High molecular weight precursor polypeptides to structural proteins of Rauscher murine leukemia virus

The synthesis of three structural proteins of Rauscher murine leukemia virus was examined by autoradiography of radiolabeled proteins immunoprecipitated from extracts of cells pulse-labeled with [³⁵S]methionine and fractionated by sodium dodecyl sulfate/polyacrylamide gel electrophoresis. Polypeptides carrying virus-specific antigenic determinants were immunoprecipitated with monospecific antiserum directed against either the Rauscher virus envelope glyco-protein gp69/71, the major core protein p30, or a small molecular weight internal protein 15. Analysis of polypeptides immunoprecipatated by anti-gp69/71 serum revealed a 90,000mmolecular weight protein as the immediate precuror of gp69/71. Very high molecular weight proteins of 260,000 and 350,000 that appeared to carry gp69/71 antigenicity were also detected. Labeling with ³H-labeled sugars revealed that the 90,000 molecular weight molecule was glycosylated but its carbohydrate moiety was deficient in fucose relative to the mature envelope protein. The synthesis of a 70,000 molecular weight polypeptide with gp69/71 antigenic determinants was observed when glycosylation was inhibited. Tryptic digest analysis confirmed the procursor-product relationships between the 70,000 molecular weight polypeptide, the 90,000 molecular weight protein and the mature envelope glycoprotein. Analysis of the polypeptides immunoprecipitated by anti-p30 serum and anti-p15 serum indicated that these virion proteins were both cleavage products of a 65,000 molecular weight precursor molecule. Tryptic digest analysis showed that the 65,000 molecular weight molecules precipitated by anti-p30 serum and anti-p15 serum were the same molecule and confirmed the precursor-product relationship of the 65,000 molecular weight precursor molecule to the virion protein p30. Possible very large precursors to the p30 and p15 proteins with molecular weights of 260,000 and 350,000 were also observed. These proteins migrated in electrophoretic gels parallel with the very large polypeptides immunoprecipitated by anti-gp69/71 serum. The 65,000 molecular weight precursor molecule was not detectable when viral-specific protein was synthesized in the presence of amino acid analogs which can inhibit precursor protein cleavage. However, a slightly larger polypeptide with a molecular weight of about 70,000 was observed. The concentrations of the very large possible precursors were not enhanced by amino acid analog incorporation. Released virions were found to contain precursor protein and intermediatesized cleavage products carrying antigenic determinants of virion proteins p30 and p15 in addition to the mature sized proteins. Precursor to gp69/71 was not detected in released virus particles.