In vivo phosphorylation of the 170-kDa form of eukaryotic DNA topoisomerase II. Cell cycle analysis

We have examined the level of incorporation of 32P into DNA topoisomerase II in vivo in chicken lymphoblastoid cells that were fractionated into the various cell cycle phases by centrifugal elutriation. We find that topoisomerase II is phosphorylated in vivo, with the level of incorporation being approximately 3.5-fold higher in the G2 + M fraction than earlier in the cell cycle. Our antibody studies have revealed that topoisomerase II antigen exists as a number of discrete polypeptide species in these cells. Of these, the 170-kDa intact polypeptide is phosphorylated approximately 4.5-fold more than several antigenic fragments that actually comprise the bulk of the topoisomerase II antigen in these cells at mitosis. Phosphorylation of the 170-kDa form of the enzyme may be involved in activation of the enzyme for its role in the disjunction of sister chromatids at anaphase.