The unique N terminus of herpes simplex virus type 1 ribonucleotide reductase large subunit is phosphorylated by casein kinase 2, which may have a homologue in Escherichia coli

Protein phosphorylation during Plasmodium berghei gametogenesis

Plasmodium gametogenesis within the mosquito midgut is a complex differentiation process involving signaling mediated by phosphorylation, which modulate metabolic routes and protein synthesis required to complete this development. However, the mechanisms leading to gametogenesis activation are poorly understood. We analyzed protein phosphorylation during Plasmodium berghei gametogenesis in vitro in serum-free medium using bidimensional electrophoresis (2-DE) combined with immunoblotting (IB) and antibodies specific to phosphorylated serine, threonine and tyrosine. Approximately 75 protein exhibited phosphorylation changes, of which 23 were identified by mass spectrometry. These included components of the cytoskeleton, heat shock proteins, and proteins involved in DNA synthesis and signaling pathways among others. Novel phosphorylation events support a role for these proteins during gametogenesis. The phosphorylation sites of six of the identified proteins, HSP70, WD40 repeat protein msi1, enolase, actin-1 and two isoforms of large subunit of ribonucleoside reductase were investigated using TiO2 phosphopeptides enrichment and tandem mass spectrometry. In addition, transient exposure to hydroxyurea, an inhibitor of ribonucleoside reductase, impaired male gametocytes exflagellation in a dose-dependent manner, and provides a resource for functional studies.

A single viral gene determines lethal cross-species neurovirulence of baboon herpesvirus HVP2

Alpha-herpesviruses can produce more severe infections in non-natural host species than in their natural host. Isolates of the baboon alpha-herpesvirus Papiine herpesvirus 2 (HVP2) are either very neurovirulent in mice (subtype nv) or non-virulent (subtype ap), but no such difference is evident in the natural baboon host. Comparative genome sequencing was used to identify subtype-specific sequence differences (SSDs) between HVP2nv and HVP2ap isolates. Some genes were identified that despite exhibiting sequence variation among isolates did not have any SSDs, while other genes had comparatively high levels of SSDs. Construction of genomic recombinants between HVP2nv and HVP2ap isolates mapped the mouse neurovirulence determinant to within three genes. Construction of gene-specific recombinants demonstrated that the UL39 ORF is responsible for determining the lethal neurovirulence phenotype of HVP2 in mice. These results demonstrate that differences in a single viral gene can determine the severity of herpesvirus infection in a non-natural host species.

The ribonucleotide reductase R1 subunits of herpes simplex virus types 1 and 2 protect cells against TNFα- and FasL-induced apoptosis by interacting with caspase-8

We previously reported that HSV-2 R1, the R1 subunit (ICP10; UL39) of herpes simplex virus type-2 ribonucleotide reductase, protects cells against apoptosis induced by the death receptor (DR) ligands tumor necrosis factor-alpha- (TNFα) and Fas ligand (FasL) by interrupting DR-mediated signaling at, or upstream of, caspase-8 activation. Further investigation of the molecular mechanism underlying HSV-2 R1 protection showed that extracellular-regulated kinase 1/2 (ERK1/2), phosphatidylinositol 3-kinase (PI3-K)/Akt, NF-κB and JNK survival pathways do not play a major role in this antiapoptotic function. Interaction studies revealed that HSV-2 R1 interacted constitutively with caspase-8. The HSV-2 R1 deletion mutant R1(1-834)-GFP and Epstein-Barr virus (EBV) R1, which did not protect against apoptosis induced by DR ligands, did not interact with caspase-8, indicating that interaction is required for protection. HSV-2 R1 impaired caspase-8 activation induced by caspase-8 over-expression, suggesting that interaction between the two proteins prevents caspase-8 dimerization/activation. HSV-2 R1 bound to caspase-8 directly through its prodomain but did not interact with either its caspase domain or Fas-associated death domain protein (FADD). Interaction between HSV-2 R1 and caspase-8 disrupted FADD-caspase-8 binding. We further demonstrated that individually expressed HSV-1 R1 (ICP6) shares, with HSV-2 R1, the ability to bind caspase-8 and to protect cells against DR-induced apoptosis. Finally, as the long-lived Fas protein remained stable during the early period of infection, experiments with the HSV-1 UL39 deletion mutant ICP6∆ showed that HSV-1 R1 could be essential for the protection of HSV-1-infected cells against FasL.

A short polypeptide from the herpes simplex virus type 2 ICP10 gene can induce antigen aggregation and autophagosomal degradation for enhanced immune presentation

It has been reported that certain polypeptides derived from aggregation-prone cellular proteins can turn soluble green fluorescent protein (GFP) into aggregates. Here we report our finding that a short peptide derived from a viral gene, ICP10 of herpes simplex virus (HSV)-2, also possesses such a property. A sequence as short as 13 amino acids from the middle region of the gene can convert GFP into an aggregation-prone toxic protein once it is fused to the C terminus. Moreover, this short peptide can direct a surrogate tumor antigen into the autophagosome/lysosome degradation pathway, drastically increasing both MHC class I and class II antigen presentation. The simultaneous induction of both arms of the T cell immune response to the tumor antigen effectively protects the immunized animals from tumor challenge. Designated VIPA (i.e., viral inducer of protein aggregation), this unique viral sequence may represent an attractive candidate as a molecular adjuvant for cancer immunotherapy and for other immunologically preventable diseases.

CK2 inhibitors increase the sensitivity of HSV-1 to interferon-β

Herpes simplex virus type 1 (HSV-1) requires the activities of cellular kinases for efficient replication. The host kinase, CK2, has been shown or is predicted to modify several HSV-1 proteins and has been proposed to affect one or more steps in the viral life cycle. Furthermore, potential cellular and viral substrates of CK2 are involved in antiviral pathways and viral counter-defenses, respectively, suggesting that CK2 regulates these processes. Consequently, we tested whether pharmacological inhibitors of CK2 impaired HSV-1 replication, either alone or in combination with the cellular antiviral factor, interferon-β (IFN-β). Our results indicate that the use of CK2 inhibitors results in a minor reduction in HSV-1 replication but enhanced the inhibitory effect of IFN-β on replication. This effect was dependent on the HSV-1 E3 ubiquitin ligase, infected cell protein 0 (ICP0), which impairs several host antiviral responses, including that produced by IFN-β. Inhibitors of CK2 did not, however, impede the ability of ICP0 to induce the degradation of two cellular targets: the promyelocytic leukemia protein (PML) and the DNA-dependent protein kinase catalytic subunit (DNA-PKcs). Notably, this effect was only apparent for HSV-1, as the CK2 inhibitors did not enhance the antiviral effect of IFN-β on either vesicular stomatitis virus or adenovirus type 5. Thus, our data suggest that the activity of CK2 is required for an early function during viral infection that assists the growth of HSV-1 in IFN-β-treated cells.

Viral serine/threonine protein kinases

Phosphorylation represents one the most abundant and important posttranslational modifications of proteins, including viral proteins. Virus-encoded serine/threonine protein kinases appear to be a feature that is unique to large DNA viruses. Although the importance of these kinases for virus replication in cell culture is variable, they invariably play important roles in virus virulence. The current review provides an overview of the different viral serine/threonine protein kinases of several large DNA viruses and discusses their function, importance, and potential as antiviral drug targets.

The TORrid affairs of viruses: effects of mammalian DNA viruses on the PI3K-Akt-mTOR signalling pathway

The successful replication of mammalian DNA viruses requires that they gain control of key cellular signalling pathways that affect broad aspects of cellular macromolecular synthesis, metabolism, growth and survival. The phosphatidylinositol 3'-kinase-Akt-mammalian target of rapamycin (PI3K-Akt-mTOR) pathway is one such pathway. Mammalian DNA viruses have evolved various mechanisms to activate this pathway to obtain the benefits of Akt activation, including the maintenance of translation through the activation of mTOR. In addition, viruses must overcome the inhibition of this pathway that results from the activation of cellular stress responses during viral infection. This Review will discuss the range of mechanisms that mammalian DNA viruses use to activate this pathway, as well as the multiple mechanisms these viruses have evolved to circumvent inhibitory stress signalling.

The ribonucleotide reductase domain of the R1 subunit of herpes simplex virus type 2 ribonucleotide reductase is essential for R1 antiapoptotic function

The R1 subunit (ICP10) of herpes simplex virus type 2 (HSV-2) ribonucleotide reductase (RR), which in addition to its C-terminal reductase domain possesses a unique N-terminal domain of about 400 aa, protects cells against apoptosis. As the NH2domain on its own is not antiapoptotic, it has been postulated that both domains of R1 or part(s) of them could be necessary for this function. Here, N- and C-terminal deletions were introduced in HSV-2 R1 to map the domain(s) involved in its antiapoptotic potential. The results showed that, whereas most of the NH2domain including part of the recently described putativeα-crystallin domain is dispensable for antiapoptotic activity, it is the integrity of the structured RR domain that is required for protection. As theα-crystallin domain appears to play an important role in protein folding and oligomerization, the N-terminal boundary of the antiapoptotic domain could not be defined precisely. In addition, this study provided evidence that overexpression of HSV-2 R2 at levels up to 30-fold more than HSV-2 R1 did not decrease protection from tumour necrosis factor alpha, indicating that the R1 surface where R2 binds is not involved in antiapoptotic activity. Importantly, this result suggests that the co-expression of both RR subunits during the lytic cycle should not affect protection from this cytokine.

Effects of pharmacological cyclin-dependent kinase inhibitors on viral transcription and replication

Cyclin-dependent kinases (CDKs) are required for replication of adeno-, papilloma- and other viruses that replicate only in dividing cells. Surprisingly, CDKs are also required for replication of HIV-1, HSV-1, and other viruses that can replicate in non-dividing cells. Since two low-molecular weight pharmacological CDK inhibitors (PCIs), flavopiridol (Flavo) and roscovitine (Rosco), appear to be non-toxic in human clinical trials against cancer, these drugs have been proposed as potential antiviral drugs. Rosco preferentially inhibits CDKs involved in cell cycle regulation (CDK1, 2, and 7) or neuronal functions (CDK5), whereas Flavo preferentially inhibits CDKs involved in cell cycle (CDK1, 2, 4, 7) or transcription (CDK7, and 9). As potential antivirals, PCIs display several advantages: (i) they are active against many different viruses, including drug-resistant strains of HIV-1 and HSV-1; (ii) PCI-resistant mutants of HIV-1 or HSV-1 have not been identified; and (iii) the antiviral effects of PCIs and conventional antivirals appear to be additive (as expected from drugs that target independent pathways). Moreover, PCIs target both the etiological agents (i.e., the virus) and the pathogenic mechanisms (i.e., unrestricted cell division) of the many diseases that include both a CDK-requiring virus and unrestricted cell division (e.g., Kaposi's sarcoma, cervical carcinoma, HIV-associated nephropathy-HIVAN). This is nicely illustrated in a recent study which demonstrated the efficacy of Flavo in a mouse model of HIVAN. Herein, we will review the involvement of CDKs in viral replication and the antiviral properties of the most extensively characterized PCIs, with special emphasis on the mechanisms of inhibition of viral transcription.

The human cytomegalovirus UL45 gene product is a late, virion-associated protein and influences virus growth at low multiplicities of infection

Human cytomegalovirus (HCMV) encodes a protein related to the large (R1) subunit of ribonucleotide reductase (RR), but does not encode the corresponding small (R2) subunit. The R1 homologue, UL45, lacks many catalytic residues, and its impact on deoxyribonucleotide (dNTP) production remains unknown. Here, UL45 is shown to accumulate at late stages of infection and to be a virion tegument protein. To study UL45 function in its genome context, UL45 was disrupted by transposon insertion. The UL45-knockout (UL45-KO) mutant exhibited a growth defect in fibroblasts at a low m.o.i. and also a cell-to-cell spread defect. This did not result from a reduced dNTP supply because dNTP pools were unchanged in resting cells infected with the mutant virus. Irrespective of UL45 expression, all cellular RR subunits - S-phase RR subunits, and the p53-dependent p53R2 - were induced by infection. p53R2 was targeted to the infected cell nucleus, suggesting that HCMV diverts a mechanism normally activated by DNA damage response. Cells infected with the UL45-KO mutant were moderately sensitized to Fas-induced apoptosis relative to those infected with the parental virus. Together with the report on the UL45-KO endotheliotropic HCMV mutant (Hahn et al., J Virol 76, 9551-9555, 2002), these data suggest that UL45 does not share the prominent antiapototic role attributed to the mouse cytomegalovirus homologue M45 (Brune et al., Science 291, 303-305, 2001).

The R1 subunit of herpes simplex virus ribonucleotide reductase has chaperone-like activity similar to Hsp27

HSV-2 R1, the R1 subunit of herpes simplex virus (HSV) ribonucleotide reductase, protects cells against apoptosis. Here, we report the presence in HSV-2 R1 of a stretch exhibiting similarity to the alpha-crystallin domain of the small heat shock proteins, a domain known to be important for oligomerization and cytoprotective activities of these proteins. Also, the HSV-2 R1 protein, which forms multimeric structures in the absence of nucleotide, displayed chaperone ability as good as Hsp27 in a thermal denaturation assay using citrate synthase. In contrast, mammalian R1, which does not contain an alpha-crystallin domain, has neither chaperone nor anti-apoptotic activity. Thus, we propose that the chaperone activity of HSV-2 R1 could play an important role in viral pathogenesis.

One-thousand-and-one substrates of protein kinase CK2?

CK2 (formerly termed "casein kinase 2") is a ubiquitous, highly pleiotropic and constitutively active Ser/Thr protein kinase whose implication in neoplasia, cell survival, and virus infection is supported by an increasing number of arguments. Here an updated inventory of 307 CK2 protein substrates is presented. More than one-third of these are implicated in gene expression and protein synthesis as being either transcriptional factors (60) or effectors of DNA/RNA structure (50) or translational elements. Also numerous are signaling proteins and proteins of viral origin or essential to virus life cycle. In comparison, only a minority of CK2 targets (a dozen or so) are classical metabolic enzymes. An analysis of 308 sites phosphorylated by CK2 highlights the paramount relevance of negatively charged side chains that are (by far) predominant over any other residues at positions n+3 (the most crucial one), n+1, and n+2. Based on this signature, it is predictable that proteins phosphorylated by CK2 are much more numerous than those identified to date, and it is possible that CK2 alone contributes to the generation of the eukaryotic phosphoproteome more so than any other individual protein kinase. The possibility that CK2 phosphosites play some global role, e.g., by destabilizing alpha helices, counteracting caspase cleavage, and generating adhesive motifs, will be discussed.

The R1 subunit of herpes simplex virus ribonucleotide reductase protects cells against apoptosis at, or upstream of, caspase-8 activation

The R1 subunit of herpes simplex virus (HSV) ribonucleotide reductase, which in addition to its C-terminal reductase domain possesses a unique N-terminal domain of about 400 amino acids, is thought to have an additional, as yet unknown, function. Here, we report that the full-length HSV-2 R1 has an anti-apoptotic function able to protect cells against death triggered by expression of R1(Delta2-357), an HSV-2 R1 subunit with its first 357 amino acids deleted. We further substantiate the R1 anti-apoptotic activity by showing that its accumulation at low level could completely block apoptosis induced by TNF-receptor family triggering. Activation of caspase-8 induced either by TNF or by Fas ligand expression was prevented by the R1 protein. As HSV R1 did not inhibit cell death mediated by several agents acting via the mitochondrial pathway (Bax overexpression, etoposide, staurosporine and menadione), it is proposed that it functions to interrupt specifically death receptor-mediated signalling at, or upstream of, caspase-8 activation. The N-terminal domain on its own did not exhibit anti-apoptotic activity, suggesting that both domains of R1 or part(s) of them are necessary for this new function. Evidence for the importance of HSV R1 in protecting HSV-infected cells against cytokine-induced apoptosis was obtained with the HSV-1 R1 deletion mutants ICP6Delta and hrR3. These results show that, in addition to its ribonucleotide reductase function, which is essential for virus reactivation, HSV R1 could contribute to virus propagation by preventing apoptosis induced by the immune system.