RNA tumor virus phosphoproteins: phosphorylation of precursor and processed polypeptides.

Monospecific antisera made against the 30,000 molecular weight major internal polypeptide (p30) and the 12,000 molecular weight phosphorylated polypeptide (pp12) of a wild mouse type C oncovirus were used to immunoprecipitate precursor polypeptides from extracts of isotopically labeled cells infected with the oncovirus. Analysis of the immunoprecipitates by SDS-polyacrylamide gel electrophoresis led to the detection of several precursor polypeptide (Pr) species containing the determinants of both p30 and pp12. These species, namely Pr>100, Pr100, Pr77, Pr62, and Pr50, were all found to be phosphorylated in pulse experiments. The polypeptide pp12 was, however, the major phosphorylated species immunoprecipitated by anti-pp12 sera in pulse and chase experiments. These data and a relatively high degree of phosphorylation in the processed pp12 suggested that phosphorylation of oncovirus protein is initiated at the polyprotein level and the phosphoprotein moiety is further phosphorylated subsequent to processing.