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Molecular and Cellular Biology, November 2000, p. 8468-8479, Vol. 20, No. 22
0270-7306/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.

Nup2p, a Yeast Nucleoporin, Functions in Bidirectional Transport of Importin alpha

Jens Solsbacher,1,dagger Patrick Maurer,1 Frank Vogel,2 and Gabriel Schlenstedt1,*

Medizinische Biochemie und Molekularbiologie, Universität des Saarlandes, 66421 Homburg,1 and Max-Delbrück-Centrum für Molekulare Medizin, 13092 Berlin,2 Germany

Received 12 May 2000/Returned for modification 7 June 2000/Accepted 29 August 2000

Import of proteins containing a classical nuclear localization signal (NLS) into the nucleus is mediated by importin alpha  and importin beta . Srp1p, the Saccharomyces cerevisiae homologue of importin alpha , returns from the nucleus in a complex with its export factor Cse1p and with Gsp1p (yeast Ran) in its GTP-bound state. We studied the role of the nucleoporin Nup2p in the transport cycle of Srp1p. Cells lacking NUP2 show a specific defect in both NLS import and Srp1p export, indicating that Nup2p is required for efficient bidirectional transport of Srp1p across the nuclear pore complex (NPC). Nup2p is located at the nuclear side of the central gated channel of the NPC and provides a binding site for Srp1p via its amino-terminal domain. We show that Nup2p effectively releases the NLS protein from importin alpha -importin and beta  and strongly binds to the importin heterodimer via Srp1p. Kap95p (importin beta ) is released from this complex by a direct interaction with Gsp1p-GTP. These data suggest that besides Gsp1p, which disassembles the NLS-importin alpha -importin beta  complex upon binding to Kap95p in the nucleus, Nup2p can also dissociate the import complex by binding to Srp1p. We also show data indicating that Nup1p, a relative of Nup2p, plays a similar role in termination of NLS import. Cse1p and Gsp1p-GTP release Srp1p from Nup2p, which suggests that the Srp1p export complex can be formed directly at the NPC. The changed distribution of Cse1p at the NPC in nup2 mutants also supports a role for Nup2p in Srp1p export from the nucleus.

* Corresponding author. Mailing address: Medizinische Biochemie und Molekularbiologie, Universität des Saarlandes, Haus 44, D-66421 Homburg, Germany. Phone: 49-6841-166522. Fax: 49-6841-166288. E-mail: bcgsch{at}med-rz.uni-sb.de.

dagger Present address: Aventis Research and Technologies GmbH, Operative Forschung, 65926 Frankfurt am Main, Germany.

Molecular and Cellular Biology, November 2000, p. 8468-8479, Vol. 20, No. 22
0270-7306/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.


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