Antigenic structure analysis of glycosylated protein 3 of porcine reproductive and respiratory syndrome virus

The function of the glycosylated protein 3 (GP3), a porcine reproductive and respiratory syndrome virus (PRRSV) associated protein is poorly known. In the present study, the gene encoding GP3 (ORF3), lacking the highly hydrophobic domain in the N- and C-termini was expressed as GST-fusion proteins in E. coli. Monoclonal antibodies (MAbs) against GP3 were developed and used to probe a series of GP3 peptides using ELISA. After precise analysis by sequential deletion of the terminal amino acid residues from each peptide, the minimal epitopes recognized by the MAbs were localized to W74CRIGHDRCGED85 and Y67EPGRSLW74. The epitope sequences were well conserved among most of the North American-type isolates, with the exception of two amino acid mutations in both epitopes in a few of these isolates. Mutational analysis revealed that these mutants were not recognized by any of the five MAbs, indicating that genetic variation could lead to altered antigenicity. Eight out of nine peptide fragments, 58–72aa, 73–87aa, 88–101aa, 102–115aa, 50–65aa, 66–81aa, 80–95aa and 94–109aa were recognized by PRRSV-positive pig serum as determined by Western blot analysis. The results herein may elucidate partially the antigenic structure of GP3 and variations of PRRSV.