Casein Kinase II Stabilizes the Activity of Human Topoisomerase IIα in a Phosphorylation-independent Manner

Previous reports have indicated that topoisomerase II (topo II) co-purifies with and is a substrate for casein kinase II. We have carried out a detailed study of the effect that purified casein kinase II has on the activity of purified recombinant human topo IIalpha. Co-incubation of topo IIalpha and casein kinase II led to an apparent activation of the topo IIalpha; however, in experiments in which topo IIalpha was preincubated at 37 degrees C with or without native casein kinase II prior to assaying for decatenation activity, it emerged that the kinase was exerting its "activating" function via a decrease in the rate of topo IIalpha enzyme inactivation during the incubation period. This stabilization of topo IIalpha by casein kinase II was ATP-independent and was observed in both mutated and truncated derivatives of topo IIalpha lacking the major casein kinase II phospho-acceptor sites, indicating the lack of a requirement for phosphorylation. Consistent with a nonenzymatic role for casein kinase II, stoichiometric quantities of kinase were required for topo IIalpha stabilization. These data indicate that casein kinase II plays a significant role in regulating human topo IIalpha protein action via stabilization against thermal inactivation.