The topoisomerase I-binding RING protein, topors, is associated with promyelocytic leukemia nuclear bodies.

We previously identified topors as a topoisomerase I-binding protein that localizes in punctate nuclear regions when expressed as a GFP fusion protein. We now demonstrate that both the GFP-topors fusion protein and endogenous topors are associated with promyelocytic leukemia (PML) nuclear bodies in exponentially growing HeLa cells. Studies using isogenic PML+/+ and PML-/- murine embryonic fibroblasts indicate that the punctate nuclear localization of topors is dependent on PML. A basic C-terminal region but not the N-terminal RING domain of topors is required for the punctate nuclear localization of this protein. Additional studies indicate that topors, but not PML, rapidly relocalizes from nuclear bodies to the nucleoplasm in cells exposed to the transcription inhibitor dichloro-1-beta-d-ribofuranolsylbenzimidazole or to the topoisomerase I-targeting drug camptothecin. These results identify topors as a new member of the group of proteins that associate dynamically with PML nuclear bodies and suggest that topors may be involved in the cellular response to camptothecin.