The Classical Nuclear Localization Signal Receptor, Importin , is Required for Efficient Transition through the G₁/S Stage of the Cell Cycle in Saccharomyces cerevisiae

There is significant evidence linking nucleocytoplasmic transport to cell cycle control. The budding yeast, Saccharomyces cerevisiae, serves as an ideal model system to study 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 /Srp1, to the G₂/M transition of the cell cycle. Here, we utilize two engineered mutants of importin /Srp1 with specific molecular defects to explore how protein import impacts 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 impacts cell cycle progression reveal a profound defect at the G₁ to S phase transition in both the 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 impacted in these mutants and provide evidence that three of these cargoes, Cdc45, Yox1, and Mcm10, are not efficiently localized to the nucleus in importin mutants. These results reveal that the classical nuclear protein import pathway makes important contributions to the G₁/S cell cycle transition.

Key Words: G1/S transition, Nuclear transport, cell cycle, importin alpha, nuclear localization signal (NLS)

Online ISS 1091-6490
Copyright 2008 by the Genetics Society of America
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