Sequence determinants of nuclear localization in the alpha and beta isoforms of human topoisomerase II

The alpha and beta isoforms of DNA topoisomerase II (topo II) are targets f or several widely used chemotherapeutic agents, and resistance to some of t hese drugs may be associated with reduced nuclear localization of the a iso form. Human topo II alpha contains a strong bipartite nuclear localization signal (NLS) sequence between amino acids 1454 and 1497 (alpha NLS1454-1497 ). In the present study, me show that human topo II alpha tagged with green fluorescence protein is still detectable in the nucleus when alpha NLS1454 -1497 has been disrupted. Seven additional regions in topo IIa containing o verlapping potential bipartite NLSs were evaluated for their nuclear target ing abilities using a beta-galactosidase reporter system. A moderately func tional NLS was identified between amino acids 1259 and 1296. When human top o II beta was examined in a similar fashion, it was found to contain two st rongly functional sequences beta NLS1522-1548 and beta NLS1538-1573 in the region of topo II beta comparable to the region in topo II alpha that conta ins the strongly functional alpha NLS1454-1497. The third, beta NLS1294-133 2 although weaker than the other two beta sequences, is significantly stron ger than the analogous alpha NLS1259-1296. Differences in the NLS sequences of human topo II isoforms may contribute to their differences in subnuclea r localization. (C) 1999 Academic Press.