Open reading frame 2 of porcine circovirus type 2 encodes a major capsid protein

Porcine circovirus 2 (PCV2), a single-stranded DNA virus associated with po st-weaning multisystemic wasting syndrome of swine, has two potential open reading frames, ORF1 and ORF2, greater than 600 nucleotides in length. ORF1 is predicted to encode a replication-associated protein (Rep) essential fo r replication of viral DNA, while ORF2 contains a conserved basic amino aci d sequence at the N terminus resembling that of the major structural protei n of chicken anaemia virus, Thus far, the structural protein(s) of PCV2 hav e not been identified. In this study, a viral structural protein of 30 kDa was identified in purified PCV2 particles. ORF2 of PCV2 was cloned into a b aculovirus expression vector and the gene product was expressed in insect c ells. The expressed ORF2 gene product had a molecular mass of 30 kDa, simil ar to that detected in purified virus particles. The recombinant ORF2 prote in self-assembled to form capsid-like particles when viewed by electron mic roscopy. Antibodies against the ORF2 protein were detected in samples of se ra obtained from pigs as early as 3 weeks after experimental infection with PCV2, These results show that the major structural protein of PCV2 is enco ded by ORF2 and has a molecular mass of 30 kDa.