Mating and Marital Fidelity in Saccharomyces cerevisiae

Abstract

Haploid cells of the budding yeast Saccharomyces cerevisiae communicate using secreted pheromones and mate to form diploid zygotes. Mating is monogamous, resulting in the fusion of precisely one cell of each mating type. Monogamous mating in crowded conditions, where cells have access to more than one potential partner, raises the question of how multiple-mating outcomes are prevented. Here we identify mutants capable of mating with multiple partners, revealing the mechanisms that ensure monogamous mating. Before fusion, cells develop polarity sites oriented toward potential partners. Competition between these polarity sites within each cell leads to disassembly of all but one focus, thus favoring a single fusion event. Fusion promotes the formation of heterodimeric complexes between subunits that are uniquely expressed in each mating type. One complex shuts off haploid-specific gene expression, and the other shuts off the ability to respond to pheromone. Zygotes able to form either complex remain monogamous, but zygotes lacking both can re-mate.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 towards 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 towards high-pheromone regions. Here we show that the Gα-MAPK interaction can enhance the efficiency of polarity site alignment between mating partners, although the magnitude of that effect depends on context. Surprisingly, however, we find no evidence that phosphorylation of Gβγ, Far1, or Bni1 contribute to the benefit conferred by Gα-MAPK interaction. The role of this interaction remains mysterious.