Biochemical characterization of CK2α and α′ paralogues and their derived holoenzymes: evidence for the existence of a heterotrimeric CK2α′-holoenzyme forming trimeric complexes
Altogether 2 holoenzymes and 4 catalytic CK2 constructs were expressed and characterized i.e. CK2α21−335β2; CK2α′-derived holoenzyme; CK2α1−335; MBP-CK2α′; His-tagged CK2α and His-tagged CK2α′. 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°C. In the case of the two holoenzymes, the CK2α′-derived holoenzyme lost ca. 90% of its activity after 14 min, whereas CK2α21−335β2 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 CK2α′-derived holoenzyme eluted at a position corresponding to a molecular mass of 105 kDa which is significantly below the elution of the CK2α21−335β2 holoenzyme (145 kDa). Calmodulin was not phosphorylated by either CK2α21−335β2 or the CK2α′-derived holoenzyme. However, in the presence of polylysine only the CK2α21−335β2 holoenzyme could use calmodulin as a substrate such as the catalytic subunits, in contrast to the CK2α′-derived holoenzyme which only phosphorylated calmodulin weakly. This attenuation may be owing to a different structural interaction between the catalytic CK2α′ subunit and non-catalytic CK2β subunit.
KeywordsProtein kinase CK2 Isozymes Inhibitors Substrate
Maltose binding protein
We thank Drs. B. Boldyreff and B. Guerra for critically reading the manuscript and Hans H. Jensen for expert technical assistance. B.B.O is supported by grant no. DP06083 from the Danish Cancer Society, K.N from the Deutsche Forschungsgemeinschaft (DFG) grant no. NI 643/1-3 and O.G.I by grant no. 002521109210 from the Danish Cancer Society; grant no. 21-04-0517 from the Danish Research Agency, the NOVO Nordisk Foundation and from a donation dedicated to Cancer Research from Karen Marie Maaløe.
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