A majority of casein kinase II alpha subunit is tightly bound to intranuclear components but not to the beta subunit

Nuclear casein kinase II (CK II) was purified from an epithelial cell line of Chironomus tentans and characterized. The intracellular distribution of CK II and its two intracellular subunits (alpha and beta) was analysed by immunoblotting. The apparent molecular weights of the alpha and beta subunits were estimated to be 36 and 28 kDa, respectively. Like other purified CK II preparations, CK II from Chironomus tentans is able to use ATP or GTP for phosphorylation of casein and phosvitin, and its activity is strongly inhibited by heparin and by the transcription inhibitor 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB). Due to their differential solubilities in NaCl and (NH4)2SO4 solutions, individual alpha and beta subunit pools could be detected. More than 85% of the total immunostainable alpha subunit and essentially all immunoreactive individual beta subunit and heterooligomeric enzyme molecules were localised to the nucleus. Unexpectedly, more than 80% of this nuclear alpha subunit was insoluble in 0.35 M NaCl, while all individual beta subunit and heterooligomeric enzyme molecules were solubilized under the same conditions. Of the 0.35 M NaCl soluble kinase fractions, the active multisubunit form of CK II precipitated in 50% (NH4)2SO4 and could thus be separated from the free beta subunit, which precipitated at 60% and 80% (NH4)2SO4. These results suggest that a major portion of the nuclear CK II alpha subunit does not form heterooligomeric structures with the beta subunit, but binds tightly to nuclear components.