Role of N-linked glycans in antigenicity, processing, and cell surface expression of bovine herpesvirus 1 glycoprotein gIV.

Glycoprotein gIV, a structural component of bovine herpesvirus type 1, stimulates high titers of virus-neutralizing antibody. The protein contains three potential sites for the addition of N-linked carbohydrates. Three mutants were constructed by oligonucleotide-directed mutagenesis, in each case changing one N-linked glycosylation site from Asn-X-Thr/Ser to Ser-X-Thr/Ser. A fourth mutant was altered at two sites. The altered forms of the gIV gene were cloned into a vaccinia virus transfer vector to generate recombinant vaccinia viruses expressing mutant proteins. Analysis of these mutants revealed that only two (residues 41 and 102) of the three (residues 41, 102, and 411) potential sites for the addition of N-linked glycans are actually utilized. Absence of glycans at residue 41 (gN1) showed no significant effect on the conformation of the protein or induction of a serum neutralizing antibody response. However, mutant proteins lacking glycans at residue 102 (gN2) or residues 41 and 102 (gN1N2) showed altered reactivity with conformation-dependent gIV-specific monoclonal antibodies. These mutants also induced significantly lower serum neutralizing antibody responses than wild-type gIV. Nonetheless, each of the mutant proteins were modified by the addition of O-glycans and transported to the cell surface. Our results demonstrate that absence of N-linked glycans at one (residue 102) or both (residues 41 and 102) utilized N-linked glycosylation sites alters the conformation but does not prevent processing and transport of gIV to the cell surface.