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The roles of yeast formins and their regulators Bud6 and Bil2 in the pheromone response

Published Online:
new hypothesis

Significance Statement

  • Formins and their regulators have been studied for many years in yeast, but almost exclusively in mitotically dividing cells, leaving their roles in the pheromone response poorly understood.

  • The authors here find that proper shmoo formation requires the formin Bni1 but not Bnr1, as well as the actin nucleation-promoting activities of Bud6, and a novel activity of Bil2 in spatially organizing Bni1 at mating projection tips.

  • This work demonstrates the physiological importance of formin spatial regulation in shaping actin networks, which in turn shapes cell morphology, and establishes an ideal in vivo system for dissecting Bni1 regulation.

In response to pheromone Saccharomyces cerevisiae extend a mating projection. This process depends on the formation of polarized actin cables which direct secretion to the mating tip and translocate the nucleus for karyogamy. Here, we demonstrate that proper mating projection formation requires the formin Bni1, as well as the actin nucleation promoting activities of Bud6, but not the formin Bnr1. Further, Bni1 is required for pheromone gradient tracking. Our work also reveals unexpected new functions for Bil2 in the pheromone response. Previously we identified Bil2 as a direct inhibitor of Bnr1 during vegetative cell growth. Here, we show that Bil2 has Bnr1-independent functions in spatially focusing Bni1-GFP at mating projection tips, and in vitro Bil2 and its binding partner Bud6 organize Bni1 into clusters that nucleate actin assembly. bil2∆ cells also display entangled Bni1-generated actin cable arrays and defects in secretory vesicle transport and nuclear positioning. At low pheromone concentrations, bil2∆ cells are delayed in establishing a polarity axis, and at high concentrations they prematurely form a second and a third mating projection. Together, these results suggest that Bil2 promotes the proper formation and timing of mating projections by organizing Bni1 and maintaining a persistent axis of polarized growth.