Nuclear import of I kappa B alpha is accomplished by a Ran-independent transport pathway

The inhibitor of kappa B alpha (I kappa B alpha) protein is able to shuttle between the cytoplasm and the nucleus. We have utilized a combination of i n vivo and in vitro approaches to provide mechanistic insight into nucleocy toplasmic shuttling by I kappa B alpha. I kappa B alpha contains multiple f unctional domains that contribute to shuttling of I kappa B alpha between t he cytoplasm and the nucleus, Nuclear import of I kappa B alpha is mediated by the central ankyrin repeat domain. Similar to previously described nucl ear import pathways, nuclear import of I kappa B alpha is temperature and A TP dependent and is blocked by a dominant-negative mutant of importin beta. However, in contrast to classical nuclear import pathways, nuclear import of I kappa B alpha is independent of soluble cytosolic factors and is not b locked by the dominant-negative RanQ69L protein. Nuclear export of I kappa B alpha is mediated by an N-terminal nuclear export sequence. Nuclear expor t of I kappa B alpha requires the CRM1 nuclear export receptor and is block ed by the dominant-negative RanQ69L protein. Our results are consistent wit h a model in which nuclear import of I kappa B alpha is mediated through di rect interactions with components of the nuclear pore complex, while nuclea r export of I kappa B alpha is mediated via a CRM1-dependent pathway.