Identification of a cellular protein substrate phosphorylated by the avian sarcoma virus-transforming gene product

The avian sarcoma virus-transforming gene product (pp60 src) appears potentially able to mediate cell transformation via phosphorylation since it is tightly associated with a protein kinase activity. We have searched for and have been able to identify a normal cellular protein that appears to be a substrate of pp60 src. The phosphorylation of this protein (34K) is transformation-specific in ASV-transformed cells of both avian and mammalian origin. Moreover, the 34K polypeptide serves as a substrate for the pp60 src phosphotransferase activity in vitro and is phosphorylated at a site identical to the major site of phosphorylation in vivo. These data suggest that upon transformation the 34,000-dalton protein is phosphorylated directly as a result of pp60 src activity.