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Molecular and Cellular Biology, November 1998, p. 6805-6815, Vol. 18, No. 11
0270-7306/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.

Cse1p Is Involved in Export of Yeast Importin alpha  from the Nucleus

Jens Solsbacher,1 Patrick Maurer,1 F. Ralf Bischoff,2 and Gabriel Schlenstedt1,*

Medizinische Biochemie, Universität des Saarlandes, 66421 Homburg,1 and Abteilung Molekulare Biologie der Mitose, Deutsches Krebsforschungszentrum, 69120 Heidelberg,2 Germany

Received 13 April 1998/Returned for modification 25 May 1998/Accepted 6 August 1998

Proteins bearing a nuclear localization signal (NLS) are targeted to the nucleus by the heterodimeric transporter importin. Importin alpha  binds to the NLS and to importin beta , which carries it through the nuclear pore complex (NPC). Importin disassembles in the nucleus, evidently by binding of RanGTP to importin beta . The importin subunits are exported separately. We investigated the role of Cse1p, the Saccharomyces cerevisiae homologue of human CAS, in nuclear export of Srp1p (yeast importin alpha ). Cse1p is located predominantly in the nucleus but also is present in the cytoplasm and at the NPC. We analyzed the in vivo localization of the importin subunits fused to the green fluorescent protein in wild-type and cse1-1 mutant cells. Srp1p but not importin beta  accumulated in nuclei of cse1-1 mutants, which are defective in NLS import but not defective in NLS-independent import pathways. Purified Cse1p binds with high affinity to Srp1p only in the presence of RanGTP. The complex is dissociated by the cytoplasmic RanGTP-binding protein Yrb1p. Combined with the in vivo results, this suggests that a complex containing Srp1p, Cse1p, and RanGTP is exported from the nucleus and is subsequently disassembled in the cytoplasm by Yrb1p. The formation of the trimeric Srp1p-Cse1p-RanGTP complex is inhibited by NLS peptides, indicating that only NLS-free Srp1p will be exported to the cytoplasm.

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

Molecular and Cellular Biology, November 1998, p. 6805-6815, Vol. 18, No. 11
0270-7306/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.


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