Partial phosphorylation in vivo of the avian retrovirus pp32 DNA endonuclease

Avian retrovirus pp32, a DNA endonuclease which is structurally related to the avian retrovirus DNA polymerase beta polypeptide, has been demonstrated to be partially phosphorylated in vivo. Unlabeled or [35S]methionine-labeled pp32 from avian sarcoma virus or avian myeloblastosis virus migrated as an electrophoretic doublet on discontinuous sodium dodecyl sulfate-polyacrylamide slab gels. However, pp32 immunoprecipitated from avian sarcoma virus labeled in vivo with [32P]orthophosphoric acid migrated as a single band, which co-electrophoresed with the slower-moving band of the doublet represented by unlabeled or 35S-labeled pp32. The presence of a slower-migrating phosphorylated band in pp32 suggests that the observed electrophoretic heterogeneity of purified pp32 is due to partial phosphorylation. Tryptic peptide analysis of 32P-labeled avian sarcoma virus beta and pp32 demonstrated that all the three labeled peptides in the beta polypeptide were also present in pp32. However, pp32 had one tryptic peptide which was preferentially labeled in comparison to the comigrating peptide found in beta digests, suggesting that phosphorylation may play a role in the processing of pp32 from beta or in the regulation of its associated DNA endonuclease activity.