The molecular biology of infectious pancreatic necrosis virus (IPNV)

Abstract

IPNV is a medium-sized, unenveloped bisegmented dsRNA-containing virus in the family Birnaviridae. Genome segment A (3097 bp) contains two overlapping open-reading frames (ORFs). A large ORF encodes a 106 kDa polyprotein (NH2-pVP2-NS protease-VP3-COOH) which is cotranslationally cleaved by the protease to generate the major capsid proteins VP2 and VP3, and a second, small ORF which overlaps the amino end of the large ORF but in a different reading frame, and encodes a 17 kDa arginine-rich minor polypeptide. Genome segment B (2784 bp) encodes a minor internal capsid polypeptide VP1 (94 kDa), which based on its size, low copy number and the presence of several conserved domains associated with RNA-dependent RNA polymerases (RdRp) of other RNA viruses, is the putative virion-associated RdRp. VP1 is present in the virion in two forms: as a free polypeptide and as a genome-linked protein (VPg) covalently attached to the 5′ ends of both genome segments. During in vitro RNA transcription, VP1 serves as a primer and remains attached to the 5′ end of the RNA thereby becoming a VPg. Transcription follows a semi-conservative, strand-displacement mechanism. In infected cells two genome-length 24S viral mRNAs lacking 3′ poly A tracts are synthesized that can hybridize to the two denatured genome segments. In vivo protein synthesis involves both polyprotein processing and internal initiation of translation at some of the in-phase methionine codons. The virus-coded protease functions only in cis and its insensitivity to a number of proteinase inhibitors suggests that it may be a novel viral protease. The putative cleavage sites on the polyprotein have been mapped to within a few amino acids but the exact boundary between $\text{pVP2}\text{NS}$ and $\text{NS}\text{VP3}$ has not been established. A universal, group-specific epitope has been mapped to near the amino terminus of VP2, whereas a serotype-specific epitope was found to be located in the middle of the polypeptide.