A Mechanistic and Computational Analysis of Pheromone Receptor Polarity Establishment in Yeast

The mating response of S. cerevisiae is directional. Mating cells must interpret the direction of the pheromone gradient and grow toward a potential partner. The pheromone receptors are the most upstream component of the mating response. They are distributed uniformly on the plasma membrane during vegetative growth, but in response to pheromone, they are rapidly internalized and subsequently polarized to the region where the mating projection will form. We have previously shown that internalization of the receptor is required for its polarization but that actin-dependent directed secretion is not. Pheromone also induces the polarization of the G protein subunits, Gα and Gβ, and this is dependent on receptor internalization. How is receptor polarity established prior to actin-dependent directed secretion? In a genetic screen for proteins that interact with Gβ, we identified Yck1, one of the two yeast casein kinases required for receptor phosphorylation and internalization. Here we show that Gβ interacts with Yck1 on the plasma membrane and inhibits receptor phosphorylation. In gradient-stimulated cells, the most concentrated region of unphosphorylated receptor anticipates the eventual shmoo site, often after moving from the presumptive default polarization site. Additionally, we show that receptor internalization is asymmetric in response to pheromone treatment. A mutant form of Gβ, whose interaction with Yck1 is diminished, does not inhibit receptor phosphorylation, and this correlates with defects in receptor polarization and gradient tracking. These data suggest that polarity is first established in response to a pheromone gradient by differential internalization of the receptor and G protein, which depends in turn, on asymmetric protection of the receptor by its G protein. We have developed a computational model that mimics gradient induced receptor polarity upstream of directed secretion and identifies key aspects of this putative feedback mechanism.