Production of PtdIns(4)P and PtdIns(4,5)P2 by plasma-membrane preparations from A431 cells was selectively stimulated in a dose-dependent manner by epidermal growth factor (EGF) in the presence of Na3VO4. Na3VO4 itself mimicked this effect, which was overcome after treatment by a specific phosphotyrosyl phosphatase isolated from A431 cells. PtdIns and PtdIns(4)P kinase activities were present in phosphotyrosyl-proteins isolated from EGF- and/or Na3VO4-stimulated A431 cells by immunoaffinity using an anti-phosphotyrosine antibody. These data suggest for the first time an EGF-dependent regulation of PtdIns 4-kinase and PtdIns(4)P 5-kinase activities by a mechanism involving a tyrosine-phosphorylation process.

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