A surface plasmon resonance study of the interactions between the component subunits of protein kinase CK2 and two protein substrates,casein and calmodulin

Surface plasmon resonance has been used to study the interaction between the subunits composing protein kinase CK2 (two catalytic, -subunits, and two regulatory, -subunits), as well as the interaction of each subunit with two types of protein substrates, casein, the phosphorylation of which is activated by the regulatory subunit, and calmodulin, which belongs to the kind of substrates on which the catalytic subunit is down regulated by the regulatory subunit. The interaction of casein with the catalytic subunit differs from the interaction with the holoenzyme. Similarly to the interaction with the regulatory subunit, the catalytic subunit interacts with the protein substrate forming a very stable, irreversible complex. The reconstituted holoenzyme, however, binds casein reversibly, displaying a binding mode similar to that displayed by the regulatory subunit. The interaction of calmodulin with the catalytic subunit gives place, like in the case of casein, to an irreversible complex. The interactions with the regulatory subunit, and with the holoenzyme were practically negligible, and the interaction with the regulatory subunit disappeared upon increasing the temperature value to close to 30°C. The presence of polylysine induced a high increase in the extent of calmodulin binding to the holoenzyme. The results obtained suggest that CK2 subunit and protein substrates share a common, or at least an overlapping site of interaction on the catalytic subunit. The interaction between both subunits would prevent substrates from binding irreversibly to subunit, and, at the same time, it would generate a new and milder site of interaction between the whole holoenzyme and the protein substrate. The main difference between casein and calmodulin would consist in the lower affinity display by the last for the new site generated upon the binding of the regulatory subunit, in the absence of polycations like polylysine.