In Vivo Analysis of Importin α Proteins Reveals Cellular Proliferation Inhibition and Substrate Specificity

Abstract

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.

Brigitte Nentwig gave excellent technical help. Dirk Görlich gave us the anti-importin β antibody. Friedrich C. Luft edited the manuscript. Dominik N. Müller helped us with statistical analysis.

The Deutsche Forschungsgemeinschaft (M.K. and T.S.), the Max Delbrueck Center for Molecular Medicine (C.Q. and M.K.), the ESF Project Berlin (project 20010019) (M.K. and B.F.), and the GIF (E.H.) gave support.