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 transport across the nuclear pore complex (NPC). We report the identification of Yrb4p, a novel Gsp1p binding protein. The 123 kDa protein was isolated from Saccharomyces cerevisiae cells and found to be related to importin‐β, the mediator of nuclear localization signal (NLS)‐dependent import into the nucleus, and to Pse1p. Like importin‐β, Yrb4p and Pse1p specifically bind to Gsp1p–GTP, protecting it from GTP hydrolysis and nucleotide exchange. The GTPase block of Gsp1p complexed to Yrb4p or Pse1p is released by Yrb1p, which contains a Gsp1p binding domain distinct from that of Yrb4p. This might reflect an in vivo function 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 across the NPC in wild‐type cells. L25 binds to Yrb4p and Pse1p and is released by Gsp1p–GTP. Consistent with its putative role as an import receptor for L25‐like proteins, Yrb4p localizes to the cytoplasm, the nucleoplasm and the NPC.