Nuclear Import of IκBα Is Accomplished by a Ran-Independent Transport Pathway

Abstract

The inhibitor of kappa B alpha (IκBα) protein is able to shuttle between the cytoplasm and the nucleus. We have utilized a combination of in vivo and in vitro approaches to provide mechanistic insight into nucleocytoplasmic shuttling by IκBα. IκBα contains multiple functional domains that contribute to shuttling of IκBα between the cytoplasm and the nucleus. Nuclear import of IκBα is mediated by the central ankyrin repeat domain. Similar to previously described nuclear import pathways, nuclear import of IκBα is temperature and ATP dependent and is blocked by a dominant-negative mutant of importin β. However, in contrast to classical nuclear import pathways, nuclear import of IκBα is independent of soluble cytosolic factors and is not blocked by the dominant-negative RanQ69L protein. Nuclear export of IκBα is mediated by an N-terminal nuclear export sequence. Nuclear export of IκBα requires the CRM1 nuclear export receptor and is blocked by the dominant-negative RanQ69L protein. Our results are consistent with a model in which nuclear import of IκBα is mediated through direct interactions with components of the nuclear pore complex, while nuclear export of IκBα is mediated via a CRM1-dependent pathway.

ACKNOWLEDGMENTS

We thank Candace Nichol for technical assistance and David J. Pintel for a critical reading of the manuscript. We thank Dirk Gorlich, Steve Adam, Iain Mattai, and Ludwig Englmeier for reagents and advice and Minoru Yoshida for his generous gift of leptomycin B.

This work was supported by American Cancer Society grant RPG-98-097-01, by a grant from the Charlotte Geyer Foundation, and by the University of Missouri Molecular Biology Program.