Pheromone gradients guide and stabilize polarity sites for mating in Saccharomyces cerevisiae

Abstract

Many cells detect and follow gradients of chemical signals to perform their functions. Yeast cells use gradients of extracellular pheromones to locate mating partners, providing a tractable model to understand how cells decode the spatial information in gradients. To mate, yeast cells must orient polarity toward the mating partner. Initial orientation is often incorrect, but mobile polarity sites then explore the cell cortex until they reach the proper position, where they stop moving and “commit” to the mating partner. How does the polarity site reach the proper position, and what causes the polarity site to stop when it gets there?To address how the polarity site reaches the proper position, we imaged the polarity sites of mating yeast cells during the search for a partner. By analyzing polarity site movement, we found that the gradient of pheromone from the mating partner biases the direction of polarity site movement to improve search efficiency. We hypothesized that upon reaching alignment, polarity sites would detect a high concentration of pheromone from the mating partner, which could cause the polarity site to stop moving. We tested this hypothesis by asking if yeast cells could commit to partners that make different amounts of pheromone. We found that commitment is robust to varying pheromone levels, and suggest that steep pheromone gradients near each partner’s polarity site trap the polarity site in place.