Activity of Recombinant α and β Subunits of Casein Kinase II from Xenopus laevis

Abstract

Casein kinase II (CKII) is a ubiquitous protein kinase, found predominantly in cell nuclei, which has two subunits in a tetrameric α2β2 or αα′2 conformation. The catalytic center is present in the α subunit which is active by itself while β is a regulatory subunit that can greatly enhance the activity of α. The cDNA genes of Xenopus laevis coding for the α and β subunits of CKII have been expressed in Escherichia coli and extensively purified. The recombinant subunits reconstitute a fully active holoenzyme when incubated in stoichiometric amounts. Mutations that change serines in positions 2 and 3 of the β subunit for glycines completely eliminate the autophosphorylation site present in this subunit but do not significantly affect the capacity of β to activate α. A fusion protein composed of glutathione transferase linked to the X. laevis CKII β subunit can also activate α. This fusion protein binds to glutathione-agarose beads and can mediate the binding of the α subunit to this matri