YRB4P, A YEAST RAN-GTP-BINDING PROTEIN INVOLVED IN IMPORT OF RIBOSOMAL-PROTEIN L25 INTO THE NUCLEUS

Gsp1p, the essential yeast Ran homologue, is a key regulator of transp ort across the nuclear pore complex (NPC), We report the identificatio n of Yrb4p, a novel Gsp1p binding protein, The 123 kDa protein was iso lated from Saccharomyces cerevisiae cells and found to be related to i mportin-beta, the mediator of nuclear localization signal (NLS)-depend ent import into the nucleus, and to Pse1p, Like importin-beta, Yrb4p a nd Pse1p specifically bind to Gsp1p-GTP, protecting it from GTP hydrol ysis and nucleotide exchange, The GTPase block of Gsp1p complexed to Y rb4p or Pse1p is released by Yrb1p, which contains a Gsp1p binding dom ain distinct from that of Yrb4p, This might reflect an in vivo functio n for Yrb1p, Cells disrupted for YRB4 are defective in nuclear import of ribosomal protein L25, but show no defect in the import of proteins containing classical NLSs, Expression of a Yrb4p mutant deficient in Gsp1p-binding is dominant-lethal and blocks bidirectional traffic acro ss the NPC in wild-type cells, L25 binds to Yrb4p and Pse1p and is rel eased by Gsp1p-GTP, Consistent with its putative role as an import rec eptor for L25-like proteins, Yrb4p localizes to the cytoplasm, the nuc leoplasm and the NPC.