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

Murine gammaherpesvirus 68 ORF52 encodes a tegument protein required for virion morphogenesis in the cytoplasm

The tegument, a semiordered matrix of proteins overlying the nucleocapsid and underlying the virion envelope, in viruses in the gamma subfamily of Herpesviridae is poorly understood. Murine gammaherpesvirus 68 (MHV-68) is a robust model for studying gammaherpesvirus virion structure, assembly, and composition, as MHV-68 efficiently completes the lytic phase and productively infects cultured cells. We have found that MHV-68 ORF52 encodes an abundant tegument protein conserved among gammaherpesviruses. Detergent sensitivity experiments revealed that the MHV-68 ORF52 protein is more tightly bound to the virion nucleocapsid than the ORF45 tegument protein but could be dissociated from particles that retained the ORF65 small capsomer protein. ORF52, tagged with enhanced green fluorescent protein or FLAG epitope, localized to the cytoplasm. A recombinant MHV-68 bacterial artificial chromosome mutant with a nonsense mutation incorporated into ORF52 exhibited viral DNA replication, expression of late lytic genes, and capsid assembly and packaging at levels near those of the wild type. However, the MHV-68 ORF52-null virus was deficient in the assembly and release of infectious virion particles. Instead, partially tegumented capsids produced by the ORF52-null mutant accumulated in the cytoplasm, containing conserved capsid proteins, the ORF64 and ORF67 tegument proteins, but virtually no ORF45 tegument protein. Thus, ORF52 is essential for the tegumentation and egress of infectious MHV-68 particles in the cytoplasm, suggesting an important conserved function in gammaherpesvirus virion morphogenesis.

Molecular characterization of the rhesus rhadinovirus (RRV) ORF4 gene and the RRV complement control protein it encodes

The diversity of viral strategies to modulate complement activation indicates that this component of the immune system has significant antiviral potential. One example is the Kaposi's sarcoma-associated herpesvirus (KSHV) complement control protein (KCP), which inhibits progression of the complement cascade. Rhesus rhadinovirus (RRV), like KSHV, is a member of the subfamily Gammaherpesvirinae and currently provides the only in vivo model of KSHV pathobiology in primates. In the present study, we characterized the KCP homologue encoded by RRV, RRV complement control protein (RCP). Two strains of RRV have been sequenced to date (H26-95 and 17577), and the RCPs they encode differ substantially in structure: RCP from strain H26-95 has four complement control protein (CCP) domains, whereas RCP from strain 17577 has eight CCP domains. Transcriptional analyses of the RCP gene (ORF4, referred to herein as RCP) in infected rhesus macaque fibroblasts mapped the ends of the transcripts of both strains. They revealed that H26-95 encodes a full-length, unspliced RCP transcript, while 17577 RCP generates a full-length unspliced mRNA and two alternatively spliced transcripts. Western blotting confirmed that infected cells express RCP, and immune electron microscopy disclosed this protein on the surface of RRV virions. Functional studies of RCP encoded by both RRV strains revealed their ability to suppress complement activation by the classical (antibody-mediated) pathway. These data provide the foundation for studies into the biological significance of gammaherpesvirus complement regulatory proteins in a tractable, non-human primate model.

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.

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.

Kaposi's sarcoma-associated herpesvirus-encoded protein kinase and its interaction with K-bZIP

The oncogenic herpesvirus, Kaposi's sarcoma-associated herpesvirus, also identified as human herpesvirus 8, contains genes producing proteins that control transcription and influence cell signaling. Open reading frame 36 (ORF36) of this virus encodes a serine/threonine protein kinase, which is designated the viral protein kinase (vPK). Our recent efforts to elucidate the role of vPK in the viral life cycle have focused on identifying viral protein substrates and determining the effects of vPK-mediated phosphorylation on specific steps in viral replication. The vPK gene was transcribed into 4.2-kb and 3.6-kb mRNAs during the early and late phases of viral reactivation. vPK is colocalized with viral DNA replication/transcription compartments as marked by a polymerase processivity factor, and K-bZIP, a protein known to bind the viral DNA replication origin (Ori-Lyt) and to regulate viral transcription. The vPK physically associated with and strongly phosphorylated K-bZIP at threonine 111, a site also recognized by the cyclin-dependent kinase Cdk2. Both K-bZIP and vPK were corecruited to viral promoters targeted by K-bZIP as well as to the Ori-Lyt region. Phosphorylation of K-bZIP by vPK had a negative impact on K-bZIP transcription repression activity. The extent of posttranslational modification of K-bZIP by sumoylation, a process that influences its repression function, was decreased by vPK phosphorylation at threonine 111. Our data thus identify a new role of vPK as a modulator of viral transcription.

The Rta/Orf50 transactivator proteins of the gamma-herpesviridae

The replication and transcription activator protein, Rta, is encoded by Orf50 in Kaposi's sarcoma-associated herpesvirus (KSHV) and other known gammaherpesviruses including Epstein-Barr virus (EBV), rhesus rhadinovirus (RRV), herpesvirus saimiri (HVS), and murine herpesvirus 68 (MHV-68). Each Rta/Orf50 homologue of each gammaherpesvirus plays a pivotal role in the initiation of viral lytic gene expression and lytic reactivation from latency. Here we discuss the Rta/Orf50 of KSHV in comparison to the Rta/Orf50s of other gammaherpesviruses in an effort to identify structural motifs, mechanisms of action, and modulating host factors.

Functional characterization of Kaposi's sarcoma-associated herpesvirus ORF45 by bacterial artificial chromosome-based mutagenesis

Open reading frame 45 (ORF45) of Kaposi's sarcoma-associated herpesvirus (KSHV) encodes an immediate-early protein. This protein is also present in virions as a tegument protein. ORF45 protein interacts with interferon regulatory factor 7 (IRF-7) and inhibits virus-induced type I interferon production by blocking activation of IRF-7. To define further the function of ORF45 and the mechanism underlying its action, we constructed an ORF45-null recombinant virus genome (BAC-stop45) by using a bacterial artificial chromosome (BAC) system. Stable 293T cells carrying the BAC36 (wild type) and BAC-stop45 genomes were generated. When monolayers of 293T BAC36 and 293T BAC-stop45 cells were induced with 12-O-tetradecanoylphorbol-13-acetate and sodium butyrate, no significant difference was found between them in overall viral gene expression and lytic DNA replication, but induced 293T BAC-stop45 cells released 10-fold fewer virions to the medium than did 293T BAC36 cells. When ORF45-null virus was used to infect cells, lower infectivity was observed than for wild-type BAC36. These results suggest that KSHV ORF45 plays roles in both early and late stages of viral infection, probably in viral ingress and egress.

Mimivirus giant particles incorporate a large fraction of anonymous and unique gene products

Acanthamoeba polyphaga mimivirus is the largest known virus in both particle size and genome complexity. Its 1.2-Mb genome encodes 911 proteins, among which only 298 have predicted functions. The composition of purified isolated virions was analyzed by using a combined electrophoresis/mass spectrometry approach allowing the identification of 114 proteins. Besides the expected major structural components, the viral particle packages 12 proteins unambiguously associated with transcriptional machinery, 3 proteins associated with DNA repair, and 2 topoisomerases. Other main functional categories represented in the virion include oxidative pathways and protein modification. More than half of the identified virion-associated proteins correspond to anonymous genes of unknown function, including 45 "ORFans." As demonstrated by both Western blotting and immunogold staining, some of these "ORFans," which lack any convincing similarity in the sequence databases, are endowed with antigenic properties. Thus, anonymous and unique genes constituting the majority of the mimivirus gene complement encode bona fide proteins that are likely to participate in well-integrated processes.

Dissecting the regions of virion-associated Kaposi's sarcoma-associated herpesvirus complement control protein required for complement regulation and cell binding

Complement, which bridges innate and adaptive immune responses as well as humoral and cell-mediated immunity, is antiviral. Kaposi's sarcoma-associated herpesvirus (KSHV) encodes a lytic cycle protein called KSHV complement control protein (KCP) that inhibits activation of the complement cascade. It does so by regulating C3 convertases, accelerating their decay, and acting as a cofactor for factor I degradation of C4b and C3b, two components of the C3 and C5 convertases. These complement regulatory activities require the short consensus repeat (SCR) motifs, of which KCP has four (SCRs 1 to 4). We found that in addition to KCP being expressed on the surfaces of experimentally infected endothelial cells, it is associated with the envelope of purified KSHV virions, potentially protecting them from complement-mediated immunity. Furthermore, recombinant KCP binds heparin, an analogue of the known KSHV cell attachment receptor heparan sulfate, facilitating infection. Treating virus with an anti-KCP monoclonal antibody (MAb), BSF8, inhibited KSHV infection of cells by 35%. Epitope mapping of MAb BSF8 revealed that it binds within SCR domains 1 and 2, also the region of the protein involved in heparin binding. This MAb strongly inhibited classical C3 convertase decay acceleration by KCP and cofactor activity for C4b cleavage but not C3b cleavage. Our data suggest similar topological requirements for cell binding by KSHV, heparin binding, and regulation of C4b-containing C3 convertases but not for factor I-mediated cleavage of C3b. Importantly, they suggest KCP confers at least two functions on the virion: cell binding with concomitant infection and immune evasion.