The apoptotic v-cyclin-CDK6 complex phosphorylates and inactivates Bcl-2.

v-cyclin encoded by Kaposi's sarcoma herpesvirus/human herpesvirus 8 (KSHV or HHV8) associates with cellular cyclin-dependent kinase 6 (CDK6) to form a kinase complex that promotes cell-cycle progression, but can also induce apoptosis in cells with high levels of CDK6. Here we show that whereas HHV8-encoded v-Bcl-2 protects against this apoptosis, cellular Bcl-2 has lost its anti-apoptotic potential as a result of an inactivating phosphorylation in its unstructured loop region. Moreover, we identify Bcl-2 as a new substrate for v-cyclin-CDK6 in vitro, and show that it is present in a complex with CDK6 in cell lysates. A Bcl-2 mutant with a S70A S87A double substitution in the loop region is not phosphorylated and provides resistance to apoptosis, indicating that inactivation of Bcl-2 by v-cyclin-CDK6 may be required for the observed apoptosis. Furthermore, the identification of phosphorylated Bcl-2 in HHV8-positive Kaposi's sarcoma indicates that HHV8-mediated interference with host apoptotic signalling pathways may encourage the development of Kaposi's sarcoma.

A novel virus-host cell membrane interaction. Membrane voltage-dependent endocytic-like entry of bacteriophage straight phi6 nucleocapsid.

Studies on the virus-cell interactions have proven valuable in elucidating vital cellular processes. Interestingly, certain virus-host membrane interactions found in eukaryotic systems seem also to operate in prokaryotes (Bamford, D.H., M. Romantschuk, and P. J. Somerharju, 1987. EMBO (Eur. Mol. Biol. Organ.) J. 6:1467-1473; Romantschuk, M., V.M. Olkkonen, and D.H. Bamford. 1988. EMBO (Eur. Mol. Biol. Organ.) J. 7:1821-1829). straight phi6 is an enveloped double-stranded RNA virus infecting a gram-negative bacterium. The viral entry is initiated by fusion between the virus membrane and host outer membrane, followed by delivery of the viral nucleocapsid (RNA polymerase complex covered with a protein shell) into the host cytosol via an endocytic-like route. In this study, we analyze the interaction of the nucleocapsid with the host plasma membrane and demonstrate a novel approach for dissecting the early events of the nucleocapsid entry process. The initial binding of the nucleocapsid to the plasma membrane is independent of membrane voltage (DeltaPsi) and the K(+) and H(+) gradients. However, the following internalization is dependent on plasma membrane voltage (DeltaPsi), but does not require a high ATP level or K(+) and H(+) gradients. Moreover, the nucleocapsid shell protein, P8, is the viral component mediating the membrane-nucleocapsid interaction.

Herpes simplex virus type 1 entry into host cells: reconstitution of capsid binding and uncoating at the nuclear pore complex in vitro.

During entry, herpes simplex virus type 1 (HSV-1) releases its capsid and the tegument proteins into the cytosol of a host cell by fusing with the plasma membrane. The capsid is then transported to the nucleus, where it docks at the nuclear pore complexes (NPCs), and the viral genome is rapidly released into the nucleoplasm. In this study, capsid association with NPCs and uncoating of the viral DNA were reconstituted in vitro. Isolated capsids prepared from virus were incubated with cytosol and purified nuclei. They were found to bind to the nuclear pores. Binding could be inhibited by pretreating the nuclei with wheat germ agglutinin, anti-NPC antibodies, or antibodies against importin beta. Furthermore, in the absence of cytosol, purified importin beta was both sufficient and necessary to support efficient capsid binding to nuclei. Up to 60 to 70% of capsids interacting with rat liver nuclei in vitro released their DNA if cytosol and metabolic energy were supplied. Interaction of the capsid with the nuclear pore thus seemed to trigger the release of the viral genome, implying that components of the NPC play an active role in the nuclear events during HSV-1 entry into host cells.

CAK-independent activation of CDK6 by a viral cyclin.

In normal cells, activation of cyclin-dependent kinases (cdks) requires binding to a cyclin and phosphorylation by the cdk-activating kinase (CAK). The Kaposi's sarcoma-associated herpesvirus encodes a protein with similarity to D-type cyclins. This KSHV-cyclin activates CDK6, alters its substrate specificity, and renders CDK6 insensitive to inhibition by the cdk inhibitor p16(INK4a). Here we investigate the regulation of the CDK6/KSHV-cyclin kinase with the use of purified proteins and a cell-based assay. We find that KSHV-cyclin can activate CDK6 independent of phosphorylation by CAK in vitro. In addition, CAK phosphorylation decreased the p16(INK4a) sensitivity of CDK6/KSHV-cyclin complexes. In cells, expression of CDK6 or to a lesser degree of a nonphosphorylatable CDK6(T177A) together with KSHV-cyclin induced apoptosis, indicating that CDK6 activation by KSHV-cyclin can proceed in the absence of phosphorylation by CAK in vivo. Coexpression of p16 partially protected cells from cell death. p16 and KSHV-cyclin can form a ternary complex with CDK6 that can be detected by binding assays as well as by conformational changes in CDK6. The Kaposi's sarcoma-associated herpesvirus has adopted a clever strategy to render cell cycle progression independent of mitogenic signals, cdk inhibition, or phosphorylation by CAK.

Kaposi's sarcoma-associated herpesvirus-encoded v-cyclin triggers apoptosis in cells with high levels of cyclin-dependent kinase 6.

Kaposi's sarcoma-associated herpesvirus (KSHV) has a key etiological role in development of Kaposi's sarcoma (KS). v-Cyclin is a KSHV-encoded homologue to D-type cyclins that associates with cellular cyclin-dependent kinase 6 (CDK6). v-Cyclin promotes S-phase entry of quiescent cells and has been suggested to execute functions of both D- and E-type cyclins. In this study, expression of v-cyclin in cells with elevated levels of CDK6 led to apoptotic cell death after the cells entered S phase. The cell death required the kinase activity of CDK6 because cells expressing a kinase-deficient form of CDK6 did not undergo apoptosis upon v-cyclin expression. Studies on the mechanisms involved in this caspase-3-mediated apoptosis indicated that it was independent of cellular p53 or pRb status, and it was not suppressed by Bcl-2. In contrast, the KSHV-encoded v-Bcl-2 efficiently suppressed v-cyclin-/CDK6-induced apoptosis, demonstrating a marked difference in the antiapoptotic properties of c-Bcl-2 and v-Bcl-2. In KS lesions, high CDK6 expression was confined to a subset of cells, some of which displayed signs of apoptosis. These results suggest that v-cyclin may exert both growth-promoting and apoptotic functions in KS, depending on factors regulating CDK6 and v-Bcl-2 levels.

Generation of infectious nucleocapsids by in vitro assembly of the shell protein on to the polymerase complex of the dsRNA bacteriophage phi 6.

A method for the in vitro uncoating of the phi 6 nucleocapsid (NC) was developed. The resulting particle, designated as the NC core, containing the genomic double-stranded (ds) RNA segments and the proteins P1, P2, P4 and P7, was not infectious but had a highly enhanced in vitro transcriptase activity compared to that of the intact NC. The NC shell protein P8 was purified by immunoaffinity chromatography, and it was shown to self-assemble to shell-like structures upon addition of calcium ions. The conditions for the self-assembly of the shell were optimized. Shell reassembly on to the NC cores restored the infectivity but resulted in a decrease of transcriptase activity. No reassembly of the shell on to RNA-less cores (procapsids) produced from a cDNA construction in Escherichia coli was observed. Our results suggest that the intracellular uncoating of the NC is the event activating the phi 6 dsRNA transcriptase and that the NC shell is necessary for infectivity, probably for the passage of the NC through the host cytoplasmic membrane. Packaging of the dsRNA segments into the procapsid appears to be a prerequisite for NC shell assembly.

Kaposi's sarcoma herpesvirus lytic replication compromises apoptotic response to p53 reactivation in virus-induced lymphomas.

Primary effusion lymphomas (PELs) are aggressive Kaposi's sarcoma herpesvirus (KSHV)-induced malignancies with median survival time <6 months post-diagnosis. Mutations in the TP53 gene seldom occur in PELs, suggesting that genetic alterations in the TP53 are not selected during PEL progression. We have reported that p53 reactivation by an inhibitor of the p53-MDM2 interaction, Nutlin-3, induces selective and massive apoptosis in PEL cells leading to efficient anti-tumor activity in a subcutaneous xenograft model for PEL. Here, we show compelling anti-tumor activity of Nutlin-3 in the majority of intraperitoneal PEL xenografts in vivo. Interestingly, our results demonstrate that spontaneous induction of viral lytic replication in tumors could drastically attenuate the p53-dependent apoptotic response to Nutlin-3. Moreover, viral reactivation compromised p53-dependent apoptosis in PEL cells treated with genotoxic anti-cancer agents doxorubicin and etoposide. We have recently demonstrated that the Ser/Thr kinases Pim 1 and 3 are required to trigger induction of the lytic replication cascade of KSHV. We have now assessed the ability of a novel Pim kinase inhibitor to restore the Nutlin-3-induced cytotoxicity in lytic PEL cells. PEL cells induced to lytic replication by phorbol esters showed 50% inhibition of active viral replication following treatment with the Pim kinase inhibitor. Importantly, co-treatment of these cells with the kinase inhibitor and Nutlin-3 resulted in a robust restoration of the Nutlin-3-induced cell death. These results highlight the potential impact of activation of viral lytic replication on disease progression and response to treatment in KSHV-induced lymphomas.

Recruitment of the tumour suppressor protein p73 by Kaposi's Sarcoma Herpesvirus latent nuclear antigen contributes to the survival of primary effusion lymphoma cells.

Kaposi's Sarcoma Herpesvirus (KSHV) is the causative agent of Kaposi's Sarcoma (KS) and two rare lymphoproliferative disorders, primary effusion lymphoma (PEL) and the plasmablastic variant of multicentric Castleman's disease (MCD). The KSHV latency-associated nuclear antigen-1 (LANA), required for the replication and maintenance of latent viral episomal DNA, is involved in the transcriptional regulation of viral and cellular genes and interacts with different cellular proteins, including the tumour suppressor p53. Here, we report that LANA also recruits the p53-related nuclear transcription factor p73, which influences cellular processes like DNA damage response, cell cycle progression and apoptosis. Both the full-length isoform TAp73α, as well as its dominant negative regulator ΔNp73α, interact with LANA. LANA affects TAp73α stability and sub-nuclear localisation, as well as TAp73α-mediated transcriptional activation of target genes. We observed that the small-molecule inhibitor Nutlin-3, which disrupts the interaction of p53 and p73 with MDM2, induces apoptotic cell death in p53 wild-type, as well as p53-mutant PEL cell lines, suggesting a possible involvement of p73. The small-molecule RETRA, which activates p73 in the context of mutant p53, leads to the induction of apoptosis in p53-mutant PEL cell lines. RNAi-mediated knockdown of p73 confirmed that these effects depend on the presence of the p73 protein. Furthermore, both Nutlin-3 and RETRA disrupt the LANA-p73 interaction in different PEL cell lines. These results suggest that LANA modulates p73 function and that the LANA-p73 interaction may represent a therapeutic target to interfere with the survival of latently KSHV-infected cells.

Anticancer compound ABT-263 accelerates apoptosis in virus-infected cells and imbalances cytokine production and lowers survival rates of infected mice.

ABT-263 and its structural analogues ABT-199 and ABT-737 inhibit B-cell lymphoma 2 (Bcl-2), BCL2L1 long isoform (Bcl-xL) and BCL2L2 (Bcl-w) proteins and promote cancer cell death. Here, we show that at non-cytotoxic concentrations, these small molecules accelerate the deaths of non-cancerous cells infected with influenza A virus (IAV) or other viruses. In particular, we demonstrate that ABT-263 altered Bcl-xL interactions with Bcl-2 antagonist of cell death (Bad), Bcl-2-associated X protein (Bax), uveal autoantigen with coiled-coil domains and ankyrin repeats protein (UACA). ABT-263 thereby activated the caspase-9-mediated mitochondria-initiated apoptosis pathway, which, together with the IAV-initiated caspase-8-mediated apoptosis pathway, triggered the deaths of IAV-infected cells. Our results also indicate that Bcl-xL, Bcl-2 and Bcl-w interact with pattern recognition receptors (PRRs) that sense virus constituents to regulate cellular apoptosis. Importantly, premature killing of IAV-infected cells by ABT-263 attenuated the production of key pro-inflammatory and antiviral cytokines. The imbalance in cytokine production was also observed in ABT-263-treated IAV-infected mice, which resulted in an inability of the immune system to clear the virus and eventually lowered the survival rates of infected animals. Thus, the results suggest that the chemical inhibition of Bcl-xL, Bcl-2 and Bcl-w could potentially be hazardous for cancer patients with viral infections.

In vitro transcription of the double-stranded RNA bacteriophage phi 6 is influenced by purine NTPs and calcium.

The double-stranded RNA bacteriophage phi 6 contains a virion-associated RNA-dependent RNA polymerase complex. Removal of the virus envelope and the nucleocapsid surface protein, P8, reveals a nucleocapsid core particle (proteins P1, P2, P4, P7) which is the viral polymerase complex, capable of synthesizing RNA strands of positive polarity. The in vitro plus strand synthesis (transcription) reaction of the particle obtained from the mature virion was optimized and its activation and inactivation were investigated. Purine nucleoside triphosphates (NTPs), binding to a low-affinity binding site in the polymerase complex, activated plus strand synthesis. GTP was the preferred NTP, but dGTP, ddGTP, and the noncleavable analog GMP-PCP could also switch on transcription. This NTP-binding site is probably different from that of the unspecific viral NTPase found in protein P4 and also from that of the rNTP-specific RNA polymerase active site. Binding of purine NTPs was sufficient for the switch-on; hydrolysis of the NTP was not required. Besides nucleotides, divalent cations had an effect on phi 6 in vitro plus strand synthesis. Magnesium ions are required for the activity but calcium ions inhibit the reaction. Manganese ions are shown to dissipate the effect of magnesium and calcium ions, leading to uncontrolled, exceptionally high level plus strand synthesis.

NTP binding induces conformational changes in the double-stranded RNA bacteriophage ø6 subviral particles.

Bacteriophage ø6 is a double-stranded RNA virus consisting of a nucleocapsid (NC) surrounded by a membrane. Beneath the NC major coat protein, P8, resides the ø6 RNA polymerase complex which is composed of four early proteins P1, P2, P4, and P7. Protein P1 forms the dodecahedral framework with which the other three proteins are associated. We have developed a new method for the isolation of stable polymerase complex particles which retain their structural integrity and polymerase activity for several days. Purine nucleotides, especially GTP, dGTP, ddGTP, and GDP, stabilized the particle efficiently. Furthermore, binding of any NTP was shown to induce conformational changes in the NC structure, as detected by alterations in the binding properties of NC-specific monoclonal antibodies. In the presence of NTPs, most of the epitopes in protein P4 become more exposed than without NTPs, while the epitopes in protein P8 were either masked or unmasked due to NTP binding. Based on the accessibility of the epitopes of protein P1 on the NC, we postulate that at least part of this protein is also accessible on the NC surface.

Purified phi 6 nucleocapsids are capable of productive infection of host cells with partially disrupted outer membranes.

Purified nucleocapsids of bacteriophage phi 6, lacking the phage lipid envelope, are unable to infect intact Pseudomonas syringae host cells. A method for studying the process by which a naked virus particle, the phi 6 nucleocapsid, penetrates the host cytoplasmic membrane was developed. Host cells were rendered competent for nucleocapsid infection by treatment with repeated washings with salt and sucrose and the subsequent addition of lysozyme. This treatment disrupts the outer membrane, permitting the nucleocapsid to reach the cytoplasmic membrane and to infect the cell. The nucleocapsid infection is blocked by monoclonal antibodies raised against the nucleocapsid shell protein P8.

Protein P4 of double-stranded RNA bacteriophage phi 6 is accessible on the nucleocapsid surface: epitope mapping and orientation of the protein.

Protein P4, an early protein of double-stranded RNA bacteriophage phi 6, is a component of the virion-associated RNA polymerase complex and possesses a nucleoside triphosphate (NTP) phosphohydrolase activity. We have produced and characterized a panel of 20 P4-specific monoclonal antibodies. Epitope mapping using truncated molecules of recombinant P4 revealed seven linear epitopes. The accessibility of the epitopes on the phi 6 nucleocapsid (NC) surface showed that at least the C terminus and an internal domain, containing the consensus sequence for NTP binding, protrude the NC shell. Four of the NC-binding antibodies distorted the integrity of the NC by releasing protein P4 and the major NC surface protein P8. This finding suggests a close contact between these two proteins. The dissociation of the NC led to the activation of the virion-associated RNA polymerase. The multimeric status of the recombinant P4 was similar to that of the virion-associated P4, indicating that no accessory virus proteins are needed for its multimerization.

Intermediates in the assembly pathway of the double-stranded RNA virus phi6.

The double-stranded RNA bacteriophage phi6 contains a nucleocapsid enclosed by a lipid envelope. The nucleocapsid has an outer layer of protein P8 and a core consisting of the four proteins P1, P2, P4 and P7. These four proteins form the polyhedral structure which acts as the RNA packaging and polymerase complex. Simultaneous expression of these four proteins in Escherichia coli gives rise to procapsids that can carry out the entire RNA replication cycle. Icosahedral image reconstruction from cryo-electron micrographs was used to determine the three-dimensional structures of the virion-isolated nucleocapsid and core, and of several procapsid-related particles expressed and assembled in E. coli. The nucleocapsid has a T = 13 surface lattice, composed primarily of P8. The core is a rounded structure with turrets projecting from the 5-fold vertices, while the procapsid is smaller than the core and more dodecahedral. The differences between the core and the procapsid suggest that maturation involves extensive structural rearrangements producing expansion. These rearrangements are co-ordinated with the packaging and RNA polymerization reactions that result in virus assembly. This structural characterization of the phi6 assembly intermediates reveals the ordered progression of obligate stages leading to virion assembly along with striking similarities to the corresponding Reoviridae structures.

Bacteriophage phi 6 envelope elucidated by chemical cross-linking, immunodetection, and cryoelectron microscopy.

Bacteriophage phi 6 is an enveloped dsRNA virus which infects the plant pathogenic Pseudomonas syringae bacterium. Using low dose cryoelectron microscopy we show that the nucleocapsid, spikeless virion, and intact virion have radii of 29, 35, and 43 nm, respectively. Thus, the membrane is 6 nm thick and the surface spikes of the receptor binding protein P3 extend 8 nm from the membrane surface. Cross-linking, immunological, and complementation evidence suggest that the spikes are formed of multimeric P3 molecules and that P3 is associated with membrane-bound protein P6. We observe that the envelope can accommodate up to 400 molecules of P3 but that the average virion contains less than one-fourth of this amount. Assembly of a very small number of P3 or truncated P3 molecules onto inactive virions restores infectivity, showing that only a few spikes are necessary for receptor binding and membrane fusion.

Genome organization of membrane-containing bacteriophage PRD1.

We have determined the nucleotide sequence of the late region (11 kbp) of the lipid-containing bacteriophage PRD1. Gene localization was carried out by complementing nonsense phage mutants with genomic clones containing specific reading frames. The localization was confirmed by sequencing the N-termini of isolated gene products as well as sequencing the N-termini of tryptic fragments of the phage membrane-associated proteins. This, with the previously obtained sequence of the early regions, allowed us to organize most of the phage genes in the phage genome.