Tyrosine kinase activity of internalized epidermal growth factor receptors

The functional state of internalized epidermal growth factor (EGF) receptors is reviewed. It is shown that in A431 cells internalized EGF-receptor complexes remain in an undissociated and nondegraded state for a long time. The internalized EGF-receptor complexes retain activated tyrosine kinase activity capable of autophosphorylation and phosphorylation of exogenous substrates. It is concluded that the activated tyrosine kinase of growth factor receptors is translocated from the plasma membrane into the cytoplasm and that the activated state is maintained for long enough to allow phosphorylation of intracellular substrates which may be inaccessible to the kinase while the latter is associated with the membrane.