Chemical Features of the Protein Kinase CK2 Polyamine Binding Site

Protein kinase CK2 is a ubiquitous eukaryotic Ser/Thr kinase whose catalytic activity is enhanced several times by polyamines. We have shown previously that the regulatory β-subunit of CK2 bears a polyamine binding site located in the region Asp51−Tyr110. In the present study, we have used spermine analogs to investigate the structural requirements of the CK2 polyamine binding site. We have observed a strong correlation between the stimulations of CK2 activity by all tested polyamines and their binding efficiencies to the enzyme. As a result, spermine was found to be the most efficient stimulator of the kinase activity and the best CK2 ligand. The effect of the pH on the stimulation of CK2 activity by spermine strongly suggests the involvement of ionic interactions between the positive charges of spermine and the negative charges of acidic amino acids of the β-subunit. Using a fusion protein made of MBP and the β-subunit region encompassing amino acid residues Asp51−Pro110, we have studied the binding of spermine as a function of the ionic strength. We show that this region delineates a functional and autonomous domain containing a binding site involved in the interaction with the four positive charges of spermine. Altogether, these results led to the elaboration of the first model defining the crucial structural parameters of a polyamine−protein interaction at the molecular level.