title: Transportin-mediated nuclear import of heterogeneous nuclear RNP proteins.
abstract: Heterogeneous nuclear ribonucleoprotein (hnRNP) A1 is an abundant nuclear protein that plays an important role in pre-mRNA processing and mRNA export from the nucleus. A1 shuttles rapidly between the nucleus and the cytoplasm, and a 38-amino acid domain, M9, serves as the bidirectional transport signal of A1. Recently, a 90-kD protein, transportin, was identified as the mediator of A1 nuclear import. In this study, we show that transportin mediates the nuclear import of additional hnRNP proteins, including hnRNP F. We have also isolated and sequenced a novel transportin homolog, transportin2, which may differ from transportin1 in its substrate specificity. Immunostaining shows that transportin1 is localized both in the cytoplasm and the nucleoplasm, and nuclear rim staining is also observed. The nuclear localization of A1 is dependent on ongoing RNA polymerase II transcription. Interestingly, a pyruvate kinase-M9 fusion, which normally localizes in the nucleus, also accumulates in the cytoplasm when RNA polymerase II is inhibited. Thus, M9 itself is a specific sensor for transcription-dependent nuclear transport. Transportin1-A1 complexes can be isolated from the cytoplasm and the nucleoplasm, but transportin1 is not detectable in hnRNP complexes. RanGTP causes dissociation of A1-transportin1 complexes in vitro. Thus, it is likely that after nuclear import, A1 dissociates from transportin1 by RanGTP and becomes incorporated into hnRNP complexes, where A1 functions in pre-mRNA processing.
authors.affiliation: Howard Hughes Medical Institute, and Department of Biochemistry and Biophysics, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania 19104-6148, USA.
authors.forename: M C
authors.initials: MC
authors.lastname: Siomi
authors.forename: P S
authors.initials: PS
authors.lastname: Eder
authors.forename: N
authors.initials: N
authors.lastname: Kataoka
authors.forename: L
authors.initials: L
authors.lastname: Wan
authors.forename: Q
authors.initials: Q
authors.lastname: Liu
authors.forename: G
authors.initials: G
authors.lastname: Dreyfuss
mesh_terms: D000818:Animals; D000911:Antibodies, Monoclonal; D000918:Antibody Specificity; D001692:Biological Transport / Q000502:physiology; D002467:Cell Nucleus / Q000737:chemistry / Q000201:enzymology*; D003593:Cytoplasm / Q000737:chemistry / Q000378:metabolism; D019204:GTP-Binding Proteins / Q000378:metabolism / Q000494:pharmacology; D006367:HeLa Cells; D000076245:Heterogeneous Nuclear Ribonucleoprotein A1; D034481:Heterogeneous-Nuclear Ribonucleoprotein Group A-B; D034541:Heterogeneous-Nuclear Ribonucleoprotein Group F-H; D034441:Heterogeneous-Nuclear Ribonucleoproteins; D006801:Humans; D028884:Karyopherins; D051379:Mice; D008807:Mice, Inbred BALB C; D008969:Molecular Sequence Data; D009687:Nuclear Proteins / Q000032:analysis / Q000737:chemistry / Q000235:genetics* / Q000276:immunology / Q000378:metabolism* / Q000494:pharmacology; D011770:Pyruvate Kinase / Q000378:metabolism; D012322:RNA Precursors / Q000378:metabolism; D018160:Receptors, Cytoplasmic and Nuclear / Q000737:chemistry / Q000235:genetics* / Q000276:immunology / Q000378:metabolism*; D012261:Ribonucleoproteins / Q000378:metabolism*; D017386:Sequence Homology, Amino Acid; D013379:Substrate Specificity; D014158:Transcription, Genetic / Q000502:physiology; D028961:beta Karyopherins; D020931:ran GTP-Binding Protein
chemical_list: D000911:Antibodies, Monoclonal; D000076245:Heterogeneous Nuclear Ribonucleoprotein A1; D034481:Heterogeneous-Nuclear Ribonucleoprotein Group A-B; D034541:Heterogeneous-Nuclear Ribonucleoprotein Group F-H; D034441:Heterogeneous-Nuclear Ribonucleoproteins; D028884:Karyopherins; D009687:Nuclear Proteins; D012322:RNA Precursors; D018160:Receptors, Cytoplasmic and Nuclear; D012261:Ribonucleoproteins; C503621:TNPO2 protein, human; C503622:Tnpo2 protein, mouse; D028961:beta Karyopherins; D011770:Pyruvate Kinase; D019204:GTP-Binding Proteins; D020931:ran GTP-Binding Protein