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© The Rockefeller University Press, 0021-9525/1999//255 $5.00
The Journal of Cell Biology, Volume 145, Number 2, , 1999 255-264

Regular Articles

CRM1-mediated Recycling of Snurportin 1 to the Cytoplasm



Efrosyni Paraskeva*, Elisa Izaurralde{ddagger}, F. Ralf Bischoff§, Jochen Huber||, Ulrike Kutay*, Enno Hartmann, Reinhard Lührmann||, and Dirk Görlich*

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

Importin β is a major mediator of import into the cell nucleus. Importin β 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 β–binding domain for binding to, and import by, importin β, 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

Abbreviations used in this paper: CBC, cap-binding complex; GAP, GTPase-activating protein; IBB domain, importin β–binding domain; NES, nuclear export signal; NLS, nuclear localization signal; NPC, nuclear pore complex; U snRNP, uridine-rich small nuclear RNP.


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