Snurportin1, an m3G‐cap‐specific nuclear import receptor with a novel domain structure

The nuclear import of the spliceosomal snRNPs U1, U2, U4 and U5, is dependent on the presence of a complex nuclear localization signal (NLS). The latter is composed of the 5′‐2,2,7‐terminal trimethylguanosine (m3G) cap structure of the U snRNA and the Sm core domain. Here, we describe the isolation and cDNA cloning of a 45 kDa protein, termed snurportin1, which interacts specifically with m3G‐cap but not m7G‐cap structures. Snurportin1 enhances the m3G‐capdependent nuclear import of U snRNPs in both Xenopus laevis oocytes and digitonin‐permeabilized HeLa cells, demonstrating that it functions as an snRNP‐specific nuclear import receptor. Interestingly, solely the m3G‐cap and not the Sm core NLS appears to be recognized by snurportin1, indicating that at least two distinct import receptors interact with the complex snRNP NLS. Snurportin1 represents a novel nuclear import receptor which contains an N‐terminal importin β binding (IBB) domain, essential for function, and a C‐terminal m3G‐cap‐binding region with no structural similarity to the arm repeat domain of importin α.

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