In vitro characterization of protein kinase CKII β mutants defective in β-β dimerization

Protein kinase CKII is composed of two catalytic (α or α′) subunits and two regulatory (β) subunits. The β subunit mediates tetramer formation through β-β homodimerization and α-β heterodimerization. In a previous study R26 and R75, point mutants of CKIIβ defective in β-β dimerization, were isolated. In the present work we characterized these CKIIβ mutants in vitro. Purified R26 and R75 bound to CKIIα but were defective in binding to CKIIβ. R75 stimulated the catalytic activity of CKII whereas R26 gave little stimulation, and poly-L-lysine increased the stimulation of catalytic activity by R26 or R75. Circular dichroism and intrinsic fluorescence data pointed to different conformational changes in R26 and R75. Molecular modeling of these mutants provides an explanation of the difference in their ability to interact with CKIIβ and to activate CKIIα.