Nuclear interactions of topoisomerase II alpha and {szligbeta} with phospholipid scramblase 1

DNA topoisomerase (topo) II modulates DNA topology and is essential for cell division. There are two isoforms of topo II ( alpha and {szligbeta}) that have limited functional redundancy, although their catalytic mechanisms appear the same. Using their COOH-terminal domains (CTDs) in yeast two-hybrid analysis, we have identified phospholipid scramblase 1 (PLSCR1) as a binding partner of both topo II alpha and {szligbeta}. Although predominantly a plasma membrane protein involved in phosphatidylserine externalization, PLSCR1 can also be imported into the nucleus where it may have a tumour suppressor function. The interactions of PLSCR1 and topo II were confirmed by pull-down assays with topo II alpha and {szligbeta} CTD fusion proteins and endogenous PLSCR1, and by co-immunoprecipitation of endogenous PLSCR1 and topo II alpha and {szligbeta} from HeLa cell nuclear extracts. PLSCR1 also increased the decatenation activity of human topo II alpha . A conserved basic sequence in the CTD of topo II alpha was identified as being essential for binding to PLSCR1 and binding of the two proteins could be inhibited by a synthetic peptide corresponding to topo II alpha amino acids 1430-1441. These studies reveal for the first time a physical and functional interaction between topo II and PLSCR1.