Design and Functional Characterization of HIV-1 Envelope Protein-Coupled T Helper Liposomes

Functionalization of experimental HIV-1 virus-like particle vaccines with heterologous T helper epitopes (T helper VLPs) can modulate the humoral immune response via intrastructural help (ISH). Current advances in the conjugation of native-like HIV-1 envelope trimers (Env) onto liposomes and encapsulation of peptide epitopes into these nanoparticles renders this GMP-scalable liposomal platform a feasible alternative to VLP-based vaccines. In this study, we designed and analyzed customizable Env-conjugated T helper liposomes. First, we passively encapsulated T helper peptides into a well-characterized liposome formulation displaying a dense array of Env trimers on the surface. We confirmed the closed pre-fusion state of the coupled Env trimers by immunogold staining with conformation-specific antibodies. These peptide-loaded Env-liposome conjugates efficiently activated Env-specific B cells, which further induced proliferation of CD4+ T cells by presentation of liposome-derived peptides on MHC-II molecules. The peptide encapsulation process was then quantitatively improved by an electrostatically driven approach using an overall anionic lipid formulation. We demonstrated that peptides delivered by liposomes were presented by DCs in secondary lymphoid organs after intramuscular immunization of mice. UFO (uncleaved prefusion optimized) Env trimers were covalently coupled to peptide-loaded anionic liposomes by His-tag/NTA(Ni) interactions and EDC/Sulfo-NHS crosslinking. EM imaging revealed a moderately dense array of well-folded Env trimers on the liposomal surface. The conformation was verified by liposomal surface FACS. Furthermore, anionic Env-coupled T helper liposomes effectively induced Env-specific B cell activation and proliferation in a comparable range to T helper VLPs. Taken together, we demonstrated that T helper VLPs can be substituted with customizable and GMP-scalable liposomal nanoparticles as a perspective for future preclinical and clinical HIV vaccine applications. The functional nanoparticle characterization assays shown in this study can be applied to other systems of synthetic nanoparticles delivering antigens derived from various pathogens.

Noncanonical NF-κB activation by the oncoprotein Tio occurs through a nonconserved TRAF3-binding motif

Members of the nuclear factor κB (NF-κB) family of transcription factors regulate many cellular functions. Activation of NF-κB signaling is commonly classified as occurring through canonical or noncanonical pathways. Most NF-κB-inducing stimuli, including the viral oncoprotein Tio, lead to a concerted activation of both NF-κB pathways; however, extensive crosstalk at multiple levels between these signaling cascades restricts the ability to discriminate between the canonical and the noncanonical effects. We showed that noncanonical NF-κB activation by Tio depends on a distinct sequence motif that directly recruits tumor necrosis factor receptor-associated factor 3 (TRAF3). Through its TRAF3-binding motif, Tio triggered a ubiquitin-independent depletion of TRAF3 from the cytosol, which prevented TRAF3 from inhibiting signaling through the noncanonical NF-κB cascade. Furthermore, the Tio-TRAF3 interaction did not affect components of the canonical NF-κB signaling pathway or the expression of target genes; thus, Tio induced noncanonical NF-κB independently of crosstalk with the canonical pathway. Together, these data identify a distinct molecular mechanism of noncanonical NF-κB activation that should enable studies into the particular functions of this pathway.

Actin-dependent activation of serum response factor in T cells by the viral oncoprotein tip

Serum response factor (SRF) acts as a multifunctional transcription factor regulated by mutually exclusive interactions with ternary complex factors (TCFs) or myocardin-related transcription factors (MRTFs). Binding of Rho- and actin-regulated MRTF:SRF complexes to target gene promoters requires an SRF-binding site only, whereas MAPK-regulated TCF:SRF complexes in addition rely on flanking sequences present in the serum response element (SRE). Here, we report on the activation of an SRE luciferase reporter by Tip, the viral oncoprotein essentially contributing to human T-cell transformation by Herpesvirus saimiri. SRE activation in Tip-expressing Jurkat T cells could not be attributed to triggering of the MAPK pathway. Therefore, we further analyzed the contribution of MRTF complexes. Indeed, Tip also activated a reporter construct responsive to MRTF:SRF. Activation of this reporter was abrogated by overexpression of a dominant negative mutant of the MRTF-family member MAL. Moreover, enrichment of monomeric actin suppressed the Tip-induced reporter activity. Further upstream, the Rho-family GTPase Rac, was found to be required for MRTF:SRF reporter activation by Tip. Initiation of this pathway was strictly dependent on Tip's ability to interact with Lck and on the activity of this Src-family kinase. Independent of Tip, T-cell stimulation orchestrates Src-family kinase, MAPK and actin pathways to induce SRF. These findings establish actin-regulated transcription in human T cells and suggest its role in viral oncogenesis.

Activation of noncanonical NF-kappaB signaling by the oncoprotein Tio

NF-kappaB transcription factors are key regulators of cellular proliferation and frequently contribute to oncogenesis. The herpesviral oncoprotein Tio, which promotes growth transformation of human T cells in a recombinant herpesvirus saimiri background, potently induces canonical NF-kappaB signaling through membrane recruitment of the ubiquitin ligase tumor necrosis factor receptor-associated factor 6 (TRAF6). Here, we show that, in addition to Tio-TRAF6 interaction, the Tio-induced canonical NF-kappaB signal requires the presence of the regulatory subunit of the inhibitor of kappaB kinase (IKK) complex, NF-kappaB essential modulator (NEMO), and the activity of its key kinase, IKKbeta, to up-regulate expression of endogenous cellular inhibitor of apoptosis 2 (cIAP2) and interleukin 8 (IL-8) proteins. Dependent on TRAF6 and NEMO, Tio enhances the expression of the noncanonical NF-kappaB proteins, p100 and RelB. Independent of TRAF6 and NEMO, Tio mediates stabilization of the noncanonical kinase, NF-kappaB-inducing kinase (NIK). Concomitantly, Tio induces efficient processing of the p100 precursor molecule to its active form, p52, as well as DNA binding of nuclear p52 and RelB. In human T cells transformed by infection with a Tio-recombinant virus, sustained expression of p100, RelB, and cIAP2 depends on IKKbeta activity, yet processing to p52 remains largely unaffected by IKKbeta inhibition. However, long term inhibition of IKKbeta disrupts the continuous growth of the transformed cells and induces cell death. Hence, the Tio oncoprotein triggers noncanonical NF-kappaB signaling through NEMO-dependent up-regulation of p100 precursor and RelB, as well as through NEMO-independent generation of p52 effector.

NFkappaB signaling is induced by the oncoprotein Tio through direct interaction with TRAF6

The transcription factor NFkappaB is a major regulator of genes involved in inflammation and oncogenesis. NFkappaB is induced upon stimulation of cellular receptors coupled to different intracellular signaling molecules. Further downstream, TRAF6 links at least two receptor pathways to take control of IkappaB, the administrator of NFkappaB activity. Here we report on a strong NFkappaB activation by Tio, a unique herpesviral oncoprotein promoting transformation of human T cells in a Src-kinase-dependent manner. NFkappaB induction by Tio is independent of Src-kinase interaction and tyrosine phosphorylation of Tio. Mutation of a glutamic acid-rich motif at the N terminus of Tio, corresponding to a TRAF6 consensus binding motif, completely abrogated NFkappaB activation. Cotransfection of a dominant negative TRAF6 construct led to a decrease in NFkappaB activation. Furthermore, we provide evidence that TRAF6 directly binds to the Tio oncoprotein. Identification of TRAF6 as the direct target of Tio describes a novel mechanism for the constitutive activation of NFkappaB through an oncoprotein.

Tyrosine phosphorylation of the Tio oncoprotein is essential for transformation of primary human T cells

Human T cells are transformed to antigen-independent permanent growth in vitro upon infection with herpesvirus saimiri subgroup C strains. The viral oncoproteins required for this process, StpC and Tip, could be replaced by Tio, the oncoprotein of herpesvirus ateles. Here we demonstrate that proliferation of lymphocytes transformed with Tio-recombinant herpesvirus saimiri required the activity of Src family kinases. Src kinases had previously been identified as interaction partners of Tio. This interaction was now shown to be independent of any of the four tyrosine residues of Tio but to be dependent on an SH3-binding motif. Mutations within this motif abrogated the transforming capabilities of Tio-recombinant herpesvirus saimiri. Furthermore, kinase interaction resulted in the phosphorylation of Tio on a single tyrosine residue at position 136. Mutation of this residue in the viral context revealed that this phosphorylation site, but none of the other tyrosine residues, was required for T-cell transformation. These data indicate that the interaction of Tio with a Src kinase is essential for both the initiation and the maintenance of T-cell transformation by recombinant herpesvirus saimiri. The requirement for the tyrosine phosphorylation site at position 136 suggests a role for Tio beyond simple deregulation of the kinase.

Herpesvirus ateles Tio can replace herpesvirus saimiri StpC and Tip oncoproteins in growth transformation of monkey and human T cells

Herpesvirus saimiri group C strains are capable of transforming human and simian T-lymphocyte populations to permanent antigen-independent growth. Two viral oncoproteins, StpC and Tip, that are encoded by a single bicistronic mRNA, act in concert to mediate this phenotype. A closely related New World monkey herpesvirus, herpesvirus ateles, transcribes a single spliced mRNA at an equivalent genome locus. The encoded protein, Tio, has sequence homologies to both StpC and Tip. We inserted the tio sequence of herpesvirus ateles strain 73 into a recombinant herpesvirus saimiri C488 lacking its own stpC/tip oncogene. Simian as well as human T lymphocytes were growth transformed by the chimeric Tio-expressing viruses. Thus, a single herpesvirus protein appears to be responsible for the oncogenic effects of herpesvirus ateles.

Primary structure of the Herpesvirus ateles genome

Herpesvirus ateles is an agent indigenous to spider monkeys (Ateles spp.) and causes fulminant lymphomas in various New World primates. Structural and genetic relatedness led to the classification of this virus as a member of the genus Rhadinovirus. It is most closely related to Herpesvirus saimiri. The 108,409-bp light DNA segment of the herpesvirus ateles strain 73 genome has two genes for U-RNA-like transcripts and 73 open reading frames, of which at least 6 show significant homologies to cellular genes (encoding complement control proteins, apoptosis-regulatory proteins, D-type cyclins, interleukin-8 receptors, and enzymes involved in nucleotide metabolism). The left terminal region of the light DNA segment bears the putative rhadinovirus oncogene tio.

Human herpesvirus 8: is it a tumor virus?

Human herpesvirus 8 (HHV-8), also termed Kaposi's sarcoma-associated herpesvirus, was identified in Kaposi's sarcoma (KS) biopsy specimens in 1994. The epidemiological data available to date indicate a strong association of HHV-8 with KS. It appears that HHV-8 is necessary for KS development. HHV-8 DNA is invariably found in all epidemiological forms of KS and primary effusion lymphomas. In contrast, HHV-8 DNA is rarely found in various tumor and nontumor tissues from patient groups not at risk of KS. Although current serology does not allow us to assess the HHV-8 prevalence in the general population, high titers of HHV-8 antibodies are almost exclusively found in KS risk groups. In addition, HHV-8 seroconversion has been shown to precede KS development. The mechanisms and genes involved in HHV-8 pathogenesis are less clear. HHV-8 belongs to a family of transforming viruses, and several candidate oncogenes have been identified by using rodent fibroblast transformation assays. However, expression of most of these genes could not be shown in latently infected tumor cells. As the HHV-8 genome encodes several cytokines and cytokine receptor homologues, HHV-8 may also promote KS pathogenesis through paraendocrine mechanisms.

Herpesvirus ateles gene product Tio interacts with nonreceptor protein tyrosine kinases

Herpesvirus ateles is a gamma-2-herpesvirus which naturally infects spider monkeys (Ateles spp.) and causes malignant lymphoproliferative disorders in various other New World primates. The genomic sequence of herpesvirus ateles strain 73 revealed a close relationship to herpesvirus saimiri, with a high degree of variability within the left terminus of the coding region. A spliced mRNA transcribed from this region was detected in New World monkey T-cell lines transformed by herpesvirus ateles in vitro or derived from T cells of infected Saguinus oedipus. The encoded viral protein, termed Tio, shows restricted homology to the oncoprotein StpC and to the tyrosine kinase-interacting protein Tip, two gene products responsible for the T-cell-transforming and oncogenic phenotype of herpesvirus saimiri group C strains. Tio was detectable in lysates of the transformed T lymphocytes. Dimer formation was observed after expression of recombinant Tio. After cotransfection, Tio was phosphorylated in vivo by the protein tyrosine kinases Lck and Src and less efficiently by Fyn. Stable complexes of these Src family kinases with the viral protein were detected in lysates of the transfected cells. Binding analyses indicated a direct interaction of Tio with the SH3 domains of Lyn, Hck, Lck, Src, Fyn, and Yes. In addition, tyrosine-phosphorylated Tio bound to the SH2 domains of Lck, Src, or Fyn. Thus, herpesvirus ateles-encoded Tio may contribute to viral T-cell transformation by influencing the function of Src family kinases.

Human herpesvirus 8--the first human Rhadinovirus

Kaposi's sarcoma (KS)-associated herpesvirus, also known as human herpesvirus 8 (HHV-8), is the first known human member of the genus Rhadinovirus. It is regularly found by polymerase chain reaction in all forms of KS, in certain types of Castleman's disease, and in body cavity-based B-cell lymphoma. Other members of this virus group occur in nonhuman primates, ungulates, rabbits, and mice and cause in part fulminant lymphomas and other neoplastic disorders of the hematopoietic system. Rhadinoviruses share a typical genome structure; most characteristically, they contain numerous sequences that appear to be sequestered from cellular DNA. We cloned and sequenced almost the complete genome of HHV-8 from a single KS biopsy specimen. Although this procedure revealed collinear organization and extensive homologies with the open reading frames of herpesvirus saimiri, genes with homology to the known oncoproteins (Stp, Tip) were not identified in the HHV-8 genome. However, HHV-8 reading frame K1, the positional analogue of Stp/Tip, was found to be significantly variable between different strains. We found, in addition, the reading frames for homologues of cellular interleukin 6, macrophage inflammatory proteins alpha and beta (MIP1 alpha and MIP1 beta, respectively), an interferon-responsive factor, and two inhibitors of apoptosis. Several of these cell-homologous genes of HHV-8 have already been shown to code for functional proteins.

The immunogenic glycoprotein gp35-37 of human herpesvirus 8 is encoded by open reading frame K8.1

Human herpesvirus 8 (HHV-8) is likely to be involved in the pathogenesis of Kaposi's sarcoma (KS) and body cavity-based lymphomas (BCBLs). The HHV-8 genome is primarily in a latent state in BCBL-derived cell lines like BCBL-1, but lytic replication can be induced by phorbol esters (R. Renne, W. Zhang, B. Herndier, M. McGrath, N. Abbey, D. Kedes, and D. E. Ganem, Nat. Med. 2:342-346, 1996). A 35- to 37-kDa glycoprotein (gp35-37) is the polypeptide most frequently and intensively recognized by KS patient sera on Western blots with induced BCBL-1 cells. Its apparent molecular mass is reduced to 30 kDa by digestion with peptide-N-glycosidase F. By searching the known HHV-8 genomic sequence for open reading frames (ORF) with the potential to encode such a glycoprotein, an additional, HHV-8-specific reading frame was identified adjacently to ORF K8. This ORF, termed K8.1, was found to be transcribed primarily into a spliced mRNA encoding a glycoprotein of 228 amino acids. Recombinant K8.1 was regularly recognized by KS patient sera in Western blots, and immunoaffinity-purified antibodies to recombinant K8.1 reacted with gp35-37. This shows that the immunogenic gp35-37 is encoded by HHV-8 reading frame K8.1, which will be a useful tool for studies of HHV-8 epidemiology and pathogenesis.

Kaposi's sarcoma-associated herpesvirus encodes a functional cyclin

Kaposi's sarcoma-associated herpesvirus (KSHV) (also called human herpesvirus 8) is consistently found in Kaposi's sarcoma lesions and in body-cavity-based lymphomas. A 17-kb KSHV lambda clone was obtained directly from a Kaposi's sarcoma lesion. DNA sequence analysis of this clone identified an open reading frame which has 32% amino acid identity and 53% similarity to the virus-encoded cyclin (v-cyclin) of herpesvirus saimiri (HVS) and 31% identity and 53% similarity to human cellular cyclin D2. This KSHV open reading frame was shown to encode a 29- to 30-kDa protein with the properties of a v-cyclin. KSHV v-cyclin protein was found to associate predominantly with cdk6, a cellular cyclin-dependent kinase known to interact with cellular type D cyclins and HVS v-cyclin. The KSHV v-cyclin was also found to associate weakly with cdk4. KSHV v-cyclin-cdk6 complexes strongly phosphorylated glutathione S-transferase-Rb fusion protein and histone H1 as substrates in vitro. Thus, KSHV v-cyclin resembles the v-cyclin of the T-lymphocyte-transforming HVS in its specificity for association with cdk6 and in its ability to strongly activate cdk6 protein kinase activity.

Human herpesvirus 8 encodes a homolog of interleukin-6

Kaposi's sarcoma is a multifocal lesion that is reported to be greatly influenced by cytokines such as interleukin-6 (IL-6) and oncostatin M. DNA sequences of a novel human gammaherpesvirus, termed human herpesvirus 8 (HHV-8) or Kaposi sarcoma-associated herpesvirus, have been identified in all epidemiological forms of Kaposi's sarcoma with high frequency. The presence of HHV-8 DNA is also clearly associated with certain B-cell lymphomas (body cavity-based lymphomas) and multicentric Castleman's disease. Sequence analysis of a 17-kb fragment revealed that adjacent to a block of conserved herpesvirus genes (major DNA-binding protein, glycoprotein B, and DNA polymerase), the genome of HHV-8 encodes structural homolog of IL-6. This cytokine is involved not only in the pathogenesis of Kaposi's sarcoma but also in certain B-cell lymphomas and multicentric Castleman's disease. The viral counterpart of IL-6 (vIL-6) has conserved important features such as cysteine residues involved in disulfide bridging or an amino-terminal signal peptide. Most notably, the region known to be involved in receptor binding is highly conserved in vIL-6. This conservation of essential features and the remarkable overlap between diseases associated with HHV-8 and diseases associated with IL-6 disregulation clearly suggest that vIL-6 is involved in HHV-8 pathogenesis.

The complement control protein homolog of herpesvirus saimiri regulates serum complement by inhibiting C3 convertase activity

The herpesvirus saimiri genome encodes a complement control protein homolog (CCPH). Stable mammalian cell transfectants expressing a recombinant transmembrane form of CCPH (mCCPH) or a 5'FLAG epitope-tagged mCCPH (5'FLAGmCCPH) conferred resistance to complement-mediated cell damage by inhibiting the lytic activity of human serum complement. The function of CCPH was further defined by showing that the mCCPH and the 5'FLAGmCCPH transfectants inhibited C3 convertase activity and effectively reduced cell surface deposition of the activated complement component, C3d.

Inhibition of complement-mediated cytolysis by the terminal complement inhibitor of herpesvirus saimiri

Herpesvirus saimiri (HVS) is a lymphotropic herpesvirus that induces T-cell transformation in vitro and causes lymphomas and leukemias in New World primates other than its natural host, the squirrel monkey. Nucleotide sequence analysis of the HVS genome revealed two open reading frames with significant homology to genes for human complement regulatory molecules. One of these genes encodes a predicted protein (designated HVSCD59) with 48% amino acid sequence identity to the human terminal complement regulatory protein CD59 (HuCD59). The CD59 homolog from squirrel monkey (SMCD59) was cloned, and the corresponding amino acid sequence showed 69% identity with HVSCD59. BALB/3T3 cells stably expressing HVSCD59, SMCD59, or HuCD59 were equally protected from complement-mediated lysis by human serum. However, only HVSCD59-expressing cells were effectively protected from complement-mediated lysis when challenged with rat serum, suggesting that HVSCD59 was less species restrictive. The complement regulatory activity of HVSCD59 and SMCD59 occurred after C3b deposition, indicating terminal complement inhibition. Treatment of BALB/3T3 stable transfectants with phosphatidylinositol-specific phospholipase C prior to complement attack decreased the complement regulatory function of HVSCD59, suggesting cell surface attachment via a glycosyl-phosphatidylinositol anchor. Cells expressing HVSCD59 effectively inhibited complement-mediated lysis by squirrel monkey serum in comparison with SMCD59-expressing cells. Finally HVSCD59-specific transcripts were detected in owl monkey cells permissive for lytic HVS replication but not in T cells transformed by HVS, which failed to produce virions. These data are the first to demonstrate a functional, virally encoded terminal complement inhibitor and suggest that HVSCD59 represents a humoral immune evasion mechanism supporting the lytic life cycle of HVS.

Genetic relationships between bovine herpesvirus 4 and the gammaherpesviruses Epstein-Barr virus and herpesvirus saimiri

The overall arrangement of genes in the unique central part of the bovine herpesvirus type 4 (BHV-4) genome has been deduced by analysis of short DNA sequences. Twenty-three genes conserved in at least one of the completely sequenced herpesviruses have been identified and localized. All of these genes encoded amino acid sequences with higher similarity to proteins of the gammaherpesviruses Epstein-Barr virus (EBV) and herpesvirus saimiri (HVS) than to the homologous products of the alphaherpesviruses varicella-zoster virus and herpes simplex virus type 1 or the betaherpesvirus human cytomegalovirus. The genome organization of BHV-4 had also an overall colinearity with that of the gammaherpesviruses EBV and HVS. Furthermore, the BHV-4 genes content and arrangement were more similar to those of HVS than to those of EBV, suggesting that BHV-4 and HVS are evolutionarily more closely related to each other than either are to EBV. BHV-4 DNA sequences were generally deficient in CpG dinucleotide. This CpG deficiency is characteristic of gammaherpesvirus genomes and suggests that the BHV-4 latent genome is extensively methylated. Despite several biological features similar to those of betaherpesviruses, BHV-4 displays the molecular characteristics of the representative members of the gammaherpesvirinae subfamily.

Herpesvirus saimiri has a gene specifying a homologue of the cellular membrane glycoprotein CD59

Herpesvirus saimiri (HSV) is a T-lymphotropic tumor virus that causes fulminant lymphomas and leukemias in various New World primates other than its natural host, the squirrel monkey (Saimiri sciureus). In the course of completing the nucleotide sequence of its genome, we identified an open reading frame of 363 nucleotides, designated HVS-15, that has no detectable homology to any other viral sequences to date. HVS-15 encodes a 121-amino-acid protein which shows significant similarities to human CD59, a phosphatidyl-inositol-glycan-anchored glycoprotein involved in T-cell activation and restriction of complement-mediated lysis. The predicted HVS-15 gene product is more similar to human CD59 than to the related murine Ly-6 antigens. A nucleotide sequence identity of 64% was found between HVS-15 and the CD59 reading frame, and a 48% identity exists between the corresponding protein sequences. The comparison of the amino acid sequences revealed a number of conserved structural features such as a similar pattern of hydrophobic termini and an identical cysteine skeleton.

Primary structure of the herpesvirus saimiri genome

This report describes the complete nucleotide sequence of the genome of herpesvirus saimiri, the prototype of gammaherpesvirus subgroup 2 (rhadinoviruses). The unique low-G + C-content DNA region has 112,930 bp with an average base composition of 34.5% G + C and is flanked by about 35 noncoding high-G + C-content DNA repeats of 1,444 bp (70.8% G + C) in tandem orientation. We identified 76 major open reading frames and a set of seven U-RNA genes for a total of 83 potential genes. The genes are closely arranged, with only a few regions of sizable noncoding sequences. For 60 of the predicted proteins, homologous sequences are found in other herpesviruses. Genes conserved between herpesvirus saimiri and Epstein-Barr virus (gammaherpesvirus subgroup 1) show that their genomes are generally collinear, although conserved gene blocks are separated by unique genes that appear to determine the particular phenotype of these viruses. Several deduced protein sequences of herpesvirus saimiri without counterparts in most of the other sequenced herpesviruses exhibited significant homology with cellular proteins of known function. These include thymidylate synthase, dihydrofolate reductase, complement control proteins, the cell surface antigen CD59, cyclins, and G protein-coupled receptors. Searching for functional protein motifs revealed that the virus may encode a cytosine-specific methylase and a tyrosine-specific protein kinase. Several herpesvirus saimiri genes are potential candidates to cooperate with the gene for saimiri transformation-associated protein of subgroup A (STP-A) in T-lymphocyte growth stimulation.

New member of the multigene family of complement control proteins in herpesvirus saimiri

A number of glycoproteins are regulators of the complement cascade and prevent damage to cells by inappropriate activation of complement. In humans, all of them are encoded by a multigene family on chromosome I and share a characteristic structural feature, the short consensus repeats of about 61 amino acids with a constant framework of cysteine, proline, and tryptophan. We found the gene for glycoproteins of analogous structure in herpesvirus saimiri, a T-lymphotropic tumor virus of New World primates. Unspliced transcripts code for a membrane-bound 65- to 75-kDa virion surface component, while spliced mRNA instructs a secreted glycoprotein of 47 to 53 kDa. Expression of complement control proteins suggests a novel mechanism of counteracting host immune defense to prevent elimination of a virus that is capable of persisting in circulating lymphocytes.

Nucleotide sequence of HSUR 6 and HSUR 7, two small RNAs of herpesvirus saimiri

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Structural organization of the conserved gene block of Herpesvirus saimiri coding for DNA polymerase, glycoprotein B, and major DNA binding protein

Lymphotropic herpesviruses such as Epstein-Barr virus and Herpesvirus saimiri are commonly grouped as gamma-herpesviruses, although overall genome organization and numerous biological properties are quite different in the viruses. To define the relationship more precisely, we sequenced the Kpnl fragments F (6.5 kb) and C (9.8 kb) of the H.saimiri strain No. 11 genome; these DNA fragments were found to contain the genes coding for equivalents of the major DNA binding protein, a putative glycoprotein transport polypeptide, the glycoprotein B, and the DNA polymerase of herpes simplex virus. This DNA segment represents the longest block of contiguous genes with pronounced sequence homologies between herpesviruses of known DNA primary structure. Comparisons confirmed that the two gamma-herpesviruses are related; the group is, however, even more diverse than the alpha-herpesviruses represented by their prototypes, herpes simplex virus and varicella-zoster virus. H. saimiri DNA is strongly depleted in the dinucleotide CpG, possibly the consequence of de novo methylation of persisting viral DNA in lymphoid cells.