The Ser/Thr kinase CK2 (previously called casein kinase 2) is composed of two catalytic chains (CK2 alpha) attached to a dimer of noncatalytic subunits (CK2 beta). CK2 is involved in suppression of apoptosis, cell survival, and tumorigenesis. To investigate these activities and possibly affect them, selective CK2 inhibitors are required. An often-used CK2 inhibitor is 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB). In a complex structure with human CK2 alpha, DRB binds to the canonical ATP cleft, but additionally it occupies an allosteric site that can be alternatively filled by glycerol. Inhibition kinetic studies corroborate the dual binding mode of the inhibitor. Structural comparisons reveal a surprising conformational plasticity of human CK2 alpha around both DRB binding sites. After local rearrangement, the allosteric site serves as a CK2 beta interface. This opens the potential to construct molecules interfering with the CK2 alpha/CK2 beta interaction.

The protein kinase CK2 (former name: ''casein kinase 2'') predominantly occurs as a heterotetrameric holoenzyme composed of two catalytic chains (CK2a) and two noncatalytic subunits (CK2b). The CK2b subunits form a stable dimer to which the CK2a monomers are attached independently. In contrast to the cyclins in the case of the cyclin-dependent kinases CK2b is no on-switch of CK2a; 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 CK2a and CK2b 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 CK2a on CK2b with an upshift of the CK2a melting temperature of more than 9°. Using isothermal titration calorimetry (ITC) we measured a dissociation constant of 12.6 nM. This high affinity between CK2a and CK2b is mainly caused by enthalpic rather than entropic contributions. Finally, we determined a crystal structure of the CK2b construct to 2.8 Å resolution and revealed by structural comparisons with the CK2 holoenzyme structure that the CK2b conformation is largely conserved upon association with CK2a, whereas the latter undergoes significant structural adaptations of its backbone.

The constitutively active Ser/Thr kinase CK2 (casein kinase 2) is used by tumor cells to acquire apoptosis resistance. CK2 exists as a heterotetrameric holoenzyme with two catalytic chains (CK2α) attached to a dimer of noncatalytic subunits (CK2β). A druggable cavity at the CK2β interface of CK2α allows the design of small molecules disturbing the CK2α/CK2β interaction and thus affecting activity, stability, and substrate specificity. We describe here the first structure of CK2α with an effective CK2β-competitive compound, namely, a 13-meric cyclic peptide derived from the C-terminal CK2β segment. Some well-ordered water molecules not visible in CK2 holoenzyme structures were detected at the interface. Driven mainly by enthalpy, the peptide binds with submicromolar affinity to CK2α, stimulates its catalytic activity, and reduces effectively the CK2α/CK2β affinity. The results provide a thermodynamic and structural rationalization of the peptide's CK2β-competitive functionality and pave thus the way to a peptidomimetic drug addressing the CK2α/CK2β interaction.

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