Expression of the Bottom-Component RNA of Cowpea Mosaic Virus: Evidence that the 60-Kilodalton VPg Precursor Is Cleaved into Single VPg and a 58-Kilodalton Polypeptide.

In cowpea protoplasts infected with cowpea mosaic virus, a bottom-component (B) RNA-encoded 60-kilodalton (60K) polypeptide is synthesized, which is membrane-bound and represents the direct precursor to the genome-bound protein VPg. The relationship between this VPg precursor and other B-RNA-encoded polypeptides was studied. Digestion of the B-RNA-encoded 170K and 84K polypeptides with Staphylococcus aureus protease V8 and subsequent analysis of the generated peptides with antiserum against VPg showed that a VPg sequence resides internally in these polypeptides. Furthermore, a new B-RNA-encoded polypeptide was detected, with a size of 58K, which differed from the 60K polypeptide only in the lack of VPg sequences. A model is presented in which the 60K VPg precursor is generated from the 200K primary translation product from B RNA and further processed to a 58K polypeptide and single VPg.