Peptide Map Comparison of Infectious Pancreatic Necrosis Virus-Specific Polypeptides

Abstract

An investigation of virus-specific protein synthesis in infectious pancreatic necrosis virus (IPNV)-infected rainbow trout gonad cells was undertaken to find a relationship between the coding capacity of the virus genome (two segments of double-stranded RNA of 2.5 × 10 6 and 2.3 × 10 6 molecular weight) and the sizes and relative amounts of virus-specific proteins. Using polyacrylamide slabgel electrophoresis and autoradiography, eight distinct virus-specific polypeptides were detected in infected, [ 35 S]methionine-labeled cells. These proteins may be grouped into three size classes on the basis of molecular weight: (i) large, α (90,000); (ii) medium, β 1 (59,000), β 2 (58,000), and β 3 (57,000); and (iii) small, γ 1 (29,000), γ 1A (28,000), γ 2 (27,000), and γ 3 (25,000). The combined molecular weight of these polypetides (373,000) is beyond the coding capacity of the virus genome. Purified IPNV contained polypeptides α, β 3 , γ 1 , and γ 1A . Pulse-chase experiments and tryptic peptide map comparisons revealed that only four of the eight intracellular proteins were primary gene products, namely, α, β 1 , γ 1 , and β 2 , with a combined molecular weight of 205,000. Of these primary gene products only the α polypeptide was found to be stable, whereas the other three underwent intracellular proteolytic cleavage during virus morphogenesis. Polypeptide β 1 was cleaved to generate β 2 and β 3 ; γ 1 was trimmed to produce γ 1A , and the only nonstructural primary gene product, γ 2 , was found to be a precursor of γ 3 . These results suggest that IPNV possesses a unique mechanism to synthesize three size classes of proteins using mRNA transcripts from two high-molecular-weight double-stranded RNA genome segments.