Transportin2 functions as importin and mediates nuclear import of HuR.

The RanGTP-binding nuclear transport receptors transportin1 (TRN1) and transportin2 (TRN2) are highly similar in sequence but are reported to function in nuclear import and export, respectively. Here we show that TRN2 possesses properties of a nuclear import receptor. TRN1/2 both interacted with a similar set of RNA-binding proteins in a RanGTP-sensitive manner. TRN2 bound RanGTP with high affinity, a feature of nuclear import receptors. As expected of an import complex, RanGTP also disrupted the interaction between TRN2 and HuR, an RNA-binding protein previously described as a TRN2 export substrate. The HuR nucleocytoplasmic shuttling signal, a sequence resembling the M9 nuclear import signal of hnRNP A1, was necessary and sufficient for TRN-mediated nuclear import of HuR in vitro. Finally, crosscompetition experiments demonstrated that HuR nucleocytoplasmic shuttling signal and M9 are imported along redundant pathways involving TRN1/2, substantiating the function of TRN2 in nuclear import.

[1]  D. Page,et al.  Two closely related human nuclear export factors utilize entirely distinct export pathways. , 2001, Molecular cell.

[2]  U. Kutay,et al.  Nucleocytoplasmic transport: taking an inventory , 2003, Cellular and Molecular Life Sciences CMLS.

[3]  Stefan Jaekel,et al.  Importin β, transportin, RanBP5 and RanBP7 mediate nuclear import of ribosomal proteins in mammalian cells , 1998, The EMBO journal.

[4]  P. Silver,et al.  Protein and RNA export from the nucleus. , 2002, Developmental cell.

[5]  I. Macara.  Transport into and out of the Nucleus , 2001, Microbiology and Molecular Biology Reviews.

[6]  A. Levy,et al.  Hypoxic Stabilization of Vascular Endothelial Growth Factor mRNA by the RNA-binding Protein HuR* , 1998, The Journal of Biological Chemistry.

[7]  J. Steitz,et al.  Delineation of mRNA Export Pathways by the Use of Cell-Permeable Peptides , 2001, Science.

[8]  J. A. Steitz,et al.  HuR and mRNA stability , 2001, Cellular and Molecular Life Sciences CMLS.

[9]  E. Izaurralde.  A novel family of nuclear transport receptors mediates the export of messenger RNA to the cytoplasm. , 2002, European journal of cell biology.

[10]  Matthias Mann,et al.  Mtr10p functions as a nuclear import receptor for the mRNA‐binding protein Npl3p , 1998, The EMBO journal.

[11]  A. Shyu,et al.  RNA stabilization by the AU‐rich element binding protein, HuR, an ELAV protein , 1998, The EMBO journal.

[12]  G. Dreyfuss,et al.  Messenger-RNA-binding proteins and the messages they carry , 2002, Nature Reviews Molecular Cell Biology.

[13]  S. Riva,et al.  Nucleo-cytoplasmic distribution of human hnRNP proteins: a search for the targeting domains in hnRNP A1. , 1995, Journal of cell science.

[14]  G. Dreyfuss,et al.  Transportin-mediated Nuclear Import of Heterogeneous Nuclear RNP Proteins , 1997, The Journal of cell biology.

[15]  B. Cullen,et al.  Nuclear mRNA export: insights from virology. , 2003, Trends in biochemical sciences.

[16]  G. Dreyfuss,et al.  A nuclear export signal in hnRNP A1: A signal-mediated, temperature-dependent nuclear protein export pathway , 1995, Cell.

[17]  J. Steitz,et al.  Overexpression of HuR, a nuclear–cytoplasmic shuttling protein, increases the in vivo stability of ARE‐containing mRNAs , 1998, The EMBO journal.

[18]  A. Radu,et al.  Karyopherin β2B participates in mRNA export from the nucleus , 2002, Proceedings of the National Academy of Sciences of the United States of America.

[19]  F. Bischoff,et al.  Identification of Two Novel RanGTP-binding Proteins Belonging to the Importin β Superfamily* , 2000, The Journal of Biological Chemistry.

[20]  F. Bischoff,et al.  Export of Importin α from the Nucleus Is Mediated by a Specific Nuclear Transport Factor , 1997, Cell.

[21]  E. Izaurralde,et al.  The interplay of nuclear mRNP assembly, mRNA surveillance and export. , 2003, Trends in cell biology.

[22]  J. Steitz,et al.  HNS, a nuclear-cytoplasmic shuttling sequence in HuR. , 1998, Proceedings of the National Academy of Sciences of the United States of America.

[23]  G. Vogel.  Berlin's Scientific Treasure House Shakes Off the Dust , 2004, Science.

[24]  G. Dreyfuss,et al.  A Novel Receptor-Mediated Nuclear Protein Import Pathway , 1996, Cell.

[25]  U. Kutay,et al.  Multiple pathways contribute to nuclear import of core histones , 2001, EMBO reports.

[26]  E. Izaurralde,et al.  NXF1/p15 heterodimers are essential for mRNA nuclear export in Drosophila. , 2001, RNA.

[27]  F. Bischoff,et al.  Identification of different roles for RanGDP and RanGTP in nuclear protein import. , 1996, The EMBO journal.

[28]  I. Kanazawa,et al.  Interaction between Mutant Ataxin-1 and PQBP-1 Affects Transcription and Cell Death , 2002, Neuron.

[29]  G. Dreyfuss,et al.  Transportin: nuclear transport receptor of a novel nuclear protein import pathway. , 1996, Experimental cell research.

[30]  Dirk Görlich,et al.  RanBP1 is crucial for the release of RanGTP from importin β‐related nuclear transport factors , 1997, FEBS letters.

[31]  B. Cullen,et al.  Definition of a Consensus Transportin-specific Nucleocytoplasmic Transport Signal* , 1999, The Journal of Biological Chemistry.

[32]  G. Dreyfuss,et al.  A nuclear localization domain in the hnRNP A1 protein , 1995, The Journal of cell biology.

[33]  Ed Hurt,et al.  A Conserved mRNA Export Machinery Coupled to pre-mRNA Splicing , 2002, Cell.