Biochemical properties of p15-associated protease in an avian RNA tumor virus

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Abstract
The viral structural protein p15 from avian myeloblastosis virus emerges from ion-exchange column chromatography along with a proteolytic activity. Protein p15 is apparently pure, as judged by sodium dodecyl sulfate[SDS]polyacrylamide gel electrophoresis and isoelectric focusing. Increase and decrease in proteolytic activity coincided exactly with increasing and decreasing amounts of p15 during ion-exchange chromatography and during size fractionation by gel filtration. The proteolytic activity cleaved various substrates such as bovine serum albumin, ovalbumin, concanavalin A and casein after denaturation by SDS and heat. Highest enzyme activity was observed around pH 5.7. As judged from its cleavage pattern and its response to proteolytic inhibitors, the proteolytic activity appears papain-like, and the protease responsible for it may be classified as a thiol protease. If added to immunoprecipitated viral polyprotein precursor Pr76, p15 resulted in cleavage of Pr76, which could be inhibited by antibodies against p15.