| Abstract: | Two retroviral protein-tyrosine kinases, v-src and v-ros, have been reported to possess phosphatidylinositol (PtdIns) kinase activity. Because the epidermal growth factor (EGF) receptor is a protein-tyrosine kinase with structural homology to p60v-src and because EGF stimulates PtdIns turnover in A431 cells, the EGF receptor has been examined for PtdIns kinase activity. Preparations of the EGF receptor, isolated from A431 cells and purified by two different methods of affinity chromatography, possessed an associated PtdIns kinase activity. This activity which co-purified with the EGF receptor represented only about 2% of the total PtdIns kinase activity of A431 membranes, and there was no correlation between the number of EGF receptors and the amount of PtdIns kinase activity in membranes from various cell types. A peptide substrate, angiotensin II, and PtdIns did not compete with each other as substrates for the protein-tyrosine and PtdIns kinase activities of the EGF receptor. When self-phosphorylated EGF receptor was fractionated by Sephacryl S-300 gel permeation chromatography, the peak of PtdIns kinase activity was separated from the comigrating peak of protein-tyrosine kinase activity and the self-phosphorylated EGF receptor. These results indicate that the protein-tyrosine kinase and PtdIns kinase activities which co-purify with the EGF receptor reside on different molecules. Angiotensin II and PtdIns did not compete as substrates for p60v-src isolated by immunoabsorption with a monoclonal antibody, suggesting that PtdIns kinase activity may also not be intrinsic to p60v-src. |
