Interactions between subunits of protein kinase CK2 and their protein substrates influences its sensitivity to specific inhibitors

Five isoforms of CK2 may exist simultaneously in yeast cells: free catalytic subunits CK2 alpha', CK2 alpha and three holoenzymatic structures composed of alpha alpha'beta beta', alpha(2)beta beta' and alpha'(2)beta beta'. Each isolated and purified form exhibits properties typical for CK2, but they differ in substrate specificity as well as in sensitivity to specific modulators. All five isoforms of protein kinase CK2 from Saccharomyces cerevisiae were examined for their binding capacity with ATP/GTP and two commonly used ATP-competitive inhibitors TBB and TBI. Enzymes were tested with protein substrates differently interacting with CK2 subunits: Elf1, Fip1, Svf1, P2B and synthetic peptide. Obtained results show that K (m) for ATP varies from 2.4-53 mu M for Elf1/CK2 alpha' and Svf1/CK2 alpha, respectively. Similar differences can be seen in case when GTP was used as phosphate donor. The inhibitory effect depends on composition of CK2/substrate complexes. Highest sensitivity to TBB shows all complexes containing alpha alpha'(II2)-I-2' isoform with K (i) values between 0.2 and 1.1 mu M. The prospect that TBB and TBI could be utilized to discriminate between different molecular forms of CK2 in yeast cells was examined. Both inhibitors, TBB as well as TBI, decreases cell growth to extents devoting interactions with different CK2 isoforms present in the cell but the presence of beta/beta'-dimer has a high importance towards sensitivity. Conceivably, a given inhibitor concentration can inhibit only selected CK2-mediated processes in the cell.