In Search of Small Molecule Inhibitors Targeting the Flexible CK2 Subunit Interface

Protein kinase CK2 is a tetrameric holoenzyme composed of two catalytic (alpha and/or alpha') subunits and two regulatory (beta) subunits. Crystallographic data paired with fluorescence imaging techniques have suggested that the formation of the CK2 holoenzyme complex within cells is a dynamic process. Although the monomeric CK2 alpha subunit is endowed with a constitutive catalytic activity, many of the plethora of CK2 substrates are exclusively phosphorylated by the CK2 holoenzyme. This means that the spatial and high affinity interaction between CK2 alpha and CK2 beta subunits is critically important and that its disruption may provide a powerful and selective way to block the phosphorylation of substrates requiring the presence of CK2 beta. In search of compounds inhibiting this critical protein-protein interaction, we previously designed an active cyclic peptide (Pc) derived from the CK2 beta carboxy-terminal domain that can efficiently antagonize the CK2 subunit interaction. To understand the functional significance of this interaction, we generated cell-permeable versions of Pc, exploring its molecular mechanisms of action and the perturbations of the signaling pathways that it induces in intact cells. The identification of small molecules inhibitors of this critical interaction may represent the first-choice approach to manipulate CK2 in an unconventional way.