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

EMBO J. 1998 Jul 15;17(14):4114-26. doi: 10.1093/emboj/17.14.4114.


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 beta 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 alpha.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Amino Acid Sequence
  • Animals
  • Biological Transport
  • Cell Extracts
  • Guanosine / analogs & derivatives
  • HeLa Cells
  • Humans
  • Microinjections
  • Molecular Sequence Data
  • Molecular Weight
  • Nuclear Localization Signals / physiology*
  • Oocytes
  • RNA Cap-Binding Proteins
  • RNA Caps / metabolism*
  • Receptors, Cytoplasmic and Nuclear* / metabolism
  • Recombinant Fusion Proteins
  • Ribonucleoproteins, Small Nuclear / metabolism*
  • Sequence Deletion
  • Sequence Homology, Amino Acid
  • Xenopus laevis


  • Cell Extracts
  • Nuclear Localization Signals
  • RNA Cap-Binding Proteins
  • RNA Caps
  • Receptors, Cytoplasmic and Nuclear
  • Recombinant Fusion Proteins
  • Ribonucleoproteins, Small Nuclear
  • SNUPN protein, human
  • Guanosine

Associated data

  • GENBANK/AF039029