Role of the pseudorabies virus gI cytoplasmic domain in neuroinvasion, virulence, and posttranslational N-linked glycosylation

The glycoproteins I and E of pseudorabies virus are important mediators of cell-to-cell spread and virulence in all animal models tested. Although the se two proteins form a complex with one another, ascribing any function to the individual proteins has been difficult. We have shown previously, using nonsense mutations, that the N-terminal ectodomain of the gE protein is su fficient for gE-mediated transsynaptic spread whereas the cytoplasmic domai n of the protein is required for full expression of virulence. These same s tudies demonstrated that the cytoplasmic domain of gE is also required for endocytosis of the protein, In this report, we describe the construction of viruses with nonsense mutations in gI that allowed us to determine the con tributions or the gI cytoplasmic domain to protein expression as well as vi rus neuroinvasion and virulence after infection of the rat eye. We also con structed double mutants with nonsense mutations in both gE and gI so that t he contributions of both the gE and gI cytoplasmic domains could be determi ned. We observed that the gI cytoplasmic domain is required for efficient p osttranslational modification of the gI protein. The gE cytoplasmic domain has no effect on gE posttranslational glycosylation, In addition, we found that infection of all gE-gI-dependent anterograde circuits projecting from the fat retina requires both ectodomains and at least one of the cytoplasmi c domains of the proteins. The gI cytoplasmic domain promotes transsynaptic spread of virus better than the gE cytoplasmic domain. Interestingly, both gE and gI cytoplasmic tails are required for virulence; lack of either one or both results in an attenuated infection. These data suggest that gE and gI play differential roles in mediating directional neuroinvasion of the r at; however, the gE and gI cytoplasmic domains most likely function togethe r to promote virulence.