Different point mutations within the conserved N-glycosylation motif of pseudorabies virus glycoprotein M result in expression of a nonglycosylated form of the protein

Glycoprotein M (gM) constitutes one of the rare examples of a nonessential glycoprotein conserved throughout all herpesvirus subfamilies. Whereas gM in wild-type pseudorabies virus (PrV) strains carries an N-glycan, gM of the attenuated strain Bartha is not glycosylated due to a point mutation in the N-glycosylation motif. Since PrV Bartha lacks glycoproteins E and I and carries a mutated gC, we analysed glycosylation of gM in isogenic PrV glycoprotein deletion mutants. Whereas gM was glycosylated normally in most mutants, two independent gC deletion mutants and a gI mutant expressed a nonglycosylated form of gM. DNA sequence analyses revealed the presence of point mutations in the N-glycosylation consensus motif. Surprisingly, mutations in strain Bartha, the two gC-deletion mutants and the gI mutant proved to be different, although all affected the N-glycosylation motif. Thus, our data show that different, apparently independent point mutations cause expression of nonglycosylated gM.