Cell Cycle Phase-specific Phosphorylation of Human Topoisomerase II

Abstract

Type II topoisomerases are essential for faithful cell division in all organisms. In human cells, the α isozyme of topoisomerase II has been implicated in catalyzing mitotic chromosome segregation via its action as a DNA unlinking enzyme. Here, we have shown that the enzymatic activity of topoisomerase IIα protein purified from HeLa cell nuclei was strongly enhanced following phosphorylation by protein kinase C. We have investigated the possibility that this kinase is involved in cell cycle phase-specific phosphorylation of topoisomerase IIα in HeLa cells. Two-dimensional tryptic phosphopeptide mapping revealed that topoisomerase IIα protein immunoprecipitated from metabolically labeled HeLa cells was differentially phosphorylated during the G/M phases of the cell cycle. To identify sites of phosphorylation and the kinase(s) responsible for this modification, oligohistidine-tagged recombinant domains of topoisomerase IIα protein were overexpressed in Escherichia coli and purified by affinity chromatography. Phosphorylation of a short fragment of the N-terminal ATPase domain of topoisomerase IIα by protein kinase C in vitro generated two phosphopeptides that co-migrated with prominent G/M phase-specific phosphopeptides from the HeLa cell-derived topoisomerase IIα protein. Site-directed mutagenesis studies indicated that phosphorylation of serine 29 generated both of these phosphopeptides. Our results implicate protein kinase C in the cell cycle phase-dependent modulation of topoisomerase IIα enzymatic activity in human cells.