ACTIVE HETERODIMERS ARE FORMED FROM HUMAN DNA TOPOISOMERASE II-ALPHA AND II-BETA ISOFORMS

DNA topoisomerase II is a nuclear enzyme essential for chromosome dyna mics and DNA metabolism. In mammalian cells, two genetically and bioch emically distinct topoisomerase II forms exist, which are designated t opoisomerase II alpha and topoisomerase II beta. In our studies of hum an topoisomerase II, we have found that a substantial fraction of the enzyme exists as alpha/beta heterodimers in HeLa cells. The ability to form heterodimers was verified when human topoisomerases II alpha and II beta mere coexpressed in yeast and investigated in a dimerization assay. Analysis of purified heterodimers shows that these enzymes main tain topoisomerase II specific catalytic activities, The natural exist ence of an active heterodimeric subclass of topoisomerase II merits at tention whenever topoisomerases II alpha and II beta function, localiz ation, and cell cycle regulation are investigated.