Epidermal growth factor (EGE) reduces the phosphorylation of a 50KD protein in A431 cells

Previous reports have shown that EGF induces the phosphorylation of the EGF receptor. In the present study the authors report the EGF sensitive reduction of the phosphorylation of a 50KD protein in solubilized A431 membranes. Membranes were prepared from A431 cell using a technique previously reported. Phosphorylation as determined by PAGE and autoradiography demonstrated an EGF sensitive phosphorylation of the EGF receptor at 170 and 150KD. A 50KD protein was phosphorylated in the absence of EGF. The presence of EGF decreased the phosphorylation of a 50KD protein. Prephosphorylation of the 50KD protein and the addition of EGF for increasing periods of time demonstrated no change in the phosphorylation level. These data suggest: (1) EGF reduces the phosphorylation of a 50KD protein. (2) The addition of EGF for increasing time periods with no change in the 50KD phosphorylation level might indicate that EGF is inhibiting a kinase rather than stimulating a phosphatase