The biosynthesis of oncovirus proteins.

The patterns of oncovirus protein biosynthesis are essentially similar for avian and mammalian viruses. In each case the four major internal structural proteins are synthesized as a precursor polypeptide of about 75 000 daltons, the product of the gag gene. Translation occurs on genome-sized mRNA. This polyprotein is cleaved in a series of steps to give the mature proteins. The mechanism and localization of cleavage have not yet been clarified. Viral reverse transcriptase, the product of the pol gene, also is translated on genome-sized mRNA as a precursor, which is a "read-through" product of the neighbouring gag gene. The two major envelope proteins are translated as a glycosylated precursor of apparent molecular weight about 90 000 from the env gene located on a sub-genomic RNA species. The precursor is transported to the plasma membrane where it may mark the site of virus budding. It is cleaved in transport or on the membrane, but the resulting two mature envelope proteins remain tied by disulfide bonds. Sarc, the protein product of the src gene that is responsible for transformation, is translated from a different viral mRNA than the structural proteins. Sarc has not been definitively characterized in any system.