Special Article

Transportin: Nuclear Transport Receptor of a Novel Nuclear Protein Import Pathway

SARA NAKIELNY, MIKIKO C. SIOMI, HARUHIKO SIOMI, W. MATTHEW MICHAEL, VICTORIA POLLARD, AND GIDEON DREYFUSS
Howard Hughes Medical Institute and Department of Biochemistry and Biophysics, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania 19104-6148

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Abstract
Many nuclear proteins are imported into the cell nucleus by the "classical" nuclear localization signal (NLS)-mediated import pathway. In this pathway, a sequence rich in basic residues in the protein interacts with a heterodimeric complex termed importin and this, along with the GTPase Ran, mediates nuclear import of the NLS-bearing protein. The heterogeneous nuclear ribonucleoprotein (hnRNP) A1 protein contains a novel nuclear localization sequence, termed M9, that does not contain any clusters of basic residues. Very recently, we showed that M9 directs import into the nucleus by a novel protein import pathway distinct from the classical NLS pathway. A 90-kilodalton protein termed transportin was identified as a protein that specifically interacts with wild-type M9 but not transport-defective M9 mutants. Transportin and an ATP-regenerating system were found to be necessary and sufficient for import of M9-containing proteins in an in vitro import assay. In this report, we provide additional evidence that transportin can interact directly with M9-containing proteins and also show that it can mediate import of full-length hnRNP A1. In addition, Ran, or a Ran-binding protein, is identified as a second protein component of this novel nuclear import pathway. Transportin relatives from Saccharomyces cerevisiae which likely serve as additional nuclear transport receptors are described.

EXPERIMENTAL CELL RESEARCH 229
261 - 266 (1996)
ARTICLE NO. 0369
Copyright © 1996 Academic Press, Inc.