The regulatory β subunit of protein kinase CK2 mediates formation of tetrameric CK2 complexes

Protein kinase CK2 is a tetrameric enzyme composed of two catalytic (alpha and/or alpha') subunits and two regulatory (beta) subunits. Because CK2 beta is synthesized in excess of CK2 alpha, we hypothesized that formation of CK2 beta homodimers precedes the incorporation of the catalytic subunits of CK2 into complexes. To test this hypothesis, we cotransfected cells with two epitope-tagged variants of CK2 beta. The results of these cotransfection studies demonstrate that interactions between two CK2 beta subunits take place in the absence of CK2 alpha. Together with results from previous biosynthetic labeling studies, these results suggest that formation of CK2 beta homodimers occurs before incorporation of catalytic subunits of CK2 into CK2 complexes. We also cotransfected Cos-7 cells with a deletion fragment of CK2 beta (i.e. Myc-beta 1-166) together with full-length hemagglutinin (HA)-tagged CK2 beta and/or CK2 alpha', Although complexes between Myc-beta 1-166 and HA-beta were readily detected, we obtained no evidence of direct interactions between Myc-beta 1-166 and HA-CK2 alpha', These results suggest that residues within the N-terminal 166 amino acids of CK2 beta are sufficient for interactions between CK2 beta subunits, whereas the C-terminal domain of CK2 beta is required for complex formation with the catalytic subunits of CK2. Finally, we observed that expression of full-length HA-beta promotes phosphorylation of Myc-beta 1-166 by HA-CK2 alpha'.