| Abstract: | A protein tyrosine kinase that phosphorylates both alpha and beta subunits of inactivated (Na+,K+)-ATPase from dog kidney was purified about 500-fold from Ehrlich ascites tumor cell membranes. The enzyme required divalent cations Mn2+, Mg2+, or Fe2+ but was inhibited by Cu2+ or Zn2+. The purified enzyme phosphorylated the beta subunit about five times faster than the alpha subunit of the (Na+,K+)-ATPase. The random polymer poly(Glu80Tyr20) was an excellent substrate while casein was only marginally phosphorylated. In contrast, the purified transforming gene product of Rous sarcoma virus phosphorylated all three substrates and the (Na+,K+)-ATPase was preferentially phosphorylated on the alpha subunit. The transforming gene product of Fujinami sarcoma visue and EGF receptor kinase from A431 cells phosphorylated (Na+,K+)-ATPase poorly whereas casein was an excellent substrate. The molecular weight of the partially purified protein tyrosine kinase from Ehrlich ascites tumor cells determined by gel filtration was about 60,000. One of two major phosphorylated phosphopeptides resolved by sodium dodecyl sulfate-polyacrylamide gel electrophoresis had an Mr of 60 kDa, thus suggesting that it might be the autophosphorylated protein tyrosine kinase. A phosphatase that hydrolyzes phosphorylated histones or poly(Glu80Tyr20) was partially purified from the same membrane. |
