 using synthetic genomics assembly methods

Here, we present a transformational approach to genome engineering of herpes simplex virus type 1 (HSV-1), which has a large DNA genome, using synthetic genomics tools. We believe this method will enable more rapid and complex modifications of HSV-1 and other large DNA viruses than previous technologies, facilitating many useful applications. Yeast transformation-associated recombination was used to clone 11 fragments comprising the HSV-1 strain KOS 152 kb genome. Using overlapping sequences between the adjacent pieces, we assembled the fragments into a complete virus genome in yeast, transferred it into an Escherichia coli host, and reconstituted infectious virus following transfection into mammalian cells. The virus derived from this yeast-assembled genome, KOS^YA, replicated with kinetics similar to wild-type virus. We demonstrated the utility of this modular assembly technology by making numerous modifications to a single gene, making changes to two genes at the same time and, finally, generating individual and combinatorial deletions to a set of five conserved genes that encode virion structural proteins. While the ability to perform genome-wide editing through assembly methods in large DNA virus genomes raises dual-use concerns, we believe the incremental risks are outweighed by potential benefits. These include enhanced functional studies, generation of oncolytic virus vectors, development of delivery platforms of genes for vaccines or therapy, as well as more rapid development of countermeasures against potential biothreats.

The Product of the Herpes Simplex Virus 2 UL16 Gene Is Critical for the Egress of Capsids from the Nuclei of Infected Cells

The herpes simplex virus (HSV) UL16 gene is conserved throughout the Herpesviridae and encodes a poorly understood tegument protein. The HSV-1 UL16 protein forms complexes with several viral proteins, including UL11, gE, VP22, and UL21. We previously demonstrated that HSV-2 UL21 was essential for virus propagation due to the failure of DNA-containing capsids (C capsids) to exit the nucleus. We hypothesized that if a UL16/UL21 complex was required for nuclear egress, HSV-2 lacking UL16 would have a phenotype similar to that of HSV-2 lacking UL21. Deletion of HSV-2 UL16 (Δ16) resulted in a 950-fold reduction in virus propagation in mouse L cell fibroblasts and a 200-fold reduction in virus propagation in Vero cells that was fully reversed upon the repair of Δ16 (Δ16R) and partially reversed by infecting UL16-expressing cells with Δ16. The kinetics of viral gene expression in cells infected with Δ16 were indistinguishable from those of cells infected with Δ16R or the parental virus. Additionally, similar numbers of capsids were isolated from the nuclei of cells infected with Δ16 and the parental virus. However, transmission electron microscopy, fluorescence in situ hybridization experiments, and fluorescent capsid localization assays all indicated a reduction in the ability of Δ16 C capsids to exit the nucleus of infected cells. Taken together, these data indicate that, like UL21, UL16 is critical for HSV-2 propagation and suggest that the UL16 and UL21 proteins may function together to facilitate the nuclear egress of capsids.IMPORTANCE HSV-2 is a highly prevalent sexually transmitted human pathogen that is the main cause of genital herpes infections and is fueling the epidemic transmission of HIV in sub-Saharan Africa. Despite important differences in the pathological features of HSV-1 and HSV-2 infections, HSV-2 is understudied compared to HSV-1. Here we demonstrate that a deletion of the HSV-2 UL16 gene results in a substantial inhibition of virus replication due to a reduction in the ability of DNA-containing capsids to exit the nucleus of infected cells. The phenotype of this UL16 mutant resembles that of an HSV-2 UL21 mutant described previously by our laboratory. Because UL16 and UL21 interact, these findings suggest that a complex containing both proteins may function together in nuclear egress.

Domain Interaction Studies of Herpes Simplex Virus 1 Tegument Protein UL16 Reveal Its Interaction with Mitochondria

The UL16 tegument protein of herpes simplex virus 1 (HSV-1) is conserved among all herpesviruses and plays many roles during replication. This protein has an N-terminal domain (NTD) that has been shown to bind to several viral proteins, including UL11, VP22, and glycoprotein E, and these interactions are negatively regulated by a C-terminal domain (CTD). Thus, in pairwise transfections, UL16 binding is enabled only when the CTD is absent or altered. Based on these results, we hypothesized that direct interactions occur between the NTD and the CTD. Here we report that the separated and coexpressed functional domains of UL16 are mutually responsive to each other in transfected cells and form complexes that are stable enough to be captured in coimmunoprecipitation assays. Moreover, we found that the CTD can associate with itself. To our surprise, the CTD was also found to contain a novel and intrinsic ability to localize to specific spots on mitochondria in transfected cells. Subsequent analyses of HSV-infected cells by immunogold electron microscopy and live-cell confocal imaging revealed a population of UL16 that does not merely accumulate on mitochondria but in fact makes dynamic contacts with these organelles in a time-dependent manner. These findings suggest that the domain interactions of UL16 serve to regulate not just the interaction of this tegument protein with its viral binding partners but also its interactions with mitochondria. The purpose of this novel interaction remains to be determined.

IMPORTANCE

The HSV-1-encoded tegument protein UL16 is involved in multiple events of the virus replication cycle, ranging from virus assembly to cell-cell spread of the virus, and hence it can serve as an important drug target. Unfortunately, a lack of both structural and functional information limits our understanding of this protein. The discovery of domain interactions within UL16 and the novel ability of UL16 to interact with mitochondria in HSV-infected cells lays a foundational framework for future investigations aimed at deciphering the structure and function of not just UL16 of HSV-1 but also its homologs in other herpesviruses.

HSV-1 tegument protein and the development of its genome editing technology

Herpes simplex virus 1 (HSV-1) is composed of complex structures primarily characterized by four elements: the nucleus, capsid, tegument and envelope. The tegument is an important viral component mainly distributed in the spaces between the capsid and the envelope. The development of viral genome editing technologies, such as the identification of temperature-sensitive mutations, homologous recombination, bacterial artificial chromosome, and the CRISPR/Cas9 system, has been shown to largely contribute to the rapid promotion of studies on the HSV-1 tegument protein. Many researches have demonstrated that tegument proteins play crucial roles in viral gene regulatory transcription, viral replication and virulence, viral assembly and even the interaction of the virus with the host immune system. This article briefly reviews the recent research on the functions of tegument proteins and specifically elucidates the function of tegument proteins in viral infection, and then emphasizes the significance of using genome editing technology in studies of providing new techniques and insights into further studies of HSV-1 infection in the future.

Novel Structure and Unexpected RNA-Binding Ability of the C-Terminal Domain of Herpes Simplex Virus 1 Tegument Protein UL21

Proteins forming the tegument layers of herpesviral virions mediate many essential processes in the viral replication cycle, yet few have been characterized in detail. UL21 is one such multifunctional tegument protein and is conserved among alphaherpesviruses. While UL21 has been implicated in many processes in viral replication, ranging from nuclear egress to virion morphogenesis to cell-cell spread, its precise roles remain unclear. Here we report the 2.7-Å crystal structure of the C-terminal domain of herpes simplex virus 1 (HSV-1) UL21 (UL21C), which has a unique α-helical fold resembling a dragonfly. Analysis of evolutionary conservation patterns and surface electrostatics pinpointed four regions of potential functional importance on the surface of UL21C to be pursued by mutagenesis. In combination with the previously determined structure of the N-terminal domain of UL21, the structure of UL21C provides a 3-dimensional framework for targeted exploration of the multiple roles of UL21 in the replication and pathogenesis of alphaherpesviruses. Additionally, we describe an unanticipated ability of UL21 to bind RNA, which may hint at a yet unexplored function.

IMPORTANCE

Due to the limited genomic coding capacity of viruses, viral proteins are often multifunctional, which makes them attractive antiviral targets. Such multifunctionality, however, complicates their study, which often involves constructing and characterizing null mutant viruses. Systematic exploration of these multifunctional proteins requires detailed road maps in the form of 3-dimensional structures. In this work, we determined the crystal structure of the C-terminal domain of UL21, a multifunctional tegument protein that is conserved among alphaherpesviruses. Structural analysis pinpointed surface areas of potential functional importance that provide a starting point for mutagenesis. In addition, the unexpected RNA-binding ability of UL21 may expand its functional repertoire. The structure of UL21C and the observation of its RNA-binding ability are the latest additions to the navigational chart that can guide the exploration of the multiple functions of UL21.

Insights into the function of tegument proteins from the varicella zoster virus

Chickenpox (varicella) is caused by primary infection with varicella zoster virus (VZV), which can establish long-term latency in the host ganglion. Once reactivated, the virus can cause shingles (zoster) in the host. VZV has a typical herpesvirus virion structure consisting of an inner DNA core, a capsid, a tegument, and an outer envelope. The tegument is an amorphous layer enclosed between the nucleocapsid and the envelope, which contains a variety of proteins. However, the types and functions of VZV tegument proteins have not yet been completely determined. In this review, we describe the current knowledge on the multiple roles played by VZV tegument proteins during viral infection. Moreover, we discuss the VZV tegument protein-protein interactions and their impact on viral tissue tropism in SCID-hu mice. This will help us develop a better understanding of how the tegument proteins aid viral DNA replication, evasion of host immune response, and pathogenesis.

The unusual fold of herpes simplex virus 1 UL21, a multifunctional tegument protein

UL21 is a conserved protein in the tegument of alphaherpesviruses and has multiple important albeit poorly understood functions in viral replication and pathogenesis. To provide a roadmap for exploration of the multiple roles of UL21, we determined the crystal structure of its conserved N-terminal domain from herpes simplex virus 1 to 2.0-Å resolution, which revealed a novel sail-like protein fold. Evolutionarily conserved surface patches highlight residues of potential importance for future targeting by mutagenesis.

The herpes simplex virus 2 UL21 protein is essential for virus propagation

Herpes simplex virus 2 (HSV-2) is an important human pathogen that is the major cause of genital herpes infections and a significant contributor to the epidemic spread of human immunodeficiency virus infections. The UL21 gene is conserved throughout the Alphaherpesvirinae subfamily and encodes a tegument protein that is dispensable for HSV-1 and pseudorabies virus replication in cultured cells; however, its precise functions have not been determined. To investigate the role of UL21 in the HSV-2 replicative cycle, we constructed a UL21 deletion virus (HSV-2 ΔUL21) using an HSV-2 bacterial artificial chromosome, pYEbac373. HSV-2 ΔUL21 was unable to direct the production of infectious virus in noncomplementing cells, whereas the repaired HSV-2 ΔUL21 strain grew to wild-type (WT) titers, indicating that UL21 is essential for virus propagation. Cells infected with HSV-2 ΔUL21 demonstrated a 2-h delay in the kinetics of immediate early viral gene expression. However, this delay in gene expression was not responsible for the inability of cells infected with HSV-2 ΔUL21 to produce virus insofar as late viral gene products accumulated to WT levels by 24 h postinfection (hpi). Electron and fluorescence microscopy studies indicated that DNA-containing capsids formed in the nuclei of ΔUL21-infected cells, while significantly reduced numbers of capsids were located in the cytoplasm late in infection. Taken together, these data indicate that HSV-2 UL21 has an early function that facilitates viral gene expression as well as a late essential function that promotes the egress of capsids from the nucleus.

Generation and Characterization of UL21-Null Herpes Simplex Virus Type 1

UL21 of herpes simplex virus type 1 (HSV-1) is an accessory gene that encodes a component of the tegument. Homologs of this protein have been identified in the alpha, beta, and gamma herpesvirus subfamilies, although their functions are unclear. To clarify the functions of UL21, we generated a UL21-null HSV-1 mutant. Growth analysis showed that the synthesis of infectious UL21-null HSV-1 in glial cells was delayed and that the overall yield was low. The plaque sizes of the UL21-null mutant were smaller than those of wild-type HSV-1. We identified several candidate UL21-interacting proteins, including intermediate filaments, by yeast two-hybrid screening. The distribution of glial fibrillary acidic protein (GFAP), which is the main component of intermediate filaments, was altered in UL21-null mutant-infected glial cells compared to wild-type virus-infected cells. These results will help clarify the function of UL21 and broaden our understanding of the life cycle of HSV.

Regulated interaction of tegument proteins UL16 and UL11 from herpes simplex virus

It is well known that proteins in the tegument (located between the viral capsid and envelope proteins) play critical roles in the assembly and budding of herpesviruses. Tegument proteins UL16 and UL11 of herpes simplex virus (HSV) are conserved among all the Herpesviridae. Although these proteins directly interact in vitro, UL16 was found to colocalize poorly with UL11 in cotransfected cells. To explain this discrepancy, we hypothesized that UL16 is initially made in an inactive form and is artificially transformed to the binding-competent state when cells are disrupted. Consistent with a regulated interaction, UL16 was able to fully colocalize with UL11 when a large C-terminal segment of UL16 was removed, creating mutant UL16(1-155). Moreover, membrane flotation assays revealed a massive movement of this mutant to the top of sucrose gradients in the presence of UL11, whereas both the full-length UL16 and the C-terminal fragment (residues 156 to 373) remained at the bottom. Further evidence for the presence of a C-terminal regulatory domain was provided by single-amino-acid substitutions at conserved cysteines (C269S, C271S, and C357S), which enabled the efficient interaction of full-length UL16 with UL11. Lastly, the binding site for UL11 was further mapped to residues 81 to 155, and to our surprise, the 5 Cys residues within UL16(1-155) are not required, even though the modification of free cysteines in UL16 with N-ethylmaleimide does in fact prevent binding. Collectively, these results reveal a regulatory function within the C-terminal region of UL16 that controls an N-terminal UL11-binding activity.