Role of Gα–MAPK interaction in mating of <i>Saccharomyces cerevisiae</i>

Guidance of cell growth or movement in response to chemical cues in the environment is critical for many cell behaviors. Budding yeast orientation of polarized growth in response to gradients of mating pheromones provides a tractable model to address how cells accurately assess small spatial differences in chemical concentrations. Pheromones bind to receptors that act through heterotrimeric G proteins to promote activation of the MAPK Fus3. Active Fus3 binds to Gα, which is thought to enhance local phosphorylation of relevant MAPK substrates to promote orientation of polarity toward high-pheromone regions. Polarity is oriented by a pathway in which Gβγ binds the scaffold protein Far1 to activate the conserved polarity regulator Cdc42, which activates the formin Bni1 to orient actin and hence growth. Gβγ, Far1, and Bni1 are all MAPK substrates whose phosphorylation could improve orientation toward high-pheromone regions. Here, we show that the Gα-MAPK interaction can enhance the efficiency of polarity-site alignment between mating partners, particularly under conditions with high Fus3 activity. Surprisingly, however, we find no evidence that phosphorylation of Gβγ, Far1, or Bni1 contribute to the benefit conferred by Gα-MAPK interaction. The precise role of this interaction remains mysterious.