BackgroundThe control of the subcellular localization of cell cycle regulators has emerged as a crucial mechanism in cell division regulation. The active transport of proteins between the nucleus and the cytoplasm is mediated by the transport receptors of the β-karyopherin family. In this work we characterized the terminal phenotype of a mutant strain in β-karyopherin Kap95, a component of the classical nuclear import pathway.ResultsWhen KAP95 was inactivated, most cells arrested at the G2/M phase of the cell cycle, which is in agreement with the results observed in mutants in the other components of this pathway. However, a number of cells accumulate at G1, suggesting a novel role of Kap95 and the classical import pathway at Start. We investigated the localization of Start transcription factors. It is known that Swi6 contains a classical NLS that interacts with importin α. Here we show that the in vivo nuclear import of Swi6 depends on Kap95. For Swi4, we identified a functional NLS between amino acids 371 and 376 that is sufficient and necessary for Swi4 to enter the nucleus. The nuclear import driven by this NLS is mediated by karyopherins Kap95 and Srp1. Inactivation of Kap95 also produces a dramatic change in the localization of Mbp1 since the protein is mainly detected in the cytoplasm. Two functionally redundant Kap95- and Srp1-dependent NLSs were identified in Mbp1 between amino acids 27-30 and 166-181. Nuclear accumulation was not completely abolished in a kap95 mutant or in the Mbp1 mutated in the two NLSs, suggesting that alternative pathways might contribute to the Mbp1 nuclear import to a lesser extent.ConclusionsKap95 plays an essential role at the initiation of the cell cycle by driving the nuclear import of Swi4, Swi6 and Mbp1, the three transcription factors responsible for the gene expression at Start. This transport depends on the specific nuclear localization signals present in cargo proteins.