Studies on the structure and function of the avian sarcoma virus transforming-gene product

The protein kinase activity associated with pp60src of a mutant of RSV temperature sensitive for transformation was shown to be sixfold more labile than that of its wild-type parent at 45 degrees C when pp60src's synthesized in vitro were compared. Thus, a mutant that is temperature sensitive for transformation has a temperature-sensitive protein kinase activity. Analysis of the levels of protein kinase activity in immunoprecipitates from cells infected with four different temperature-sensitive mutants of RSV led to the surprising finding that two mutants had barely detectable levels of protein kinase activity even at the permissive temperature, whereas two others had levels of activity at the nonpremissive temperatures that were as great as 40% that of wild-type pp60src. Protein kinase activity of pp60src of NY68 was partially renatured when cells were shifted from 41 degrees C to 36 degrees C. This reactivation occurred in less than an hour and did not require protein synthesis. It was found that pp60src synthesized in vitro is phosphorylated. Finally, the methionine-containing tryptic peptides of pp60sarc immunoprecipitated from uninfected chick cells were very similar to those of viral pp60src.