Importin alpha can migrate into the nucleus in an importin beta- and Ran-independent manner

A classical nuclear localization signal (NLS)-containing protein is transported into the nucleus via the formation of a NLS-substrate/importin alpha/beta complex. In this study, we found that importin alpha migrated into the nucleus without the addition of importin beta, Ran or any other soluble factors in an in vitro transport assay. A mutant importin alpha lacking the importin beta-binding domain efficiently entered the nucleus. Competition experiments showed that this import pathway for importin alpha is distinct from that of importin beta. These results indicate that importin alpha alone can enter the nucleus via a novel pathway in an importin beta- and Ran-independent manner. Furthermore, this process is evolutionarily conserved as similar results were obtained in Saccharomyces cerevisiae. Moreover, the import rate of importin alpha differed among individual nuclei of permeabilized cells, as demonstrated by time-lapse experiments. This heterogeneous nuclear accumulation of importin alpha was affected by the addition of ATP, but not ATPgammaS. These results suggest that the nuclear import machinery for importin alpha at individual nuclear pore complexes may be regulated by reaction(s) that require ATP hydrolysis.