Evidence for separable functions of Srp1p, the yeast homolog of importin alpha (Karyopherin alpha): role for Srp1p and Sts1p in protein degradation.

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.