The nuclear export receptor Xpo1p forms distinct complexes with NES transport substrates and the yeast ran binding protein 1 (Yrb1p)

Xpo1p (Crm1p) is the nuclear export receptor for proteins containing a leuc ine-rich nuclear export signal (NES). Xpo1p, the NES-containing protein, an d GTP-bound Ran form a complex in the nucleus that translocates across the nuclear pore. We have identified Yrb1p as the major Xpo1p-binding protein i n Saccharomyces cerevisiae extracts in the presence of GTP-bound Gsp1p (yea st Ran). Yrb1p is cytoplasmic at steady-state but shuttles continuously bet ween the cytoplasm and the nucleus. Nuclear import of Yrb1p is mediated by two separate nuclear targeting signals. Export from the nucleus requires Xp o1p, but Yrb1p does not contain a leucine-rich NES. Instead, the interactio n of Yrb1p with Xpo1p is mediated by Gsp1p-GTP. This novel type of export c omplex requires the acidic C-terminus of Gsp1p, which is dispensable for th e binding to importin beta -like transport receptors. A similar complex wit h Xpo1p and Gsp1p-GTP can be formed by Yrb2p, a relative of Yrb1p predomina ntly located in the nucleus. Yrb1p also functions as a disassembly factor f or NES/Xpo1p/Gsp1p-GTP complexes by displacing the NES protein from Xpo1p/G sp1p. This Yrb1p/Xpo1p/Gsp1p complex is then completely dissociated after G TP hydrolysis catalyzed by the cytoplasmic GTPase activating protein Rna1p.