Replication-Competent Herpes Simplex Virus 1 Isolates Selected from Cells Transfected with a Bacterial Artificial Chromosome DNA Lacking Only the U _L 49 Gene Vary with Respect to the Defect in the U _L 41 Gene Encoding Host Shutoff RNase

Abstract

To generate a null U L 49 gene mutant of herpes simplex virus 1 (HSV-1), we deleted from the viral DNA, encoded as a bacterial artificial chromosome (BAC), the U L 49 open reading frame and, in a second step, restored it. Upon transfection into Vero cells, the BAC-⌬U L 49 DNA yielded foci of degenerated cells that could not be expanded and a few replication-competent clones. The replication-competent viral clones derived from independent transfections yielded viruses that expressed genes with some delay, produced smaller plaques, and gave lower yields than wild-type virus. A key finding is that the independently derived replication-competent viruses lacked the virion host shutoff (vhs) activity expressed by the RNase encoded by the U L 41 gene. One mutant virus expressed no vhs protein, whereas two others, derived from independent transfections, produced truncated vhs proteins consistent with the spontaneous in-frame deletion. In contrast, cells infected with the virus recovered upon transfection of the BAC-U L 49R DNA (R-U L 49) accumulated a full-length vhs protein, indicating that in the parental BAC-⌬U L 49 DNA, the U L 41 gene was intact. We conclude that expression of the vhs protein in the absence of U L 49 protein is lethal, a conclusion bolstered by the evidence reported elsewhere that in transfected cells vhs requires both VP16 and VP22, the product of U L 49, to be neutralized.Of the 84 different known proteins encoded by herpes simplex virus 1 (HSV-1), at least 4 proteins, all located in the tegument of the virion, interact with mRNAs. Of these, the proteins encoded by the U S 11, U L 47, and U L 49 open reading frames (ORFs) bind RNAs, whereas the fourth, encoded by the U L 41 ORF, acts as an RNase (reviewed in reference 44). Apart from potential regulatory functions, interest in the RNA binding proteins stemmed from the observation that virions package mRNAs (40,42). Moreover, in the course of studies of this phenomenon, our laboratories reported that VP22, the product of the U L 49 ORF, transports the mRNA from infected to uninfected cells for expression prior to viral infection (41). The studies reported here were initially designed to determine the contribution of VP22 to the packaging of mRNAs in the virion. We show that a series of ⌬U L 49 mutants derived from independent transfections of viral DNA lacking the U L 49 ORF yielded recombinant viruses defective in the U L 41 gene. On the basis of the results reported in this article and in parallel studies published by Taddeo et al. (52), we conclude that VP22 and VP16 are both required for the replication of viruses encoding functional U L 41 protein. Relevant to this report are the following.The shutoff of cellular protein synthesis, a function designated virion host shutoff (vhs), was first identified and mapped to the U L 41 ORF by Frenkel and colleagues (25,26,47). Intensive studies carried out over 2 decades demonstrated that the U L 41 product is a ␥ 2 protein, that it is packaged in the virion, and that it mediates the degradation of mRNA...

Herpes Simplex Virus 2 Virion Host Shutoff Endoribonuclease Activity Is Required To Disrupt Stress Granule Formation

“…HSV strains carrying large deletions in the UL41 gene encoding vhs have been reported to have two particular attributes: cells infected with these viruses form spontaneous SGs late in infection (27,41,74), and virus is unable to block activation of doublestranded RNA-dependent eIF2 kinase (protein kinase R [PKR]) that arises as a result of infection (68,75). In keeping with the first attribute, we demonstrate here that specific removal of vhs endoribonuclease activity also results in spontaneous SG formation.…”

“…3 support the existence of such a component. We have observed that disruption of SG formation continues to late times postinfection (26), when the endoribonuclease activity of vhs is predicted to be suppressed via the interaction of vhs with VP16 and VP22 (65)(66)(67)(68). Thus, another viral component may be required later in infection to ensure the continued disruption of SG formation.…”

The Virion Host Shutoff RNase Plays a Key Role in Blocking the Activation of Protein Kinase R in Cells Infected with Herpes Simplex Virus 1

“…HSV-1 VHS selectively suppresses expression of PKR in HT1080 cell lines. In order to verify the involvement of the VHS in the control of PKR, HT1080 cells were transfected with plasmids expressing wild-type or catalytically inactive VHS protein under the control of the cytomegalovirus (CMV) promoter either alone or along with plasmids expressing two other tegument proteins, U L 48 (VP16) and U L 49 (VP22), previously reported to regulate the function of VHS (26,29). The cells were harvested 48 h after transfection, and lysates were electrophoretically separated in a denaturing polyacrylamide gel, transferred to nitrocellulose membranes, and probed with antibodies against total and phosphorylated forms of PKR and against GAPDH as a loading control.…”

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

“…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 evidence that VHS-RNase interacts with ICP27 emerged from studies designed to determine the role of this α protein in the degradation of viral mRNAs (33).…”