Nuclear phosphoproteins : I. Isolation and characterization of a phosphoprotein fraction from calf thymus nuclei

Abstract

A phosphoprotein fraction has been isolated from calf thymus nuclei which contains about 1.3% alkali-labile phosphorus by wt. The enzymatic reaction in which serine and threonine residues in the protein are phosphorylated by the terminal phosphate of ATP (and other nucleoside triphosphates) has been extensively studied in vitro. The purified protein fraction contains endogenous kinase activity, so that protein phosphorylation proceeds without the need for any added enzyme. Radioactive phosphate groups incorporated in this reaction are stable to incubation in a medium containing an excess of unlabeled ATP, indicating that the enzyme activity responsible for the rapid "turnover" of protein-phosphate groups seen in intact nuclei is not an inherent part of the phosphoprotein. The reaction between ATP and the nuclear phosphoprotein is only slightly reversible, making it appear unlikely that this phosphoprotein functions as a high-energy phosphate reservoir or as a high-energy intermediate in oxidative phosphorylation.