Mechanisms of Chemotropism in Fungi: Saccharomyces cerevisiae as a Model

Abstract

Budding yeast decode pheromone gradients to locate mating partners, providing a model of chemotropism in fungi. How yeast polarize toward a single partner in crowded environments is unclear. Initially, cells often polarize in unproductive directions, but then they relocate the polarity site until two partners’ polarity sites align, whereupon the cells “commit” to each other by stabilizing polarity to promote fusion. Using live-cell fluorescence microscopy, computational modeling, and quantitative autocorrelation analyses, I address the role of the early mobile polarity sites, finding that commitment by either partner failed if just one partner was defective in generating, orienting, or stabilizing its mobile polarity sites. Mobile polarity sites were enriched for pheromone receptors and G proteins, suggesting that such sites engage in an exploratory search of the local pheromone landscape, stabilizing only when they detect elevated pheromone levels. Mobile polarity sites were also enriched for pheromone secretion factors, and simulations suggest that only focal secretion at polarity sites would produce high pheromone concentrations at the partner’s polarity site, triggering commitment.