Anti-pp60^src antibodies are substrates for EGF-stimulated protein kinase

Abstract

Epidermal growth factor (EGF) stimulates phosphorylation of its own receptor at a tyrosine residue¹. Similarly, the viral gene product pp60^src, which is responsible for cellular transformation by avian sarcoma virus (ASV), phosphorylates itself and immunoglobulin directed against pp60^src at tyrosine residues^2,3. This unusual site of phosphorylation catalysed by two membrane-associated protein kinases^4–7 involved in growth control prompted us to study the immunological relatedness of the EGF-stimulated protein kinase and the pp60^src. Using anti-pp60^src antisera, we attempted to immunoprecipitate the EGF-stimulated protein kinase solubilized from plasma membranes. We report here that neither the EGF-stimulated kinase nor the EGF receptor were immunoprecipitable by anti-pp60^src sera. However, anti-pp60^src IgG served as a specific substrate for the EGF-stimulated kinase, suggesting a close similarity between the EGF-stimulated kinase and pp60^src.