Signals That Dictate Nuclear, Nucleolar, and Cytoplasmic Shuttling of the γ(1)34.5 Protein of Herpes Simplex Virus Type 1

The γ(1)34.5 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 γ(1)34.5 protein is implicated in viral egress and interacts with proliferating cell nuclear antigen. In this report, we show that the γ(1)34.5 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 γ(1)34.5 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. R(215)A and R(216)A substitutions in the bipartite motif disrupt this activity. Intriguingly, leptomycin B, an inhibitor of nuclear export, blocks the cytoplasmic accumulation of the γ(1)34.5 protein. L(134)A and L(136)A substitutions in the leucine-rich motif completely excluded the γ(1)34.5 protein from the cytoplasm. These results suggest that the γ(1)34.5 protein continuously shuttles between the nucleus, nucleolus, and cytoplasm, which may be a requirement for the different activities of the γ(1)34.5 protein in virus-infected cells.