Infection of cells by herpes simplex virus type 1 (HSV-1) triggers host cell shutoff whereby mRNAs are degraded and cellular protein synthesis is diminished. However, virus protein translation continues because the translational apparatus in HSV-infected cells is maintained in an active state. Surprisingly, poly(A)-binding protein 1 (PABP1), a predominantly cytoplasmic protein that is required for efficient translation initiation, is partially relocated to the nucleus during HSV-1 infection. This relocalization occurred in a time-dependent manner with respect to virus infection. Since HSV-1 infection causes cell stress, we examined other cell stress inducers and found that oxidative stress similarly relocated PABP1. An examination of stress-induced kinases revealed similarities in HSV-1 infection and oxidative stress activation of JNK and p38 mitogen-activated protein (MAP) kinases. Importantly, PABP relocalization in infection was found to be independent of the viral protein ICP27. The depletion of PABP1 by small interfering RNA (siRNA) knockdown had no significant effect on viral replication or the expression of selected virus late proteins, suggesting that reduced levels of cytoplasmic PABP1 are tolerated during infection.The lytic replication cycle of herpes simplex virus type 1 (HSV-1) can be divided into three phases, immediate-early (IE), early (E), and late (L), that occur in a coordinated sequential gene expression program. IE proteins can regulate E and L gene expression, which produces proteins involved in DNA replication, capsid production, and virion assembly. HSV infection results in host cell shutoff to facilitate the efficient production of viral proteins. First, mRNA is degraded by the virion-associated vhs protein, and then ICP27, a multifunctional regulator of gene expression, inhibits pre-mRNA splicing. As most viral mRNAs are intronless, this abrogates the production of stable cellular mRNAs that can be exported to the cytoplasm and compete for translation with viral mRNAs (44).HSV mRNAs are capped and polyadenylated and so are translated via a normal cap-dependent mechanism. Translation initiation, during which translationally active ribosomes are assembled, is a tightly regulated process (21). Eukaryotic initiation factor 4F (eIF4F) (composed of eIF4E, eIF4G, and eIF4A) that binds the cap at the 5Ј end of the mRNA promotes the recruitment of the 40S ribosomal subunit and associated factors, including eIF2-GTP initiator tRNA. The recognition of the start codon then promotes large ribosomal subunit joining. Poly(A)-binding protein 1 (PABP1), which binds and multimerizes on mRNA poly(A) tails, enhances translation initiation through interactions with the eIF4G component of the eIF4F cap-binding complex (20,29,32,51) to circularize the mRNA in a "closed-loop" conformation (24). Key protein-RNA and protein-protein interactions in the translation initiation complex are strengthened by this PABP1-mediated circularization (12).HSV-1 maintains active viral translation in the face of host translatio...

Herpes simplex virus type 1 (HSV-1) ICP27 protein is an essential regulator of viral gene expression with roles at various levels of RNA metabolism in the nucleus. Using the tethered function assay, we showed a cytoplasmic activity for ICP27 in directly enhancing mRNA translation in vivo in the absence of other viral factors. The region of ICP27 required for translational stimulation maps to the C terminus. Furthermore, in infected cells, ICP27 is associated with polyribosomes, indicating a function in translation during the lytic cycle.The multifunctional ICP27 phosphoprotein (40) is essential for herpes simplex virus type 1 (HSV-1) replication, has counterparts in all the herpesvirus families (3, 15), and is necessary for efficient expression of early (37) and late (17,20,26,27,31) viral genes. While ICP27 influences viral transcription (12,17,24), much evidence indicates that it acts posttranscriptionally. An RNA binding protein (23, 36), ICP27 inhibits pre-mRNA splicing (2,11,14,34,35), stimulates pre-mRNA 3Ј processing (18, 19), affects mRNA stability (1), and shuttles between the nucleus and the cytoplasm (25), promoting viral RNA nuclear export (4,13,32,35). An RGG box is required for RNA binding (23, 32), and C-terminal regions, including a zinc finger-like motif, have roles in transactivation and repression of reporter and viral genes (1, 27-28). Interestingly, two recent reports have provided indirect evidence for a role in the cytoplasm. First, ICP27 was shown to stimulate the polysomal association of HSV-1 VP16 RNA and the levels of VP16 protein (6); however, it remains to be determined whether this is due to a direct effect of ICP27 on translation. Second, association of ICP27 with translation factors eIF3, eIF4G, and cytoplasmic poly(A)-binding protein 1 (PABP) was reported (7), suggesting a potential role in viral and/or host mRNA translation or mRNA stability. Here we present data demonstrating that ICP27 directly stimulates mRNA translation.

Regulation of mRNA translation is a major control point for gene expression and is critical for life. Of central importance is the complex between cap-bound eukaryotic initiation factor 4E (eIF4E), eIF4G, and poly(A) tail-binding protein (PABP) that circularizes mRNAs, promoting translation and stability. This complex is often targeted to regulate overall translation rates, and also by mRNA-specific translational repressors. However, the mechanisms of mRNA-specific translational activation by RNA-binding proteins remain poorly understood. Here, we address this deficit, focusing on a herpes simplex virus-1 protein, ICP27. We reveal a direct interaction with PABP that is sufficient to promote PABP recruitment and necessary for ICP27-mediated activation. PABP binds several translation factors but is primarily considered to activate translation initiation as part of the PABP-eIF4G-eIF4E complex that stimulates the initial cap-binding step. Importantly, we find that ICP27-PABP forms a complex with, and requires the activity of, eIF4G. Surprisingly, ICP27-PABP-eIF4G complexes act independently of the effects of PABP-eIF4G on cap binding to promote small ribosomal subunit recruitment. Moreover, we find that a cellular mRNA-specific regulator, Deleted in Azoospermia-like (Dazl), also employs the PABP-eIF4G interaction in a similar manner. We propose a mechanism whereby diverse RNA-binding proteins directly recruit PABP, in a non-poly(A) tail-dependent manner, to stimulate the small subunit recruitment step. This strategy may be particularly relevant to biological conditions associated with hypoadenylated mRNAs (e.g., germ cells/neurons) and/or limiting cytoplasmic PABP (e.g., viral infection, cell stress). This mechanism adds significant insight into our knowledge of mRNA-specific translational activation and the function of the PABP-eIF4G complex in translation initiation.D espite the importance of translational control, the mechanisms by which specific subsets of mRNAs are translationally regulated are only well defined in a handful of cases. Nevertheless, it is clear that regulation is most often mediated by factors recruited to the 3′ untranslated region (UTR) of mRNAs and frequently occurs at the level of initiation (1). Initiation is a multistep process involving several mRNA-dependent steps, each of which requires eukaryotic initiation factors (eIFs) (2). Initially, the m 7 GpppX cap is bound by eIF4F, comprising a large scaffold protein, eIF4G, bound to the cap-binding protein, eIF4E, and an RNA helicase, eIF4A. The small (40S) ribosomal subunit, initiator tRNA, and associated initiation factors are then recruited as a 43S preinitiation complex. Recruitment is facilitated by eIF4A-dependent removal of RNA secondary structure and by the interaction of 40S-associated eIF3 with eIF4G. The 43S small ribosomal subunit complex then scans the 5′ UTR to locate a start codon, recognition of which promotes release of initiation factors and joining of the large (60S) ribosomal subunit to form an 80S ribosome. Like the cap,...

Vaccination against tick-borne encephalitis (TBE) is based on the use of formalin-inactivated, culture-derived whole-virus vaccines. Immune response following vaccination is primarily directed to the viral envelope (E) protein, the major viral surface antigen. In Europe, two TBE vaccines are available in adult and pediatric formulations, namely FSME-IMMUN® (Pfizer) and Encepur® (GlaxoSmithKline). Herein, we analyzed the content of these vaccines using mass spectrometry (MS). The MS analysis revealed that the Encepur vaccine contains not only proteins of the whole virus particle, but also viral non-structural protein 1 (NS1). MS analysis of the FSME-IMMUN vaccine failed due to the high content of human serum albumin used as a stabilizer in the vaccine. However, the presence of NS1 in FSME-IMMUN was confirmed by immunization of mice with six doses of this vaccine, which led to a robust anti-NS1 antibody response. NS1-specific Western blot analysis also detected anti-NS1 antibodies in sera of humans who received multiple doses of either of these two vaccines; however, most vaccinees who received ≤3 doses were negative for NS1-specific antibodies. The contribution of NS1-specific antibodies to protection against TBE was demonstrated by immunization of mice with purified NS1 antigen, which led to a significant (p < 0.01) prolongation of the mean survival time after lethal virus challenge. This indicates that stimulation of anti-NS1 immunity by the TBE vaccines may increase their protective effect.