Immunoprecipitation of labeled extracts from murine leukemia virus-infected cells with antisera specific for internal structural (gag) proteins yields three major gag-related polyproteins with molecular weights of 180,000 (Pr180gag"P"), 80,000, and 65,000 (Pr65ag. It has been shown by others that Pr65'ag is the immediate precursor of the internal structural (gag) protein, and that Pr1809'w-l°i s the precursor to reverse transcriptase. In studies reported here, the 80,000dalton gag-related polyprotein from Moloney strain murine leukemia virus (M-MuLV)-infected cells was found to be glycosylated by the following criteria: (i) incorporation of [3H]mannose, (ii) a change in electrophoretic mobility upon digestion with endoglycosidase H, and (iii) a change in electrophoretic mobility when glycosylation was inhibited by treatment of the cells with tunicamycin during labeling. The 80,000-dalton gag polyprotein has therefore been designated GpP80'"9. The unglycosylated form of GpP80'ga was a polypeptide of 75,000 daltons. A comparison of [3H]mannose and [3H]galactose labeling experiments suggested that GpP809ga is further glycosylated to yield a glycopolypeptide of 95,000 daltons. This 95,000-dalton polypeptide is relatively rapidly cleaved to yield two glycopeptides of 55,000 and 40,000 daltons which are released into the cell culture fluid, as soluble proteins. Cell-free translation of M-MuLV genomic RNA resulted in two major gag-related products of 75,000 and 65,000 daltons. The 65,000-dalton gag-related cell-free translation product comigrated with Pr65'qg, and the 75,000-dalton cell-free product comigrated with the unglycosylated form of GpP809'9. Both of the gag-related cell-free translation products could be labeled with [3S]formyl methionine, which is incorporated only as the N-terminal amino acid during translation. Other investigators have shown that GpP809'9 and Pr659'9 differ at their N-termini, and these results combined with those reported here suggest that GpP809(l and Pr65gag are translated from two separate initiation sites in M-MuLV RNA. The virion proteins of RNA tumor viruses are produced in infected cells by cleavage of three polyprotein precursors (reviewed in 4, 7). All of the internal structural proteins, products of the gag gene, are initially present in a single precursor polyprotein which ranges in size from 60,000 to 80,000 daltons, and the individual internal structural proteins are produced by a series of proteolytic cleavages (12, 21, 23, 30, 37, 38). Viral reverse transcriptase, the product of the pol gene, is initially synthesized as part of a large 180,000-dalton polypeptide which also contains most, if not all, of the gag polypeptides (Prl80"g"w°") (12, 15). The envelope proteins, products of the env gene, are initially synthesized from subgenomic length mRNA (10, 11, 31) as a glycosylated polyprotein of approximately 80,000 to 90,000 daltons (6, 18, 22, 30, 39). In murine leukemia virus (MuLV)-infected cells, two major gag-related polyproteins are observed by pulse labeling: a polypept...
