Rat liver membranes contain a 120 kDa glycoprotein which serves as a substrate for the tyrosine kinases of the receptors for insulin and epidermal growth factor.

The receptors for insulin and epidermal growth factor possess tyrosine-specific protein kinase activity which may play a role in mediating the biological actions of these two peptides. We have identified a 120 kDa glycoprotein (pp120) in rat liver plasma membranes which can be phosphorylated by the insulin receptor in a cell-free system and in intact cultured hepatoma cells. In the present report, we have demonstrated in a cell-free system that solubilized epidermal growth factor receptors can phosphorylate tyrosine residues in pp120.