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© The Rockefeller University Press, 0021-9525/1997//1181 $5.00
The Journal of Cell Biology, Volume 138, Number 6, , 1997 1181-1192

Article

Transportin-mediated Nuclear Import of Heterogeneous Nuclear RNP Proteins



Mikiko C. Siomi, Paul S. Eder, Naoyuki Kataoka, Lili Wan, Qing Liu, and Gideon Dreyfuss

Howard Hughes Medical Institute, and Department of Biochemistry and Biophysics, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania 19104-6148

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.

Abbreviations used in this paper: IBB, importin β binding domain; GST, glutathione-S-transferase; hn, heterogeneous nuclear; NLS, nuclear localization signal; NPC, nuclear pore complex; pol II, polymerase II.

Address all correspondence to Gideon Dreyfuss, Howard Hughes Medical Institute, University of Pennsylvania School of Medicine, Philadelphia, PA 19104-6148. Tel.: (215) 898-0398. Fax: (215) 573-2000.


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