Active DNA Topoisomerase IIα Is a Component of the Salt-stable Centrosome Core

Recently, we reported that the monoclonal antibody specific for human DNA topoisomerase IIα, Ki-S1, stains not only the nuclei of human A431 cells but also extranuclear structures suggestive of centrosomes (Meyer, K. N., Kjeldsen, E., Straub, T., Knudsen, B. K., Kikuchi, A., Hickson, I. D., Kreipe, H., and Boege, F. (1997) J. Cell Biol. 136, 775–788). Here, we confirm colocalization of Ki-S1 with the centrosomal marker γ-tubulin. In addition, we show labeling of centrosomes by peptide antibodies against the N and C termini of human topoisomerase IIα. Probing Western blots of isolated centrosomes with topoisomerase IIα antibodies, we demonstrate a protein band of 170 kDa. Moreover, isolated centrosomes exhibited DNA decatenation and relaxation activity correlated to the amount of topoisomerase IIα protein in the same way as seen in the pure recombinant enzyme. Topoisomerase IIα epitopes could not be removed from centrosomes by salt extraction, DNase treatment, or RNase treatment, procedures that completely removed the enzyme from nuclei. Taken together, these observations suggest that active topoisomerase IIα is bound tightly to the centrosome in a DNA-independent manner. Because such centrosomal topoisomerase IIα was also present in quiescent lymphocytes devoid of topoisomerase IIα in the nuclei, we assume that it might be a long-lived storage form.