Capsid structure of simian cytomegalovirus from cryoelectron microscopy: evidence for tegument attachment sites

Comprehensive Analysis of the Tegument Proteins Involved in Capsid Transport and Virion Morphogenesis of Alpha, Beta and Gamma Herpesviruses

Herpesviruses are enveloped and have an amorphous protein layer surrounding the capsid, which is termed the tegument. Tegument proteins perform critical functions throughout the viral life cycle. This review provides a comprehensive and comparative analysis of the roles of specific tegument proteins in capsid transport and virion morphogenesis of selected, well-studied prototypes of each of the three subfamilies of Herpesviridae i.e., human herpesvirus-1/herpes simplex virus-1 (Alphaherpesvirinae), human herpesvirus-5/cytomegalovirus (Betaherpesvirinae) and human herpesvirus -8/Kaposi's sarcomavirus (Gammaherpesvirinae). Most of the current knowledge is based on alpha herpesviruses, in particular HSV-1. While some tegument proteins are released into the cytoplasm after virus entry, several tegument proteins remain associated with the capsid and are responsible for transport to and docking at the nucleus. After replication and capsid formation, the capsid is enveloped at the nuclear membrane, which is referred to as primary envelopment, followed by de-envelopment and release into the cytoplasm. This requires involvement of at least three tegument proteins. Subsequently, multiple interactions between tegument proteins and capsid proteins, other tegument proteins and glycoproteins are required for assembly of the virus particles and envelopment at the Golgi, with certain tegument proteins acting as the central hub for these interactions. Some redundancy in these interactions ensures appropriate morphogenesis.

Tegument Protein pp150 Sequence-Specific Peptide Blocks Cytomegalovirus Infection

Human cytomegalovirus (HCMV) tegument protein pp150 is essential for the completion of the final steps in virion maturation. Earlier studies indicated that three pp150nt (N-terminal one-third of pp150) conformers cluster on each triplex (Tri1, Tri2A and Tri2B), and extend towards small capsid proteins atop nearby major capsid proteins, forming a net-like layer of tegument densities that enmesh and stabilize HCMV capsids. Based on this atomic detail, we designed several peptides targeting pp150nt. Our data show significant reduction in virus growth upon treatment with one of these peptides (pep-CR2) with an IC₅₀ of 1.33 μM and no significant impact on cell viability. Based on 3D modeling, pep-CR2 specifically interferes with the pp150–capsid binding interface. Cells pre-treated with pep-CR2 and infected with HCMV sequester pp150 in the nucleus, indicating a mechanistic disruption of pp150 loading onto capsids and subsequent nuclear egress. Furthermore, pep-CR2 effectively inhibits mouse cytomegalovirus (MCMV) infection in cell culture, paving the way for future animal testing. Combined, these results indicate that CR2 of pp150 is amenable to targeting by a peptide inhibitor, and can be developed into an effective antiviral.

Atomic structures and deletion mutant reveal different capsid-binding patterns and functional significance of tegument protein pp150 in murine and human cytomegaloviruses with implications for therapeutic development

Cytomegalovirus (CMV) infection causes birth defects and life-threatening complications in immunosuppressed patients. Lack of vaccine and need for more effective drugs have driven widespread ongoing therapeutic development efforts against human CMV (HCMV), mostly using murine CMV (MCMV) as the model system for preclinical animal tests. The recent publication (Yu et al., 2017, DOI: 10.1126/science.aam6892) of an atomic model for HCMV capsid with associated tegument protein pp150 has infused impetus for rational design of novel vaccines and drugs, but the absence of high-resolution structural data on MCMV remains a significant knowledge gap in such development efforts. Here, by cryoEM with sub-particle reconstruction method, we have obtained the first atomic structure of MCMV capsid with associated pp150. Surprisingly, the capsid-binding patterns of pp150 differ between HCMV and MCMV despite their highly similar capsid structures. In MCMV, pp150 is absent on triplex Tc and exists as a “Λ”-shaped dimer on other triplexes, leading to only 260 groups of two pp150 subunits per capsid in contrast to 320 groups of three pp150 subunits each in a “Δ”-shaped fortifying configuration. Many more amino acids contribute to pp150-pp150 interactions in MCMV than in HCMV, making MCMV pp150 dimer inflexible thus incompatible to instigate triplex Tc-binding as observed in HCMV. While pp150 is essential in HCMV, our pp150-deletion mutant of MCMV remained viable though with attenuated infectivity and exhibiting defects in retaining viral genome. These results thus invalidate targeting pp150, but lend support to targeting capsid proteins, when using MCMV as a model for HCMV pathogenesis and therapeutic studies.

Cytomegalovirus (CMV) infection is a leading viral cause of birth defects and could be deadly to AIDS patients and organ transplant recipients. Absence of effective vaccines and potent drugs against human CMV (HCMV) infections has motivated animal-based studies, mostly based on the mouse model with murine CMV (MCMV), both for understanding pathogenesis of CMV infections and for developing therapeutic strategies. Distinct from other medically important herpesviruses (those responsible for cold sores, genital herpes, shingles and several human cancers), CMV contains an abundant phosphoprotein, pp150, which is a structurally, immunogenically, and regulatorily important tegument protein and a potential drug target. Here, we used cryoEM with localized reconstruction method to obtain the first atomic structure of MCMV. The structure reveals that the organization patterns of the capsid-associated tegument protein pp150 are different in MCMV and HCMV, despite their highly similar capsid structures. We also show that deleting pp150 did not eliminate MCMV infection in contrast to pp150’s essential role in HCMV infections. Our results have significant implication to the current practice of using mouse infected with MCMV for HCMV therapeutic development.

Different functional states of fusion protein gB revealed on human cytomegalovirus by cryo electron tomography with Volta phase plate

Human cytomegalovirus (HCMV) enters host by glycoprotein B (gB)-mediated membrane fusion upon receptor-binding to gH/gL-related complexes, causing devastating diseases such as birth defects. Although an X-ray crystal structure of the recombinant gB ectodomain at postfusion conformation is available, the structures of prefusion gB and its complex with gH/gL on the viral envelope remain elusive. Here, we demonstrate the utility of cryo electron tomography (cryoET) with energy filtering and the cutting-edge technologies of Volta phase plate (VPP) and direct electron-counting detection to capture metastable prefusion viral fusion proteins and report the structures of glycoproteins in the native environment of HCMV virions. We established the validity of our approach by obtaining cryoET in situ structures of the vesicular stomatitis virus (VSV) glycoprotein G trimer (171 kD) in prefusion and postfusion conformations, which agree with the known crystal structures of purified G trimers in both conformations. The excellent contrast afforded by these technologies has enabled us to identify gB trimers (303kD) in two distinct conformations in HCMV tomograms and obtain their in situ structures at up to 21 Å resolution through subtomographic averaging. The predominant conformation (79%), which we designate as gB prefusion conformation, fashions a globular endodomain and a Christmas tree-shaped ectodomain, while the minority conformation (21%) has a columnar tree-shaped ectodomain that matches the crystal structure of the "postfusion" gB ectodomain. We also observed prefusion gB in complex with an "L"-shaped density attributed to the gH/gL complex. Integration of these structures of HCMV glycoproteins in multiple functional states and oligomeric forms with existing biochemical data and domain organization of other class III viral fusion proteins suggests that gH/gL receptor-binding triggers conformational changes of gB endodomain, which in turn triggers two essential steps to actuate virus-cell membrane fusion: exposure of gB fusion loops and unfurling of gB ectodomain.

Structural Aspects of Betaherpesvirus-Encoded Proteins

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

A pUL25 dimer interfaces the pseudorabies virus capsid and tegument

Inside the virions of α-herpesviruses, tegument protein pUL25 anchors the tegument to capsid vertices through direct interactions with tegument proteins pUL17 and pUL36. In addition to promoting virion assembly, both pUL25 and pUL36 are critical for intracellular microtubule-dependent capsid transport. Despite these essential roles during infection, the stoichiometry and precise organization of pUL25 and pUL36 on the capsid surface remain controversial due to the insufficient resolution of existing reconstructions from cryo-electron microscopy (cryoEM). Here, we report a three-dimensional (3D) icosahedral reconstruction of pseudorabies virus (PRV), a varicellovirus of the α-herpesvirinae subfamily, obtained by electron-counting cryoEM at 4.9 Å resolution. Our reconstruction resolves a dimer of pUL25 forming a capsid-associated tegument complex with pUL36 and pUL17 through a coiled coil helix bundle, thus correcting previous misinterpretations. A comparison between reconstructions of PRV and the γ-herpesvirus Kaposi's sarcoma-associated herpesvirus (KSHV) reinforces their similar architectures and establishes important subfamily differences in the capsid-tegument interface.

Synthetic DNA approach to cytomegalovirus vaccine/immune therapy

There is no licensed vaccine or cure for human cytomegalovirus (CMV), a ubiquitous β-herpes virus that infects 60-95 % of adults worldwide. Infection is a major cause of congenital abnormalities in newborns, contributes to development of childhood cerebral palsy and medulloblastoma, can result in severe disease in immunocompromised patients, and is a major impediment during successful organ transplantation. While CMV has been increasingly associated with numerous inflammatory diseases and cancers, only recently has it been correlated with increased risk of heart disease in adults, the number-one killer in the USA. These data, among others, suggest that subclinical CMV infection, or microinfection, in healthy individuals may play more of a causative role than an epiphenomenon in development of CMV-associated pathologies. Due to the myriad of diseases and complications associated with CMV, an efficacious vaccine would be highly valuable in reducing human morbidity and mortality as well as saving billions of dollars in annual health-care costs and disability adjusted life years (DALY) in the developing world. Therefore, the development of a safe efficacious CMV vaccine or immune therapy is paramount to the public health. This review aims to provide a brief overview on aspects of CMV infection and disease and focuses on current vaccine strategies. The use of new synthetic DNA vaccines might offer one such approach to this difficult problem.

The A, B, Cs of herpesvirus capsids

Assembly of herpesvirus nucleocapsids shares significant similarities with the assembly of tailed dsDNA bacteriophages; however, important differences exist. A unique feature of herpesviruses is the presence of different mature capsid forms in the host cell nucleus during infection. These capsid forms, referred to as A-, B-, and C-capsids, represent empty capsids, scaffold containing capsids and viral DNA containing capsids, respectively. The C-capsids are the closest in form to those encapsidated into mature virions and are considered precursors to infectious virus. The evidence supporting A- and B-capsids as either abortive forms or assembly intermediates has been lacking. Interaction of specific capsid forms with viral tegument proteins has been proposed to be a mechanism for quality control at the point of nuclear egress of mature particles. Here, we will review the available literature on these capsid forms and present data to debate whether A- and B-capsids play an important or an extraneous role in the herpesvirus life cycle.

The large tegument protein pUL36 is essential for formation of the capsid vertex-specific component at the capsid-tegument interface of herpes simplex virus 1

Herpesviruses have a characteristic particle structure comprising an icosahedral capsid, which contains the DNA genome and is, in turn, surrounded by a proteinaceous tegument layer and a lipid envelope. In herpes simplex virus, the interaction between the capsid and tegument is limited to the capsid vertices and involves two minor capsid proteins, pUL17 and pUL25, and the large inner tegument protein pUL36. pUL17 and pUL25 form a heterodimeric structure, the capsid vertex-specific component (CVSC), that lies on top of the peripentonal triplexes, while pUL36 has been reported to connect the CVSC to the penton. In this study, we used virus mutants with deletions in the genes for pUL36 and another inner tegument protein, pUL37, to analyze the contributions of these proteins to CVSC structure. Using electron cryomicroscopy and icosahedral reconstruction of mutants that express pUL17 and pUL25 but not pUL36, we showed that in contrast to accepted models, the CVSC is not formed from pUL17 and pUL25 on their own but requires a contribution from pUL36. In addition, the presence of full-length pUL36 results in weak density that extends the CVSC toward the penton, suggesting either that this extra density is formed directly by pUL36 or that pUL36 stabilizes other components of the vertex-tegument interface.

IMPORTANCE Herpesviruses have complex particles that are formed as a result of a carefully controlled sequence of assembly steps. The nature of the interaction between two of the major particle compartments, the icosahedral capsid and the amorphous tegument, has been extensively studied, but the identity of the interacting proteins and their roles in forming the connections are still unclear. In this study, we used electron microscopy and three-dimensional reconstruction to analyze virus particles formed by mutants that do not express particular interacting proteins. We show that the largest viral protein, pUL36, which occupies the layer of tegument closest to the capsid, is essential for formation of structurally normal connections to the capsid. This demonstrates the importance of pUL36 in the initial stages of tegument addition and provides new insights into the process of virus particle assembly.

Organization of capsid-associated tegument components in Kaposi's sarcoma-associated herpesvirus

Capsid-associated tegument proteins have been identified in alpha- and betaherpesviruses to play an essential role in viral DNA packaging. Whether and how such tegument proteins exist in gammaherpesviruses have been mysteries. Here, we report a 6-Å-resolution cryo-electron microscopy (cryo-EM) structure of Kaposi's sarcoma-associated herpesvirus (KSHV) virion, a member of the oncogenic gammaherpesvirus subfamily. The KSHV virion structure reveals, for the first time, how capsid-associated tegument proteins are organized in a gammaherpesvirus, with five tegument densities capping each penton vertex, a pattern highly similar to that in alphaherpesvirus but completely different from that in betaherpesvirus. Each KSHV tegument density can be divided into three prominent regions: a penton-binding globular region, a helix-bundle stalk region, and a β-sheet-rich triplex-binding region. Fitting of the crystal structure of the truncated HSV-1 UL25 protein (the KSHV ORF19 homolog) and secondary structure analysis of the full-length ORF19 established that ORF19 constitutes the globular region with an N-terminal, 60-amino-acid-long helix extending into the stalk region. Matching secondary structural features resolved in the cryo-EM density with secondary structures predicted by sequence analysis identifies the triplex-binding region to be ORF32, a homolog of alphaherpesvirus UL17. Despite the high level of tegument structural similarities between KSHV and alphaherpesvirus, an ORF19 monomer in KSHV, in contrast to a UL25 dimer in alphaherpesviruses, binds each penton subunit, an observation that correlates with conformational differences in their pentons. This newly discovered organization of triplex-ORF32-ORF19 also resolves a long-standing mystery surrounding the virion location and conformation of alphaherpesvirus UL25 protein.

IMPORTANCE

Several capsid-associated tegument proteins have been identified in the alpha- and betaherpesvirus subfamilies of the Herpesviridae. These tegument proteins play essential roles in viral propagation and are potential drug targets for curbing herpesvirus infections. However, no such tegument proteins have been identified for gammaherpesviruses, the third herpesvirus subfamily, which contains members causing several human cancers. Here, by high-resolution cryo-EM, we show the three-dimensional structure of the capsid-associated tegument proteins in the prototypical member of gammaherpesviruses, KSHV. The cryo-EM structure reveals that the organization of KSHV capsid-associated tegument proteins is highly similar to that in alphaherpesvirus but completely different from that in betaherpesvirus. Structural analyses further localize ORF19 and ORF32 proteins (the alphaherpesvirus UL25 and UL17 homologs in KSHV, respectively) in the KSHV capsid-associated tegument cryo-EM structure. These findings also resolve a long-standing mystery regarding the location and conformation of alphaherpesvirus UL25 protein inside the virion.

Recovery of an HMWP/hmwBP (pUL48/pUL47) complex from virions of human cytomegalovirus: subunit interactions, oligomer composition, and deubiquitylase activity

We report that the human cytomegalovirus (HCMV) high-molecular-weight tegument protein (HMWP, pUL48; 253 kDa) and the HMWP-binding protein (hmwBP, pUL47; 110 kDa) can be recovered as a complex from virions disrupted by treatment with 50 mM Tris (pH 7.5), 0.5 M NaCl, 0.5% NP-40, and 10 mM dithiothreitol [DTT]. The subunit ratio of the complex approximates 1:1, with a shape and structure consistent with an elongated heterodimer. The HMWP/hmwBP complex was corroborated by reciprocal coimmunoprecipitation experiments using antipeptide antibodies and lysates from both infected cells and disrupted virus particles. An interaction of the amino end of pUL48 (amino acids [aa] 322 to 754) with the carboxyl end of pUL47 (aa 693 to 982) was identified by fragment coimmunoprecipitation experiments, and a head-to-tail self-interaction of hmwBP was also observed. The deubiquitylating activity of pUL48 is retained in the isolated complex, which cleaves K11, K48, and K63 ubiquitin isopeptide linkages.

IMPORTANCE

Human cytomegalovirus (HCMV, or human herpesvirus 5 [HHV-5]) is a large DNA-containing virus that belongs to the betaherpesvirus subfamily and is a clinically important pathogen. Defining the constituent elements of its mature form, their organization within the particle, and the assembly process by which it is produced are fundamental to understanding the mechanisms of herpesvirus infection and developing drugs and vaccines against them. In this study, we report isolating a complex of two large proteins encoded by HCMV open reading frames (ORFs) UL47 and UL48 and identifying the binding domains responsible for their interaction with each other and of pUL47 with itself. Our calculations indicate that the complex is a rod-shaped heterodimer. Additionally, we determined that the ubiquitin-specific protease activity of the ORF UL48 protein was functional in the complex, cleaving K11-, K48-, and K63-linked ubiquitin dimers. This information builds on and extends our understanding of the HCMV tegument protein network that is required to interface the HCMV envelope and capsid.

Role of human cytomegalovirus tegument proteins in virion assembly

Like other herpesviruses, human cytomegalovirus (HCMV) contains a unique proteinaceous layer between the virion envelope and capsid, termed the tegument. Upon infection, the contents of the tegument layer are delivered to the host cell, along with the capsid and the viral genome, where they facilitate the initial stages of virus replication. The tegument proteins also play important roles in virion assembly and this dual nature makes them attractive potential targets for antiviral therapies. While our knowledge regarding tegument protein function during the initiation of infection has been the subject of intense study, their roles in assembly are much less well understood. In this review, we will focus on recent studies that highlight the functions of HCMV tegument proteins during assembly, and pose key questions for further investigation.

Highly acidic C-terminal region of cytomegalovirus pUL96 determines its functions during virus maturation independently of a direct pp150 interaction

Tegument proteins pp150 and pUL96 function at a late step in cytomegalovirus (CMV) maturation. Here, we show that pp150 interacts directly with pUL96; however, the N-terminal region of pp150 and the C-terminal region of pUL96, which are critical for these proteins to function, are not required for this interaction. Moreover, the largely dispensable C-terminal region of pp150 is critical for pp150-pUL96 interaction. To further study the role of pUL96, several point and clustered mutations were engineered into the CMV Towne bacterial artificial chromosome (Towne-BAC) genome, replacing the conserved negatively charged C-terminal residues of pUL96. Although individual point mutations (E122A, D124A, and D125A) reduced virus growth slightly, the clustered mutations of 122EVDDAV127 significantly reduced virus growth, produced small syncytial plaque phenotypes, and impacted a late stage of virus maturation. When the UL96 C-terminal alanine conversion mutant (B6-BAC) virus was serially passaged in cell culture, it gained a plaque size comparable to that of Towne-BAC, displayed an altered restriction fragment length pattern, and replicated with increased growth kinetics. Whole-genome sequencing of this passaged virus (UL96P10) and the similarly passaged Towne-BAC virus revealed major differences only in the RNA4.9 and UL96 regions. When one of the mutations in the UL96 coding region was engineered into the B6-BAC virus, it significantly increased the plaque size and rescued the virus growth rate. Thus, accumulation of compensatory mutations only in UL96 in this revertant and the specific involvement of functionally dispensable regions of pp150 in the pUL96-pp150 interaction point toward a role for pUL96 in virus maturation that does not depend upon pp150.

IMPORTANCE

Human cytomegalovirus causes significant medical problems in newborns, as well as in people with low immunity. In this study, we investigated the functions of two essential virus proteins, pp150 and pUL96, and determined the impact of their mutual interaction on virus replication. These studies provide valuable information that is critical for the development of targeted antiviral therapies.

Protein interactions in the murine cytomegalovirus capsid revealed by cryoEM

Cytomegalovirus (CMV) is distinct among members of the Herpesviridae family for having the largest dsDNA genome (230 kb). Packaging of large dsDNA genome is known to give rise to a highly pressurized viral capsid, but molecular interactions conducive to the formation of CMV capsid resistant to pressurization have not been described. Here, we report a cryo electron microscopy (cryoEM) structure of the murine cytomegalovirus (MCMV) capsid at a 9.1 Å resolution and describe the molecular interactions among the ∼3000 protein molecules in the MCMV capsid at the secondary structure level. Secondary structural elements are resolved to provide landmarks for correlating with results from sequence-based prediction and for structure-based homology modeling. The major capsid protein (MCP) upper domain (MCPud) contains α-helices and β-sheets conserved with those in MCPud of herpes simplex virus type 1 (HSV-1), with the largest differences identified as a "saddle loop" region, located at the tip of MCPud and involved in interaction with the smallest capsid protein (SCP). Interactions among the bacteriophage HK97-like floor domain of MCP, the middle domain of MCP, the hook and clamp domains of the triplex proteins (hoop and clamp domains of TRI-1 and clamp domain of TRI-2) contribute to the formation of a mature capsid. These results offer a framework for understanding how cytomegalovirus uses various secondary structural elements of its capsid proteins to build a robust capsid for packaging its large dsDNA genome inside and for attaching unique functional tegument proteins outside.

The smallest capsid protein mediates binding of the essential tegument protein pp150 to stabilize DNA-containing capsids in human cytomegalovirus

Human cytomegalovirus (HCMV) is a ubiquitous herpesvirus that causes birth defects in newborns and life-threatening complications in immunocompromised individuals. Among all human herpesviruses, HCMV contains a much larger dsDNA genome within a similarly-sized capsid compared to the others, and it was proposed to require pp150, a tegument protein only found in cytomegaloviruses, to stabilize its genome-containing capsid. However, little is known about how pp150 interacts with the underlying capsid. Moreover, the smallest capsid protein (SCP), while dispensable in herpes simplex virus type 1, was shown to play essential, yet undefined, role in HCMV infection. Here, by cryo electron microscopy (cryoEM), we determine three-dimensional structures of HCMV capsid (no pp150) and virion (with pp150) at sub-nanometer resolution. Comparison of these two structures reveals that each pp150 tegument density is composed of two helix bundles connected by a long central helix. Correlation between the resolved helices and sequence-based secondary structure prediction maps the tegument density to the N-terminal half of pp150. The structures also show that SCP mediates interactions between the capsid and pp150 at the upper helix bundle of pp150. Consistent with this structural observation, ribozyme inhibition of SCP expression in HCMV-infected cells impairs the formation of DNA-containing viral particles and reduces viral yield by 10,000 fold. By cryoEM reconstruction of the resulting “SCP-deficient” viral particles, we further demonstrate that SCP is required for pp150 functionally binding to the capsid. Together, our structural and biochemical results point to a mechanism whereby SCP recruits pp150 to stabilize genome-containing capsid for the production of infectious HCMV virion.

Human cytomegalovirus (HCMV) causes birth defects in newborns and life-threatening complications in immunocompromised individuals, such as AIDS patients and organ transplant recipients. The smallest capsid protein (SCP) – only 8 kDa molecular mass as compared to the 155 kDa major capsid protein – has been demonstrated to be essential for HCMV growth, but is dispensable in herpes simplex virus type 1. These seemingly contradictory observations have been a paradox. Here, we solve this paradox by high resolution cryo electron microscopy (cryoEM), in conjunction with functional studies using ribozyme inhibition. Our structural comparisons of HCMV virion and capsid reveal molecular interactions at the secondary structure level and suggest that SCP might contribute to capsid binding of pp150, an essential, cytomegalovirus-specific tegument protein. SCP-deficient particles generated by ribozyme inhibition of SCP-expression in HCMV-infected cells show no pp150 tegument density, demonstrating that SCP is required for the functional binding of pp150 to the capsid. Our results suggest that SCP recruits pp150 to stabilize the HCMV nucleocapsid to enable encapsidation of the genome, which is more densely packaged in HCMV than in other herpesviruses. Overall, this study not only resolves the above paradox, but also illustrates the passive acquisition of a new, essential function by SCP in the production of infectious HCMV virions.

Structure of the pseudorabies virus capsid: comparison with herpes simplex virus type 1 and differential binding of essential minor proteins

The structure of pseudorabies virus (PRV) capsids isolated from the nucleus of infected cells and from PRV virions was determined by cryo-electron microscopy (cryo-EM) and compared to herpes simplex virus type 1 (HSV-1) capsids. PRV capsid structures closely resemble those of HSV-1, including distribution of the capsid vertex specific component (CVSC) of HSV-1, which is a heterodimer of the pUL17 and pUL25 proteins. Occupancy of CVSC on all PRV capsids is near 100%, compared to ~50% reported for HSV-1 C-capsids and 25% or less that we measure for HSV-1 A- and B-capsids. A PRV mutant lacking pUL25 does not produce C-capsids and lacks visible CVSC density in the cryo-EM-based reconstruction. A reconstruction of PRV capsids in which green fluorescent protein was fused within the N-terminus of pUL25 confirmed previous studies with a similar HSV-1 capsid mutant localizing pUL25 to the CVSC density region that is distal to the penton. However, comparison of the CVSC density in a 9-Å-resolution PRV C-capsid map with the available crystal structure of HSV-1 pUL25 failed to find a satisfactory fit, suggesting either a different fold for PRV pUL25 or a capsid-bound conformation for pUL25 that does not match the X-ray model determined from protein crystallized in solution. The PRV capsid imaged within virions closely resembles C-capsids with the addition of weak but significant density shrouding the pentons that we attribute to tegument proteins. Our results demonstrate significant structure conservation between the PRV and HSV capsids.

Diverse immune evasion strategies by human cytomegalovirus

Members of the Herpesviridae family have the capacity to undergo both lytic and latent infection to establish a lifelong relationship with their host. Following primary infection, human cytomegalovirus (HCMV) can persist as a subclinical, recurrent infection for the lifetime of an individual. This quiescent portion of its life cycle is termed latency and is associated with periodic bouts of reactivation during times of immunosuppression, inflammation, or stress. In order to exist indefinitely and establish infection, HCMV encodes a multitude of immune modulatory mechanisms devoted to escaping the host antiviral response. HCMV has become a paradigm for studies of viral immune evasion of antigen presentation by both major histocompatibility complex (MHC) class I and II molecules. By restricting the presentation of viral antigens during both productive and latent infection, HCMV limits elimination by the human immune system. This review will focus on understanding how the virus manipulates the pathways of antigen presentation in order to modulate the host response to infection.

Identification of a varicella-zoster virus replication inhibitor that blocks capsid assembly by interacting with the floor domain of the major capsid protein

A novel anti-varicella-zoster virus compound, a derivative of pyrazolo[1,5-c]1,3,5-triazin-4-one (coded as 35B2), was identified from a library of 9,600 random compounds. This compound inhibited both acyclovir (ACV)-resistant and -sensitive strains. In a plaque reduction assay under conditions in which the 50% effective concentration of ACV against the vaccine Oka strain (V-Oka) in human fibroblasts was 4.25 μM, the 50% effective concentration of 35B2 was 0.75 μM. The selective index of the compound was more than 200. Treatment with 35B2 inhibited neither immediate-early gene expression nor viral DNA synthesis. Twenty-four virus clones resistant to 35B2 were isolated, all of which had a mutation(s) in the amino acid sequence of open reading frame 40 (ORF40), which encodes the major capsid protein (MCP). Most of the mutations were located in the regions corresponding to the "floor" domain of the MCP of herpes simplex virus 1. Treatment with 35B2 changed the localization of MCP in the fibroblasts infected with V-Oka but not in the fibroblasts infected with the resistant clones, although it did not affect steady-state levels of MCP. Overexpression of the scaffold proteins restored the normal MCP localization in the 35B2-treated infected cells. The compound did not inhibit the scaffold protein-mediated translocation of MCP from the cytoplasm to the nucleus. Electron microscopic analysis demonstrated the lack of capsid formation in the 35B2-treated infected cells. These data indicate the feasibility of developing a new class of antivirals that target the herpesvirus MCPs and inhibit normal capsid formation by a mechanism that differs from those of the known protease and encapsidation inhibitors. Further biochemical studies are required to clarify the precise antiviral mechanism.

Viral and host control of cytomegalovirus maturation

Maturation in herpesviruses initiates in the nucleus of the infected cell, with encapsidation of viral DNA to form nucleocapsids, and concludes with envelopment in the cytoplasm to form infectious virions that egress the cell. The entire process of virus maturation is orchestrated by protein-protein interactions and enzymatic activities of viral and host origin. Viral tegument proteins play important roles in maintaining the structural stability of capsids and directing the acquisition of virus envelope. Envelopment occurs at modified host membranes and exploits host vesicular trafficking. In this review, we summarize current knowledge of and concepts in human cytomegalovirus (HCMV) maturation and their parallels in other herpesviruses, with an emphasis on viral and host factors that regulate this process.

The UL36 tegument protein of herpes simplex virus 1 has a composite binding site at the capsid vertices

Herpesviruses have an icosahedral nucleocapsid surrounded by an amorphous tegument and a lipoprotein envelope. The tegument comprises at least 20 proteins destined for delivery into the host cell. As the tegument does not have a regular structure, the question arises of how its proteins are recruited. The herpes simplex virus 1 (HSV-1) tegument is known to contact the capsid at its vertices, and two proteins, UL36 and UL37, have been identified as candidates for this interaction. We show that the interaction is mediated exclusively by UL36. HSV-1 nucleocapsids extracted from virions shed their UL37 upon incubation at 37°C. Cryo-electron microscopy (cryo-EM) analysis of capsids with and without UL37 reveals the same penton-capping density in both cases. As no other tegument proteins are retained in significant amounts, it follows that this density feature (∼100 kDa) represents the ordered portion of UL36 (336 kDa). It binds between neighboring UL19 protrusions and to an adjacent UL17 molecule. These observations support the hypothesis that UL36 plays a major role in the tegumentation of the virion, providing a flexible scaffold to which other tegument proteins, including UL37, bind. They also indicate how sequential conformational changes in the maturing nucleocapsid control the ordered binding, first of UL25/UL17 and then of UL36.

Procapsid assembly, maturation, nuclear exit: dynamic steps in the production of infectious herpesvirions

Herpesviruses, a family of animal viruses with large (125-250 kbp) linear DNA genomes, are highly diversified in terms of host range; nevertheless, their virions conform to a common architecture. The genome is confined at high density within a thick-walled icosahedral capsid with the uncommon (among viruses, generally) but unvarying triangulation number T = 16. The envelope is a membrane in which some 11 different viral glycoproteins are implanted. Between the capsid and the envelope is a capacious compartment called the tegument that accommodates ∼20-40 different viral proteins (depending on which virus) destined for delivery into a host cell. A strong body of evidence supports the hypothesis that herpesvirus capsids and those of tailed bacteriophages stem from a distant common ancestor, whereas their radically different infection apparatuses - envelope on one hand and tail on the other - reflect subsequent coevolution with divergent hosts. Here we review the molecular components of herpesvirus capsids and the mechanisms that regulate their assembly, with particular reference to the archetypal alphaherpesvirus, herpes simplex virus type 1; assess their duality with the capsids of tailed bacteriophages; and discuss the mechanism whereby, once DNA packaging has been completed, herpesvirus nucleocapsids exit from the nucleus to embark on later stages of the replication cycle.

Cryo electron tomography of herpes simplex virus during axonal transport and secondary envelopment in primary neurons

During herpes simplex virus 1 (HSV1) egress in neurons, viral particles travel from the neuronal cell body along the axon towards the synapse. Whether HSV1 particles are transported as enveloped virions as proposed by the ‘married’ model or as non-enveloped capsids suggested by the ‘separate’ model is controversial. Specific viral proteins may form a recruitment platform for microtubule motors that catalyze such transport. However, their subviral location has remained elusive. Here we established a system to analyze herpesvirus egress by cryo electron tomography. At 16 h post infection, we observed intra-axonal transport of progeny HSV1 viral particles in dissociated hippocampal neurons by live-cell fluorescence microscopy. Cryo electron tomography of frozen-hydrated neurons revealed that most egressing capsids were transported independently of the viral envelope. Unexpectedly, we found not only DNA-containing capsids (cytosolic C-capsids), but also capsids lacking DNA (cytosolic A-/B-capsids) in mid-axon regions. Subvolume averaging revealed lower amounts of tegument on cytosolic A-/B-capsids than on C-capsids. Nevertheless, all capsid types underwent active axonal transport. Therefore, even few tegument proteins on the capsid vertices seemed to suffice for transport. Secondary envelopment of capsids was observed at axon terminals. On their luminal face, the enveloping vesicles were studded with typical glycoprotein-like spikes. Furthermore, we noted an accretion of tegument density at the concave cytosolic face of the vesicle membrane in close proximity to the capsids. Three-dimensional analysis revealed that these assembly sites lacked cytoskeletal elements, but that filamentous actin surrounded them and formed an assembly compartment. Our data support the ‘separate model’ for HSV1 egress, i.e. progeny herpes viruses being transported along axons as subassemblies and not as complete virions within transport vesicles.

Herpes simplex virus 1 (HSV1) establishes lifelong latent infections in the peripheral nervous system. After reactivation, progeny viral particles travel within sensory neurons towards sites of initial infection. There are conflicting reports what type of viral structures are transported: some studies observed non-enveloped capsids traveling while others reported transport of fully enveloped viruses within vesicles. Here, we used cryo electron tomography to analyze the three-dimensional architecture of HSV1 in axons of hippocampal neurons. In mid-axonal regions we found predominantly non-enveloped capsids. Interestingly, we observed both genome-containing and empty capsids that differed significantly in the amount of bound proteins. Viral protein recruitment thus varied between the different cytosolic capsid types, but effective transport occurred despite these differences. Furthermore, we observed three-dimensional snapshots of secondary capsid envelopment in axon terminals. Altogether, this study provides valuable structural detail on axonal HSV1 particles supporting the notion that viral subassemblies are conveyed along the axons to be assembled only after axonal transport.

Cytomegalovirus pUL96 is critical for the stability of pp150-associated nucleocapsids

Maturation of human cytomegalovirus (HCMV) initiates with nucleocapsids that egress from the nucleus and associate with a juxtanuclear cytoplasmic assembly compartment, where virion envelopment and release are orchestrated. Betaherpesvirus conserved proteins pp150 (encoded by UL32) and pUL96 are critical for HCMV growth in cell culture. pp150 is a capsid-proximal tegument protein that preserves the integrity of nucleocapsids during maturation. pUL96, although expressed as an early protein, acts late during virus maturation, similar to pp150, based on the comparable antigen distribution in UL96, UL32, or UL96/UL32 dual mutant virus-infected cells. pp150 associates with nuclear capsids prior to DNA encapsidation, whereas both pp150 and pUL96 associate with extracellular virus, suggesting that pUL96 is added after pp150. In the absence of pUL96, capsid egress from the nucleus continues; however, unlike wild-type virus infection, pp150 accumulates in the nuclear, as well as in the cytoplasmic, compartment. Ultrastructural evaluation of a UL96 conditional mutant revealed intact nuclear stages but aberrant nucleocapsids accumulating in the cytoplasm comparable to the known phenotype of UL32 mutant virus. In summary, pUL96 preserves the integrity of pp150-associated nucleocapsids during translocation from the nucleus to the cytoplasm.

Yeast two hybrid analyses reveal novel binary interactions between human cytomegalovirus-encoded virion proteins

Human cytomegalovirus (HCMV) is the largest human herpesvirus and its virion contains many viral encoded proteins found in the capsid, tegument, and envelope. In this study, we carried out a yeast two-hybrid (YTH) analysis to study potential binary interactions among 56 HCMV-encoded virion proteins. We have tested more than 3,500 pairwise combinations for binary interactions in the YTH analysis, and identified 79 potential interactions that involve 37 proteins. Forty five of the 79 interactions were also identified in human cells expressing the viral proteins by co-immunoprecipitation (co-IP) experiments. To our knowledge, 58 of the 79 interactions revealed by YTH analysis, including those 24 that were also identified in co-IP experiments, have not been reported before. Novel potential interactions were found between viral capsid proteins and tegument proteins, between tegument proteins, between tegument proteins and envelope proteins, and between envelope proteins. Furthermore, both the YTH and co-IP experiments have identified 9, 7, and 5 interactions that were involved with UL25, UL24, and UL89, respectively, suggesting that these “hub” proteins may function as the organizing centers for connecting multiple virion proteins in the mature virion and for recruiting other virion proteins during virion maturation and assembly. Our study provides a framework to study potential interactions between HCMV proteins and investigate the roles of protein-protein interactions in HCMV virion formation or maturation process.

Biochemical and structural characterization of the capsid-bound tegument proteins of human cytomegalovirus

Human cytomegalovirus (HCMV) is the most genetically and structurally complex human herpesvirus and is composed of an envelope, a tegument, and a dsDNA-containing capsid. HCMV tegument plays essential roles in HCMV infection and assembly. Using cryo electron tomography (cryoET), here we show that HCMV tegument compartment can be divided into two sub-compartments: an inner and an outer tegument. The inner tegument consists of densely-packed proteins surrounding the capsid. The outer tegument contains those components that are loosely packed in the space between the inner tegument and the pleomorphic glycoprotein-containing envelope. To systematically characterize the inner tegument proteins interacting with the capsid, we used chemical treatment to strip off the entire envelope and most tegument proteins to obtain a tegumented capsid with inner tegument proteins. SDS-polyacrylamide gel electrophoresis analyses show that only two tegument proteins, UL32-encoded pp150 and UL48-encoded high molecular weight protein (HMWP), remains unchanged in their abundance in the tegumented capsids as compared to their abundance in the intact particles. Three-dimensional reconstructions by single particle cryo electron microscopy (cryoEM) reveal that the net-like layer of icosahedrally-ordered tegument densities are also the same in the tegumented capsid and in the intact particles. CryoET reconstruction of the tegumented capsid labeled with an anti-pp150 antibody is consistent with the biochemical and cryoEM data in localizing pp150 within the ordered tegument. Taken together, these results suggest that pp150, a betaherpesvirus-specific tegument protein, is a constituent of the net-like layer of icosahedrally-ordered capsid-bound tegument densities, a structure lacking similarities in alpha and gammaherpesviruses.

A role for the small GTPase Rab6 in assembly of human cytomegalovirus

In human-cytomegalovirus (HCMV)-infected cells, the localization of the viral protein pp150 to the virus assembly compartment (AC) is dependent on its direct interaction with the cellular protein Bicaudal D1 through a dynein- and microtubule-dependent mechanism. We found that the small GTPase Rab6 also interacts indirectly with pp150 through its interaction with Bicaudal D1. Inhibition of Rab6 activity in HCMV-infected cells interrupted the intracellular trafficking of pp150, significantly reducing infectious virus production without affecting the formation of the AC, arguing for an important function for this cellular GTPase in the intracellular localization of pp150 during virus assembly.

Cytomegalovirus UL103 controls virion and dense body egress

Human cytomegalovirus UL103 encodes a tegument protein that is conserved across herpesvirus subgroups. Mutant viruses lacking this gene product exhibit dramatically reduced accumulation of cell-free virus progeny and poor cell-to-cell spread. Given that viral proteins and viral DNA accumulate with normal kinetics in cells infected with mutant virus, UL103 appears to function during the late phase of replication, playing a critical role in egress of capsidless dense bodies and virions. Few dense bodies were observed in the extracellular space in mutant virus-infected cells in the presence or absence of the DNA encapsidation inhibitor 2-bromo-5,6-dichloro-1-(β-d-ribofuranosyl)benzimidazole. Upon reversal of encapsidation inhibition, UL103 had a striking impact on accumulation of cell-free virus, but not on accumulation of cell-associated virus. Thus, UL103 plays a novel and important role during maturation, regulating virus particle and dense body egress from infected cells.

Three-dimensional visualization of gammaherpesvirus life cycle in host cells by electron tomography

Gammaherpesviruses are etiologically associated with human tumors. A three-dimensional (3D) examination of their life cycle in the host is lacking, significantly limiting our understanding of the structural and molecular basis of virus-host interactions. Here, we report the first 3D visualization of key stages of the murine gammaherpesvirus 68 life cycle in NIH 3T3 cells, including viral attachment, entry, assembly, and egress, by dual-axis electron tomography. In particular, we revealed the transient processes of incoming capsids injecting viral DNA through nuclear pore complexes and nascent DNA being packaged into progeny capsids in vivo as a spool coaxial with the putative portal vertex. We discovered that intranuclear invagination of both nuclear membranes is involved in nuclear egress of herpesvirus capsids. Taken together, our results provide the structural basis for a detailed mechanistic description of gammaherpesvirus life cycle and also demonstrate the advantage of electron tomography in dissecting complex cellular processes of viral infection.

HCMV-encoded glycoprotein M (UL100) interacts with Rab11 effector protein FIP4

The envelope of human cytomegalovirus (HCMV) consists of a large number of glycoproteins. The most abundant glycoprotein in the HCMV envelope is the glycoprotein M (UL100), which together with glycoprotein N (UL73) form the gM/gN protein complex. Using yeast two-hybrid screening, we found that the gM carboxy-terminal cytoplasmic tail (gM-CT) interacts with FIP4, a Rab11-GTPase effector protein. Depletion of FIP4 expression in HCMV-infected cells resulted in a decrease in infectious virus production that was also associated with an alteration of the HCMV assembly compartment (AC) phenotype. A similar phenotype was also observed in HCMV-infected cells that expressed dominant negative Rab11(S25N). Recently, it has been shown that FIP4 interactions with Rab11 and additionally with Arf6/Arf5 are important for the vesicular transport of proteins in the endosomal recycling compartment (ERC) and during cytokinesis. Surprisingly, FIP4 interaction with gM-CT limited binding of FIP4 with Arf5/Arf6; however, FIP4 interaction with gM-CT did not prevent recruitment of Rab11 into the ternary complex. These data argued for a contribution of the ERC during cytoplasmic envelopment of HCMV and showed a novel FIP4 function independent of Arf5 or Arf6 activity.

Bicaudal D1-dependent trafficking of human cytomegalovirus tegument protein pp150 in virus-infected cells

Human cytomegalovirus (HCMV) virion assembly takes place in the nucleus and cytoplasm of infected cells. The HCMV virion tegument protein pp150 (ppUL32) is an essential protein of HCMV and has been suggested to play a role in the cytoplasmic phase of HCMV assembly. To further define its role in viral assembly and to identify host cell proteins that interact with pp150 during viral assembly, we utilized yeast two-hybrid analyses to detect an interaction between pp150 and Bicaudal D1 (BicD1), a protein thought to play a role in trafficking within the secretory pathway. BicD1 is known to interact with the dynein motor complex and the Rab6 GTPase. The interaction between pp150 and BicD1 was confirmed by coimmunoprecipitation and fluorescence resonance energy transfer. Depletion of BicD1 with short hairpin RNA (shRNA) caused decreased virus yield and a defect in trafficking of pp150 to the cytoplasmic viral assembly compartment (AC), without altering trafficking to the AC of another essential tegument protein, pp28, or the viral glycoprotein complex gM/gN. The C terminus of BicD1 has been previously shown to interact with the GTPase Rab6, suggesting a potential role for Rab6-mediated vesicular trafficking in HCMV assembly. Finally, overexpression of the N terminus of truncated BicD1 acts in a dominant-negative manner and leads to disruption of the AC and a decrease in the assembly of infectious virus. This phenotype was similar to that observed following overexpression of dynamitin (p50) and provided additional evidence that morphogenesis of the AC and virus assembly were dynein dependent.

Time-dependent transformation of the herpesvirus tegument

All herpesviruses have a layer of protein called the tegument that lies between the virion membrane and the capsid. The tegument consists of multiple, virus-encoded protein species that together can account for nearly half the total virus protein. To clarify the structure of the tegument and its attachment to the capsid, we used electron microscopy and protein analysis to examine the tegument of herpes simplex virus type 1 (HSV-1). Electron microscopic examination of intact virions revealed that whereas the tegument was asymmetrically distributed around the capsid in extracellular virions, it was symmetrically arranged in cell-associated virus. Examination of virions after treatment with nonionic detergent demonstrated that: (i) in extracellular virus the tegument was resistant to removal with Triton X-100 (TX-100), whereas it was lost nearly completely when cell-associated virus was treated in the same way; (ii) the tegument in TX-100-treated extracellular virions was asymmetrically distributed around the capsid as it is in unextracted virus; and (iii) in some images, tegument was seen to be linked to the capsid by short, regularly spaced connectors. Further analysis was carried out with extracellular virus harvested from cells at different times after infection. It was observed that while the amount of tegument present in virions was not affected by time of harvest, the amount remaining after TX-100 treatment increased markedly as the time of harvest was increased from 24 h to 64 h postinfection. The results support the view that HSV-1 virions undergo a time-dependent change in which the tegument is transformed from a state in which it is symmetrically organized around the capsid and extractable with TX-100 to a state where it is asymmetrically arranged and resistant to extraction.

Tegument proteins of human cytomegalovirus

SUMMARY

Human cytomegalovirus (HCMV) is a common, medically relevant human herpesvirus. The tegument layer of herpesvirus virions lies between the genome-containing capsids and the viral envelope. Proteins within the tegument layer of herpesviruses are released into the cell upon entry when the viral envelope fuses with the cell membrane. These proteins are fully formed and active and control viral entry, gene expression, and immune evasion. Most tegument proteins accumulate to high levels during later stages of infection, when they direct the assembly and egress of progeny virions. Thus, viral tegument proteins play critical roles at the very earliest and very last steps of the HCMV lytic replication cycle. This review summarizes HCMV tegument composition and structure as well as the known and speculated functions of viral tegument proteins. Important directions for future investigation and the challenges that lie ahead are identified and discussed.

Control of cytoplasmic maturation events by cytomegalovirus tegument protein pp150

Cytomegalovirus replication depends upon a betaherpesvirus-conserved 150-kDa tegument phosphoprotein (pp150; encoded by UL32) that supports the final steps in virion maturation at cytoplasmic assembly compartments. Amino acid substitutions were introduced into conserved region 1 (CR1) and CR2 of pp150, affecting a region that may interact with nucleocapsids. Two independent CR2 point mutants (N201A and G207A) failed to support viral replication in evaluations by a transient complementation assay or after reconstruction into recombinant viruses. An assembly compartment-like cytoplasmic inclusion developed in UL32 mutant virus-infected cells that was similar to that of wild-type virus-infected cells. The cellular localization of the trans-Golgi marker Golgin-97 suggested differences in the organization of the assembly compartment compared to that of wild-type virus-infected cells. Replication-defective CR2 point mutants exhibited the same phenotype as that of a virus carrying a complete deletion of the UL32 open reading frame in these assays. Electron micrographs of fibroblasts at 3 or 5 days postinfection with a deletion mutant (DeltaUL32) grown on UL32-complementing cells showed a similar number and morphology of capsids in the nucleus, but the cytoplasmic region associated with virion assembly appeared highly vesiculated and contained few recognizable nucleocapsids or complete virus particles. These data demonstrate that the principle role of pp150 is to retain nucleocapsid organization through secondary envelopment at the assembly compartment.

Deletion or green fluorescent protein tagging of the pUL35 capsid component of pseudorabies virus impairs virus replication in cell culture and neuroinvasion in mice

To facilitate tracing of virion movement, the non-essential capsid proteins pUL35 of herpes simplex virus type 1 and pseudorabies virus (PrV) have been tagged with green fluorescent protein (GFP). However, the biological relevance of PrV pUL35 and the functionality of the fusion proteins have not yet been investigated in detail. We generated PrV mutants either lacking the 12 kDa UL35 gene product, or expressing GFP fused to the N terminus of pUL35. Remarkably, both mutants exhibited significant replication defects in rabbit kidney cells, which could be corrected in pUL35-expressing cells. After intranasal infection of mice both mutants showed delayed neuroinvasion, and survival times of the animals were extended to 3 days, compared with 2 days after wild-type infection. Thus, fusion of pUL35 with GFP resulted in a non-functional protein, which has to be considered for the use of corresponding mutants in tracing studies.

Multimerization of tegument protein pp28 within the assembly compartment is required for cytoplasmic envelopment of human cytomegalovirus

Human cytomegalovirus (HCMV) UL99-encoded pp28 is an essential tegument protein required for envelopment and production of infectious virus. Nonenveloped virions accumulate in the cytoplasm of cells infected with recombinant viruses with the UL99 gene deleted. Previous results have suggested that a key function of pp28 in the envelopment of infectious HCMV is expressed after the protein localizes in the assembly compartment (AC). In this study, we investigated the potential role of pp28 multimerization in the envelopment of the infectious virion. Our results indicated that pp28 multimerized during viral infection and that interacting domains responsible for self-interaction were localized in the amino terminus of the protein (amino acids [aa] 1 to 43). The results from transient-expression and/or infection assays indicated that the self-interaction took place in the AC. A mutant pp28 molecule containing only the first 35 aa failed to accumulate in the AC, did not interact with pp28 in the AC, and could not support virus replication. In contrast, the first 50 aa of pp28 was sufficient for the self-interaction within the AC and the assembly of infectious virus. Recombinant viruses encoding an in-frame deletion of aa 26 to 33 of pp28 were replication competent, whereas infectious virus was not recovered from HCMV BACs lacking aa 26 to 43. These findings suggested that the accumulation of pp28 was a prerequisite for multimerization of pp28 within the AC and that pp28 multimerization in the AC represented an essential step in the envelopment and production of infectious virions.

Functions of human cytomegalovirus tegument proteins prior to immediate early gene expression

Proteins within the tegument layer of herpesviruses such as human cytomegalovirus (HCMV) are released into the cell upon entry when the viral envelope fuses with the cell membrane. These proteins are fully formed and active, and they mediate key events at the very start of the lytic infectious cycle, including the delivery of the viral genome to the nucleus and the initiation of viral gene expression. This review examines what is known about tegument protein function prior to the immediate early (IE) phase of the viral lytic replication cycle and identifies key questions that need to be answered to better understand how these proteins promote HCMV infection so that antiviral treatments that target these important viral regulators can be developed.

Unique structures in a tumor herpesvirus revealed by cryo-electron tomography and microscopy

Gammaherpesviruses, including the human pathogens Epstein-Barr virus and Kaposi's sarcoma-associated herpesvirus, are causative agents of lymphomas and other malignancies. The structural characterization of these viruses has been limited due to difficulties in obtaining adequate amount of virion particles. Here we report the first three-dimensional structural characterization of a whole gammaherpesvirus virion by an emerging integrated approach of cryo-electron tomography combined with single-particle cryo-electron microscopy, using murine gammaherpesvirus-68 (MHV-68) as a model system. We found that the MHV-68 virion consists of distinctive envelope and tegument compartments, and a highly conserved nucleocapsid. Two layers of tegument are identified: an inner tegument layer tethered to the underlying capsid and an outer, flexible tegument layer conforming to the overlying, pleomorphic envelope, consistent with the sequential viral tegumentation process inside host cells. Surprisingly, comparison of the MHV-68 virion and capsid reconstructions shows that the interactions between the capsid and inner tegument proteins are completely different from those observed in alpha and betaherpesviruses. These observations support the notion that the inner layer tegument across different subfamilies of herpesviruses has evolved significantly to confer specific characteristics related to viral-host interactions, in contrast to a highly conserved capsid for genome encapsidation and protection.

Sequence requirements for localization of human cytomegalovirus tegument protein pp28 to the virus assembly compartment and for assembly of infectious virus

The human cytomegalovirus UL99 open reading frame encodes a 190-amino-acid (aa) tegument protein, pp28, that is myristoylated and phosphorylated. pp28 is essential for assembly of infectious virus, and nonenveloped virions accumulate in the cytoplasm of cells infected with recombinant viruses with a UL99 deletion. pp28 is localized to the endoplasmic reticulum-Golgi intermediate compartment (ERGIC) in transfected cells, while in infected cells, it is localized together with other virion proteins in a juxtanuclear compartment termed the assembly compartment (AC). We investigated the sequence requirements for pp28 trafficking to the AC and assembly of infectious virus. Our studies indicated that the first 30 to 35 aa were required for localization of pp28 to the ERGIC in transfected cells. Mutant forms of pp28 containing only the first 35 aa localized with other virion structural proteins to cytoplasmic compartments early in infection, but localization to the AC at late times required a minimum of 50 aa. In agreement with previous reports, we demonstrated that the deletion of a cluster of acidic amino acids (aa 44 to 59) prevented wild-type trafficking of pp28 and recovery of infectious virus. A recombinant virus expressing only the first 50 aa was replication competent, and this mutant, pp28, localized to the AC in cells infected with this virus. These findings argued that localization of pp28 to the AC was essential for assembly of infectious virus and raised the possibility that amino acids in the amino terminus of pp28 have additional roles in the envelopment and assembly of the virion other than simply localizing pp28 to the AC.

Electron cryomicroscopy comparison of the architectures of the enveloped bacteriophages phi6 and phi8

The enveloped dsRNA bacteriophages phi6 and phi8 are the two most distantly related members of the Cystoviridae family. Their structure and function are similar to that of the Reoviridae but their assembly can be conveniently studied in vitro. Electron cryomicroscopy and three-dimensional icosahedral reconstruction were used to determine the structures of the phi6 virion (14 A resolution), phi8 virion (18 A resolution), and phi8 core (8.5 A resolution). Spikes protrude 2 nm from the membrane bilayer in phi6 and 7 nm in phi8. In the phi6 nucleocapsid, 600 copies of P8 and 72 copies of P4 interact with the membrane, whereas in phi8 it is only P4 and 60 copies of a minor protein. The major polymerase complex protein P1 forms a dodecahedral shell from 60 asymmetric dimers in both viruses, but the alpha-helical fold has apparently diverged. These structural differences reflect the different host ranges and entry and assembly mechanisms of the two viruses.

Enzymatic activities of human cytomegalovirus maturational protease assemblin and its precursor (pPR, pUL80a) are comparable: [corrected] maximal activity of pPR requires self-interaction through its scaffolding domain

Herpesviruses encode an essential, maturational serine protease whose catalytic domain, assemblin (28 kDa), is released by self-cleavage from a 74-kDa precursor (pPR, pUL80a). Although there is considerable information about the structure and enzymatic characteristics of assemblin, a potential pharmacologic target, comparatively little is known about these features of the precursor. To begin studying pPR, we introduced five point mutations that stabilize it against self-cleavage at its internal (I), cryptic (C), release (R), and maturational (M) sites and at a newly discovered "tail" (T) site. The resulting mutants, called ICRM-pPR and ICRMT-pPR, were expressed in bacteria, denatured in urea, purified by immobilized metal affinity chromatography, and renatured by a two-step dialysis procedure and by a new method of sedimentation into glycerol gradients. The enzymatic activities of the pPR mutants were indistinguishable from that of IC-assemblin prepared in parallel for comparison, as determined by using a fluorogenic peptide cleavage assay, and approximated rates previously reported for purified assemblin. The percentage of active enzyme in the preparations was also comparable, as determined by using a covalent-binding suicide substrate. An unexpected finding was that, in the absence of the kosmotrope Na2SO4, optimal activity of pPR requires interaction through its scaffolding domain. We conclude that although the enzymatic activities of assemblin and its precursor are comparable, there may be differences in how their catalytic sites become fully activated.

The amino-conserved domain of human cytomegalovirus UL80a proteins is required for key interactions during early stages of capsid formation and virus production

Assembly of many spherical virus capsids is guided by an internal scaffolding protein or group of proteins that are often cleaved and eliminated in connection with maturation and incorporation of the genome. In cytomegalovirus there are at least two proteins that contribute to this scaffolding function; one is the maturational protease precursor (pUL80a), and the other is the assembly protein precursor (pUL80.5) encoded by a shorter genetic element within UL80a. Yeast GAL4 two-hybrid assays established that both proteins contain a carboxyl-conserved domain that is required for their interaction with the major capsid protein (pUL86) and an amino-conserved domain (ACD) that is required for their self-interaction and for their interaction with each other. In the work reported here, we demonstrate that when the ACD is deleted (deltaACD) or disrupted by a point mutation (L47A), the bacterially expressed mutant protein sediments as a monomer during rate-velocity centrifugation, whereas the wild-type protein sediments mainly as oligomers. We also show that the L47A mutation reduces the production of infectious virus by at least 90%, results in the formation of irregular nuclear capsids, gives rise to tube-like structures in the nucleus that resemble the capsid core in cross-section and contain UL80 proteins, slows nuclear translocation of the major capsid protein, and may slow cleavage by the maturational protease. We provide physical corroboration that mutating the ACD disrupts self-interaction of the UL80 proteins and biological support for the proposal that the ACD has a critical role in capsid assembly and production of infectious virus.

Betaherpesvirus-conserved cytomegalovirus tegument protein ppUL32 (pp150) controls cytoplasmic events during virion maturation

The UL32 gene of human cytomegalovirus (CMV) encodes a prominent betaherpesvirus-conserved virion tegument protein, called pp150 (basic phosphoprotein/ppUL32), that accumulates within a cytoplasmic inclusion adjacent to the nucleus at late times during infection. Using a UL32 deletion mutant (DeltaUL32-BAC) (where BAC is bacterial artificial chromosome), we demonstrate that pp150 is critical for virion maturation in the cytoplasmic compartment. Cotransfection of a pp150 expression plasmid with DeltaUL32-BAC DNA led to complementation of the replication defect with focus formation due to secondary spread. Deletion of the amino terminus of pp150 or disruption of the betaherpesvirus conserved regions, CR1 and CR2, revealed these regions to be critical for replication. In contrast, deletion of the carboxyl terminus only partially compromised maturation while disruption of glycosylation sites had no effect. An African green monkey CMV UL32 homolog complemented DeltaUL32-BAC replication but murine CMV M32 failed to complement, consistent with evolutionary divergence of rodent and primate cytomegaloviruses. Infection with DeltaUL32-BAC showed normal expression of all kinetic classes of viral genes and replication of viral DNA, with accumulation of viral DNA-containing particles in the cytoplasm; however, mutant virus did not spread to adjacent cells. In contrast to this block in virion infectivity, cell-to-cell transfer of pp65-containing particles was observed, suggesting that release of dense bodies continued in the absence of pp150. These observations demonstrate that pp150 is critical for virion egress, possibly at the stage of final envelopment.

High-molecular-weight protein (pUL48) of human cytomegalovirus is a competent deubiquitinating protease: mutant viruses altered in its active-site cysteine or histidine are viable

We show here that the high-molecular-weight protein (HMWP or pUL48; 253 kDa) of human cytomegalovirus (HCMV) is a functionally competent deubiquitinating protease (DUB). By using a suicide substrate probe specific for ubiquitin-binding cysteine proteases (DUB probe) to screen lysates of HCMV-infected cells, we found just one infected-cell-specific DUB. Characteristics of this protein, including its large size, expression at late times of infection, presence in extracellular virus particles, and reactivity with an antiserum to the HMWP, identified it as the HMWP. This was confirmed by constructing mutant viruses with substitutions in two of the putative active-site residues, Cys24Ile and His162Ala. HMWP with these mutations either failed to bind the DUB probe (C24I) or had significantly reduced reactivity with it (H162A). More compellingly, the deubiquitinating activity detected in wild-type virus particles was completely abolished in both the C24I and H162A mutants, thereby directly linking HMWP with deubiquitinating enzyme activity. Mutations in these active-site residues were not lethal to virus replication but slowed production of infectious virus relative to wild type and mutations of other conserved residues. Initial studies, by electron microscopy, of cells infected with the mutants revealed no obvious differences at late times of replication in the general appearance of the cells or in the distribution, relative numbers, or appearance of virus particles in the cytoplasm or nucleus.

Human cytomegalovirus virion proteins

Human cytomegalovirus (HCMV) is the largest member of the family of human herpesviruses. The number of virus encoded proteins and the complexity of their functions in the life cycle of this virus are reflected in the size of its genome. There continues to be some controversy surrounding the exact protein coding capacity of the virus with estimates ranging from 160 open reading frames to more than 200 open reading frames. Very recent studies using mass spectrometry to determine the viral proteome suggests that the number of viral proteins may be even greater than previous estimates. The proteins of the virion capsid have readily identifiable homologous proteins in the capsid of the more extensively studied herpes simplex virus, likely because of similar capsid structure and assembly pathways. In contrast, the tegument and the envelope of HCMV contain a significant number of proteins that lack structural homology to proteins found in either alpha or gamma-herpesviruses. This brief overview discusses some of the general features and possible functions of the HCMV virion structural proteins in the replicative cycle of this virus.

Budding events in herpesvirus morphogenesis

Herpes virions are complex particles that consist of more than 30 different virally encoded proteins. The molecular basis of how this complicated structure is assembled is only recently beginning to emerge. After replication in the host cell nucleus viral DNA is incorporated into preformed capsids, which leave the nucleus by a first budding event at the inner nuclear membrane resulting in the formation of primary enveloped virions in the perinuclear space. The primary envelope then fuses with the outer leaflet of the nuclear membrane thereby releasing nucleocapsids into the cytoplasm. Final envelopment, including the acquisition of more than 15 tegument and more than 10 envelope (glyco) proteins occurs by budding into Golgi-derived vesicles. Mature virions are released after fusion of the vesicle membrane with the plasma membrane of the cell. Thus, herpesvirus morphogenesis requires two different budding steps, which are distinct not only in the subcellular compartments in which they occur but also by the viral proteins involved. This review summarizes recent advances in our understanding of the two herpesvirus budding events.

A novel class of herpesvirus with bivalve hosts

Ostreid herpesvirus 1 (OsHV-1) is the only member of the Herpesviridae that has an invertebrate host and is associated with sporadic mortality in the Pacific oyster (Crassostrea gigas) and other bivalve species. Cryo-electron microscopy of purified capsids revealed the distinctive T=16 icosahedral structure characteristic of herpesviruses, although the preparations examined lacked pentons. The gross genome organization of OsHV-1 was similar to that of certain mammalian herpesviruses (including herpes simplex virus and human cytomegalovirus), consisting of two invertible unique regions (U(L), 167.8 kbp; U(S), 3.4 kbp) each flanked by inverted repeats (TR(L)/IR(L), 7.6 kbp; TR(S)/IR(S), 9.8 kbp), with an additional unique sequence (X, 1.5 kbp) between IR(L) and IR(S). Of the 124 unique genes predicted from the 207 439 bp genome sequence, 38 were members of 12 families of related genes and encoded products related to helicases, inhibitors of apoptosis, deoxyuridine triphosphatase and RING-finger proteins, in addition to membrane-associated proteins. Eight genes in three of the families appeared to be fragmented. Other genes that did not belong to the families were predicted to encode DNA polymerase, the two subunits of ribonucleotide reductase, a helicase, a primase, the ATPase subunit of terminase, a RecB-like protein, additional RING-like proteins, an ion channel and several other membrane-associated proteins. Sequence comparisons showed that OsHV-1 is at best tenuously related to the two classes of vertebrate herpesviruses (those associated with mammals, birds and reptiles, and those associated with bony fish and amphibians). OsHV-1 thus represents a third major class of the herpesviruses.

Essential function of the pseudorabies virus UL36 gene product is independent of its interaction with the UL37 protein

The large tegument protein encoded by the UL36 gene of pseudorabies virus (PrV) physically interacts with the product of the adjacent UL37 gene (B. G. Klupp, W. Fuchs, H. Granzow, R. Nixdorf, and T. C. Mettenleiter, J. Virol. 76:3065-3071, 2002). To analyze UL36 function, two PrV recombinants were generated by mutagenesis of an infectious PrV full-length clone in Escherichia coli: PrV-DeltaUL36F exhibited a deletion of virtually the complete UL36 coding region, whereas PrV-UL36BSF contained two in-frame deletions of 238 codons spanning the predicted UL37 binding domain. Coimmunoprecipitation experiments confirmed that the mutated gene product of PrV-UL36BSF did not interact with the UL37 protein. Like the previously described PrV-DeltaUL37 (B. G. Klupp, H. Granzow, and T. C. Mettenleiter, J. Virol. 75:8927-8936, 2001) but in contrast to PrV-DeltaUL36F, PrV-UL36BSF was able to replicate in rabbit kidney (RK13) cells, although maximum virus titers were reduced ca. 50-fold and plaque diameters were reduced by ca. 45% compared to wild-type PrV. PrV-DeltaUL36F was able to productively replicate after repair of the deleted gene or in a trans-complementing cell line. Electron microscopy of infected RK13 cells revealed that PrV-UL36BSF and phenotypically complemented PrV-DeltaUL36F were capable of nucleocapsid formation and egress from the nucleus by primary envelopment and deenvelopment at the nuclear membrane. However, reenvelopment of nucleocapsids in the cytoplasm was blocked. Only virus-like particles without capsids were released efficiently from cells. Interestingly, cytoplasmic nucleocapsids of PrV-UL36BSF but not of PrV-DeltaUL36F were found in large ordered structures similar to those which had previously been observed with PrV-DeltaUL37. In summary, our results demonstrate that the interaction between the UL36 and UL37 proteins is important but not strictly essential for the formation of secondary enveloped, infectious PrV particles. Furthermore, UL36 possesses an essential function during virus replication which is independent of its ability to bind the UL37 protein.

RNAs are packaged into human cytomegalovirus virions in proportion to their intracellular concentration

The assembly of human cytomegalovirus (HCMV) virions is a complex process and involves the incorporation of viral transcripts. These RNAs are delivered to the newly infected cells and have the potential to be translated in the absence of HCMV gene expression. We have quantified the relative amount of RNAs in HCMV virions and infected cells with real-time reverse transcription-PCR and observed that viral and cellular RNAs are packaged in proportion to the amount of RNA within the cell at the time of assembly. To determine whether cis elements influenced RNA packaging, we constructed a recombinant HCMV mutant virus that expressed the yellow fluorescence protein (YFP) gene fused to the virion RNA UL21.5. We also constructed a mutant virus in which the UL21.5 transcription unit was replaced with the YFP gene. YFP RNA was incorporated into both viruses, indicating that RNA is incorporated in the absence of a virus-specific signal motif. Furthermore, with in situ hybridization, packaged transcripts were observed throughout the cytoplasm of the infected cells, including the site of virus assembly. Several proteins that nonspecifically interact with RNA, including the tegument protein pp28, were found within HCMV virions. These studies demonstrate that both viral and cellular RNAs are nonspecifically incorporated into HCMV, potentially through interactions with several virion proteins.

Identification of proteins associated with murine gammaherpesvirus 68 virions

Murine gammaherpesvirus 68 (MHV68 [also known as gammaHV-68]) is distinguished by its ability to replicate to high titers in cultured cells, making it an excellent candidate for studying gammaherpesvirus virion composition. Extracellular MHV68 virions were isolated, and abundant virion-associated proteins were identified by mass spectrometry. Five nucleocapsid protein homologues, the tegument protein homologue encoded by open reading frame (ORF) 75c, and envelope glycoproteins B and H were detected. In addition, gene products from MHV68 ORF20, ORF24, ORF28, ORF45, ORF48, and ORF52 were identified in association with virions, suggesting that these gammaherpesvirus genes are involved in the early phase of infection or virion assembly and egress.