EGF is internalized and degraded

Thirty years

ago, cell biologists were convinced that protein hormones and cells had a superficial relationship. Although steroid hormones such as testosterone could squeeze through the cell membrane to deliver commands, their protein counterparts never got beyond receptors on the cell's surface. Graham Carpenter and Stanley Cohen (both then at Vanderbilt University) overturned the conventional wisdom with their study of epidermal growth factor (EGF), a protein hormone that spurs fibroblasts to duplicate their DNA and divide.

When the pair steeped human fibroblasts in EGF tagged with radioactive iodine, they found that the amount of radioactivity affixed to the cell's surface peaked after ∼30 to 40 min, and then plummeted (Carpenter and Cohen, 1976). To track the missing radioactivity, Carpenter and Cohen soaked cells in labeled EGF before shifting them to a hormone-free mixture. The hot iodine returned to solution, the researchers discovered, but not as...