Abstract
We have identified a novel pathway for protein import into the nucleus. Although the product of Saccharomyces cerevisiae gene MSN5 was previously shown to function as a karyopherin (Kap) for nuclear export of various proteins, we discovered a nuclear import pathway mediated by Msn5p (also referred to as Kap142p). We have purified from yeast cytosol a complex containing Kap142p and the trimeric replication protein A (RPA), which is required for multiple aspects of DNA metabolism, including DNA replication, DNA repair, and recombination. In wild-type cells, RPA was localized primarily to the nucleus but, in a KAP142 deletion strain, RPA was mislocalized to the cytoplasm and the strain was highly sensitive to bleomycin (BLM). BLM causes DNA double-strand breaks and, in S. cerevisiae, the DNA damage is repaired predominantly by RPA-dependent homologous recombination. Therefore, our results indicate that in wild-type cells a critical portion of RPA was imported into the nucleus by Kap142p. Like several other import-related Kap-substrate complexes, the endogenous RPA-Kap142p complex was dissociated by RanGTP, but not by RanGDP. All three RPA genes are essential for viability, whereas KAP142 is not. Perhaps explaining this disparity, we observed an interaction between RPA and Kap95p in a strain lacking Kap142p. This interaction could provide a mechanism for import of RPA into the nucleus and cell viability in the absence of Kap142p. Together with published results (Kaffman, A., N.M. Rank, E.M. O'Neill, L.S. Huang, and E.K. O'Shea. 1998. Nature. 396:482-486; Blondel, M., P.M. Alepuz, L.S. Huang, S. Shaham, G. Ammerer, and M. Peter. 1999. Genes Dev. 13:2284-2300; DeVit, M.J., and M. Johnston. 1999. Curr. Biol. 9:1231-1241; Mahanty, S.K., Y. Wang, F.W. Farley, and E.A. Elion. 1999. Cell. 98:501-512) our data indicate that the karyopherin Kap142p is able to mediate nuclear import of one set of proteins and nuclear export of a different set of proteins.