A 34-kd protein with strong homology to ras-like proteins inhibits epidermal growth factor activity.

Epidermal growth factor (EGF) and its analog, transforming growth factor-alpha, are felt to be important in oncogenesis. When malignant rabbit fibroma virus infects RK-13 rabbit kidney cells, a 34-kd protein that inhibits the effects of EGF on certain target cell lines is produced. We have purified this protein using high-pressure liquid chromatography and gel electrophoresis. This purified protein abolishes EGF-induced cellular proliferation. It also causes the EGF receptor-bearing A431 carcinoma cell line to stop proliferating in vitro. This purified 34-kd EGF inhibitor (EGFI) redirects cellular protein phosphorylation in the presence or absence of EGF. Whereas EGF increases phosphorylation of cellular proteins in normal rat kidney cells, clone 49F, and A431 EGFI generally decreases it. Both EGF and EGFI cause increased protein production in A431 and normal rat kidney cells. The major species of protein synthesized by cells seem invariant to EGFI, with or without EGF. The partial protein sequence of two fragments of EGFI shows striking similarity to two ras like proteins. Possible means by which such a ras-like protein might inhibit EGF-induced cellular proliferation are discussed. Therefore, a purified 34-kd ras-like protein inhibits EGF-induced cellular proliferation and changes the targets for cellular protein phosphorylation. Studies are in progress to characterize this protein further, both structurally and functionally.