Signals that dictate nuclear, nucleolar, and cytoplasmic shuttling of the γ₁34.5 protein of herpes simplex virus type 1

The γ₁34.5 protein of herpes simplex virus type 1 (HSV-1) is required for viral neurovirulence in vivo. In infected cells, this viral protein prevents the shutoff of protein synthesis mediated by double-stranded-RNA-dependent protein kinase PKR. This is accomplished by recruiting protein phosphatase 1 to dephosphorylate the α subunit of translation initiation factor eIF-2 (eIF-2α). Moreover, the γ₁34.5 protein is implicated in viral egress and interacts with proliferating cell nuclear antigen. In this report, we show that the γ₁34.5 protein encoded by HSV-1(F) is distributed in the nucleus, nucleolus, and cytoplasm in transfected or superinfected cells. Deletion analysis revealed that the Arg-rich cluster from amino acids 1 to 16 in the γ₁34.5 protein functions as a nucleolar localization signal. The region from amino acids 208 to 236, containing a bipartite basic amino acid cluster, is able to mediate nuclear localization. R²¹⁵A and R²¹⁶A substitutions in the bipartite motif disrupt this activity. Intriguingly, leptomycin B, an inhibitor of nuclear export, blocks the cytoplasmic accumulation of the γ₁34.5 protein. L¹³⁴ A and L¹³⁶ A substitutions in the leucine-rich motif completely excluded the γ₁34.5 protein from the cytoplasm. These results suggest that the γ₁34.5 protein continuously shuttles between the nucleus, nucleolus, and cytoplasm, which may be a requirement for the different activities of the γ₁34.5 protein in virus-infected cells.