Deletion of the Herpes Simplex Virus VP22-encoding Gene (UL49) Alters the Expression, Localization, and Virion Incorporation of ICP0

The role of the herpes simplex virus tegument protein VP22 is not yet known. Here we describe the characterization of a virus in which the entire VP22 open reading frame has been deleted. We show that VP22 is not essential for virus growth but that virus lacking VP22 (Delta22) displays a cell-specific replication defect in epithelial MDBK cells. Virus particles assembled in the absence of VP22 show few obvious differences to wild-type (WT) particles, except for a moderate reduction in glycoproteins gD and gB. In addition, the Delta22 virus exhibits a general delay in the initiation of virus protein synthesis, but this is not due to a glycoprotein-related defect in virus entry. Intriguingly, however, the absence of VP22 has an obvious effect on the intracellular level of the immediate-early (IE) protein ICP0. Moreover, following translocation from the nucleus to the cytoplasm, ICP0 is unable to localize to the characteristic cytoplasmic sites observed in a WT infection. We demonstrate that, in WT-infected cells, VP22 and ICP0 are concentrated in the same cytoplasmic sites. Furthermore, we show that, while ICP0 and ICP4 are components of WT extracellular virions, the altered localization of ICP0 in the cytoplasm of Delta22-infected cells correlates with an absence of both ICP0 and ICP4 from Delta22 virions. Hence, while a role has not yet been defined for virion IE proteins in virus infection, our results suggest that their incorporation is a specific event requiring the tegument protein VP22. This report provides the first direct evidence that VP22 influences virus assembly.