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J. Cell Biol., Volume 145, Number 2, April 19, 1999 255-264

CRM1-mediated Recycling of Snurportin 1 to the Cytoplasm

Efrosyni Paraskeva,* Elisa Izaurralde,Dagger F. Ralf Bischoff,§ Jochen Huber,parallel Ulrike Kutay,* Enno Hartmann, Reinhard Lührmann,parallel and Dirk Görlich*

* Zentrum für Molekulare Biologie der Universität Heidelberg, D-69120 Heidelberg, Germany; Dagger  University of Geneva, Department of Molecular Biology, CH-1211 Geneva 4, Switzerland; § Abteilung Molekulare Biologie der Mitose, Deutsches Krebsforschungszentrum, D-69120 Heidelberg, Germany; parallel  Institut für Molekularbiologie und Tumorforschung, D-35037 Marburg, Germany; and  Zentrum Biochemie und Molekulare Zellbiologie, Abteilung Biochemie II, D-37073 Göttingen, Germany

Importin beta  is a major mediator of import into the cell nucleus. Importin beta  binds cargo molecules either directly or via two types of adapter molecules, importin alpha , for import of proteins with a classical nuclear localization signal (NLS), or snurportin 1, for import of m3G-capped U snRNPs. Both adapters have an NH2-terminal importin beta -binding domain for binding to, and import by, importin beta , and both need to be returned to the cytoplasm after having delivered their cargoes to the nucleus. We have shown previously that CAS mediates export of importin alpha . Here we show that snurportin 1 is exported by CRM1, the receptor for leucine-rich nuclear export signals (NESs). However, the interaction of CRM1 with snurportin 1 differs from that with previously characterized NESs. First, CRM1 binds snurportin 1 50-fold stronger than the Rev protein and 5,000-fold stronger than the minimum Rev activation domain. Second, snurportin 1 interacts with CRM1 not through a short peptide but rather via a large domain that allows regulation of affinity. Strikingly, snurportin 1 has a low affinity for CRM1 when bound to its m3G-capped import substrate, and a high affinity when substrate-free. This mechanism appears crucial for productive import cycles as it can ensure that CRM1 only exports snurportin 1 that has already released its import substrate in the nucleus.

Key words: nuclear transport;  nuclear pore complex;  importin;  exportin;  snurportin 1


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