In vitro cleavage of avian retrovirus gag proteins by viral protease p15

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Avian myeloblastosis virus contains a proteolytic activity that can cleave in vitro the viral precursor polypeptide Pr76 gag. This substrate was prepared by radioactive labeling in vivo followed by immune precipitation, polyacrylamide gel electrophoresis in presence of sodium dodecyl sulfate, and elution from the gel. The major products of this reaction include the mature virion proteins p27 and p15, as well as an unstable fragment containing both of these proteins. Several other fragments are also formed, but mature p12 and the major p19 species are not. The cleavage of undenatured Pr76 bound to antibodies and formallin-fixed Staphylococcus yields similar fragments. The viral proteolytic enzyme is indistinguishable from the structural protein p15. Cleavage of Pr76 by p15 is optimal in the pH range 4–7 and is stimulated by salt. The activity of the enzyme is not inhibited by reagents specific for proteases with serine at their active sites, but is partially inhibited by reagents specific for thiols. Proteolysis is highly specific. Under the conditions used for Pr76 cleavage, p15 does not introduce breaks into mixtures of cellular proteins eluted in parallel to Pr76 from SDS-containing gels. However, it does fragment proteins that contain all or parts of the amino acid sequence of Pr76. These proteins include the precursor polypeptide for viral reverse transcriptase (Pr180 gag-pol), a virus-related protein found in uninfected gs + chick cells (P120), viral proteins from cells infected with avian erythroblastosis virus (P75) or with avian myelocytomatosis virus MC29 (P110).