Nuclear export of Far1p in response to pheromones requires the export receptor Msn5p/Ste21p

Far1p is a bifunctional protein that is required to arrest the cell cycle a nd to establish cell polarity during yeast mating. Far1p is localized predo minantly in the nucleus but accumulates in the cytoplasm in cells exposed t o pheromones. Here we show that Far1p functions in both subcellular compart ments: nuclear Far1p is required to arrest the cell cycle, whereas cytoplas mic Far1p is involved in the establishment of cell polarity. The subcellula r localization of Far1p is regulated by two mechanisms: (1) Far1p contains a functional bipartite nuclear localization signal (NLS), and (2) Far1p is exported from the nucleus by Msn5p/Ste21p, a member of the exportin family. Cells deleted for Msn5p/Ste21p failed to export Far1p in response to phero mones, whereas overexpression of Msn5p/Ste21p was sufficient to accumulate Far1p in the cytoplasm in the absence of pheromones. Msn5p/Ste21p was local ized in the nucleus and interacted with Far1p in a manner dependent on GTP- bound Gsp1p. Two-hybrid analysis identified a small fragment within Far1p t hat is necessary and sufficient for binding to Msn5p/Ste21p, and is also re quired to export Far1p in vivo. Finally, similar to Delta msn5/ste21 strain s, cells expressing a mutant Far1p, which can no longer be exported, exhibi t a mating defect, but are able to arrest their cell cycle in response to p heromones. Taken together, our results suggest that nuclear export of Far1p by Msn5p/Ste21p coordinates the two separable functions of Far1p during ma ting.