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We have identified and substantially purified a tyrosine protein kinase from normal ,wqvine brain that is immunologically related to tl4e product of the Rous sarcoma virus oncogene (pp6O0V). The enzyme, a 61-kDa protein (p61), is solubilized with detergent from bovine cerebral cortical membranes and purified by column chromatography.In the purest preparations, this protein is phosphorylated only on tyrosine, but it can also be a substrate for serine-and threonine-specific protein kinases. The p61 protein phosphorylates the heavy chain of immunoglobulins from rabbits bearing Rous sarcoma virus-induced tumors (TBR IgO but not normal IgG. TBR IgG precipitates the 61-kDa phosptoprotein and protein kinase activity from purified preparations. The activity of the purified brain tyrosine kinase is 10 times higher in the presence of 7-10 mM Mn2+ and 6 mM Mg2' than it is with 6 mM Mg2+ alone. With Mn2+, the p61 enzyme has a Km for ATP of 2 pM. All preparations of p61 also contain a 64-kDa protein (p64) that is phosphorylated on tyrosine. Measurement of the Stokes radius of p61 and p64 by gel filtration shows that they are not physically associated in buffer containing the nonionic detergent Lubrol 12A9. The p64 protein is not precipitated by TBR IgG. We do not know whether p64 is only a substrate for the p61 tyrosine kinase or is itself a kinase.The discovery by Collett and Erikson (1) that the translation product of the transforming gene of Rous sarcoma virus is a protein kinase opened a new interest in the relationship between protein phosphorylation and the control of cell growth. Unlike previously known protein kinases, the protein kinase associated with the transforming protein specifically catalyzes the phosphorylation of tyrosine residues (2-4). Within a year after the observation of this unusual modification, several different classes of RNA tumor viruses were found to contain genes for transforming proteins that are tyrosine-specific protein kinases. Furthermore, the cellular genome contains DNA homologous to the viral oncogene, and uninfected cells express low levels of a protein closely related to the one encoded by the virus (reviewed in refs. 5 and 6). It is not known what the function of the protooncogene or of its product is in normal cells. It seems likely that it does serve some important role, since the protooncogene has been closely conserved over evolution and genes' homologous to avian and mammalian protooncogenes are found in fishes, Drosophila, and sponges (7-9). Although tyrosine kinase from virally transformed cells has been purified (10), the product of the normal cellular src gene has not yet been purified in quantities sufficient for biochemical studies (11). We now report the substantial purification from normal bovine brain of a 61-kDa tyrosine-specific protein kinase (p61) which seems to be immunologically related to the transforming protein of Rous sarcoma virus.The enzyme purifies together with a 64-kDa protein that is phosphorylated on a tyrosine residue (p64). The function of thi...

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2016-11-15T19:40:59