The interaction of CK2α and CK2β, the subunits of protein kinase CK2, requires CK2β in a preformed conformation and is enthalpically driven

The protein kinase CK2 (former name: casein kinase 2'') predominantly occurs as a heterotetrameric holoenzyme composed of two catalytic chains (CK2 alpha) and two noncatalytic subunits (CK2 beta). The CK2 beta subunits form a stable dimer to which the CK2 alpha monomers are attached independently. In contrast to the cyclins in the case of the cyclin-dependent kinases CK2 beta is no on-switch of CK2 alpha; rather the formation of the CK2 holoenzyme is accompanied with an overall change of the enzyme's profile including a modulation of the substrate specificity, an increase of the thermostability, and an allocation of docking sites for membranes and other proteins. In this study we used C-terminal deletion variants of human CK2 alpha and CK2 beta that were enzymologically fully competent and in particular able to form a heterotetrameric holoenzyme. With differential scanning calorimetry (DSC) we confirmed the strong thermostabilization effect of CK2 alpha on CK2 beta with an upshift of the CK2 alpha melting temperature of more than 9 degrees. Using isothermal titration calorimetry (ITC) we measured a dissociation constant of 12.6 nM. This high affinity between CK2 alpha and CK2 beta is mainly caused by enthalpic rather than entropic contributions. Finally, we determined a crystal structure of the CK2 beta construct to 2.8 angstrom resolution and revealed by structural comparisons with the CK2 holoenzyme structure that the CK2 beta conformation is largely conserved upon association with CK2 alpha, whereas the latter undergoes significant structural adaptations of its backbone.