Translation of mouse mammary tumor virus RNA: Precursor polypeptides are phosphorylated during processing

The synthesis of viral polypeptides of the mouse mammary tumor virus (MMTV) was studied by pulse-labeling of MMTV-producing cells and by translating MMTV virion RNA in vitro, in Xenopus laevis oöcytes. Virus-related polypeptides were detected by means of immunoprecipitation withm monospecific antisera against the major viral proteins gp49 and p24 and analysis of the immunoprecipitates on polyacrylamide gels. In pulse-labeled MMTV-producing cells (Mm5mt/c1), a precursor polypeptide of 73,000 daltons was immunoprecipitated by anti-p24 serum (Pr73 gag ). Pr73 gag co-migrated with the 73,000-dalton glycosylated precursor for the envelope proteins (Pr73 env ) immunoprecipitated by anti-gp49 serum. Pr73 gag was, during chase, converted into a 76,000-dalton polypeptide, also reacting with the anti-p24 serum (Pr76 gag ). After prolonged incubation, the mature internal protein p24 was synthesized. Pulse-labeling with 32P and subsequent chasing revealed that phosphate was incorporated into Pr76 gag and not into Pr73 gag . Isolated virion 70 S RNA of MMTV, microinjected into Xenopus oöcytes, gave rise to synthesis of Pr73 gag , Pr76 gag , and p24, all immunoprecipitated by anti-p24 serum, and the viral core proteins p14 and p10, precipitated by polyvalent anti-MMTV serum. 70 S RNA did not instruct synthesis of the viral envelope glycoproteins.