Cellular Synthesis and Modification of Murine Hepatitis Virus Polypeptides

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Abstract
Mouse L fibroblasts infected with mouse hepatitis virus, MHV3, and radiolabeled with 35S-methionine, contained, in addition to the 3 major structural polypeptides, p24, p56 and p180, 2 additional ones, p22 and p50. Of these 5 polypeptides, only 3, p22, p56 and p180, were labeled in infected cells during a 2 min 35S-methionine pulse and are, thus, presumed to be immediate translation productions. Pulse-chase and chymotryptic peptide mapping experiments showed apparent precursor-product relatiohnships between p56-p50 and between p22-p24. Protein synthesis in infected cells was synchronized at the initiation stage by pre-exposure to hypertonic medium. Using a 0.5 min pulse-10 min chase sequence to limit incorporation of 35S-methionine to stretches of .apprx. 100 amino acids adjacent to translational initiation sites, it was found that all 3 polypeptides, p22, p56 and p180, contained radiolabel. Translation of the 3 major structural proteins (or precursors) is initiated independently rather than at a single site, as in the cases of other positive-strand RNA viruses such as polio or Semliki Forest virus.