Molecular and Cellular Biochemistry 191: 75–83, 1999. © 1999 Kluwer Academic Publishers. Printed in the Netherlands. Interactions of protein kinase CK2 subunits

Several approaches have been used to study the interactions of the subunits of protein kinase CK2. The inactive mutant of CK2a that has Asp 156 mutated to Ala (CK2 a A156) is able to bind the CK2 b subunit and to compete effectively in this binding with wild-type subunits a and a ¢. The interaction between CK2 a A156 and CK2 b was also demonstrated by transfection of epitope-tagged cDNA constructs into COS-7 cells. Immunoprecipitation of epitope-tagged CK2a A156 coprecipitated the b subunit and vice-versa. The assay of the CK2 activity of the extracts obtained from cells transiently transfected with these different subunits yielded some surprising results: The CK2 specific phosphorylating activity of these cells transfected with the inactive CK2a A156 was considerably higher than the control cells transfected with vectors alone. Assays of the immunoprecipitated CK2 a A156 expressed in these cells, however, demonstrated that the mutant was indeed inactive. It can be concluded that transfection of the inactive CK2 a A156 affects the endogenous activity of CK2. Transfection experiments with CK2 a and b subunits and CK2 a A156 were also used to confirm the interaction of CK2 with the general CDK inhibitor p21WAF1/CIP1 co-transfected into these cells. Finally a search in the SwissProt databank for proteins with properties similar to those derived from the amino acid composition of CK2 b indicated that CK2 b is related to protein phosphatase 2A and to other phosphatases as well as to a subunit of some ion-transport ATPases. (Mol Cell Biochem 191: 75–83, 1999) Key words: protein kinase, CK2, p21, protein phosphatase 2