Herpes simplex virus VP16 rescues viral mRNA from destruction by the virion host shutoff function.

Abstract

Herpes simplex virus (HSV) virions contain two regulatory proteins that facilitate the onset of the lytic cycle: VP16 activates transcription of the viral immediate‐early genes, and vhs triggers shutoff of host protein synthesis and accelerated turnover of cellular and viral mRNAs. VP16 and vhs form a complex in infected cells, raising the possibility of a regulatory link between them. Here we show that viral protein synthesis and mRNA levels undergo a severe decline at intermediate times after infection with a VP16 null mutant, culminating in virtually complete translational arrest. This phenotype was rescued by a transcriptionally incompetent derivative of VP16 that retains vhs binding activity, and was eliminated by inactivating the vhs gene. These results indicate that VP16 dampens vhs activity, allowing HSV mRNAs to persist in infected cells. Further evidence supporting this hypothesis came from the demonstration that a stably transfected cell line expressing VP16 was resistant to host shutoff induced by superinfecting HSV virions. Thus, in addition to its well known function as a transcriptional activator, VP16 stimulates viral gene expression at a post‐transcriptional level, by sparing viral mRNAs from degradation by one of the virus‐induced host shutoff mechanisms.

Packaging of the Virion Host Shutoff (Vhs) Protein of Herpes Simplex Virus: Two Forms of the Vhs Polypeptide Are Associated with Intranuclear B and C Capsids, but Only One Is Associated with Enveloped Virions

“…An observation that it is absent from B and C capsid preparations that contain Vhs would be an important control that those preparations are not contaminated with enveloped virions. It was also of interest to look for VP16 because Vhs and VP16 can be coimmunoprecipitated from infected cells (58,65,69), and newly synthesized VP16 appears to play an important role in controlling the activity of newly synthesized Vhs (36).…”

Selective degradation of mRNAs by the HSV host shutoff RNase is regulated by the U L 47 tegument protein

“…VP16 was the first viral protein shown to interact with VHS and, in the context of infected cells, shown to abrogate the RNase activity (13). The demonstration that neutralization of VHS activity required VP22 emerged from the observation that deletion of the gene encoding VP22 resulted in the selection of mutants defective in RNase activity (32).…”

“…The primary function of VHSRNase appears to be the degradation of mRNAs made in response to infection, but it also prevents, by a mechanism not fully understood, the activation of protein kinase R (10-12). At late stages in the viral replicative cycle, virion host shutoff (VHS)-RNase activity is blocked or neutralized by two late proteins, virion protein (VP) 16 and VP22, encoded by U L 48 and U L 49 ORFs, respectively (13,14). The RNase cleaves mRNA in polyribosomes (15).…”

The herpes simplex virus 1 U L 41 gene-dependent destabilization of cellular RNAs is selective and may be sequence-specific

“…Because viral mRNA is transcribed at a higher rate than cellular mRNA, viral proteins do accumulate. At middle or late times after infection, vhs interacts with the ␣-trans-inducing factor also known as VP16 encoded by the U L 48 ORF and it becomes inactive (6). The function of vhs appears to be twofold, to enable efficient transition from ␣ to ␤ and ␥ protein synthesis and to block host responses to infection.…”