Nuclear sequestration of the exchange factor Cdc24 by Far1 regulates cell polarity during yeast mating

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Abstract
Cytoskeletal rearrangements during the cell cycle and in response to signals are regulated by small Rho-type GTPases, but it is not known how these GTPases are activated in a spatial and temporal manner. Here we show that Cdc24, the guanine-nucleotide exchange factor for the yeast GTPase Cdc42, is sequestered in the cell nucleus by Far1. Export of Cdc24 to a site of cell polarization is mediated by two mechanisms. At bud emergence, activation of the G1 cyclin-dependent kinase Cdc28-Cln triggers degradation of Far1 and, as a result, relocation of Cdc24 to the cytoplasm. Cells overexpressing a non-degradable Far1 were unable to polarize their actin cytoskeleton because they failed to relocate Cdc24 to the incipient bud site. In contrast, in response to mating pheromones, the Far1-Cdc24 complex is exported from the nucleus by Msn5. This mechanism ensures that Cdc24 is targeted to the site of receptor-associated heterotrimeric G-protein activation at the plasma membrane, thereby allowing polarization of the actin cytoskeleton along the morphogenetic gradient of pheromone. Either degradation of Far1 or its nuclear export by Msn5 was sufficient for cell growth, suggesting that the two mechanisms are redundant for cell viability. Taken together, our results indicate that Far1 functions as a nuclear anchor for Cdc24. This sequestration regulates cell polarity in response to pheromones by restricting activation of Cdc42 to the site of pheromone receptor activation.