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 th e subunits composing protein kinase CK2 (two catalytic, alpha -subunits, an d two regulatory, beta -subunits), as well as the interaction of each subun it with two types of protein substrates, casein, the phosphorylation of whi ch 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 subun it differs from the interaction with the holoenzyme. Similarly to the inter action with the regulatory subunit, the catalytic subunit interacts with th e protein substrate forming a very stable, irreversible complex. The recons tituted holoenzyme, however, binds casein reversibly, displaying a binding mode similar to that displayed by the regulatory subunit. The interaction o f calmodulin with the catalytic subunit gives place, like in the case of ca sein, to an irreversible complex. The interactions with the regulatory subu nit, and with the holoenzyme were practically negligible, and the interacti on with the regulatory subunit disappeared upon increasing the temperature value to close to 30 degreesC. The presence of polylysine induced a high in crease in the extent of calmodulin binding to the holoenzyme. The results o btained suggest that CK2 beta 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 alpha subunit, and, at the same time, it would generate a new and milder site of interaction between the whole holoenzyme and the pro tein substrate. The main difference between casein and calmodulin would con sist in the lower affinity display by the last for the new site generated u pon the binding of the regulatory subunit, in the absence of polycations li ke polylysine.