The major envelope protein, GP5, of a European porcine reproductive and respiratory syndrome virus contains a neutralization epitope in its N-terminal ectodomain

A set of neutralizing monoclonal antibodies (mAbs) directed against the GP(5) protein of European type porcine reproductive and respiratory syndrome virus (PRRSV) has been produced previously (Weiland et al., 1999). This set reacted with a plaque-purified virus (PPV) subpopulation of Dutch isolate Intervet-10 (1-10), but not with the European prototype PRRSV LV. In order to map the neutralization epitope in the GP(5) protein of the PPV strain, the ORF5 nucleotide sequence of PPV was determined. When the amino acid sequence derived from this nucleotide sequence was compared with that of PRRSV LV, four amino acid differences were found. Using site-directed mutagenesis, we showed that a proline residue at position 24 of the GP(5) sequence of the PPV strain enabled recognition by the neutralizing mAbs. Pepscan analysis demonstrated that the epitope recognized by the neutralizing mAbs stretched from residues 29 to 35. Surprisingly, the reactivity of the mAbs in the Pepscan system was independent of the presence of a proline in position 24. Moreover, residue 24 is located within the predicted signal peptide, implying that either the signal peptide is not cleaved or is cleaved due to the presence of Pro(24) such that the epitope remains intact. Our results demonstrate the presence of a neutralization epitope in the N-terminal ectodomain of the GP(5) protein of PRRSV and imply a role for the ectodomain of GP(5) in the infection of PRRSV.