Sequence Determinants of Nuclear Localization in the α and β Isoforms of Human Topoisomerase II

Abstract The α and β isoforms of DNA topoisomerase II (topo II) are targets for several widely used chemotherapeutic agents, and resistance to some of these drugs may be associated with reduced nuclear localization of the α isoform. Human topo IIα contains a strong bipartite nuclear localization signal (NLS) sequence between amino acids 1454 and 1497 (αNLS 1454–1497 ). In the present study, we show that human topo IIα tagged with green fluorescence protein is still detectable in the nucleus when αNLS 1454–1497 has been disrupted. Seven additional regions in topo IIα containing overlapping potential bipartite NLSs were evaluated for their nuclear targeting abilities using a β-galactosidase reporter system. A moderately functional NLS was identified between amino acids 1259 and 1296. When human topo IIβ was examined in a similar fashion, it was found to contain two strongly functional sequences βNLS 1522–1548 and βNLS 1538–1573 in the region of topo IIβ comparable to the region in topo IIα that contains the strongly functional αNLS 1454–1497 . The third, βNLS 1294–1332 , although weaker than the other two β sequences, is significantly stronger than the analogous αNLS 1259–1296 . Differences in the NLS sequences of human topo II isoforms may contribute to their differences in subnuclear localization.