EPITOPE MAPPING OF THE NUCLEOCAPSID PROTEIN OF EUROPEAN AND NORTH-AMERICAN ISOLATES OF PORCINE REPRODUCTIVE AND RESPIRATORY SYNDROME VIRUS

Two major genotypes of porcine reproductive and respiratory syndrome v irus (PRRSV) have been described, which correspond to the European and North American isolates, PRRSV nucleocapsid (N) protein has been iden tified as the most immunodominant viral protein, The N genes from two PRRSV isolates, Olot/91 (European) and Quebec 807/94 (North American), were cloned and expressed in: (i) baculovirus under the control of th e polyhedrin promoter and (ii) Escherichia coli using the pET3x system , The N protein from both isolates was expressed much more efficiently in E. coli as a fusion protein than in baculovirus, The antigenicity of the protein was similar in both systems and it was recognized by a collection of 48 PRRSV-positive pig sera, The antigenic structure of t he PRRSV N protein was investigated using seven monoclonal antibodies (MAbs) and overlapping fragments of the protein expressed in E, coli, Four MAbs recognized two discontinuous epitopes that were present in t he partially folded protein, or at least a large fragment comprising t he first 78 residues. The other three MAbs revealed the presence of a common antigenic site localized in the central region of the protein ( amino acids 50-66), This region is well conserved among different isol ates of European and North American origin and is the most hydrophilic region of the protein. However, this epitope, although recognized by the MAbs and many pig sera, is not useful for diagnostic purposes. Mor eover, none of the N protein fragments were able to mimic the antigeni city of the entire protein.