Poly(A)-Binding Protein 1 Partially Relocalizes to the Nucleus during Herpes Simplex Virus Type 1 Infection in an ICP27-Independent Manner and Does Not Inhibit Virus Replication

Abstract

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...

Nuclear Relocalization of Polyadenylate Binding Protein during Rift Valley Fever Virus Infection Involves Expression of the NSs Gene

“…It did, and additionally, transcription inhibition in the absence of infection led to PABP1 relocalization. We noted a more robust relocalization in cells that were both infected with RVFV MP12rLuc and treated with actinomycin D. As various cell stressors have been shown to lead to PABP1 relocalization, it is possible that factors related to the stress of infection also contribute to relocalization of PABP1 (41,61,62). We concluded that PABP1 relocalization is likely secondary to the NSs host transcription block.…”

“…Herpes simplex virus (HSV) proteins ICP27 and UL47 both bind to PABP1, leading to a decrease in PABP1's interaction with binding partners eIF4G and PABP-interacting protein 2 (40). Additionally, PABP1 partially relocalizes to the nucleus during HSV-1 infection (40,41). Rotavirus protein NSP3A functionally replaces and evicts PABP1 from the eIF4F complex and leads to nuclear relocalization of PABP1 (42,43).…”

Viral and cellular mRNA-specific activators harness PABP and eIF4G to promote translation initiation downstream of cap binding

“…Thus, ICP27 may function in this capacity by targeting small subunit recruitment via PABP-eIF4G to promote the translation of specific viral mRNAs, two of which (VP16 and ICP5) have been identified to date. Intriguingly, PABP is relocalized to the nucleus in an ICP27-independent manner at late times postinfection (10,11), suggesting that ICP27 may provide a means to direct limiting amounts of cytoplasmic PABP to a subset of viral mRNAs late in infection (7,8). Moreover, because PABP nuclear export is largely mRNA-driven in mammalian cells (12), and ICP27 promotes export of viral mRNAs (17), it is possible that the ICP27-PABP interaction may allow nuclear-retained PABP to be recruited and exported on these mRNAs, enabling their efficient translation.…”

The “Bridge” in the Epstein-Barr Virus Alkaline Exonuclease Protein BGLF5 Contributes to Shutoff Activity during Productive Infection

“…Interestingly, cellular expression of PABPC is altered upon infection by members of all herpesvirus subfamilies. The alphaherpesvirus HSV causes nuclear accumulation of PABPC in infected cells (11,45), similar to the gammaherpesviruses. PABPC relocalization by HSV, however, occurs as a consequence of the combined expression of both ICP27, inhibiting mRNA splicing, and the vhs protein (11,31).…”