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Processing of the env Gene Products of Moloney Murine Leukaemia Virus

¹ Department of Tumor Virology, The University of Texas at Houston, M. D. Anderson Hospital and Tumor Institute, Houston, Texas 77030, U.S.A.
and² National Cancer Institute, Laboratory of DNA Tumor Viruses, Bethesda, Maryland 20205, U.S.A.

The initial env gene polyproteins present in Moloney murine leukaemia virus (M-MuLV)-infected NIH/3T3 cells were examined to determine their relationship to each other as well as their role in generating env gene products gp70, p15(E) and p12(E). Steady-state labelling with [³H]glucosamine revealed anti-gp69/71 immunoprecipitable proteins of mol. wt. 93000 (gPr93^env), 83000 (gPr83^env) and 70000 (gp70), whereas similar labelling with [³H]fucose showed only two bands of anti-gp69/71 immunoprecipitable radioactivity migrating in SDS-polyacrylamide gels with gPr93^env and gp70. Pulse-chase experiments employing [³H]leucine labelling instead of labelled sugars failed to detect gPr93^env using similar techniques. The gPr83^env was the only polypeptide detected in 15 min [³H]leucine pulselabellings, whereas gPr83^env, gp70, p15(E) and p12(E) were detected in chase experiments using appropriate antisera in immunoprecipitation experiments. Pretreatment of infected cells with tunicamycin, an inhibitor of glycosylation, allowed the synthesis of a major band at mol. wt. 62000 (Pr62^env) and a minor band of 73000 mol. wt. at the expense of gPr83^env. In pulse-chase experiments conducted in the presence of tunicamycin, Pr62^env increased during the early chase period but disappeared during the later stages of the chase. No product of Pr62^env was detected. Cation-exchange chromatography of tryptic digests of radioactive tyrosine-labelled gPr83^env, Pr62^env and gp70 showed sequence relationships among the three proteins. Comparison of the two-dimensional fingerprints of [³H]leucine-labelled gPr83^env and the mature proteins gp70 and p12(E) support their precursor-product relationship. Of interest is the observation that gp70 and p12(E) seemed to share a few leucine-containing tryptic peptides. These results provide strong evidence that gPr83^env is the primary product of the env gene which, upon tunicamycin treatment, is synthesized as a subglycosylated protein, Pr62^env. It appears that gPr83^env undergoes further modification of its core oligosaccharide structure as detected by fucosylation to yield gPr93^env. Our inability to detect gPr93^env by [³H]leucine labellings suggests a close chronological relationship between fucosylation and cleavage of the precursor polyprotein, suggesting that cleavage of gPr93^env yields gp70 and p15(E). The latter is further cleaved to yield p12(E) plus a polypeptide containing the C-terminal end of p15(E).

Keywords: MuLV envelope proteins, gp70, p15(E), p12(E), M-MuLV env gene products

Received 5 August 1981; accepted 23 November 1981.

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