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 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.
Institut für Molekularbiologie und Tumorforschung, Emil-Mannkopff-Strasse 2, D-35037 Marburg, Germany.
J
J
Huber
U
U
Cronshagen
M
M
Kadokura
C
C
Marshallsay
T
T
Wada
M
M
Sekine
R
R
Lührmann
D000595:Amino Acid Sequence; D000818:Animals; D001692:Biological Transport; D002457:Cell Extracts; D006151:Guanosine / Q000031:analogs & derivatives; D006367:HeLa Cells; D006801:Humans; D008845:Microinjections; D008969:Molecular Sequence Data; D008970:Molecular Weight; D019913:Nuclear Localization Signals / Q000502:physiology*; D009865:Oocytes; D039381:RNA Cap-Binding Proteins; D012315:RNA Caps / Q000378:metabolism*; D018160:Receptors, Cytoplasmic and Nuclear / Q000378:metabolism; D011993:Recombinant Fusion Proteins; D017411:Ribonucleoproteins, Small Nuclear / Q000378:metabolism*; D017384:Sequence Deletion; D017386:Sequence Homology, Amino Acid; D014982:Xenopus laevis
D002457:Cell Extracts; D019913:Nuclear Localization Signals; D039381:RNA Cap-Binding Proteins; D012315:RNA Caps; D018160:Receptors, Cytoplasmic and Nuclear; D011993:Recombinant Fusion Proteins; D017411:Ribonucleoproteins, Small Nuclear; C114090:SNUPN protein, human; D006151:Guanosine