Post-translational modifications of retrovirus gag gene-encoded polyproteins include proteolytic cleavage, phosphorylation, and glycosylation. To study the sequence of these events, we labeled JLS-V9 cells chronically infected with Rauscher murine leukemia virus in pulse-chase experiments with the radioactive precursors [3S]methionine, [14C]mannose, [3H]glucosamine, and [32P]phosphate. Newly synthesized gag polyproteins which incorporated label, and the modified products derived from them, were identified by immunoprecipitation of cell lysates with anti-p30 rabbit serum, followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography. Pulse-chase experiments were carried out in the presence as well as in the absence of tunicamycin, an inhibitor of glycosylation. Among the three major polyproteins synthesized in the absence of tunicamycin, two were found to be glycosylated but not phosphorylated. These were designated gPr80gag and gP949"9. Both shared identical [3S]methionine peptides with Pr659'9 and p30. Of the two nonglycosylated precursors, Pr65gag and Pr759"9, only Pr659`9 was found to be detectably phosphorylated, and Pr759ag could be readily identified only when glycosylation was inhibited. On the basis of these results, a scheme for the post-translational modification of gag polyproteins is proposed. According to this scheme the gag gene-encoded polyproteins are processed from a common precursor, Pr759ag, by two divergent pathways: one leading through the intermediate Pr659'9 to internal virion components via cleavage and phosphorylation and the other via tunicamycin-sensitive mannosylation to the intermediate gPr809"9, which is further glycosylated to yield cell surface polyprotein gP94ga*.
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