Three-dimensional structure of the human herpesvirus 8 capsid

Stunning symmetries involved in the self-assembly of the HSV-1 capsid

Herpes simplex virus-1 (HSV-1) is an enveloped dsDNA virus, infecting ~ 67% of humans. Here, we present the essential components of the HSV-1, focusing on stunning symmetries on the capsid. However, little is known about how the symmetries are involved dynamically in the self-assembly process. We suggest small angle X-ray scattering as a suitable method to capture the dynamics of self-assembly. Furthermore, our understanding of the viruses can be expanded by using an integrative approach that combines heterogeneous types of data, thus promoting new diagnostic tools and a cure for viral infections.

DNA-Packing Portal and Capsid-Associated Tegument Complexes in the Tumor Herpesvirus KSHV

Assembly of Kaposi's sarcoma-associated herpesvirus (KSHV) begins at a bacteriophage-like portal complex that nucleates formation of an icosahedral capsid with capsid-associated tegument complexes (CATCs) and facilitates translocation of an ∼150-kb dsDNA genome, followed by acquisition of a pleomorphic tegument and envelope. Because of deviation from icosahedral symmetry, KSHV portal and tegument structures have largely been obscured in previous studies. Using symmetry-relaxed cryo-EM, we determined the in situ structure of the KSHV portal and its interactions with surrounding capsid proteins, CATCs, and the terminal end of KSHV's dsDNA genome. Our atomic models of the portal and capsid/CATC, together with visualization of CATCs' variable occupancy and alternate orientation of CATC-interacting vertex triplexes, suggest a mechanism whereby the portal orchestrates procapsid formation and asymmetric long-range determination of CATC attachment during DNA packaging prior to pleomorphic tegumentation/envelopment. Structure-based mutageneses confirm that a triplex deep binding groove for CATCs is a hotspot that holds promise for antiviral development.

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.

The KSHV portal protein ORF43 is essential for the production of infectious viral particles

Herpesvirus capsid assembly involves cleavage and packaging of the viral genome. The Kaposi's sarcoma-associated herpesvirus (KSHV) open reading frame 43 (orf43) encodes a putative portal protein. The portal complex functions as a gate through which DNA is packaged into the preformed procapsids, and is injected into the cell nucleus upon infection. The amino acid sequence of the portal proteins is conserved among herpesviruses. Here, we generated an antiserum to ORF43 and determined late expression kinetics of ORF43 along with its nuclear localization. We generated a recombinant KSHV mutant, which fails to express ORF43 (BAC16-ORF43-null). Assembled capsids were observed upon lytic induction of this virus; however, the released virions lacked viral DNA and thus could not establish infection. Ectopic expression of ORF43 rescued the ability to produce infectious particles. ORF43 antiserum and the recombinant ORF43-null virus can provide an experimental system for further studies of the portal functions and its interactions.

Activation and counteraction of antiviral innate immunity by KSHV: an Update

The innate immune responses triggering production of type I interferons and inflammatory cytokines constitute a nonspecific innate resistance that eliminates invading pathogens including viruses. The activation of innate immune signaling through pattern recognition receptors (PRRs) is by sensing pathogen-associated molecular patterns derived from viruses. According to their distribution within cells, PRRs are classified into three types of receptors: membrane, cytoplasmic, and nuclear. Kaposi's sarcoma-associated herpesvirus (KSHV), a large DNA virus, replicates in the nucleus. Its genome is protected by capsid proteins during transport in the cytosol. Multiple PRRs are involved in KSHV recognition. To successfully establish latent infection, KSHV has evolved to manipulate different aspects of the host antiviral innate immune responses. This review presents recent advances in our understanding about the activation of the innate immune signaling in response to infection of KSHV. It also reviews the evasion strategies used by KSHV to subvert host innate immune detection for establishing a persistent infection.

Human Papilloma Virus-Type 16 (HPV-16) & Human Herpes Virus-Type 8 (HHV-8) infections were found to be co-existing major cancer-contributing factors. Individualized, safe, effective treatment of hopelessly advanced cancer patients with metastasis by combining 4 methods of effective treatment: 1) Optimal dose of Vitamin D3 using its 10 unique beneficial effects, 2) Selective Drug Uptake Enhancement Method, 3) Stimulation of newly discovered Thymus gland representation areas on the back of each hand, & 4) Identification & removal of co-existing cancer-contributing factors

Previous study indicated Human Papilloma Virus-Type 16 (HPV-16) has important role in genesis of ovarian cancer, uterus cancer, & breast cancer. Omura, Y. noticed HPV-16 virus co-exists with single-cell parasite Toxoplasma Gondii (T.G.) infection in most of the cancer tissues he examined. Therefore, he thought some unknown virus must exist in the T.G. In early 2018, he found there is strong Human Herpes Virus-Type 8 (HHV-8) infection in every cell of Toxoplasma Gondii he examined. Thus, we found in every cancer cells there are co-existing cancer-promoting factors of HPV-16 & HHV-8 infection associated with Toxoplasma Gondii. As a common source of these infections in human cancer patients, we found most egg yolk has co-existing HPV-16, HHV-8 & Toxoplasma Gondii, but egg whites are free of these infections & can be eaten safely as protein supply. Many cats and dogs also have these infections. As most important, safe treatment of cancer, we found repeated optimal dose of Vitamin D3 with optimal interval of about 8 hours is ideal because of 10 unique, beneficial effects discovered by him. Individually determined one optimal dose of Vitamin D3 has most effective anti-cancer effects in hopeless, advanced, cancer patients by reducing extremely high Integrin α5β1 of over 1500ng to normal value of 0.002∼0.004ng and 8-OH-dG (proportional to DNA mutation which is required for cancer growth) of over 60∼80ng to normal value of 0.1∼0.25ng for average 8 hours and 6000∼9000ng of strong pathogenic infection of HPV-16 & HHV-8 in cancer tissue reduced to safe range of less than 500ng by markedly increased urinary excretion. Concerning beneficial effect of optimal dose of Vitamin D3, all the previous studies failed to use individualized optimal dose. As a result, they could not properly evaluate beneficial effects & side effects and many reports claimed there were no significant beneficial effects including anti-cancer effects. Although optimal dose of Vitamin D3 can eliminate these viral infections very significantly but not less than 500∼300ng. Cancer incidence is only significant when these viruses are above 6000ng inside the body. Thus, we can keep potentially serious patients with strong cancer-contributing virus infections at relatively safe low degree of infection of much less than 3000ng. Our ideal treatment for advanced cancer patients is a combination of 1) Individually determined Optimal dose of Vitamin D3 with optimal interval of about 8 hours, 2) Selective Drug Uptake Enhancement Method by manual stimulation organ representation area of pathological organs at hands, 3) Stimulation of Thymus gland representation area of the back of the hand which was discovered by Omura, Y. in 2017, & 4) Identification & removal of multiple co-existing cancer-promoting factors. This combination of 4 new methods of treatment was found to be most effective & safe treatment for not only hopelessly advanced cancer patients but also many other difficult medical problems.

KSHV and the Role of Notch Receptor Dysregulation in Disease Progression

Kaposi's sarcoma-associated herpesvirus (KSHV) is the causative agent of two human cancers, Kaposi's Sarcoma (KS) and primary effusion lymphoma (PEL), and a lymphoproliferation, Multicentric Castleman's Disease (MCD). Progression to tumor development in KS is dependent upon the reactivation of the virus from its latent state. We, and others, have shown that the Replication and transcriptional activator (Rta) protein is the only viral gene product that is necessary and sufficient for viral reactivation. To induce the reactivation and transcription of viral genes, Rta forms a complex with the cellular DNA binding component of the canonical Notch signaling pathway, recombination signal binding protein for Jk (RBP-Jk). Formation of this Rta:RBP-Jk complex is necessary for viral reactivation to occur. Expression of activated Notch has been shown to be dysregulated in KSHV infected cells and to be necessary for cell growth and disease progression. Studies into the involvement of activated Notch in viral reactivation have yielded varied results. In this paper, we review the current literature regarding Notch dysregulation by KSHV and its role in viral infection and cellular pathogenesis.

Kaposi's Sarcoma-Associated Herpesvirus: Epidemiology and Molecular Biology

Kaposi's sarcoma-associated herpesvirus (KSHV), also known as Human herpesvirus 8 (HHV-8), is a member of the lymphotropic gammaherpesvirus subfamily and a human oncogenic virus. Since its discovery in AIDS-associated KS tissues by Drs. Yuan Chang and Patrick Moore, much progress has been made in the past two decades. There are four types of KS including classic KS, endemic KS, immunosuppressive therapy-related KS, and AIDS-associated KS. In addition to KS, KSHV is also involved in the development of primary effusion lymphoma (PEL) and certain types of multicentric Castleman's disease. KSHV manipulates numerous viral proteins to promote the progression of angiogenesis and tumorigenesis. In this chapter, we review the epidemiology and molecular biology of KSHV and the mechanisms underlying KSHV-induced diseases.

Four levels of hierarchical organization, including noncovalent chainmail, brace the mature tumor herpesvirus capsid against pressurization

Like many double-stranded DNA viruses, tumor gammaherpesviruses Epstein-Barr virus and Kaposi's sarcoma-associated herpesvirus withstand high internal pressure. Bacteriophage HK97 uses covalent chainmail for this purpose, but how this is achieved noncovalently in the much larger gammaherpesvirus capsid is unknown. Our cryoelectron microscopy structure of a gammaherpesvirus capsid reveals a hierarchy of four levels of organization: (1) Within a hexon capsomer, each monomer of the major capsid protein (MCP), 1,378 amino acids and six domains, interacts with its neighboring MCPs at four sites. (2) Neighboring capsomers are linked in pairs by MCP dimerization domains and in groups of three by heterotrimeric triplex proteins. (3) Small (∼280 amino acids) HK97-like domains in MCP monomers alternate with triplex heterotrimers to form a belt that encircles each capsomer. (4) One hundred sixty-two belts concatenate to form noncovalent chainmail. The triplex heterotrimer orchestrates all four levels and likely drives maturation to an angular capsid that can withstand pressurization.

KSHV: pathways to tumorigenesis and persistent infection

Kaposi's sarcoma-associated herpesvirus (KSHV; also known as human herpesvirus 8) is the etiologic agent of Kaposi's sarcoma, primary effusion lymphoma, and multicentric Castleman's disease. These cancers often occur in the context of immunosuppression, which has made KSHV-associated malignancies an increasing global health concern with the persistence of the AIDS epidemic. KSHV has also been linked to several acute inflammatory diseases. KSHV exists between a lytic and latent lifecycle, which allows the virus to transition between active replication and quiescent infection. KSHV encodes a number of proteins and small RNAs that are thought to inadvertently transform host cells while performing their functions of helping the virus persist in the infected host. KSHV also has an arsenal of components that aid the virus in evading the host immune response, which help the virus establish a successful lifelong infection. In this comprehensive chapter, we will discuss the diseases associated with KSHV infection, the biology of latent and lytic infection, and individual proteins and microRNAs that are known to contribute to host cell transformation and immune evasion.

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.

An alternative Kaposi's sarcoma-associated herpesvirus replication program triggered by host cell apoptosis

Kaposi's sarcoma-associated herpesvirus (KSHV) is linked to several neoplastic diseases: Kaposi's sarcoma, primary effusion lymphoma (PEL), and multicentric Castleman's disease (MCD). KSHV replicates actively, via a controlled gene expression program, but can also remain latent. It had been thought that the transition from latent to lytic replication was controlled exclusively by the replication and transcription activator protein RTA (open reading frame 50 [ORF50] gene product). A dominant-negative (DN) ORF50 mutant, ORF50ΔSTAD, blocks gene expression and replication. We produced a PEL cell line derivative containing both latent KSHV genomes and an inducible ORF50ΔSTAD. We unexpectedly found that induction of apoptosis triggered high-level viral replication, even when DN ORF50ΔSTAD was present, suggesting that apoptosis triggers KSHV replication through a distinct RTA-independent pathway. We verified that apoptosis triggers KSHV replication independent of RTA using ORF50 small interfering RNA (siRNA) and also showed that caspase activity is required to trigger KSHV replication. We showed that when apoptosis triggers KSHV replication, the kinetics of late gene expression is accelerated by 12 to 24 h and that virus produced following apoptosis has reduced infectivity. KSHV therefore appears to replicate via two distinct pathways, a conventional pathway requiring RTA, with slower replication kinetics, producing virus with higher infectivity, and an alternative apoptosis-triggered pathway that does not require RTA, has faster replication kinetics, and produces virus with lower infectivity. The existence of a distinct apoptosis-triggered, accelerated replication pathway may have evolutionary advantages for the virus and clinical significance for the treatment of KSHV-associated neoplasms. It also provides further evidence that KSHV can sense and react to its environment.

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.

Functional characterization of Kaposi's sarcoma-associated herpesvirus small capsid protein by bacterial artificial chromosome-based mutagenesis

A systematic investigation of interactions amongst KSHV capsid proteins was undertaken in this study to comprehend lesser known KSHV capsid assembly mechanisms. Interestingly the interaction patterns of the KSHV small capsid protein, ORF65 suggested its plausible role in viral capsid assembly pathways. Towards further understanding this, ORF65-null recombinant mutants (BAC-∆65 and BAC-stop65) employing a bacterial artificial chromosome (BAC) system were generated. No significant difference was found in both overall viral gene expression and lytic DNA replication between stable monolayers of 293T-BAC36 (wild-type) and 293T-BAC-ORF65-null upon induction with 12-O-tetradecanoylphorbol-13-acetate, though the latter released 30-fold fewer virions to the medium than 293T-BAC36 cells. Sedimentation profiles of capsid proteins of ORF65-null recombinant mutants were non-reflective of their organization into the KSHV capsids and were also undetectable in cytoplasmic extracts compared to noticeable levels in nuclear extracts. These observations collectively suggested the pivotal role of ORF65 in the KSHV capsid assembly processes.

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.

Small capsid protein pORF65 is essential for assembly of Kaposi's sarcoma-associated herpesvirus capsids

Kaposi's sarcoma-associated herpesvirus (KSHV) is the etiologic agent for KS tumors, multicentric Castleman's disease, and primary effusion lymphomas. Like other herpesvirus capsids, the KSHV capsid is an icosahedral structure composed of six proteins. The capsid shell is made up of the major capsid protein, two triplex proteins, and the small capsid protein. The scaffold protein and the protease occupy the internal space. The assembly of KSHV capsids is thought to occur in a manner similar to that determined for herpes simplex virus type 1 (HSV-1). Our goal was to assemble KSHV capsids in insect cells using the baculovirus expression vector system. Six KSHV capsid open reading frames were cloned and the proteins expressed in Sf9 cells: pORF25 (major capsid protein), pORF62 (triplex 1), pORF26 (triplex 2), pORF17 (protease), pORF17.5 (scaffold protein), and also pORF65 (small capsid protein). When insect cells were coinfected with these baculoviruses, angular capsids that contained internal core structures were readily observed by conventional electron microscopy of the infected cells. Capsids were also readily isolated from infected cells by using rate velocity sedimentation. With immuno-electron microscopy methods, these capsids were seen to be reactive to antisera to pORF65 as well as to KSHV-positive human sera, indicating the correct conformation of pORF65 in these capsids. When either virus expressing the triplex proteins was omitted from the coinfection, capsids did not assemble; similar to observations made in HSV-1-infected cells. If the virus expressing the scaffold protein was excluded, large open shells that did not attain icosahedral structure were seen in the nuclei of infected cells. The presence of pORF65 was required for capsid assembly, in that capsids did not form if this protein was absent as judged by both by ultrastructural analysis of infected cells and rate velocity sedimentation experiments. Thus, a novel outcome of this study is the finding that the small capsid protein of KSHV, like the major capsid and triplex proteins, is essential for capsid shell assembly.

Virion-wide protein interactions of Kaposi's sarcoma-associated herpesvirus

Herpesvirus virions are highly organized structures built through specific protein-protein interactions. Thus, revelation of the protein interactions among virion proteins will shed light on the processes and the mechanisms of virion formation. Recently, we identified 24 virion proteins of Kaposi's sarcoma-associated herpesvirus (KSHV), using a proteomic approach (F. X. Zhu et al., J. Virol. 79:800-811, 2005). In the current study, a comprehensive analysis of protein-protein interaction between KSHV virion proteins was carried out using yeast two-hybrid (Y2H) and coimmunoprecipitation (co-IP) approaches. Every pairwise combination between KSHV tegument and capsid proteins, between tegument and envelope proteins, and among tegument proteins was tested for possible binary interaction. Thirty-seven protein-protein interactions were identified by both Y2H and co-IP analyses. The results revealed interactions between tegument and capsid proteins such as that of open reading frame 64 (ORF64) with ORF25 (major capsid protein [MCP]), ORF62 (triplex-1 [TRI-1]), and ORF26 (TRI-2). Many interactions were detected among the tegument proteins. ORF64 was found to interact with several tegument proteins including ORF11, ORF21, ORF33, ORF45, ORF63, ORF75, and ORF64 itself, suggesting that ORF64 may serve as a hub protein and play a role in recruiting tegument proteins during tegumentation and virion assembly. Our investigation also revealed redundant interactions between tegument proteins and envelope glycoproteins. These interactions are believed to contribute to final envelopment in virion assembly. Overall, this study allows us to establish a virion-wide protein interaction map, which provides insight into the architecture of the KSHV virion and sets up a foundation for exploring the functions of these proteins in viral particle assembly.

Chapter 5: Imaging in depth: controversies and opportunities

One enduring challenge of biological imaging is achieving depth of penetration-into cells, tissues, and animals. How deeply can we probe and with what resolution and efficacy? These are critical issues as microscopists seek to push ever deeper, while resolving structural details and observing specific molecular events. In this guide to depth-appropriate modalities, standard optical platforms such as confocal and two-photon microscopes are considered along with complementary imaging modalities that range in depth of penetration. After an introduction to basic techniques, the trade-offs and limitations that distinguish competing technologies are considered, with emphasis on the visualization of subcellular structures and dynamic events. Not surprisingly, there are differences of opinion regarding imaging technologies, as highlighted in a section on point-scanning and Nipkow-disk style confocal microscopes. Confocal microscopy is then contrasted with deconvolution and multi-photon imaging modalities. It is also important to consider the detectors used by current instruments (such as PMTs and CCD cameras). Ultimately specimen properties, in conjunction with instrumentation, determine the depth at which subcellular operations and larger-scale biological processes can be visualized. Relative advantages are mentioned in the context of experiment planning and instrument-purchase decisions. Given the rate at which new optical techniques are being invented, this report should be viewed as a snapshot of current capabilities, with the goal of providing a framework for thinking about new developments.

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.

Cryo-electron tomography of Kaposi's sarcoma-associated herpesvirus capsids reveals dynamic scaffolding structures essential to capsid assembly and maturation

Kaposi's sarcoma-associated herpesvirus (KSHV) is a recently discovered DNA tumor virus that belongs to the gamma-herpesvirus subfamily. Though numerous studies on KSHV and other herpesviruses, in general, have revealed much about their multilayered organization and capsid structure, the herpesvirus capsid assembly and maturation pathway remains poorly understood. Structural variability or irregularity of the capsid internal scaffolding core and the lack of adequate tools to study such structures have presented major hurdles to earlier investigations employing more traditional cryo-electron microscopy (cryoEM) single particle reconstruction. In this study, we used cryo-electron tomography (cryoET) to obtain 3D reconstructions of individual KSHV capsids, allowing direct visualization of the capsid internal structures and systematic comparison of the scaffolding cores for the first time. We show that B-capsids are not a structurally homogenous group; rather, they represent an ensemble of "B-capsid-like" particles whose inner scaffolding is highly variable, possibly representing different intermediates existing during the KSHV capsid assembly and maturation. This information, taken together with previous observations, has allowed us to propose a detailed pathway of herpesvirus capsid assembly and maturation.

Direct visualization of the putative portal in the Kaposi's sarcoma-associated herpesvirus capsid by cryoelectron tomography

Genetic and biochemical studies have suggested the existence of a bacteriophage-like, DNA-packaging/ejecting portal complex in herpesviruses capsids, but its arrangement remained unknown. Here, we report the first visualization of a unique vertex in the Kaposi's sarcoma-associated herpesvirus (KSHV) capsid by cryoelectron tomography, thus providing direct structural evidence for the existence of a portal complex in a gammaherpesvirus. This putative KSHV portal is an internally localized, umbilicated structure and lacks all of the external machineries characteristic of portals in DNA bacteriophages.

Molecular biology of KSHV in relation to AIDS-associated oncogenesis

KSHV has been established as the causative agent of KS, PEL, and MCD, malignancies occurring more frequently in AIDS patients. The aggressive nature of KSHV in the context of HIV infection suggests that interactions between the two viruses enhance pathogenesis. KSHV latent infection and lytic reactivation are characterized by distinct gene expression profiles, and both latency and lytic reactivation seem to be required for malignant progression. As a sophisticated oncogenic virus, KSHV has evolved to possess a formidable repertoire of potent mechanisms that enable it to target and manipulate host cell pathways, leading to increased cell proliferation, increased cell survival, dysregulated angiogenesis, evasion of immunity, and malignant progression in the immunocompromised host. Worldwide, approximately 40.3 million people are currently living with HIV infection. Of these, a significant number are coinfected with KSHV. The complex interplay between the two viruses dramatically elevates the risk for development of KSHV-induced malignancies, KS, PEL, and MCD. Although HAART significantly reduces HIV viral load, the entire T-cell repertoire and immune function may not be completely restored. In fact, clinically significant immune deficiency is not necessary for the induction of KSHV-related malignancy. Because of variables such as lack of access to therapy noncompliance with prescribed treatment, failure to respond to treatment and the development of drug-resistant strains of HIV, KSHV-induced malignancies will continue to present as major health concerns.

Rhesus monkey rhadinovirus: a model for the study of KSHV

Rhesus monkey rhadinovirus (RRV) is one of the closest phylogenetic relatives to the human pathogen Kaposi sarcoma-associated herpesvirus (KSHV)-a gamma-2 herpesvirus and the etiologic agent of three malignancies associated with immunosuppression. In contrast to KSHV, RRV displays robust lytic-phase growth in culture, replicating to high titer, and therefore holds promise as an effective model for studying primate gammaherpesvirus lytic gene transcription as well as virion structure, assembly, and proteomics. More recently, investigators have devised complementary latent systems of RRV infection, thereby also enabling the characterization of the more restricted latent transcriptional program. Another benefit of working with RRV as a primate gammaherpesvirus model is that its efficient lytic growth makes genetic manipulation easier than that in its human counterpart. Exploiting this quality, laboratories have already begun to generate mutant RRV, setting the stage for future work investigating the function of individual viral genes. Finally, rhesus macaques support experimental infection with RRV, providing a natural in vivo model of infection, while similar nonhuman systems have remained resistant to prolonged KSHV infection. Recently, dual infection with RRV and a strain of simian immunodeficiency virus (SIV) has led to a lymphoproliferative disorder (LPD) reminiscent of multicentric Castleman disease (MCD)--a clinical manifestation of KSHV infection in a subset of immunosuppressed patients. RRV, in short, shows a high degree of homology with KSHV yet is more amenable to experimental manipulation both in vitro and in vivo. Taken together, these qualities ensure its current position as one of the most relevant viral models of KSHV biology and infection.

Mass spectrometric analyses of purified rhesus monkey rhadinovirus reveal 33 virion-associated proteins

The repertoire of proteins that comprise intact gammaherpesviruses, including the human pathogen Kaposi's sarcoma-associated herpesvirus (KSHV), is likely to have critical functions not only in viral structure and assembly but also in the early stages of infection and evasion of the host's rapidly deployed antiviral defenses. To develop a better understanding of these proteins, we analyzed the composition of rhesus monkey rhadinovirus (RRV), a close phylogenetic relative of KSHV. Unlike KSHV, RRV replicates to high titer in cell culture and thus serves as an effective model for studying primate gammaherpesvirus structure and virion proteomics. We employed two complementary mass spectrometric approaches and found that RRV contains at least 33 distinct virally encoded proteins. We have assigned 7 of these proteins to the capsid, 17 to the tegument, and 9 to the envelope. Of the five gammaherpesvirus-specific tegument proteins, three have no known function. We also found three proteins not previously associated with a purified herpesvirus and an additional seven that represent new findings for a member of the gamma-2 herpesviruses. Detergent extraction resulted in particles that contained six distinct tegument proteins in addition to the expected capsid structural proteins, suggesting that this subset of tegument components may interact more directly with or with higher affinity for the underlying capsid and, in turn, may play a role in assembly or transport of viral or subviral particles during entry or egress.

Human herpesvirus 8--a novel human pathogen

In 1994, Chang and Moore reported on the latest of the gammaherpesviruses to infect humans, human herpesvirus 8 (HHV-8) [1]. This novel herpesvirus has and continues to present challenges to define its scope of involvement in human disease. In this review, aspects of HHV-8 infection are discussed, such as, the human immune response, viral pathogenesis and transmission, viral disease entities, and the virus's epidemiology with an emphasis on HHV-8 diagnostics.

Three-dimensional localization of the smallest capsid protein in the human cytomegalovirus capsid

The smallest capsid proteins (SCPs) of the human herpesviruses differ substantially in size and sequence and are thought to impart some unique aspects of infection to their respective viruses. We used electron cryomicroscopy and antibody labeling to show that the 8-kDa SCP of human cytomegalovirus is attached only to major capsid protein subunits of the hexons, not the pentons. Thus, the SCPs of different herpesviruses illustrate that a protein can evolve significantly in sequence, structure, and function, while preserving its role in the architecture of the virus by binding to a specific partner in a specific oligomeric state.

Virion proteins of Kaposi's sarcoma-associated herpesvirus

The proteins that compose a herpesvirus virion are thought to contain the functional information required for de novo infection, as well as virion assembly and egress. To investigate functional roles of Kaposi's sarcoma-associated herpesvirus (KSHV) virion proteins in viral productive replication and de novo infection, we attempted to identify virion proteins from purified KSHV by a proteomic approach. Extracellular KSHV virions were purified from phorbol-12-tetradecanoate-13-acetate-induced BCBL-1 cells through double-gradient ultracentrifugation, and their component proteins were resolved by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Thirty prominent protein bands were excised and subjected to high-performance liquid chromatography ion trap mass spectrometric analysis. This study led to the identification of 24 virion-associated proteins. These include five capsid proteins, eight envelope glycoproteins, six tegument proteins, and five proteins whose locations in the virions have not yet been defined. Putative tegument proteins encoded by open reading frame 21 (ORF21), ORF33, and ORF45 were characterized and found to be resistant to protease digestion when purified virions were treated with trypsin, confirming that they are located within the virion particles. The ORF64-encoded large tegument protein was found to be associated with capsid but sensitive to protease treatment, suggesting its unique structure and array in KSHV virions. In addition, cellular beta-actin and class II myosin heavy chain type A were found inside KSHV virions and associated with tegument-capsid structure. Identification of KSHV virion proteins makes it possible to study the functional roles of these virion proteins in KSHV replication and pathogenicity.

De novo infection with rhesus monkey rhadinovirus leads to the accumulation of multiple intranuclear capsid species during lytic replication but favors the release of genome-containing virions

Rhesus monkey rhadinovirus (RRV) is one of the closest phylogenetic relatives to the human pathogen Kaposi's sarcoma-associated herpesvirus (KSHV), yet it has the distinct experimental advantage of entering efficiently into lytic replication and growing to high titers in culture. RRV therefore holds promise as a potentially attractive model with which to study gammaherpesvirus structure and assembly. We have isolated RRV capsids, determined their molecular composition, and identified the genes encoding five of the main capsid structural proteins. Our data indicate that, as with other herpesviruses, lytic infection with RRV leads to the synthesis of three distinct intranuclear capsid species. However, in contrast to the inefficiency of KSHV maturation following reactivation from latently infected B-cell lines (K. Nealon, W. W. Newcomb, T. R. Pray, C. S. Craik, J. C. Brown, and D. H. Kedes, J. Virol. 75:2866-2878, 2001), de novo infection of immortalized rhesus fibroblasts with RRV results in the release of high levels of infectious virions with genome-containing C capsids at their center. Together, our findings argue for the use of RRV as a powerful model with which to study the structure and assembly of gammaherpesviruses and, specifically, the human rhadinovirus,KSHV.

Three-dimensional structures of the A, B, and C capsids of rhesus monkey rhadinovirus: insights into gammaherpesvirus capsid assembly, maturation, and DNA packaging

Rhesus monkey rhadinovirus (RRV) exhibits high levels of sequence homology to human gammaherpesviruses, such as Kaposi's sarcoma-associated herpesvirus, and grows to high titers in cell cultures, making it a good model system for studying gammaherpesvirus capsid structure and assembly. We have purified RRV A, B, and C capsids, thus for the first time allowing direct structure comparisons by electron cryomicroscopy and three-dimensional reconstruction. The results show that the shells of these capsids are identical and are each composed of 12 pentons, 150 hexons, and 320 triplexes. Structural differences were apparent inside the shells and through the penton channels. The A capsid is empty, and its penton channels are open. The B capsid contains a scaffolding core, and its penton channels are closed. The C capsid contains a DNA genome, which is closely packaged into regularly spaced density shells (25 A apart), and its penton channels are open. The different statuses of the penton channels suggest a functional role of the channels during capsid maturation, and the overall structural similarities of RRV capsids to alphaherpesvirus capsids suggest a common assembly and maturation pathway. The RRV A capsid reconstruction at a 15-A resolution, the best achieved for gammaherpesvirus particles, reveals overall structural similarities to alpha- and betaherpesvirus capsids. However, the outer regions of the capsid, including densities attributed to the Ta triplex and the small capsomer-interacting protein (SCIP or ORF65), exhibit prominent differences from their structural counterparts in alphaherpesviruses. This structural disparity suggests that SCIP and the triplex, together with tegument and envelope proteins, confer structural and potentially functional specificities to alpha-, beta-, and gammaherpesviruses.

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.

Molecular genetics of Kaposi's sarcoma-associated herpesvirus (human herpesvirus-8) epidemiology and pathogenesis

Kaposi's sarcoma had been recognized as unique human cancer for a century before it manifested as an AIDS-defining illness with a suspected infectious etiology. The discovery of Kaposi's sarcoma-associated herpesvirus (KSHV), also known as human herpesvirus-8, in 1994 by using representational difference analysis, a subtractive method previously employed for cloning differences in human genomic DNA, was a fitting harbinger for the powerful bioinformatic approaches since employed to understand its pathogenesis in KS. Indeed, the discovery of KSHV was rapidly followed by publication of its complete sequence, which revealed that the virus had coopted a wide armamentarium of human genes; in the short time since then, the functions of many of these viral gene variants in cell growth control, signaling apoptosis, angiogenesis, and immunomodulation have been characterized. This critical literature review explores the pathogenic potential of these genes within the framework of current knowledge of the basic herpesvirology of KSHV, including the relationships between viral genotypic variation and the four clinicoepidemiologic forms of Kaposi's sarcoma, current viral detection methods and their utility, primary infection by KSHV, tissue culture and animal models of latent- and lytic-cycle gene expression and pathogenesis, and viral reactivation from latency. Recent advances in models of de novo endothelial infection, microarray analyses of the host response to infection, receptor identification, and cloning of full-length, infectious KSHV genomic DNA promise to reveal key molecular mechanisms of the candidate pathogeneic genes when expressed in the context of viral infection.

Three-dimensional localization of pORF65 in Kaposi's sarcoma-associated herpesvirus capsid

Of the six herpesvirus capsid proteins, the smallest capsid proteins (SCPs) share the least sequence homology among herpesvirus family members and have been implicated in virus specificity during infection. The herpes simplex virus-1 (HSV-1) SCP was shown to be horn shaped and to specifically bind the upper domain of each major capsid protein in hexons but not in pentons. In Kaposi's sarcoma-associated herpesvirus (KSHV), the protein encoded by the ORF65 gene (pORF65) is the putative SCP but its location remains controversial due to the absence of such horn-shaped densities from both the pentons and hexons of the KSHV capsid reconstructions. To directly locate the KSHV SCP, we have used electron cryomicroscopy and three-dimensional reconstruction techniques to compare the three-dimensional structure of KSHV capsids to that of anti-pORF65 antibody-labeled capsids. Our difference map shows prominent antibody densities bound to the tips of the hexons but not to pentons, indicating that KSHV SCP is attached to the upper domain of the major capsid protein in hexons but not to that in pentons, similar to HSV-1 SCP. The lack of horn-shaped densities on the hexons indicates that KSHV SCP exhibits structural features that are substantially different from those of HSV-1 SCP. The location of SCP at the outermost regions of the capsid suggests a possible role in mediating capsid interactions with the tegument and cytoskeletal proteins during infection.

The ORF45 protein of Kaposi's sarcoma-associated herpesvirus is associated with purified virions

Kaposi's sarcoma-associated herpesvirus (KSHV) ORF45 is encoded by an immediate-early gene in the KSHV genome. This protein was recently shown to interact with interferon regulatory factor 7 and inhibit virus-mediated alpha/beta interferon induction (Zhu et al., Proc. Natl. Acad. Sci. USA 99:5573-5578, 2002). ORF45 was characterized as a phosphorylated protein, and it is localized in the cytoplasm of infected cells. In this report, we provide evidence that ORF45 is associated with KSHV virions. (i) ORF45 was detected in gradient-purified virions by Western blotting along with known structural proteins of KSHV including gB, K8.1, and major capsid protein. In contrast, ORF50/Rta, K8alpha, and ORF59/PF8 were not detected in the same virion preparation. (ii) ORF45 comigrates with KSHV virions in sucrose gradient ultracentrifugation. (iii) Virion-associated ORF45 was resistant to trypsin digestion but became sensitive after the virions were treated with detergent which destroys the viral envelope. (iv) ORF45 remained associated with tegument-nucleocapsid complex when virion-specific glycoproteins were removed after detergent treatment. (v) An ORF45 protein band was visualized by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of extensively purified KSHV virions and identified by mass spectrometry. (vi) By immunoelectron microscopy, virus-like structures were specifically stained by anti-ORF45 antibody. Based on the evidence, we conclude that ORF45 is associated with purified KSHV virions and appears to be a tegument protein. The presence of ORF45 in KSHV virions raised the possibility that this protein may be delivered to host cells at the start of infection and therefore have the opportunity to act at the very early stage of the infection, suggesting an important role of ORF45 in KSHV primary infection.

Reversible aggregation of mouse prion protein derivatives with PrPSC-like structural properties

Three carbamylated derivatives of reduced mouse prion protein (mPrP) were isolated during the aborted oxidative folding in the presence of urea. These three prion protein derivatives (mPrP-a, mPrP-b, and mPrP-c) exist as monomer in the acidic solution (pH < 2.0) and exhibit prevalent random coil structure. However, they undergo rapid aggregation and transformation to a predominant beta-sheet structure upon exposure to ionic buffer with pH greater than 3.0. The stability of aggregates of mPrP conformers is in part dependent upon the time that they were allowed to develop. The nascent aggregates comprise a significant fraction of loosely packed mPrP monomers that can be dissociated by treatment with strong acidic solution. Matured aggregates acquired through prolonged sample incubation contain more tightly packed mPrP monomers that cannot be dissociated by strong acid but can be disaggregated by denaturant. The properties of reversible aggregation of mPrP-a, mPrP-b, and mPrP-c bear a striking resemblance to that observed with aggregates of hamster PrPSC.

Studying Large Viruses

In this article we have attempted to describe some structural aspects of large viruses. Although this may seem a straightforward task, it is complicated by the fact that large viruses do not represent a distinctive class of organisms and any grouping under this heading will include a range of unrelated viruses with different structures, replication strategies, and host types. To simplify matters we limited our definition to dsDNA viruses with genomes of 100 kbp or larger. However, even this restricted grouping includes viruses with diverse and seemingly unrelated structures. Furthermore, few if any structural features are exclusive to large viruses and most of what appears distinctive about their structure or assembly can also be found in smaller, and usually better characterized, viruses. Therefore we have not attempted to provide a comprehensive catalog of the properties of large viruses but have tried to illustrate particular structural points with examples from a few of the better known forms, notably herpes simplex virus (HSV) and phage T4. The two techniques used to provide rigorous analyses of virus structures are X-ray crystallography and electron cryomicroscopy with computer-assisted reconstruction. To date, X-ray crystallography has been successful only with smaller viruses, and what is known about the structures of these large viruses has come primarily from electron cryomicroscopy. However, with the notable exception of the HSV capsid, such studies have been limited in extent and of relatively low resolution, and the information obtained has been confined largely to describing the spatial distributions and relationships between the subunits. Nevertheless, these studies have given us our clearest insights into the biology of these complex particles and increases in resolution promise to extend these insights by bridging the gap between gross and atomic structures, as exemplified by the identification and mapping of secondary structural elements in the HSV capsid.

Handedness of the herpes simplex virus capsid and procapsid

The capsid of herpes simplex virus has an icosahedral surface lattice with a nonskew triangulation number, T=16. Nevertheless, the proteins arrayed on this lattice necessarily have an intrinsic handedness. We have determined the handedness of both the herpes simplex virus type 1 capsid and its precursor procapsid by a cryoelectron microscopic tilting method.

Spectrum of Kaposi's sarcoma-associated herpesvirus, or human herpesvirus 8, diseases

Human herpesvirus 8 (HHV-8), also known as Kaposi's sarcoma-associated herpesvirus (KSHV), discovered in 1994, is a human rhadinovirus (gamma-2 herpesvirus). Unlike other human herpesviruses (herpes simplex virus, Epstein-Barr virus, varicella-zoster virus, cytomegalovirus, HHV-6, and HHV-7), it is not widespread in the general population and has many unique proteins. HHV-8 is strongly associated with all subtypes of Kaposi's sarcoma (KS), multicentric Castleman's disease, and a rare form of B-cell lymphoma, primary effusion lymphoma. In addition, HHV-8 DNA sequences have been found in association with other diseases, but the role of the virus in these diseases is largely unconfirmed and remains controversial. The seroprevalence of HHV-8, based on detection of latent and lytic proteins, is 2 to 5% in healthy donors except in certain geographic areas where the virus is endemic, 80 to 95% in classic KS patients, and 40 to 50% in HIV-1 patients without KS. This virus can be transmitted both sexually and through body fluids (e.g., saliva and blood). HHV-8 is a transforming virus, as evidenced by its presence in human malignancies, by the in vitro transforming properties of several of its viral genes, and by its ability to transform some primary cells in culture. It is not, however, sufficient for transformation, and other cofactors such as immunosuppressive cytokines are involved in the development of HHV-8-associated malignancies. In this article, we review the biology, molecular virology, epidemiology, transmission, detection methods, pathogenesis, and antiviral therapy of this newly discovered human herpesvirus.

Molecular interactions and viral stability revealed by structural analyses of chemically treated cypovirus capsids

Cytoplasmic polyhedrosis virus (CPV, genus Cypovirus) is a unique member of the family Reoviridae which lacks the outer protective shells that exist in all other members, yet exhibits unusual stability. We have analyzed the effects of different acidic, basic, detergent, and urea treatments on CPV capsids. The integrity of the CPV capsids was unaffected under high-pH conditions that disrupted the orthoreovirus inner core, consistent with its ability to maintain structural integrity in extremely alkaline environments during infection. However, it was sensitive to low pH, detergents, and urea, similarly to other viruses in this family. The three-dimensional structure comparisons by electron cryomicroscopy of the intact empty CPV capsid with the "spikeless" capsid whose turrets were removed by chemical treatments revealed the interaction footprint of the turret on the capsid shell. The observed structural changes associated with the removal of the turret suggest critical structural roles of the turret in maintaining capsid integrity in addition to its enzymatic activities.

The molecular pathology of Kaposi's sarcoma-associated herpesvirus

Kaposi's sarcoma (KS)-associated herpesvirus (KSHV) is the eighth and most recently identified human herpesvirus (HHV-8). KSHV was discovered in 1994 by Chang et al. who used representational difference analysis to search for DNA sequences present in AIDS-associated KS but not in adjacent normal skin [1]. The virus has since been shown to be specifically associated with all forms of this disease and has fulfilled all of Hill's criteria for causation (reviewed in ). KSHV is also found in all cases of primary effusion lymphoma and in a plasmablastic variant of multicentric Castleman's disease. Over the last few years a wealth of data has been gained on the role of KSHV genes during infection. This review is an attempt to assemble this information into a more complete picture of how KSHV may cause disease.

Capsid structure of Kaposi's sarcoma-associated herpesvirus, a gammaherpesvirus, compared to those of an alphaherpesvirus, herpes simplex virus type 1, and a betaherpesvirus, cytomegalovirus

The capsid of Kaposi's sarcoma-associated herpesvirus (KSHV) was visualized at 24-A resolution by cryoelectron microscopy. Despite limited sequence similarity between corresponding capsid proteins, KSHV has the same T=16 triangulation number and much the same capsid architecture as herpes simplex virus (HSV) and cytomegalovirus (CMV). Its capsomers are hexamers and pentamers of the major capsid protein, forming a shell with a flat, close-packed, inner surface (the "floor") and chimney-like external protrusions. Overlying the floor at trigonal positions are (alpha beta(2)) heterotrimers called triplexes. The floor structure is well conserved over all three viruses, and the most variable capsid features reside on the outer surface, i.e., in the shapes of the protrusions and triplexes, in which KSHV resembles CMV and differs from HSV. Major capsid protein sequences from the three subfamilies have some similarity, which is closer between KSHV and CMV than between either virus and HSV. The triplex proteins are less highly conserved, but sequence analysis identifies relatively conserved tracts. In alphaherpesviruses, the alpha-subunit (VP19c in HSV) has a 100-residue N-terminal extension and an insertion near the C terminus. The small basic capsid protein sequences are highly divergent: whereas the HSV and CMV proteins bind only to hexons, difference mapping suggests that the KSHV protein, ORF65, binds around the tips of both hexons and pentons.

Lytic replication of Kaposi's sarcoma-associated herpesvirus results in the formation of multiple capsid species: isolation and molecular characterization of A, B, and C capsids from a gammaherpesvirus

Despite the discovery of Epstein-Barr virus more than 35 years ago, a thorough understanding of gammaherpesvirus capsid composition and structure has remained elusive. We approached this problem by purifying capsids from Kaposi's sarcoma-associated herpesvirus (KSHV), the only other known human gammaherpesvirus. The results from our biochemical and imaging analyses demonstrate that KSHV capsids possess a typical herpesvirus icosahedral capsid shell composed of four structural proteins. The hexameric and pentameric capsomers are composed of the major capsid protein (MCP) encoded by open reading frame 25. The heterotrimeric complexes, forming the capsid floor between the hexons and pentons, are each composed of one molecule of ORF62 and two molecules of ORF26. Each of these proteins has significant amino acid sequence homology to capsid proteins in alpha- and betaherpesviruses. In contrast, the fourth protein, ORF65, lacks significant sequence homology to its structural counterparts from the other subfamilies. Nevertheless, this small, basic, and highly antigenic protein decorates the surface of the capsids, as does, for example, the even smaller basic capsid protein VP26 of herpes simplex virus type 1. We have also found that, as with the alpha- and betaherpesviruses, lytic replication of KSHV leads to the formation of at least three capsid species, A, B, and C, with masses of approximately 200, 230, and 300 MDa, respectively. A capsids are empty, B capsids contain an inner array of a fifth structural protein, ORF17.5, and C capsids contain the viral genome.