Biochemical characterization of CK2alpha and alpha' paralogues and their derived holoenzymes: evidence for the existence of a heterotrimeric CK2alpha'-holoenzyme forming trimeric complexes.

Altogether 2 holoenzymes and 4 catalytic CK2 constructs were expressed and characterized i.e. CK2alpha(2)1-335 beta2; CK2alpha'-derived holoenzyme; CK2alpha1-335; MBP-CK2alpha'; His-tagged CK2alpha and His-tagged CK2alpha'. The two His-tagged catalytic subunits were expressed in insect cells, all others in Escherichia coli. IC50 studies involving the established CK2 inhibitors DMAT, TBBt, TBBz, apigenin and emodin were carried out and the Ki values calculated. Although the differences in the Ki values found were modest, there was a general tendency showing that the CK2 holoenzymes were more sensitive towards the inhibitors than the free catalytic subunits. Thermal inactivation experiments involving the individual catalytic subunits showed an almost complete loss of activity after only 2 min at 45 degrees C. In the case of the two holoenzymes, the CK2alpha'-derived holoenzyme lost ca. 90% of its activity after 14 min, whereas CK2alpha2(1-335) beta2 only showed a loss of ca. 40% by this time of incubation. Gel filtration analyses were performed at high (500 mM) and low (150 mM) monovalent salt concentrations in the absence or presence of ATP. At 500 mM NaCl the CK2alpha'-derived holoenzyme eluted at a position corresponding to a molecular mass of 105 kDa which is significantly below the elution of the CK2alpha(2)1-335 beta2 holoenzyme (145 kDa). Calmodulin was not phosphorylated by either CK2alpha2(1-335) beta2 or the CK2alpha'-derived holoenzyme. However, in the presence of polylysine only the CK2alpha(2)1-335 beta2 holoenzyme could use calmodulin as a substrate such as the catalytic subunits, in contrast to the CK2alpha'-derived holoenzyme which only phosphorylated calmodulin weakly. This attenuation may be owing to a different structural interaction between the catalytic CK2alpha' subunit and non-catalytic CK2beta subunit.