CK2, a protein kinase of the next millennium

Abstract

Protein kinase CK2 is a ubiquitous eukaryotic protein kinase composed of two catalytic subunits, α and/or α′, and two regulatory subunits, β. In order to define similarities and dissimilarities between the α and α ′ catalytic subunits, which might account for their particular cellular functions, different forms of the enzyme were expressed in Sf9 cells and their properties determined. Both catalytic subunits were expressed separately, and also along with the regulatory β subunit, in order to obtain free α and α′, as well as α2β2 and α′2β2 forms. Our results confirm that the b subunit acts to stabilize the α and α′ subunits and also influences the substrate specificity and kinetic properties of the enzyme. Although significant differences between the specificities of holoenzymes α2β2 and α′2β2 as determined using a number of substrates were not detected, autophosphorylation studies on α2β2 and α′2β2 revealed significant differences in this property. The regulatory subunit β was phosphorylated less rapidly by the α′ subunit than by the α subunit, and the extent of phosphorylation of β by α was also greater than that of β by α′. It was also noted that the thermo-stability and the extent of its activation by NaCl were greater for αβ2 than for α′β2. These different properties may relate to distinct functions of the two form of CK2.