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

The nuclear localization signal (NLS) of spliceosomal U snRNPs is composed of the U snRNA's 2,2,7-trimethyl-guanosine (m(3)G)- cap and the Sm core domain. The m(3)G-cap is specifically bound by snurportin1, which contains an NH2-terminal importin-beta binding (IIB) domain and a COOH-terminal m(3)G-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 impa IIB domain (IBBimpalpha), nuclear U1 snRNP import was Ran dependent. Furthermore, beta-galactosidase (betaGal) containing a snurportin1 IIB domain, but not IIBimpalpha- 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

Crystal structure of the spliceosomal 15.5 kD protein bound to a U4 snRNA fragment.

and Lin, 1991). It is thought that the U4/U6 interaction is made and broken in each cycle of splicing. The structural rearrangements of the U4 and U6 snRNAs are evolution