A novel nuclear localization signal in human DNA topoisomerase I

DNA topoisomerase (topo) I is a nuclear enzyme that plays an important role in DNA metabolism. Based on conserved nuclear targeting sequences, four cl assic nuclear localization signals (NLSs) have been proposed at the N termi nus of human topo I, but studies with yeast have suggested that only one of them (amino acids (aa) 150-156) is sufficient to direct the enzyme to the nucleus. In this study, we expressed human topo I fused to enhanced green f luorescent protein (EGFP) in mammalian cells and demonstrated that whereas aa 150-156 are sufficient for nuclear localization, the nucleolar localizat ion requires as 157-199. More importantly, we identified a novel NLS within aa 117-146. In contrast to the classic MLSs that are rich in basic amino a cids, the novel NLS identified in this study is rich in acidic amino acids, Furthermore, this novel NLS alone is sufficient to direct not only EGFP in to the nucleus but also topo I; and the EGFP topo I fusion driven by the no vel NLS is as active in vivo as the wild-type topo I in response to the top o I inhibitor topotecan. Together, our results suggest that human topo I ca rries two independent NLSs that have opposite amino acid compositions.