VP22 core domain from Herpes simplex virus 1 reveals a surprising structural conservation in both the Alpha- and Gammaherpesvirinae subfamilies

Liquid-liquid Phase Separation in Viral Function

An emerging set of results suggests that liquid-liquid phase separation (LLPS) is the basis for the formation of membrane-less compartments in cells. Evidence is now mounting that various types of virus-induced membrane-less compartments and organelles are also assembled via LLPS. Specifically, viruses appear to use intracellular phase transitions to form subcellular microenvironments known as viral factories, inclusion bodies, or viroplasms. These compartments - collectively referred to as viral biomolecular condensates - can be used to concentrate replicase proteins, viral genomes, and host proteins that are required for virus replication. They can also be used to subvert or avoid the intracellular immune response. This review examines how certain DNA or RNA viruses drive the formation of viral condensates, the possible biological functions of those condensates, and the biophysical and biochemical basis for their assembly.

Duck plague virus tegument protein vp22 plays a key role in the secondary envelopment and cell-to-cell spread

Duck plague virus (DPV) is one of the major infectious and fatal diseases of geese, ducks, and other wild waterfowl. The DPV UL49 gene product VP22 is one of the most abundant tegument proteins. However, the role of the DPV VP22 is enigmatic to be clarified. In this study, we found deletion of the UL49 gene resulted in reduced viral growth curve and smaller plaque size in duck embryo fibroblast (DEF) cells, confirming that DPV VP22 is required for efficient viral growth in vitro. In addition, deletion of the UL49 gene inhibited the secondary envelopment of the virus, the release of viral particles, and the spread of viruses between cells. Our study signified the importance of VP22 for DPV secondary envelopment, release, cell-to-cell spread, and accumulation of viral RNA. These findings provide a basis for further study of the function of VP22 in DPV or other herpesviruses.

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.

Structural basis of higher order oligomerization of KSHV inhibitor of cGAS

Kaposi's sarcoma-associated herpesvirus (KSHV) inhibitor of cyclic GMP-AMP synthase (cGAS) (KicGAS) encoded by ORF52 is a conserved major tegument protein of KSHV and the first reported viral inhibitor of cGAS. In our previous study, we found that KicGAS is highly oligomerized in solution and that oligomerization is required for its cooperative DNA binding and for inhibiting DNA-induced phase separation and activation of cGAS. However, how KicGAS oligomerizes remained unclear. Here, we present the crystal structure of KicGAS at 2.5 Å resolution, which reveals an "L"-shaped molecule with each arm of the L essentially formed by a single α helix (α1 and α2). Antiparallel dimerization of α2 helices from two KicGAS molecules leads to a unique "Z"-shaped dimer. Surprisingly, α1 is also a dimerization domain. It forms a parallel dimeric leucine zipper with the α1 from a neighboring dimer, leading to the formation of an infinite chain of KicGAS dimers. Residues involved in leucine zipper dimer formation are among the most conserved residues across ORF52 homologs of gammaherpesviruses. The self-oligomerization increases the valence and cooperativity of interaction with DNA. The resultant multivalent interaction is critical for the formation of liquid condensates with DNA and consequent sequestration of DNA from being sensed by cGAS, explaining its role in restricting cGAS activation. The structure presented here not only provides a mechanistic understanding of the function of KicGAS but also informs a molecular target for rational design of antivirals against KSHV and related viruses.

Varicella-Zoster virus ORF9 is an antagonist of the DNA sensor cGAS

Varicella-Zoster virus (VZV) causes chickenpox and shingles. Although the infection is associated with severe morbidity in some individuals, molecular mechanisms that determine innate immune responses remain poorly defined. We found that the cGAS/STING DNA sensing pathway was required for type I interferon (IFN) induction during VZV infection and that recognition of VZV by cGAS restricted its replication. Screening of a VZV ORF expression library identified the essential VZV tegument protein ORF9 as a cGAS antagonist. Ectopically or virally expressed ORF9 bound to endogenous cGAS leading to reduced type I IFN responses to transfected DNA. Confocal microscopy revealed co-localisation of cGAS and ORF9. ORF9 and cGAS also interacted directly in a cell-free system and phase-separated together with DNA. Furthermore, ORF9 inhibited cGAMP production by cGAS. Taken together, these results reveal the importance of the cGAS/STING DNA sensing pathway for VZV recognition and identify a VZV immune antagonist that partially but directly interferes with DNA sensing via cGAS.

Cooperative DNA binding mediated by KicGAS/ORF52 oligomerization allows inhibition of DNA-induced phase separation and activation of cGAS

Cyclic GMP-AMP synthase (cGAS) is a key DNA sensor that detects aberrant cytosolic DNA arising from pathogen invasions or genotoxic stresses. Upon binding to DNA, cGAS is activated and catalyzes the synthesis of cyclic GMP-AMP (cGAMP), which induces potent antimicrobial and antitumor responses. Kaposi sarcoma-associated herpesvirus (KSHV) is a human DNA tumor virus that causes Kaposi sarcoma and several other malignancies. We previously reported that KSHV inhibitor of cGAS (KicGAS) encoded by ORF52, inhibits cGAS enzymatic activity, but the underlying mechanisms remained unclear. To define the inhibitory mechanisms, here we performed in-depth biochemical and functional characterizations of KicGAS, and mapped its functional domains. We found KicGAS self-oligomerizes and binds to double stranded DNA cooperatively. This self-oligomerization is essential for its DNA binding and cGAS inhibition. Interestingly, KicGAS forms liquid droplets upon binding to DNA, which requires collective multivalent interactions with DNA mediated by both structured and disordered domains coordinated through the self-oligomerization of KicGAS. We also observed that KicGAS inhibits the DNA-induced phase separation and activation of cGAS. Our findings reveal a novel mechanism by which DNA viruses target the host protein phase separation for suppression of the host sensing of viral nucleic acids.

Evasion of Intracellular DNA Sensing by Human Herpesviruses

Sensing of viral constituents is the first and critical step in the host innate immune defense against viruses. In mammalian cells, there are a variety of pathogen recognition receptors (PRRs) that detect diverse pathogen-associated molecular patterns (PAMPs) including viral RNA and DNA. In the past decade, a number of host DNA sensors have been discovered and the underlying sensing mechanisms have been elucidated. Herpesviruses belong to a large family of enveloped DNA viruses. They are successful pathogens whose elaborate immune evasion mechanisms contribute to high prevalence of infection among their hosts. The three subfamilies of herpesviruses have all been found to employ diverse and overlapping strategies to interfere with host DNA sensing. These strategies include masking viral DNA or the DNA sensor, degradation of the DNA sensor, and post-transcriptional modification of the DNA sensor or its adaptor protein. In this review, we will discuss the current state of our knowledge on how human herpesviruses use these strategies to evade DNA-induced immune responses. Comprehensive understanding of herpesvirus immune-evasion mechanisms will aid in the development of vaccines and antivirals for herpesvirus-associated diseases.

Role of the DNA Binding Activity of Herpes Simplex Virus 1 VP22 in Evading AIM2-Dependent Inflammasome Activation Induced by the Virus

AIM2 is a cytosolic DNA sensor of the inflammasome, which induces critical innate immune responses against various invading pathogens. Earlier biochemical studies showed that the binding of AIM2 to DNA triggered the self-oligomerization of AIM2, which is essential for AIM2 inflammasome activation. We recently reported that VP22, a virion tegument protein of herpes simplex virus 1 (HSV-1), inhibited activation of the AIM2 inflammasome in HSV-1-infected cells by preventing AIM2 oligomerization. VP22 binds non-specifically to DNA; however, its role in HSV-1 replication is unclear. We investigated the role of VP22 DNA binding activity in the VP22-mediated inhibition of AIM2 inflammasome activation. We identified a VP22 domain encoded by amino acids 227 to 258 as the minimal domain required for its binding to DNA in vitro Consecutive alanine substitutions in this domain substantially impaired the DNA binding activity of VP22 in vitro and attenuated the inhibitory effect of VP22 on AIM2 inflammasome activation in an AIM2 inflammasome reconstitution system. The inhibitory effect of VP22 on AIM2 inflammasome activation was completely abolished in macrophages infected with a recombinant virus harboring VP22 with one of the consecutive alanine substitutions, similar to the effect of a VP22-null mutant virus. These results suggested that the DNA binding activity of VP22 is critical for VP22-mediated AIM2 inflammasome activation in HSV1-infected cells.IMPORTANCE VP22, a major component of the HSV-1 virion tegument, is conserved in alphaherpesviruses and has structural similarity to ORF52, a component of the virion tegument that is well-conserved in gammaherpesviruses. Although the potential DNA binding activity of VP22 was discovered decades ago, its significance in the HSV-1 life cycle is poorly understood. Here, we show that the DNA binding activity of VP22 is critical for the inhibition of AIM2 inflammasome activation induced in HSV-1-infected cells. This is the first report to show a role for the DNA binding activity of VP22 in the HSV-1 life cycle, allowing the virus to evade AIM2 inflammasome activation, which is critical for its replication in vivo.

Regulation and inhibition of the DNA sensor cGAS

Cell-autonomous sensing of nucleic acids is essential for host defence against invading pathogens by inducing antiviral and inflammatory cytokines. cGAS has emerged in recent years as a non-redundant DNA sensor important for detection of many viruses and bacteria. Upon binding to DNA, cGAS synthesises the cyclic dinucleotide 2'3'-cGAMP that binds to the adaptor protein STING and thereby triggers IRF3- and NFκB-dependent transcription. In addition to infection, the pathophysiology of an ever-increasing number of sterile inflammatory conditions in humans involves the recognition of DNA through cGAS. Consequently, the cGAS/STING signalling axis has emerged as an attractive target for pharmacological modulation. However, the development of cGAS and STING inhibitors has just begun and a need for specific and effective compounds persists. In this review, we focus on cGAS and explore how its activation by immunostimulatory DNA is regulated by cellular mechanisms, viral immune modulators and small molecules. We further use our knowledge of cGAS modulation by cells and viruses to conceptualise potential new ways of pharmacological cGAS targeting.

The Role of VP16 in the Life Cycle of Alphaherpesviruses

The protein encoded by the UL48 gene of alphaherpesviruses is named VP16 or alpha-gene-transactivating factor (α-TIF). In the early stage of viral replication, VP16 is an important transactivator that can activate the transcription of viral immediate-early genes, and in the late stage of viral replication, VP16, as a tegument, is involved in viral assembly. This review will explain the mechanism of VP16 acting as α-TIF to activate the transcription of viral immediate-early genes, its role in the transition from viral latency to reactivation, and its effects on viral assembly and maturation. In addition, this review also provides new insights for further research on the life cycle of alphaherpesviruses and the role of VP16 in the viral life cycle.

Alphaherpesvirus Major Tegument Protein VP22: Its Precise Function in the Viral Life Cycle

Alphaherpesviruses are zoonotic pathogens that can cause a variety of diseases in humans and animals and severely damage health. Alphaherpesvirus infection is a slow and orderly process that can lie dormant for the lifetime of the host but may be reactivated when the immune system is compromised. All alphaherpesviruses feature a protein layer called the tegument that lies between the capsid and the envelope. Virus protein (VP) 22 is one of the most highly expressed tegument proteins; there are more than 2,000 copies of this protein in each viral particle. VP22 can interact with viral proteins, cellular proteins, and chromatin, and these interactions play important roles. This review summarizes the latest literature and discusses the roles of VP22 in viral gene transcription, protein synthesis, virion assembly, and viral cell-to-cell spread with the purpose of enhancing understanding of the life cycle of herpesviruses and other pathogens in host cells. The molecular interaction information herein provides important reference data.

Conserved Outer Tegument Component UL11 from Herpes Simplex Virus 1 Is an Intrinsically Disordered, RNA-Binding Protein

A distinguishing morphological feature of all herpesviruses is the multiprotein tegument layer located between the nucleocapsid and lipid envelope of the virion. Tegument proteins play multiple roles in viral replication, including viral assembly, but we do not yet understand their individual functions or how the tegument is assembled and organized. UL11, the smallest tegument protein, is important for several distinct processes in replication, including efficient virion morphogenesis and cell-cell spread. However, the mechanistic understanding of its role in these and other processes is limited in part by the scant knowledge of its biochemical and structural properties. Here, we report that UL11 from herpes simplex virus 1 (HSV-1) is an intrinsically disordered, conformationally dynamic protein that undergoes liquid-liquid phase separation (LLPS) in vitro Intrinsic disorder may underlie the ability of UL11 to exert multiple functions and bind multiple partners. Sequence analysis suggests that not only all UL11 homologs but also all HSV-1 tegument proteins contain intrinsically disordered regions of different lengths. The presence of intrinsic disorder, and potentially, the ability to form LLPS, may thus be a common feature of the tegument proteins. We hypothesize that tegument assembly may involve the formation of a biomolecular condensate, driven by the heterogeneous mixture of intrinsically disordered tegument proteins.IMPORTANCE Herpesvirus virions contain a unique tegument layer sandwiched between the capsid and lipid envelope and composed of multiple copies of about two dozen viral proteins. However, little is known about the structure of the tegument or how it is assembled. Here, we show that a conserved tegument protein UL11 from herpes simplex virus 1, a prototypical alphaherpesvirus, is an intrinsically disordered protein that undergoes liquid-liquid phase separation in vitro Through sequence analysis, we find intrinsically disordered regions of different lengths in all HSV-1 tegument proteins. We hypothesize that intrinsic disorder is a common characteristic of tegument proteins and propose a new model of tegument as a biomolecular condensate.

Identification of Marek's Disease Virus VP22 Tegument Protein Domains Essential for Virus Cell-to-Cell Spread, Nuclear Localization, Histone Association and Cell-Cycle Arrest

VP22 is a major tegument protein of alphaherpesviruses encoded by the UL49 gene. Two properties of VP22 were discovered by studying Marek's disease virus (MDV), the Mardivirus prototype; it has a major role in virus cell-to-cell spread and in cell cycle modulation. This 249 AA-long protein contains three regions including a conserved central domain. To decipher the functional VP22 domains and their relationships, we generated three series of recombinant MDV genomes harboring a modified UL49 gene and assessed their effect on virus spread. Mutated VP22 were also tested for their ability to arrest the cell cycle, subcellular location and histones copurification after overexpression in cells. We demonstrated that the N-terminus of VP22 associated with its central domain is essential for virus spread and cell cycle modulation. Strikingly, we demonstrated that AAs 174-190 of MDV VP22 containing the end of a putative extended alpha-3 helix are essential for both functions and that AAs 159-162 located in the putative beta-strand of the central domain are mandatory for cell cycle modulation. Despite being non-essential, the 59 C-terminal AAs play a role in virus spread efficiency. Interestingly, a positive correlation was observed between cell cycle modulation and VP22 histones association, but none with MDV spread.

Decreased expression of the immediate early protein, ICP4, by deletion of the tegument protein VP22 of equine herpesvirus type 1

VP22 is a major tegument protein of equine herpesvirus type 1 (EHV-1). In the present study, we examined functions of VP22 in EHV-1 replication by viral protein expression analyses in cells infected with the VP22-deficient virus. The expressions of several viral proteins in the cells infected with the VP22-deficient virus were lower than those in the cells infected with the parent virus. One of the weakly expressed proteins was identified as ICP4, which is a major regulatory protein encoded by an immediate early gene of EHV-1. A real-time PCR analysis showed that the mRNA expression of ICP4 was the same in cells infected with the parent and VP22-deficient viruses. Hence, VP22 appears to promote synthesis of ICP4 post-transcriptionally.

A Conserved Leucine Zipper Motif in Gammaherpesvirus ORF52 Is Critical for Distinct Microtubule Rearrangements

Productive viral infection often depends on the manipulation of the cytoskeleton. Herpesviruses, including rhesus monkey rhadinovirus (RRV) and its close homolog, the oncogenic human gammaherpesvirus Kaposi's sarcoma-associated herpesvirus/human herpesvirus 8 (KSHV/HHV8), exploit microtubule (MT)-based retrograde transport to deliver their genomes to the nucleus. Subsequently, during the lytic phase of the life cycle, the maturing viral particles undergo orchestrated translocation to specialized regions within the cytoplasm, leading to tegumentation, secondary envelopment, and then egress. As a result, we hypothesized that RRV might induce changes in the cytoskeleton at both early and late stages of infection. Using confocal imaging, we found that RRV infection led to the thickening and acetylation of MTs emanating from the MT-organizing center (MTOC) shortly after viral entry and more pronounced and diffuse MT reorganization during peak stages of lytic gene expression and virion production. We subsequently identified open reading frame 52 (ORF52), a multifunctional and abundant tegument protein, as being the only virally encoded component responsible for these cytoskeletal changes. Mutational and modeling analyses indicated that an evolutionarily conserved, truncated leucine zipper motif near the N terminus as well as a strictly conserved arginine residue toward the C terminus of ORF52 play critical roles in its ability to rearrange the architecture of the MT cytoskeleton. Taken together, our findings combined with data from previous studies describing diverse roles for ORF52 suggest that it likely binds to different cellular components, thereby allowing context-dependent modulation of function.

IMPORTANCE A thorough understanding of the processes governing viral infection includes knowledge of how viruses manipulate their intracellular milieu, including the cytoskeleton. Altering the dynamics of actin or MT polymerization, for example, is a common strategy employed by viruses to ensure efficient entry, maturation, and egress as well as the avoidance of antiviral defenses through the sequestration of key cellular factors. We found that infection with RRV, a homolog of the human pathogen KSHV, led to perinuclear wrapping by acetylated MT bundles and identified ORF52 as the viral protein underlying these changes. Remarkably, incoming virions were able to supply sufficient ORF52 to induce MT thickening and acetylation near the MTOC, potentially aiding in the delivery viral genomes to the nucleus. Although the function of MT alterations during late stages of infection requires further study, ORF52 shares functional and structural similarities with alphaherpesvirus VP22, underscoring the evolutionary importance of MT cytoskeletal manipulations for this virus family.

Kaposi's Sarcoma-Associated Herpesvirus Inhibitor of cGAS (KicGAS), Encoded by ORF52, Is an Abundant Tegument Protein and Is Required for Production of Infectious Progeny Viruses

Although Kaposi's sarcoma-associated herpesvirus (KSHV) ORF52 (also known as KSHV inhibitor of cGAS [KicGAS]) has been detected in purified virions, the roles of this protein during KSHV replication have not been characterized. Using specific monoclonal antibodies, we revealed that ORF52 displays true late gene expression kinetics and confirmed its cytoplasmic localization in both transfected and KSHV-infected cells. We demonstrated that ORF52 comigrates with other known virion proteins following sucrose gradient centrifugation. We also determined that ORF52 resides inside the viral envelope and remains partially associated with capsid when extracellular virions are treated with various detergents and/or salts. There results indicate that ORF52 is a tegument protein abundantly present in extracellular virions. To characterize the roles of ORF52 in the KSHV life cycle, we engineered a recombinant KSHV ORF52-null mutant virus and found that loss of ORF52 results in reduced virion production and a further defect in infectivity. Upon analysis of the virion composition of ORF52-null viral particles, we observed a decrease in the incorporation of ORF45, as well as other tegument proteins, suggesting that ORF52 is important for the packaging of other virion proteins. In summary, our results indicate that, in addition to its immune evasion function, KSHV ORF52 is required for the optimal production of infectious virions, likely due to its roles in virion assembly as a tegument protein.

IMPORTANCE

The tegument proteins of herpesviruses, including Kaposi's sarcoma-associated herpesvirus (KSHV), play key roles in the viral life cycle. Each of the three subfamilies of herpesviruses (alpha, beta, and gamma) encode unique tegument proteins with specialized functions. We recently found that one such gammaherpesvirus-specific protein, ORF52, has an important role in immune evasion during KSHV primary infection, through inhibition of the host cytosolic DNA sensing pathway. In this report, we further characterize ORF52 as a tegument protein with vital roles during KSHV lytic replication. We found that ORF52 is important for the production of infectious viral particles, likely through its role in virus assembly, a critical process for KSHV replication and pathogenesis. More comprehensive investigation of the functions of tegument proteins and their roles in viral replication may reveal novel targets for therapeutic interventions against KSHV-associated diseases.

Crystal Structure of Human Herpesvirus 6B Tegument Protein U14

The tegument protein U14 of human herpesvirus 6B (HHV-6B) constitutes the viral virion structure and is essential for viral growth. To define the characteristics and functions of U14, we determined the crystal structure of the N-terminal domain of HHV-6B U14 (U14-NTD) at 1.85 Å resolution. U14-NTD forms an elongated helix-rich fold with a protruding β hairpin. U14-NTD exists as a dimer exhibiting broad electrostatic interactions and a network of hydrogen bonds. This is first report of the crystal structure and dimerization of HHV-6B U14. The surface of the U14-NTD dimer reveals multiple clusters of negatively- and positively-charged residues that coincide with potential functional sites of U14. Three successive residues, L424, E425 and V426, which relate to viral growth, reside on the β hairpin close to the dimer's two-fold axis. The hydrophobic side-chains of L424 and V426 that constitute a part of a hydrophobic patch are solvent-exposed, indicating the possibility that the β hairpin region is a key functional site of HHV-6 U14. Structure-based sequence comparison suggests that U14-NTD corresponds to the core fold conserved among U14 homologs, human herpesvirus 7 U14, and human cytomegalovirus UL25 and UL35, although dimerization appears to be a specific feature of the U14 group.

Human herpesvirus 6B (HHV-6B), a causative agent of exanthema subitum for children and immunocompromised adults, encodes numerous tegument proteins that constitute the viral matrix. HHV-6B U14 is a tegument protein essential for viral propagation, and additionally it interacts with host factors such as tumor suppressor p53 and cellular protein EDD, thereby regulating host cell responses. Here, we report the molecular structure of HHV-6B U14 at an atomic resolution. The N-terminal domain of U14 (U14-NTD) adopts an elongated, helix-rich fold without any significant overall similarity to known structures. U14-NTD forms a 100 kDa homodimer through electrostatic interactions and a wide hydrogen bond network. The U14-NTD homodimer displays four clusters of electrostatic potential with deep grooves, implying multiple binding sites for other viral or host proteins. U14-NTD corresponds to the core fold shared by homologous proteins of human herpesvirus 7 (HHV-7) and of human cytomegalovirus, although dimerization seems to be specific to HHV-6 and HHV-7. The U14-NTD structure provides clues to promote further analysis on the role and behavior of U14 in the pathogenesis of HHV-6. It also leads to a comprehensive understanding of the U14 homologs in beta herpesviruses, and furthermore contributes to the overall knowledge about tegument proteins in herpesviruses.

Cell-penetrating peptides as noninvasive transmembrane vectors for the development of novel multifunctional drug-delivery systems

Unique characteristics, such as nontoxicity and rapid cellular internalization, allow the cell-penetrating peptides (CPPs) to transport hydrophilic macromolecules into cells, thus, enabling them to execute biological functions. However, some CPPs have limitations due to nonspecificity and easy proteolysis. To overcome such defects, the CPP amino acid sequence can be modified, replaced, and reconstructed for optimization. CPPs can also be used in combination with other drug vectors, fused with their preponderances to create novel multifunctional drug-delivery systems that increase the stability during blood circulation, and also develop novel preparations capable of targeted delivery, along with sustainable and controllable release. Further improvements in CPP structure can facilitate the penetration of macromolecules into diverse biomembrane structures, such as the blood brain barrier, gastroenteric mucosa, and skin dermis. The ability of CPP to act as transmembrane vectors improves the clinical application of some biomolecules to treat central nervous system diseases, increase oral bioavailability, and develop percutaneous-delivery dosage form.

Viral evasion of DNA-stimulated innate immune responses

Cellular sensing of virus-derived nucleic acids is essential for early defenses against virus infections. In recent years, the discovery of DNA sensing proteins, including cyclic GMP-AMP synthase (cGAS) and gamma-interferon-inducible protein (IFI16), has led to understanding of how cells evoke strong innate immune responses against incoming pathogens carrying DNA genomes. The signaling stimulated by DNA sensors depends on the adaptor protein STING (stimulator of interferon genes), to enable expression of antiviral proteins, including type I interferon. To facilitate efficient infections, viruses have evolved a wide range of evasion strategies, targeting host DNA sensors, adaptor proteins and transcription factors. In this review, the current literature on virus-induced activation of the STING pathway is presented and we discuss recently identified viral evasion mechanisms targeting different steps in this antiviral pathway.

Tegument Assembly and Secondary Envelopment of Alphaherpesviruses

Alphaherpesviruses like herpes simplex virus are large DNA viruses characterized by their ability to establish lifelong latent infection in neurons. As for all herpesviruses, alphaherpesvirus virions contain a protein-rich layer called "tegument" that links the DNA-containing capsid to the glycoprotein-studded membrane envelope. Tegument proteins mediate a diverse range of functions during the virus lifecycle, including modulation of the host-cell environment immediately after entry, transport of virus capsids to the nucleus during infection, and wrapping of cytoplasmic capsids with membranes (secondary envelopment) during virion assembly. Eleven tegument proteins that are conserved across alphaherpesviruses have been implicated in the formation of the tegument layer or in secondary envelopment. Tegument is assembled via a dense network of interactions between tegument proteins, with the redundancy of these interactions making it challenging to determine the precise function of any specific tegument protein. However, recent studies have made great headway in defining the interactions between tegument proteins, conserved across alphaherpesviruses, which facilitate tegument assembly and secondary envelopment. We summarize these recent advances and review what remains to be learned about the molecular interactions required to assemble mature alphaherpesvirus virions following the release of capsids from infected cell nuclei.

Conserved tegument protein complexes: Essential components in the assembly of herpesviruses

One of the structural components of herpesviruses is a protein layer called the tegument. Several of the tegument proteins are highly conserved across the herpesvirus family and serve as a logical focus for defining critical interactions required for viral assembly. A number of studies have helped to elucidate a role for conserved tegument proteins in the process of secondary envelopment during the course of herpesviral assembly. This review highlights how these tegument proteins directly contribute to bridging the nucleocapsid and envelope of virions during secondary envelopment.

Inhibition of cGAS DNA Sensing by a Herpesvirus Virion Protein

Invading viral DNA can be recognized by the host cytosolic DNA sensor, cyclic GMP-AMP (cGAMP) synthase (cGAS), resulting in production of the second messenger cGAMP, which directs the adaptor protein STING to stimulate production of type I interferons (IFNs). Although several DNA viruses are sensed by cGAS, viral strategies targeting cGAS are virtually unknown. We report here that Kaposi's sarcoma-associated herpesvirus (KSHV) ORF52, an abundant gammaherpesvirus-specific tegument protein, subverts cytosolic DNA sensing by directly inhibiting cGAS enzymatic activity through a mechanism involving both cGAS binding and DNA binding. Moreover, ORF52 homologs in other gammaherpesviruses also inhibit cGAS activity and similarly bind cGAS and DNA, suggesting conserved inhibitory mechanisms. Furthermore, KSHV infection evokes cGAS-dependent responses that can limit the infection, and an ORF52 null mutant exhibits increased cGAS signaling. Our findings reveal a mechanism through which gammaherpesviruses antagonize host cGAS DNA sensing.