Subnuclear Localization of Human Topoisomerase I.

Human topoisomerase I is partitioned between the nucleolus and the nucleoplasm in the interphase cells. Under unstressed conditions it is concentrated in the first compartment but nucleolar concentration of the full length protein is lost after inactivation of relaxation activity. Due to the above, subnuclear localization of topoisomerase I is linked with DNA relaxation activity of topoisomerase I. Looking for other factors responsible for subnuclear distribution of topoisomerase I, we studied here localization of the fluorescently tagged fragments and point mutants of topoisomerase I in HeLa cells. We found that two regions of topoisomerase I, the N-terminal and the linker domains, were critical for subnuclear localization of the enzyme. The linker domain and the distal region of the N-terminal domain directed topoisomerase I to the nucleolus, whereas the remaining region of the N-terminal domain was responsible for the nucleoplasmic localization. The effects exhibited by the regions which contributed to nuclear distribution of topoisomerase I were independent of DNA relaxation activity. Localization mutations in both domains complemented one another giving the wild-type phenotype for the double mutant. These results suggest a two-stage model of regulation of partitioning of topoisomerase I between the nucleolus and the nucleoplasm. The first stage is a net of interactions provided by the N-terminal and the linker domains. The other stage, accessible only if the first net is balanced, is driven by DNA relaxation activity. J. Cell. Biochem. 118: 407-419, 2017.