Casein kinase II copurifies with yeast DNA topoisomerase II and re-activates the dephosphorylated enzyme.

Mitotic division in yeast requires the activity of topoisomerase II, a DNA topology modifying enzyme that is able to disentangle sister chromatids after DNA replication. Previous work has shown that topoisomerase II is a phosphoprotein in intact yeast cells. We show here that when dephosphorylated in vitro, topoisomerase II is unable to cleave or decatenate kinetoplast DNA. An efficient kinase activity that modifies topoisomerase II on seven major sites was found to copurify with the enzyme purified from yeast. Characterization of this kinase, analysis of phosphotryptic peptides, and studies with a yeast mutant deficient in casein kinase II, indicate that the copurifying kinase is casein kinase II (CKII). Topoisomerase II itself has no self-phosphorylating activity. Modification of topoisomerase II by the copurifying kinase is sufficient to restore decatenation activity after dephosphorylation by alkaline phosphatase. The CKII target sites have been mapped to multiple serine and threonine residues on 4 tryptic fragments within the C-terminal 350 amino acids of yeast topoisomerase II. These results are consistent with a model in which the C-terminal domain of topoisomerase II is a negative regulatory domain that is neutralized by phosphorylation.