Virus-coded origin of a 32,000-dalton protein from avian retrovirus cores: structural relatedness of p32 and the beta polypeptide of the avian retrovirus DNA polymerase

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Abstract
A 32,000 dalton protein (p32) located in avian retrovirus cores was immunoprecipitated from [35S]methionine-labeled avian myeloblastosis virus (AMV) propagated in cultured chicken embryo fibroblast cells by an antiserum preparation (sarc III) derived from tumor-bearing hamsters injected with cloned and passaged cells from an avian sarcoma virus-induced primary hamster tumor. Since sarc III serum apparently contained antibodies only to virus-coded proteins and not to chicken cellular proteins, the immunoprecipitation of p32 from AMV by sarc III serum strongly suggested that p32 is virus coded. The origin of p32 was more definitively established by demonstrating the existence of a structural relationship between p32 and the AMV DNA polymerase. AMV p32 cross-reacted with the .beta. polypeptide of AMV .alpha..beta. DNA polymerase in radioimmunoprecipitation and radioimmunoprecipitation inhibition assays, indicating that p32 and .beta. share common antigenic determinants. This relationship was clarified by sodium dodecyl sulfate-polyacrylamide gel electrophoretic analysis of the peptides generated by limited proteolytis of 125I-labeled AMV DNA polymerase polypeptides and of 125I-labeled AMV p32 by chymotrypsin or Staphylococcus aureus V-8 protease. The peptides which appeared during proteolytic digestion of p32 were a subset of those produced by digestion of the .beta. polypeptide; however, p32 had no discernible peptides in common with the .alpha. polypeptide. All of the peptides produced by limited proteolysis of .beta. were present in the digests of either p32 or .alpha.. Apparently, p32 is derived by cleavage of the .beta. polypeptide of AMV DNA polymerase, presumably at a site near or identical to that at which .alpha. is generated from .beta. by proteolytic cleavage.