The autophosphorylation and p34 cdc2 phosphorylation sites of casein kinase-2 β-subunit are not essential for reconstituting the fully-active heterotetrameric holoenzyme

Two mutants of human casein kinase-2 β-subunit with short deletions at either their amino (Δ1–4) or carboxy (Δ209–215) terminal side have been created that have lost the capability to undergo autophosphorylation and p34 cdc2 mediated phosphorylation, respectively. Both mutants give rise to reconstituted CK2 holoenzymes displaying basal catalytic activity, thermostability and responsiveness to polylysine, identical to those of wild-type holoenzyme, whose reconstitution, moreover, is not affected by previous phosphorylation of the β-subunit at either its N-terminal or C-terminal sites. Unlike the wild-type β and β(Δ209–215), however, β(Δ1–4) fails to confer to the reconstituted holoenzyme the typical responsiveness to NaCl stimulation. These results suggest that while neither the autophosphorylation nor the p34 cdc2 phosphorylation sites are required for conferring a stable structure and full catalytic activity to CK2, the autophosphorylation site is implicated in the NaCl-dependent fine tuning of CK2 activity.