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

Abstract

Protein kinase CK2 is a tetrameric enzyme composed of two catalytic (α and/or α') subunits and two regulatory (β) subunits. Because CK2β is synthesized in excess of CK2α, we hypothesized that formation of CK2β 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 CK2β. The results of these cotransfection studies demonstrate that interactions between two CK2β subunits take place in the absence of CK2α. Together with results from previous biosynthetic labeling studies, these results suggest that formation of CK2β homodimers occurs before incorporation of catalytic subunits of CK2 into CK2 complexes. We also cotransfected Cos-7 cells with a deletion fragment of CK2β (i.e. Myc-β1- 166) together with full-length hemagglutinin (HA)-tagged CK2β and/or CK2α'. Although complexes between Mycβ1-166 and HA-β were readily detected, we obtained no evidence of direct interactions between Myc-β1-166 and HA- CK2α'. These results suggest that residues within the N-terminal 166 amino acids of CK2β are sufficient for interactions between CK2β subunits, whereas the C-terminal domain of CK2β is required for complex formation with the catalytic subunits of CK2. Finally, we observed that expression of full- length HA-β promotes phosphorylation of Myc-β1-166 by HA-CK2α'.