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Cellular Synthesis and Modification of Murine Hepatitis Virus Polypeptides

Department of Microbiology and Immunology, University of Western Ontario, London, Ontario, N6A 5C1, Canada

Mouse L fibroblasts infected with mouse hepatitis virus, MHV3, and radiolabelled with ³⁵S-methionine, contained, in addition to the three major structural polypeptides, p24, p56 and p180, two additional ones, p22 and p50. Of these total five polypeptides, only three, p22, p56 and p180, were labelled in infected cells during a 2 min ³⁵S-methionine pulse and are, therefore, presumed to be immediate translation products. Pulse-chase and chymotryptic peptide mapping experiments showed apparent precursor-product relationships between p56 and p50 and between p22 and 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 approx. 100 amino acids adjacent to translational initiation sites, it was found that all three polypeptides, p22, p56 and p180 contained radiolabel. It is thus apparent that translation of the three 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.

Received 27 October 1980; accepted 23 January 1981.