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Published online 28 January 2002. doi:10.1083/jcb.200108114
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© The Rockefeller University Press, 0021-9525/2002/2/467 $5.00
The Journal of Cell Biology, Volume 156, Number 3, February 4, 2002 467-479

Article

The importin-ß binding domain of snurportin1 is responsible for the Ran- and energy-independent nuclear import of spliceosomal U snRNPs in vitro



Jochen Huber1, Achim Dickmanns1,2 and Reinhard Lührmann1

1 Department of Cellular Biochemistry, Max Planck Institute of Biophysical Chemistry, D-37077 Göttingen, Germany
2 RNA Metabolism and Neuronal Diseases, Max Planck Institute of Biochemistry, D-82152 Martinsried, Germany

Address correspondence to Reinhard Lührmann, Max Planck Institute of Biophysical Chemistry, Am Fassberg 11, D-37077 Göttingen. Tel.: 49-551-201-1405. Fax: 49-551-201-1197. E-mail: reinhard.luehrmann{at}mpi-bpc.mpg.de

The nuclear localization signal (NLS) of spliceosomal U snRNPs is composed of the U snRNA's 2,2,7-trimethyl-guanosine (m3G)-cap and the Sm core domain. The m3G-cap is specifically bound by snurportin1, which contains an NH2-terminal importin-ß binding (IBB) domain and a COOH-terminal m3G-cap–binding region that bears no structural similarity to known import adaptors like importin-{alpha} (imp{alpha}). Here, we show that recombinant snurportin1 and importin-ß (impß) are not only necessary, but also sufficient for U1 snRNP transport to the nuclei of digitonin-permeabilized HeLa cells. In contrast to imp{alpha}–dependent import, single rounds of U1 snRNP import, mediated by the nuclear import receptor complex snurportin1–impß, did not require Ran and energy. The same Ran- and energy-independent import was even observed for U5 snRNP, which has a molecular weight of more than one million. Interestingly, in the presence of impß and a snurportin1 mutant containing an imp{alpha} IBB domain (IBBimp{alpha}), nuclear U1 snRNP import was Ran dependent. Furthermore, ß-galactosidase (ßGal) containing a snurportin1 IBB domain, but not IBBimp{alpha}-ßGal, was imported into the nucleus in a Ran-independent manner. Our results suggest that the nature of the IBB domain modulates the strength and/or site of interaction of impß with nucleoporins of the nuclear pore complex, and thus whether or not Ran is required to dissociate these interactions.

Key Words: spliceosomal U snRNPs; IBB domain; snurportin 1; nucleo-cytoplasmic transport; energy and Ran requirements


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