ABSTRACT
Purified rat epididymal nuclei possess a cyclic AMP-independent protein kinase activity that phosphorylates of casein. The enzymic activity was solubilized by treating intact nuclei with 1 M (NH4)2SO4. One major peak of kinase activity was obtained when the solubilized enzyme preparation was subjected to diethylaminoethyl-Sephadex chromatography. The activity of the kinase was dependent on a bivalent metal ion such as Mg2+, Co2+, Ca2+ or Mn2+. NaCl (0·3 M) caused a further activation (approx. 200%) of the metal (Co2+)- dependent enzyme. The apparent Km values of the enzyme for casein, ATP and Co2+ are approx. 0·6 mg/ml, 10 μΜ and 2·2 mM respectively. The enzyme was maximally active at pH 5·5. The enzyme showed high specificity for phosphorylation of the acidic protein casein but did not phosphorylate basic proteins, such as histones and protamine. The properties of the nuclear protein kinase were clearly different from those of the cytosolic enzymes previously characterized.