Interplay between nuclear import and cell cycle control

The bi-directional transport of proteins across the nuclear envelope via nuclear pore complexes is a highly regulated process that involves many different factors. The classical nuclear import pathway is the most studied mechanism for the transport of proteins into the nucleus. During classical nuclear import, proteins are shuttled into the nucleus by an internal targeting sequence termed the classical nuclear localization signal (cNLS) via interactions with nuclear import receptors. The cNLS receptor, importin alpha recognizes the cNLS-containing protein or cargo in the cytoplasm as the initial step in the transport process. In an effort to investigate how the classical nuclear transport mechanism affects a fundamental process in biology, we examined the interplay between nuclear transport and progression through the cell cycle in budding yeast, Saccharomyces cerevisiae. In eukaryotes, a subset of proteins are transported across the nuclear envelope in a cell cycle dependent manner. Previous studies support a link between nuclear transport and the cell cycle but these studies did not demonstrate a direct role of the nuclear transport machinery in cell cycle regulation. We first demonstrated that the cNLS cargo binding affinity for importin alpha dictates the rate of import of a cNLS cargo into the nucleus, suggesting that cargo binding to the receptor regulates the import process. We next hypothesized that temporally regulated interactions between specific cNLS cargoes and importin alpha are required for progression through the cell cycle. Results of our study exploiting importin alpha mutants with defects in cargo binding and release showed that a defect in cNLS cargo recognition causes a profound delay in progression through the G1/S transition of the cell cycle. In conclusion, cNLS cargo recognition by the classical nuclear import receptor, importin alpha is required for efficient transition through the G1/S stage of the cell cycle. This is the first study to demonstrate a direct role of importin alpha in regulation of the G1/S phase of the cell cycle. In conclusion, this work defines the rate-limiting step in the classical nuclear import process and then reveals that the G1/S stage of the cell cycle is strongly dependent on classical protein import.