In vivo analysis of importin α proteins reveals cellular proliferation inhibition and substrate specificity

The "classical" nuclear import pathway depends on importins α and β. Humans have only one importin β, while six α importins have been described. Whether or not distinct α importins are essential for specific import pathways in living human cells is unclear, We used RNA interference technology to specifically down-regulate the expression of ubiquitously expressed human α importins in HeLa cells. Down-regulation of importins α3, α5, α7, and β strongly inhibited HeLa cell proliferation, while down-regulation of importins α1 and α4 had only a minor effect or no effect. Nucleoplasmin import was not prevented by down-regulation of any α importin, indicating that the importin α/β pathway was generally not affected. In contrast, importin α3 or α5 down-regulation specifically inhibited the nuclear import of the Ran guanine nucleotide exchange factor, RCC1. Coinjection of recombinant α importins and RCC1 into down-regulated cells demonstrated that these transport defects were specifically caused by the limited availability of importin α3 in both cases. Thus, importin α3 is the only α importin responsible for the classical nuclear import of RCC1 in living cells.