Replication of Herpes Simplex Virus 1 Depends on the γ 1 34.5 Functions That Facilitate Virus Response to Interferon and Egress in the Different Stages of Productive Infection

Abstract

ABSTRACT The ability of the γ 1 34.5 protein to suppress the PKR response plays a crucial role in herpes simplex virus pathogenesis. In this process, the γ 1 34.5 protein associates with protein phosphatase 1 to form a large complex that dephosphorylates eIF-2α and thereby prevents translation shutoff mediated by PKR. Accordingly, γ 1 34.5 null mutants are virulent in PKR-knockout mice but not in wild-type mice. However, γ 1 34.5 deletion mutants, with an extragenic compensatory mutation, inhibit PKR activity but remain avirulent, suggesting that the γ 1 34.5 protein has additional functions. Here, we show that a substitution of the γ 1 34.5 gene with the NS1 gene from influenza A virus renders viral resistance to interferon involving PKR. The virus replicates as efficiently as wild-type virus in SK-N-SH and CV-1 cells. However, in mouse 3T6 cells, the virus expressing the NS1 protein grows at an intermediate level between the wild-type virus and the γ 1 34.5 deletion mutant. This decrease in growth, compared to that of the wild-type virus, is due not to an inhibition of viral protein synthesis but rather to a block in virus release or egress. Virus particles are predominantly present in the nucleus and cytoplasm. Notably, deletions in the amino terminus of the γ 1 34.5 protein lead to a significant decrease in virus growth in mouse 3T6 cells, which is independent of eIF-2α dephosphorylation. In correlation, a series of deletions in the amino-terminal domain impair nuclear as well as cytoplasmic egress. These results indicate that efficient viral replication depends on the γ 1 34.5 functions required to prevent the PKR response and to facilitate virus egress in the different stages during virus infection.