Originally published as Genetics Published Articles Ahead of Print on November 3, 2008.

Genetics, Vol. 181, 105-118, January 2009, Copyright © 2009
doi:10.1534/genetics.108.097303

The Classical Nuclear Localization Signal Receptor, Importin-{alpha}, Is Required for Efficient Transition Through the G1/S Stage of the Cell Cycle in Saccharomyces cerevisiae

Kanika F. Pulliam, Milo B. Fasken, Laura M. McLane, John V. Pulliam and Anita H. Corbett1

Department of Biochemistry, Emory University School of Medicine, Atlanta, Georgia 30322

1 Corresponding author: Department of Biochemistry, Room 4117, Emory University School of Medicine, Rollins Research Center, 1510 Clifton Rd. NE, Atlanta, GA 30322.
E-mail: acorbe2{at}emory.edu

There is significant evidence linking nucleocytoplasmic transport to cell cycle control. The budding yeast, Saccharomyces cerevisiae, serves as an ideal model system for studying transport events critical to cell cycle progression because the nuclear envelope remains intact throughout the cell cycle. Previous studies linked the classical nuclear localization signal (cNLS) receptor, importin-{alpha}/Srp1, to the G2/M transition of the cell cycle. Here, we utilize two engineered mutants of importin-{alpha}/Srp1 with specific molecular defects to explore how protein import affects cell cycle progression. One mutant, Srp1-E402Q, is defective in binding to cNLS cargoes that contain two clusters of basic residues termed a bipartite cNLS. The other mutant, Srp1-55, has defects in release of cNLS cargoes into the nucleus. Consistent with distinct in vivo functional consequences for each of the Srp1 mutants analyzed, we find that overexpression of different nuclear transport factors can suppress the temperature-sensitive growth defects of each mutant. Studies aimed at understanding how each of these mutants affects cell cycle progression reveal a profound defect at the G1 to S phase transition in both srp1-E402Q and srp1-55 mutants as well as a modest G1/S defect in the temperature-sensitive srp1-31 mutant, which was previously implicated in G2/M. We take advantage of the characterized defects in the srp1-E402Q and srp1-55 mutants to predict candidate cargo proteins likely to be affected in these mutants and provide evidence that three of these cargoes, Cdc45, Yox1, and Mcm10, are not efficiently localized to the nucleus in importin-{alpha} mutants. These results reveal that the classical nuclear protein import pathway makes important contributions to the G1/S cell cycle transition.


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Genetics 2009 181: NP. [Full Text]