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 distri bution of CK II and its two intracellular subunits (alpha and beta) wa s analysed by immunoblotting. The apparent molecular weights of the al pha and beta subunits were estimated to be 36 and 28 kDa, respectively . Like other purified CK II preparations, CK II from Chironomus tentan s is able to use ATP or GTP for phosphorylation of casein and phosviti n, and its activity is strongly inhibited by heparin and by the transc ription inhibitor 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DR B). Due to their differential solubilities in NaCl and (NH4)(2)SO4 sol utions, individual alpha and beta subunit pools could be detected. Mor e than 85% of the total immunostainable alpha subunit and essentially all immunoreactive individual beta subunit was insoluble in 0.35 M NaC l, while all individual beta subunit and heterooligomeric enzyme molec ules were solubilized under the same conditions. Of the 0.35 M NaCl so luble kinase fractions, the active multisubunit form of CK II precipit ated in 50% (NH4)(2)SO4 and thus could be separated from the free beta subunit, which precipitated at 60% and 80% (NH4)(2)SO4. 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.