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

The inhibitor of kappa B alpha (IkBa) 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 IkBa. IkBa contains multiple functional domains that contribute to shuttling of IkBa between the cytoplasm and the nucleus. Nuclear import of IkBa is mediated by the central ankyrin repeat domain. Similar to previously described nuclear import pathways, nuclear import of IkBa is temperature and ATP dependent and is blocked by a dominant-negative mutant of importin b. However, in contrast to classical nuclear import pathways, nuclear import of IkBa is independent of soluble cytosolic factors and is not blocked by the dominant-negative RanQ69L protein. Nuclear export of IkBa is mediated by an N-terminal nuclear export sequence. Nuclear export of IkBa 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 IkBa is mediated through direct interactions with components of the nuclear pore complex, while nuclear export of IkBa is mediated via a CRM1-dependent pathway.