Kap104p-mediated nuclear import. Nuclear localization signals in mRNA-binding proteins and the role of Ran and Rna.
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.
SciCrunch is a data sharing and display platform. Anyone can create a custom portal where they can select searchable subsets of hundreds of data sources, brand their web pages and create their community. SciCrunch will push data updates automatically to all portals on a weekly basis. User communities can also add their own data to scicrunch, however this is not currently a free service.