Abstract

The small Ras-like GTPase Ran plays an essential role in the transport of macromolecules in and out of the nucleus and has been implicated in spindle (1,2 ) and nuclear envelope formation (3,4 ) during mitosis in higher eukaryotes. We identified Saccharomyces cerevisiae open reading frame YGL164c encoding a novel RanGTP-binding protein, termed Yrb30p. The protein competes with yeast RanBP1 (Yrb1p) for binding to the GTP-bound form of yeast Ran (Gsp1p) and is, like Yrb1p, able to form trimeric complexes with RanGTP and some of the karyopherins. In contrast to Yrb1p, Yrb30p does not coactivate but inhibits RanGAP1(Rna1p)-mediated GTP hydrolysis on Ran, like the karyopherins. At steady state, Yrb30p localizes exclusively to the cytoplasm, but the presence of a functional nuclear export signal and the localization of truncated forms of Yrb30p suggest that the protein shuttles between nucleus and cytoplasm and is exported via two alternative pathways, dependent on the nuclear export receptor Xpo1p/Crm1p and on RanGTP binding. Whereas overproduction of the full-length protein and complete deletion of the open reading frame reveal no obvious phenotype, overproduction of C-terminally truncated forms of the protein inhibits yeast vegetative growth. Based on these results and the exclusive conservation of the protein in the fungal kingdom, we hypothesize that Yrb30p represents a novel modulator of the Ran GTPase switch related to fungal lifestyle.