Kap104p is a Saccharomyces cerevisiae nuclear import receptor for two essential mRNA-binding proteins, Nab2p and Nab4p/Hrp1p. We demonstrate direct binding of Kap104p to each of these substrates. We have defined the nuclear localization signals in both Nab2p and Nab4p/Hrp1p by Kap104p binding in vitro and KAP104-dependent nuclear import in vivo. The nuclear localization signals map to similar arginine/glycine-rich RNA-binding domains in both proteins and are thus termed rg-nuclear localization signals to distinguish them from classical nuclear localization signals. We also demonstrate that Kap104p, like other known beta-karyopherins (or importins), interacts directly with the small GTPase Ran/Gsp1. However, unlike other known import factors, Ran binding is not sufficient to mediate release of substrates from Kap104p; efficient Ran-GTP-mediated substrate release requires RNA. Also, addition of Kap104p to Nab2p and Nab4p/Hrp1p prebound to single-stranded DNA-cellulose stimulated release of both proteins from the resin. We suggest a simple cycle in which Nab2p and Nab4p/Hrp1p, upon import, are released in the nucleus at sites of transcription by the concerted action of Ran-GTP and binding to newly synthesized mRNA. The resulting ribonucleoprotein complexes are exported to the cytoplasm, where Kap104p rebinds to Nab2p and Nab4p/Hrp1p, contributing to their release from mRNA.
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