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 × 106 and 2.3 × 106 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, [35S]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, β1 (59,000), β2 (58,000), and β3 (57,000); and (iii) small, γ1 (29,000), γ1A (28,000), γ2 (27,000), and γ3 (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, β3, γ1, and γ1A. Pulse-chase experiments and tryptic peptide map comparisons revealed that only four of the eight intracellular proteins were primary gene products, namely, α, β1, γ1, and β2, 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 β1 was cleaved to generate β2 and β3; γ1 was trimmed to produce γ1A, and the only nonstructural primary gene product, γ2, was found to be a precursor of γ3. 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.
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