Cytoplasmic and nuclear distribution of casein kinase II: characterization of the enzyme uptake by bovine adrenocortical nuclear preparation

Abstract

Casein kinase II (CK II) is a ubiquitous protein kinase that has been found in both nuclear and soluble subcellular fractions and whose precise cellular functions and mechanisms of control remain to be clarified. Using immunocytochemical localization, it was observed that the intracellular distribution of CK II exhibited a striking shift toward an increased nuclear concentration during active proliferation of bovine adrenocortical cells in primary culture. The interaction of CK II with purified adrenocortical cell nuclear preparation was thus examined in vitro. CK II was found to rapidly associated with nuclei in a temperature-dependent and saturable process, resulting in a tight binding of the kinase to nuclear components, as shown by various extraction procedures. This association resulted in a concentration of the kinase in the nuclear preparation about 100-fold that in the medium and exhibited two types of binding sites with Ka of 109 and 107 M-1, respectively. The nuclear CK II uptake was dependent upon the presence of ATP and was stimulated by a kinase activator such as spermine, although the enzyme activity did not appear to be required for the process. These observations would be in line with a pore-mediated, energy-dependent nuclear uptake of the kinase. Since a number of potential nuclear CK II targets have been reported, including the oncoprotein myc, it is suggested that the nuclear translocation of the kinase as characterized in vitro may have a biological significance in living cell, especially in the control of nuclear activities related to cell proliferation and the mechanism of action of growth factors.