Structural components of mouse mammary tumor virus. I. Polypeptides of the virion.

Mouse mammary tumor virus (mMTV) obtained from MJY-alpha cell cultures and analyzed by polyacrylamide gel electrophoresis possesses four major polypeptides and six to eight minor components. Three of the major proteins, with estimated molecular weights of 60,000, 52,000 and 37,000, are glycoproteins, as demonstrated by labeling with [³H]glucosamine. Labeling with [³⁵S]sulfate revealed significant amounts of sulfate associated with the 60,000 (gp60) and 52,000 (gp52) glycoproteins, whereas sulfate was minimally incorporated into the 37,000 (gp37) dalton glycoprotein. At least three glycopeptide species containing both [³H]glucosamine and [³⁵S]sulfate labels were obtained after extensive Pronase digestion of mMTV, indicating that [³⁵S]sulfate is covalently linked to the carbohydrate component of mMTV glycoproteins. Variations in the polypeptide pattern of mMTV were observed when virions were grown and labeled under different culture conditions. The amount of gp60 decreased substantially, with an apparent increase in a minor polypeptide at 33,000 daltons, if virions were harvested from stationary cultures or if culture medium was not changed daily during viral harvests. When virions containing gp60 were incubated with medium from stationary cultures or with stationary cell layers, the level of gp60 decreased with a corresponding increase in the amount of radioactivity at 33,000 daltons. Cleavage of gp60 and gp52 was demonstrated by treatment of purified mMTV virions with trypsin or alpha-chymotrypsin (1–150 μg/ml) for 1 min to 3 hr. Virions were morphologically unaltered after incubation with either protease; however, both gp60 and gp52 were absent from the polypeptide patterns. The data suggest that gp52 is cleaved to form 33,000, 22,000, and possibly 37,000 dalton glycoproteins which remain associated with virions. Other mMTV virion polypeptides were resistant to treatment with protease.