Nonstructural Components Of Polyproteins Encoded By Replication-Defective Mammalian Transforming Retroviruses Are Phosphorylated And Have Associated Protein-Kinase Activity

Translation of Abelson murine leukemia (AbLV) 50 S genomic RNA by microinjection into Xenopus laevis oocytes results in synthesis of a protein indistinguishable in tryptic peptide composition, molecular weight, and type C viral structural components from the 120,000 M r viral-specific polyprotein expressed in AbLV-transformed mink cells. Post-translational cleavage of this polyprotein, designated AbLV P120, can be achieved by the introduction of avian myeloblastosis virus AMV RNA, which encodes an enzyme with specificity for viral polyprotein cleavage sites, to the oocyte translation system. Polyproteins encoded by AbLV, a second transforming isolate of mouse origin designated AK-T8 and feline sarcoma virus (FeSV) are each shown to be highly phosphorylated. Sites of phosphorylation are not restricted to their known phosphorylated structural components, but also include components encoded by their acquired cellular sequences. Immunoprecipitates of AbLV and FeSV polyproteins obtained both from cells and from pseudotype virions, are shown to contain protein kinase activity. Whether this activity is a function of the polyprotein itself or represents a cellular enzyme associated with the viral polyprotein in the form of an enzyme-substrate complex is, as yet, not resolved. By taking advantage of its efficient incorporation into pseudotype virions, AbLV P120 was purified to homogeneity by a combination of agarose gel filtration and ion-exchange chromatography.