Polypeptides of Mason-Pfizer monkey virus. II. Synthesis and processing of the env gene products.

Mason-Pfizer monkey virus (M-PMV), the prototype D-type retrovirus, encodes two glycosylated virion proteins, gp20 and gp70. The polyprotein precursor to these proteins was identified by immunoprecipitation of pulse labeled M-PMV-infected cells with an antiserum raised against gp70, the major glycoprotein of the virus. The relationship of this precursor to the two viral glycoproteins was verified by tryptic peptide mapping, which demonstrated that gp20 and gp70 were independent products of the env gene. The types and degree of glycosylation of the precursor and its products was investigated by tunicamycin inhibition of glycosylation, endo-beta-N-acetyl glucosaminidase H (Endo-H) and endo-beta-N-acetylglucosaminidase F (Endo-F) catalyzed removal of glycosylated residues. The results suggest that the precursor, a molecule with a mol wt of 86,000, is composed of approximately 55,000 Da of protein to which 14-15 oligosaccharide chains are attached. The precursor is cleaved post-translationally to yield the two glycoproteins of M-PMV, gp70 and gp20. Most, if not all, of the glycan units associated with the gp70 molecule are of the complex variety, as shown by their resistance to Endo-H cleavage. The gp20 molecule, on the other hand, appears to contain a single glycan unit predominantly of the high mannose type since this side chain is sensitive to digestion by Endo-H.