The importin-beta binding domain of snurportin1 is responsible for the Ran- and energy-independent nuclear import of spliceosomal U snRNPs in vitro |
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Authors: | Huber Jochen Dickmanns Achim Lührmann Reinhard |
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Institution: | 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 |
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Abstract: | 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-beta binding (IBB) domain and a COOH-terminal m3G-cap--binding region that bears no structural similarity to known import adaptors like importin-alpha (impalpha). Here, we show that recombinant snurportin1 and importin-beta (impbeta) are not only necessary, but also sufficient for U1 snRNP transport to the nuclei of digitonin-permeabilized HeLa cells. In contrast to impalpha-dependent import, single rounds of U1 snRNP import, mediated by the nuclear import receptor complex snurportin1-impbeta, 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 impbeta and a snurportin1 mutant containing an impalpha IBB domain (IBBimpalpha), nuclear U1 snRNP import was Ran dependent. Furthermore, beta-galactosidase (betaGal) containing a snurportin1 IBB domain, but not IBBimpalpha-betaGal, 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 impbeta with nucleoporins of the nuclear pore complex, and thus whether or not Ran is required to dissociate these interactions. |
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Keywords: | spliceosomal U snRNPs IBB domain snurportin 1 nucleo-cytoplasmic transport energy and Ran requirements |
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