The linear C-terminal regions of epidermal growth factor (EGF) and transforming growth factor-alpha bind to different epitopes on the human EGF receptor

Epidermal growth factor (EGF) and transforming growth factor-alpha (TGF alp ha) bind with similar affinities in a competitive fashion to the human EGF receptor, and basically induce similar mitogenic responses. In spite of the fact that EGF and TGF alpha are structurally alike, it is still not clear if the two growth factors bind the receptor in an identical manner. The obs ervation that the 13A9 antibody blocks binding of TGF alpha, but not that o f EGF, to the human EGF receptor [Winkler, O'Connor, Winget and Fendly (198 9) Biochemistry 28, 6373-6378] suggests that their binding characteristics are not identical. In the present study we have made use of a set of EGF/TG F alpha chimaeric molecules to show that the 13A9 antibody blocks receptor binding of ligands with TGF alpha sequences, but not of ligands with EGF se quences, in their C-terminal linear regions. Using HaCaT human keratinocyte cells in culture, it was determined that ligands that are able to bind the EGF receptor in the presence of 13A9 are also able to induce calcium relea se from intracellular stores in these cells, indicating that these ligands have the ability to activate the EGF receptor in the presence of the antibo dy. From these data it is concluded that the flexible C-terminal linear dom ains of EGF and TGF alpha bind to separate sequences on the EGF receptor, s uch that the binding domain of TGF alpha, but not that of EGF, overlaps wit h the binding epitope of the 13A9 antibody.