Distinct energy requirement for nuclear import and export of importin β in living cells

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Abstract
Importin β can shuttle between the nucleus and cytoplasm through the nuclear pore complex (NPC). This study deals with the issue of how the energy is utilized during the NPC passage of importin β. In chilled or ATP-depleted cells, importin β was transported into the nucleus, while the nuclear export of importin β was inhibited. Further, it was found that the nuclear export inhibition of importin β is not due to nuclear retention via binding to nucleoporins or nuclear importin α. These data show that the nuclear export of importin β involves energy-requiring step(s) in living cells.