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Mol Biol Cell. 2013 Sep;24(18):2932-42. doi: 10.1091/mbc.E13-03-0118. Epub 2013 Jul 24.

Identification and characterization of Drosophila Snurportin reveals a role for the import receptor Moleskin/importin-7 in snRNP biogenesis.

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Curriculum in Genetics and Molecular Biology, University of North Carolina, Chapel Hill, NC 27599 Departments of Biology, University of North Carolina, Chapel Hill, NC 27599 Departments of Genetics, University of North Carolina, Chapel Hill, NC 27599 Program in Molecular Biology and Biotechnology, University of North Carolina, Chapel Hill, NC 27599 Lineberger Comprehensive Cancer Center, University of North Carolina, Chapel Hill, NC 27599.


Nuclear import is an essential step in small nuclear ribonucleoprotein (snRNP) biogenesis. Snurportin1 (SPN1), the import adaptor, binds to trimethylguanosine (TMG) caps on spliceosomal small nuclear RNAs. Previous studies indicated that vertebrate snRNP import requires importin-β, the transport receptor that binds directly to SPN1. We identify CG42303/snup as the Drosophila orthologue of human snurportin1 (SNUPN). Of interest, the importin-β binding (IBB) domain of SPN1, which is essential for TMG cap-mediated snRNP import in humans, is not well conserved in flies. Consistent with its lack of an IBB domain, we find that Drosophila SNUP (dSNUP) does not interact with Ketel/importin-β. Fruit fly snRNPs also fail to bind Ketel; however, the importin-7 orthologue Moleskin (Msk) physically associates with both dSNUP and spliceosomal snRNPs and localizes to nuclear Cajal bodies. Strikingly, we find that msk-null mutants are depleted of the snRNP assembly factor, survival motor neuron, and the Cajal body marker, coilin. Consistent with a loss of snRNP import function, long-lived msk larvae show an accumulation of TMG cap signal in the cytoplasm. These data indicate that Ketel/importin-β does not play a significant role in Drosophila snRNP import and demonstrate a crucial function for Msk in snRNP biogenesis.

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