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Molecular and Cellular Biology, August 2000, p. 6062-6073, Vol. 20, No. 16
0270-7306/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.

Evidence for Separable Functions of Srp1p, the Yeast Homolog of Importin alpha  (Karyopherin alpha ): Role for Srp1p and Sts1p in Protein Degradation

Michelle M. Tabb,dagger Prasad Tongaonkar, Loan Vu, and Masayasu Nomura*

Departments of Microbiology and Molecular Genetics and Biological Chemistry, University of California, Irvine, Irvine, California 92697-1700

Received 2 March 2000/Returned for modification 18 April 2000/Accepted 25 May 2000

Srp1p (importin alpha ) functions as the nuclear localization signal (NLS) receptor in Saccharomyces cerevisiae. The srp1-31 mutant is defective in this nuclear localization function, whereas an srp1-49 mutant exhibits defects that are unrelated to this localization function, as was confirmed by intragenic complementation between the two mutants. RPN11 and STS1 (DBF8) were identified as high-dosage suppressors of the srp1-49 mutation but not of the srp1-31 mutation. We found that Sts1p interacts directly with Srp1p in vitro and also in vivo, as judged by coimmunoprecipitation and two-hybrid analyses. Mutants of Sts1p that cannot interact with Srp1p are incapable of suppressing srp1-49 defects, strongly suggesting that Sts1p functions in a complex with Srp1p. STS1 also interacted with the second suppressor, RPN11, a subunit of the 26S proteasome, in the two-hybrid system. Further, degradation of Ub-Pro-beta -galactosidase, a test substrate for the ubiquitin-proteasome system, was defective in srp1-49 but not in srp1-31. This defect in protein degradation was alleviated by overexpression of either RPN11 or STS1 in srp1-49. These results suggest a role for Srp1p in regulation of protein degradation separate from its well-established role as the NLS receptor.

* Corresponding author. Mailing address: University of California, Irvine, Department of Biological Chemistry, 240D Med Sci I, Irvine, CA 92697-1700. Phone: (949) 824-4564. Fax: (949) 824-3201. E-mail: mnomura{at}uci.edu.

dagger Present address: Department of Developmental and Cell Biology, University of California, Irvine, Irvine, CA 92697-2300.

Molecular and Cellular Biology, August 2000, p. 6062-6073, Vol. 20, No. 16
0270-7306/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.


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