[The tyrosine kinase activity of the internalized epidermal growth factor receptor in A-431 cells].

The interaction between epidermal growth factor (EGF) and specific membrane receptors in A-431 cells results in stimulation of its tyrosine kinase (TK) activity. The intrinsic tyrosine kinase phosphorylates both the receptor itself and different intracellular substrates. Upon EGF binding, clustering and internalization of EGF-receptor (EGF-R) complexes are observed. In the present study by incubating A-431 cells with EGF at 4 or at 37 degrees C, eluting surface-bound EGF we were able to compare the TK-activity of internalized EGF receptors to those localized on the plasma membrane. By the method of the phosphorylation of exogenic substrate poly (Glu/Tyr) 4:1 in vitro it was shown that the total TK-activity was equal to 3 microM PO4- incorporated/mg protein/min. Under conditions, when 40% EGF-R complexes were internalized their activity consisted of 46% of the total after inactivation 60% surface-bound receptors. We draw a conclusion that the TK-activity of EGF-receptors is remained during their internalization.