A RANBP1 MUTATION WHICH DOES NOT VISIBLY AFFECT NUCLEAR IMPORT MAY REVEAL ADDITIONAL FUNCTIONS OF THE RAN GTPASE SYSTEM

a nuclear GTPase, and a number of interacting proteins, including regu lators RanGEF1 and RanGAP1, are involved in nucleocytoplasmic transpor t. We have identified a new temperature-sensitive mutation in budding yeast YRB1 gene, which encodes Ran-binding protein-1 (RanBP1), In cont rast to other yrb1 alleles, the new mutation (yrb1-21) does not cause visible defects in import of nuclear proteins Npl3p, histone H2B, or p -galactosidase fused to a nuclear localization signal. We hypothesize that the inviability of mutant cells at the restrictive temperature is caused by an additional essential function of RanBP1 other than nucle ar import. This function may be revealed by the terminal phenotypes of yrb1-21, which include failure of the mitotic spindles to properly al ign along the mother-bud axis and accumulation of cells in late mitosi s or G1 phase of the cell cycle. These features are shared, in part, b y a mutation in RanGEF1, but not in RanGAP1. The yrb1-21 allele suppre sses a RanGEF1 mutation, indicating that RanGEF1 and RanBP1 may be inv olved in the same essential function, (C) 1998 Academic Press.