G proteins mediate changes in cell shape by stabilizing the axis of polarity.

Upon exposure to mating pheromone, yeast cells change their form to pear-shaped shmoos. We looked at pheromone-dependent cell shape changes in mutants that are unable to orient growth during mating and unable to choose a bud site. In these double mutants, cell surface growth, secretion sites, cytoskeleton, and pheromone receptors are spread out, explaining why these cells are round. In contrast, polarity establishment proteins localize to discrete sites in these mutants. However, the location of these sites wanders. Thus, these mutants are able to initiate polarized growth but fail to maintain the location of growth sites. Our results demonstrate that stabilization of the growth axis requires positional signaling from either the pheromone receptor or specific bud site selection proteins.