AnAmino-Terminal Deletion Mutation ofPseudorabies Virus Glycoprotein glll Affects Protein Localization andRNA Accumulation

We have constructed a pseudorabies virus mutant that contains virtually a complete deletion of the predicted signal sequence coding region for a nonessential envelope glycoprotein, gIII. No signal sequence mutants have been reported previously for a herpesvirus glycoprotein. Through endoglycosidase treatments and pulse-chase analysis, we have determined that the mutant gIII protein is not posttranslationally modified like the wild-type polypeptide, but rather is present as a single, stable species within the infected cell. The mutant polypeptide cannot be detected in the virus envelope, nor is it aberrantly localized to the tissue culture medium. Immunofluorescence studies have indicated that the mutant protein also is not localized to the surfaces of infected cells. In addition, Northern (RNA) and slot blot analyses, as well as in vitro translation experiments using infected-cell cytoplasmic RNA, have indicated that the mutant gIII allele is expressed at lower levels than the wild-type gene is. This is despite the fact that no alterations have been made upstream of the gIII coding sequence. From these results, it appears that the first 22 amino acids of the wild-type gIII protein define a necessary signal peptide that is responsible for at least the correct initiation of translocation and subsequent glycosylation of the gIII envelope glycoprotein within infected cells.