title: Nuclear import of U snRNPs requires importin beta.
abstract: Macromolecules that are imported into the nucleus can be divided into classes according to their nuclear import signals. The best characterized class consists of proteins which carry a basic nuclear localization signal (NLS), whose transport requires the importin alpha/beta heterodimer. U snRNP import depends on both the trimethylguanosine cap of the snRNA and a signal formed when the Sm core proteins bind the RNA. Here, factor requirements for U snRNP nuclear import are studied using an in vitro system. Depletion of importin alpha, the importin subunit that binds the NLS, is found to stimulate rather than inhibit U snRNP import. This stimulation is shown to be due to a common requirement for importin beta in both U snRNP and NLS protein import. Saturation of importin beta-mediated transport with the importin beta-binding domain of importin alpha blocks U snRNP import both in vitro and in vivo. Immunodepletion of importin beta inhibits both NLS-mediated and U snRNP import. While the former requires re-addition of both importin alpha and importin beta, re-addition of importin beta alone to immunodepleted extracts was sufficient to restore efficient U snRNP import. Thus importin beta is required for U snRNP import, and it functions in this process without the NLS-specific importin alpha.
authors.affiliation: European Molecular Biology Laboratory, Meyerhofstrasse 1, D-69117 Heidelberg, Germany.
authors.forename: I
authors.initials: I
authors.lastname: Palacios
authors.forename: M
authors.initials: M
authors.lastname: Hetzer
authors.forename: S A
authors.initials: SA
authors.lastname: Adam
authors.forename: I W
authors.initials: IW
authors.lastname: Mattaj
mesh_terms: D001692:Biological Transport / Q000187:drug effects; D002467:Cell Nucleus / Q000378:metabolism*; D006151:Guanosine / Q000031:analogs & derivatives; D028884:Karyopherins; D008954:Models, Biological; D019913:Nuclear Localization Signals; D009687:Nuclear Proteins / Q000145:classification / Q000235:genetics / Q000378:metabolism*; D010446:Peptide Fragments / Q000378:metabolism; D039381:RNA Cap-Binding Proteins; D012315:RNA Caps; D016601:RNA-Binding Proteins / Q000378:metabolism; D017414:Ribonucleoprotein, U4-U6 Small Nuclear / Q000378:metabolism; D017411:Ribonucleoproteins, Small Nuclear / Q000378:metabolism*; D014909:Wheat Germ Agglutinins / Q000494:pharmacology
chemical_list: D028884:Karyopherins; D019913:Nuclear Localization Signals; D009687:Nuclear Proteins; D010446:Peptide Fragments; D039381:RNA Cap-Binding Proteins; D012315:RNA Caps; D016601:RNA-Binding Proteins; D017414:Ribonucleoprotein, U4-U6 Small Nuclear; D017411:Ribonucleoproteins, Small Nuclear; D014909:Wheat Germ Agglutinins; D006151:Guanosine