Disruption of the YRB2 gene retards nuclear protein export, causing a profound mitotic delay, and can be rescued by overexpression of XPO1/CRM1

Disruption of the YRB2 gene encoding a nuclear Ran-binding protein homologous to Yrb1p/RanBP1 makes Saccharomyces cerevisiae cold sensitive for colony-formation, but not for growth in liquid medium. Schizosaccharomyces pombe Hba1p, which is homologous to Saccharomyces cerevisiae Yrb2p, rescued the cold sensitivity of Delta yrb2 cells, When released from an ct factor block, Delta yrb2 cells underwent a prolonged delay at the short spindle stage of mitosis with a normal level of C1b/p34(CDC28) kinase activity, but there was no chromosome loss, this being consistent with the finding that Delta yrb2 was synthetic lethal with neither Delta mad1 nor Delta mad3. The cold sensitive colony-formation of Delta yrb2 cells was rescued by both XPO1/CRM1 and GSP1, but not CDCS, carried on a multicopy vector. XPO1/CRM1 rescued Delta yrb2 even in a single copy. Consistent with such a tight functional interaction, Xpo1p/Crm1p directly bound to Yrb2p, but not Yrb1p, and Delta yrb2 cells were found to have a defect in nuclear export signal (NES)-dependent nuclear protein export. From these results together, the ability of Xpo1/Crm1p to export NES-proteins is suggested to be enhanced by both Yrb2p and Gsp1p, and thereby disruption of YRB2 retards nuclear protein export, resulting in the mitotic delay.