Antagonistic effects of NES and NLS motifs determine S-cerevisiae Rna1p subcellular distribution

Nucleus/cytosol exchange requires a GTPase, Ran. In yeast Rna1p is the GTPa se activating protein for Ran (RanGAP) and Prp20p is the Ran GDP/GTP exchan ge factor (GEF). RanGAP is primarily cytosolic and GEF is nuclear. Their su bcellular distributions led to the prediction that Ran-GTP hydrolysis takes place solely in the cytosol and GDP/GTP exchange solely in the nucleus. Cu rrent models propose that the Ran-GTP/Ran-GDP gradient across the nuclear m embrane determines the direction of exchange. We provide three lines of evi dence that Rna1p enters and leaves the nuclear interior. (1) Rna1p possesse s leucine-rich nuclear export sequences (NES) that are able to relocate a p assenger karyophilic protein to the cytosol; alterations of consensus resid ues re-establish nuclear location. (2) Rna1p possesses other sequences that function as a novel nuclear localization sequence able to deliver a passen ger cytosolic protein to the nucleus. (3) Endogenous Rna1p location is depe ndent upon Xpo1p/Crm1p, the yeast exportin for leucine-rich NES-containing proteins. The data support the hypothesis that Rna1p exists on both sides o f the nuclear membrane, perhaps regulating the Ran-GTP/Ran-GDP gradient, pa rticipating in a complete RanGTPase nuclear cycle or serving a novel functi on.