125I-EGF binding to cultured human fibroblasts: initial localization and fate of hormone determined by quantitative electron microscopic autoradiography

The potent mitogen epidermal growth factor (EGF) binds to specific receptors on human fibroblasts. In the present study we have used a quantitative EM autoradiographic approach to visualize the events involved in the binding process. When 125I-EGF is incubated at 40C for 120 min., labeled EGF primarily localizes to the plasma membrane of the fibroblast but when incubated at 370C for 120 min., over 2/3 of the labeled material is internalized by the cell. The internalized radioactivity is primarily localized to lysosomes. These studies demonstrate a temperature-dependent internalization of EGF following initial binding to specific plasma membrane receptors