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Synthesis and Glycosylation of Polyprotein Precursors to the Internal Core Proteins of Friend Murine Leukemia Virus

Synthesis and post-translational processing of murine leukemia virus proteins were analyzed in a murine cell line (Eveline) that produces large amounts of Friend lymphatic leukemia virus. Immunoprecipitation of l-[35S]methionine-labeled cell extracts demonstrated that several different virus-specific proteins antigenically related to the virion core (gag) proteins p12 and p30 become radioactive within 1 min of labeling and exhibit labeling kinetics characteristic of primary translation products. The most abundant of these were proteins with molecular weights of 75,000 and 65,000. There were, in addition, two large glycosylated polyproteins with apparent molecular weights of 220,000 and 230,000, which were precipitated by antisera to p30 or p12 but not by antiserum to the major envelope glycoproteins gp69/71. Several lines of evidence, including labeling with d-[3H]glucosamine and binding to insolubilized lectins, suggested that the 75,000-dalton internal core polyprotein is slowly processed to form a glycoprotein with an apparent molecular weight of 93,000. On the contrary, the 65,000-dalton protein appeared to be an immediate precursor to the virion core proteins. Its processing can involve intermediates containing p30 and p12 antigens with molecular weights of 50,000 and 40,000; however, the latter did not appear to be obligatory intermediates. The detection of the 40,000-dalton protein suggested that the genes for p30 and p12 are adjacent on the viral genome. These results indicated that there are several pathways of synthesis and post-translational processing of polyprotein precursors to the gag proteins and that several of these polyproteins are glycosylated. A comparison of gag precursor processing in rapidly growing, slowly growing, and stationary cells indicated that different pathways are favored under different conditions of cell growth. Our analysis of envelope glycoprotein synthesis has confirmed the existence of two rapidly labeled 90,000-dalton glycoproteins, which appear to be precursors to the envelope glycoproteins gp69/71.