Defects in the process of mitosis may lead to abnormal chromosome segregation and aneuploidy, a hallmark of cancer cells. The chromosomal passenger proteins (CPPs) Aurora B, Survivin and INCENP play an important role as regulators of cell division. As mitosis progresses, the three proteins sequentially co-localize to the centromeres, the spindle midzone and the midbody. Aurora B, Survivin and INCENP are already expressed at G2, when the nuclear envelope is still intact, but how their localization is regulated at this stage is poorly characterized. Before nuclear envelope breakdown (NEB), active nucleocytoplasmic transport determines the localization and function of many cellular proteins, and we have previously shown that Survivin is a nuclear shuttling protein. Here, we have used epitope-tagged proteins and immunofluorescence to investigate the mechanisms that modulate the nucleocytoplasmic localization of the CPPs, and the kinase activity of Aurora B before NEB. We show that Aurora B, like Survivin, undergoes CRM1-mediated nuclear shuttling, although both proteins appear to lack functional transport signals of the “classical” type. INCENP, on the other hand, contains multiple autonomous nuclear localization signals, and resides more stably in the nucleus. Contrasting with the closely related localization of Aurora B and Survivin during mitosis, their nuclear shuttling is regulated in a largely independent manner: co-expression experiments indicate that the nucleocytoplasmic distribution of Aurora B, but not of Survivin, is modulated by INCENP and the nuclear Survivin isoform Survivin-ΔEx3. We further show that INCENP and Survivin-ΔEx3, but not Survivin, modulate Aurora B kinase acitvity before NEB, as indicated by in situ analysis of pre-mitotic phosphorylation of the Aurora B target histone H3. In summary, our results characterize novel mechanisms that contribute to regulate the localization and kinase function of Aurora B before NEB, identify a novel role for the Survivin-ΔEx3 isoform, and provide new insight on the regulation of Aurora B kinase activity by INCENP before mitosis.

[Proc Amer Assoc Cancer Res, Volume 46, 2005]