pp60src-dependent protein phosphorylation in membranes from Rous sarcoma virus-transformed chicken embryo fibroblasts

The putative transforming protein of S13 avian erythroblastosis virus is a transmembrane glycoprotein with an associated protein kinase activity

S13 is an avian retrovirus that transforms both fibroblasts and erythroblasts. The gene product responsible for the oncogenic effects of S13 is the env-related glycoprotein gp155. In this report we show that gp155 is a transmembrane protein with a 55-kDa cytoplasmic domain. Pulse-chase analysis shows that gp155 was cleaved posttranslationally into two glycosylated proteins, gp85 and gp70. In addition, we show that a tyrosine protein kinase activity is associated only with the gp70 protein in microsomes and in immune complexes.

Protein phosphorylation at tyrosine is induced by the v-erbB gene product in vivo and in vitro

The v-erbB gene product of avian erythroblastosis virus (AEV) has extensive homology with the receptor for epidermal growth factor (EGF). We report here that chicken embryo fibroblasts (CEF) transformed by AEV show enhanced tyrosine phosphorylation of a number of cellular polypeptides, including the 36 kd protein, which is phosphorylated in avian sarcoma virus-transformed fibroblasts, and the 42 kd protein, which is phosphorylated in mitogen-stimulated cells. CEF infected by AEV mutants with deletions in v-erbA showed enhanced tyrosine phosphorylation, whereas CEF infected by mutants with deletions in v-erbB did not. When membranes from AEV-transformed cells were incubated with gamma-32P-ATP, both the v-erbB gene product and the 36 kd cellular protein became phosphorylated at tyrosine. These results indicate that the v-erbB protein induces tyrosine phosphorylation in vivo and in vitro, and suggest that, like the EGF receptor, it possesses tyrosine-specific protein kinase activity.