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Mol Cell Biol, July 1998, p. 4141-4148, Vol. 18, No. 7
0270-7306/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.

Functional Conservation of the Transportin Nuclear Import Pathway in Divergent Organisms

Mikiko C. Siomi,1 Micheline Fromont,2 Jean-Christophe Rain,2 Lili Wan,1 Fan Wang,1 Pierre Legrain,2 and Gideon Dreyfuss1 *

Howard Hughes Medical Institute and Department of Biochemistry and Biophysics, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania 19104-6148,1 and Laboratoire du Métabolisme des ARN, CNRS URA 1300, Institut Pasteur, Paris 75724, France2

Received 11 February 1998/Returned for modification 2 April 1998/Accepted 21 April 1998

Human transportin1 (hTRN1) is the nuclear import receptor for a group of pre-mRNA/mRNA-binding proteins (heterogeneous nuclear ribonucleoproteins [hnRNP]) represented by hnRNP A1, which shuttle continuously between the nucleus and the cytoplasm. hTRN1 interacts with the M9 region of hnRNP A1, a 38-amino-acid domain rich in Gly, Ser, and Asn, and mediates the nuclear import of M9-bearing proteins in vitro. Saccharomyces cerevisiae transportin (yTRN; also known as YBR017c or Kap104p) has been identified and cloned. To understanding the nuclear import mediated by yTRN, we searched with a yeast two-hybrid system for proteins that interact with it. In an exhaustive screen of the S. cerevisiae genome, the most frequently selected open reading frame was the nuclear mRNA-binding protein, Nab2p. We delineated a ca.-50-amino-acid region in Nab2p, termed NAB35, which specifically binds yTRN and is similar to the M9 motif. NAB35 also interacts with hTRN1 and functions as a nuclear localization signal in mammalian cells. Interestingly, yTRN can also mediate the import of NAB35-bearing proteins into mammalian nuclei in vitro. We also report on additional substrates for TRN as well as sequences of Drosophila melanogaster, Xenopus laevis, and Schizosaccharomyces pombe TRNs. Together, these findings demonstrate that both the M9 signal and the nuclear import machinery utilized by the transportin pathway are conserved in evolution.

* Corresponding author. Mailing address: Howard Hughes Medical Institute & Department of Biochemistry & Biophysics, University of Pennsylvania School of Medicine, Philadelphia, PA 19104-6148. Phone: (215) 898-0398. Fax: (215) 573-2000. E-mail: gdreyfuss{at}hhmi.upenn.edu.

Mol Cell Biol, July 1998, p. 4141-4148, Vol. 18, No. 7
0270-7306/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.


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