Phosphatidylinositol 3-phosphate is present in normal and transformed fibroblasts and is resistant to hydrolysis by bovine brain phospholipase C II.
The transforming protein of polyoma virus, middle T antigen, associates with two cellular enzymes, pp60c-src, a protein tyrosine kinase, and a phosphatidylinositol kinase that forms phosphatidylinositol 3-phosphate. The formation of a ternary complex of these proteins is essential for complete transformation and maximal tumor induction by the virus. A mutant virus encoding an altered middle T protein that activates pp60c-src but fails to bind phosphatidylinositol kinase is partially defective in transformation. We have confirmed, using an enzymological method, that the product of the in vitro reaction catalyzed by middle T-pp60c-src-phosphatidylinositol kinase complexes is phosphatidylinositol 3-phosphate (PtdIns(3)P), as previously reported (Whitman, M., Downes, C. P., Keeler, M., Keller, T., and Cantley, L. (1988) Nature 332, 644-646). PtdIns(3)P is present in normal as well as virus-infected and transformed cells at levels ranging from 0.6 to 2.6% of the major phosphatidylinositol phosphate isomer, phosphatidylinositol 4-phosphate (PtdIns(4)P). Steady-state levels of PtdIns(3)P do not appear to be affected by the expression of middle T in cells. PtdIns(3)P is not hydrolyzed by bovine brain phospholipase C II, which readily cleaves PtdIns(4)P and other phosphatidylinositols. This result underscores the likelihood that the metabolism of PtdIns(3)P is distinct from that of PtdIns(4)P and raises further questions regarding a possible role of PtdIns(3)P in normal and neoplastic cell growth.
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Department of Internal Medicine, Washington University School of Medicine, St. Louis, Missouri 63110.
Majerus
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Gorga
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Young
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Benjamin
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D000818:Animals; D001921:Brain / Q000201:enzymology*; D002460:Cell Line; D002471:Cell Transformation, Neoplastic; D002851:Chromatography, High Pressure Liquid; D005347:Fibroblasts / Q000378:metabolism; D006868:Hydrolysis; D007295:Inositol Phosphates / Q000302:isolation & purification / Q000378:metabolism; D007527:Isoenzymes / Q000378:metabolism*; D018129:Phosphatidylinositol Phosphates; D010716:Phosphatidylinositols / Q000378:metabolism*; D010738:Type C Phospholipases / Q000378:metabolism*
D007295:Inositol Phosphates; D007527:Isoenzymes; D018129:Phosphatidylinositol Phosphates; D010716:Phosphatidylinositols; C055525:phosphatidylinositol 3-phosphate; D010738:Type C Phospholipases